In engineering, perfect and universal solutions don’t exist. Each case requires the designer or engineer to identify and implement the best solution for that project’s task, budget, and requirements. One such area is electrical motors; if your application requires precise control, servo and stepper motors are usually the right components for the task. Both motors perform the same general job — rotating a shaft a specific number of degrees or turns — but they do so differently. Each method also comes with important advantages and disadvantages.
What is a Stepper Motor and How Does it Work?
Stepper motors divide a motor’s full rotation into several steps. They accomplish this by using a permanent magnet or iron rotor and a series of electromagnets surrounding it. These components make up the stationary part of the motor, also known as the stator. The motor activates these electromagnets in a carefully controlled sequence to advance the rotor on step at a time. (For a more in depth tutorial, read “What’s New in Stepper Motors?”)
This activation is the result of a variety of driver circuits, and the magnetic arrangement determines the motor’s precision. You’ll commonly see precision levels of 1.8 degrees per step (360 degrees divided by 200 magnetic steps), but this type of division is not universal. The motors can attain sub-step resolution using a fractional stepping technique as well. For this technique, the motor activates multiple phases at once, and users can achieve even greater control by activating phases in an analog method called “microstepping.”
What is a Servo Motor and How Does it Work?
Servo motors, like stepper motor, come in a wide range of shapes, sizes, and price points. You can find inexpensive hobby micro-servos for a few dollars, and these motors feature a variable resistor that allows them to move to an angular position and return there even when an external force moves them. More expensive industrial servos can cost thousands of dollars and feature position and speed feedback. Users often couple these motors with gearboxes to increase their torque as well. You can command servo motors using external control hardware to travel to a certain position and turn in a controlled manner to obey the command.
Closed Loop (Servo) vs. Open Loop (Stepper)
While you can choose between many different types of servos and steppers with a wide range of prices and capabilities, the fundamental difference comes down to feedback. What is the difference between open loop and closed loop?
- Stepper motors: no feedback. Stepper motors can be an excellent choice, but they don’t offer feedback on whether they’ve traveled their intended distance. If there’s a glitch in the system or physical impediment while spinning, they can “skip steps,” then continue along as if nothing is wrong.
- Servo motors: feedback featured. If a servo motor is commanded to go to a certain position, it will do its best to get there. In less capable setups, this may mean a motor constantly struggling to attain a certain position, or near-instant error feedback when it can’t correct itself in more capable systems.
Choosing the Right Stepper Motor
With the proper motor type, either motor may be appropriate for your application. Evaluate your build to see if any of these factors are essential:
- Sensors. If you’re using a stepper, you’ll probably need to have some way to ”zero” the system to a known point before the system can do its designated task. Stepper setups may also use this zero sensor or others to verify their processes. Adding a sensor can mean that using a servo motor will be more complicated or expensive in the long run, so you’ll want to consider your needs before you choose your motor.
- Speed vs. Torque. Another factor to consider is that stepper motors tend to lose torque as speed increases, which means a servo motor may be a better choice for higher speeds. Steppers do, however, exert excellent torque at low speeds.
- Angular precision. If your application depends on speed over angular precision, another choice would be a so-called brushless DC motor. They act as a sort of hybrid-controlled motor optimized for speed — e.g., they spin up to exactly 2000 RPM — not rotational accuracy.
Finally, “normal” commutated DC motors may be an appropriate solution in many cases; while they don’t feature any built-in feedback, you can add an encoder or a simple pin to measure how many times the motor turns.