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Mechanism design of the flapper actuator in Chinese Braille display
Sensors and Actuators A 135 (2007) 680–689
Mechanism design of the flapper actuator in Chinese Braille display Fung-Huei Yeh, Shih-Hao Liang ∗ Mechanical and Electro-Mechanical Engineering, Tamkang University, Tamsui, Taipei County 251, Taiwan, ROC Received 3 March 2006; received in revised form 2 June 2006; accepted 5 July 2006 Available online 22 August 2006
Abstract This report presents the mechanism design of flapper type actuator used in a Chinese Braille display called Golden Braille display (GBD) in Taiwan. An actuator is the most important part to manipulate the Braille dot in Braille display. The proposed flapper actuator is designed with numerical analysis, including contact mechanics, heat transfer, and electromagnetics to ensure the feasibility and durability of the design. Experiments are conducted to verify the simulation results of numerical analysis. The optimal results indicate that the flapper should operate at 6 V with a 330 enameled coil. The thrust force of Braille dot reaches 15.9 gf and the temperature rising of flapper is well restrained at 42.9 ◦ C when the cooling blowers are equipped in GBD. Mechanics analysis shows that even when the flapper’s trust force reaches 20 gf to interact with finger’s pressing force, the absolutely maximum stress of flapper’s armature is only 1.16E−1% of its yield stress and the Braille dot of Braille pin is only 1.19E−2% of its yield stress. The negligible stress loading indicates the structure of flapper is sufficiently robust. Our investigation also shows that GBD is a durable and economically viable alternative of Braille display. © 2006 Elsevier B.V. All rights reserved. Keywords: Braille display; Flapper actuator; Computer aided analysis; Contact mechanics; Heat transfer; Electromanetics
1. Introduction Braille display is a device with raised-characters for providing the visually impaired people with the means to read information. A Braille display comprises both mechanical and electronic parts for displaying a line of Braille codes, created by Louis Braille in the 19th century to replace the uncomfortably tactile recognition of line-type characters. A Braille cell contains a cell of 3 × 2 dots array representing a character, with every dot being convex or flat to the touch of the fingertip. In comparison, a tactile display called Optacon was developed by Linvill [1] for producing the visual shape letters mapping from text characters on the screen in 1960s. Optacon contained a 6 × 24 array of vertical vibrating rods, and the users seated their fingertips on vibrating rods to identify a tactile image of each letter scanned by the lens. The operation of Optacon was inconvenient and time consuming for reading printed article. Today, Braille codes are still the most popular way for the tactile recognition of characters with finger. At present, the number of dots in a Braille cell is usually six or eight, arranged as 3 × 2 or 4 × 2. The standard
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Corresponding author. Tel.: +886 2 2621 5656x2012; fax: +886 2 2620 9745. E-mail address: [email protected] (S.-H. Liang).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.07.013
Chinese Braille display uses two or three Braille cells to present a Chinese character. Fig. 1 shows the Braille codes in English and Chinese form of the character ”Braille”. A Braille display uses an actuator to move a Braille pin up or down to control the convex or flatness of a Braille dot. There are various types of actuators used in the current Braille display, such as solenoid, relay, piezo-electric beam, shape-memory alloy (SMA), and electrorheological (ER) fluid. Frisken-Gibson et al. [2] arranged the solenoids in two vertical layers with one layer interleaving with the other to improve the tactile resolution. Sixty-four solenoids (8 × 8 pins) were arranged in two layers and each solenoid provided four different levels of raised height. Roberts et al. [3] developed another creatively designed mechanism with only three solenoids and rotating wheel to minimize the volume of display. The Braille cell was located on the outer ring of the rotating wheel like the tread on a tire. The rotating angle of wheel was controlled by software for displaying enough Braille codes. Braille pins were pushed or pulled by only three solenoids, and the pin retention device inside the wheel included two layers of three tracks for holding Braille pins in upward or downward movement. Sriskanthan and Subramanian [4] used relays as actuators and integrated 40 Braille cells into a complete Braille display. Users can use this Braille display for receiving the Braille
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Fig. 1. Braille codes and the standard Chinese Braille cell.
codes transferred from personal computers (PC). However, Srikanthan neither disclosed the design of the relay actuator in the report nor analyzed and evaluated the durability of the design. Piezo-electric actuated Braille displays are the most popular commercialized product for displaying Braille codes. A simply supported piezo-electric beam needs at least 200 V for raising a Braille pin. A unique design of Braille display [5] was to integrate a piezo-electrical Braille cell into the outer edge of pantograph’s extremity. The user reads the Braille cell and moves cursor through the text by using the horizontally bi-dimensional pantograph. The main shortcoming of using the one-cell Braille display is that the user’s finger cannot brush across the Braille cell; instead, the finger is sitting on the refreshable Braille cell, which does not suit the user’s reading habit. Yobas et al. [6,7] recently used micro-electro-mechanical systems (MEMS) for developing the pneumatic actuator manipulated by piezoelectric valve, and successfully presented a preliminary result on making a single Braille dot. SMA wire [8–10] and ER fluid [11] were also used as actuators. Inputting the current into the two ends of an SMA wire can cause the wire to shrink. A shrinking wire can push or pull a pin depending on the different actuating mechanisms, but this system needs a cooling device to eliminate the large amount of generated heat. On the other hand, inputting the current on specific area of the ER fluid can govern the shape of the raised dot. ER fluid covered with compliant rubber needs high voltage to operate, and the high voltage inside the system of ER fluid must be monitored for the safety reason. A creative flapper actuator for Braille display is developed in Taiwan. This report presents a study on the design and analysis of a flapper actuating mechanism used in Braille display. The actuating principle of a flapper is also based on the electromagnetic theory, similar to solenoid and relay. Electromagnetic actuator has several advantages such as fast response time, larger
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force producing, and achieving with low voltage [12]. Solenoid and relay are assembled with iron core and enameled coil. The iron core wound with the enameled coil carries current produces magnetic force. Solenoid is used to as actuator or sensor because of its moveable iron core makes reversibility of electric and magnetic effect. The iron core of solenoid actuator is a moveable part as the moving transducer itself, and the current flow in enameled coil can produce the magnetic force for driving iron core. On the other hand, relay is only used to mechanically switch an electronic circuit on or off. Relay’s actuating mechanism consists of fixed iron core for enhancing magnetic force, coil for producing magnetic field, moveable armature for manipulating electrodes, driving electrodes mechanism for circuit switching purpose, and a spring for pulling the armature back to its quiescent position. However, the flapper presented in this study is a new design with a different actuating mechanism from those of solenoid and relay. Flapper in this study does not consist of the driving electrodes mechanism in relay because of the circuit switching function is not needed in actuating the Braille dot. The spring used in relay’s armature is also omitted in flapper, and the armature in flapper can restore to its quiescent position by gravity effect. Golden Braille display (GBD) is a flapper-actuated Braille display developed in Taiwan. This report presents the analysis of feasibility and durability of the design by using contact mechanics, heat transfer, and electromagnetics theories as well as empirical experiments, and tests by visually impaired people, which few other reports have done. GBD has 45 Braille cells and each Braille cell has eight Braille dots. There are altogether 360 flapper actuators in a GBD display. The reason for using flapper is that it has a longer lifetime in comparison with the peizo-electric actuator. The driving force of flapper has the same intensity when the pressing force of finger is less than the driving force, while the driving force of the most popular beamtype piezo-electric actuator varies with the pressing force of a finger. Moreover, the feeling of brushing through the flapper type Braille display most resembles that of touching the paper with Braille dot; the sense of touching is better and more comfortable than beam-type piezo-electric actuator. Every flapper-actuated Braille cell is made as an individual module and, therefore, can be used in hot plug and is easy for repair. Still another benefit for using flapper is the manufacturing cost of flapper is only USD $8 per Braille cell, compared to USD $35 per piezoelectric actuator Braille cell. In short, GBD provides longer duration and lower manufacturing cost than piezo-electric type does. The flapper actuator of GBD is mainly composed of iron core, enameled coil, and armature mechanism. The flapper’s volume and the standard stroke of Braille dot further limit the geometry of flapper actuator design, which will be described in greater details in the following sections. The rest of the report is organized as follows: Section 2 describes the structure of the flapper type actuator and the Braille display. Section 3 explains the basic theories behind the design of the flapper type actuator. Section 4 is devoted to the numerical analysis and the empirical experiments, followed by Section 5, conclusion.
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Fig. 3. Parts of flapper type actuator used in Chinese Braille display.
The models of flapper are divided into two types according to different types of armatures, and the Braille pins also have four different lengths. The complete assemblage of the actuating mechanism of a Braille cell is shown in Fig. 4. Fig. 2. Principle of the magnetic field production.
