Friction estimation in a planar electrohydraulic manipulator

An automated machine for industrial fish head cutting has been designed and built in our laboratory. A planar electrohydraulic manipulator is used to quickly move a sharp pneumatically actuated blade to its desired position. There are two lubricated metal guideways under the blade. The objective of this experimental investigation is to understand the nature of friction present in the system and corresponding effects. At first, a simple open-loop procedure is used to obtain the steady-state friction-velocity behavior at very low velocities. Online estimation of dynamic friction (in the closed-loop system) is considered afterwards. A nonlinear reduced-order observer, introduced in the literature, for simultaneous estimation of friction and velocity, is applied to the system. Experimental results indicate that this estimator does not work well. Thus, in its original form, it is not suitable. A modification is proposed herein, to obtain good performance. Experiments with the modified observer show satisfactory estimation of velocity and friction.