Author Topic: Liener scale Feedback  (Read 1054 times)

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Offline Boots

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Liener scale Feedback
« on: September 24, 2018, 11:14:28 PM »
I have an Enco 10x54 Mill and have 4 Mitsubishi servo motors and drives  (HF-KE73KW1-s100). I am very interested in Closed Loop using Linear scales. Is it possible to use the linear scales as the feed back? Does it matter which linear scale you use; Optical or Magnetic ? etc. Please explain how to set it up. I am planning to purchasing the 7766 stepper, 7737 and the pendent. What else will we need? 


Online joeaverage

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Re: Liener scale Feedback
« Reply #1 on: September 25, 2018, 08:06:37 AM »
Hi,
why? Are the servos not equipped with encoders?

Craig
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Offline Boots

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Re: Liener scale Feedback
« Reply #2 on: September 25, 2018, 12:54:49 PM »
The encoders on the servos are fine. I want to use linear scales as the feedback loop because it will give a true position of the table, backlash, belt moment etc. The encoders feedback is good but does not take into consideration the actual position of the table. I saw in the documentation that you can use liner scales as feed back, is this correct? please show how it would be setup and which linear scales would be compatible - thanks.

Online joeaverage

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Re: Liener scale Feedback
« Reply #3 on: September 25, 2018, 02:51:01 PM »
Hi,
yes it is possible and it is done for exactly the reasons you have mentioned. It does complicate the issue in a big way.
The Hicon when the feedback part is enabled will probably do it.

Galill controllers are known for their applicability to these situations, they are expensive.

I use C5 grade ground ballscrews which are accurate to less than 18um over 300mm and a cyclic accuracy of less than 8um.
As a consequence if I know the rotational position (encoder) of the screw I have a fairly accurate notion of where the table is. Most industrial
CNC machinery operates that way with C5 or C3 ballscrews.

Craig

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Offline Boots

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Re: Liener scale Feedback
« Reply #4 on: September 25, 2018, 05:02:43 PM »
Hi Craig,
Thanks for the feed back - "When you say it complicates the issue in a big way" please explain more, I want to know what I am getting in to. Also do you have a block diagram of how it would be connected? I assume the 7766 board to the 7737 to the motors and then linear encoders back to the 7766? - thanks again for responding

Online joeaverage

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Re: Liener scale Feedback
« Reply #5 on: September 26, 2018, 03:51:01 AM »
Hi,
I do not have a Hicon so what follows is my best guess, there would need to be several confirmations of various features
before this could be considered practical.

In the diagram attached note that the Hicon unit (which includes the add-on boards) receives Step/Direction input from Mach and compares
that to the Linear Encoder feedback. The Hicon would be required to process that via its PID processor and output an analogue voltage
to your servo/drive. The servo/Drive would have to be in Velocity mode. The rotary encoder of the servo would still be hooked up to the drive
per normal, its just that it is not used as a positional measure but rather velocity measurement. The output shaft is connected to the axis and finally
the linear encoder is connected to the axis and measures the table position.

This arrangement is shown with the Hicon but could equally describe a Galill controller..

Lets take a few components and look at the requirements to make it work.

The first is the Linear Encoder. It will be hooked to the Hicon so it will require an output that the Hicon can decode, namely A and B quadrature
channels and probably an Index signal. This could be a problem, I have seen linear scales with quadrature output but don't recall seeing one with an Index
signal and yet I suspect that the Hicon requires it. This would best be discussed with Vital Systems.

The second issue, still related to the Linear Encoder is the quality of the encoder itself. Have a look at Hidenhan and Reinshaw websites to see what they
offer, and make sure you are sitting comfortably when you read the price. What sort of corners have to be cut by Chinese (and others) manufacturers
to sell linear scales at the prices they do?

I'm of the opinion that to get decent linear scales, not Heidenhan or Reinshaw, just decent ones is likely to be quite expensive, I'm guessing $200 per axis.

The Hicon itself is not cheap, the basic board is $600 and the addon board and activation cost another $600.

I assume you already have analogue input servos and drives?

What is not certain is that the Hicon can do the PID part without additional on-board programming. The Hicon does offer that low level programming
but could require some very clever programming on your part. There again I would rely on Vital Systems advice as to what is possible 'out of the
box' and what extra may be required to get this setup to work.

Note that this general arrangement is not uncommon, it is uncommon in CNC lathes and mills, but is used in semiconductor wafer processing.
Semiconductor machines require extreme precision. The absolute best (Heidenhan or Reinshaw) linear scales are probably only just good enough.
Some use LVDT (Linear Voltage Differential Transformer) transducers and all the high precision (and tricky) stuff use laser interferometry. Zeeman splitting/stabilization
will get you to about 10nm and iodine cells are required to get you to a few nm.

I make the point that in those situations where linear position feedback is employed the feedback components (scales/LVDTs/interferometers) are very
VERY expensive. While you may not require the sort of precision these technologies seek to achieve I think you can rely of the fact that you will spend
significant money trying to do linear feedback. I suspect rather more than it would cost to fit C5 or better ballscrews.

Craig

My wife left with my best friend...
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Offline Boots

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Re: Liener scale Feedback
« Reply #6 on: September 26, 2018, 03:17:26 PM »
Hi Craig,

Thanks for the info - and sharing your understanding of the tech involved. I will talk with Vital about the index signal and see if it is needed. Sometimes your mind takes you places reality can't follow. This maybe our problem here;  a great idea but not realistic. Again thanks for all your input, you have been very helpful. Ill follow up with Vital - you guys are the best!

With Respect, Boots-2b1

Offline a3dcreator

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Re: Liener scale Feedback
« Reply #7 on: October 19, 2018, 06:47:28 PM »
I have  a Hicon with closed loop and magnetic linear scales.
It works. The annoying thing is that when you are jogging you don't want this feature on. because it tries to compensate each time you try to jog. Causing a small jump every time you do so. That is my only gripe with the whole system. There is no easy way to toggle this feature on and off. As far as i know.

Offline Boots

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Re: Liener scale Feedback
« Reply #8 on: October 21, 2018, 10:08:34 PM »
Hi A3dcretor,
Thanks for responding - I am planning to setup the 7766 and 7737 with linear scales also. A couple of questions. When you wired it are you coming for the servo back to the 7737 board and then bring the linear scale output to J7 on the 7766 Board, how did you wire it? also the small adjustments do they affect the normal running of the Gcode programs, such as an arc, does it create problems?

Offline a3dcreator

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Re: Liener scale Feedback
« Reply #9 on: November 30, 2018, 02:56:50 PM »
Boy I am really sorry i never replied. I didn't get a message from the server stating there was a reply to this thread.

If you have Linear scales you want to go from the Linear scales to the 7737 board. You also have to make sure your linear scales are relative, not absolute. They also have to be quadrature encoders.

My setup is a little different than yours. I have clearpath servo motors. they keep track of their own positions. So All i do is connect my linear scales to the 7737 board.

it does affect the gcode. thats the whole point. after every move it compensates for the error so that there is no large build up as the code runs. It does not create problems. In my experience its actually better with thatn without.