TABLE OF CONTENTS In a closed-loop stepping motor system the rotor position is detected and fed back to the control unit. Each step command is issued only when the motor has responded satisfactorily to the previous command and so there is no possibility of the motor losing synchronism.
A schematic closed-loop control is shown in Fig. 7.1. Initially the system is stationary with one or more phases excited. The target position is loaded into the downcounter and a pulsed START signal is applied to the control unit, which immediately passes a step command to the phase sequence generator. Consequently there is a change in excitation and the motor starts to accelerate at a rate dictated by the load parameters.
As the first step nears completion the position detector generates a pulse which is sent to both the downcounter and the control unit. The downcounter is decremented and therefore contains the position of the load relative to the target. Note here the contrast between closed and open-loop control schemes; with open-loop control the downcounter is able to record only the number of step commands sent to the motor and there is no guarantee that these steps have been executed. With closed-loop control, however, the downcounter is recording actual load position.
The position detector pulse sent to the control unit is used to generate the next step command. For larger loads the time taken to reach the first step position is longer and therefore the...
