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Shape memory micro-actuation for a gastro-intestinal intervention system
Sensors and Actuators 77 Ž1999. 157–166 www.elsevier.nlrlocatersna
Shape memory micro-actuation for a gastro-intestinal intervention system Dominiek Reynaerts ) , Jan Peirs, Hendrik Van Brussel Katholieke UniÕersiteit LeuÕen, DiÕision of Production Engineering, Machine Design and Automation, Celestijnenlaan 300 B, B-3001 HeÕerlee, Belgium Received 2 July 1997; accepted 24 March 1999
Abstract This paper describes the design of a prototype gastro-intestinal intervention system based on an inchworm-type of mobile robot. This type of device is a kind of vehicle for inspection through the colon, something that is currently impossible due to the large number of turns in the intestinal system. Eventually, tools for intervention can be added in the future. The overall system is about 95 mm long and has a diameter of 15 mm. The robot consists of three main modules: an extension and contraction module, a two degree-of-freedom bending module and two locking modules. All these modules are to be actuated by shape memory elements. The main part of the paper describes a modular actuator for realising bending motion. The design can be compared to a single vertebra, where stacking several elements can form a spinal column. It will be shown that this design greatly facilitates the control. The electromechanical interconnection of the different parts was an integral part of the design as well as techniques for selectively addressing the different actuators. In order to increase the performance of the proposed design, the last part of the paper discusses some production aspects of the shape memory elements. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Gastro-intestinal; Shape memory alloy; Micro-actuator; Micro-electromechanical system; Microvalve; Minimal invasive surgery
1. Introduction The idea of miniature mobile robots goes back to the GNAT robot developed at MIT, artificial intelligence lab w1x. The idea behind this GNAT is an insect-like, completely autonomous, robot that can be mass-produced. Although this idea originally looked very futuristic, the growing interest in micro-electromechanical systems ŽMEMS. caused the research community to consider these systems to be within the reach of current technology. The terminology in this field is very poorly defined, but for the purpose of this paper, a miniature robot is defined as proposed by Dario et al. w2x: a robot with a size in the order of a few cubic centimetres that operates in a workspace and generates forces comparable to those applicable by human operators during fine manipulation. The most promising future for this kind of robots probably lies in the field of inspection and intervention in medical as well as in industrial applications. Within this
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field, several prototypes have been proposed: Aoshima and Yabuta w3x proposed a miniature robot using piezo-elements; A giant magnetostrictive alloy actuated robot has been proposed by Fukuda et al. w4x. The same author also presented a micro optical robotic system with cordless optical power supply w5x; Ikuta w6x presented the MEDIWORM, an active microrobot actuated by shape memory alloys; a similar idea was also proposed by Hesselbach and Stork w7x. Some of these designs are based on the principle of the inchworm actuator originally commercialised by Burleigh Instrument. The inchworm principle was mostly used to extend the stroke of piezoelectric linear drives w8,9x and consists of two clamping modules and one expansion module. By intermittent clamping, expanding, and reclamping, these systems can creep over a rod or inside a tube. For all these systems, compact actuation is required. Shape Memory Alloy ŽSMA. actuators offer the advantage of extremely high power-to-volume ratios that are comparable to those for hydraulic actuation w10x. Moreover, they enable a very simple direct drive actuator design and an electrical current using simple resistive heating can di-
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 1 9 1 - 0
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rectly drive them. Shape memory alloys have been used for a wide range of applications w11x. Honma et al. w12x and Kuribayashi w13x presented shape memory based micro-actuators. In the medical field, most applications are based on the superelastic properties of SMAs or on a one-way actuation like clamping. ‘Real’ actuators are mostly used in minimal invasive surgery tools like endoscopes. Some examples of this latter category are the active catheter tip developed by Dario and Montesi w14x, the active endoscope presented by Ikuta et al. w15x, or the miniature active catheter developed by Guo et al. w16x. The proposed research is part of a Brite-Euram fundamental research project studying shape memory alloy micro-actuators for medical applications. A first development within this project consisted of an actively controlled implantable drug delivery device w17x. The second part of this project concerned the development of a gastro-intestinal intervention system based on an inchworm-type of mobile robot. Semi-autonomous gastro-intestinal intervention systems have only been addressed in the recent past w6,18x. This paper first describes the general lay-out of the designed gastro-intestinal intervention system. Afterwards, a modular shape memory actuator for realising bending motion is discussed in more detail. A final part of the paper discusses the production aspects of the shape memory elements.
2. Design of a gastro-intestinal intervention system When MEMS were proposed, operation in the blood stream was probably the most popular application among the projected ideas. This kind of ‘free floating’ devices have to cope with a number of severe problems: the reaction of the immunosystem on strange objects, the clothing of the blood around these objects, the required
Fig. 2. Inchworm-type of robot for inspection of the colon.
ability to navigate in a viscous environment, if necessary against the blood flow direction. All these problems were largely underestimated. In view of the current state-of-theart in microsystem technology, it was therefore decided to build a more realistic application consisting of a gastro-intestinal inspection and intervention system. In fact, this can also be considered as a mobile robot navigating through a pipe, in this case, the colon, but the scale is quite different. Fig. 1 shows a simplified representation of the human colon w19x. The colon has an average diameter of about 50 mm. The smallest radius is about 20 to 30 mm and is located at the bending portion between the rectum and the sigmoid colon. The transverse colon, which is 400 to 500 mm long, is the largest and most mobile part of the colon. Due to its horizontal position, the breathing process affects its movements. According to Sturges and Laowattana w19x, the use of a colonoscope is impeded by the peristaltic action of the gut, attempting to expel the device. The main problem for inspecting the colon with an endoscopic device is the large number of turns, more specifically at the sigmoid, which have to be taken when entering the human intestinal system. It is therefore also extremely difficult to manoeuvre a classical endoscope around the bends of the colon without damaging the gut. Therefore, the proposed design aims at a more advanced mobile system for colonoscopy. The system that is developed is basically an inchwormtype of mobile robot. Fig. 2 shows that, compared to the conventional inchworm with two clamping and an extension actuator, the proposed design has two additional bending degrees of freedom. This gastro-intestinal intervention system has an outer diameter of maximum 15 mm, which is acceptable for colonoscopes. Table 1 summarises the specifications for the design. Table 1 Specifications for inchworm design
Fig. 1. Diagram of the human colon Žfrom Ref. w19x..
Module
Number
Diameter Žmm.
Length Žmm.
Stroke
Translation Clamping Bending
1 2 2
15 15 15
10 32.5 10
10 mm ø 15–50 mm "458
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As shown in this table, each module has a specific functionality that is to be realised within a very narrow volume. Ø The translation module has a length of 10 mm and has to realise a stroke of 10 mm. This expansion can be realised with a spring-type SMA actuator. Also, the design has to guarantee sufficient lateral stiffness. Ø The clamping module has a length of 32.5 mm must be able to clamp in a human colon with a diameter ranging from 15 to 50 mm. The gut is a highly flexible and slippery environment and in the same time it is very fragile and sensitive to damage. Therefore, a clamping system based on balloons seems most appropriate. This approach was already proposed by Slatkin et al. w18x. Ø The bending module has a length of 10 mm. It has to realise a bending from y458 to 458. Both modules are stacked in such a way that a two degree-of-freedom system is obtained. Several design teams within the project develop the different parts. In first instance, the interface between the different actuators is a flat disk of 15 mm diameter. After completion of the first prototype, a further integration of the different actuators will be considered. For instance, empty space in one module can be used for the other actuators. For this first design, the forces and torques to be developed should be sufficient to support the weight of the device. A more detailed specification will be made after experimental testing. No commercial endoscopic inchworms are yet existing, so this design would be a completely new product. The proposed specs are on-the-edge, but appear to be reasonable for a first prototype.
3. Design of a modular bending actuator 3.1. Bending actuator general concept Fig. 3 shows that the actuator can be compared to a single vertebra, where stacking several elements can form a spinal column. Each vertebra is controlled in a binary
Fig. 3. Vertebral design of a bending actuator.
