Assessing the usability of remote control servos for admittance-controlled haptic finger manipulators

Robotic devices that are able to manipulate the fingers can support the study of robot-assisted motor learning. Currently no devices are available that provide a transparent haptic environment and provide a platform to study motor learning. To cut down on costs it is proposed to use remote control (RC) servos with admittance control. In this study five RC servos are tested to evaluate their controller and passive dynamic properties. Frequency and step response are evaluated and passive dynamics are estimated using a model fit. With a fitted frequency response, system stability is evaluated for different human impedances. The high speed servos have lowest passive inertia (2·10-4 kgm2) and highest bandwidth (20 Hz). The communication protocol of RC servos causes a delay of more than 5 ms from change in setpoint to change in output. Stability analysis shows that the high speed servos have largest stability regions. Simulations show that reducing the virtual inertia and damping makes the system more susceptible to unstable behavior. At this moment however the passive dynamics of the setup are more transparent than the virtual inertia (1·10-3 kgm2) and damping that can be simulated with an admittance controller. A possible cause lies with the communication delay and high gearing present in RC servos.