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Design of a microgripper for micromanipulation of microcomponents using SMA wires and flexible hinges
Available online at www.sciencedirect.com
Sensors and Actuators A 141 (2008) 144–150
Design of a microgripper for micromanipulation of microcomponents using SMA wires and flexible hinges J.H. Kyung a,∗ , B.G. Ko a , Y.H. Ha b , G.J. Chung a a
Intelligence Machine System Research Center, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-Gu, Daejeon 305-343, Republic of Korea b Gyeonam TechnoPark, 136 Jungangno Changwon, Gyeongnam 641-739, Republic of Korea Received 27 October 2005; received in revised form 5 July 2007; accepted 5 July 2007 Available online 25 July 2007
Abstract For this study, a simple and easily manufacturable microgripper was designed utilizing shape memory alloy (SMA) wire. First, a microgripper with flexible hinge structures was designed to enable the fine handling of micro parts. The structural design of the microgripper was refined by repeated FE analyses. The characteristics of its major components, including SMA wire and a strain gauge, were investigated by experiment, and on the basis of the results, a controller was designed and fabricated for the control of the gripping force of the microgripper. In addition, the PI control gain of the controller was determined by considering the characteristics of the SMA wire, which has hysteresis response characteristics to heating and cooling. Gripping force control testing was conducted using the fabricated microgripper and a PI controller, in order to verify the performance of the designed microgripper. © 2007 Elsevier B.V. All rights reserved. Keywords: Microgripper; Shape memory alloy wire; Flexible hinge; PI control; Gripping force control
1. Introduction Recently, devices for handling micro parts for the industries of electronics, information technology, optics, medicine, and bio-technology have been developed. Accordingly, the development of a microgripper which fulfils the core role in fine handling is urgently required [1,2]. Many studies have been conducted to develop microgrippers capable of carrying out fine handling. Most of them were designed to have sufficient strength using thin silicon or stainless sheets; however, they were not easy to fabricate due to their complicated structures. A piezoelectric actuator and shape memory alloy (SMA) has been widely used for the actuators [3–6]. On the one hand, Ni–Ti shape memory alloy has the merits of a higher power-to-weight ratio and corrosion resistance. Additionally, SMA wire is cheap and does not require further processing as it can just be cut and used. On the other hand, its demerits are low energy efficiency, limited bandwidth to heating and cooling, and smaller deformation [7,8]. ∗
Corresponding author. E-mail address: [email protected] (J.H. Kyung).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.07.013
In works where micro parts are manipulated using a microgripper, the gripping force has to be controlled to prevent damage to them, even under external disturbance or uncertainties [9,10]. In this study, a simple, compactly structured, and easy to fabricate microgripper was designed using SMA wire, and a structural analysis was conducted using the finite element method (FEM). The gripping force was determined with the strain gauge at the tip of the microgripper to constitute the PI controller, and a study on the gripping force control was conducted. The performance of the microgripper was verified by experiments. 2. Structure and principle of operation Fig. 1 shows a conceptual drawing of the microgripper proposed in this study. The microgripper consists of gripping jaws, SMA wires, two flexible hinges (A, B), a stainless body, and a strain gauge. The gripping jaw is used to manipulate the micro parts, with tips designed to open and close by up to 120 m maximum. The flexible hinge A is a rotating axis that converts the linear motion of the SMA wire to a rotating motion. The flexible hinge B is
J.H. Kyung et al. / Sensors and Actuators A 141 (2008) 144–150
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Fig. 2. Microgripper: (a) open and (b) closed.
of the gripping force is transferred to this flexible hinge B. Therefore, the gripping force can be measured with the strain gauge adhered to the flexible hinge B.
Fig. 1. Conceptual drawing of microgripper.
