What is a Servo?

[northwest]

Okay, don't look to me for an answer, this is a question, I feel I need to really understand what these are and do, my experience of servo's are those little (usually) black boxes with a pivot lever on to controlling a swash plate, rudder or aelierons on a model flying machine, these "servo's" that are spoken of in the CNC world look like complicated motors to me. and what the hell is a "closed loop stepper"? Who closed it and why?
I feel I need to understand this in order to progress my understanding of CNC further.

 

[m_c]

A servo is essentially an actuator that uses some form of closed loop control, to control it's position.
In the model servo case, the RC controller sends a varying PWM output, that tells the servo where it wants the servo to be. The servo then continually moves the actuator to keep it where it's been requested to be. If you've played with one, you'll know that if you push the arm and overcome the servo power, the servo will move it back in to position once you release the arm. This is because the servo has an inbuilt position sensor (possibly just a basic potentiometer, or possibly magnetic encoder)

This is the same as a servo motor system.
Your controller tells the servo drive what it wants, and the drive moves the motor to try and make that happen, using a digital encoder (old systems used an analogue resolver) to monitor the motors actual position.

In the case of a closed loop stepper system, basic stepper systems run open loop, where your stepper drive simple outputs the required power to cause the motor to step. The drive has no* ability to know if the motor has actually moved, hence it's described as an open loop system. It's a system that runs blind.
In the case of a closed loop stepper system, they add an encoder so the drive can monitor the actual motor position, so it knows if the motor has actually moved or not.


*I say no ability, but some better modern stepper drives can actually detect motor stalling, but they are still open loop systems.

 

[voipio]

Closed loop means there is positional feedback from something, either a transducer of some sort, or possibly a DRO output. In a closed loop system, the feedback allows the system to "know" where it is positioned, and supply drive to the motor, be it stepper or other type, to get it to where it needs to be. An open loop system has no idea what its position is. It has been designed to move x steps in order to change the position by y inches/mm or whatever. Open loop systems will usually perform some sort of startup calibration routine to determine where "zero," or some known reference position, is using a limit switch of some sort, be it a switch, opto device, or magnetic device.

 

[Screwdriver]

This Old Tony has a complete series on how his stuff works. This intro for example is excellent:

 

[Hood]

Biggest difference between a stepper and a servo when talking CNC machines is a stepper, whether closed loop or standard, has max torque at standstill and it drops with speed. A servo on the other hand usually has a fairly consistent torque throughout its speed range which is its rated torque. It will also have a peak torque where it can give more torque for short periods and this can be 3-4 times the rated torque so this lends itself well to a CNC system as you can get fast acceleration.
Another big difference is a stepper will have limited rpm, probably 1-2000rpm dependant on size where a servo can be 3 times that or more depending on the motor, I have some servos that are 6000rpm rated.

A couple of random torque/speed graphs.

The stepper you posted the last day.

One of the Servos I have on the plasma.

 

[Jack Ryan]

Okay, don't look to me for an answer, this is a question, I feel I need to really understand what these are and do, my experience of servo's are those little (usually) black boxes with a pivot lever on to controlling a swash plate, rudder or aelierons on a model flying machine, these "servo's" that are spoken of in the CNC world look like complicated motors to me. and what the hell is a "closed loop stepper"? Who closed it and why?
I feel I need to understand this in order to progress my understanding of CNC further.

A servo is a closed loop control system where the difference between something’s desired speed or position and its actual speed or position is used as a correction so that the desired speed or position can be attained. The control is continuous so that if the speed or position changes or drifts, the difference signal is used to correct it.

This applies irrespective of the mechanism that is used to effect the change.

Sometimes a system that uses a servo control system is called, in a particular context, a servo. That is an industry slang.

The servos in aircraft – particularly the servo tab and similar, are neither servos nor are they closed loop feedback control systems. It seems someone just liked the name, so they called some trim tabs servos (perhaps because it is easier to say than Flettner tabs).

Jack

 

[minimutly]

When I was in the mod servos were two 3 phase devices linked together, you turned one and the remote one followed. Used a lot in old radar systems etc. Probably now obsolete.

 

[daleyd]

When I was in the mod servos were two 3 phase devices linked together, you turned one and the remote one followed. Used a lot in old radar systems etc. Probably now obsolete.

Ah synchros and servos - all my control theory during my apprenticeship revolved around those in the early 90’s - and they were already out of date for about 30 years then

I was still working on kit up until I left in 2001 that had been designed in the 60’s and still used these for positional control. Very rugged and worked well, but could be a nightmare to fault-find if things went wrong. They could position 20 tonnes of gun turret to within a couple of seconds of arc and hold it there perfectly.

 

[minimutly]

I did my apprenticeship in the early 80s, yes they had loads of obsolete stuff, some we never bothered to learn about - but I well remember making a loom for a camera built by or from a design by a German company - something called a Kinetheodelite. Had at least two 50 pin plessey plugs to cock up...

 

[daleyd]

I did my apprenticeship in the early 80s, yes they had loads of obsolete stuff, some we never bothered to learn about - but I well remember making a loom for a camera built by or from a design by a German company - something called a Kinetheodelite. Had at least two 50 pin plessey plugs to cock up...

I think we were the last class that got taught the old dc conversion machinery, there were no more apprentice classes that came through for about 2 years after us (normally 3 intakes a year) and they changed the courses a lot to remove a lot of the old/obsolete stuff. They had a ward-Leonard motor-generator set that we used to teach DC speed control theory even though that kit hadn’t been used in anger for decades.