SAW force sensor based on reflective delay line quasi-mirror topology

We present an example of a temperature self-compensated sensitive element design for a force sensor and of an interrogation algorithm that helps to increase working distance while satisfying limited bandwidth requirement. The SAW sensitive element installation in the Roberval parallelogram based force sensor enables the antisymmetric deformation distribution along the SAW propagation path. The sensitive element design is based on a single port reflective delay line with reflectors arranged on both sides of the IDT. Each reflector consists of several segments with 10 grooves in each segment and the distances between the IDT and the reflectors ensure that the reflections from the left reflector segments fall in the middle of the gap between the reflections of the right reflector segments. The impulse response consists of series of responses from all the segments, followed by a triple transit tail. When temperature changes, the impulse response shape remains unchanged, while when a force is applied, the opposite stress in the left and right sides of the delay line determines the intersection of the pulses from the left side with the pulses from the right side. If a relatively long (1.8 μs) interrogation RF burst is applied the device output response amplitude decreases with force increase. Fabricated devices with central frequency 433 MHz have confirmed modeled characteristics and operation principles. The working distance with 10 mW interrogation signal reached 30 m.