A stepper motor converts a train of input pulses into a precisely defined increment in the mechanical shaft position, where each pulse moves the shaft through a fixed angle. The motor's position can then be commanded to move and hold at one of these steps without any feedback sensor (i.e. an open loop Stepper Motor control), as long as the motor is carefully sized to the application in respect to torque and speed. Stepper motor control provides this input train of pulses to command the motor to move to the desired position or at the desired speed.
Stepper motor control constant voltage drives are used to apply a constant positive or negative voltage to each winding to drive motion. However, it is winding current, not voltage, that applies torque to the stepper motor shaft. The current (I) in each winding is related to the applied voltage (V) by the winding inductance (L) and the winding resistance (R). Thus, these are also known as L/R drives. To obtain high torque at high speeds requires a large drive voltage with a low resistance and low inductance. With an L/R drive it is possible to control a low voltage resistive motor with a higher voltage drive simply by adding an external resistor in series with each winding. However, since it wastes power in the resistors, and generates heat, it is therefore considered a low-performing option.
Constant current drives generate a constant current in each winding rather than applying a constant voltage. On each new step, a high voltage is initially applied to the winding, causing the current in the winding to rise quickly since dI/dt = V/L where V is large. The current in each winding is monitored by the stepper motor controller, usually by measuring the voltage across a small sense resistor in series with each winding. When the current exceeds a specified current limit, the voltage is turned off. When the winding current drops below the specified limit, the voltage is turned on again. In this way, the current is held relatively constant for a particular step position. This requires additional electronics to sense winding currents and control the switching, but it allows stepper motors to be driven with higher torque at higher speeds than L/R drives.
Portescap’s Microstepping Drives provide substantial benefits for increased system resolution, low noise and smooth motion. The power stage is designed to suit low electrical time constant motors and achieve a high dynamic performance. Some advantages of our EDM-453 microstepping drive include:
• Resolution switch from 1/2 up to 1/64 microsteps
• High angular resolution in static and dynamic
• Smooth operation, low noise
• Boost mode for higher torque / acceleration output
• Stand-by mode for optimized heat management
• Chopper control mode selectable between regenerative and freewheeling
• Clock frequency up to 150 kHz
• All inputs opto-isolated
• Short circuit and over temperature protection