Improving performance of an energy efficient hydraulic circuit

Abstract

Hydraulic circuits with fast dynamic response are often characterized by low power efficiency; on the other hand, energy-efficient circuits under certain circumstances, can demonstrate slow transient responses. Continuously rising energy costs combined with the demand on high performance has necessitated that hydraulic circuits become more efficient yet still demonstrate superior dynamic response. This thesis introduces a new hydraulic circuit configuration which demonstrates high dynamic performance and high efficiency.

A pump-controlled hydraulic motor system was used as the basis of the study because of its high circuit efficiency. This is primarily because there is no power loss between the pump and motor. To improve the dynamic response of the pump, a DC motor was designed to control the pump swashplate (and hence flow rate) directly. The pump and DC motor were mathematically modeled and their parameters were experimentally identified. Based on the model and experimental results, a nonlinear PID controller was designed for the DC motor. By means of the DC motor’s quick dynamic response (in the order of 10 ms), the DC motor controlled pump demonstrated a fast dynamic response with a rise time of 15 to 35 ms depending on the pump pressure.

As the dynamic response speed of the pump flow rate was increased, overshoot of the hydraulic motor output also increased. To reduce this overshoot, a bypass flow control circuit was designed to bypass part of the flow during the transient. Due to the unique operating requirements of the bypass flow control system, a PID controller with a resetable integral gain was designed for the valve to reduce the rise time of the bypass control valve. The feasibility ("proof of concept") of the bypass flow control concept was first established using simulation techniques. The simulation results showed that the bypass flow control system could significantly reduced the overshoot of the hydraulic motor rotational speed.

The bypass controller was applied to the experimental test circuit. The transient results for the pump-controlled motor system with the bypass flow control are presented under a constant resistive and an inertial load. The test results showed that the bypass flow control could reduce the overshoot of the hydraulic motor rotational speed by about 50%. The relative efficiency of the circuit with the bypass flow control system was 1% to 5% lower for the particular pump-controlled system that was used in this study. For a pump/motor that does not demonstrate significant flow ripple of the magnitude experienced in this study, the relative efficiency would be the same as the pump/motor system without bypass. It was concluded that the proposed bypass control system, combined with the DC motor-swashplate driven pump, could be used to create an energy efficient circuit with excellent dynamic transient responses.