Force Sensor with Quartz Resonators by Differential Method

Abstract

The output of the force sensor using quartz resonators is variations in frequency according to change in an external force. The output can be easily converted into a digital signal through a frequency counter. The output of the frequency counter can be directly connected to microprocessor. The force sensor is robust to electrical noises and operates with low electric power. On the other hand, though a quartz resonator is fragile to an impulsive force, the impulsive force applying on a quartz resonator can be damped through a mechanical filter such as a silicone rubber.First, characteristics of a rectangular AT-cut quartz resonator as a force sensor under a static force are discussed. Second, a differential method that compensates temperature dependence of sensitivity and improves force sensitivity is proposed. A pair of rectangular quartz resonators is mounted to a holder that can impose the tensile stress to one and the compressive stress to the other by an external force. The relationship between the force directions of a pair of quartz resonators where the temperature coefficient of the sensitivity becomes zero is derived. Third, responses of the force sensor under a dynamic force are measured. Consequently, it is revealed that the output is able to respond to the force. Lastly, an example of a grasping force sensor that consists of two pairs of rectangular quartz resonators mounted on a robot hand is proposed. Two pairs of differential structures is used in the sensor so that the sensitivity of the sensor is independent of a grasping position on the robot hand. A grasping position on the robot hand is also able to measured by the grasping force sensor.