2. Structure of flapper type actuator and Braille display The structure of flapper was designed to actuate a Braille dot upward by inputting the current. When the current the flapper carries vanishes, the gravity effect will draw the Braille dot downward to the original position. As described earlier, each Braille cell consists of eight Braille dots, and a complete GBD has 45 Braille cells. 2.1. Electromagnetic parts and Braille pin The operation principle of flapper is based on the electromagnetics theory. Fixed irons core inside the enameled coil and a rotatable armature form the flapper actuator. A long enameled coil wrapped in helix carries the electric current, which produces the magnetic field lines. An iron core put inside the enameled coil enhances the magnetic force field shown in Fig. 2. The magnetic field lines traveling along the axis inside the iron core bend outside around the enameled coil, and then re-enter the other end to form a closed loop. The pole of iron core produces a strong magnetic force and an iron armature is attracted by the pole to drive a Braille pin. The mechanism of force-producing structure of flapper used in GBD is shown in Fig. 3. Part A is an iron core for enhancing magnetic force. Part B is a plastic frame to cover and bind the iron core for electrically insulating purpose, and the enameled coil C is wound on the surface of the plastic frame. The yoke D is used to fix the iron core, to enhance the magnetic field, and to become the fulcrum of armature. Part E is an iron made armature actuated by iron core to push the Braille pin F upward. Part G is an armature clip for limiting the degree of freedom (DOF) of armature in only one rotation direction.
2.2. Operations of flapper There are two opposite conditions of flapper in operation. One is the flapper carries no current, and no magnetic force is produced to attract the armature. The gravity effect causes the point e3 in armature and the Braille pin to move downward, as shown in Fig. 5. There is a small air gap between point e1 in armature (part E) and part A, and point e3 in part E is at its lowest position. The other condition is the flapper carrying current, and the armature resembling a lever mechanism that is attracted to push a Braille pin upward. The armature has a contact location e1 with part A that produces the magnetic force. The point e2 in armature is a fulcrum clipped by armature clip G, and the only one DOF of armature at point e2 is the rotation about zaxis. Point e3 is lifted to contact Braille pin when point e1 is
Fig. 4. Assembled mechanism of a Braille cell.
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Fig. 5. Operational principle of a flapper to manipulate a Braille pin.
attracted to contact part A; meanwhile, the Braille pin is pushed upward. Point e2 in armature must not have translation but allows rotation effect when the armature is pulled to lift Braille pins. The mechanical interference between armature and armature clip can be avoided when the rotational angle of point e2 is limited within a specific range. Point e2 can be treated as a hinge joint when the armature is actuated, and the turning angle between e1 e2 and e2 e3 can change the vertical stroke at point e3 . e2 e3 is in horizontal position when the flapper is not carrying current, and there is a small angle R between e1 e2 and a vertical line. The flapper carrying current then pulls e1 e2 part of armature, and point e3 produces the upward displacement h. Displacement h can be defined as: h = e2 e3 × sin(R)
Fig. 6. CAD designed Braille cell module and the manufactured prototype.
(1)
The rotation angle is small, so R can be estimated as: R=
h e2 e3
(2)
The length of flapper refers to the length of armature e2 e3 . The angle R then can be easily estimated when the upper stroke h of a Braille pin reaches the standard 0.7 mm. The length ratio of two lever arms of armature is approximately 1:3 (e1 e2 :e2 e3 ). This specific length ratio magnifies three times the upward displacement of Braille pin of the air gap between the iron core and armature. This design allows the use of a smaller space of air gap in the flapper to generate a larger displacement of Braille pin. The small space of air gap also has the advantage of requiring less current to attract the armature. 2.3. Braille cell module and Braille display There are eight Braille pins forming a complete Braille cell. The combination of Braille cell mechanism and electronic circuit is called a Braille cell module. Each individual flapper actuates an individual Braille pin. The full functional prototype of a Braille cell module was originally manufactured from the designed CAD model shown in Fig. 6. Eight flappers are soldered on a circuit board with a tactile surface which contains eight vertical holes for holding eight Braille pins. People can put their fingertip on the tactile surface to identify a Braille code
Fig. 7. Overall structure of a Braille display.
when each flapper is controlled to push the Braille pin upward or maintain the pin in the flat status. GBD has a row of 45 Braille cells to display the Braille codes. The entire machine contains a machine case, a motherboard, 45 Braille cell modules, a power supply, and two cooling blowers. Fig. 7 shows only one Braille cell module plugged in the 23rd slot of the motherboard for clarity. The protocol is compatible with LPT port, COM port, USB port, and wireless Bluetooth for transmitting the Braille codes from PC to GBD. 3. Basic theory This study adopts the theories of contact mechanics, heat transfer and electromagnetics for efficient designing and ana-
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lyzing of the feasibility of flapper structure. Contact mechanics can analyze the mechanism to ensure the durability of flapper structure. Heat transfer and electromagnetics are used to obtain the electrical parameters of flapper [13], and the object is to find the feasible setting of the heat generated by the enameled coil and the electromagnetic force produced by the flapper. After applying the aforementioned theories in the design processes, the final flapper can be more durable for GBD operation. 3.1. Contact analysis of flapper by contact mechanics Contact mechanics in finite element method (FEM) [14] can analyze the intensity of pressure and deformation in flapper parts. When the flapper actuator is actuated and the user’s fingertip presses the Braille dot, the armature is contacted with Braille pin and iron core. The mechanical part in flapper is under the externally applied force and the flapper structure is deformed. The contact analysis therefore can ensure the flapper structure is durable for Braille display in practical use. The numerical techniques are useful to the analysis of the contact problem because of the flexibility and robustness for analyzing the complicated geometry of objects and various contact conditions. The contact mechanics of FEM techniques can be divided into node-to-node technique, node-to-surface technique, and surface-to-surface technique. The contact analysis of flapper can use the node-to-node technique. The simplified node-to-node technique can be used to analyze the contact conditions when the movement of parts is considered and the contact location is known beforehand. Penalty approximation is used to solve the contact problem in FEM to enforce contact compatibility. When two nodes in FEM mesh contact each other, a stiffness relationship between two contact areas is specified. A spring with stiffness k between two contact nodes must be provided when the contact condition occurs. The spring between two contact nodes therefore deforms with a displacement quantity δ, and the equilibrium equation is: F = kδ
(3)
where k is the contact stiffness and F is called the contact force. The contact stiffness in penalty method depends on the contact material. 3.2. Thermal analysis of Braille cell module Limiting the temperature increase in flapper is necessary for guarding the thermal damage in the flapper, electronic ICs, and plastic parts of the Braille cell. The enameled coil of flapper must be supplied sufficient current to sustain the upward force for a Braille pin. Because of the electric resistance, the enameled coil generates heat when the current is applied. Consequently, the flapper temperature rises. The electric resistance of enameled coil increases with the rising temperature, and this effect should be estimated. The current inside the enameled coil decays as the resistance of the enameled coil increases by temperature rising, and the Braille cell module finally reaches a thermally steady state. The thermal analysis of Braille cell module would
adopt unsteady time analysis because the electric resistance of enameled coil depends on temperature. The relation between temperature and copper made enameled coil is expressed as: T f − Ti Rf = Ri · 1 + (4) 234.5 + Ti Ri is the electric resistance of enameled coil in the initial state without current carrying and Rf is the electric resistance of enameled coil in the state of thermal balance. Ti is the initial temperature of enameled coil and Tf is the temperature in the state of thermal balance. The flapper has the heat rate q˙ G , which depends on temperature T in thermal balance state stated in Eq. (5): q˙ G = q˙ G (T ) =
V2 Rf
(5)
The heat source of GBD is mainly from the flapper. Every Braille cell module has eight flappers, and a GBD with 45 Braille cells has 360 flappers altogether. The entire GBD generates maximum heat when the 360 flappers lift their own Braille pins at the same time. The Braille cell module located near the middle of 45 Braille cell modules will be at the highest temperature when the maximum heat generation state occurs. The compact 2D heat transfer equation in (6) expresses the thermal behavior of a Braille cell module. ρc
(T∞ − Tp ) qflux ∂T ∂2 T ∂2 T = k 2 + k 2 + h∞ + + q˙ G ∂t ∂x ∂y L L
(6)
where ρ is the density, c the specific heat, and k is the conductivity. h∞ and T∞ , respectively, are the heat transfer coefficient and the ambient temperature of air. L is the thickness of circuit board, t the time, and T is the temperature parameter. This heat transfer equation can be numerically solved by finite difference time domain (FDTD) method [15]. 3.3. Electromagnetic analysis The upward force of Braille pin should be over 15 gf for comfortable Braille reading. The attracting force of flapper supported by direct current can be analyzed and designed based on the static magnetic theorem [16]. Field intensity H and flux density B obey Eqs. (7) and (8): ∇ ×H =J