made to be thinner than A in order to concentrate most of the gripping force, which is measured with the strain gauge. The SMA wire, which is the actuator, is composed of Ni–Ti alloy with a diameter of 100 m to cope with the strength of hinge A and perform the fastest movement possible. The SMA wire used in this study was Flexinol 100 LT of Mondotronics Inc., and has main properties as shown in Table 1. The principle of operation of the proposed microgripper is as follows: first, when the two SMA wires are energized to generate heat, the wire begins to contract lengthwise at the activating temperature. According to the contraction of the SMA wire, the gripping jaw is closed with the flexible hinge A as shown in Fig. 2(b). When the currents in the two SMA wires are cut off, the wires become cooler, expand back to their original sizes, and the elastic recovery force of the flexible hinge A opens the gripping jaw as shown in Fig. 2(a). When a micro part is gripped with the opening and closing movements of the proposed microgripper, stress is concentrated in the flexible hinge B, which is thinner than hinge A, and most
3. Structural analysis Through the structural analysis of the proposed microgripper, the structurally weak points were identified, and the microgripper could then be redesigned to avoid local stress concentration. Stress and elongation were calculated with FEM, on the basis of which the microgripper was redesigned to achieve a homogeneous stress profile along its overall length. The 3D modeling and FE analysis of the proposed gripper was conducted using the Autodesk’s MDT (mechanical desktop power pack). First, an analysis was conducted to determine the width of the flexible hinge A. As the load condition for the structural analysis, the recovery force of the SMA wire was set to 3 N. The
Table 1 SMA wire properties Thermal Activation start temperature (austenite start temperature) (◦ C) Activation finish temperature (austenite finish temperature) (◦ C) Relaxation start temperature (martensite start temperature) (◦ C) Relaxation finish temperature (martensite finish temperature) (◦ C)
68 78 52 42
Speed (in still air, at 20 ◦ C) Typical contraction speed (s) Relaxation speed (s)
1.0 0.8
Electrical Linear resistance (/m) Recommended current (mA) Recommended power (W/m)
150 180 4.86
Material Density (g/cm3 ) Maximum recovery force (MPa) Recommended recovery force (MPa) Recommended deformation force (MPa) Maximum deformation ratio (%) Recommend deformation ratio (%)
6.45 469 150 28 8 3–5
Fig. 3. FE analysis result (von Mises stress).
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Fig. 4. FE analysis result (gripping displacement).
thickness of the microgripper was set to 1 mm, taking into consideration the convenience of manufacturing. The microgripper was designed to satisfy a safety factor of 1.2. In order to implement the jaws’ maximum opening width of 120 m, structural analyses had been carried out by varying the width of the flexible hinge A. Finally, at a hinge A width of 0.55 mm, the maximum stress was calculated to be 295 MPa as shown in Fig. 3 and the maximum displacement of the gripper jaw was 123 m as described in Fig. 4, thus meeting the design criteria. In the analysis for determining the width of the flexible hinge B, since the strain gauge is adhered to hinge B only, it is important to design hinge B so that the gripping force is not transmitted to
hinge A in order to obtain accurate measurement of the gripping force. In addition, the design safety factor 1.2 was also applied to hinge B. In order to meet the design strength at hinge B, structural analyses were carried out by varying the width of hinge B. According to the analysis, with the width of hinge B at 0.3 mm, the maximum stresses at A and B were calculated to be 22 MPa and 75.5 MPa, respectively. It could be confirmed that most of the gripping force was transmitted to hinge B and the calculated maximum stress fell within the allowable tensile strength range, considering the safety factor. Fig. 5 shows the gripping force analysis result. The maximum gripping force of the microgripper was calculated to be 0.33 N.
Fig. 5. Maximum gripping force (=0.33 N) (factor of safety = 1.2).
J.H. Kyung et al. / Sensors and Actuators A 141 (2008) 144–150
Fig. 6. Photograph of the fabricated microgripper.
The proposed microgripper was designed taking the above results of the analyses in account and fabricated using STS304 material, whose yield strength is 360 MPa. 4. Fabrication of microgripper Fig. 6 shows the microgripper manufactured by electric discharge machining. Experiments for a repetitive open and close motion were conducted by open loop control, observing the microgripper through a 10 × 10 magnification microscope. Fig. 7 shows the open and close motion of the microgripper.
Fig. 7. Manufactured microgripper: (a) open and (b) closed.
5. Experiments with the microgripper The microgripper was fabricated with the following components: gripper body, SMA wire, and strain gauge. Initially, experiments were conducted to investigate the characteristics of its components. The SMA wire used in this study contracts when heated and recovers its original length when cooled. To investigate such properties, a measuring apparatus, shown in Fig. 8, was fabricated using an encoder and a spring with its stiffness, 27 N/m. When the SMA wire is energized, it rotates the encoder according to the length of contraction. When the current is cut off, the wire extends to its original length and rotates the encoder in the opposite direction by the spring. The model of encoder used was Autonics’ ENB-600-3-1. The change of length l of the SMA wire according to the rotation angle of the encoder can be calculated using the following equation: l = rθ =
πD × encoder count 600
(1)
where D is the rotating diameter of the encoder and 600 is the number of pulse generated by the encoder per rotation. In the test, the length of the SMA wire was 34 mm, electric resistance was 6.4 . Fig. 9 shows the result of the experiment, confirming the hysteresis of the SMA wire at heating and cooling.
