This paper presents a novel application of direct force control to test biological specimens using a serial manipulator with 6 degrees of freedom Direct force control compares actual force/moment values with desired values of load. The error is compensated by a proportional/integral controller (PI), a damping factor implemented with the velocity of the robot and acting in the direction of the force and a feedforward compensation. The controller works with a frequency of 0.5 kHz which enhances its performance due to the direct force feedback loop. A fresh porcine cervical spine C2-C4 was used. All muscle tissues were removed while leaving intact all ligaments and bony tissue. The specimen was loaded separately with ±3 Nm in every spatial axis. The mean errors in the unconstrained axes in the present study were less than 1.70 N and 0.32 Nm. Direct force control of 6 axes with a high controller frequency of 0.5 kHz developed in this methodology shows a successful procedure to perform biomechanical in-vitro tests. The controller demonstrated the ability to maintain zero load targets in the unconstrained axes. This control approach allows the application of pure moments in order to perform in vitro biomechanical experiments with spine segments.
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