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Dielectric Electroactive Polymer Membrane Actuator with Ring-type Electrode as Driving Component of a Tactile Actuator
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 1537 – 1540
30th Eurosensors Conference, EUROSENSORS 2016
Dielectric electroactive polymer membrane actuator with ring-type electrode as driving component of a tactile actuator Rui Zhua,b,*, Ulrike Wallrabe b, Matthias C Wapler b, Peter Woias b, Ulrich Mescheder a a
Department of Mechanical & Medical Engineering and Institute Microsystem Technology (iMST), Robert-Gerwig-Platz 1, Furtwangen 78120, Germany Institut für Mikrosystemtechnik (IMTEK), Georges-Köhler-Allee 102, Freiburg 79110, Germany
Abstract We present a driving concept of a tactile actuator based on a dielectric electroactive polymer using a ring-type electrode. This electrode is located around a deflectable membrane. The fabrication process and tests of the tactile actuator including integration into a hydraulic amplifier design are presented proving the concept of the novel actuation. The tactile actuator exhibits a maximum measured deflection of 124±7 µm for an applied voltage of 4200 V. The membrane thickness and the total thickness of the actuator including hydraulic amplifier were 30 and about 570 µm, respectively. © Authors. Published by Elsevier Ltd. This ©2016 2016The The Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Dielectric electroactive polymer; DEAP; Tactile actuator; Tactile display
1. Introduction Tactile actuators can be used in tactile displays which can provide information to humans through the tactile sense in the form of Braille letters or more complex two dimensional images. Thus, such tactile devices can be used as reading devices and, e. g, in navigation systems for blind people. However, such tactile devices are also interesting for people who are not visually impaired as they can receive multidimensional information simultaneously to vision, hearing and olfaction or use the devices in a vision and hearing limited environment. However, at present the application range of tactile device is restricted because the commercially available tactile actuators are relatively large in size and expensive. Hence, the final target of the present research is miniaturization * Corresponding author. Tel.: +49-7723-920-2101; fax: +49-7723-920-2633. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.455
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Fig. 1. Sketch of the proposed tactile actuator with integrated hydraulic amplifier (a) 3D sketch, (b) cross section view of inactive actuator, (c) cross section view of active actuator
and cost reduction of tactile actuators while keeping a reliable perception of the tactile information. Actuation principles such as dielectric electroactive polymer (DEAP) [1, 2], piezo electric materials [3], shape memory alloy [4], thermal expansion [5] and pneumatic [6] have been investigated in the literature. Due to scaling effects, the generated deformation stroke is severely reduced with miniaturization, regardless of the chosen actuation principle, which results in insufficient stroke of such a tactile actuator. Thus, mechanisms like flexural beams [7], buckling deformation [1] and hydraulic amplification [2] have been employed to amplify the resulting deformation of a miniaturized actuator. AS DEAP provides a larger actuation strain (63% for silicone) [8], it is considered as a material that has good potential for miniaturized actuators. Due to the sandwich structure and softness of DEAP, buckling deformation and hydraulic principles are suitable amplification mechanisms. The aim of this paper is to introduce the concept of a ring-type electrode DEAP membrane on a tactile actuator, namely its design, fabrication and test of a DEAP actuator combined with a hydraulic displacement amplification mechanism. The analysis of a complete actuator system will be published in another paper. 2. Design A schematic sketch of the system (actuator and amplifier) is shown in Fig. 1. Instead of using a soft electrode (e.g. graphite) covering the full DEAP membrane as suggested in [2] to actuate the lower membrane, a novel configuration comprising a ring-type electrode is proposed. As shown in Fig. 1, a chamber in a silicon substrate with different sizes of the opening on the two sides is filled with oil and encapsulated by two Polydimethylsiloxane (PDMS) membranes of which the lower membrane forms the DEAP actuator. One electrode of the actuator is provided by the highly Borondoped surface of the Si substrate while the second electrode below the PDMS membrane has a ring-type layout covering only the rigid part around the deflectable membrane to prohibit electrode fracture and avoid stiffening of the actuated membrane by the electrode itself. An out of plane deformation is formed in the deflectable region when a voltage is applied to the electrodes. The deflection is amplified by the hydraulic amplifier, so that a larger deflection is achieved in the upper membrane. 