Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: pofo on September 23, 2010, 12:46:12 AM
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Has anyone used an Acu-Rite glass scale with the Position-Trac feature as a positioning system on Mach 3? Position-Trac encodes the absolute position of the axis into the reference output.
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I really don't think there would be any point in doing this unless you are saying that the accuracy of servo mounted encoders is not good enough.
Tweakie.
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Tweakie,
In this system (Millpwr), the glass encoders are used for the feedback loop. There are no encoders on the motor. But that function is ok. I am able to get pulses for the x and y movement.
What I would like to do is use the reference output of the glass encoder to home the axis. On this encoder, they have a reference track that has reference pulses somehow encoded such that the absolute position of the table can be determined. I thought I just had one pulse in the middle of the table that I could use as a reference for home, but instead, I have many pulses I don't know what to do with. So I need to know how to use the reference track. They call it Position-Trac. I'm guessing that the reference pulses are somehow spaced differently along the scale so that the encoder pulses can be counted between reference pulses to determine where you are; Meaning the distance between reference pulses is different along the scale so that the count would increment by one for each reference pulse. It's just a guess and I haven't the means yet to count pulses to see if that is true.
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Right - I see what you are trying to achieve now.
As far as I am aware Mach does not operate in closed loop mode.
With servo systems the feedback loop is closed within the servo driver and errors corrected at that point rather than within Mach itself.
This does not mean to say that Mach cannot be operated in closed loop mode but you might just have to be the first person to do it :)
Good luck,
Tweakie.
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Understood about Mach. I'm using a DSPMC controller with Mach that is closed loop. I'm posting here just in hopes that someone has come across this and has the information.
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Can the DSPMC not do index homing?
The way my servo drives handle it is I get Mach to tell them to home, the drive moves the motor until it sees a home switch then it seeks out the index pulse and sets home, then the drive informs Mach it has homed and Mach sets the machine coords.
Hood
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Good idea. I don't have my DSPMC yet, but that sounds like it would work.
But I'd like to take advantage of the absolute positioning and not have to put in limit/home switches. I'd like the controller to read its current position and then know directly from that, which way to go to get home.
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I'd like the controller to read its current position and then know directly from that, which way to go to get home.
My drives do that, if the switch is active they will back off until its inactive then will seek the switch again. So all that is needed is that the switch is activated and kept that way in that direction.
Actually my drives can be configured many ways for homing but thats the way I choose.
Hood
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I want to do it from a cold start, when the controller has no idea where the table is. That is the purpose of the Position-Trac coding. When you boot up, the controller doesn't know where the table is and with Position Trac, it can determine the absolute position of the table by reading the Position Trac coding.
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Mine can do it from a cold start, granted I have to reference the axis but the switch is either active or inactive so the drive knows which way the initial move should be.
It may be possible for the DSPMC to read the coding on your scales but afraid I know nothing about the DSPMC's inner workings.
Hood
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IF memeory serves me someone already did it. The system uses a speacially marked scale that has a positional pattern encoded into the scale ouput.
The system used a math calculation from the signal to decode its location. From a cold start all you had to do was start a slow move and as soon as it passed one of the signals you could determen where you were at then set that as current location and then you had to move to Machine zero and reset Machine coords.
OR something similar
I think is was on the yahoo forum years ago. I will try to find the info I have a copy of it somewhere here. SOMEWHERE
(;-) TP
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Yes! That is it. It would be great if you can locate it. I've been searching with no luck.
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What model scales do you have? Have you tried acurite support to explain the sequencing???
Once you understand how the position sequenced itself the rest would be easy.
(;-) TP
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I found part of the discussion it was for a differrent manf http://www.heidenhain.com/fileadmin/pdb/media/img/571_470-24.pdf
But the process in Mach should be the same. The trick is in how you reset Mach's machine zero value so it represents the calculated encoder position values.
Mach support discussion:: http://www.machsupport.com/forum/index.php/topic,9771.0.html
Still looking for the other discussion BUT i am afraid it went to an email discussion so it MAY be gone by now.
(;-) TP
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OOPS that was not a different MANF it was a different model.
(;-) TP
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Thanks. Those two links are big help. The Heidenhaim link is enough to get me going. Acurite was no help, which I don't understand. How can they sell a product but not have a tech sheet explaining one of the features.