The choice between a servo and stepper motor is an important decision in any system design. In general, the higher the performance requirement (high precision, variable load, high duty cycle, high torque vs. speed) the more likely a servo will be the choice. Where purchase cost is critical, performance is not crucial, and simple set-up is needed, a stepper solution is a cost-effective option.
The following chart compares the performance of brush/brushless servo and stepper motors:
With stepper motors, closing the position loop via an encoder is not an effective means of providing increased precision. Whereas the encoder will provide position verification, the step motor is unable to “servo” to ensure high dynamic performance and repeatability. Adding an encoder and using a microstep driver makes the relative costs of servo and stepper systems roughly equivalent, yet the performance remains decidedly better with servos.
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Thank you for your kind description and comparison.
The power parameter may weight a lot when speaking about robots. Isn’t it ? of course it is depended also on the accuracy and other relevant needed parameters.
Good email but designed to create a bit of controversy. Closed loop stepper and position verification stepper are two different animals. In the olden days (old school) closed loop steppers were really just position verification systems. Move to a location and check to see where we are, if there is any error, make a correction. These days there are actual 50 pole (stepper motor) vector drives. Several drive and control companies have created drives that will actually drive a stepper motor as though it were a servo motor. Traditional servo motors have low inertia rotors with 6 to 8 poles which are designed for high acceleration applications or applications that require very high speeds 3500 to 5000 rpm’s. Steppers on the other hand have heavier, higher inertia rotors with 50 poles designed for high torque at low speeds. The heavier inertia rotors are also better for inertial mis-matches between rotor and system. So ultimately the 50 pole servo is ideal for slow speed, high torque applications for many of the reasons already discussed but regurgitated. 1) They are better for inertial mis-matches and may not require a gear box that are typically required for traditional servo motors, 2) they can be turned back into a steppers at zero speed. Meaning no dither or hunting 3) Stepper drives provide a fixed voltage and current that ultimately defines a torque curve. Once the torque at required speed is exceeded the motor stalls. Well as a vector drive, current is a variable so the motor has a little bit more available torque at certain speeds. Back emf is a given so for higher speed apps this is not a good animal. 3) However 80 percent of the automation apps require speeds of less than 20rps so for most applications speed is not the issue. 4) The holy grail of reasons is they are much cheaper… All servo means is that there is some level of feedback to which a control can make a desired error correction. So full circle which is better? The application should define the choice however with a little know how one can be a little more clever about the decision making process.