(7)
∇ ×B =0
(8)
According to the variant material properties, Eq. (9) shows the relationship of B and H: B = μH
(9)
If the property of magnetic material is not linear, then permeability μ is a function of B that can be expressed as: μ=
B H(B)
(10)
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By the magnetic vector potential approach, flux density B resulted from vector potential A yields (11): B =∇ ×A
(11)
The definition of B in Eq. (11) always satisfies Eq. (8); thus Eq. (11) can be rewritten as following form: 1 ∇ ×A =J (12) ∇× μ(B) If the property of material is linear and isotropic, Eq. (12) can be simplified as: 1 − ∇ 2A = J μ
(13)
The Maxwell stress tensor is defined as a magnetic force per unit area produced by the magnetic filed on a surface. The differential form of force can be written as: 1 (H(B · n) + B(H · n) − H(B · n)) (14) 2 n is a vector starting from a point on surface and is normal to the surface. The net force on the armature is obtained by creating a surface, which completely encloses the surface of armature and integrates the magnetic stress with the surface. The value of attracting force is calculated from Eq. (14) by the discretized method of FEM. dF =
4. Results of numerical analysis and experiment The numerical analysis based on the theory described in Section 3 can obtain the value of stress, strain, temperature, and thrust force in flapper. The value of stress and strain in armature and Braille pin can be obtained by using contact mechanics to ensure the solid and durable mechanism. The temperature of flapper can be estimated from thermal analysis to guard overheating situations in GBD, and the thrust force of flapper can be estimated by electromagnetic analysis to guarantee comfortable Braille reading. The experiment of thrust force and temperature in flapper is used to confirm the feasibility and comfort of machine use. 4.1. Results of numerical mechanics analysis The static analysis of flapper’s mechanism emphasizes the stress and strain of armature and Braille pin. The purpose is to ensure the flapper’s mechanism to stay below the elastic limits in operation, and the permanent deformation in flapper’ mechanism is not allowed. The test of Braille display by visually impaired people shows that a minimum of 15 gf for the upward force is required to provide ease of use and correct Braille identification. The boundary conditions of numerical analysis match the actual situation of flapper operation. The numerical analysis is performed under the interaction situation of the actuating force f1 and pressing force f2 shown in Fig. 5. At the initial state, the enameled coil of flapper does not carry current. The stress and strain does not occur in the armature and Braille pin except the negligible effect by the gravity effect. The iron core
Fig. 8. FEM mesh and the location of concentrated stress in A type armature.
produces attracting force f1 to the armature when the current flows into the enameled coil. Meanwhile, the point e3 in armature pushes the Braille pin 0.7 mm upward. When a user’s finger is pressing the Braille dot, a downward force f2 is produced in point e3 . In simulation, the pressing force f2 is set as 20 gf to ensure the higher safety factor in flapper structure. The iron core always draws the point e1 in armature when the attracting force is able to be against the downward force f2 . The observation of simulated results is focused on the estimated strain and stress in armature and Braille pin. The elastic displacement transformed from the strain is also an important issue in the design process. The A and B type armature is analyzed by FEM with suitable mesh under the effects of electromagnetic attracting force and downward force produced by a fingertip. The FEM mesh of A type armature is shown in Fig. 8. The maximum of absolute stress and of absolute stain both occur in the notch of the armature. The yield stress of wrought iron is 210 MPa (1 MPa = 9.81 × 105 gf/cm2 ), and the results of numerical analysis show that the armature is firm enough. The values of stress and strain occurred in the notch of A and B types of armature are shown in Table 1. The results indicate the iron made armature is under the elastic limit with an extremely small loading. Under the same FEM analysis condition with Braille pins, the critical values of stress and strain occur on the tip of Braille pins. The Braille pins of four different lengths are modeled and analyzed to obtain the location of maximum absolute values of Table 1 Mechanics results of A and B type armature (pressing force f2 = 20 gf) Type of armature
A
B
Vertical displacement at point e3 (cm) Minimum stress (gf/cm2 ) Percentage of minimum stress compared with yield stress (210 MPa for wrought iron) (%) Maximum stress (gf/cm2 ) Percentage of maximum stress compared with yield stress (210 MPa for wrought iron) (%) Minimum strain Maximum strain
−3.54E−3 −253801 1.23E−1
−2.67E−3 −288015 1.40E−1
342463 1.66E−1
292438 1.42E−1
−1.15E−4 1.56E−4
−1.40E−4 1.43E−4
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Fig. 10. The complete arrangement of relays in GBD.
Fig. 9. FEM mesh and the location of concentrated stress in A type Braille pin.
stress and strain. The FEM mesh of A type Braille pin is shown in Fig. 9. The maximum of absolute stress and absolute strain occur in the Braille dot. The Braille pin is made of acrylonitrile butadiene styrene (ABS) material with the mechanical property of 35 MPa yield stress. All types of the Braille pins are below the elastic limit. The complete numerical results of four types of Braille pins are show in Table 2. 4.2. Numerical results of thermal and electromagnetic analysis After the structural design of flapper mechanism is confirmed, thermal analysis and electromagnetic analysis are both performed to obtain the optimal operating parameters of flapper. The operating parameters of flapper include supplied voltage, twist turns of enameled coil, and the cooling system capacity.
The aforementioned operating parameters affect the temperature rising and the decaying thrust force of flapper actuator. The thermal analysis is focused on the Braille cell module, which consists of eight flappers that generate heat. When the 45 Braille cells in GBD operate at the same time and the inlet cooling vents are arranged evenly on the Braille machine case, the maximum temperature will occur near the middle Braille cell module of the Braille display. The intensity of heat generation in a flapper depends on the setting of supplied voltage and the internal electric resistance of enameled coil. There are eight flappers in the FDTD grid model, and the FDTD numerical grid of a Braille cell module is shown in Fig. 10. From the analysis results, the maximum temperature occurs in the no. 3 flapper. The balanced temperature of Braille cell module depends on the mounted air-cooling system capacity. There are two cooling methods used in electronic machine. One is the natural convection which the heat is dissipated by natural airflow, and the other is force convection which is equipped with blowers to speed up the rate of cooling. The temperature in a Braille cell module using natural convection method is higher than that using force convection method. The thrust force of flapper actuator would decay owing to the temperature increase of enameled coil. The material of twisted enameled coil is the copper wire with 0.05 mm diameter. The electric resistance of copper increases as the temperature rises. The current in enameled coil therefore decreases under the same supplied voltage; meanwhile, the thrust force drops below the force requirement. On the condition of fixed voltage, the FDTD thermal model can estimate the final electric resistance of enameled coil. The input parameter, the current in enameled coil, thus
Table 2 Mechanics results of A, B, C, and D type Braille pin (pressing force f2 = 20 gf) Type of Braille pin
A
B
C
D
Vertical displacement at Braille dot (cm) Minimum stress (gf/cm2 ) Percentage of minimum stress compared with yield stress (35 MPa for ABS polymer) (%) Maximum stress (gf/cm2 ) Percentage of maximum stress compared with yield stress (35 MPa for ABS polymer) (%) Minimum strain Maximum strain
−2.73E−7 −2708 7.89E−3
−4.22E−7 −2749 8.01E−3
−5.02E−8 −2666 7.76E−3
−5.31E−7 −4072 1.19E−2
212 6.20E−4
158 4.63E−4
119 3.48E−4
119 3.47E−4
−3.73E−7 1.62E−8
−3.77E−7 1.10E−8
−3.42E−7 6.65E−9
−6.94E−7 4.75E−9
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Fig. 11. Magnetic flux field of the flapper actuator.
could be obtained to calculate the electromagnetic force. The FEM electromagnetic model of flapper is shown in Fig. 11. The combination of yoke and iron core enhances the magnetic field. A wide range of input parameters are modeled for numerical analysis to select the optimal system behaviors of temperature and thrust force. The supplied voltage in the numerical simulation varies from 2 to 7 V, the initial electric resistance of enameled coil is from 260 to 360 , the ambient temperature is set as 27 ◦ C, and the cooling conditions include nature convection without blowers and force convection with two blowers. The numerical results are shown in Fig. 12. Temperature and
thrust force are the criteria for selecting the design parameters. The temperature should be below 75 ◦ C if the blower is damaged and the cooling method becomes natural condition. With the resistance of enameled coil with 0.05 mm diameter set as 330 due to the space limitation of Braille cell module, the temperature of flapper reaches the critical working point 80 ◦ C when there are no blowers and the supplied voltage is 7 V, as shown in Table 3. For such a high temperature resulted from the above scenario, the nature convection is ineffective to dissipate the heat in the limited space. The additional two blowers in the backside of GBD could lower the temperature to 49.43 ◦ C in 7 V
Fig. 12. Results of thermal and electromagnetic analysis under natural and force convection.
