Many servo-hydraulic control applications require high bandwidth realtime force
tracking. Some specimen dynamics greatly limit the force bandwidth obtainable with
standard PID loops. In these cases, the force control bandwidth is much lower than its
position control counterpart. When the specimen includes resonant modes in the
frequency range of interest, these resonant poles result in complex zeros in the open loop
transfer function, which adversely affect the loop shape. These low frequency complex
zeros are a consequence of the “highly inertial specimen” characteristics where the
frequency of the zeros is inversely proportional to the square root of the mass.
This paper presents a servo-hydraulic model to study this problem and design control
laws which are implemented on an experimental test bench. An evaluation of a dynamic
inversion controller shows a lack in robustness to model uncertainties. Included in this
paper is the design of an H-infinity loop shaping controller which exhibits high gain
disturbance rejection and robust stability with an order of magnitude improvement in the
force reference tracking bandwidth. The controller also includes an inverse model prefilter,
which increases the force reference tracking bandwidth higher than the displacement control (without pre-filter) tracking bandwidth.