Design and Control of a Large-Range Nil-Stiffness Electro-Magnetic Active Force Sensor

Abstract

Active force sensors are key instruments to get around the tradeoff between the sensitivity and the measurement range of conventional passive force sensors. Thanks to their quasi-infinite stiffness in closed loop, active sensors can be applied for force measurements on samples with a wide range of stiffness without interference with the mechanical parameters of the sensor. MEMS (Micro-Electro Mechanical Systems) active force sensors have been wildly developed in the literature but they are ill adapted for force measurements at the Newton level needed in meso-scale robotics. In this article, a novel structure for a meso-scale active force sensor is proposed for the measurement of forces from the milli-newton to the newton.This novel meso-scale sensor is based on a nil-stiffness guidance and an electromagnetic actuation. This paper deals with its design, identification, calibration and closed loop control. The sensor exhibits nil-stiffness characteristic in open loop and an almost infinite stiffness in closed loop. This allows measuring forces with a large range of gradients. First experiments shows the ability of this new sensor architecture to measure low frequency forces up to 0.8N with a precision of 0.03 N and a closed loop –20 dB cutoff frequency of 73.9Hz.