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Table 3 Numerical results of a Braille cell module (330 ) Supplied voltage (V)
Convection condition
Temperature of flapper (◦ C)
Thrust force (gf)
6.5
With blower Without blower
46.41 77.48
18.14 15.27
7
With blower Without blower
49.43 85.66
19.82 16.58
and the thrust force of flapper is also improved to 19.82 gf. The optimal design should take into account the temperature rising if the blower is out of function for the safety reason. The supplied voltage of 7 and 6.5 V is not the optimal selection because of the high temperature in nature convection. The supplied voltage of 6 V matches our needs since the temperature is less than 70 ◦ C in nature convection. Hence, the designed parameters of flapper include 6 V as the supplied voltage, 330 as the resistance of enameled coil, and force convection with blowers as the cooling system. 4.3. Empirical experimental of actuator in GBD The experiment of thrust force and temperature rising of flapper is conducted to confirm the actual operation of GBD. Although the simulation of actuating mechanism shows the flapper structure is strong enough for use, and the electric resistance of enameled coil and the supplied voltage indicate the thrust force is enough below the safety operating temperature, additional experiment is further conducted for comparison with the numerical analysis results, including gluing the thermal couple on the surface of flapper to measure the temperature, and the force gauge for obtaining the value of thrust force. 4.3.1. Experiment method of flapper temperature and thrust force First, each flapper in a Braille cell module is glued with a thermal sensor called thermal couple. The resistance of enameled coil is 330 and the supplied voltage is 6 V. The temperature data are stored in PC through the temperature reader with time until the steady state is reached. Because the maximum temperature occurs in the no. 3 flapper from the results of thermal analysis, the experiment therefore glues the thermal couple on the flapper with maximum temperature in each Braille cell module. There are 45 temperature values obtained from the temperature reader at a time. A force gauge is fabricated on a vertical test stand with precise vertical movement, and the vertical test stand provides a rigid platform to ensure the vertical thrust force of Braille dot is accurately read. The force gauge presses the 0.7 mm upward Braille pin downward to measure the thrust force. After the 360 flapper actuators operated for 2 h, all the Braille cell modules reach the thermally steady state. The temperature of Braille display is unchanged and the ambient temperature remains at 27 ◦ C. The force gauge reads the minimum thrust force in each Braille cell module.
Fig. 13. Temperature measured from 45 Braille cells in a complete Braille display.
Table 4 Comparison of numerical and experimental results (330 , 6 V) Convection state
The maximum temperature of flapper (◦ C)
The trust force at Braille dot (gf)
Numerical model
With blower Without blower
43.61 69.91
16.21 13.81
Experiment
With blower Without blower
42.9 70.0
15.9 13.1
4.3.2. Comparison of experiment and simulation results When all the 360 actuators are input the continuous electric current, the heat production of flapper would be maximum. From the simulation results, the optimal operating point of flapper is set as 6 V and 330 . The temperature of flapper would be below 69.91 ◦ C and the thrust force would maintain 13.81 gf in nature convection. Under the above condition, the higher temperature causes the thrust force of flapper decay to the lowest value of 13.81 gf. On the other hand, the results of the experiments show that the highest temperature occurs around the central of GBD. The steady temperature in flapper and tactile surface in each Braille cell module in nature convection is shown in Fig. 13. Table 4 shows that the experimental results match the simulation results in both highest temperature in flapper and the lowest decayed thrust force occur on the same Braille cell module. Hence, both simulation and experiment conforms the feasibility and durability of the flapper actuator design. 5. Conclusion and future work In this report, we have presented a flapper-actuated Braille display called GBD. The flapper electromagnetic actuator is designed in accordance with the numerical analysis tools, such
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as contact mechanics, heat transfer and electromagnetic analysis to ensure the feasibility and the durability of the design. In addition to the simulated results of the numerical analysis, empirical experiments are also conducted to measure the actual parameters and performance criteria. The results of both numerical analysis and empirical experiments shows that the GBD is regarded as a promising alternative in the aspects of the structure design geometry, and the durability in terms of actuating mechanism, thermal and electromagnetic analyses. The simulation results of the numerical contact analysis shows that if the pressing force of fingers increases to 20 gf and the thrust force is sufficient to push against the pressed force, the concentrated stress in the notch of armature is below the 1.16E−1% of its yield stress. Similarly, the Braille dot of the Braille pins is 1.19E−2% of its yield stress. These imply that both armature and the Braille pins are within the elastic limit. The interaction of pressing force and thrust force therefore would not deform the shape of actuating mechanism. In addition, the simulated results of the numerical thermal and electromagnetic analysis show that the thrust force of Braille pin is 16.21 gf when the optimal electrical operating point of flapper is set as 6 V, the electric resistance of enameled coil is made as 330 at 20 ◦ C, and the GBD is equipped with two blowers. The experiment results also confirm the numerical analysis results. When the operating point of flapper is set as equipping no blower at ambient temperature 27 ◦ C and the flapper is set as 6 V and 330 , the thrust force of Braille pin is decayed to 13.1 gf and the temperature of flapper is raised to 70.0 ◦ C. On the other hand, the experiment with two blowers, the thrust force of flapper is increased to 15.9 gf and the temperature of flapper is decreased at 42.9 ◦ C. Another benefit of the proposed flapper actuator is that the low manufacturing cost suggests that the flapper actuator is economically viable. As there are more than 1000 units of GBD are manufactured for preliminary test use, the durability of the flapper actuator Braille display are proven to be functional and economically viable. The suggestions from the visually impaired people that have tested GBD include modifications for improving the comfortable and correct reading of Braille codes in GBD. While there is no reported dissatisfaction in the use of the Braille display, our research will focus on numerous directions for further improvement, such as lowering the heat generation of flapper, reducing the volume and weight of Braille display, and decreasing the noise from blowers. Acknowledgements This study was supported by the National Science Council, Taiwan, Republic of China Grant (NSC94-2614-E-032-001). The authors also wish to thank professor Jya-Jang Tsai, Hsuan Chuang University, Taiwan, for his discussion on the results of this study.
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References [1] J.G. Linvill, Development progress on a microelectronic tactile facsimile reading aid for the blind, IEEE Trans. Audio Electroacoust. 17 (4) (1969) 271–274. [2] S.F. Frisken-Gibson, P. Bach-Y-Rita, W.J. Tompkins, J.G. Webster, A 64solenoid, four-level fingertip search display for the blind, IEEE Trans. Biomed. Eng. 34 (12) (1987) 963–965. [3] J. Roberts, O. Slattery, D. Kardos, B. Swope, New technology enables many-fold reduction in the cost of refreshable Braille displays, in: Fourth Annual ACM Conference on Assistive Technologies, Virginia, USA, 2000, pp. 42–49. [4] N. Sriskanthan, K.R. Subramanian, Braille display terminal for personal computers, IEEE Trans. Consumer Electron. 36 (2) (1990) 121–128. [5] C. Ramstein, Combining haptic and Braille technologies: design issues and pilot study, in: Second Annual ACM Conference on Assistive Technologies, British Columbia, Canada, 1996, pp. 37–44. [6] L. Yobas, M. Huff, F. Lisy, D.M. Durand, A novel bulk-micromachined electrostatic microvalve with a curved-compliant structure applicable for a pneumatic tactile display, J. Microelectromech. Syst. 10 (2001) 187– 196. [7] L. Yobas, D.M. Durand, G.G. Skebe, F.J. Lisy, M.A. Huff, A novel integrable microvalve for refreshable Braille display system, J. Microelectromech. Syst. 12 (2003) 252–263. [8] Y. Haga, W. Makishi, K. Iwami, K. Totsu, K. Nakamura, M. Esashi, Dynamic Braille display using SMA coil actuator and magnetic latch, Sens. Actuators A 119 (2005) 316–322. [9] P.S. Wellman, W.J. Peine, R.D. Howe, Mechanical design and control of a high-bandwidth shape memory alloy tactile display, in: International Symposium of Experimental Robotics, Barcelona, Spain, 1997, pp. 55– 66. [10] P.M. Taylor, A. Moser, A. Creed, A sixty-four element tactile display using shape memory alloy wires, Displays 18 (1998) 163–168. [11] P.M. Taylor, D.M. Pollet, A. Hosseini-Sianaki, C.J. Varley, Advances in an electrorheological fluid based tactile array, Displays 18 (1998) 135– 141. [12] H. Guckel, T. Earles, J. Klein, J.D. Zook, T. Ohnstein, Electromagnetic linear actuator with inductive position sensing, Sens. Actuators A 53 (1996) 386–391. [13] F.H. Yeh, H.S. Tsay, S.H. Liang, Application of an adaptive-network-based fuzzy inference system for the optimal design of a Chinese Braille display, Biomed. Eng. Appl. Basis Commun. 17 (1) (2005) 50–60. [14] K.J. Bathe, Finite Element Procedures, Prentice-Hall, New Jersey, 1996. [15] S.V. Patankar, Numerical Heat Transfer and Fluid Flow, Hemisphere, New York, 1980. [16] M. Plonus, Applied Electromagnetics, McGraw-Hill, New York, 1978.
