An electrohydraulic force control system for large-scale force calibration

Calibration of large-scale force sensors using force standard machines (FSMs) has found increasing requirements but faces difficulty in fulfilling the demands of high pressure, low measurement uncertainty, and enough force-holding duration. To cope with this difficulty, this paper proposes an electrohydraulic force control system for large-scale force calibration, which is composed of a coarse force control unit and a fine force control unit. The coarse force control unit consists of a variable-speed pump source and a single-rod cylinder as a hydraulic booster. In the fine force control unit, an asymmetric double-rod hydraulic cylinder is firstly presented to adjust the oil volume with a high resolution into or out of the working cylinder, such that force control can be achieved with good stability performance. A design criterion is also given to determine key system parameters. Moreover, two practical controllers are designed for coarse force control and fine force control, which both consist of a feedforward term and a feedback term. Two force control tests of 1000 kN and 5000 kN are carried out using an actual FSM to validate the proposed system. The test results show that the generated forces can be controlled within 1000±0.05 kN and 5000±0.1 kN from the indicator values of the force sensor that has been calibrated by another FSM. When using the system for calibration using two sensors, the response rates of the different sensors will differ. Considering the limited force capacity of the FSM, an additional force simulation test of 15000 kN is also implemented to show the high-pressure loading ability of the proposed system.