Step motors are prized for their ability to provide precise positioning without a feedback mechanism or closed loop control system. This inherent precision is owed to the fact that hybrid step motors have toothed rotors and stator that create an electromechanical gearing system to increase the resolution provided by rotating the stator field by 50X. Move the field by 90° (one full step) and the motor shaft moves by just 1.8°.
If the stator field is rotated at a constant rate from a precise quartz crystal clock, the open loop velocity is also excellent. But how accurate is this positioning? According to the specification of a typical hybrid step motor, the device can position within ±0.09°. The factors affecting this positional accuracy are the precision to which the stator and rotor components are punched and assembled, the concentricity of the air gap, and the current balance of the two phases as provided by the step motor driver.
This chart illustrates the actual position error measurement of a step motor and driver using a high resolution measurement system in the motion control laboratory at Applied Motion Products’ California headquarters.
The data we've provided so far references static positional accuracy: the motor has arrived at its intended position and stopped moving. What if you need the motor to be at an exact position while it is still moving? Several “on-the-fly” applications require such precision, including labeling and scanning. To determine positional accuracy at speed, we must consider both the inherent electromechanical errors shown above, plus the rotor lag angle. When a step motor is driven open loop, the stator field is rotated by the driver, and, if the laws of physics are not violated by expecting more torque than the motor can deliver, the rotor follows. The torque produced is a function of the difference between the electrical angles of the stator and rotor, called lag angle. This follows a mostly sinusoidal relationship as shown below.
If we accelerate the motor hard and use, say, 50% of the available torque, we can calculate the lag angle using the arcsine function to determine that the rotor will lag behind the intended position by 30 electrical degrees (0.6° at the shaft) at any given time.
In reality, step motors typically perform much better than this because high torque utilization normally occurs when accelerating. Once a constant speed has been achieved, the torque demand is reduced to that required to overcome friction.