Stepper motors offer a means to achieve speed control without the usage of closed loop mechanisms such as shaft encoders or resolvers. On a microstepping internal indexer driver, this open loop control is obtained by modulating the frequency at the STEP input. Each pulse at the STEP input, becomes a mechanical step motion at the stepper motor. Hence, it is safe to say that since we know what frequency we are applying at the STEP input, we then know the actual step rate the stepper motor is moving at. This will hold true as long as the right parametric values, such as current, voltage and torque, are maintained within reasonable levels throughout the application’s operation.

Unfortunately, we cannot just apply any frequency or step rate to any given stepper motor. Due to the mechanisms behind the revolving magnetic field at the stator and the permanent magnet at the rotor, a stepper motor can only start moving if the requested speed is smaller than a parameter given by the motor’s manufacturer and referred to as the starting frequency (denominated FS). For example, if the FS for a particular stepper motor is 300 steps per second (SPS), it will most likely not be possible to start the motor at a frequency of 400 SPS.

Since the application may require speed rates larger than the FS, it is then very important to subject the motor commutation through an acceleration profile which starts at a speed rate lower than its maximum FS and increases speed accordingly until reaching the desired speed.

Figure 2-1 Typical Stepper Acceleration and Deceleration Profile

Figure 2-1 shows a typical acceleration and deceleration profile where:

Starting Speed is a STEP frequency lower than the motor’s rated FS at which the motor will start moving. Measured in steps per second (SPS), where STEPS refers to full steps.

Acceleration Rate is a factor of how much the STEP frequency will be increased on a per second basis. Measured in steps per second per second (SPSPS).

Desired Speed is the STEP frequency the application requires the motor to move at. It marks the STEP frequency at which the acceleration profile concludes. Measured in steps per second.

Deceleration Speed is a factor of how much the STEP frequency will be decreased on a per second basis. Measured in steps per second per second (SPSPS).

Stopping Speed is the STEP frequency at which the deceleration profile and the motor will be stopped. In this application note, stopping speed is taken to be the same as the starting speed. Measured in steps per second.