3. Fabrication The actuator system fabrication process flow is shown in Figure 2. At first, a 2.9 µm SiO2 layer is grown on a one side polished (100) silicon wafer (thickness 510 µm). Then, photoresist is spin coated on both sides of the wafer; the photoresist on the polished side is exposed and structured with squares to pattern the large openings of the chamber, aligned along Si<111>. Next, the SiO2 layer is structured at the bottom side in buffered HF, and the photoresist layers are removed afterwards. Then the wafer is completely etched through anisotropically in KOH; the residual SiO2 layer is then removed in buffered HF. This creates a hydraulic chamber, which has a large opening on its polished side and a small opening on its unpolished side. Thirdly, to provide a good conducting layer as the upper electrode of DEAP,
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Rui Zhu et al. / Procedia Engineering 168 (2016) 1537 – 1540
1) (100) silicon wafer, 500 µm thick
2) 2.5 µm SiO2 grown on silicon wafer
3) Structuring SiO2 and etching silicon wafer in KOH
4) Removal of SiO2 and borondoping on lower side of wafer
5) Bond PDMS membrane on upper side of Si after plasma activation of PDMS
6) Oil filling of Si-cavity
Silicon Oil SiO2 7) Encapsulation of cavity with PDMS
PDMS Gold Boron-doped silicon
8) Compliant Au-electrode coating by sputtering Fig. 2. Fabrication process of the tactile actuator
a
Upper membrane
b
Lower membrane Ring-type electrode Width of electrode
Fig.3. Optical image of a single tactile actuator. The electrode width and side length of upper and lower membrane are 750, 750 and 1500 µm, respectively. (a) Upper side of actuator, (b) Lower side of actuator
the polished side of the silicon wafer is boron-doped (sheet resistance 0.9 Ω·cm). Thus, the inner rim of bottom electrode of the DEAP membrane is defined by the opening of the hydraulic chamber. In the next step, a PDMS membrane is formed by curing of spin coated liquid PDMS on a polymer wafer (Adwilld-218) for 30 min at 90 °C. The liquid PDMS is mixed by a static mixer (K-System syringes, 10:1) of Sylgard 184 and its curing agent. Spincoating is done at spin speed and time of 2200 rpm and 45 s, respectively. The achieved PDMS membrane thickness is 30 µm. Then the PDMS membrane is bonded onto the unpolished side of the chip after the surface activation of PDMS and Si-wafer by O2 plasma. Subsequently, a thin film of liquid PDMS is spin coated at 6000 rpm during 240 s on a plastic wafer and is partially transferred to the polished side. In the last step, the chamber is filled with “Fomblin Y” oil and encapsulated by the defined PDMS membrane. The PDMS membrane is glued on the silicon chip using film of uncured PDMS film under pressure load of 500 g/cm2 at 90 °C for 2 h. Finally, a gold lower ring electrode is sputtered on the DEAP membrane using a shadow mask. A photo of a fabricated actuator is shown in Fig. 3. 4. Results A complete actuator system with a 30 µm thick PDMS membrane, a 1500 µm wide upper and a 750 µm wide lower membrane was fabricated and tested. The center deflection of the upper membrane was measured using a Zygo New View 7100 at increasing applied voltage in steps of 300 V. In Fig. 4, the results for two layouts are shown: layout 1, with the second electrode designed as ring-type (black squares, Fig. 2, 8) solid); layout 2, with second electrode fully covering the lower membrane (red circles, Fig. 2, 8) striped). Layout 1 clearly shows a threshold voltage of 1600 V at which the deflection starts to increase linearly with the applied voltage. Layout 2 shows a smaller slope at high voltages with a tendency to saturate at voltages above 3000 V, however, with a larger deflection at lower voltage and smoother transition around the threshold (around 1100 V). The maximum voltage is limited by the electric breakdown of about 4500 V for a 30 µm PDMS membrane, corresponding to a breakdown field strength of PDMS of about 150 V/µm. The maximum center deflection at 4200 V is 124±7 µm and 88±7 for the layout 1 and 2, respectively.
