A stepper motor (or step motor) is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor’s position can then be commanded to move and hold at one of these steps without any feedback sensor (an open-loop controller), as long as the motor is carefully sized to the application1. A servomotor is a rotary actuator that allows for precise control of angular position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors. Servomotors are used in applications such as robotics, CNC machinery or automated manufacturing. 2.
A stepper motor’s shaft has permanent magnets attached to it. Around the body of the motor is a series of coils that create a magnetic field that interacts with the magnets. When these coils are turned on and off the magnetic field causes the rotor to move. As the coils are turned on and off in sequence the motor will rotate forward or reverse. This sequence is called the phase pattern and there are several types of patterns that will cause the motor to turn.
To make a stepper motor rotate, you must constantly turn on and off the coils. If you simply energise one coil the motor will just jump to that position and stay there resisting change. The ability to stay put at one position rigidly is often an advantage of stepper motors.
Because steppers can be controlled by turning coils on and off, they are easy to control using digital circuitry and micro controllers. The controller simply energises the coils in a certain pattern and the motor will move accordingly. At any given time the computer will know the position of the motor since the number of steps given can be tracked.
If you take a normal DC servo motor, it has one coil with two wires. If you attach a battery to those wires the motor will spin. Reversing the polarity will reverse the direction. In a DC motor, the speed and current draw is a affected by the load. For applications in which the exact position of the motor must be known, a feedback device like an encoder must be used and is not optional like with a stepper motor. The control circuitry to use is much more complex than the circuitry that controls a stepper motor.
Some performance differences between stepper motors and servo motors are the result of their respective design. One rotation of a stepper motor requires more current exchanges through the windings than a servo motor. The stepper motor’s design results in torque degradation at higher speeds when compared to a servo. Using a higher driving bus voltage reduces this effect by mitigating the electrical time constant of the windings. Conversely, a high pole count has a beneficial effect at lower speeds giving the stepper motor a torque advantage over the same size servo motor.
Another difference is the way each motor type is controlled. Traditional stepper motors are cheaper to operate, since no encoder is necessary for most positioning applications. Furthermore, stepper motors are simpler to commission and maintain than servos. They are less expensive, especially in small motor applications. They don’t lose steps or require encoders if operated within their design limits. Steppers are stable at rest and hold their position without any fluctuation, especially with dynamic loads.
Servos are excellent in applications requiring speeds greater than 2,000 RPM and for high torque at high speeds or requiring high dynamic response. Steppers are excellent at speeds less than 2,000 RPM and for low to medium acceleration rates and for high holding torque.
More on these motor types:
- Source: https://en.wikipedia.org/wiki/Stepper_motor ↩
- Source: https://en.wikipedia.org/wiki/Servomotor ↩