Motion control experts

Just wanted to see if there's any recommended resources for motion control education.

I have an application I'm working on with a linear motor for a testing machine.

I've tried various ways of doing this to this point but trying to understand how to best calculate my position target based on a varying frequency.

This test will oscillate from 0.1hz up to 500hz frequency during its cycle.

The max travel is set at 220mm this is a hard max limit (the actual value will be inversely proportional to the frequency)

The other hard limit is 2G acceleration for all tests.

Complex problem for me as a motion noob. Any tips appreciated.

Why not buy a motion controller that has swept sine wave capability and is meant for testing applications?
An RMC70 will do this easily.

https://www.youtube.com/watch?v=Zu3MC4LJpu8&t=88s


What often gets screwed up is the design of the test system. My motion senses say OH-OH when the sine waves go over 10 HZ. It is easy to shuffle electrons around at 500 Hz not physical things.
Hardly anybody gets this right.
The peak acceleration is equal to the amplitude*(2*PI*HZ)^2
So even if the amplitude is reduced to 0.001m or 1 mm at 500 Hz the acceleration is about 1000g. In addition, the natural frequency needs to be about 4 times the 500 Hz.


I/we have a lot of experience with testing systems. Few get it right or understand the limitations.

More information...
This is on an existing test system which is changing brands of its motion controller.

My goal is more or less at the very least replicating, but hopefully improving (in some way/shape/form) the functionality.

Appreciate the feedback.

Does the current system go to 500Hz?
If so, at what amplitude?
What feed back are you using?
I doubt the system is moving at 500 Hz in closed loop. If only injecting at 500 Hz signal, how do you know the actuator is really moving at 500 Hz?

I can think of only 1 thing I have that gets close to 500Hz motion (besides the obvious audio speaker which can get to 20kHz). I have an oscillating tool that is rated for 20,000 Oscillations per minute (OPM).

https://www.makitatools.com/products/details/XMT03Z

That’s roughly 333Hz. Some are rated a little faster at 21000 or 22000 OPM but I couldn’t find any near the 30000 OPM that would equate to 500HZ.

Peter Nachtwey said:

Does the current system go to 500Hz?
If so, at what amplitude?
What feed back are you using?
I doubt the system is moving at 500 Hz in closed loop. If only injecting at 500 Hz signal, how do you know the actuator is really moving at 500 Hz?

Click to expand...

Because this system is in a lab environment currently, they use a secondary system to record and verify all of the commands vs actual. This cabinet obviously isn't part of the end result but is great to have for development purposes.

for the device we tested initially, at 500hz with a 2g acceleration limit, the amplitude was .0002mm.

The motor has a encoder obviously which I was lead to believe is used, but they also use a linear glass scale for position feedback. I don't know which brand or what Model (I will be asking this today).

p.s. these questions are helpful to me in considering things I overlooked while observing so thanks for that.

linear scale used has a .002mm period.

I had this in my notes from my initial review.

What does a 0.002mm period mean.
I think of terms of range of counts and resolution.
Also, how do measure amplitude of 0.0002mm? You need nano meter range resolution.
Also, with 0.0002mm amplitude and 500 HZ the acceleration is 0.2 g. Is there an extra zero somewhere?


If all you want is a vibration it may be easier to make a cam with a lobe the size you want and rotate it at 500 Hz similar to what is done in engines.

I hate guessing but I'm guessing 0.002mm is the period of whatever the transducer is physically measuring. Most likely a glass scale for stability.

The image is from a Heidenhain brochure for very high accuracy scales. Notice that the period isn't directly proportional to accuracy. The way the feature is interpolated has a play in it as well. Here is one example of scale like this.

Heidenhain LIP style scale

There are probably other devices made for labs to measure these super small motions but I'm not aware of them.

I want to know what the application is. This is definitely not a normal application.
Our controllers are used for many testing applications. When the application requires high frequency, normal closed loop control is not possible. Instead the controller simply generates a sine wave at what the desired frequency is and measures the response. This is open loop. One of our guys uses a library he download from fftw.org to process the data. I check his results with the python library that isn't as fast but it is good for verification. This is definitely not done on a PLC.

I agree Peter. He says:

for the device we tested initially, at 500hz with a 2g acceleration limit, the amplitude was .0002mm.

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They are not controlling amplitude. Just measuring what it is. But he didn’t say if they actually reached 500Hz or not. Is that the excitation signal or the DUT frequency? Too many gaps.

Again we don’t have the information we want. They are looking for improvements but improvements in what?

We have used the adaptive control routine in the Delta controllers with good success. I have even taken it further where I phase multiple adaptive tests so the peaks never line up. This way the flow demand on the system stays relatively constant. Otherwise if the peaks line up, there is a huge demand for flow to the system.

Sorry for the delay getting back.

Correct to both peter and norm, the amplitude at the max(Fixed) acceleration is 0.0002.

From a function standpoint there's not really anything to improve as it's been working for some years as I understand. The customer has pointed out an annoyance with the messaging back and forth with the PC and controller that I can remedy.

The machine is testing transducers to verify their outputs are within specifications.

I need to see if I can go to the customer again to see the machine. I've only seen it once and through this process I've developed a lot more questions about it.

@Peter your cam suggestion came across my mind earlier as well, but I had not yet used the software tool yet. spent some of the day today getting familiar with it. This will probably be the next route I check.

I am trying to make it as friendly as possible for any changes that may need to be made in the future.

dwoodlock said:

From a function standpoint there's not really anything to improve as it's been working for some years as I understand.

Click to expand...

I really doubt it has been working as you say.

The customer has pointed out an annoyance with the messaging back and forth with the PC and controller that I can remedy.

Click to expand...

That may be what irritates the customer the most but making physical things move at that speed is very difficult. The motion controller is only moving electrons. It is the motors and mechanics that are difficult to move.

The machine is testing transducers to verify their outputs are within specifications.

Click to expand...

They must be using a glass scale or magnetic tape.

I need to see if I can go to the customer again to see the machine. I've only seen it once and through this process I've developed a lot more questions about it.

Click to expand...

Yes

@Peter your cam suggestion came across my mind earlier as well, but I had not yet used the software tool yet. spent some of the day today getting familiar with it. This will probably be the next route I check.

Click to expand...

I didn't make myself clear. It has often been suggested to me "why use a motion controller at all?"
Use a set or pair of physical cams. Two cams can be machined that will make a physical thing move between them in a sine wave. The two cams will be 180 degrees out of phase so they will take turns pushing the object. This is kind of like a car valve only it is more like a desmodromic mechanism that pushes in two direction unlike a car valve that has a cam the pushes in one direction against a spring.

Obviously a separate pair of cams must be machined for difference amplitudes. A motor then needs to spin the cams at the desired speeds/rpm. This is a simple brute force approach. The down side is the mechanical setup time between testing different amplitudes. However a machinist/mechanic can make the changes. To achieve 500Hz the cams would need to be spinning at 30000 RPM which is still a lot.