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Stepper vs. Servo motors, what is the difference?

Two types of motor technology exist in the laser industry, one is a Stepper motor and another is a Servo motor. Both technologies are employed in laser engraving machines, but what exactly is the difference between these two control approaches? Which one is better for your equipment?

Stepper Motorsimage-motor-stepper

Stepper motors are relatively inexpensive, and provide the same or greater accuracy as servo motors. Sufficiently powerful stepper motors for a given application do not lose steps. Stepper motors are no more likely to lose steps than a servo encoder is to pass bad information back to the controller. This hoax is pure hype generated by manufacturers who are either unfamiliar with stepper motors, or who are distorting facts to market their particular product. If for some reason a stepper motor does encounter an obstacle it can’t overcome, such as a thick piece of material, it will simply skip steps, hurting nothing. If a servo motor encounters the same obstacle, it will sit there and fight itself until it breaks a gear tooth or burns up.

If a stepper motor goes bad, which can happen to both types of motors, replacement cost is under $200. No elaborate tuning process is needed to keep it functioning accurately. Best of all, its excellent low rpm torque lends itself perfectly to shape cutting, without the need for backlash producing gearboxes. The low speed torque makes it possible to use a timing belt and pulley reduction, with virtually no backlash.

bunny-tortoise

Rapid traverse moves faster than 80 inches per second are virtually never used, as the typical laser tube cannot fire fast enough to keep up with a faster moving motor. Also the vast majority of laser cutting movements with any motor is at very slow motor speeds, to allow for clean and smooth edge quality cutting through the entire thickness of the material. Also the distance between pieces being cut rarely provides sufficient time for ramping (acceleration and deceleration). Don’t be taken in by claims of super fast speeds. The more important issue in laser engraving and especially laser cutting applications is how slow and smooth a move a machine can make.

The use of dual Y axis motors is easier and more precise with stepper motors than with servo motors, which require instructions to be passed along from motor to motor in kind of a daisy chain, rather than directly to both motors from the controller. With steppers, the two motor drivers receive identical simultaneous instructions from the computer, one in reverse of the other, reducing the chance of miscommunication.

Servo motors

image-motor-servoServo motors are much more expensive than steppers — perhaps double to even triple the price, or more. They are generally just as accurate, if maintained in a proper state of tune, however they rely on encoders to provide positioning information back to the computer. Thus the complexity of the system is at least doubled, with no accuracy advantage, greater initial cost, and more maintenance issues. The “closed loop” rhetoric that some manufacturers play up sounds convincing to the uninitiated, but provides no benefit over a simpler and more reliable stepper system.

Servo motors are available in larger sizes than stepper motors, and powerful servos are generally used on heavy machines with gantry carriages in the 500 to 1,000 lb range. They offer no advantage whatsoever on lighter machines, such as a CheckMate Laser system or even its competitors.

Stepper vs. Servo Motors

The following will attempt to justly compare stepper vs. servo motors assuming the following:

  • The motors are of equal rated power
  • Both motors compared are of same quality
  • Servo is equipped with an encoder
  • Stepper is not equipped with an encoder
  • The driver is assumed to provide the same features excluding feedback options
  • The Servo motors are of DC Brushed type

Keep in mind that this is a comparison only and not a determination on which motor is “better.”. Both motors should be viewed as separate technologies, both of which are capable motors given the correct setup and use.

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 Laser applications. Stepper motors are also very versatile in their use for automation and Laser applications. Because of their simplicity stepper motors may be found on anything from printers to clocks.
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. 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 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 damaged by mechanical overloads.
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 setup. 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.

The green selections in the Stepper (11) vs Servo (11) comparison chart above are the selected category winner, which results in a tie. Of course, its always up for debate depending on the motor and usage of the motor.

The stepper vs. servo motor debate is a hot topic among some Laser manufacturers. You may see many websites describing why the motor they use on their product is better. Our feeling is that either motor is a capable choice given the right setup. The “debate” of stepper vs servo motors is pointless. Use the information above to help guide your decision. Most will be swayed by cost alone.

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