Fig. 8. Test apparatus for SMA wire’s characteristics.
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Fig. 12. Proportional control. Fig. 9. SMA wire characteristics.
6. Test for gripping force control
Fig. 10. Force sensor calibration chart of strain vs. force.
The gripping force of the fabricated microgripper was measured with the strain gauge mounted on the flexible hinge B. Therefore, the relation between the strain measured with the strain gauge and the gripping force has to be determined. By removing one of the two gripper jaws and exerting force at the remaining jaw, the strain on hinge B was measured with the strain gauge. Fig. 10 shows the strain on hinge B according to the exerted force, wherein the strain is in linear proportion with the force. The strain gauge and strain amplifier used in the experiment were KYOWA’s KyoKSN-2-120-E5-11 and DPM-712B, respectively.
Since the microgripper can manipulate micro parts, the gripping force has to be controlled using force feedback controller in order to prevent damage to the parts. Therefore, a controller which can control the gripping force of the microgripper needs to be designed. In this study, a PI controller was used to control the gripping force of the microgripper. Fig. 11 shows a conceptual diagram of the closed loop control system including the controller. The gain value of the designed controller was obtained by the following experiment. The control experiment was conducted by varying the proportional gain P, the result of which is shown in Fig. 12, where the horizontal axis is the time and the vertical axis is the gripping force. The rising time and normal status error reduce according to the increase in proportional gain. However, when the proportional gain exceeds a certain criteria (P = 4.0), the response of the microgripper becomes unstable. The normal status error of the microgripper response can be easily removed by an integral controller. Step response experiments were conducted on the closed loop system by varying the integrate gain I. Fig. 13 shows the optimal response of the microgripper obtained by repeated experiments, where the proportional and integrate gain are P = 1.3 and I = 0.01, respectively. Using the PI controller as described above, the experiments for verifying the gripping force control performance of the microgripper were conducted as follows. Currents were applied in the form of continuous step and sine waves to measure the resultant trajectory change of the gripping force. Fig. 14 shows
Fig. 11. Closed loop system.
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7. Conclusion
Fig. 13. Proportional–integrate control.
In this study, an easy-to-manufacture microgripper with a simple shape and structure was developed using shape memory alloy wire. Prior to the control of the gripping force of the developed microgripper, the characteristics of the major components including the SMA wire and strain gauge were investigated by experiments, and a controller was designed on the basis of the experiment results. The PI controller was designed considering the characteristics of the SMA wire, and its gain values were determined by trial and error. Using the designed PI controller, gripping force trajectory tracking experiments were conducted to verify the performance of the designed controller. References
Fig. 14. Force tracking performance for the step trajectory.
the gripping force tracking performance to the step wave input, and Fig. 15 shows the result to the sine wave input. In both cases, the force trajectory tracking performances were adequate for the intended application, proving that the designed controller satisfies its desired performance.
[1] P. Dario, M.C. Carrozza, A. Benvenuto, A. Menciassi, Micro-systems in biomedical applications, J. Micromech. Microeng. 10 (2000) 235– 244. [2] J. Sun, Z. Zhong, Finite element alalysis of a new PZT actuator for hard disk drives, Sens. Actuator A: Phys. 100 (2002) 257–263. [3] J. Speich, M. Goldfarb, A compliant-mechanism-based three degree-offreedom manipulator for small-scale manipulation, Robotica 18 (2000) 95–104. [4] S.K. Nah, Zhong Z, Design of microrobots with microgrippers for manipulation of micro-parts, in: Proceedings of the 3rd International Conference on Experimental Mechanics (ICEM 2004), Singapore, November 29–December 1, 2004, pp. 1–6, paper A05-6. [5] P. Krulevitch, A.P. Lee, P.B. Ramsey, J.C. Trevino, J. Hamiliton, M.A. Northrup, Thin film shape memory alloy microactuators, J. Microelectromech. Syst. 5 (4) (1996) 270–282. [6] M. Kohl, B. Krevet, E. Just, SMA microgripper system, Sens. Actuator A 97–98 (2002) 646–652. [7] R. Stalmans, J. Van Humbeeck, Shape Memory Alloys: Functional and Maart, Presented at the Seminar on Smart Materials and Technologies Sensors, Control Systems and Regulators, Czech Republic, Prague, October 1995, pp. 1–10. [8] J. Peirs, D. Reynaerts, H. Van Brussel, The true power of SMA microactuation, in: Proceedings of the Micromechanics Europe Workshop MME’01, Cork, Ireland, September 16–18, 2001, pp. 217–220. [9] S.B. Choi, Y.M. Han, J.H. Kim, C.C. Cheong, Force tracking control of a flexible gripper featuring shape memory alloy actuators, Mechatronics 11 (2001) 677–690. [10] A. Keith, Williams, T.-C. George, Chiu, J. Robert, Bernhard, Nonlinear control of a shape memory alloy adaptive tuned vibration absorber, J. Sound Vib. 288 (2005) 1131–1155.
