Researchers at Carnegie Mellon University have come up with a solid-state clutch they say is three to 30 times lighter than ordinary clutch mechanisms with the same holding force. Consisting of little more than a couple sheets of aluminized Mylar, the clutch consumes 340 to 750 times less energy compared to previous devices while operating at four to 20 times less voltage than previous electrostatic components used in robots.
The clutch uses electrostatic adhesion as a holding mechanism. The CMU group says they have been investigating the idea for some time but their early prototypes were plagued by high voltage requirements and undesired sticking when the clutch was turned off. Eventually the group settled on a design that overcomes these difficulties. It consists of a dielectric layer made of a material called Luxprint that separates the two sheets of aluminum-coated Mylar. This structure behaves a little like a capacitor. With no voltage applied, the two Mylar sheets can slide back and forth. Application of a voltage develops a strong electrostatic field between the two sheets that holds them together and prevents them from sliding. The electrostatic field can develop quickly, and clutch consumes little power because the electrons don’t flow across the Luxprint. One benefit of the solid-state clutch is that its degree of slip can be controlled just by adjusting the amount of applied voltage.
A promising application area for the clutch is in exoskeletons and prosthetics. Researchers say for testing purposes, they incorporated the electrostatic clutch and a spring into an ankle exoskeleton. During 150 consecutive steps of walking, the clutch was used to engage the spring while the foot was on the ground and disengage it during the swing forward. Researchers saw an average force of about 100 N on the device, and the efficiency of the clutch/spring was 95%. Five clutched springs placed in parallel produced a peak force of 501 N with all clutches engaged and a peak force of 14 N with no clutches engaged, for a 36X change in stiffness.
The engagement and release times for the new clutch are relatively quick: Tests at CMU showed that the clutch fully engages and releases in less than 30 msec. The average capacitance was 21.8 ± 5.3 nF. The clutched spring performed 2.61 ± 0.33 J of negative work and 2.14 ± 0.29 J of positive work for each step of the exoskeleton. The electroadhesive clutch had a total mass of 11 gm and consumed 0.6 mW of power during walking. Researchers say this is a three-fold improvement in weight and a factor of 340 improvement in power consumption compared to the best clutches used in similar applications.
More information is available from the CMU group: https://engineering.cmu.edu/media/feature/2016/07_07_collins_ankle_exoskeleton.html