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STEPPERS OR BRUSHLESS SERVOS?

We supply both stepper and brushless AC servo motors and drives for machine automation. We do not supply DC brushed servomotors and drives which are older technology. Both steppers and brushless servomotors are similar in construction but the servomotors have feedback devices which enable closed loop operation. There are a lot of varying opinions about the pros and cons of either style of motor, so this will help you decide which to use. It isn't as simple as one being better than the other. Overall machine functionality is highly dependent on the controller and software and is more important than just comparing one style of motor with the other.

Simplicity
Stepper systems are easier to build, install and maintain because there are no feedback devices. This means fewer parts and connections to fail.

Cost
A stepper system will always be much lower in cost than a brushless servo system of the same torque capability.

Feedback
Brushless servomotors have feedback devices so the system will run in closed loop. This means that if an unexpected load is applied, the motor will correct its position. Stepper motors are usually run in open loop without feedback, although advanced controllers sometimes have encoder feedback. For this reason stepper motors are usually selected and run with a torque safety margin.

Because servomotors have feedback devices, they can be driven closer to their maximum torque capacity. They can also run at much higher speeds. However, this is only feasible if your mechanical components such as ballscrews and gearboxes will permit it.

Feedback is good but not always perfect. Because a servo system is driven by an error signal (difference between actual and required position) there can be following errors. Servo systems can also have overshoot and dither problems, which do not occur when using steppers motors.

Inertia Sensitivity
Servomotors are more sensitive to changes in load mass. Normally, the machine components are much heavier than the load, but when heavy loads are applied, servomotor tuning can be affected.

Resolution
Brushless servomotors usually have a resolution of between 500 and 4000 counts/rev. Two phase stepper motors are manufactured with a nominal resolution of 200 steps/rev. However, by switching and regulating motor currents from the drive, 4000 steps/rev can be achieved. Some stepper drives are capable of resolutions as high as 50000 pulses/rev.

High resolution in stepper motors is good for very smooth motion, but it is not always perfect. Very high resolution means the controller producing pulses must switch very fast and this is not always possible. It is also impractical to aim for the highest resolution possible (like 50000 steps/rev) when errors caused by mechanical components (gear backlash, belt stretch, ballscrew end play) will be many times greater than one motor step.

Digital Control
Because the movement of a stepper motor is defined by the number of pulses fed into the drive, they are ideally suited to digital control from computers and PLCs and other digital circuits. Most brushless servomotors use an analog +/-10V control and resolver or encoder feedback, requiring a more sophisticated and costly controller.

Standardisation
Nearly all stepper motors conform to the NEMA flange dimensions so they can be easily be replaced, even between different brands.

Speed
Stepper motors can run between 0 and 3000 rpm, although continuous running should not occur over 1500 rpm or motor overheating will occur. Brushless servomotors can usually run at continuous speeds of 3000 rpm without this problem and in some cases, upto 6000rpm.

Noise
Is quiet operation necessary? Stepper motors are inherently noisy. This won't be a problem in a noisy factory, but could be very annoying in a laboratory environment so attenuation may be necessary. Brushless servomotors don't have this problem.

Shaft Power
The largest stepper motors available are 65 frame size, 3 stack. This motor, with the right drive, can deliver around 2000 Watts of shaft power. Brushless servomotors can provide much higher power.

Power Consumption
When in motion, stepper motors have their full rated motor current through the motor windings, regardless of load requirement. Even while at rest, there is still some holding torque current, locking the motor shaft. In situations where energy is limited (eg. battery powered applications) this is not efficient. A servomotor will only consume current on demand, depending on the difference between theoretical shaft position and actual position as sensed by the encoder.

AUTOMATED MOTION SYSTEMS PTY. LTD. - WESTERN AUSTRALIA