magnetic field in a predetermined direction e g from south pole to north pole since each revolution of the output shaft 14 produces four rising edge signals the first signal marks the initiation of a revolution and the fifth signal marks its termination similarly the second and sixth rising edge signals are used to mark initiation and termination of one revolution of the motor 12 offset from the first revolution by approximately 1 4 revolution a limiting value of t 2 t 2 limit can be calculated for any measured value of t 1 in counts from the table of inverse velocities fig6 is a plot of t 2 limit as a function of t 1 t 2 limit is potentially able to serve as the desired characteristic limiting function employing the time counts of the microprocessor the measured value of t 2 t 2 measured can be compared with t 2 limit if t 2 measured is greater than t 2 limit it is an indication that the system is decelerating at a rate greater than a limiting value of u03b1 corresponding to t 2 limit and u03c9 2 if t 2 measured is sustained at a value greater than t 2 limit for a predetermined period of time then the microprocessor reverses the direction of the motor determination of the predetermined period of time is described later in this description of the preferred embodiment the microprocessor stores a discrete number of values of t 1 and corresponding t 2 limit values when t 1 measured equals one of the stored values of t1 then t 2 limit equals.