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Maximum deflection (µm)
150
Square ring Membrane fully covered by second electrode Fit line
100
50
0 0
1000
2000
3000
4000
Applied voltage (V)
Fig. 4. Measured center deflection of upper membrane against the applied voltage
5. Conclusion Using a ring-type electrode around the PDMS membrane a pronounced buckling mode of the DEAP actuator is observed. Due to the layout of the electrodes, fracture of the electrode under dynamic operation as well as stiffening of the very soft PDMS membrane is avoided. Compared to the membrane with a fully covered electrode, the ring electrode design provides a sharp transition from an actuation based on a squeezing effect to a buckling mode resulting in a larger slope. The buckling amplitude is amplified by a hydraulic principle resulting in a total membrane deflection of upper membrane (acting a bump for tactile sensing) of around 120 µm which is suitable for tactile perception through fingertips. DEAP need rather large voltage. The maximum voltage of the DEAP actuator is limited by electric breakdown at around 150 V/µm which is consistent with material data of the PDMS that we used (Sylgard 184). Already with this first proof of concept of using PDMS as DEAP actuator, the effectiveness of a ring-shaped electrode configuration and hydraulic amplification for tactile sensing could be demonstrated; Further work to characterize the obtained actuation force and understand the behavior of the system including layout rules for the electrode area and deflecting membrane area is needed. Acknowledgements The work was funded by Germany BMBF (Bundesministeriums für Bildung und Forschung) within the project iView (intelligente, vibrotactil induzierte Wahrnehmung) and by the EU and the state of Baden-Wuerttemberg within the Center of Applied Research ZAFH-AAL. References [1] =. Yu, W. Yuan, P. Brochu, B. Chen, Z. Liu, Q. Pei, Large-strain, rigid-to-rigid deformation of bistable electroactive polymers, Appl. Phys. Lett., 95 (2009) 192904 [2] H. S. Lee, H. Phung, D.-H. Lee, U. K. Kim, C. T. Nguyen, H. Moon, J. C. Koo, J. Nam, H. R. Choi, Design analysis and fabrication of arrayed tactile display based on dielectric elastomer actuator, Sensors and Actuators A: Physical, 205 (2014) 191–198. [3] H.-c. Cho, B.-s. Kim, J.-j. Park, J.-b. Song, Development of a Braille Display using Piezoelectric Linear Motors, SICE-ICASE International Joint Conference, 2006, pp. 1917–1921. [4] Y. Haga,W. Makishi, K. Iwami, K. Totsu, K. Nakamura, M. Esashi, Dynamic Braille display using SMA coil actuator and magnetic latch, Sensors and Actuators A: Physical, 119 (2005) 316–322. [5] H.-J. Kwon, S. W. Lee, S. S. Lee, Braille dot display module with a PDMS membrane driven by a thermopneumatic actuator, Sensors and Actuators A: Physical, 154 (2009) 238–246. [6] X. Wu, S.-H. Kim, H. Zhu, C.-H. Ji, and M. G. Allen, A Refreshable Braille Cell Based on Pneumatic Microbubble Actuators, J. Microelectromech. Syst, 21 (2012) 908–916. [] http://web.metec-ag.de/braille%20cell%20p16.html, accessed 2016. 07. 14 [] Y. Bar-Cohen, Electroactive polymer (EAP) actuators as artificial muscles: Reality, potential, and challenges, 2nd ed. Bellingham, Wash.: SPIE Press, 2004.
