Electrostatic Clutch with Variable Frictional Force at Low Driving Voltage Using Charge Accumulative Gel for Kinesthetic Haptics

As interest in Virtual Reality (VR) and Augmented Reality (AR) increases, the demand for kinesthetic haptic feedback gloves is also rapidly rising. Generally, kinesthetic haptic gloves utilize electromagnetic motors due to their fast response time and precise controllability. [1] However, they have limitations, including low force density and high energy consumption. To address these issues, electrostatic clutches which generate frictional force using electrostatic forces between two electrodes have been utilized. [2] The electrostatic clutches have a typical capacitor structure, enabling an extremely thin design and low energy consumption. The electrostatic force is proportional to the permittivity of the dielectric material and the operating voltage. Therefore, conventional electrostatic clutch-based haptic gloves use high input voltages ranging from hundreds to thousands of volts, resulting in bulky control circuits and potential interference with surrounding wearable devices. [5]<br/>To develop the electrostatic clutch, we selected polyvinyl chloride (PVC) gel for a high frictional force at low driving voltage. PVC gel is highly plasticized PVC that is fabricated to a 40 μm thickness by using a bar coating method. The PVC gel has distinctive electrical characteristics. When a voltage is applied to the PVC gel, the negative charges and plasticizers are accumulated near the anode. [4] Also, electrostatic force is generated between the accumulated negative charges and the anode. Due to the short distance between the anode and the negative charges, high electrostatic forces are produced. [5] Consequently, the PVC gel based electrostatic clutch generated a high frictional shear stress of 20 N/cm² at 100 V.<br/>However, due to the mechanical properties of the PVC gel and the electrode film of clutch, several issues arise. The tackiness of the PVC gel generates a high frictional shear stress of 10 N/cm² even without applying voltage. Additionally, since the film is not perfectly flat and not in full contact with the dielectric, a high pull-in voltage of over 40V is needed for full contact via electrostatic zipping. To address these issues, a dielectric liquid was applied as a lubricant over the surface of the PVC gel. The dielectric liquid reduced the coefficient of friction, resulting in a low frictional shear stress of 0.35 N/cm² at 0 V. Additionally, it increases electrostatic force in proportion to its permittivity relative to air. [6] Therefore, the electrostatic zipping occurred even at 20 V. Consequently, a wide range of frictional shear stress, from 0.35 N/cm^{&lt;span style="font-size:10.8333px"&gt;2&lt;/span&gt;} to 18.9 N/cm², could be generated within 100 V.<br/>A kinesthetic haptic glove was designed using a PVC gel based electrostatic clutch. A film-type clutch with a thickness of 5 mm was attached to the back of the hand, and the generated frictional force was transmitted to the fingertips through wire tendons. This compact and comfortable kinesthetic haptic glove enabled users to experience sensations of volume and viscosity in a VR environment.<br/><br/>This work was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIT) (CRC23021-000)<br/><br/>[1] Guo, Yuan et al., Actuators 110, (2021)<br/>[2] Yin, Jessica et al., Adv. Func. Mat. 31 (2021)<br/>[3] Hinchet, Ronan et al., Adv. Mat. Tech. 5 (2020)<br/>[4] Xia, H et al., J. Mater. Chem. C 114, (2010)<br/>[5] Kim, Hyunwoo et al., Adv. Int. Sys. 3 (2021)<br/>[6] Taghavi, Majid et al., Sci. Robotics 3 (2018)