A typical force sensor is composed of a moveable shuttle suspended by flexures, and thus can be modeled as a spring-mass-damper system. When an external force is applied, the shuttle experiences a finite amount of displacement based on the stiffness defined by the suspension structures. The shuttle displacement detected by the transduction mechanism is then translated into force by using the stiffness of the system. However, the stiffness of the flexures becomes displacement-dependent when larger forces are applied, which in return results in a nonlinear force-displacement curve. On the other hand, if the external force to be measured is too small, the displacement occurring in the system would be too small to be detected by the sensors accurately.
Hence, the stiffness of the system plays a highly critical role in order to define sensor performance characteristics, including sensitivity, measurement range, and bandwidth. Considering the wide range of applications, force sensors usually need to be designed in accordance with the application’s sensitivity, measurement range, and bandwidth requirements.
LDCN is developing novel solutions to minimize the stiffness dependence of the force sensors. This can be achieved by introducing actuators to the system and using displacement feedback to nullify any displacement that would occur in a typical force sensor. Here, the system becomes a function of the control signal rather than the stiffness; it can detect a wide range of forces, from a few μN to the mN range, without a loss of accuracy.
While this approach allows larger forces to be measured and thus substantially increases the measurement range, the device’s initial stiffness is still a limiting factor against measuring smaller forces.
Left: A zero-displacement MEMS force sensor with electrostatic actuators and electrothermal sensors, as reported in Applied Physics Letters.
Right: A MEMS force sensor with piezoresistive sensors and a stiffness management mechanism.