Servo processor "upgrade module" - looking for volonteers.

[H.O]

I'm sorry John, I know you didn't.

Like I said, from my experience the UHU chip tops out at ~130kHz, because the web-site said 150kHz easily and ~300kHz under optimal conditions I initially thought that the diferential reciever chip and filters on the HP-board played a part in me only getting 130kHz. But the HP-board has prooved to work nicely at speeds around 1Mhz when used with my module.

To me it sounds like the UHU chips will fit the bill nicely in your case, especially if you already have them on hand. A 4:1 reduction is probably around the highest you can go in one stage with a belt reduction. Your optimal seems to somewhere between 10 and 20:1 so perhaps two stacked 4:1 reductions giving a total of 16:1 driving your 4" circumference pulley could work.

Then you'd have 3000rpm/16*4=750IPM rapids, the maximum step and encoder frequency would be 500*4*3000/60=100kHz which the UHU will handle nicely and the resolution as specified by the encoder alone would be 4/(500*4*16)=0.000125 (32000 steps/inch). A 0.5Nm (70oz-in) continous torque motor would give you 70*16/(4/PI/2))/16=110lbs of force minus losses. Not too shabby depending on the overall effeciency of the drive train. (What's the torque rating of the your motors?)

You could reduce the needed amount of reduction by using a smaller pulley for the actual axis drive but that on the other hand makes for less belt engagement around the pulley, which I think is quite small already. (Not that I know the size and/or requirement of the machine you're building.)

Lots and lots to think about and many variables to play with, as you've noticed.

Thanks!
/Henrik.

(I hope my math is correct this time, I don't guarantee it though....)

[jcdammeyer]

[QUOTE=H.O;791140]A 0.5Nm (70oz-in) continous torque motor would give you 70*16/(4/PI/2))/16=110lbs of force minus losses. Not too shabby depending on the overall effeciency of the drive train. (What's the torque rating of the your motors?)
QUOTE]

Henrik,

As a separate project from CNC on my mill using DC Servo's I also have the need for a small CNC router for cutting plastic and MDF wood. So I'm building up the JGRO router. Work is slow at the moment so every $ I don't spend on something is a dollar earned so to speak which is why I'm building rather than buying.

The Steppers allocated for this project are 280 oz-in motors that on a good day may do 750 RPM. Initial plans are for 10 TPI ACME all thread lead screws but odds are I'll end up changing over to belts. The discussion about this project is what then shifted over to DC Servo's instead of steppers. That's when I started doing the math for encoder lines per rev and maximum input for Uli's UHU ATMEL processor.

Thanks.

John

[LZ1TWB]

Hi all,

Henrik, I ran trough the userguide but did not find the thing I looked for. Have you made a parameter, similar to the error count on the original chip, and how can you make sure it is on position/ or not? Maybe check the actual axis with a dial, or use an external DRO for this. Sorry I may have missed this one.

It seems that you use an additional DIP socket to bring up the board above the surrounding components. I've seen some sockets here with quite long legs. I may need something like that here as the reset capacitor is near and might have a conflict.

Todor

PS: We were performing at a local airshow last weekend, anyone interested in our hobby can see some pics here: http://www.facebook.com/album.php?ai...4&l=1cd566e0b1

[H.O]

Hi Todor,
The UHU-chip, and this is from my understandig as I don't have access to its actual code, works by triggering an interrupt service routine on each edge of the A and B channel signals. The interrupt service routine then "manually" steps the internal position register up or down.

At each quadrature state there are only two valid states that the encoder can transition to, one in each direction. Like this: AB-Ab-ab-aB-AB-Ab-ab-aB and so on. At state 'ab' the next state HAS to be either Ab or aB depending on which direction the encoder is rotating. If the next state detected is anything else then the code knows that it have missed at least one state and increments the "O-count" as an indication of this. Now you may think that if it is at state ab and then detects AB why not just count two "steps"? Because AB is two "counts ahead" in BOTH directions so it can not know if it should count up or down.


This module however does not count the encoder pulses with software. There is a built in 16 bit hardware quadrarute counter in the uC used on the module. The "only" software involved with actually tracking the encoder is to increment or decrement the upper word of the 32bit position register when the 16 bit hardware counter over- or underflows.

You can query the module for the current target position (GT-command) and match this to the expected number determined by the steps/unit and the number of pulses sent out from the CNC system. This will verify that you aren't loosing (or gaining) any step-pulses due to noise or wrong step signal polarity etc.

You can also query the module for the actual position (GP-command). It should match the target position once the motor is stationary. However, this will NOT tell you if the ACTUAL position is wrong due to noise on the encoder lines etc - there's simply know way to determine that software wise. Likewise on the UHU, if noise forces the encoder signals to another state (which at the time happens to be valid) it will not know the difference between the noise and an actual transition of the encoder signals.

If you should find yourself in a position where you think noise on the encoder lines are causing trouble you can activate the internal digital filter of the module (see the Y-setting) and set it to a frequency close to your highest possible encoder frequency + some margin.

To VERIFY that you are on position and no pulses gets lost due to noise or whatever you have to resort to actually measuring the position with a DTI, DRO etc.



As for the IC-socket, yes. On the HP-UHU I had to use one 20-pin IC socket to get the board to clear the USB connector. You could stack a couple of ordinary sockets if you want, or you could mount the "offending" capacitor on the bottom side of the PCB - whatever works.

Sorry for the verbose answer. Let me know if there's anything else!

/Henrik.

[tenmetalman]

H.O.
the module's were here when I got home. they look great My goal is to get a couple of hours in on the project tonight. I turned on the air in my shop (the current outside temp. is 95f. @6:15pm). my plan is to get back up & running with Uli's chip then change over. I'll probably drive you nuts wanting my hand held. Paul

[H.O]

Thanks for letting me know Paul!
That sounds like a good aproach, it'll verify that everything works as expected before trying the module. Please do the initial test with the motor not actually driving anything.

Apart from the fact that module doesn't work with the "UHU specific" terminal, the process of setting it up is pretty much the same. Don't worry about asking questions if you're uncertain but also don't be afraid of simply trying things - that's how you usually find problems ;-)

I'll be here if you need me.
/Henrik.

[kreutz]

Hello Henrik, Paul and John;

Just to let you all know that I am still alive although I haven't had enough free time to keep on visiting the forum and my PM box has been full for a long time. Just came back from Spain where I spent a month on vacation far away from my PC and cell phone.

Best regards,

Kreutz

[jcdammeyer]

Welcome back. I had trouble shedding my cell phone in Greece this spring. Found an internet cafe with low booze prices right by the beach. The family spent it getting wet. I drank beer and surfed with my iPhone.

John Dammeyer

[H.O]

Yes, welocme back Kreutz!
I hope you'll check in now and then, as time allows.

/Henrik.

[tenmetalman]

Kreutz,
Holidays are always too short, glad to know your alive & kicking. I can't keep up with my "stuff" and I'm retired !
best to you & the family. looked at the wife's site not long ago & it was looking great as as always
Paul

[kreutz]

Thanks guys, I am glad you are all OK and still on the CNC wagon... I am planning to come back here more often, it will probably take still a while to start working on my CNC stuff.

My first year's goal was to start a new enterprise and make it profitable; Goal accomplished!. My second year's goal is to keep it profitable (while working less hours a week) having at least as much free time as before so I can enjoy my hobbies.

Best regards,

Kreutz.

[jcdammeyer]

So watcha up to?

John Dammeyer