Biographies Jin-Ho Kyung received a BS degree in mechanical engineering from Hankook Aviation University, Geonggi-do, Korea in 1985, MS degree from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea in 1988 and PhD degree from KAIST in 2003. He is employed by Korea Institute of Machinery and Materials and works as a principal researcher. His research interests include microgripper design, micromachine tool system design and parallel kinematic machine design. B.G. Ko received a BS degree in 2003 and MS degree in 2003 from Kunsan University, Kunsan, Korea. His research interests include the design and control of microgrippers. Fig. 15. Force tracking performance for the sine trajectory.
Young-Ho Ha received a PhD in mechanical engineering from Korea Advanced Institute of Science and Technology (KAIST) in 1995. He is employed by
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Gyeongnam Technopark and works as a chief researcher. His research interests include robotics, electromechanical system and laser applications. Gwang-Jo Chung received a BS degree in electrical engineering from Yonsei University, Seoul, Korea in 1977, MS degree from Yonsei University in 1983 and
a PhD in control engineering from Kyungnam University, Kyungnam, Korea in 1994. He is employed by Korea Institute of Machinery and Materials and works as a principal researcher. His research interests include robotics, electrical system design and industrial robot applications.
Sensors and Actuators A 141 (2008) 144–150
Design of a microgripper for micromanipulation of microcomponents using SMA wires and flexible hinges J.H. Kyung a,∗ , B.G. Ko a , Y.H. Ha b , G.J. Chung a a
Intelligence Machine System Research Center, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-Gu, Daejeon 305-343, Republic of Korea b Gyeonam TechnoPark, 136 Jungangno Changwon, Gyeongnam 641-739, Republic of Korea Received 27 October 2005; received in revised form 5 July 2007; accepted 5 July 2007 Available online 25 July 2007
Abstract For this study, a simple and easily manufacturable microgripper was designed utilizing shape memory alloy (SMA) wire. First, a microgripper with flexible hinge structures was designed to enable the fine handling of micro parts. The structural design of the microgripper was refined by repeated FE analyses. The characteristics of its major components, including SMA wire and a strain gauge, were investigated by experiment, and on the basis of the results, a controller was designed and fabricated for the control of the gripping force of the microgripper. In addition, the PI control gain of the controller was determined by considering the characteristics of the SMA wire, which has hysteresis response characteristics to heating and cooling. Gripping force control testing was conducted using the fabricated microgripper and a PI controller, in order to verify the performance of the designed microgripper. © 2007 Elsevier B.V. All rights reserved. Keywords: Microgripper; Shape memory alloy wire; Flexible hinge; PI control; Gripping force control
1. Introduction Recently, devices for handling micro parts for the industries of electronics, information technology, optics, medicine, and bio-technology have been developed. Accordingly, the development of a microgripper which fulfils the core role in fine handling is urgently required [1,2]. Many studies have been conducted to develop microgrippers capable of carrying out fine handling. Most of them were designed to have sufficient strength using thin silicon or stainless sheets; however, they were not easy to fabricate due to their complicated structures. A piezoelectric actuator and shape memory alloy (SMA) has been widely used for the actuators [3–6]. On the one hand, Ni–Ti shape memory alloy has the merits of a higher power-to-weight ratio and corrosion resistance. Additionally, SMA wire is cheap and does not require further processing as it can just be cut and used. On the other hand, its demerits are low energy efficiency, limited bandwidth to heating and cooling, and smaller deformation [7,8]. ∗
Corresponding author. E-mail address: [email protected] (J.H. Kyung).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.07.013
In works where micro parts are manipulated using a microgripper, the gripping force has to be controlled to prevent damage to them, even under external disturbance or uncertainties [9,10]. In this study, a simple, compactly structured, and easy to fabricate microgripper was designed using SMA wire, and a structural analysis was conducted using the finite element method (FEM). The gripping force was determined with the strain gauge at the tip of the microgripper to constitute the PI controller, and a study on the gripping force control was conducted. The performance of the microgripper was verified by experiments. 2. Structure and principle of operation Fig. 1 shows a conceptual drawing of the microgripper proposed in this study. The microgripper consists of gripping jaws, SMA wires, two flexible hinges (A, B), a stainless body, and a strain gauge. The gripping jaw is used to manipulate the micro parts, with tips designed to open and close by up to 120 m maximum. The flexible hinge A is a rotating axis that converts the linear motion of the SMA wire to a rotating motion. The flexible hinge B is
J.H. Kyung et al. / Sensors and Actuators A 141 (2008) 144–150
145
Fig. 2. Microgripper: (a) open and (b) closed.