ScienceDirect Procedia Engineering 168 (2016) 1537 – 1540
30th Eurosensors Conference, EUROSENSORS 2016
Dielectric electroactive polymer membrane actuator with ring-type electrode as driving component of a tactile actuator Rui Zhua,b,*, Ulrike Wallrabe b, Matthias C Wapler b, Peter Woias b, Ulrich Mescheder a a
Department of Mechanical & Medical Engineering and Institute Microsystem Technology (iMST), Robert-Gerwig-Platz 1, Furtwangen 78120, Germany Institut für Mikrosystemtechnik (IMTEK), Georges-Köhler-Allee 102, Freiburg 79110, Germany
Abstract We present a driving concept of a tactile actuator based on a dielectric electroactive polymer using a ring-type electrode. This electrode is located around a deflectable membrane. The fabrication process and tests of the tactile actuator including integration into a hydraulic amplifier design are presented proving the concept of the novel actuation. The tactile actuator exhibits a maximum measured deflection of 124±7 µm for an applied voltage of 4200 V. The membrane thickness and the total thickness of the actuator including hydraulic amplifier were 30 and about 570 µm, respectively. © Authors. Published by Elsevier Ltd. This ©2016 2016The The Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Dielectric electroactive polymer; DEAP; Tactile actuator; Tactile display
1. Introduction Tactile actuators can be used in tactile displays which can provide information to humans through the tactile sense in the form of Braille letters or more complex two dimensional images. Thus, such tactile devices can be used as reading devices and, e. g, in navigation systems for blind people. However, such tactile devices are also interesting for people who are not visually impaired as they can receive multidimensional information simultaneously to vision, hearing and olfaction or use the devices in a vision and hearing limited environment. However, at present the application range of tactile device is restricted because the commercially available tactile actuators are relatively large in size and expensive. Hence, the final target of the present research is miniaturization * Corresponding author. Tel.: +49-7723-920-2101; fax: +49-7723-920-2633. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.455
1538
Rui Zhu et al. / Procedia Engineering 168 (2016) 1537 – 1540
Fig. 1. Sketch of the proposed tactile actuator with integrated hydraulic amplifier (a) 3D sketch, (b) cross section view of inactive actuator, (c) cross section view of active actuator
and cost reduction of tactile actuators while keeping a reliable perception of the tactile information. Actuation principles such as dielectric electroactive polymer (DEAP) [1, 2], piezo electric materials [3], shape memory alloy [4], thermal expansion [5] and pneumatic [6] have been investigated in the literature. Due to scaling effects, the generated deformation stroke is severely reduced with miniaturization, regardless of the chosen actuation principle, which results in insufficient stroke of such a tactile actuator. Thus, mechanisms like flexural beams [7], buckling deformation [1] and hydraulic amplification [2] have been employed to amplify the resulting deformation of a miniaturized actuator. AS DEAP provides a larger actuation strain (63% for silicone) [8], it is considered as a material that has good potential for miniaturized actuators. Due to the sandwich structure and softness of DEAP, buckling deformation and hydraulic principles are suitable amplification mechanisms. The aim of this paper is to introduce the concept of a ring-type electrode DEAP membrane on a tactile actuator, namely its design, fabrication and test of a DEAP actuator combined with a hydraulic displacement amplification mechanism. The analysis of a complete actuator system will be published in another paper. 2. Design A schematic sketch of the system (actuator and amplifier) is shown in Fig. 1. Instead of using a soft electrode (e.g. graphite) covering the full DEAP membrane as suggested in [2] to actuate the lower membrane, a novel configuration comprising a ring-type electrode is proposed. As shown in Fig. 1, a chamber in a silicon substrate with different sizes of the opening on the two sides is filled with oil and encapsulated by two Polydimethylsiloxane (PDMS) membranes of which the lower membrane forms the DEAP actuator. One electrode of the actuator is provided by the highly Borondoped surface of the Si substrate while the second electrode below the PDMS membrane has a ring-type layout covering only the rigid part around the deflectable membrane to prohibit electrode fracture and avoid stiffening of the actuated membrane by the electrode itself. An out of plane deformation is formed in the deflectable region when a voltage is applied to the electrodes. The deflection is amplified by the hydraulic amplifier, so that a larger deflection is achieved in the upper membrane. 