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Fig. 4. Design concept of a single vertebra actuator.
mode. This means that a single vertebra has only two powered positions Žleft bending or right bending. and the central position in the unpowered condition. This also means that the complete ‘spinal column’ can only reach a discrete number of positions. However, for the envisaged endoscopic application this is more than sufficient. Fig. 4 shows a design concept for a single vertebra actuator. The actuator consists of a central rotating part actuated by two antagonistic SMA actuators. The central element also contains two return springs with constant and equal torque. This means that the unpowered position corresponds to the neutral position. Fig. 5 shows a photograph of a preliminary prototype. For this specific example, the actuator has a stroke of "158 and a diameter of 17 mm. The major parts of this prototype are made of brass, but could be easily made in another material Že.g., plastics.. As each actuator is controlled in a binary mode, only a mechanism for selection of the different actuators has to be provided. The electrical control of the different actuators
Fig. 5. Prototype of a single vertebra actuator.
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Fig. 6. Electromechanical connection of the actuators.
can be realised with a bus system. Each single vertebra contains an integrated electronic circuit to enable actuator selection, or eventually selection of several actuators within one single bending module. With plastic modules, this integration can be realised by integrating electrical connections on the parts of the module with the SIL 1 technology w17x. Fig. 6 shows the principle of electromechanical connection for the different modules. Adjacent modules can be rotated 908 to each other so that a two-dimensional bending actuator is obtained. In this case, five signals are provided: a supply voltage for the electronics Vcc , a supply voltage for the SMA elements Vs , a common ground, a select channel, and a clock channel. Fig. 7 shows two principles for driving the different actuators based on the above signals. The first principle Žfrequency modulation based communication. realises actuator selection with a filter or a phase locked loop. This allows simultaneous actuation using only one signal wire carrying a frequency modulated selection signal. No clock signals are required. This design has the drawback that all modules differ by their filter, which is a disadvantage for mass production and results in a decreased degree of modularity. The second principle Žserial communication. uses digital addressing of the different modules. This system also enables simultaneous actuation but requires an additional clock channel. In this case, all modules differ by their address. As it is much easier to change a digital address, this solution is preferable for real mass production.
considered as a flat disk with diameter 15 mm. The desired stroke of an actuator is "158. This means that three vertebras are required for realising the bending movement specified in Table 1. The external torque corresponds to the weight of the links to rotate. It is estimated to be 6 Nmm. Other requirements are a sufficiently high electrical resistance for control by an electronic circuit embedded in the vertebra and the possibility for miniaturisation. The assumed SMA material characteristics can be found in the upper left corner of Table 2. It is assumed that the return force to deform a cold shape memory element is one-third of the force it generates when heated. Table 2 compares nine different actuation principles for the vertebra actuator. In each row of Table 2, a comparison is made between pure antagonistic SMAs, an SMA working against an elastic joint, and a combination of both. Ø The pure antagonistic system has two stable powered positions, but is not stable when not powered. The force required to deform a SMA element is 1r3 of the force it generates. Consequently, the activated SMA has to generate force for external use and for deformation of his antagonist as expressed in Eq. Ž1.. By simplifying this equation, this means that the SMA has to generate a torque 1.5 times higher than the external torque. MSM A G Mext q
1 3
MSMA ´ MSMA G
3 2
Mext
Eq. Ž1. is the Torque balance for antagonistic system. Ø The SMA in combination with an elastic joint has one powered position and also a stable neutral position. The elastic joint serves as both spring and joint. The activated SMA has to produce the external torque but also to deform the spring as expressed by Eq. Ž2.. In the unpowered situation, the spring has to be able to deform the SMA and to generate the external torque. The torque generated by the bias spring is assumed to be constant. This is correct if super-elastic joints are used Žsee Section 4.. Combination
3.2. Detailed actuator design This section describes a general study of different alternatives for the vertebra actuator design. The vertebra is
1
ŽInjection moulded parts Spritzgiesteile mit Integrierten Leiterzugen ¨ with integrated conducting pads.. SIL starts from a plastic part that is galvanically covered with copper and tin. The tin layer is patterned by laser evaporation. Afterwards, this layer is used as a mask to etch the underlying copper. Finally, tin is removed and the result is a structured copper layer.
Ž 1.
Fig. 7. Two principles for selective addressing of the actuators.
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Table 2 Comparisons of actuation principles for the vertebra actuator
of both formulas leads to the conclusion that the SMA has to generate 3 times the external torque. MSM A G Mext q Mspring Mspring G Mext q
1
¶ •´ M ß
MSMA
SM A G 3 Mext
Ž 2.
3 Ž . Eq. 2 is the Torque balance for SMA with bias spring Žsuper-elastic joint.. Ø The combination of an antagonistic system and an elastic joint creates three stable positions, two powered and one unpowered. Here the torque of the SMA is 6 times higher than the external torque. MSM A G Mext q Mspring q Mspring G Mext q
1 3
1 3
¶ MSMA
MSMA
•´ M
ß
SM A G 6 Mext
Ž 3. Eq. Ž3. is the Torque balance for combined system.
The columns of Table 2 show different mountings in which the SMA material can be used: a straight wire, a pulley system, or a spring. Ø For the simple wire system, the maximum wire length of 15 mm Ži.e., the diameter of the module. is an additional constraint. The lever follows from the wire length, the maximum strain and the specified stroke. Lever and specified torque define the force and consequently, also the wire thickness and resistance. Ø The pulley system and the spring system assume that the maximum possible lever is used in order to reduce the forces and the section of the wire. For the pulley system, the force can be calculated from the specified torque and the known lever. Wire length is calculated from lever and stroke. Ø The SMA springs are calculated with the normal spring formulas where the E-modulus is replaced by the ratio t SM A rg SMA . This is correct under the following assumptions. Ž1. The maximum strain is proportional to the stress.
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Ž2. The formula is only used to calculate full stroke deformation: assumption 1 is only valid for the maximum strain at a specific stress. Ž3. The strain is proportional to the distance to the centre of the wire: zero in the centre, maximum at the surface. Together with assumption 1, this means that stress and strain are proportional to the distance to the centre as for normal springs. Therefore, the E-modulus can be replaced by t SM A rg SMA . The conclusions of this table can be summarised as follows. Ø The torque to be generated by the SMAs is higher than the required external torque: the pure antagonistic approach asks only 1.5 times the external torque, the system with the elastic joint and one SMA element requires already 3 times the external torque and the combination of antagonistic SMAs with elastic joints requires the SMAs to generate a 6 times higher torque. In the last case, the SMAs have to generate 36 Nmm instead of 6 Nmm. The combined system asks more, but offers more functionality: three stable positions instead of two and the neutral position is unpowered leading to lower power consumption as the inchworm will mainly move straightforward such that the bending actuator will be used only during a small part of the intervention. Ø None of the nine combinations satisfies all constraints for this specific design: Ž1. For straight SMA wires, the forces in the SMA, the mechanical connections and the joint are very high. As a result, thick wires are needed such that the electrical resistance is extremely low. Furthermore, these wires are not flexible such that bending forces of these wires become important. Ž2. The pulley system has a high electrical resistance but requires a very long wire passed over a number of pulleys. Depending on the case, at least five or 10 of these pulleys per SMA wire are needed. These cause friction and are not suited for miniaturisation. Ž3. SMA springs offer the easiest way for
construction of the actuator but their problem is again the extremely low electrical resistance. Ø The problem with the straight wire system can be solved by replacing the thick SMA wire by a number of thin SMA wires mechanically in parallel and electrically in series Žwithout using pulleys of course.. This way, the electrical resistance is higher and the thin wires are much more flexible. Problem is the equal load distribution over the different wires. Cutting the pattern out of a ribbon solves this problem. This technique is explained more in detail in Section 5. Ø Final conclusion: the selected design is the straight wire system with antagonistic SMAs, super-elastic joints, and a patterned ribbon.