Biographies Fung-Huei Yeh received his BS and MS degrees in mechanical engineering from Tamkang University, Taiwan, in 1980 and 1985, respectively. He received his PhD in mechanical engineering from Tatung University, Taiwan, in 1992. He is currently an associate professor of mechanical and eletro-mechanical engineering at Tamkang University, Taiwan. His research interests include visual devices and systems, medical devices and systems, and information assistive technology. Shih-Hao Liang received his BS and MS degree in mechanical engineering from Tamkang University, Taiwan, in 1999 and 2001. He is currently a PhD candidate at Department of Mechanical and Electro-Mechanical Engineering, Tamkang University. His research focuses on numerical analysis, soft computing, and the design of actuator for Braille display.
Mechanism design of the flapper actuator in Chinese Braille display Fung-Huei Yeh, Shih-Hao Liang ∗ Mechanical and Electro-Mechanical Engineering, Tamkang University, Tamsui, Taipei County 251, Taiwan, ROC Received 3 March 2006; received in revised form 2 June 2006; accepted 5 July 2006 Available online 22 August 2006
Abstract This report presents the mechanism design of flapper type actuator used in a Chinese Braille display called Golden Braille display (GBD) in Taiwan. An actuator is the most important part to manipulate the Braille dot in Braille display. The proposed flapper actuator is designed with numerical analysis, including contact mechanics, heat transfer, and electromagnetics to ensure the feasibility and durability of the design. Experiments are conducted to verify the simulation results of numerical analysis. The optimal results indicate that the flapper should operate at 6 V with a 330 enameled coil. The thrust force of Braille dot reaches 15.9 gf and the temperature rising of flapper is well restrained at 42.9 ◦ C when the cooling blowers are equipped in GBD. Mechanics analysis shows that even when the flapper’s trust force reaches 20 gf to interact with finger’s pressing force, the absolutely maximum stress of flapper’s armature is only 1.16E−1% of its yield stress and the Braille dot of Braille pin is only 1.19E−2% of its yield stress. The negligible stress loading indicates the structure of flapper is sufficiently robust. Our investigation also shows that GBD is a durable and economically viable alternative of Braille display. © 2006 Elsevier B.V. All rights reserved. Keywords: Braille display; Flapper actuator; Computer aided analysis; Contact mechanics; Heat transfer; Electromanetics
1. Introduction Braille display is a device with raised-characters for providing the visually impaired people with the means to read information. A Braille display comprises both mechanical and electronic parts for displaying a line of Braille codes, created by Louis Braille in the 19th century to replace the uncomfortably tactile recognition of line-type characters. A Braille cell contains a cell of 3 × 2 dots array representing a character, with every dot being convex or flat to the touch of the fingertip. In comparison, a tactile display called Optacon was developed by Linvill [1] for producing the visual shape letters mapping from text characters on the screen in 1960s. Optacon contained a 6 × 24 array of vertical vibrating rods, and the users seated their fingertips on vibrating rods to identify a tactile image of each letter scanned by the lens. The operation of Optacon was inconvenient and time consuming for reading printed article. Today, Braille codes are still the most popular way for the tactile recognition of characters with finger. At present, the number of dots in a Braille cell is usually six or eight, arranged as 3 × 2 or 4 × 2. The standard
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Corresponding author. Tel.: +886 2 2621 5656x2012; fax: +886 2 2620 9745. E-mail address: [email protected] (S.-H. Liang).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.07.013
Chinese Braille display uses two or three Braille cells to present a Chinese character. Fig. 1 shows the Braille codes in English and Chinese form of the character ”Braille”. A Braille display uses an actuator to move a Braille pin up or down to control the convex or flatness of a Braille dot. There are various types of actuators used in the current Braille display, such as solenoid, relay, piezo-electric beam, shape-memory alloy (SMA), and electrorheological (ER) fluid. Frisken-Gibson et al. [2] arranged the solenoids in two vertical layers with one layer interleaving with the other to improve the tactile resolution. Sixty-four solenoids (8 × 8 pins) were arranged in two layers and each solenoid provided four different levels of raised height. Roberts et al. [3] developed another creatively designed mechanism with only three solenoids and rotating wheel to minimize the volume of display. The Braille cell was located on the outer ring of the rotating wheel like the tread on a tire. The rotating angle of wheel was controlled by software for displaying enough Braille codes. Braille pins were pushed or pulled by only three solenoids, and the pin retention device inside the wheel included two layers of three tracks for holding Braille pins in upward or downward movement. Sriskanthan and Subramanian [4] used relays as actuators and integrated 40 Braille cells into a complete Braille display. Users can use this Braille display for receiving the Braille
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Fig. 1. Braille codes and the standard Chinese Braille cell.
codes transferred from personal computers (PC). However, Srikanthan neither disclosed the design of the relay actuator in the report nor analyzed and evaluated the durability of the design. Piezo-electric actuated Braille displays are the most popular commercialized product for displaying Braille codes. A simply supported piezo-electric beam needs at least 200 V for raising a Braille pin. A unique design of Braille display [5] was to integrate a piezo-electrical Braille cell into the outer edge of pantograph’s extremity. The user reads the Braille cell and moves cursor through the text by using the horizontally bi-dimensional pantograph. The main shortcoming of using the one-cell Braille display is that the user’s finger cannot brush across the Braille cell; instead, the finger is sitting on the refreshable Braille cell, which does not suit the user’s reading habit. Yobas et al. [6,7] recently used micro-electro-mechanical systems (MEMS) for developing the pneumatic actuator manipulated by piezoelectric valve, and successfully presented a preliminary result on making a single Braille dot. SMA wire [8–10] and ER fluid [11] were also used as actuators. Inputting the current into the two ends of an SMA wire can cause the wire to shrink. A shrinking wire can push or pull a pin depending on the different actuating mechanisms, but this system needs a cooling device to eliminate the large amount of generated heat. On the other hand, inputting the current on specific area of the ER fluid can govern the shape of the raised dot. ER fluid covered with compliant rubber needs high voltage to operate, and the high voltage inside the system of ER fluid must be monitored for the safety reason. A creative flapper actuator for Braille display is developed in Taiwan. This report presents a study on the design and analysis of a flapper actuating mechanism used in Braille display. The actuating principle of a flapper is also based on the electromagnetic theory, similar to solenoid and relay. Electromagnetic actuator has several advantages such as fast response time, larger
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force producing, and achieving with low voltage [12]. Solenoid and relay are assembled with iron core and enameled coil. The iron core wound with the enameled coil carries current produces magnetic force. Solenoid is used to as actuator or sensor because of its moveable iron core makes reversibility of electric and magnetic effect. The iron core of solenoid actuator is a moveable part as the moving transducer itself, and the current flow in enameled coil can produce the magnetic force for driving iron core. On the other hand, relay is only used to mechanically switch an electronic circuit on or off. Relay’s actuating mechanism consists of fixed iron core for enhancing magnetic force, coil for producing magnetic field, moveable armature for manipulating electrodes, driving electrodes mechanism for circuit switching purpose, and a spring for pulling the armature back to its quiescent position. However, the flapper presented in this study is a new design with a different actuating mechanism from those of solenoid and relay. Flapper in this study does not consist of the driving electrodes mechanism in relay because of the circuit switching function is not needed in actuating the Braille dot. The spring used in relay’s armature is also omitted in flapper, and the armature in flapper can restore to its quiescent position by gravity effect. Golden Braille display (GBD) is a flapper-actuated Braille display developed in Taiwan. This report presents the analysis of feasibility and durability of the design by using contact mechanics, heat transfer, and electromagnetics theories as well as empirical experiments, and tests by visually impaired people, which few other reports have done. GBD has 45 Braille cells and each Braille cell has eight Braille dots. There are altogether 360 flapper actuators in a GBD display. The reason for using flapper is that it has a longer lifetime in comparison with the peizo-electric actuator. The driving force of flapper has the same intensity when the pressing force of finger is less than the driving force, while the driving force of the most popular beamtype piezo-electric actuator varies with the pressing force of a finger. Moreover, the feeling of brushing through the flapper type Braille display most resembles that of touching the paper with Braille dot; the sense of touching is better and more comfortable than beam-type piezo-electric actuator. Every flapper-actuated Braille cell is made as an individual module and, therefore, can be used in hot plug and is easy for repair. Still another benefit for using flapper is the manufacturing cost of flapper is only USD $8 per Braille cell, compared to USD $35 per piezoelectric actuator Braille cell. In short, GBD provides longer duration and lower manufacturing cost than piezo-electric type does. The flapper actuator of GBD is mainly composed of iron core, enameled coil, and armature mechanism. The flapper’s volume and the standard stroke of Braille dot further limit the geometry of flapper actuator design, which will be described in greater details in the following sections. The rest of the report is organized as follows: Section 2 describes the structure of the flapper type actuator and the Braille display. Section 3 explains the basic theories behind the design of the flapper type actuator. Section 4 is devoted to the numerical analysis and the empirical experiments, followed by Section 5, conclusion.
