Valve Timing Control for a Digital Displacement Hydraulic Motor Using an Angle-Domain Repetitive Controller

A digital displacement hydraulic motor uses actively controlled on/off valves instead of a valve plate for porting fluid. Displacement can then be varied by varying the timing of the on/off valves. To minimize throttling energy loss, the valve timing must also consider the precompression and decompression of the compressible fluid. In this paper, a rotary on/off valve, whose rotation controls the valve timing, is used. The desired motion of the rotary valve that encodes the ideal valve timing is a function of, and is periodic with respect to, the motor crank shaft angle. As the desired valve trajectory is not periodic in time unless the motor speed is constant, the conventional internal model based repetitive control is not applicable. A recently developed angle-domain repetitive control is used instead to control the rotary valve motion to achieve the desired valve timing. Experiments show that the proposed controller achieves good tracking performance for a broad range of varying motor speeds. The maximum error is reduced by six to ten times with the new control scheme over baseline fixed period repetitive or proportional-integral controllers. This in turn leads to efficiency gains for the digital hydraulic motor up to 45%.