4. Construction of an optimised actuator prototype Fig. 8 shows top, side and isometric view of the actuator design. The body of the actuator consists of two parts, an upper and a lower part, that have an identical form but one of them is rotated 1808. They are made of aluminium by electro-discharge machining ŽEDM., final versions could be made of plastic by use of injection moulding. Both parts are connected to each other via two super-elastic joints. When the upper SMA strip is heated, it shortens and forces the upper part to rotate to the right. When the lower strip is heated, the upper part rotates to the left. The stroke in both directions is 158 and is limited mechanically. Heating both strips at the same time has to be avoided, as this will overload both strips and the joint. When no strip is heated, the elastic joints hold the actuator in its middle position. A mechanical stop guarantees a fixed distance between the SMA strip and the axis of rotation. A 0.15-mm thin glass plate assures thermal and electrical isolation between the SMA strip and the aluminium body. Assembly of all parts is done by gluing. To connect
Fig. 8. Bending actuator design.
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Fig. 9. Stacking of three actuators to obtain specified stroke.
the glass plate to the body and the SMA strip to the glass plate, epoxy glue is used. To obtain the specified stroke of "458, three actuators have to be used as shown in Fig. 9. Activation of one, two, or three actuators causes, respectively 158, 308 or 458 rotation. Seven useful combinations are possible leading to following positions: y458, y308, y158, 08, 158, 308, 458. The actuator contains two super-elastic joints with dimensions shown in Fig. 10. Bending is limited to the narrowed zone of 0.12 mm thickness. Two joints, one on each side of the actuator, are used to enhance stiffness and strength in the other directions. They are made of a CuAlNi alloy by EDM. Rolled strips are an alternative but were not available in this material. A super-elastic joint is an elastic joint made of superelastic material. As all elastic joints, they have a number of advantages over normal rotary joints. Ø The joint acts also as bias spring such that no extra spring has to be included in the design. This leads to a reduction in the number of parts and a simplification of the design, both improving the possibilities for miniaturisation. Ø An elastic joint exhibits no friction or backlash. Elastic joints suffer from a limited stroke and a return force that can be substantial. However, these disadvantages do not apply for this development. Super-elastic materials have a non-linear characteristic that permits to build actuators that are stiff in their neutral position while the forces to bend them to their extreme position are kept low. A high stiffness Žor high Young’s modulus. in the neutral position is important to avoid small disturbing forces from bending the actuator. With a normal elastic material this high modulus results in high elastic forces at the end of the
Fig. 10. Dimensions of super-elastic joint Žin mm..
Fig. 11. Characteristic of super-elastic joint.
stroke of the joint. This effect can be minimised by use of super-elastic materials. The material behaves linear until a stress of 450 MPa is reached. Then the material deforms without any significant increase in stress. During recovery of the material, the stress is about 50 MPa lower. Superelastic behaviour is closely related to the shape memory effect. The difference is that super-elastic materials have transformation temperatures far below ambient temperature. These transformation temperatures increase proportional with stress, such that at a certain stress level Žhere 450 MPa., the martensitic transformation temperature exceeds the ambient temperature. At this point, the material starts to deform without any significant increase in stress. As long as the stress remains below this stress level, the material behaves linear. The super-elastic deformation is 100% reversible. In fact, the material used is monocrystalline and has better characteristics than polycrystalline material such as larger strains, less hysteresis and a flatter plateau. Fig. 11 compares the bending characteristics of the super-elastic joints used in the prototype with normal
Fig. 12. Prototype of bending actuator.
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Fig. 13. The bending actuator in the left, middle, and right position.
elastic ones. An elastic joint with the same stiffness in the neutral position needs a 5 times higher torque to bend it to its extreme position. An elastic joint with the same maximum torque has a 5 times lower stiffness in the neutral position. Fig. 12 shows a photograph of a first prototype. It requires a current of at least 0.8 A. Faster motion can be achieved by increasing the current to 1.2 A Žeach time for a full stroke of 158 left or right.. The resistance of the strip is 0.3 W for the part on the actuator itself and 1 W between the two clamps. This results in a power consumption of 0.2 to 0.4 W. With the depicted system in a laboratory environment without any extra cooling, a bandwidth of 0.25 Hz can be attained. If, as explained in Section 5, patterned strips are used the electrical characteristics will improve. These power values do not pose any problem for application in a gastro-intestinal inspection system. The inchworm system as depicted on Fig. 2 has an umbilical system supplying air and water for flushing the colon. Flushing is absolutely required to move forward in the colon. Both media could
therefore also be used for cooling purposes. The strips are melt spun ribbons, 1.5 mm wide and 40 mm thick that can generate a force of 6 N. As the lever is 0.5 mm, the torque generated by the SMA is 3 Nmm, such that the external torque Ž6 times smaller. is 0.5 Nmm Ž12 times lower than specified.. This can be increased by using wider and thicker strips, but these were not available. Thicker strips should be carefully used because their flexibility is lower. The final prototype measures 15 mm in diameter and is 4 mm high which is according to the specs in Table 1. Fig. 13 shows the prototype in its three stable positions: bent to the left, middle position and bent to the right.
5. Production of the actuator elements An important problem with SMA actuators is the low electrical resistance, in particular when the electric current has to be supplied by integrated electronic circuitry. Increasing the resistance is possible by replacing the ribbon by a number of wires mechanically in parallel and electri-
Fig. 14. Different steps in the production of a thin film actuator.
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tive solution is HF Ž5%. and H 2 0 2 Ž95%.. But all hydrogen containing etching products must be avoided because of hydrogen brittleness of NiTi. Furthermore, most photoresists do not withstand these strong acids. The solution is to use electrochemical etching with a weak acid that does not contain hydrogen. FeCl 3 was used and showed good results.
6. Conclusion
Fig. 15. Ribbons cut with EDM Žtop., CO 2 laser Žmiddle. and excimer laser Žbottom..
cally in series. Suppose a ribbon is cut into n mechanically parallel wires that are put electrically in series. This causes a multiplication of the resistance by n 2 . When the total consumed power remains the same, the required current is n times lower and the voltage is n times higher. It is important that the different wires mechanically are loaded equally. Therefore, the wires have to be kept together until they are fixed to the actuated device. The thermal properties of the parallel wire system are superior to those of the flat ribbon, as a larger contact surface is available for the cooling the same cross-section. Cooling is the limiting factor for the bandwidth of a shape memory actuator w10x. Fig. 14 shows the different steps in the production of such a thin film actuator. For realising a patterned ribbon, several technologies are possible: EDM, laser cutting, and etching. All three of these technologies are tested. WireEDM has the disadvantage that a starting hole is needed for the wire unless the wire starts from the side of the workpiece. The upper photograph of Fig. 15 shows a ribbon cut with EDM. The wires are 0.3 mm wide and 5.5 mm long. A Differential Scanning Calorimetry ŽDSC. test was performed on the material after cutting. The transformation temperatures shifted 108C upwards, but this may be caused by the mechanical test performed on it. Laser cutting is faster and allows more complex patterns. Laser cutting was performed in an argon atmosphere because of the high reactivity of titanium with oxygen. To avoid movement of the tiny wires, the ribbon was glued with strain gauge glue on a substrate. An aluminiumoxide plate was used as substrate to avoid welding of the ribbon on the substrate. The middle photograph of Fig. 15 shows the CO 2 laser cut ribbon with wires of 0.2 mm wide and 5 mm long. The cut surfaces are very rough, which is very disadvantageous for fatigue. As shown on the lower photograph of Fig. 15, superior results were obtained with an excimer laser. Also etching was investigated. A major problem here is the high resistance of NiTi to etching products. An effec-
This paper described the design of a prototype gastrointestinal intervention system based on an inchworm-type of mobile robot. The proposed device can be a kind of vehicle for inspection through the colon, something that is currently impossible due to the large number of turns in the intestinal system. The prototype has three main modules: an extension and contraction module, a locking module, and a two degree-of-freedom bending module. All modules are actuated by SMA elements. This paper merely concentrated on a modular actuator for realising bending motion. The design can be compared to a single vertebra, where stacking several elements can form a spinal column. The concept takes into consideration the electromechanical interconnection of the different parts and also techniques for selectively addressing the different actuators are proposed. A detailed comparison between several design alternatives shows that for this application, a straight wire system with antagonistic actuators, is the best, although not ideal, solution. In order to enhance the performance of the proposed system, several technologies for producing straight SMA elements with high electrical resistance were discussed. Excimer laser cutting gave the best results.
