Abstract
This paper describes the development of a load torque detection technique of stepping motors by using sensed back electromotive forces (emfs). One of the advantages of stepping motors is that the motors can be operated by open loop control. However, the stepping motor driven by open loop control method loses synchronism with input pulses when the load torque exceeds the pull out torque limit. Therefore the control system must be designed with enough margin to cope with unexpected increase of load torque. In order to use stepping motors more efficiently, detection of the applied load torque is very useful. The authors developed a new method to detect the load torque without additional torque sensors by comparing the back emfs induced in the coils of the stepping motors and energizing voltages to the coils. The influence of the load torque on the phase difference between voltages supplied to the coil and back emfs is made clear theoretically. Based on the theoretical considerations, authors devoloped and examined torque sensing method by two ways. First one is to detect time difference between input drive pulses and pulses made of back emf by comparators. The second one is to detect the phase difference continuously by processing the back emfs with drive voltages. Experimental results show the effectiveness of both methods. And as an example of application, such a control where the stepping motor changes its direction of rotation at a certain torque is demonstrated successfully.
