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Author Topic: 360 deg A Axis homing using an encoder  (Read 6073 times)

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360 deg A Axis homing using an encoder
« on: March 05, 2008, 08:59:57 PM »
I have a tangential cutting machine with the A axis stepper motor rotating a pizza wheel.

I wish to install a Encoder on the A axis so I can reference this axis to a home 0deg position using the encoder as position feedback

I imagine setting up a DRO for the Axis is the way to go??? or are there other solutions??

I did read some where that Gecko may be building a closed loop stepper drive

Has any one elso installed encoders on a rotating A axis with stepper for monitoring postion??

Cheers
Jason

Offline stirling

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Re: 360 deg A Axis homing using an encoder
« Reply #1 on: March 06, 2008, 04:24:26 AM »
Hi Jason - I havn't done this but I have some thoughts that may help a tad.

As I understand it Mach doesn't provide the mechanism for closing the loop of positional encoders per se. For example when using servos, the loop has to be closed by the drive hardware. But I don't think this is the issue for you anyway.

I don't know off the top of my head if Gecko are or not but I don't think a closed loop drive will help you with your objective anyway. In order to implement a home function Mach needs to know about it.

Your problem is that an encoder pulses when the motor moves - it has no notion of position by itself. What you need to know is when it's at a particular position which you can call home. Remember that servo driven axis allways have encoders but they still use home switches. (turn the machine off and on again and then try to find home with a positional encoder)

I can't see any way round it - to implement a home function you need a home switch - not a rotary encoder. Happy to be corrected though.
« Last Edit: March 06, 2008, 06:37:28 AM by stirling »

Offline jimpinder

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Re: 360 deg A Axis homing using an encoder
« Reply #2 on: March 06, 2008, 08:52:59 AM »
Why do you need positional feedback. If you set the Mach DRO to zero, it will keep track of where the axis is - if you are using stepper motors it simply counts and says I have moved so far and puts it up on the DRO.

If the A axis is rotating I don't know how many steps per unit you have it set on, or even what your units are, but that is easy enough, and if that is 360 degrees, then Mach 3 can put out the correct number of pulses to turn the wheel a known amount. Since it is rotary, you can have it set at anything - 10, 36, 100, 180, 360 etc. Mach 3 will keep track of this position on the A axis DRO in the units of your choosing.

The only thing you need to know is a zero position. You can either set this manually, before you start machining, which I find adequate for most of my machining, and providing you don't force the axis and loose steps, it is absolutely accurate,

I have no idea what a pizza wheel is - do you mean a rotary table - if so, you could use a reflective optical sensor on the side of the table. I used one on my spindle, the reflective part being a small "mirror" from my grand-daughters box of "stickies". It works perfectly and can be connected by a single wire to Mach 3.
Not me driving the engine - I'm better looking.
Re: 360 deg A Axis homing using an encoder
« Reply #3 on: March 06, 2008, 03:44:39 PM »
Thank you for the responses.

A pizza wheel is a rotary knife similar to the one you use to cut pizza's with. This sits on the end of the A axis.

When I refernce all axises home all is ok for the X,Y and Z however because I'm using the tangential feature in mach my A axis never ends up in a 0 deg position. So I have to visually align the A axis and zero. As the machine is up againt a wall it is hard to acurately check if the a is exactly in the correct zero position. If it is a few deg off the knife is dragged around the table. this dragging/ not inline with direction of travel increased the load to a point where the stepper looses steps.

Regarding the encoder. Haven't encoders been used for linear DRO's before. ie just to measure distances??

Iam using a GREX so may be I could get a pulse to voltage converter and use a analog input for measurment of the A axis angle??


Jason
Re: 360 deg A Axis homing using an encoder
« Reply #4 on: March 06, 2008, 04:10:54 PM »
Hello Jason,
Here is a suggestion. If your motor is a dual shaft, mount a small index disc on the upper end. Maybe 2" dia with a  small notch, just like the spindle index on the lathe or mill. Use a index sensor like the one from CNC4pc as a home switch. Clamp the disc to the shaft with a set screw and adjust it radially until it homes exactly where you want it to. Set it up in Homing/Limits just like the other axis's.
Just a notion,
RC
http://www.cnc4pc.com/Store/osc/product_info.php?cPath=25&products_id=129
« Last Edit: March 06, 2008, 04:16:15 PM by Overloaded »
Re: 360 deg A Axis homing using an encoder
« Reply #5 on: March 06, 2008, 07:26:19 PM »
Thanks Overload,

Regarding this optical switch. With homing and limits being operated by this type of switch, what would be the results if during normal operation the swicth was activated.
Some times due to the code the axis will pass through zero and number of times and in either direction

Offline Chip

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Re: 360 deg A Axis homing using an encoder
« Reply #6 on: March 06, 2008, 07:59:47 PM »
Hi, Jason

If you set it up as Home only, It will be ignored unless you Reff All Home.

Hope this Helps, Chip
Re: 360 deg A Axis homing using an encoder
« Reply #7 on: March 06, 2008, 10:10:57 PM »
Thanks Chip...and also, it will always go to home in the same direction and can set the dro to 0.000 or any value you set in the "Home Off" setting. The slot doesn't matter much either. The sensor will only see one edge of it, then reverse. If I understand the man.correctly, this arrangement, if kept clean is more accurate and reliable than a typical limit switch.
RC
Re: 360 deg A Axis homing using an encoder
« Reply #8 on: March 07, 2008, 12:45:27 AM »
Thanks for your help.

I will definetly go down this path for setting up this A axis

Thanks again