Real-time FPGA control of a hexapod robot for 6-DOF biomechanical testing

A hexapod robotic test system has been developed to enable complex six degree of freedom (6-DOF) testing of bones, joints, soft tissues, artificial joints and other medical and surgical devices. The device employs six permanent-magnet servomotor driven ballscrews to actuate the system, and measures the displacement response using incremental encoders and loads using a six axis load-cell. The mechanism incorporates a unique design which mitigates many of the issues arising from load-cell compliance, common to most other serial and parallel mechanisms for material testing. This was achieved through a non-collocated design which raises additional challenges. Achieving high bandwidth control of the hexapod also presents challenges, and was achieved using a combination of LabVIEW real-time running on a floating-point Intel processor, along with LabVIEW FPGA running on 16bit Xilinx FPGAs. In this paper the following unique aspects of this hexapod are discussed: the mitigation of load-cell compliance, non-collocated control, implementation of the controller on a real-time platform, and finally technical solutions to solve the complex forward-kinematics solution in real-time. Finally, the results from testing a high-density polymer cylindrical specimen are presented.