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Fig. 3. Parts of flapper type actuator used in Chinese Braille display.
The models of flapper are divided into two types according to different types of armatures, and the Braille pins also have four different lengths. The complete assemblage of the actuating mechanism of a Braille cell is shown in Fig. 4. Fig. 2. Principle of the magnetic field production.
2. Structure of flapper type actuator and Braille display The structure of flapper was designed to actuate a Braille dot upward by inputting the current. When the current the flapper carries vanishes, the gravity effect will draw the Braille dot downward to the original position. As described earlier, each Braille cell consists of eight Braille dots, and a complete GBD has 45 Braille cells. 2.1. Electromagnetic parts and Braille pin The operation principle of flapper is based on the electromagnetics theory. Fixed irons core inside the enameled coil and a rotatable armature form the flapper actuator. A long enameled coil wrapped in helix carries the electric current, which produces the magnetic field lines. An iron core put inside the enameled coil enhances the magnetic force field shown in Fig. 2. The magnetic field lines traveling along the axis inside the iron core bend outside around the enameled coil, and then re-enter the other end to form a closed loop. The pole of iron core produces a strong magnetic force and an iron armature is attracted by the pole to drive a Braille pin. The mechanism of force-producing structure of flapper used in GBD is shown in Fig. 3. Part A is an iron core for enhancing magnetic force. Part B is a plastic frame to cover and bind the iron core for electrically insulating purpose, and the enameled coil C is wound on the surface of the plastic frame. The yoke D is used to fix the iron core, to enhance the magnetic field, and to become the fulcrum of armature. Part E is an iron made armature actuated by iron core to push the Braille pin F upward. Part G is an armature clip for limiting the degree of freedom (DOF) of armature in only one rotation direction.
2.2. Operations of flapper There are two opposite conditions of flapper in operation. One is the flapper carries no current, and no magnetic force is produced to attract the armature. The gravity effect causes the point e3 in armature and the Braille pin to move downward, as shown in Fig. 5. There is a small air gap between point e1 in armature (part E) and part A, and point e3 in part E is at its lowest position. The other condition is the flapper carrying current, and the armature resembling a lever mechanism that is attracted to push a Braille pin upward. The armature has a contact location e1 with part A that produces the magnetic force. The point e2 in armature is a fulcrum clipped by armature clip G, and the only one DOF of armature at point e2 is the rotation about zaxis. Point e3 is lifted to contact Braille pin when point e1 is
Fig. 4. Assembled mechanism of a Braille cell.
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Fig. 5. Operational principle of a flapper to manipulate a Braille pin.
attracted to contact part A; meanwhile, the Braille pin is pushed upward. Point e2 in armature must not have translation but allows rotation effect when the armature is pulled to lift Braille pins. The mechanical interference between armature and armature clip can be avoided when the rotational angle of point e2 is limited within a specific range. Point e2 can be treated as a hinge joint when the armature is actuated, and the turning angle between e1 e2 and e2 e3 can change the vertical stroke at point e3 . e2 e3 is in horizontal position when the flapper is not carrying current, and there is a small angle R between e1 e2 and a vertical line. The flapper carrying current then pulls e1 e2 part of armature, and point e3 produces the upward displacement h. Displacement h can be defined as: h = e2 e3 × sin(R)
Fig. 6. CAD designed Braille cell module and the manufactured prototype.
(1)
The rotation angle is small, so R can be estimated as: R=
h e2 e3
(2)
The length of flapper refers to the length of armature e2 e3 . The angle R then can be easily estimated when the upper stroke h of a Braille pin reaches the standard 0.7 mm. The length ratio of two lever arms of armature is approximately 1:3 (e1 e2 :e2 e3 ). This specific length ratio magnifies three times the upward displacement of Braille pin of the air gap between the iron core and armature. This design allows the use of a smaller space of air gap in the flapper to generate a larger displacement of Braille pin. The small space of air gap also has the advantage of requiring less current to attract the armature. 2.3. Braille cell module and Braille display There are eight Braille pins forming a complete Braille cell. The combination of Braille cell mechanism and electronic circuit is called a Braille cell module. Each individual flapper actuates an individual Braille pin. The full functional prototype of a Braille cell module was originally manufactured from the designed CAD model shown in Fig. 6. Eight flappers are soldered on a circuit board with a tactile surface which contains eight vertical holes for holding eight Braille pins. People can put their fingertip on the tactile surface to identify a Braille code
Fig. 7. Overall structure of a Braille display.
when each flapper is controlled to push the Braille pin upward or maintain the pin in the flat status. GBD has a row of 45 Braille cells to display the Braille codes. The entire machine contains a machine case, a motherboard, 45 Braille cell modules, a power supply, and two cooling blowers. Fig. 7 shows only one Braille cell module plugged in the 23rd slot of the motherboard for clarity. The protocol is compatible with LPT port, COM port, USB port, and wireless Bluetooth for transmitting the Braille codes from PC to GBD. 3. Basic theory This study adopts the theories of contact mechanics, heat transfer and electromagnetics for efficient designing and ana-
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lyzing of the feasibility of flapper structure. Contact mechanics can analyze the mechanism to ensure the durability of flapper structure. Heat transfer and electromagnetics are used to obtain the electrical parameters of flapper [13], and the object is to find the feasible setting of the heat generated by the enameled coil and the electromagnetic force produced by the flapper. After applying the aforementioned theories in the design processes, the final flapper can be more durable for GBD operation. 3.1. Contact analysis of flapper by contact mechanics Contact mechanics in finite element method (FEM) [14] can analyze the intensity of pressure and deformation in flapper parts. When the flapper actuator is actuated and the user’s fingertip presses the Braille dot, the armature is contacted with Braille pin and iron core. The mechanical part in flapper is under the externally applied force and the flapper structure is deformed. The contact analysis therefore can ensure the flapper structure is durable for Braille display in practical use. The numerical techniques are useful to the analysis of the contact problem because of the flexibility and robustness for analyzing the complicated geometry of objects and various contact conditions. The contact mechanics of FEM techniques can be divided into node-to-node technique, node-to-surface technique, and surface-to-surface technique. The contact analysis of flapper can use the node-to-node technique. The simplified node-to-node technique can be used to analyze the contact conditions when the movement of parts is considered and the contact location is known beforehand. Penalty approximation is used to solve the contact problem in FEM to enforce contact compatibility. When two nodes in FEM mesh contact each other, a stiffness relationship between two contact areas is specified. A spring with stiffness k between two contact nodes must be provided when the contact condition occurs. The spring between two contact nodes therefore deforms with a displacement quantity δ, and the equilibrium equation is: F = kδ
(3)
where k is the contact stiffness and F is called the contact force. The contact stiffness in penalty method depends on the contact material. 3.2. Thermal analysis of Braille cell module Limiting the temperature increase in flapper is necessary for guarding the thermal damage in the flapper, electronic ICs, and plastic parts of the Braille cell. The enameled coil of flapper must be supplied sufficient current to sustain the upward force for a Braille pin. Because of the electric resistance, the enameled coil generates heat when the current is applied. Consequently, the flapper temperature rises. The electric resistance of enameled coil increases with the rising temperature, and this effect should be estimated. The current inside the enameled coil decays as the resistance of the enameled coil increases by temperature rising, and the Braille cell module finally reaches a thermally steady state. The thermal analysis of Braille cell module would
adopt unsteady time analysis because the electric resistance of enameled coil depends on temperature. The relation between temperature and copper made enameled coil is expressed as: T f − Ti Rf = Ri · 1 + (4) 234.5 + Ti Ri is the electric resistance of enameled coil in the initial state without current carrying and Rf is the electric resistance of enameled coil in the state of thermal balance. Ti is the initial temperature of enameled coil and Tf is the temperature in the state of thermal balance. The flapper has the heat rate q˙ G , which depends on temperature T in thermal balance state stated in Eq. (5): q˙ G = q˙ G (T ) =
V2 Rf
(5)
