Servo or Steppers

[BAR553]

Can someone explain the differance between steppers and servo's. Pro's con's etc.

[vger]

This question has been gone over and over....

Here is a link to a thread concerning that... and a quote from one of the posts...

http://www.cnczone.com/forums/linear...epper_cnc.html

Top Ten Stepper Advantages:

1) Stable. Can drive a wide range of frictional and inertial loads.
2) Needs no feedback. The motor is also the position transducer.
3) Inexpensive relative to other motion control systems.
4) Standardized frame size and performance.
5) Plug and play. Easy to setup and use.
6) Safe. If anything breaks, the motor stops.
7) Long life. Bearings are the only wear-out mechanism.
8) Excellent low speed torque. Can drive many loads without gearing.
9) Excellent repeatability. Returns to the same location accurately.
10) Overload safe. Motor cannot be damaged by mechanical overload.

Top Ten DC Servo Advantages:

1) High output power relative to motor size and weight.
2) Encoder determines accuracy and resolution.
3) High efficiency. Can approach 90% at light loads.
4) High torque to inertia ratio. Can rapidly accelerate loads.
5) Has "reserve" power. 2-3 times continuous power for short periods.
6) Has "reserve" torque. 5-10 times rated torque for short periods.
7) Motor stays cool. Current draw proportional to load.
8) Usable high speed torque. Maintains rated torque to 90% of NL RPM
9) Audibly quiet at high speeds.
10) Resonance and vibration free operation.

Top Ten Stepper Disadvantages:

1) Low efficiency. Motor draws substantial power regardless of load.
2) Torque drops rapidly with speed (torque is the inverse of speed).
3) Low accuracy. 1:200 at full load, 1:2000 at light loads.
4) Prone to resonance. Requires micro-stepping to move smoothly.
5) No feedback to indicate missed steps.
6) Low torque to inertia ratio. Cannot accelerate loads very rapidly.
7) Motor gets very hot in high performance configurations.
8) Motor will not "pick up" after momentary overload.
9) Motor is audibly very noisy at moderate to high speeds.
10) Low output power for size and weight.

Top Ten DC Servo (brush type) Disadvantages (besides higher relative cost):

1) Requires "tuning" to stabilize feedback loop.
2) Motor "runs away" when something breaks. Safety circuits required.
3) Complex. Requires encoder.
4) Brush wear limits life to 2,000 hrs. Service is then required.
5) Peak torque is limited to a 1% duty cycle.
6) Motor can be damaged by sustained overload.
7) Bewildering choice of motors, encoders, servo drives.
8) Power supply current 10 times average to use peak torque. See (5).
9) Motor develops peak power at higher speeds. Gearing often required.
10) Poor motor cooling. Ventilated motors are easily contaminated.

Steve

[Al_The_Man]

The description of DC servo's #4 can be a bit misleading as evidentally the 2,000hrs was based on 2000hrs at Continuous Current rating of the motor.
In actual practical usage it is not uncommon for them to run for 10~15yrs on a set of brushes.
Servo's are closed loop through a PID loop, and can position to the finest resolution of the encoder.
Steppers are open loop and can only position to the nearest step or half step.
Both servo's and steppers have maximum torque at zero rpm, servo torque curve is fairly flat up to maximum rpm, stepper torque will fall off as rpm increases, steps can be taken to a degree in stepper drive design to maintain correct current in order to maintain rpm.
Al.

[packrat]

Originally Posted by Al_The_Man Servo's are closed loop through a PID loop, and can position to the finest resolution of the encoder. Steppers are open loop and can only position to the nearest step or half step. Al.

Steppers are some times also used with encoders/closed loop.

cary

[vger]

Al,

Very true about the brush life... There is a servo driven machine I have been maintaning for about 20 years and the brush life seems to be about 5 years. The machine has 2 servos that drive multiple flat belts on a pair of tables to transport sheets of paper (held down by vacuum) into position for microfilming. The servos run from dead stop to about half speed then to a stop (opto sensor controlled) fairly quickly. It can flip the sheets and film both sides much faster than a person can feed the individual sheets. No acceleration / deceleration, just flat out run, stop, run, stop. It's pretty hard on the servo's and the mechanicals, but the brushes seem to last about 5 years. The machine is soon to be scrapped so guess who gets the guts!

Cary,
Yes very true, but still limited to the minimum step size of the steppers/driver. I've got 4 fractional stepper drivers that do only 25,000 steps / rev with standard 200 step motors. I've thought of using those with some 2000 ppr encoders and a custom step/direction circuit to do closed loop, but haven't found the time.

Steve

[BAR553]

Thanks

Simple and plug and play are my needs today..........