question about CNC stepper motors ...
- Thread starter WilleeCue
- Start date
RBC
Deceased
Your wallet will tell you.
Actually Joey, there are some servo systems you can get into these days for not much more than a higher end stepper system.
Here's a question for you.
Which motors have a greater following error, servo's or steppers?
Royce
RBC
Deceased
Joey
Actually no, it's servo's.
Consider this.
Steppers are almost always 200 steps per revolution, or 1.8°. Since each step is basically a magnetic pole, it can't get more than about 35 to 40% behind, or it will jump to the next pole and be out of position.
Servo's are always behind. The command goes out for movement, and the motor starts to turn. Then the feedback comes back to the drive for it to make corrections. It's always behind because it can only react to what's already happened. There are drives that can learn and be pro active, but that's only for canned type cycles that are repeated over and over again.
Now you also have to keep in mind that servo's are typically much higher rpm motors, so they can use more gear reduction. Steppers are low rpm and can't reach the high rpm's so they typically use much less gear reduction.
In the end, a good stepper system can be as accurate, in lighter load conditions, as is really necessary. A good servo system can operate faster, and still hold very tight tolerances even under heavy loads.
A good stepper system will do all we need and more. But it's really cool to see a good fast servo system really working quick.
Royce
RBC
Deceased
Joey
I forgot to talk about the feedback.
Servo's have some kind of encoder feedback to tell the drive and/or controller where the axis is and how fast it's going.
In many cases, the encoder is either on the end of the motor or on the ballscrew. This works pretty well, but any backlash or other mechanical slack or play can still cause it to be out of position. Play can also cause all kinds of tuning issues because the load is inconsistent. I have 2 servo machines where the encoders are on the motors. They both have high quality ground ball screws so there isn't any backlash. I have one where the encoder is on the end of the ballscrews which are belt driven with gear reduction to the motor. Both setups are similar and work very well. My CHNC has resolution to .00001". I dare anyone to try to hold wood and phenolic to those tolerances!
With higher end machines, the encoders are linear. Basically, they are mounted along the axis of travel similar to the glass scales used on DRO's for a manual machine lathe or mill. These linear encoders can read down to microns in movement and because they are directly coupled to the actual movement of the axis, they are considered to be "true closed loop". Backlash and such won't cause any real misspositioning, but can certainly cause tuning issues if it's bad enough. I don't have any machines like that.
I do have an old pick and place machine that actually uses linear motors. No screws, belts or anything. It's a row of magnets with a puck, the stator, in the middle that moves with the axis. It does use linear encoders. But I'm a long way away from trying to figure out how to drive all of that. It would be overkill for a cue machine anyway.
Royce
I forgot to talk about the feedback.
Servo's have some kind of encoder feedback to tell the drive and/or controller where the axis is and how fast it's going.
In many cases, the encoder is either on the end of the motor or on the ballscrew. This works pretty well, but any backlash or other mechanical slack or play can still cause it to be out of position. Play can also cause all kinds of tuning issues because the load is inconsistent. I have 2 servo machines where the encoders are on the motors. They both have high quality ground ball screws so there isn't any backlash. I have one where the encoder is on the end of the ballscrews which are belt driven with gear reduction to the motor. Both setups are similar and work very well. My CHNC has resolution to .00001". I dare anyone to try to hold wood and phenolic to those tolerances!
With higher end machines, the encoders are linear. Basically, they are mounted along the axis of travel similar to the glass scales used on DRO's for a manual machine lathe or mill. These linear encoders can read down to microns in movement and because they are directly coupled to the actual movement of the axis, they are considered to be "true closed loop". Backlash and such won't cause any real misspositioning, but can certainly cause tuning issues if it's bad enough. I don't have any machines like that.
I do have an old pick and place machine that actually uses linear motors. No screws, belts or anything. It's a row of magnets with a puck, the stator, in the middle that moves with the axis. It does use linear encoders. But I'm a long way away from trying to figure out how to drive all of that. It would be overkill for a cue machine anyway.
Royce
Thanks for the great scoop Royce.