Motor drive for CNC machine

[Bill]

Hello everyone,

I know that the subject of motor drives for CNC machines is less discussed in electronics, more particularly in digital electronics, and even there are not many publications on it in scientific journals, I would like those who have had experience or knowledge on this subject shares it with us, because recently I have just exploited this field of pure electronics precisely with Mitsubishi variators of the A/B/C series, I have just discovered this immense and interesting world in electronics.

Thanks !

[EPAIII]

I have some experience and have done some calculations. Where would you like to start?

In general there are two types of CNC motor controls: OPEN loop and CLOSED loop.

An open loop system determines what you want to do and then issues the necessary commands to the motor drivers to accomplish it. A system that uses stepper motors is a good example. You want to move 1mm to produce a 1mm cut. The system "knows" how many steps the motor must step to produce the 1mm movement in the tool or blank part. It then issues that many steps along with the direction to the motor controller. Finally the motor controller issues the pulsed step current to the stepper motor. What is lacking here is any verification that the desired movement actually took place. That is why it is OPEN loop: there is no feedback to ensure that the desired movement actually took place.

A closed loop system, also called a servo system will accept the same desired 1mm cut/movement information or coordinates to move to. It also has position sensors (DROs) that provide the present coordinates or relative positions of the tool and the part blank. It compares that desired position information with the present location or coordinates and generates a difference or error signal. That difference or error signal is amplified to the appropriate level and sent to the motor drivers which, in turn activate the motor in the desired direction. The control system continues to monitor the present position and generating constant difference or error signals until the desired, 1mm motion has been completed.

The essential difference is the open loop system issues commands to the motor(s) and ASSUMES the motors were able to accomplish those commands. The closed loop system measures the error between a desired position and the actual position and CONTINUES to run the motors until that error becomes zero.

That's the first thing about CNC that I think all should know. There are many more. Is there any specific aspect that you are particularly interested in?

PS: I used the stepper motor as an example of a open loop system. However, stepper motors can also be used in closed loop or servo systems. And there are some motors that have the feedback device built into the motor. Those motors are called servo motors and are usually used in closed loop systems.

[jmelson]

I'm a (minor) developer on the LinuxCNC project, and make motor drives and controllers for low-end CNC systems.  Your question is REALLY open-ended, I have no idea what part of CNC you want to discuss.  Can you narrow it down a bit?
Jon

[Benta]

Your question is REALLY open-ended

Agree. Are we talking spindle motor or servos here? Size? Power? Voltage? Speeds?...

[thm_w]

Look at their previous posts, they go nowhere: https://www.eevblog.com/forum/profile/?area=showposts;u=942464
Don't waste your effort.

[Benta]

Yeah. Close it.