of the gripping force is transferred to this flexible hinge B. Therefore, the gripping force can be measured with the strain gauge adhered to the flexible hinge B.
Fig. 1. Conceptual drawing of microgripper.
made to be thinner than A in order to concentrate most of the gripping force, which is measured with the strain gauge. The SMA wire, which is the actuator, is composed of Ni–Ti alloy with a diameter of 100 m to cope with the strength of hinge A and perform the fastest movement possible. The SMA wire used in this study was Flexinol 100 LT of Mondotronics Inc., and has main properties as shown in Table 1. The principle of operation of the proposed microgripper is as follows: first, when the two SMA wires are energized to generate heat, the wire begins to contract lengthwise at the activating temperature. According to the contraction of the SMA wire, the gripping jaw is closed with the flexible hinge A as shown in Fig. 2(b). When the currents in the two SMA wires are cut off, the wires become cooler, expand back to their original sizes, and the elastic recovery force of the flexible hinge A opens the gripping jaw as shown in Fig. 2(a). When a micro part is gripped with the opening and closing movements of the proposed microgripper, stress is concentrated in the flexible hinge B, which is thinner than hinge A, and most
3. Structural analysis Through the structural analysis of the proposed microgripper, the structurally weak points were identified, and the microgripper could then be redesigned to avoid local stress concentration. Stress and elongation were calculated with FEM, on the basis of which the microgripper was redesigned to achieve a homogeneous stress profile along its overall length. The 3D modeling and FE analysis of the proposed gripper was conducted using the Autodesk’s MDT (mechanical desktop power pack). First, an analysis was conducted to determine the width of the flexible hinge A. As the load condition for the structural analysis, the recovery force of the SMA wire was set to 3 N. The
Table 1 SMA wire properties Thermal Activation start temperature (austenite start temperature) (◦ C) Activation finish temperature (austenite finish temperature) (◦ C) Relaxation start temperature (martensite start temperature) (◦ C) Relaxation finish temperature (martensite finish temperature) (◦ C)
68 78 52 42
Speed (in still air, at 20 ◦ C) Typical contraction speed (s) Relaxation speed (s)
1.0 0.8
Electrical Linear resistance (/m) Recommended current (mA) Recommended power (W/m)
150 180 4.86
Material Density (g/cm3 ) Maximum recovery force (MPa) Recommended recovery force (MPa) Recommended deformation force (MPa) Maximum deformation ratio (%) Recommend deformation ratio (%)
6.45 469 150 28 8 3–5
Fig. 3. FE analysis result (von Mises stress).