Acknowledgements This research was sponsored by the Brite-Euram programme of the European Union, project number BE-759693 and contract number BRE2-CT93-0579 and by the Belgian programme on Interuniversity Poles ŽIUAP4-24. of attraction initiated by the Belgian State, Prime Minister’s Office, Science Policy Programming. The authors assume the scientific responsibility of this paper. D. Reynaerts is a postdoctoral Fellow of the Fund for Scientific Research Flanders—Belgium ŽF.W.O..
References w1x A.M. Flynn, R.A. Brooks, W.M. Wells III, D.S. Barrett, Intelligence for miniature robots, Sensors and Actuators 20 Ž1989. 187–196. w2x P. Dario, R. Valleggi, M.C. Carrozza, M.C. Montesi, M. Cocco, Microactuators for microrobots: a critical survey, Workshop Digest Micromechanics Europe, Leuven, Belgium, June 1–2, 1992, pp. 141–173. w3x S. Aoshima, T. Yabuta, A miniature mobile robot using piezo
166
w4x
w5x
w6x
w7x
w8x
w9x
w10x
w11x
w12x w13x w14x
w15x
w16x
D. Reynaerts et al.r Sensors and Actuators 77 (1999) 157–166 elements, Tutorial on Piezoelectric Smart Systems of the IEEE Int. Conf. on Robotics and Automation, Sacramento, CA, USA, April 9–11, 1991, pp. 49–97. T. Fukuda, H. Hosokai, H. Ohyama, H. Hashimoto, F. Arai, Giant Magnetostrictive Alloy ŽGMA. applications to micro mobile robot as a micro actuator without power supply cables, Proc. IEEE Micro Electro Mechanical Systems, Nara, Japan, Jan. 30–Feb. 2, 1991, pp. 210–215. T. Fukuda, H. Ishihara, Micro optical robotic system with cordless optical power supply, Proc. Int. Conf. on Intelligent Robots and Systems, Yokohama, Japan, July 26–30, 1993, pp. 1703–1708. K. Ikuta, The application of microrminiature mechatronics to medical robotics, Proc. Int. Conf. on Intelligent Robots and Systems, Tokyo, Japan, Nov. 1988, pp. 9–14. J. Hesselbach, H. Stork, Electrically controlled shape memory actuators for use in handling systems, Proc. 1st Int. Conf. on Shape Memory and Superelastic Technologies, Monterey, CA, USA, March 7–10, 1994, pp. 277–282. E. Shamoto, T. Moriwaki, Walking drive—a new precision feed drive, Proc. IMACSrSICE Int. Symp. on Robotics, Mechatronics and Manufacturing Systems, Kobe, Japan, September 10–16, 1992, pp. 111–116. W. Krause, W. Schinkothe, Lineardirectantriebe fur ¨ ¨ die Feinwerktechnik,wLinear direct drives for precision machinesx Feinwerktechnik und Messtechnik 98 Ž1990. 7–8. D. Reynaerts, H. Van Brussel, Shape memory alloy based electrical actuation for robotic applications, Proc. 1st Int. Conf. on Shape Memory and Superelastic Technologies, Monterey, CA, USA, March 7–10, 1994, pp. 271–276. W. Van Moorleghem, D. Reynaerts, H. Van Brussel, J. Van Humbeeck, General discussion: the use of shape memory actuators, in: H. Borgmann, K. Lenz ŽEds.., Proc. 4th Int. Conf. on New Actuators, Bremen, Germany, June 15–17, 1994, pp. 225–227. D. Honma, Y. Miwa, N. Iguchi, Application of shape memory effect to digital control actuator, Bull. JSME 27 Ž230. Ž1984. 1737–1742. K. Kuribayashi, Millimeter-sized joint actuator using a shape memory alloy, Sensors and Actuators 20 Ž1989. 57–64. P. Dario, M.C. Montesi, Shape memory alloy microactuators for minimally invasive surgery, Proc. 1st Int. Conf. on Shape Memory and Superelastic Technologies, Monterey, CA, USA, March 7–10, 1994, pp. 427–433. K. Ikuta, M. Tsukamoto, S. Hirose, Shape memory alloy servo actuator system with electric resistance feedback and application for active endoscope, Proc. IEEE Int. Conf. on Robotics and Automation, Philadelphia, PA, USA, April 25–29, 1988, pp. 427–430. S. Guo, T. Fukuda, K. Kosuge, F. Arai, M. Negoro, K. Nakabayashi, Micro active catheter system with multi degrees of freedom, Proc. IEEE Int. Conf. on Robotics and Automation, San Diego, CA, USA, May 8–13, 1994, pp. 2290–2295.
w17x D. Reynaerts, J. Peirs, H. Van Brussel, Design of a shape memory actuated implantable drug delivery system, tutorial on microrobotic systems for minimal invasive surgery of the IEEE Int. Conf. on Robotics and Automation, Minneapolis, MN, USA, April 22–28, 1996, pp. 106–112. w18x B. Slatkin, J. Burdick, W. Grundfest, The development of a robotic endoscope, Proc. IEEE Int. Conf. on Robotics and Automation, Nagoya, Aichi, Japan, May 21–27, 1995, pp. 162–171. w19x R.H. Sturges Jr., S. Laowattana, A flexible, tendon-controlled device for endoscopy, Int. J. Robotics Res. 12 Ž1993. 121–131. Dominiek Reynaerts received his Mechanical Engineering degree from the Katholieke Universiteit Leuven, Belgium, in 1986. He has been working at the Mechanical Engineering Department of that same university, as a research assistant Ž1986–1991., and since 1996 as senior research assistant of the F.W.O. ŽFund for Scientific Research, Flanders.. He obtained his PhD degree in mechanical engineering in 1995 with the thesis ‘Control methods and actuation technology for whole-hand dexterous manipulation’, and became Assistant Professor at the Katholieke Universiteit Leuven in 1997. His research activities are in design and control of multi-fingered grippers, shape memory alloy actuators, precision mechanics, and micromechanical systems. He is a member of IEEE. Jan Peirs graduated as Mechanical Engineer ŽK.U. Leuven, 1993. and, as a research assistant at the Division of Production Engineering, Machine Design and Automation of K.U. Leuven, he is currently working towards a PhD Degree in Mechanical Engineering. His research interests include the design of shape memory alloy micro-actuators for medical applications and micromechanical systems in general. Hendrik Van Brussel is mechanical engineer ŽHTI-Oostende, Belgium, 1965. and electronics engineer ŽK.U. Leuven, Belgium, 1968.. He received a PhD Degree in Mechanical Engineering in 1971 from K.U. Leuven. From 1971 until 1973, he was ABOS-expert at the Metal Industries Development Centre in Bandung, Indonesia, where he set up an engineering research centre, and Associate Professor at ITB, Bandung. In 1973, he became lecturer at K.U. Leuven and is now full professor in automation and Head of the Department of Mechanical Engineering. He was a pioneer in robotics research in Europe and an active promoter of the mechatronics idea as a new paradigm for concurrent machine design. He has published more than 200 papers on different aspects of robotics, mechatronics and flexible automation. His present research interests are shifting towards holonic manufacturing systems and precision engineering, including microrobotics. He is Fellow of SME and IEEE and in 1994, he received a honorary doctor degree from the ‘Politehnica’ University in Bucharest, Romania and from RWTH, Aachen, Germany. He is also a corresponding member of the Royal Academy of Sciences, Literature and Fine Arts of Belgium and an active member of CIRP ŽInternational Institution for Production Engineering Research..
