Servo or Stepper?
Servo and stepper control systems are widely used to provide torque to motion systems where positional and speed accuracy are desired. The basic difference between the two systems is the type of motor used and how it is controlled. Although both types of motor can be used in most applications, they are designed and constructed for very different performance characteristics. Understanding their differences can give you the best balance of design, such as performance and cost.
Positioning
Essentially, servo motor (both brushed and brushless) operates in closed loop system. It requires a feedback sensor, usually an encoder, to tell the position of the motor so that the motion controller can close down the positional error. This closed loop architecture provides extremely high positional accuracy and reliability. However, to achieve the desired performance, the system must be properly tuned.
Unlike servo, stepper motor usually operates in open loop system. This means that it does not require any feedback sensor. Stepper motors achieve the accurate position by discreet steps. It can often achieve up to a few thousand steps per revolution with micro-stepping amplifiers. Operating within the designed boundaries, stepper motors provides superior positional accuracy.
Torque vs Speed
The torque of stepper motors drop rapidly and significantly when operating at high speed. As a rule-of-thumb, stepper motors should be avoided in applications that operate higher than 2000 rpm. Stepper motors are excellent choice for low speed applications for its torque capability with respect to its size. Operating stepper motors beyond the rated torque may result in stalling of the motors or miss-step.
Servo motors, on the other hand, are designed to operate at a wide range of speed and able to maintain the torque. They are better suited in high-speed applications then stepper motors.
One of the differentiating characteristics of stepper motors are the holding torque. Even when power is off, stepper motors' detent torque can hold a load in place. Servo motors do not exhibit holding torque, they hold loads in place by means of constantly compensate for position errors by the motion controller.
Figure 1: Stepper and servo torque-speed capability.
Current and Heat
Stepper motors generate significant amount of heat due to the constant current operation. Servo motors, on the other hand, are controlled with only the required current to hold or move the loads. This heat generation, or constant current consumption, can be an important factor in some applications.
Verdict
Servo control systems offer superb performances in applications require high speed, high torque and involved in dynamic load conditions Stepper control systems are simpler and less expensive. It is suitable for applications that requires high holding torque and low-to-medium acceleration.