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Fig. 4. FE analysis result (gripping displacement).
thickness of the microgripper was set to 1 mm, taking into consideration the convenience of manufacturing. The microgripper was designed to satisfy a safety factor of 1.2. In order to implement the jaws’ maximum opening width of 120 m, structural analyses had been carried out by varying the width of the flexible hinge A. Finally, at a hinge A width of 0.55 mm, the maximum stress was calculated to be 295 MPa as shown in Fig. 3 and the maximum displacement of the gripper jaw was 123 m as described in Fig. 4, thus meeting the design criteria. In the analysis for determining the width of the flexible hinge B, since the strain gauge is adhered to hinge B only, it is important to design hinge B so that the gripping force is not transmitted to
hinge A in order to obtain accurate measurement of the gripping force. In addition, the design safety factor 1.2 was also applied to hinge B. In order to meet the design strength at hinge B, structural analyses were carried out by varying the width of hinge B. According to the analysis, with the width of hinge B at 0.3 mm, the maximum stresses at A and B were calculated to be 22 MPa and 75.5 MPa, respectively. It could be confirmed that most of the gripping force was transmitted to hinge B and the calculated maximum stress fell within the allowable tensile strength range, considering the safety factor. Fig. 5 shows the gripping force analysis result. The maximum gripping force of the microgripper was calculated to be 0.33 N.
Fig. 5. Maximum gripping force (=0.33 N) (factor of safety = 1.2).
J.H. Kyung et al. / Sensors and Actuators A 141 (2008) 144–150
Fig. 6. Photograph of the fabricated microgripper.
The proposed microgripper was designed taking the above results of the analyses in account and fabricated using STS304 material, whose yield strength is 360 MPa. 4. Fabrication of microgripper Fig. 6 shows the microgripper manufactured by electric discharge machining. Experiments for a repetitive open and close motion were conducted by open loop control, observing the microgripper through a 10 × 10 magnification microscope. Fig. 7 shows the open and close motion of the microgripper.
Fig. 7. Manufactured microgripper: (a) open and (b) closed.
5. Experiments with the microgripper The microgripper was fabricated with the following components: gripper body, SMA wire, and strain gauge. Initially, experiments were conducted to investigate the characteristics of its components. The SMA wire used in this study contracts when heated and recovers its original length when cooled. To investigate such properties, a measuring apparatus, shown in Fig. 8, was fabricated using an encoder and a spring with its stiffness, 27 N/m. When the SMA wire is energized, it rotates the encoder according to the length of contraction. When the current is cut off, the wire extends to its original length and rotates the encoder in the opposite direction by the spring. The model of encoder used was Autonics’ ENB-600-3-1. The change of length l of the SMA wire according to the rotation angle of the encoder can be calculated using the following equation: l = rθ =
πD × encoder count 600
(1)
where D is the rotating diameter of the encoder and 600 is the number of pulse generated by the encoder per rotation. In the test, the length of the SMA wire was 34 mm, electric resistance was 6.4 . Fig. 9 shows the result of the experiment, confirming the hysteresis of the SMA wire at heating and cooling.
Fig. 8. Test apparatus for SMA wire’s characteristics.
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Fig. 12. Proportional control. Fig. 9. SMA wire characteristics.
6. Test for gripping force control
Fig. 10. Force sensor calibration chart of strain vs. force.
The gripping force of the fabricated microgripper was measured with the strain gauge mounted on the flexible hinge B. Therefore, the relation between the strain measured with the strain gauge and the gripping force has to be determined. By removing one of the two gripper jaws and exerting force at the remaining jaw, the strain on hinge B was measured with the strain gauge. Fig. 10 shows the strain on hinge B according to the exerted force, wherein the strain is in linear proportion with the force. The strain gauge and strain amplifier used in the experiment were KYOWA’s KyoKSN-2-120-E5-11 and DPM-712B, respectively.
Since the microgripper can manipulate micro parts, the gripping force has to be controlled using force feedback controller in order to prevent damage to the parts. Therefore, a controller which can control the gripping force of the microgripper needs to be designed. In this study, a PI controller was used to control the gripping force of the microgripper. Fig. 11 shows a conceptual diagram of the closed loop control system including the controller. The gain value of the designed controller was obtained by the following experiment. The control experiment was conducted by varying the proportional gain P, the result of which is shown in Fig. 12, where the horizontal axis is the time and the vertical axis is the gripping force. The rising time and normal status error reduce according to the increase in proportional gain. However, when the proportional gain exceeds a certain criteria (P = 4.0), the response of the microgripper becomes unstable. The normal status error of the microgripper response can be easily removed by an integral controller. Step response experiments were conducted on the closed loop system by varying the integrate gain I. Fig. 13 shows the optimal response of the microgripper obtained by repeated experiments, where the proportional and integrate gain are P = 1.3 and I = 0.01, respectively. Using the PI controller as described above, the experiments for verifying the gripping force control performance of the microgripper were conducted as follows. Currents were applied in the form of continuous step and sine waves to measure the resultant trajectory change of the gripping force. Fig. 14 shows
Fig. 11. Closed loop system.