Shape memory micro-actuation for a gastro-intestinal intervention system Dominiek Reynaerts ) , Jan Peirs, Hendrik Van Brussel Katholieke UniÕersiteit LeuÕen, DiÕision of Production Engineering, Machine Design and Automation, Celestijnenlaan 300 B, B-3001 HeÕerlee, Belgium Received 2 July 1997; accepted 24 March 1999
Abstract This paper describes the design of a prototype gastro-intestinal intervention system based on an inchworm-type of mobile robot. This type of device is a kind of vehicle for inspection through the colon, something that is currently impossible due to the large number of turns in the intestinal system. Eventually, tools for intervention can be added in the future. The overall system is about 95 mm long and has a diameter of 15 mm. The robot consists of three main modules: an extension and contraction module, a two degree-of-freedom bending module and two locking modules. All these modules are to be actuated by shape memory elements. The main part of the paper describes a modular actuator for realising bending motion. The design can be compared to a single vertebra, where stacking several elements can form a spinal column. It will be shown that this design greatly facilitates the control. The electromechanical interconnection of the different parts was an integral part of the design as well as techniques for selectively addressing the different actuators. In order to increase the performance of the proposed design, the last part of the paper discusses some production aspects of the shape memory elements. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Gastro-intestinal; Shape memory alloy; Micro-actuator; Micro-electromechanical system; Microvalve; Minimal invasive surgery
1. Introduction The idea of miniature mobile robots goes back to the GNAT robot developed at MIT, artificial intelligence lab w1x. The idea behind this GNAT is an insect-like, completely autonomous, robot that can be mass-produced. Although this idea originally looked very futuristic, the growing interest in micro-electromechanical systems ŽMEMS. caused the research community to consider these systems to be within the reach of current technology. The terminology in this field is very poorly defined, but for the purpose of this paper, a miniature robot is defined as proposed by Dario et al. w2x: a robot with a size in the order of a few cubic centimetres that operates in a workspace and generates forces comparable to those applicable by human operators during fine manipulation. The most promising future for this kind of robots probably lies in the field of inspection and intervention in medical as well as in industrial applications. Within this
) Corresponding author. Tel.: q32-16-322-640; Fax: q32-16-322-987; E-mail: [email protected]
field, several prototypes have been proposed: Aoshima and Yabuta w3x proposed a miniature robot using piezo-elements; A giant magnetostrictive alloy actuated robot has been proposed by Fukuda et al. w4x. The same author also presented a micro optical robotic system with cordless optical power supply w5x; Ikuta w6x presented the MEDIWORM, an active microrobot actuated by shape memory alloys; a similar idea was also proposed by Hesselbach and Stork w7x. Some of these designs are based on the principle of the inchworm actuator originally commercialised by Burleigh Instrument. The inchworm principle was mostly used to extend the stroke of piezoelectric linear drives w8,9x and consists of two clamping modules and one expansion module. By intermittent clamping, expanding, and reclamping, these systems can creep over a rod or inside a tube. For all these systems, compact actuation is required. Shape Memory Alloy ŽSMA. actuators offer the advantage of extremely high power-to-volume ratios that are comparable to those for hydraulic actuation w10x. Moreover, they enable a very simple direct drive actuator design and an electrical current using simple resistive heating can di-
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 1 9 1 - 0
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rectly drive them. Shape memory alloys have been used for a wide range of applications w11x. Honma et al. w12x and Kuribayashi w13x presented shape memory based micro-actuators. In the medical field, most applications are based on the superelastic properties of SMAs or on a one-way actuation like clamping. ‘Real’ actuators are mostly used in minimal invasive surgery tools like endoscopes. Some examples of this latter category are the active catheter tip developed by Dario and Montesi w14x, the active endoscope presented by Ikuta et al. w15x, or the miniature active catheter developed by Guo et al. w16x. The proposed research is part of a Brite-Euram fundamental research project studying shape memory alloy micro-actuators for medical applications. A first development within this project consisted of an actively controlled implantable drug delivery device w17x. The second part of this project concerned the development of a gastro-intestinal intervention system based on an inchworm-type of mobile robot. Semi-autonomous gastro-intestinal intervention systems have only been addressed in the recent past w6,18x. This paper first describes the general lay-out of the designed gastro-intestinal intervention system. Afterwards, a modular shape memory actuator for realising bending motion is discussed in more detail. A final part of the paper discusses the production aspects of the shape memory elements.
2. Design of a gastro-intestinal intervention system When MEMS were proposed, operation in the blood stream was probably the most popular application among the projected ideas. This kind of ‘free floating’ devices have to cope with a number of severe problems: the reaction of the immunosystem on strange objects, the clothing of the blood around these objects, the required
Fig. 2. Inchworm-type of robot for inspection of the colon.
ability to navigate in a viscous environment, if necessary against the blood flow direction. All these problems were largely underestimated. In view of the current state-of-theart in microsystem technology, it was therefore decided to build a more realistic application consisting of a gastro-intestinal inspection and intervention system. In fact, this can also be considered as a mobile robot navigating through a pipe, in this case, the colon, but the scale is quite different. Fig. 1 shows a simplified representation of the human colon w19x. The colon has an average diameter of about 50 mm. The smallest radius is about 20 to 30 mm and is located at the bending portion between the rectum and the sigmoid colon. The transverse colon, which is 400 to 500 mm long, is the largest and most mobile part of the colon. Due to its horizontal position, the breathing process affects its movements. According to Sturges and Laowattana w19x, the use of a colonoscope is impeded by the peristaltic action of the gut, attempting to expel the device. The main problem for inspecting the colon with an endoscopic device is the large number of turns, more specifically at the sigmoid, which have to be taken when entering the human intestinal system. It is therefore also extremely difficult to manoeuvre a classical endoscope around the bends of the colon without damaging the gut. Therefore, the proposed design aims at a more advanced mobile system for colonoscopy. The system that is developed is basically an inchwormtype of mobile robot. Fig. 2 shows that, compared to the conventional inchworm with two clamping and an extension actuator, the proposed design has two additional bending degrees of freedom. This gastro-intestinal intervention system has an outer diameter of maximum 15 mm, which is acceptable for colonoscopes. Table 1 summarises the specifications for the design. Table 1 Specifications for inchworm design
Fig. 1. Diagram of the human colon Žfrom Ref. w19x..
Module
Number
Diameter Žmm.
Length Žmm.
Stroke
Translation Clamping Bending
1 2 2
15 15 15
10 32.5 10
10 mm ø 15–50 mm "458
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As shown in this table, each module has a specific functionality that is to be realised within a very narrow volume. Ø The translation module has a length of 10 mm and has to realise a stroke of 10 mm. This expansion can be realised with a spring-type SMA actuator. Also, the design has to guarantee sufficient lateral stiffness. Ø The clamping module has a length of 32.5 mm must be able to clamp in a human colon with a diameter ranging from 15 to 50 mm. The gut is a highly flexible and slippery environment and in the same time it is very fragile and sensitive to damage. Therefore, a clamping system based on balloons seems most appropriate. This approach was already proposed by Slatkin et al. w18x. Ø The bending module has a length of 10 mm. It has to realise a bending from y458 to 458. Both modules are stacked in such a way that a two degree-of-freedom system is obtained. Several design teams within the project develop the different parts. In first instance, the interface between the different actuators is a flat disk of 15 mm diameter. After completion of the first prototype, a further integration of the different actuators will be considered. For instance, empty space in one module can be used for the other actuators. For this first design, the forces and torques to be developed should be sufficient to support the weight of the device. A more detailed specification will be made after experimental testing. No commercial endoscopic inchworms are yet existing, so this design would be a completely new product. The proposed specs are on-the-edge, but appear to be reasonable for a first prototype.
3. Design of a modular bending actuator 3.1. Bending actuator general concept Fig. 3 shows that the actuator can be compared to a single vertebra, where stacking several elements can form a spinal column. Each vertebra is controlled in a binary
Fig. 3. Vertebral design of a bending actuator.
