Author Topic: loosing steps - captive encoder  (Read 7408 times)

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Offline tony66

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loosing steps - captive encoder
« on: May 19, 2012, 01:12:03 AM »
Hi
I'm new here
I'm having a Rotwerk mill (sieg x2) - I'm not having any problems but i found this item at CNC4PC : captive encoder for nema23/34 steppermotors
how do I implement the encoder to read the steps - give feedback to mach3 - and make mach3 to do the "adjustment" of the stepper movement?

example: mach3 moves the steppermotor 10mm => 2000 steps, the stepper "looses" 3 steps so fact is the spindle only moves 9,985mm

IDEA: the captive encoder give feedback to mach3 and moves the spindle additional 3 steps (also have to work if the stepper moves too many steps!!!)

How do I do that?

Tony
Denmark
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Offline Hood

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Re: loosing steps - captive encoder
« Reply #1 on: May 19, 2012, 05:38:48 AM »
You dont, Mach is open loop, ie it sends out pulses and expects your axis to respond.
If using servos there is a closed loop between drives and motors but if using steppers then there is no loop to close.
Hood

Offline tony66

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Re: loosing steps - captive encoder
« Reply #2 on: May 19, 2012, 02:25:37 PM »
Hi Hood

check out this link here:

http://www.cnc4pc.com/Store/osc/product_info.php?products_id=363

it does the same as servoes but with steppermotors

Tony
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Offline Hood

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Re: loosing steps - captive encoder
« Reply #3 on: May 19, 2012, 02:29:06 PM »
The link is to capacitive encoders, was that what you were meaning to link to?
Hood

Offline tony66

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Re: loosing steps - captive encoder
« Reply #4 on: May 19, 2012, 02:33:02 PM »
Hi Hood

yes - its for nema23/34 steppermotors............ I think it would be nice to mount it on my steppers to make sure they ALWAYS go where they are supposed to go :O)

Tony
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Offline Hood

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Re: loosing steps - captive encoder
« Reply #5 on: May 19, 2012, 02:41:33 PM »
They are just encoders, they will not allow Mach to close the loop as Mach is Open Loop. Nothing can be done to change that.
There is a board you can buy from Ron Rogers that accepts encoder inputs and it will fault Mach if the encoder values are different from where Mach thinks the axis is. It however is not closed loop as it will not correct on the fly, it will just fault and allow you to restart and update from encoder values if you wish. Whether that will save a part depends on the error value you have set and the tolerance of the part you are machining, having a well set up stepper system should never need that.

Servos are closed loop from drive to motor, Mach tells the drive to move to a position and the servo drive will read the encoder and move the axis whilst watching the encoder to make sure it gets there at the correct velocity.

Hood

Offline tony66

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Re: loosing steps - captive encoder
« Reply #6 on: May 19, 2012, 02:46:43 PM »
Hi Hood

OK - that sounds reasonable - thanx for the enlightenment

but............. ain't it possible to "trick" mach3 to do what the servoes are doing? by using the encoders and a plugin or something like that?

Tony


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Offline Hood

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Re: loosing steps - captive encoder
« Reply #7 on: May 19, 2012, 02:53:49 PM »
With a servo system it is the drives that close the loop to the motors, Mach still just assumes that the axis has done as requested, the difference being the drive makes sure it has. You can not do that with a stepper system as stepper drives do not close the loop.
If you have a well designed stepper system it should be very accurate, problems come when people try to drive the motors too fast or with too high an acceleration or have motors that are not powerful enough for the task in hand.
Hood

Offline tony66

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Re: loosing steps - captive encoder
« Reply #8 on: May 19, 2012, 02:59:43 PM »
my system works perfect......... no stalling - no shaking - no nothing
just keeps running and running

I have been playing a lot with motortuning - to get best performance - with full load on the spindle
I think that's why my system is running without any issues at all

Tony
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Offline wobbles

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Re: loosing steps - captive encoder
« Reply #9 on: June 16, 2012, 02:04:11 AM »
http://www.kellyware.com/max_stepper/index.htm has encoder feedback to stepper controller. not sure if it works with mach3