Tool change Challenge

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13 Nov 2010 19:07 - 13 Nov 2010 20:35 #5290 by garymcrobertpdx
Tool change Challenge: I wish to use CAT 40 tool holders
on the mill I am constructing. The CAT 40 tool holder
has two alignment / drive notches and fits into the spindle
in only one rotational orientation.

I need to be able to position the mill spindle to a predictable
point of rotation in order to exchange tool holders.

The spindle encoder is a quad 100 CPR with an index
signal output. (US Digital E6).

Technically I should be able to resolve 1 / 400 of a revolution.
+ - 3 or 4 degrees would be adequate to make this work.

I have yet to select a spindle motor and drive due to my
lack of understanding of how to obtain this sort of control.

Hopefully the HAL software will provide a way to accomplish
this or some other clever method can be devised.
Last edit: 13 Nov 2010 20:35 by BigJohnT. Reason: remove EM character so message shows up

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13 Nov 2010 20:40 - 13 Nov 2010 20:41 #5291 by BigJohnT
Replied by BigJohnT on topic Re:Tool change Challenge
My BP Discovery 308 uses BT30 tooling and it rotates the spindle one direction to the index (I assume) then back 180 degrees to line up the drive dogs then swaps out the tools. My 308 has a servo drive in the spindle so I assume they are just moving it like a rotary axis or something. I've not given this a lot of thought but need to be prepared for the day my old DOS control give it up.

I had to edit your post to remove the EM-dash character (long dash) as the board software is funny about anything other than the normal keyboard characters and will not show your message...

John
Last edit: 13 Nov 2010 20:41 by BigJohnT.

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13 Nov 2010 22:15 #5292 by andypugh
Replied by andypugh on topic Re:Tool change Challenge
There are at least two ways to do this.

You can just use a spring-loaded tool holder, then rotate the spindle slowly till the tool drops in (detected by a microswitch) then clamp the drawbar.
I have seen examples on Youtube but can't see one right now.

Alternatively, you can quite possibly run your spindle motor as a servo, using the encoder. I think you would need a Flux Vector drive. I found I could nearly do it with my lathe, and that VFD is very old (mid 80s) and does not actually bother trying to move the spindle at less than 100rpm, which has obvious drawbacks.


I am optimistic that I will be able to do a lot better with the vector drive on the milling machine. (that has a BT30 spindle and home-made pneumatic drawbar)

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14 Nov 2010 02:07 #5293 by garymcrobertpdx
One method I am investigating is using a custom M100
Code that launches a script that issues Halcmd instructions.

I have used this method to toggle parport bits to operate
Air valves.

Perhaps using Halcmd instructions I could induce the
spindle to rotate very slowly 30 to 15 RPM when the encoder
index bit is detected, a stop and brake command is sent .
Hopefully stopping the spindle in the correct orientation
allow a Reliable tool exchange.

This approach should work with any spindle drive
Type that can rotate slowly and has a positive brake

I have not yet learned enough about the HAL software
to know if it is a viable strategy. If there is someone
In this user group that could offer some HAL guidance
it would be gratefully accepted.

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14 Nov 2010 02:44 #5294 by andypugh
Replied by andypugh on topic Re:Tool change Challenge
garymcrobertpdx wrote:

Perhaps using Halcmd instructions I could induce the
spindle to rotate very slowly 30 to 15 RPM when the encoder
index bit is detected, a stop and brake command is sent .


I was discussing this on IRC earlier, and I think there might be a rather easy way to do this.

You could potentially set up HAL with a PID controller that runs all the time, but the output of which is generally ignored, wired to a hal mux function.
Normally the mux function passes through the spindle-speed command, but when tool-change-request goes high then the mux switches to pass-through the PID command, which attempts to drive the spindle to the zero position.
This will require the spindle to be zeroed to the index, and also that the PID works modulo one full rev, or it will try to unwind all the spindle revs since startup. However overall it seems like a rather simple scheme.

I have not yet learned enough about the HAL software
to know if it is a viable strategy. If there is someone
In this user group that could offer some HAL guidance
it would be gratefully accepted.


I do keep meaning to write a simple intro for the Wiki, but have not yet done so. This post has a precis though:
www.linuxcnc.org/component/option,com_ku...5/lang,english/#4973

All the hal functions are listed towards the bottom of this page "realtime components and kernel modules"
www.linuxcnc.org/docview/html/

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14 Nov 2010 11:44 - 14 Nov 2010 11:48 #5298 by BigJohnT
Replied by BigJohnT on topic Re:Tool change Challenge
garymcrobertpdx wrote:

I have not yet learned enough about the HAL software
to know if it is a viable strategy. If there is someone
In this user group that could offer some HAL guidance
it would be gratefully accepted.


The HAL intro in the manual is a good place to start. It hopefully gets a newbie up to speed on HAL with the basics.

www.linuxcnc.org/docview/html//hal_basic_hal.html

Andy, I'm wondering out loud if a comp would be nice to have as all tool changers that use BT and CAT tool holders need to do the spindle orientation prior to swapping out the tools or would there be too many types of spindle drives to do this in a comp.

John
Last edit: 14 Nov 2010 11:48 by BigJohnT.

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14 Nov 2010 15:33 #5300 by andypugh
Replied by andypugh on topic Re:Tool change Challenge
I have been thinking the same thing about a comp.

Inputs would be encoder value, spindle speed/direction command and the tool-change request bit.
In normal operation it would pass-through the spindle speed command. Once the tool-chage-request bit goes high it would ramp the spindle speed down, then run a PID controller to home the spindle.

That would be quite a lot of replicated code (mainly the PID), though a PID controller is a simple thing so that isn't a huge problem.

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09 Dec 2010 00:01 #5931 by garymcrobertpdx
Still undecided as how to resolve the tool change challenge.
I am deliberating on two solutions

Option #1 Use a DC servomotor like a Homeshops 850 oz-in
and a drive like the Gecko 320. This will allow positioning the
spindle with plenty of accuracy and spin my cutter at around 3,000
RPM. Since the Gecko drive has a step and direction input I hope to
be able to reconfigure the HAL code to use A step generator to control
the spindle rather than the PWM generator.

The one downside is less horsepower than I would like. Using the
servomotors I have seen with a gecko 320 delivers a shaft power output
less than 1HP, I would prefer 2HP. I continue looking for alternative
motors and drives that could provide more HP.

Option #2 Use a Variable frequency drive like a Durapulse GS3 this has
A flux vector mode (as suggested by Andy also thanks for the video clip)
and a inverter grade induction motor. I have talked with techs that work
for vendors of these types of drives and have received conflicting opinions
as to whether I could position the spindle with enough accuracy for my
tool change. I suspect this would work but it could also be an expensive
disappointment!

It would be very helpful if I could locate some one who has real world
experience using these new variable frequency drives, particularly if they
are using EMC2 also.

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09 Dec 2010 00:16 #5932 by BigJohnT
Replied by BigJohnT on topic Re:Tool change Challenge
To get the best exposure to your question I would also post it to the mailing list as some prefer that medium.

John

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