3. Fabrication The actuator system fabrication process flow is shown in Figure 2. At first, a 2.9 µm SiO2 layer is grown on a one side polished (100) silicon wafer (thickness 510 µm). Then, photoresist is spin coated on both sides of the wafer; the photoresist on the polished side is exposed and structured with squares to pattern the large openings of the chamber, aligned along Si<111>. Next, the SiO2 layer is structured at the bottom side in buffered HF, and the photoresist layers are removed afterwards. Then the wafer is completely etched through anisotropically in KOH; the residual SiO2 layer is then removed in buffered HF. This creates a hydraulic chamber, which has a large opening on its polished side and a small opening on its unpolished side. Thirdly, to provide a good conducting layer as the upper electrode of DEAP,
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Rui Zhu et al. / Procedia Engineering 168 (2016) 1537 – 1540
1) (100) silicon wafer, 500 µm thick
2) 2.5 µm SiO2 grown on silicon wafer
3) Structuring SiO2 and etching silicon wafer in KOH
4) Removal of SiO2 and borondoping on lower side of wafer
5) Bond PDMS membrane on upper side of Si after plasma activation of PDMS
6) Oil filling of Si-cavity
Silicon Oil SiO2 7) Encapsulation of cavity with PDMS
PDMS Gold Boron-doped silicon
8) Compliant Au-electrode coating by sputtering Fig. 2. Fabrication process of the tactile actuator
a
Upper membrane
b
Lower membrane Ring-type electrode Width of electrode
Fig.3. Optical image of a single tactile actuator. The electrode width and side length of upper and lower membrane are 750, 750 and 1500 µm, respectively. (a) Upper side of actuator, (b) Lower side of actuator
the polished side of the silicon wafer is boron-doped (sheet resistance 0.9 Ω·cm). Thus, the inner rim of bottom electrode of the DEAP membrane is defined by the opening of the hydraulic chamber. In the next step, a PDMS membrane is formed by curing of spin coated liquid PDMS on a polymer wafer (Adwilld-218) for 30 min at 90 °C. The liquid PDMS is mixed by a static mixer (K-System syringes, 10:1) of Sylgard 184 and its curing agent. Spincoating is done at spin speed and time of 2200 rpm and 45 s, respectively. The achieved PDMS membrane thickness is 30 µm. Then the PDMS membrane is bonded onto the unpolished side of the chip after the surface activation of PDMS and Si-wafer by O2 plasma. Subsequently, a thin film of liquid PDMS is spin coated at 6000 rpm during 240 s on a plastic wafer and is partially transferred to the polished side. In the last step, the chamber is filled with “Fomblin Y” oil and encapsulated by the defined PDMS membrane. The PDMS membrane is glued on the silicon chip using film of uncured PDMS film under pressure load of 500 g/cm2 at 90 °C for 2 h. Finally, a gold lower ring electrode is sputtered on the DEAP membrane using a shadow mask. A photo of a fabricated actuator is shown in Fig. 3. 4. Results A complete actuator system with a 30 µm thick PDMS membrane, a 1500 µm wide upper and a 750 µm wide lower membrane was fabricated and tested. The center deflection of the upper membrane was measured using a Zygo New View 7100 at increasing applied voltage in steps of 300 V. In Fig. 4, the results for two layouts are shown: layout 1, with the second electrode designed as ring-type (black squares, Fig. 2, 8) solid); layout 2, with second electrode fully covering the lower membrane (red circles, Fig. 2, 8) striped). Layout 1 clearly shows a threshold voltage of 1600 V at which the deflection starts to increase linearly with the applied voltage. Layout 2 shows a smaller slope at high voltages with a tendency to saturate at voltages above 3000 V, however, with a larger deflection at lower voltage and smoother transition around the threshold (around 1100 V). The maximum voltage is limited by the electric breakdown of about 4500 V for a 30 µm PDMS membrane, corresponding to a breakdown field strength of PDMS of about 150 V/µm. The maximum center deflection at 4200 V is 124±7 µm and 88±7 for the layout 1 and 2, respectively.
