Schneider electric make a 2 wire inductive proximity sensor that we use to detect "belt slip" on conveyor belts driven by electric motors.
There are 2 versions, XSA-V11373 (slow) and XSA-V12373 (fast).
We wire these in series with the coil of the motor drive contactor. During the 10 seconds or so the units ignore the speed of the pulses on the end of the sensor (to allow the belt to get upto speed) and then after this delay, if the speed drops below the "set point" they go open circuit and stop the power to the coil.
