Simple, Passive, Non-Contact Class A Multiplexing Method Using Programmable Surface Acoustic Wave Devices

This paper describes a novel technique for class A sensor-to-module multiplexing using surface acoustic wave (SAW) technology. This technique converts information on the status of N switches to a series of N pulses in the time domain. The basic enabling technology is a programmable SAW reflector, which may be programmed to either a nonreflecting or partially reflecting state by closing or opening a switch connected to the reflector. The SAW device consists of a transducer and N programmable reflectors printed on a piezoelectric substrate such as lithium niobate.

To read the status of the switches, an electronic module sends an interrogating rf pulse to the transducer. At a series of later times determined by the spacing of the reflectors from the transducer and the SAW velocity, return pulses are generated whose amplitudes are determined by the corresponding switch settings. Coupling between the electronic module and SAW device utilizes a pair of coupling coils. This allows the status of switches on a rotating or moving object such as a steering wheel, tire, or door, to be read without making any electrical contact to the moving object. Only the switches, the SAW device, and a coil, all passive components, need to mounted on the moving object. A system to read 15 switches on a steering hub has been developed utilizing a simple SAW device and low-cost electronics, and it is straightforward to extend the system to a higher number of switches.