Variable speed control of hydraulically driven weight stacks in a 540 kN force standard machine

In order to expedite calibrations of force measuring instruments, variable speed control of linkage-weight stacks has been newly introduced to the 540 kN dead-weight type force standard machine (DWM) of the National Metrology Institute of Japan. In contrast with actuators driven by AC servo motors which are adopted in DWMs of relatively small capacities, the hydraulic actuator required more gradual control of the weight moving speed to prevent weight oscillation that will deteriorate calibration quality. Use of a non-contact gap sensor also solved the problems of detecting contact and release between the loading frame, the first weight, and the supporting frame and providing a reference for changing the weight speed. The benefits of introducing variable speed control have been verified by experiment.