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7. Conclusion
Fig. 13. Proportional–integrate control.
In this study, an easy-to-manufacture microgripper with a simple shape and structure was developed using shape memory alloy wire. Prior to the control of the gripping force of the developed microgripper, the characteristics of the major components including the SMA wire and strain gauge were investigated by experiments, and a controller was designed on the basis of the experiment results. The PI controller was designed considering the characteristics of the SMA wire, and its gain values were determined by trial and error. Using the designed PI controller, gripping force trajectory tracking experiments were conducted to verify the performance of the designed controller. References
Fig. 14. Force tracking performance for the step trajectory.
the gripping force tracking performance to the step wave input, and Fig. 15 shows the result to the sine wave input. In both cases, the force trajectory tracking performances were adequate for the intended application, proving that the designed controller satisfies its desired performance.
[1] P. Dario, M.C. Carrozza, A. Benvenuto, A. Menciassi, Micro-systems in biomedical applications, J. Micromech. Microeng. 10 (2000) 235– 244. [2] J. Sun, Z. Zhong, Finite element alalysis of a new PZT actuator for hard disk drives, Sens. Actuator A: Phys. 100 (2002) 257–263. [3] J. Speich, M. Goldfarb, A compliant-mechanism-based three degree-offreedom manipulator for small-scale manipulation, Robotica 18 (2000) 95–104. [4] S.K. Nah, Zhong Z, Design of microrobots with microgrippers for manipulation of micro-parts, in: Proceedings of the 3rd International Conference on Experimental Mechanics (ICEM 2004), Singapore, November 29–December 1, 2004, pp. 1–6, paper A05-6. [5] P. Krulevitch, A.P. Lee, P.B. Ramsey, J.C. Trevino, J. Hamiliton, M.A. Northrup, Thin film shape memory alloy microactuators, J. Microelectromech. Syst. 5 (4) (1996) 270–282. [6] M. Kohl, B. Krevet, E. Just, SMA microgripper system, Sens. Actuator A 97–98 (2002) 646–652. [7] R. Stalmans, J. Van Humbeeck, Shape Memory Alloys: Functional and Maart, Presented at the Seminar on Smart Materials and Technologies Sensors, Control Systems and Regulators, Czech Republic, Prague, October 1995, pp. 1–10. [8] J. Peirs, D. Reynaerts, H. Van Brussel, The true power of SMA microactuation, in: Proceedings of the Micromechanics Europe Workshop MME’01, Cork, Ireland, September 16–18, 2001, pp. 217–220. [9] S.B. Choi, Y.M. Han, J.H. Kim, C.C. Cheong, Force tracking control of a flexible gripper featuring shape memory alloy actuators, Mechatronics 11 (2001) 677–690. [10] A. Keith, Williams, T.-C. George, Chiu, J. Robert, Bernhard, Nonlinear control of a shape memory alloy adaptive tuned vibration absorber, J. Sound Vib. 288 (2005) 1131–1155.
Biographies Jin-Ho Kyung received a BS degree in mechanical engineering from Hankook Aviation University, Geonggi-do, Korea in 1985, MS degree from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea in 1988 and PhD degree from KAIST in 2003. He is employed by Korea Institute of Machinery and Materials and works as a principal researcher. His research interests include microgripper design, micromachine tool system design and parallel kinematic machine design. B.G. Ko received a BS degree in 2003 and MS degree in 2003 from Kunsan University, Kunsan, Korea. His research interests include the design and control of microgrippers. Fig. 15. Force tracking performance for the sine trajectory.
Young-Ho Ha received a PhD in mechanical engineering from Korea Advanced Institute of Science and Technology (KAIST) in 1995. He is employed by
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Gyeongnam Technopark and works as a chief researcher. His research interests include robotics, electromechanical system and laser applications. Gwang-Jo Chung received a BS degree in electrical engineering from Yonsei University, Seoul, Korea in 1977, MS degree from Yonsei University in 1983 and
a PhD in control engineering from Kyungnam University, Kyungnam, Korea in 1994. He is employed by Korea Institute of Machinery and Materials and works as a principal researcher. His research interests include robotics, electrical system design and industrial robot applications.