DIY Servo Controller - Initial Mill XY Test

by Ivan Hamilton 9/13/2008 7:35:00 PM

After months of putting it off, I jumped in and slapped two motors on to my mill.

I've already identified that the XY mechanisms on my mill aren't going to be up to the task (they have excessive backlash in both the bearing mount & the nuts).

Therefore, it's not worth spending too much time on the installation, as I'll need to redo it when I upgrade the mechanisms. I still hadn't done a "real world test" on my DIY servos & controller, and was keen to see the mill's table move.

Enter the angle grinder, 2 short pieces of RHS and some hose clamps...

So, what did I learn?

Keyboard: The wireless flexible keyboard I got is great. Chuck it where you need it.

Current Draw: It's not unusual in my 1:1 setup to see the motors draw 10A @ 12V. Hopefully this should drop when we switch to 36V. w

H-bridge Temperature: The H-bridge chips got hot. Real hot. I couldn't keep my finger on the heat sink, hot. Since pairs of H-bridges share heat sinks, I swapped the Y axis onto a different pair than the X axis. Then they both got hot... How hot? Not that hot: 57°C. I put a small fan near and let it continue to run: 32°C (ambient was 25°C). Not neccessary, but a larger heat sink or small fan would keep it cool.

PID Tuning: The inertia & friction of the table's screws changed the tuning requirements quite dramatically.

Encoder Inputs: Since I only used 2 of 4 servo channels on my controller, the other 2 encoder inputs were left floating. Phantom transitions on these lines fire interrupts and unnecessarily consume controller time.

Limit Switches: This was a simple test, and no limit switches were installed. Luckily, the flexible coupling slipped when it attempted to go beyond the table's travel. Limit switches are a must.

DIY Flexible couplings: OK, small length of hose with hose clamps. While there is some twist in the soft hose I used, when you consider the 0.2mm of existing screw backlash (@ 3mm/rev) equals 24°, the <10° of hose twist & <6° of servo error is quite acceptable (for now).

The Z axis hasn't had anything done yet, but that's OK. Baby steps. Although it would be easier, I don't want to automate the quill since it's got a lot of play in its rack & pinion setup. I should get moving on the ball screw upgrade and automate the column.

Even with only the X & Y axes done, the machine is transformed. I'm happy to perform any required Z movement manually for the time being.

All in all... I'm pleased, and it's quite exciting (chips will fly soon).

Currently rated 2.1 by 18 people

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Tags:

CNC | Electronics | Mechanics

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Comments

9/13/2008 10:18:33 PM

Good on ya mate, about time you put your servo's into action !!

Nice soundtrack btw :~)

Epineh

9/29/2008 4:18:37 AM

hi Ivan congratulation on your servo test it seem sew promising Ar you going to release the
schematic and firmware



thx hesham

hesham morsy

9/29/2008 9:49:20 AM

When it's complete enough, I do plan to release the details. There's still more to do before I would make anyone else try to live with it.

Ivan Hamilton

9/29/2008 10:39:06 AM

hi Ivan thank you for the fast reply

i think the most intrsted part that in yor Controller is the ability
to move the 4 moter simultaneous with one brain (microcontroller) and the genius Idea
to separate the H-Bridge that will gave large variety for example 4 l298 and 4 small servo


my first cnc design
www.cnczone.com/.../showthread.php

hesham morsy

12/29/2008 8:15:14 PM

hi ivam there is something good here
MOSFETs and half-bridge IRFP260 IR2184

http://cnczone.com/forums/showthread.php?t=69640
also a Video of 2 heavy servo
depositfiles.com/en/files/tse...iles/tseutbpe8

hesham morsy

7/15/2009 11:08:15 AM

I am doing something of the same interest and will be taking note on this .Thank

Registry Cleaner

7/16/2009 6:46:45 AM

Nice to share this information, really helpfull

Xpatmaniak

8/1/2009 12:07:46 PM

Thanks - Just the info I was looking for.. My search ends here..

Låne Penge

8/4/2009 4:44:30 AM

its a great website thanks

Travel Indonesia

9/16/2009 6:20:52 PM

"the other 2 encoder inputs were left floating" can that cause an issue or is the unit able to cope with a number of encoder inputs being left not used?

boiler insurance

9/19/2009 1:37:00 AM

Cool post - thanks a lot m8!

Fly Europa

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