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Fig. 4. Design concept of a single vertebra actuator.
mode. This means that a single vertebra has only two powered positions Žleft bending or right bending. and the central position in the unpowered condition. This also means that the complete ‘spinal column’ can only reach a discrete number of positions. However, for the envisaged endoscopic application this is more than sufficient. Fig. 4 shows a design concept for a single vertebra actuator. The actuator consists of a central rotating part actuated by two antagonistic SMA actuators. The central element also contains two return springs with constant and equal torque. This means that the unpowered position corresponds to the neutral position. Fig. 5 shows a photograph of a preliminary prototype. For this specific example, the actuator has a stroke of "158 and a diameter of 17 mm. The major parts of this prototype are made of brass, but could be easily made in another material Že.g., plastics.. As each actuator is controlled in a binary mode, only a mechanism for selection of the different actuators has to be provided. The electrical control of the different actuators
Fig. 5. Prototype of a single vertebra actuator.
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Fig. 6. Electromechanical connection of the actuators.
can be realised with a bus system. Each single vertebra contains an integrated electronic circuit to enable actuator selection, or eventually selection of several actuators within one single bending module. With plastic modules, this integration can be realised by integrating electrical connections on the parts of the module with the SIL 1 technology w17x. Fig. 6 shows the principle of electromechanical connection for the different modules. Adjacent modules can be rotated 908 to each other so that a two-dimensional bending actuator is obtained. In this case, five signals are provided: a supply voltage for the electronics Vcc , a supply voltage for the SMA elements Vs , a common ground, a select channel, and a clock channel. Fig. 7 shows two principles for driving the different actuators based on the above signals. The first principle Žfrequency modulation based communication. realises actuator selection with a filter or a phase locked loop. This allows simultaneous actuation using only one signal wire carrying a frequency modulated selection signal. No clock signals are required. This design has the drawback that all modules differ by their filter, which is a disadvantage for mass production and results in a decreased degree of modularity. The second principle Žserial communication. uses digital addressing of the different modules. This system also enables simultaneous actuation but requires an additional clock channel. In this case, all modules differ by their address. As it is much easier to change a digital address, this solution is preferable for real mass production.
considered as a flat disk with diameter 15 mm. The desired stroke of an actuator is "158. This means that three vertebras are required for realising the bending movement specified in Table 1. The external torque corresponds to the weight of the links to rotate. It is estimated to be 6 Nmm. Other requirements are a sufficiently high electrical resistance for control by an electronic circuit embedded in the vertebra and the possibility for miniaturisation. The assumed SMA material characteristics can be found in the upper left corner of Table 2. It is assumed that the return force to deform a cold shape memory element is one-third of the force it generates when heated. Table 2 compares nine different actuation principles for the vertebra actuator. In each row of Table 2, a comparison is made between pure antagonistic SMAs, an SMA working against an elastic joint, and a combination of both. Ø The pure antagonistic system has two stable powered positions, but is not stable when not powered. The force required to deform a SMA element is 1r3 of the force it generates. Consequently, the activated SMA has to generate force for external use and for deformation of his antagonist as expressed in Eq. Ž1.. By simplifying this equation, this means that the SMA has to generate a torque 1.5 times higher than the external torque. MSM A G Mext q
1 3
MSMA ´ MSMA G
3 2
Mext
Eq. Ž1. is the Torque balance for antagonistic system. Ø The SMA in combination with an elastic joint has one powered position and also a stable neutral position. The elastic joint serves as both spring and joint. The activated SMA has to produce the external torque but also to deform the spring as expressed by Eq. Ž2.. In the unpowered situation, the spring has to be able to deform the SMA and to generate the external torque. The torque generated by the bias spring is assumed to be constant. This is correct if super-elastic joints are used Žsee Section 4.. Combination
3.2. Detailed actuator design This section describes a general study of different alternatives for the vertebra actuator design. The vertebra is
1
ŽInjection moulded parts Spritzgiesteile mit Integrierten Leiterzugen ¨ with integrated conducting pads.. SIL starts from a plastic part that is galvanically covered with copper and tin. The tin layer is patterned by laser evaporation. Afterwards, this layer is used as a mask to etch the underlying copper. Finally, tin is removed and the result is a structured copper layer.
Ž 1.
Fig. 7. Two principles for selective addressing of the actuators.
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Table 2 Comparisons of actuation principles for the vertebra actuator
of both formulas leads to the conclusion that the SMA has to generate 3 times the external torque. MSM A G Mext q Mspring Mspring G Mext q
1
¶ •´ M ß
MSMA
SM A G 3 Mext
Ž 2.
3 Ž . Eq. 2 is the Torque balance for SMA with bias spring Žsuper-elastic joint.. Ø The combination of an antagonistic system and an elastic joint creates three stable positions, two powered and one unpowered. Here the torque of the SMA is 6 times higher than the external torque. MSM A G Mext q Mspring q Mspring G Mext q
1 3
1 3
¶ MSMA
MSMA
•´ M
ß
SM A G 6 Mext
Ž 3. Eq. Ž3. is the Torque balance for combined system.
The columns of Table 2 show different mountings in which the SMA material can be used: a straight wire, a pulley system, or a spring. Ø For the simple wire system, the maximum wire length of 15 mm Ži.e., the diameter of the module. is an additional constraint. The lever follows from the wire length, the maximum strain and the specified stroke. Lever and specified torque define the force and consequently, also the wire thickness and resistance. Ø The pulley system and the spring system assume that the maximum possible lever is used in order to reduce the forces and the section of the wire. For the pulley system, the force can be calculated from the specified torque and the known lever. Wire length is calculated from lever and stroke. Ø The SMA springs are calculated with the normal spring formulas where the E-modulus is replaced by the ratio t SM A rg SMA . This is correct under the following assumptions. Ž1. The maximum strain is proportional to the stress.
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Ž2. The formula is only used to calculate full stroke deformation: assumption 1 is only valid for the maximum strain at a specific stress. Ž3. The strain is proportional to the distance to the centre of the wire: zero in the centre, maximum at the surface. Together with assumption 1, this means that stress and strain are proportional to the distance to the centre as for normal springs. Therefore, the E-modulus can be replaced by t SM A rg SMA . The conclusions of this table can be summarised as follows. Ø The torque to be generated by the SMAs is higher than the required external torque: the pure antagonistic approach asks only 1.5 times the external torque, the system with the elastic joint and one SMA element requires already 3 times the external torque and the combination of antagonistic SMAs with elastic joints requires the SMAs to generate a 6 times higher torque. In the last case, the SMAs have to generate 36 Nmm instead of 6 Nmm. The combined system asks more, but offers more functionality: three stable positions instead of two and the neutral position is unpowered leading to lower power consumption as the inchworm will mainly move straightforward such that the bending actuator will be used only during a small part of the intervention. Ø None of the nine combinations satisfies all constraints for this specific design: Ž1. For straight SMA wires, the forces in the SMA, the mechanical connections and the joint are very high. As a result, thick wires are needed such that the electrical resistance is extremely low. Furthermore, these wires are not flexible such that bending forces of these wires become important. Ž2. The pulley system has a high electrical resistance but requires a very long wire passed over a number of pulleys. Depending on the case, at least five or 10 of these pulleys per SMA wire are needed. These cause friction and are not suited for miniaturisation. Ž3. SMA springs offer the easiest way for
construction of the actuator but their problem is again the extremely low electrical resistance. Ø The problem with the straight wire system can be solved by replacing the thick SMA wire by a number of thin SMA wires mechanically in parallel and electrically in series Žwithout using pulleys of course.. This way, the electrical resistance is higher and the thin wires are much more flexible. Problem is the equal load distribution over the different wires. Cutting the pattern out of a ribbon solves this problem. This technique is explained more in detail in Section 5. Ø Final conclusion: the selected design is the straight wire system with antagonistic SMAs, super-elastic joints, and a patterned ribbon.
