Velocity Control for the da Vinci Research Kit

The da Vinci Research Kit (dVRK) consists of open-source electronics and software that provides access to all levels of control but, until now, has relied primarily on an inner motor current control loop in analog hardware and an outer position control loop on the PC. In this work, we present a low-level velocity control loop, implemented on the FPGA, as an alternative to the PC-based (existing) position controller or (potential) velocity controller. The proposed velocity controller takes advantage of hardware-based measurement of the encoder period, which is inversely proportional to velocity. To avoid division on the FPGA, we implement closed loop control of the encoder period. Our implementation requires the controller gains to be adjusted based on the reference period; thus, the PC supplies both the reference period and the adjusted gains. We evaluate the proposed controller against a conventional implementation of a velocity control loop wrapped around the existing position control loop on the PC. The results demonstrate that the proposed controller yields improvements in tracking performance and disturbance rejection. The proposed velocity controller will be released open source to the dVRK community.