Increasing Admittance of Industrial Robots by Velocity Feedback Inner-Loop Shaping

Admittance and impedance controllers are often purely feedforward, using measured external force or motion, respectively, to generate a reference for an inner-loop controller. In this case, the range of dynamics which can be rendered is limited by the inner-loop, which causes, e.g. contact stability issues for low admittance industrial robots in stiff contact. When both position and force are measured, feedback control can be added to more flexibly reshape the rendered dynamics. This paper uses velocity feedback to increase the admittance of motion-controlled industrial robots in force control applications. This allows an industrial robot with a lower intrinsic admittance, which may be needed for payload, speed, or accuracy, to realize a higher admittance by control, allowing lighter manual guidance and safer contact. This is achieved by a modified disturbance observer, where an inverse dynamic model estimates external forces and amplifies them with positive feedback. This approach is compared with using positive velocity feedback with a shaping filter. Here, velocity reference calculated by the virtual admittance model is modified by the DOB (Dist-Add) or the positive velocity feedback (Vel-Add). When combined with an outer-loop admittance controller, these methods can render a higher admittance while maintaining contact stability compared to standard feedforward admittance control.