MAGNETORESISTIVE FORCE SENSORS FOR USE IN AN INSTRUMENTED PEN

The project involves the design of a magnetoresistive force sensor to provide a feedback element in an instrumented pen. The pen, together w ith a functional electrical stimulation system, would allow writing to be achieved by tetraplegic patients. Experiments have been carried ou t to define the force range and force sensor design required for succe ssful writing. Considering that tangential and normal forces are to be measured, magnetoresistive sensors are selected as the transduction t echnique. Experiments with current-carrying conductors and permanent m agnets have been carried out to define the magnetic source for the sen sors, in an attempt to optimize the system for low power consumption. Cast and sintered alloy magnets are finally chosen. If this type of pe rmanent magnet is put in an orthogonal position with respect to the se nse element plane, a lateral displacement sensitivity of -40 mV/mm in a range of 1.5 mm can be achieved, with low sensitivity to the separat ion between the magnet and the sensor (-5 mV/mm).