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Rui Zhu et al. / Procedia Engineering 168 (2016) 1537 – 1540
Maximum deflection (µm)
150
Square ring Membrane fully covered by second electrode Fit line
100
50
0 0
1000
2000
3000
4000
Applied voltage (V)
Fig. 4. Measured center deflection of upper membrane against the applied voltage
5. Conclusion Using a ring-type electrode around the PDMS membrane a pronounced buckling mode of the DEAP actuator is observed. Due to the layout of the electrodes, fracture of the electrode under dynamic operation as well as stiffening of the very soft PDMS membrane is avoided. Compared to the membrane with a fully covered electrode, the ring electrode design provides a sharp transition from an actuation based on a squeezing effect to a buckling mode resulting in a larger slope. The buckling amplitude is amplified by a hydraulic principle resulting in a total membrane deflection of upper membrane (acting a bump for tactile sensing) of around 120 µm which is suitable for tactile perception through fingertips. DEAP need rather large voltage. The maximum voltage of the DEAP actuator is limited by electric breakdown at around 150 V/µm which is consistent with material data of the PDMS that we used (Sylgard 184). Already with this first proof of concept of using PDMS as DEAP actuator, the effectiveness of a ring-shaped electrode configuration and hydraulic amplification for tactile sensing could be demonstrated; Further work to characterize the obtained actuation force and understand the behavior of the system including layout rules for the electrode area and deflecting membrane area is needed. Acknowledgements The work was funded by Germany BMBF (Bundesministeriums für Bildung und Forschung) within the project iView (intelligente, vibrotactil induzierte Wahrnehmung) and by the EU and the state of Baden-Wuerttemberg within the Center of Applied Research ZAFH-AAL. References [1] =. Yu, W. Yuan, P. Brochu, B. Chen, Z. Liu, Q. Pei, Large-strain, rigid-to-rigid deformation of bistable electroactive polymers, Appl. Phys. Lett., 95 (2009) 192904 [2] H. S. Lee, H. Phung, D.-H. Lee, U. K. Kim, C. T. Nguyen, H. Moon, J. C. Koo, J. Nam, H. R. Choi, Design analysis and fabrication of arrayed tactile display based on dielectric elastomer actuator, Sensors and Actuators A: Physical, 205 (2014) 191–198. [3] H.-c. Cho, B.-s. Kim, J.-j. Park, J.-b. Song, Development of a Braille Display using Piezoelectric Linear Motors, SICE-ICASE International Joint Conference, 2006, pp. 1917–1921. [4] Y. Haga,W. Makishi, K. Iwami, K. Totsu, K. Nakamura, M. Esashi, Dynamic Braille display using SMA coil actuator and magnetic latch, Sensors and Actuators A: Physical, 119 (2005) 316–322. [5] H.-J. Kwon, S. W. Lee, S. S. Lee, Braille dot display module with a PDMS membrane driven by a thermopneumatic actuator, Sensors and Actuators A: Physical, 154 (2009) 238–246. [6] X. Wu, S.-H. Kim, H. Zhu, C.-H. Ji, and M. G. Allen, A Refreshable Braille Cell Based on Pneumatic Microbubble Actuators, J. Microelectromech. Syst, 21 (2012) 908–916. [] http://web.metec-ag.de/braille%20cell%20p16.html, accessed 2016. 07. 14 [] Y. Bar-Cohen, Electroactive polymer (EAP) actuators as artificial muscles: Reality, potential, and challenges, 2nd ed. Bellingham, Wash.: SPIE Press, 2004.