[Nasir Sattar]

I am new to the CNC Machines. I have a few questions regarding the working of CNC servo drive and servo motor.
i have read about the servo drive that after getting an input signal from the machine controller it amplify it and give output voltage to the servo motor for moving. the servo motor give feedback regarding its position to the servo drive. The differential amplifier see the difference,amplify it and keep on giving the power voltage to the motor until the input and feedback signal difference comes to zero. when the difference is zero.it stops the motor.Now my question is that linear scale transducer attached to the machine axis also monitor the position of the servo.
what is the function of that linear transducer apart from displaying the machine axis position.
1- I have a confusion that the linear transducer is used for what? although the cnc can get its position from the servo feedback.
2- How to troubleshoot a servo drive for problems?
3- Another big problem is that if there comes a problem kindly someone advise me i am confused where to start the troubleshooting.
4-Also How to install a new servo drive to the machine,
Thanks in advance

[awerby]

You're asking too many questions at once, Nasir. If you want answers, break this down into 4 separate posts, and give more information about the system you're trying to build, or fix, or whatever.

But to address the first question: in general, servo-driven mills don't use linear transducers for positional feedback; they usually rely on rotary encoders mounted on the backs of the servo motors.

[CarbideBob]

Ok, this gets a little messy.

You try to control two things in your servo loop.
First is speed, the velocity side. You want to maintain a certain speed so that your cutting rate per tooth remains somewhat near a constant.
The second is the position loop, you do not want to over or undercut the part.
(maybe not really second, for many controlling the position loop is king as you don't want bad parts)

The simplest way is to just use the motor's encoder to control both.
Here slop in the drive assembly is ignored. The error is fed back to the control and it puts out more or less power as needed to follow the path you want.
If the motor/screw + lash is off you just "tweak" you your part program to make good stuff.
Don't worry about the amplification in between, this just matches your control out to what the amps want to see to move.

Once you introduce a scale or transducer on the load the system gets a tad more complicated.
Now you have to balance the speed loop in the amps with the position loop from the real machine position.
Normally I would teach a person to work with ignoring the second scale until they can get the loop working right with the motor encoder first.

If you start with a true dual loop system things will be a bit overwhelming.
You are asking some deep questions here.
A bit more detail (amps, control type, machine, and such) on what you are trying to do may get you more help.

A servo loop depends on errors to run.
Get this first.
No error, no movement. This is how it all works.
(yes for you servo wizards I am not looking at feedforward here)
Knowing that while moving the system is never where the computer commanded it is the key to understanding how these systems work.
Past this you get deep into a crazy world of electrical and mechanical time constants.
Why is it behind, are all axis the same amount behind in the time domain? A deep rabbit hole where machine designers love to live.

There are three different control modes for most servos.
Position control, used by step/direction or some digital drives.
Velocity control, think as speed control, used by most major cnc machine builders.
Torque control, used by lower end boards and really the base control of any motor.
You move a motor by commanding power, hence torque, but you can put some of the thinking into the amplifier.
The more you put in the amp the less control you get over what is happening but you need to not think so hard on the control side.

Clear as mud, yes ?
It's not a straightforward problem. Full of weird little problems.
But you can make it work in most cases and do a decent job ignoring the fact that it won't work quite right all the time.

Mostly with a servo you command more power and it goes faster.
Too much out and you go faster than you want to, overshooting where you want to be.

Bob