Stepper Vs. Servo Motors
Characteristics Servo Motor (DC Brushed Stepper (Hybrid)
|Cost || The cost for a servo motor and servo motor system is higher than that of a stepper motor system with equal power rating. ||This feature would have to go to |
stepper motors. Steppers are generally cheaper than servo motors that
have the same power rating.
|Versatility || Servo motors are very versatile in their use for automation and CNC applications. || Stepper motors are also very |
versatile in their use for automation and CNC applications. Because of
their simplicity stepper motors may be found on anything from printers
|Reliability || This is a toss up because it depends on the environment and how well the motor is protected. ||The stepper takes this category only because it does not require an encoder which may fail. |
|Frame Sizes || Servo motors are availible in a |
wide variety of frame sizes, from small to large motors capable of
running huge machines. Many of the motors come in NEMA standard sized.
| Stepper motors do not have as many size selections |
as servo motors in the large sizes. However stepper motors may still be
found in a variety of NEMA frame sizes.
|Setup Complexity || Servo motors require tuning of the (PID) closed loop variable circuit to obtain correct motor function. || Stepper motors are almost plug-and-play. They require only the motor wires to be wired to the stepper motor driver. |
|Motor Life || The brushes on servo motors must be replaced every 2000 hours of operation. Also encoders may need replacing. || The bearing on stepper motors are the only wearing parts. That gives stepper motors a slight edge on life. |
|Low Speed High Torque || Servo motors will do fine with low speed applications given low friction and the correct gear ratio || Stepper motors provide most torque at low speed (RPM). |
|High speed High Torque || Servo motors maintain their rated torque to about 90% of their no load RPM. || Stepper motors lose up to 80% of their maximum torque at 90% of their maximum RPM. |
|Repeatability || Servo motors can have very good repeatability if setup correctly. The encoder quality can also play into repeatability. || Because of the way stepper motors |
are constructed and operate they have very good repeatability with
little or no tuning required.
|Overload Safety || Servo motors may malfunction if overloaded mechanically. || Stepper motors are unlikely to be damages by mechanical overload. |
|Power to Weight/Size ratio || Servo motors have an excellent power to weight ratio given their efficiency. || Stepper motors are less efficient than servo motors which usually means a smaller power to weight/size ratio. || |
|Efficiency || Servo motors are very efficient. Yielding 80-90% efficiency given light loads. || Stepper motors consume a lot of power given their |
output, much of which is converted to heat. Stepper motors are usually
about 70% efficient but this has some to do with the stepper driver.
|Flexibility in motor resolution || Since the encoder on a servo motor determines the motor resolution servos have a wide range of resolutions available. || Stepper motors usually have 1.8 or 0.9 degree |
resolution. However thanks to micro-stepping steppers can obtain higher
resolutions. This is up to the driver and not the motor.
|Torque to Inertia Ratio || Servo motors are very capable of accelerating loads. || Stepper motors are also capable of accelerating |
loads but not as well as servo motors. Stepper motors may stall and skip
steps if the motor is not powerful enough.
|Least Heat production || Since the current draw of a servo motor is proportional to the load applied, heat production is very low. || Stepper motors draw excess current regardless of load. The excess power is dissipated as heat. || |
|Reserve Power and Torque || A servo motor can supply about 200% of the continuous power for short periods. || Stepper motors do not have reserve power. However stepper motors can brake very well. |
|Noise || Servo motors produce very little noise. || Stepper motors produce a slight hum due to the control process. However a high quality driver will decrease the noise level. |
|Resonance and Vibration || Servo motors do not vibrate or have resonance issues. || Stepper motors vibrate slightly and have some resonance issues because of how the stepper motor operates. || |
|Availability || Servo motors are not as readily available to the masses as are stepper motors. || Stepper motors are far easier to find than quality servo motors. |
|Motor Simplicity || Servo motors are more mechanically complex due to their internal parts and the external encoders. || Stepper motors are very simple in design with no designed consumable parts. || |
|Direct Drive Capability || Servo motors usually require more gearing ratios due |
to their high RPM. It is very rare to see a direct drive servo motor
| Stepper motors will work fine in |
direct drive mode. Many people simple use a motor couple and attach the
motor shaft directly to the leadscrew or ballscrew.
|Power Range || Because servo motors are available in DC and AC servo motors have a very wide power availability range. || The power availability range for stepper motors is not that of servo motors. |