The heat source of GBD is mainly from the flapper. Every Braille cell module has eight flappers, and a GBD with 45 Braille cells has 360 flappers altogether. The entire GBD generates maximum heat when the 360 flappers lift their own Braille pins at the same time. The Braille cell module located near the middle of 45 Braille cell modules will be at the highest temperature when the maximum heat generation state occurs. The compact 2D heat transfer equation in (6) expresses the thermal behavior of a Braille cell module. ρc
(T∞ − Tp ) qflux ∂T ∂2 T ∂2 T = k 2 + k 2 + h∞ + + q˙ G ∂t ∂x ∂y L L
(6)
where ρ is the density, c the specific heat, and k is the conductivity. h∞ and T∞ , respectively, are the heat transfer coefficient and the ambient temperature of air. L is the thickness of circuit board, t the time, and T is the temperature parameter. This heat transfer equation can be numerically solved by finite difference time domain (FDTD) method [15]. 3.3. Electromagnetic analysis The upward force of Braille pin should be over 15 gf for comfortable Braille reading. The attracting force of flapper supported by direct current can be analyzed and designed based on the static magnetic theorem [16]. Field intensity H and flux density B obey Eqs. (7) and (8): ∇ ×H =J
(7)
∇ ×B =0
(8)
According to the variant material properties, Eq. (9) shows the relationship of B and H: B = μH
(9)
If the property of magnetic material is not linear, then permeability μ is a function of B that can be expressed as: μ=
B H(B)
(10)
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By the magnetic vector potential approach, flux density B resulted from vector potential A yields (11): B =∇ ×A
(11)
The definition of B in Eq. (11) always satisfies Eq. (8); thus Eq. (11) can be rewritten as following form: 1 ∇ ×A =J (12) ∇× μ(B) If the property of material is linear and isotropic, Eq. (12) can be simplified as: 1 − ∇ 2A = J μ
(13)
The Maxwell stress tensor is defined as a magnetic force per unit area produced by the magnetic filed on a surface. The differential form of force can be written as: 1 (H(B · n) + B(H · n) − H(B · n)) (14) 2 n is a vector starting from a point on surface and is normal to the surface. The net force on the armature is obtained by creating a surface, which completely encloses the surface of armature and integrates the magnetic stress with the surface. The value of attracting force is calculated from Eq. (14) by the discretized method of FEM. dF =
4. Results of numerical analysis and experiment The numerical analysis based on the theory described in Section 3 can obtain the value of stress, strain, temperature, and thrust force in flapper. The value of stress and strain in armature and Braille pin can be obtained by using contact mechanics to ensure the solid and durable mechanism. The temperature of flapper can be estimated from thermal analysis to guard overheating situations in GBD, and the thrust force of flapper can be estimated by electromagnetic analysis to guarantee comfortable Braille reading. The experiment of thrust force and temperature in flapper is used to confirm the feasibility and comfort of machine use. 4.1. Results of numerical mechanics analysis The static analysis of flapper’s mechanism emphasizes the stress and strain of armature and Braille pin. The purpose is to ensure the flapper’s mechanism to stay below the elastic limits in operation, and the permanent deformation in flapper’ mechanism is not allowed. The test of Braille display by visually impaired people shows that a minimum of 15 gf for the upward force is required to provide ease of use and correct Braille identification. The boundary conditions of numerical analysis match the actual situation of flapper operation. The numerical analysis is performed under the interaction situation of the actuating force f1 and pressing force f2 shown in Fig. 5. At the initial state, the enameled coil of flapper does not carry current. The stress and strain does not occur in the armature and Braille pin except the negligible effect by the gravity effect. The iron core
Fig. 8. FEM mesh and the location of concentrated stress in A type armature.
produces attracting force f1 to the armature when the current flows into the enameled coil. Meanwhile, the point e3 in armature pushes the Braille pin 0.7 mm upward. When a user’s finger is pressing the Braille dot, a downward force f2 is produced in point e3 . In simulation, the pressing force f2 is set as 20 gf to ensure the higher safety factor in flapper structure. The iron core always draws the point e1 in armature when the attracting force is able to be against the downward force f2 . The observation of simulated results is focused on the estimated strain and stress in armature and Braille pin. The elastic displacement transformed from the strain is also an important issue in the design process. The A and B type armature is analyzed by FEM with suitable mesh under the effects of electromagnetic attracting force and downward force produced by a fingertip. The FEM mesh of A type armature is shown in Fig. 8. The maximum of absolute stress and of absolute stain both occur in the notch of the armature. The yield stress of wrought iron is 210 MPa (1 MPa = 9.81 × 105 gf/cm2 ), and the results of numerical analysis show that the armature is firm enough. The values of stress and strain occurred in the notch of A and B types of armature are shown in Table 1. The results indicate the iron made armature is under the elastic limit with an extremely small loading. Under the same FEM analysis condition with Braille pins, the critical values of stress and strain occur on the tip of Braille pins. The Braille pins of four different lengths are modeled and analyzed to obtain the location of maximum absolute values of Table 1 Mechanics results of A and B type armature (pressing force f2 = 20 gf) Type of armature
A
B
Vertical displacement at point e3 (cm) Minimum stress (gf/cm2 ) Percentage of minimum stress compared with yield stress (210 MPa for wrought iron) (%) Maximum stress (gf/cm2 ) Percentage of maximum stress compared with yield stress (210 MPa for wrought iron) (%) Minimum strain Maximum strain
−3.54E−3 −253801 1.23E−1
−2.67E−3 −288015 1.40E−1
342463 1.66E−1
292438 1.42E−1
−1.15E−4 1.56E−4
−1.40E−4 1.43E−4
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Fig. 10. The complete arrangement of relays in GBD.
Fig. 9. FEM mesh and the location of concentrated stress in A type Braille pin.
stress and strain. The FEM mesh of A type Braille pin is shown in Fig. 9. The maximum of absolute stress and absolute strain occur in the Braille dot. The Braille pin is made of acrylonitrile butadiene styrene (ABS) material with the mechanical property of 35 MPa yield stress. All types of the Braille pins are below the elastic limit. The complete numerical results of four types of Braille pins are show in Table 2. 4.2. Numerical results of thermal and electromagnetic analysis After the structural design of flapper mechanism is confirmed, thermal analysis and electromagnetic analysis are both performed to obtain the optimal operating parameters of flapper. The operating parameters of flapper include supplied voltage, twist turns of enameled coil, and the cooling system capacity.
The aforementioned operating parameters affect the temperature rising and the decaying thrust force of flapper actuator. The thermal analysis is focused on the Braille cell module, which consists of eight flappers that generate heat. When the 45 Braille cells in GBD operate at the same time and the inlet cooling vents are arranged evenly on the Braille machine case, the maximum temperature will occur near the middle Braille cell module of the Braille display. The intensity of heat generation in a flapper depends on the setting of supplied voltage and the internal electric resistance of enameled coil. There are eight flappers in the FDTD grid model, and the FDTD numerical grid of a Braille cell module is shown in Fig. 10. From the analysis results, the maximum temperature occurs in the no. 3 flapper. The balanced temperature of Braille cell module depends on the mounted air-cooling system capacity. There are two cooling methods used in electronic machine. One is the natural convection which the heat is dissipated by natural airflow, and the other is force convection which is equipped with blowers to speed up the rate of cooling. The temperature in a Braille cell module using natural convection method is higher than that using force convection method. The thrust force of flapper actuator would decay owing to the temperature increase of enameled coil. The material of twisted enameled coil is the copper wire with 0.05 mm diameter. The electric resistance of copper increases as the temperature rises. The current in enameled coil therefore decreases under the same supplied voltage; meanwhile, the thrust force drops below the force requirement. On the condition of fixed voltage, the FDTD thermal model can estimate the final electric resistance of enameled coil. The input parameter, the current in enameled coil, thus
Table 2 Mechanics results of A, B, C, and D type Braille pin (pressing force f2 = 20 gf) Type of Braille pin
A
B
C
D
Vertical displacement at Braille dot (cm) Minimum stress (gf/cm2 ) Percentage of minimum stress compared with yield stress (35 MPa for ABS polymer) (%) Maximum stress (gf/cm2 ) Percentage of maximum stress compared with yield stress (35 MPa for ABS polymer) (%) Minimum strain Maximum strain
−2.73E−7 −2708 7.89E−3
−4.22E−7 −2749 8.01E−3
−5.02E−8 −2666 7.76E−3
−5.31E−7 −4072 1.19E−2
212 6.20E−4
158 4.63E−4
119 3.48E−4
119 3.47E−4
−3.73E−7 1.62E−8
−3.77E−7 1.10E−8
−3.42E−7 6.65E−9
−6.94E−7 4.75E−9
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Fig. 11. Magnetic flux field of the flapper actuator.
