Off-centre load-insensitive digital quartz resonator force sensor
A digital output force sensor has been developed utilising a unique elastic structure and an AT-cut quartz crystal resonator (QXR) based on an energy trapping effect as sensing element. The force-sensitive QXR is preloaded into the elastic structure, through which a force proportional to the applied force is exerted on the QXR, and the force measurement is accomplished by measuring frequency changes. The elastic structure consists of several flexible hinges and has the advantages of no mechanical friction and no clearance, and therefore nonelastic effects of the digital force sensor are minimised. The elastic structure is specially configured to avoid measurement error induced by off-centre loads. To compensate the temperature coefficient of sensitivity, a dual-harmonic-mode self-temperature-sensing method is employed to measure the applied force and the in situ temperature simultaneously with the same QXR. A practical design is developed and the dependence of the measurement range on the elastic structure dimensions as well as the changes of the quality factor of QXR before and after preloading are discussed. FEM simulation and experimental results show that the force sensor has good performance and is insensitive to off-centre load. Detailed specifications of the force sensor are given.