4. Construction of an optimised actuator prototype Fig. 8 shows top, side and isometric view of the actuator design. The body of the actuator consists of two parts, an upper and a lower part, that have an identical form but one of them is rotated 1808. They are made of aluminium by electro-discharge machining ŽEDM., final versions could be made of plastic by use of injection moulding. Both parts are connected to each other via two super-elastic joints. When the upper SMA strip is heated, it shortens and forces the upper part to rotate to the right. When the lower strip is heated, the upper part rotates to the left. The stroke in both directions is 158 and is limited mechanically. Heating both strips at the same time has to be avoided, as this will overload both strips and the joint. When no strip is heated, the elastic joints hold the actuator in its middle position. A mechanical stop guarantees a fixed distance between the SMA strip and the axis of rotation. A 0.15-mm thin glass plate assures thermal and electrical isolation between the SMA strip and the aluminium body. Assembly of all parts is done by gluing. To connect
Fig. 8. Bending actuator design.
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Fig. 9. Stacking of three actuators to obtain specified stroke.
the glass plate to the body and the SMA strip to the glass plate, epoxy glue is used. To obtain the specified stroke of "458, three actuators have to be used as shown in Fig. 9. Activation of one, two, or three actuators causes, respectively 158, 308 or 458 rotation. Seven useful combinations are possible leading to following positions: y458, y308, y158, 08, 158, 308, 458. The actuator contains two super-elastic joints with dimensions shown in Fig. 10. Bending is limited to the narrowed zone of 0.12 mm thickness. Two joints, one on each side of the actuator, are used to enhance stiffness and strength in the other directions. They are made of a CuAlNi alloy by EDM. Rolled strips are an alternative but were not available in this material. A super-elastic joint is an elastic joint made of superelastic material. As all elastic joints, they have a number of advantages over normal rotary joints. Ø The joint acts also as bias spring such that no extra spring has to be included in the design. This leads to a reduction in the number of parts and a simplification of the design, both improving the possibilities for miniaturisation. Ø An elastic joint exhibits no friction or backlash. Elastic joints suffer from a limited stroke and a return force that can be substantial. However, these disadvantages do not apply for this development. Super-elastic materials have a non-linear characteristic that permits to build actuators that are stiff in their neutral position while the forces to bend them to their extreme position are kept low. A high stiffness Žor high Young’s modulus. in the neutral position is important to avoid small disturbing forces from bending the actuator. With a normal elastic material this high modulus results in high elastic forces at the end of the
Fig. 10. Dimensions of super-elastic joint Žin mm..
Fig. 11. Characteristic of super-elastic joint.
stroke of the joint. This effect can be minimised by use of super-elastic materials. The material behaves linear until a stress of 450 MPa is reached. Then the material deforms without any significant increase in stress. During recovery of the material, the stress is about 50 MPa lower. Superelastic behaviour is closely related to the shape memory effect. The difference is that super-elastic materials have transformation temperatures far below ambient temperature. These transformation temperatures increase proportional with stress, such that at a certain stress level Žhere 450 MPa., the martensitic transformation temperature exceeds the ambient temperature. At this point, the material starts to deform without any significant increase in stress. As long as the stress remains below this stress level, the material behaves linear. The super-elastic deformation is 100% reversible. In fact, the material used is monocrystalline and has better characteristics than polycrystalline material such as larger strains, less hysteresis and a flatter plateau. Fig. 11 compares the bending characteristics of the super-elastic joints used in the prototype with normal
Fig. 12. Prototype of bending actuator.
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Fig. 13. The bending actuator in the left, middle, and right position.
elastic ones. An elastic joint with the same stiffness in the neutral position needs a 5 times higher torque to bend it to its extreme position. An elastic joint with the same maximum torque has a 5 times lower stiffness in the neutral position. Fig. 12 shows a photograph of a first prototype. It requires a current of at least 0.8 A. Faster motion can be achieved by increasing the current to 1.2 A Žeach time for a full stroke of 158 left or right.. The resistance of the strip is 0.3 W for the part on the actuator itself and 1 W between the two clamps. This results in a power consumption of 0.2 to 0.4 W. With the depicted system in a laboratory environment without any extra cooling, a bandwidth of 0.25 Hz can be attained. If, as explained in Section 5, patterned strips are used the electrical characteristics will improve. These power values do not pose any problem for application in a gastro-intestinal inspection system. The inchworm system as depicted on Fig. 2 has an umbilical system supplying air and water for flushing the colon. Flushing is absolutely required to move forward in the colon. Both media could
therefore also be used for cooling purposes. The strips are melt spun ribbons, 1.5 mm wide and 40 mm thick that can generate a force of 6 N. As the lever is 0.5 mm, the torque generated by the SMA is 3 Nmm, such that the external torque Ž6 times smaller. is 0.5 Nmm Ž12 times lower than specified.. This can be increased by using wider and thicker strips, but these were not available. Thicker strips should be carefully used because their flexibility is lower. The final prototype measures 15 mm in diameter and is 4 mm high which is according to the specs in Table 1. Fig. 13 shows the prototype in its three stable positions: bent to the left, middle position and bent to the right.
5. Production of the actuator elements An important problem with SMA actuators is the low electrical resistance, in particular when the electric current has to be supplied by integrated electronic circuitry. Increasing the resistance is possible by replacing the ribbon by a number of wires mechanically in parallel and electri-
Fig. 14. Different steps in the production of a thin film actuator.
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tive solution is HF Ž5%. and H 2 0 2 Ž95%.. But all hydrogen containing etching products must be avoided because of hydrogen brittleness of NiTi. Furthermore, most photoresists do not withstand these strong acids. The solution is to use electrochemical etching with a weak acid that does not contain hydrogen. FeCl 3 was used and showed good results.
6. Conclusion
Fig. 15. Ribbons cut with EDM Žtop., CO 2 laser Žmiddle. and excimer laser Žbottom..
cally in series. Suppose a ribbon is cut into n mechanically parallel wires that are put electrically in series. This causes a multiplication of the resistance by n 2 . When the total consumed power remains the same, the required current is n times lower and the voltage is n times higher. It is important that the different wires mechanically are loaded equally. Therefore, the wires have to be kept together until they are fixed to the actuated device. The thermal properties of the parallel wire system are superior to those of the flat ribbon, as a larger contact surface is available for the cooling the same cross-section. Cooling is the limiting factor for the bandwidth of a shape memory actuator w10x. Fig. 14 shows the different steps in the production of such a thin film actuator. For realising a patterned ribbon, several technologies are possible: EDM, laser cutting, and etching. All three of these technologies are tested. WireEDM has the disadvantage that a starting hole is needed for the wire unless the wire starts from the side of the workpiece. The upper photograph of Fig. 15 shows a ribbon cut with EDM. The wires are 0.3 mm wide and 5.5 mm long. A Differential Scanning Calorimetry ŽDSC. test was performed on the material after cutting. The transformation temperatures shifted 108C upwards, but this may be caused by the mechanical test performed on it. Laser cutting is faster and allows more complex patterns. Laser cutting was performed in an argon atmosphere because of the high reactivity of titanium with oxygen. To avoid movement of the tiny wires, the ribbon was glued with strain gauge glue on a substrate. An aluminiumoxide plate was used as substrate to avoid welding of the ribbon on the substrate. The middle photograph of Fig. 15 shows the CO 2 laser cut ribbon with wires of 0.2 mm wide and 5 mm long. The cut surfaces are very rough, which is very disadvantageous for fatigue. As shown on the lower photograph of Fig. 15, superior results were obtained with an excimer laser. Also etching was investigated. A major problem here is the high resistance of NiTi to etching products. An effec-
This paper described the design of a prototype gastrointestinal intervention system based on an inchworm-type of mobile robot. The proposed device can be a kind of vehicle for inspection through the colon, something that is currently impossible due to the large number of turns in the intestinal system. The prototype has three main modules: an extension and contraction module, a locking module, and a two degree-of-freedom bending module. All modules are actuated by SMA elements. This paper merely concentrated on a modular actuator for realising bending motion. The design can be compared to a single vertebra, where stacking several elements can form a spinal column. The concept takes into consideration the electromechanical interconnection of the different parts and also techniques for selectively addressing the different actuators are proposed. A detailed comparison between several design alternatives shows that for this application, a straight wire system with antagonistic actuators, is the best, although not ideal, solution. In order to enhance the performance of the proposed system, several technologies for producing straight SMA elements with high electrical resistance were discussed. Excimer laser cutting gave the best results.
