Abstract: A method of and apparatus for controlling the resultant velocity of a member is provided, the resultant velocity being derived by driving the member in two component directions, simultaneously. First sensor means derive a signal indicative of the resultant velocity, the derived signal being compared with a preselected reference signal to derive an error signal constituting a resultant velocity demand signal which is integrated to obtain a resultant amount of movement demand signal. The resultant amount of movement demand signal is selected from `look-up` reference table memory means and corresponding desired values derived for the amounts of movement of the member in the two component directions. The desired values are compared with signals derived from the first and second sensor means sensing the movement of member and error signals are obtained for controlling the driving of the member in the two component directions.

Abstract: In a P-I-D process control where a process variable is controlled employing the three mode equation:Output=K.sub.1 E+K.sub.2 .intg.Edt+K.sub.3 (dE/dt)the sign of the derivative is compared with the sign of the error. If the derivative and error signs are unlike then the derivative term is discarded. Discarding the derivative term when its sign is unlike that of the error E, prevents the derivative opposing the return of the process variable towards the desired value or "set point.