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PhenixRising
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Posted: 02:04pm 16 Sep 2024

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My question:

Quote As a for-example, what is a typical PID sample time for a machine-tool axis?

ChatGPT:

Quote
A typical PID sample time for controlling a machine-tool axis can range from 1 millisecond (ms) to 10 milliseconds (ms), depending on the specific application, the performance requirements, and the hardware capabilities. Here’s a general breakdown:

1 ms (1000 Hz): This is typical for high-precision CNC (Computer Numerical Control) machine tools or robotic systems where precise and fast motion control is crucial. For such systems, the machine must respond quickly to minimize errors, and faster sampling helps maintain accurate control of the axis position and speed.

5-10 ms (100-200 Hz): Many general-purpose machine tools, such as milling machines, lathes, or industrial robots, operate with a sample time in this range. It provides a good balance between performance and system resources, ensuring stable control without overwhelming the processing system.

Factors Affecting PID Sample Time:
Machine Tool Type: High-speed, high-precision tools (e.g., in aerospace or automotive manufacturing) require shorter sample times.

Control System Performance: Faster PID loops require more computational resources, so the sample time depends on the processing capability of the control system.

Axis Dynamics: Faster-moving or more responsive axes (e.g., lightweight gantries or spindles) may need quicker sample times to maintain control stability.

Error Tolerance: Applications that require tight positional accuracy demand faster feedback to minimize the time the system takes to correct errors.
In summary, for a typical machine-tool axis, a sample time between 1 ms and 10 ms is common, but more precise applications lean towards the lower end of that range.

I asked this because I have, historically always used a 1KHz sample time...just because it's common but I have been playing around with 250Hz (settick 4) sample times and experiencing a surprisingly nice response.

There are endless publications (I think I have them all

) re: the actual PID but sample-rates are rarely covered.

I have an app-note, somewhere, about positioning to a few nano-metres

This was a piezo-electric motor on air-bearings. Galil-Motion have the capability of reaching 16KHz sample-rates but they found that there was nothing to be gained beyond 4KHz.
Our PicoMite can handle 1KHz without breaking a sweat.
Edited 2024-09-17 00:07 by PhenixRising

Andy-g0poy
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Joined: 07/03/2023
Location: United Kingdom
Posts: 75

Posted: 03:08pm 16 Sep 2024

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This will depend entirely on the machine.

If the machine can move the head at 1M/s the a sample rate of 1Khz will only give you a resolution of 1mm which is no good at all.

This is assuming that you are talking about driving the head directly. Things change if the machine does some of the work, and you tell it to move 100mm then you wait until it says done. Sample rates don't matter so much then.

General machining will want to be good to 0.001mm so you need to use that with the machine speed to work out the sample rate. Higher precision work could go down to 0.0001mm then after that you get to things like the pizo controlled systems.

Generally the slower the machine moves, the better resolution you get for a particular sample rate. It's not a one size fits all situation.

Andy

PhenixRising
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Posted: 03:41pm 16 Sep 2024

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Andy-g0poy said This will depend entirely on the machine.

If the machine can move the head at 1M/s the a sample rate of 1Khz will only give you a resolution of 1mm which is no good at all.

This is assuming that you are talking about driving the head directly. Things change if the machine does some of the work, and you tell it to move 100mm then you wait until it says done. Sample rates don't matter so much then.

General machining will want to be good to 0.001mm so you need to use that with the machine speed to work out the sample rate. Higher precision work could go down to 0.0001mm then after that you get to things like the pizo controlled systems.

Generally the slower the machine moves, the better resolution you get for a particular sample rate. It's not a one size fits all situation.

Andy

That is irrelevant. Mechanical elements have a limited bandwith. 1KHz is the magic 10 X multiplier for a machine with a bandwidth of 100Hz. That would have to be a tiny machine with hardly any mass/inertia.
Edited 2024-09-17 01:46 by PhenixRising

PhenixRising
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Posted: 04:10pm 16 Sep 2024

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Machine moves are a different thing entirely:

In my case, machine-moves are incremented @500Hz.

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