Acknowledgements This research was sponsored by the Brite-Euram programme of the European Union, project number BE-759693 and contract number BRE2-CT93-0579 and by the Belgian programme on Interuniversity Poles ŽIUAP4-24. of attraction initiated by the Belgian State, Prime Minister’s Office, Science Policy Programming. The authors assume the scientific responsibility of this paper. D. Reynaerts is a postdoctoral Fellow of the Fund for Scientific Research Flanders—Belgium ŽF.W.O..
References w1x A.M. Flynn, R.A. Brooks, W.M. Wells III, D.S. Barrett, Intelligence for miniature robots, Sensors and Actuators 20 Ž1989. 187–196. w2x P. Dario, R. Valleggi, M.C. Carrozza, M.C. Montesi, M. Cocco, Microactuators for microrobots: a critical survey, Workshop Digest Micromechanics Europe, Leuven, Belgium, June 1–2, 1992, pp. 141–173. w3x S. Aoshima, T. Yabuta, A miniature mobile robot using piezo
166
w4x
w5x
w6x
w7x
w8x
w9x
w10x
w11x
w12x w13x w14x
w15x
w16x
D. Reynaerts et al.r Sensors and Actuators 77 (1999) 157–166 elements, Tutorial on Piezoelectric Smart Systems of the IEEE Int. Conf. on Robotics and Automation, Sacramento, CA, USA, April 9–11, 1991, pp. 49–97. T. Fukuda, H. Hosokai, H. Ohyama, H. Hashimoto, F. Arai, Giant Magnetostrictive Alloy ŽGMA. applications to micro mobile robot as a micro actuator without power supply cables, Proc. IEEE Micro Electro Mechanical Systems, Nara, Japan, Jan. 30–Feb. 2, 1991, pp. 210–215. T. Fukuda, H. Ishihara, Micro optical robotic system with cordless optical power supply, Proc. Int. Conf. on Intelligent Robots and Systems, Yokohama, Japan, July 26–30, 1993, pp. 1703–1708. K. Ikuta, The application of microrminiature mechatronics to medical robotics, Proc. Int. Conf. on Intelligent Robots and Systems, Tokyo, Japan, Nov. 1988, pp. 9–14. J. Hesselbach, H. Stork, Electrically controlled shape memory actuators for use in handling systems, Proc. 1st Int. Conf. on Shape Memory and Superelastic Technologies, Monterey, CA, USA, March 7–10, 1994, pp. 277–282. E. Shamoto, T. Moriwaki, Walking drive—a new precision feed drive, Proc. IMACSrSICE Int. Symp. on Robotics, Mechatronics and Manufacturing Systems, Kobe, Japan, September 10–16, 1992, pp. 111–116. W. Krause, W. Schinkothe, Lineardirectantriebe fur ¨ ¨ die Feinwerktechnik,wLinear direct drives for precision machinesx Feinwerktechnik und Messtechnik 98 Ž1990. 7–8. D. Reynaerts, H. Van Brussel, Shape memory alloy based electrical actuation for robotic applications, Proc. 1st Int. Conf. on Shape Memory and Superelastic Technologies, Monterey, CA, USA, March 7–10, 1994, pp. 271–276. W. Van Moorleghem, D. Reynaerts, H. Van Brussel, J. Van Humbeeck, General discussion: the use of shape memory actuators, in: H. Borgmann, K. Lenz ŽEds.., Proc. 4th Int. Conf. on New Actuators, Bremen, Germany, June 15–17, 1994, pp. 225–227. D. Honma, Y. Miwa, N. Iguchi, Application of shape memory effect to digital control actuator, Bull. JSME 27 Ž230. Ž1984. 1737–1742. K. Kuribayashi, Millimeter-sized joint actuator using a shape memory alloy, Sensors and Actuators 20 Ž1989. 57–64. P. Dario, M.C. Montesi, Shape memory alloy microactuators for minimally invasive surgery, Proc. 1st Int. Conf. on Shape Memory and Superelastic Technologies, Monterey, CA, USA, March 7–10, 1994, pp. 427–433. K. Ikuta, M. Tsukamoto, S. Hirose, Shape memory alloy servo actuator system with electric resistance feedback and application for active endoscope, Proc. IEEE Int. Conf. on Robotics and Automation, Philadelphia, PA, USA, April 25–29, 1988, pp. 427–430. S. Guo, T. Fukuda, K. Kosuge, F. Arai, M. Negoro, K. Nakabayashi, Micro active catheter system with multi degrees of freedom, Proc. IEEE Int. Conf. on Robotics and Automation, San Diego, CA, USA, May 8–13, 1994, pp. 2290–2295.
w17x D. Reynaerts, J. Peirs, H. Van Brussel, Design of a shape memory actuated implantable drug delivery system, tutorial on microrobotic systems for minimal invasive surgery of the IEEE Int. Conf. on Robotics and Automation, Minneapolis, MN, USA, April 22–28, 1996, pp. 106–112. w18x B. Slatkin, J. Burdick, W. Grundfest, The development of a robotic endoscope, Proc. IEEE Int. Conf. on Robotics and Automation, Nagoya, Aichi, Japan, May 21–27, 1995, pp. 162–171. w19x R.H. Sturges Jr., S. Laowattana, A flexible, tendon-controlled device for endoscopy, Int. J. Robotics Res. 12 Ž1993. 121–131. Dominiek Reynaerts received his Mechanical Engineering degree from the Katholieke Universiteit Leuven, Belgium, in 1986. He has been working at the Mechanical Engineering Department of that same university, as a research assistant Ž1986–1991., and since 1996 as senior research assistant of the F.W.O. ŽFund for Scientific Research, Flanders.. He obtained his PhD degree in mechanical engineering in 1995 with the thesis ‘Control methods and actuation technology for whole-hand dexterous manipulation’, and became Assistant Professor at the Katholieke Universiteit Leuven in 1997. His research activities are in design and control of multi-fingered grippers, shape memory alloy actuators, precision mechanics, and micromechanical systems. He is a member of IEEE. Jan Peirs graduated as Mechanical Engineer ŽK.U. Leuven, 1993. and, as a research assistant at the Division of Production Engineering, Machine Design and Automation of K.U. Leuven, he is currently working towards a PhD Degree in Mechanical Engineering. His research interests include the design of shape memory alloy micro-actuators for medical applications and micromechanical systems in general. Hendrik Van Brussel is mechanical engineer ŽHTI-Oostende, Belgium, 1965. and electronics engineer ŽK.U. Leuven, Belgium, 1968.. He received a PhD Degree in Mechanical Engineering in 1971 from K.U. Leuven. From 1971 until 1973, he was ABOS-expert at the Metal Industries Development Centre in Bandung, Indonesia, where he set up an engineering research centre, and Associate Professor at ITB, Bandung. In 1973, he became lecturer at K.U. Leuven and is now full professor in automation and Head of the Department of Mechanical Engineering. He was a pioneer in robotics research in Europe and an active promoter of the mechatronics idea as a new paradigm for concurrent machine design. He has published more than 200 papers on different aspects of robotics, mechatronics and flexible automation. His present research interests are shifting towards holonic manufacturing systems and precision engineering, including microrobotics. He is Fellow of SME and IEEE and in 1994, he received a honorary doctor degree from the ‘Politehnica’ University in Bucharest, Romania and from RWTH, Aachen, Germany. He is also a corresponding member of the Royal Academy of Sciences, Literature and Fine Arts of Belgium and an active member of CIRP ŽInternational Institution for Production Engineering Research..