Servo Motor vs Stepper Motor: Understanding the Differences
When accurate positioning and speed control is needed, most engineers turn to servo motors or stepper motors, but what is the difference between the two, and which is the right choice?
When engineers hear the term “motor”, they typically think of AC and DC motors. These motors are found in constant motion applications, like a flywheel or a belt conveyor. In industrial automation, you can find two other types of motors: servo motors and stepper motors. These motors are used in applications where precise motion, velocity, and torque are required. In this article, we compare the differences between servo motors and stepper motors and show typical applications for both.
Figure 1. Servo motors differ from continuous motion motors due to their feedback loop and drive system. Image used courtesy of Siemens
What is a Servo Motor?
A servo motor is really quite similar to a normal continuous motion motor—there are AC and DC servo motors, brushed and brushless motors, and synchronous or asynchronous (induction) motors. What sets servo motors apart from regular motors is the feedback loop and the drive system.
Every servo motor has an encoder and drive controller: the encoder reports the position of the shaft back to the drive controller, thus making a closed-loop system. The drive system amplifies and filters the incoming voltage before applying current to the motor. The PLC will tell the drive system the position and velocity of the motor. The speed of the motor is controlled by fairly typical PWM and sine-wave signals, depending on the AC or DC nature of the motor.
Just like a typical AC motor, adjusting the frequency will change the speed of the motor. Similar to a typical DC motor, varying the voltage will change the speed. With an AC servo motor, the drive will accept an electric signal from a PLC or other controller, and the drive system will output current at a specific frequency until the desired position of the motor shaft is reached.
Figure 2. Stepper motors do not need encoders because the segments are divided accurately. Image used courtesy of Oriental Motor
What is a Stepper Motor?
A stepper motor still uses a rotating magnetic field to convert electrical current into rotational motion but has a much higher density of magnetic poles. Stepper motors divide a full rotation into many equal segments. When one magnetic pole is energized, the output shaft will move one segment, or one ‘step’, giving the name ‘stepper’ motor. If the poles are energized in sequence at a high frequency, the motion will appear to be a smooth rotation.
This type of control is typically done with special output cards that can handle the high output frequency required. As the density of segments increases, so does the accuracy of the position of the output shaft. Stepper motors do not typically have encoders because the segments are accurately divided. A stepper motor can be commanded to rotate a specific number of degrees by simply counting the number of times the outputs are energized.
Figure 3. Servo motors work well in pick-and-place applications because of their speed and accuracy. Image used courtesy of Weiss World
Applications of Servo Motors
Typically, a servo motor is combined with a gearbox, drive controller, and a linear actuator, which can be a lead screw or belt and pulley system that converts rotary motion to linear motion. This system is used to accurately and precisely move tooling from one point to another.
Within the PLC, the gearbox ratio and the actuator ratios are entered into a scaling algorithm so that when the PLC commands the servo to move a specific distance at a specific velocity, the drive system will be able to calculate the specific frequency and stop the servo after the encoder reports back the correct number of rotations. This type of system is typically found in pick-and-place stations where a component needs to be picked up from one location and placed at another location. Servo motors are a great tool for pick-and-place stations because of their speed and accuracy.
An articulating robot has six servo motors and gearboxes that move an end effector around in a controlled manner. With the high torque capacity of servo motors, robots can move large payloads throughout their working envelope. Servo motors also have the ability to accurately control their torque output. This feature makes servo motors an excellent choice for pressing applications.
Figure 4. Stepper motors are suitable for machining because of their controlled movements. Image used courtesy of Laguna Tools
Applications of Stepper Motors
Stepper motors have unique features that make them excellent candidates for highly specific motion profiles. They have the capability of using full torque at a standstill—the speed can be controlled by the frequency of the pulses to each pole and can rotate at very slow speeds without the risk of overheating.
With these special features and a smaller price tag, stepper motors can typically be found in 3D printing machines, CNC machining centers, welding equipment, and small inexpensive robotics. The small form factor, DC voltage operation, and open-loop design make stepper motors a great choice for hobbyists and small electronics. Stepper motors work well in applications where small slow movements need to be made.
Stepper Motors vs Servo Motors
The overall goal of both the servo motor and the stepper motor is to provide accurate speed control and positioning, yet these two motor designs are different from each other.
A servo motor is closer in design to a typical motor but with a closed loop system and advanced controller technology. A stepper motor is the most different in design and doesn’t require a closed-loop system to achieve positioning, although many stepper systems benefit from the addition of an encoder.
Some of the applications for these motors do overlap because of their similar goals, but when higher speeds and higher torques are required, a servo motor is the better choice. When slower speeds and higher accuracy positioning are required, a stepper motor is the better choice.
Torque
When designing a motion system, there are many considerations to take into account before choosing a stepper motor or servo motor. I like to start with torque. If your system is going to require a lot or even a significant amount of torque, then a servo motor is the better solution. A unique feature of the stepper motor is the holding torque at zero speed. If my application requires the tooling to be locked in place or hold position, a stepper motor might be the better fit.
Accuracy
With regard to accuracy, a typical servo motor is accurate to about +- 0.02 degrees, whereas a stepper motor is accurate to +- 0.005 degrees.
Responsiveness
The last criteria I would consider is the responsiveness of the application. Stepper motors, by design, are quick to change position—simply fire a few outputs and the output shaft will have to rotate a couple of degrees. Servo motors allow for a small amount of error in position before they start to move, this delay may seem short to the human eye, but when moving a cutting tool along a rotating shaft, that delay could cause imperfection in the surface finish.
Importance of Motor Selection
It is important to analyze your application and make sure you have selected the correct motor for the job, otherwise, you could be spending a lot more money than you need to, or you might wish you had spent a bit more for the right performance.