could be obtained to calculate the electromagnetic force. The FEM electromagnetic model of flapper is shown in Fig. 11. The combination of yoke and iron core enhances the magnetic field. A wide range of input parameters are modeled for numerical analysis to select the optimal system behaviors of temperature and thrust force. The supplied voltage in the numerical simulation varies from 2 to 7 V, the initial electric resistance of enameled coil is from 260 to 360 , the ambient temperature is set as 27 ◦ C, and the cooling conditions include nature convection without blowers and force convection with two blowers. The numerical results are shown in Fig. 12. Temperature and
thrust force are the criteria for selecting the design parameters. The temperature should be below 75 ◦ C if the blower is damaged and the cooling method becomes natural condition. With the resistance of enameled coil with 0.05 mm diameter set as 330 due to the space limitation of Braille cell module, the temperature of flapper reaches the critical working point 80 ◦ C when there are no blowers and the supplied voltage is 7 V, as shown in Table 3. For such a high temperature resulted from the above scenario, the nature convection is ineffective to dissipate the heat in the limited space. The additional two blowers in the backside of GBD could lower the temperature to 49.43 ◦ C in 7 V
Fig. 12. Results of thermal and electromagnetic analysis under natural and force convection.
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Table 3 Numerical results of a Braille cell module (330 ) Supplied voltage (V)
Convection condition
Temperature of flapper (◦ C)
Thrust force (gf)
6.5
With blower Without blower
46.41 77.48
18.14 15.27
7
With blower Without blower
49.43 85.66
19.82 16.58
and the thrust force of flapper is also improved to 19.82 gf. The optimal design should take into account the temperature rising if the blower is out of function for the safety reason. The supplied voltage of 7 and 6.5 V is not the optimal selection because of the high temperature in nature convection. The supplied voltage of 6 V matches our needs since the temperature is less than 70 ◦ C in nature convection. Hence, the designed parameters of flapper include 6 V as the supplied voltage, 330 as the resistance of enameled coil, and force convection with blowers as the cooling system. 4.3. Empirical experimental of actuator in GBD The experiment of thrust force and temperature rising of flapper is conducted to confirm the actual operation of GBD. Although the simulation of actuating mechanism shows the flapper structure is strong enough for use, and the electric resistance of enameled coil and the supplied voltage indicate the thrust force is enough below the safety operating temperature, additional experiment is further conducted for comparison with the numerical analysis results, including gluing the thermal couple on the surface of flapper to measure the temperature, and the force gauge for obtaining the value of thrust force. 4.3.1. Experiment method of flapper temperature and thrust force First, each flapper in a Braille cell module is glued with a thermal sensor called thermal couple. The resistance of enameled coil is 330 and the supplied voltage is 6 V. The temperature data are stored in PC through the temperature reader with time until the steady state is reached. Because the maximum temperature occurs in the no. 3 flapper from the results of thermal analysis, the experiment therefore glues the thermal couple on the flapper with maximum temperature in each Braille cell module. There are 45 temperature values obtained from the temperature reader at a time. A force gauge is fabricated on a vertical test stand with precise vertical movement, and the vertical test stand provides a rigid platform to ensure the vertical thrust force of Braille dot is accurately read. The force gauge presses the 0.7 mm upward Braille pin downward to measure the thrust force. After the 360 flapper actuators operated for 2 h, all the Braille cell modules reach the thermally steady state. The temperature of Braille display is unchanged and the ambient temperature remains at 27 ◦ C. The force gauge reads the minimum thrust force in each Braille cell module.
Fig. 13. Temperature measured from 45 Braille cells in a complete Braille display.
Table 4 Comparison of numerical and experimental results (330 , 6 V) Convection state
The maximum temperature of flapper (◦ C)
The trust force at Braille dot (gf)
Numerical model
With blower Without blower
43.61 69.91
16.21 13.81
Experiment
With blower Without blower
42.9 70.0
15.9 13.1
4.3.2. Comparison of experiment and simulation results When all the 360 actuators are input the continuous electric current, the heat production of flapper would be maximum. From the simulation results, the optimal operating point of flapper is set as 6 V and 330 . The temperature of flapper would be below 69.91 ◦ C and the thrust force would maintain 13.81 gf in nature convection. Under the above condition, the higher temperature causes the thrust force of flapper decay to the lowest value of 13.81 gf. On the other hand, the results of the experiments show that the highest temperature occurs around the central of GBD. The steady temperature in flapper and tactile surface in each Braille cell module in nature convection is shown in Fig. 13. Table 4 shows that the experimental results match the simulation results in both highest temperature in flapper and the lowest decayed thrust force occur on the same Braille cell module. Hence, both simulation and experiment conforms the feasibility and durability of the flapper actuator design. 5. Conclusion and future work In this report, we have presented a flapper-actuated Braille display called GBD. The flapper electromagnetic actuator is designed in accordance with the numerical analysis tools, such
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as contact mechanics, heat transfer and electromagnetic analysis to ensure the feasibility and the durability of the design. In addition to the simulated results of the numerical analysis, empirical experiments are also conducted to measure the actual parameters and performance criteria. The results of both numerical analysis and empirical experiments shows that the GBD is regarded as a promising alternative in the aspects of the structure design geometry, and the durability in terms of actuating mechanism, thermal and electromagnetic analyses. The simulation results of the numerical contact analysis shows that if the pressing force of fingers increases to 20 gf and the thrust force is sufficient to push against the pressed force, the concentrated stress in the notch of armature is below the 1.16E−1% of its yield stress. Similarly, the Braille dot of the Braille pins is 1.19E−2% of its yield stress. These imply that both armature and the Braille pins are within the elastic limit. The interaction of pressing force and thrust force therefore would not deform the shape of actuating mechanism. In addition, the simulated results of the numerical thermal and electromagnetic analysis show that the thrust force of Braille pin is 16.21 gf when the optimal electrical operating point of flapper is set as 6 V, the electric resistance of enameled coil is made as 330 at 20 ◦ C, and the GBD is equipped with two blowers. The experiment results also confirm the numerical analysis results. When the operating point of flapper is set as equipping no blower at ambient temperature 27 ◦ C and the flapper is set as 6 V and 330 , the thrust force of Braille pin is decayed to 13.1 gf and the temperature of flapper is raised to 70.0 ◦ C. On the other hand, the experiment with two blowers, the thrust force of flapper is increased to 15.9 gf and the temperature of flapper is decreased at 42.9 ◦ C. Another benefit of the proposed flapper actuator is that the low manufacturing cost suggests that the flapper actuator is economically viable. As there are more than 1000 units of GBD are manufactured for preliminary test use, the durability of the flapper actuator Braille display are proven to be functional and economically viable. The suggestions from the visually impaired people that have tested GBD include modifications for improving the comfortable and correct reading of Braille codes in GBD. While there is no reported dissatisfaction in the use of the Braille display, our research will focus on numerous directions for further improvement, such as lowering the heat generation of flapper, reducing the volume and weight of Braille display, and decreasing the noise from blowers. Acknowledgements This study was supported by the National Science Council, Taiwan, Republic of China Grant (NSC94-2614-E-032-001). The authors also wish to thank professor Jya-Jang Tsai, Hsuan Chuang University, Taiwan, for his discussion on the results of this study.
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Biographies Fung-Huei Yeh received his BS and MS degrees in mechanical engineering from Tamkang University, Taiwan, in 1980 and 1985, respectively. He received his PhD in mechanical engineering from Tatung University, Taiwan, in 1992. He is currently an associate professor of mechanical and eletro-mechanical engineering at Tamkang University, Taiwan. His research interests include visual devices and systems, medical devices and systems, and information assistive technology. Shih-Hao Liang received his BS and MS degree in mechanical engineering from Tamkang University, Taiwan, in 1999 and 2001. He is currently a PhD candidate at Department of Mechanical and Electro-Mechanical Engineering, Tamkang University. His research focuses on numerical analysis, soft computing, and the design of actuator for Braille display.