Stepper motors consist of a rotor with permanent magnets and a stationary stator that carries the windings. When current runs through the stator windings, it generates a magnetic flux distribution that interacts with the magnetic field distribution of the rotor to apply a turning force. Stepper motors feature very high pole counts, typically 50 or more. The stepper motor driver energizes each pole in sequence so that the rotor turns in a series of increments, or steps. Because of the very high pole count, the motion appears to be continuous.
Stepper motors have a number of positive attributes. Because they generate incremental motion, they are generally run open loop, eliminating the cost and complexity of an encoder or resolver. The high pole count allows them to generate very high torque at zero speed. They are compact and generally economical (see figure 1).
Figure 1: Stepper motors deliver good performance at an economical price point for applications requiring low speed, acceleration, and accuracy. (Courtesy of Kollmorgen)
Stepper motors also have performance limitations. You can think of a stepper motor as a spring-mass system. The motor needs to break friction to begin turning and move the load, at which point the rotor is not fully controlled. As a result, a command to advance by five steps may only result in the motor turning four steps – or six. If the drive commands a motor to advance 200 steps, however, it will do so to within just a few steps, which at that point represents an error of a few percent. Although we command stepper motors with a resolution of typically between 25,000 and 50,000 counts per revolution, because the motor is a spring-mass system under load, our typical resolution is 2000 to 6000 counts per revolution. Still, at these resolutions, even a 200-step move corresponds to a fraction of a degree.
Figure 2: As these torque versus speed curves show, stepper motors deliver peak torque at zero speed with torque falling off as speed increases (green). In contrast, servo-motor torque remains roughly constant across the operating range (blue and red). (Courtesy of Kollmorgen)
Like stepper motors, servo motors have many implementations. Let’s consider the most common design, which incorporates a rotor with permanent magnets and a stationary stator with the windings. Here too, the current creates a magnetic field distribution that acts on the rotor to develop torque. Servo motors have significantly lower pole counts than stepper motors, however. As a result, they must be run closed-loop.
Figure 3: With closed-loop feedback, servo motors deliver high accuracy positioning coupled with better speed and acceleration than stepper motors. The trade-off is increased cost, size, and complexity. (Courtesy of Kollmorgen)
Servo motors offer an undeniable performance advantage. In terms of repeatability, however, stepper motors can be quite competitive. This point brings up a common misconception about stepper motors, which is the myth of lost motion. As we discussed previously, the mass-spring nature of a stepper motor may result in a few lost steps. Because the drive is commanding the stepper to move to an angular location, lost steps are not carried over from rotation to rotation, however. Rotation to rotation, stepper motors are highly repeatable. Look for a more detailed discussion of this topic in a future blog post.
What to Know to Specify the Right Motor
- Torque requirements
- Speed requirements
- Desired acceleration
- Load mass and inertia
- Size limitations
To summarize, stepper motors are good solutions for applications with low speed, low acceleration, and low accuracy requirements. Stepper motors also tend to be compact and inexpensive. This makes these motors a good fit for medical, biotech, security and defense, and semiconductor manufacturing applications. Servo motors are a better choice for systems requiring high speed, high acceleration, and high accuracy. The trade-off is a higher cost and complexity. Servo motors are typically used in packaging, converting, web processing, and similar applications.
When your application is forgiving but your budget is not, consider a stepper motor. If performance is the most important aspect, a servo motor will do the job but be prepared to pay more.
About the Author
Bill Lackey is vice president of engineered sales, Motion Solutions.