Stepper Servo Motors

Although many designers prefer to relegate stepper motors to the realm of low-cost low-performance technology, a new technique is bringing the step motors a fresh lease of life. This new drive technique is the stepper servo, and it uses the generic stepper motor, yet extracts significantly more performance out of it. The technique requires adding an encoder and operating the motor effectively as a commuted two-phase brushless DC motor.

While the inclusion of an encoder makes the stepper servo idea non-suitable for low-cost applications, designers are increasingly considering the technique an alternate approach to applications requiring a brushless DC motor.

This is because the cost of a stepper servo motor is considerably less than a comparable brushless DC motor, while the former actually outperforms brushless DC motors in areas of torque output and acceleration. Therefore, designers are considering the stepper servo motor as a candidate for high-speed applications such as coil winding, point-to-point moves, textile equipment, high-speed electronic cams, and more.

Stepper motors are easy to use, making them popular. They maintain their position without external aids such as encoders. Neither do they require a servo control loop when designers use them for positioning, as other DC motors do. Their brushless operation, high torque output, and low cost are their biggest advantages. However, their limited speed range, noisy operation, and vibrations are their main disadvantages.

Being a multi-phase device, stepper motors require the excitation of multiple coils and driving control waveforms for their operation. The usual configuration for stepper motors will have 1.8 mechanical degrees for a full step of 90 electrical degrees—making it 200 full steps for every mechanical rotation. Other stepper motors may have 7.2- or 0.9-degree configurations in place of the customary 1.8.

A stepper servo motor has an encoder attached to the shaft. For a typical 1.8-degree stepper motor, the resolution of the encoder must be of the order of 2000 counts per mechanical rotation. The encoder verifies the final position of the rotor through a traditional step motor control scheme.

The stepper servo motor operates more like a brushless DC motor, with the actual encoder position commuting the phase angle, instead of the commanded position. The phase angle and amplitude of the driving waveform need to vary continuously depending on the output from a position PID loop. This allows the motor to servo to the commanded position.

The presence of the encoder frees the stepper servo from losing steps—the encoder determines the location. The motor operation is now more efficient, causing much lower heat generation. Traditional stepper motors require driving at large currents adequate for handling worst-case motions.

Traditional stepper motors always have problems achieving positional accuracy. With the encoder driving the stepper servo motor to its location, these vagaries of position do not arise. The encoder frees the stepper servo motor from the restrictions of the 1.8 degrees per step of the regular stepper motor. Simply increase the resolution of the encoder to get better positional accuracy.

The addition of the encoder also produces a smooth acceleration to the desired position without the customary bouncing and noise.