Direct-Current Motor Speed Control Using a PID Discrete Controller

The presented paper aims at designing a PID discrete type controller, obtained using Kalman Method, to control the speed of a Direct-Current Motor. Such a DC motor is used as the actuator for the driving mechanisms that load or unload billets from a rotary hearth furnace, to yield seamless tube pipes through hot rolling. For each supplying machine, from or towards the furnace, a direct-current motor supplied through a direct-current converter is being used. The mathematical model of this actuator is obtained as a second order transfer function, starting from the system of equations that describe its working in transitory regime. To obtain the PID discrete control law that can be easily implemented on a microcontroller, few steps must be performed. First the continuous mathematical model is converted into a pulse transfer function, and further on, the steps from Kalman designing method are applied. The initial controller is a higher order controller that generates good transient regime specification, for the step response of the controlled structure (meaning the motor's speed), however the control signal variation is not feasible, due to high negative values. Adjustments are applied, by substituting the real negative pole, yielding a PID discrete controller. The resulted control signal generates a feasible range and the discrete control law is much simpler, thus being easier to implement to ensure the imposed speed of the motors and to assure the machines correct positioning. The results assessment is made via simulation, using MATLAB computer software.