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  #1  
Old Mon 31 December 2007, 07:54
TBrown
Just call me: Tom
 
Clinton,IN
United States of America
Changing Shopbot board to G -code

I don't know if I'm on the right track or not.
To get Shopbot to use G-code do I need to change to a Break Out Board and use db25 pin Lpt port?
If this is correct, which BOB is best?
I've looked at PMD X, Bob Cambell, CNC4PC.
Bob Cambell does not have charge pump, how important is this?
Equipment now on Shopbot:
4- Gecko 202
4- Vexta A6497-9412KTG, 2 Phase, 1 Amp, 4.39 ohm
1- 44 volt power supply
1- computer power supply 5v & 12v
Any help is welcome.
Thanks
Tom
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  #2  
Old Tue 01 January 2008, 09:12
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Tom,
As far as I have been told, the A6497-9412KTG motors that shopbot uses are slightly modified PK296A1A-SG3.6 motors that have only four wires - meaning that they can only be connected Bipolar Series. Using Mariss's latest power supply formula, that means that you can use a power supply up to 80VDC without harming the motors. (1000 X SQRT(0.0308) = 175V, but the Gecko stepper drivers are limited to 80VDC maximum.)

I use the PMDX-122 breakout board. That is the only model breakout board that I've ever tried, but it seems to be an excellent product. I bought two of them (just in case), and the second one is still on the shelf.

The charge pump is a fail-safe device that stops the stepper-pulse-train if communication is lost between the computer and the breakout board. It's a nice feature, but, if I understand electronics, if the computer becomes disconnected from the breakout board, the stepper pulse train will also become disconnected.

So, other than buying a breakout board and possibly beefing up your power supply (and adding at least a 10,000 uF filter capactitor to the power supply's output), you have all that you'll need for the basic electronics.

(Don't forget to order a copy of Mach 3.)
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  #3  
Old Tue 01 January 2008, 15:21
TBrown
Just call me: Tom
 
Clinton,IN
United States of America
Mike

That sounds rel good, I will go ahead and get the PMD X board, maybe 2 because I plan to add another Z axis.
Yes, I now use Turbocnc on a homemade router, but will soon get Mach 3 purchased.
The 44 volt P S is what was operating the ShopBot before I purchased it, do you think I still need to add the cap?
I will be using the same Geckos and motors.

Thanks for the help.
Tom
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  #4  
Old Tue 01 January 2008, 17:50
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Tom
if you share the steps on the output on the x axis for the 2 motors you will free up the output you need to drive the second z axis This will save the cost of a second board. Sean. I have a picture of the shared axis wiring on my thread.
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  #5  
Old Tue 01 January 2008, 19:32
TBrown
Just call me: Tom
 
Clinton,IN
United States of America
Sean
I checked your schemetic, didn't realize that would work.
It's great to get all this info, I really didn't want to go with ShopBot code.
By the way, your build looks great.
Thanks
Tom
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  #6  
Old Wed 02 January 2008, 04:50
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Tom,
I didn't realize it either until Gerald reminded me that this machine is NOT a closed loop feedback servo device. Steppers are dumb...they only take data and instruction one direction. The MM gantry stays square by mechanical means and not by software...so their was no reason not to try it. I have already assembled my cabinet and run all the motors and can attest to 5 Geckos running on (1) PMDX. x1 & x2 shared, y, z, a independent. Good luck with your build.
Sean
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  #7  
Old Wed 02 January 2008, 05:40
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Tom,
The 44VDC power supply that Shopbot uses will usually work just fine with or without the external capacitor. The purpose of the external capacitor is to act as a very weak battery to store electricity for 1/120th of a second with a standard linear power supply and for a much shorter period of time for a switching power supply. Because of the way stepper motors work, they sometimes need a lot of electricity and sometimes very little electricity. Sometimes, when you have four or five motors working in unison, all of the motors make an electricity 'withdrawal' from the power supply at the same time. Without the capacitor, the voltage can and does drop, and the motors stutter.

A large capacitor costs about $10 to $35. It never hurts to have one and sometimes helps a great deal. (The Gecko stepper drivers are much higher performance than the original stepper drivers that Shopbot supplied. Mariss, at Gecko, highly recommends the external capacitor.)
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  #8  
Old Wed 02 January 2008, 07:50
TBrown
Just call me: Tom
 
Clinton,IN
United States of America
Sean
I'll do that, Thanks

Mike
Ok on the cap.
Would I gain a lot by building a transformer like MM uses?
Thanks
Tom
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  #9  
Old Wed 02 January 2008, 09:32
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Tom,
The rule of thumb is that speed is most influenced by voltage and torque is most influenced by current (amps). If I were you, I would start with what you have (with the addition of the external capacitor) and then update the controller with a 70V power supply after you've got the machine running. The speed difference between 44V and 70V would not be significant and possibly might be almost the same, depending on how smoothly your rollers glide, how heavy your gantry is, and how you have your ramps set.
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  #10  
Old Thu 03 January 2008, 08:44
BernardR
Just call me: Bernard
 
Georgetown Texas
United States of America
The original inclusion of the charge pump into the MachX series was to prevent machine movement while the computer was doing it's boot procedure and before the Mach Engine took control. The only time you should have any problems is if there is a power failure while you are out of the room with all the equipment running, at such time there could be spurious movement after the power restores. Even this can probably be avoided by changing your BIOS option to non automatic restart.

I would recommend that you read Step_motor_basics.pdf on the Gecko site, in particular pay attention to the star method of power distribution and the inclusion of 470uF 100VDC working capacitors directly across the power supply pins of the Geckos.
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  #11  
Old Thu 03 January 2008, 09:09
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Bernard,
The 470uF 100V capacitors written about in Mariss's paper was required with the G201 stepper drivers. The G202 has the capacitor built in. The G203v does not require that capacitor; however, it can be added to give additional filtering. I have not found a need to use it with my G203v stepper drivers.

There's really no such thing as too much capacitance. However, there is a point of diminishing returns. I've found that having 5% or less ripple on a power supply works for most applications. The large filter capacitors keep the ripple to that point or less. The G202 and the G203v handle the 'fly-wheel' ripple that the stepper drives tend to create.
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  #12  
Old Thu 03 January 2008, 09:55
BernardR
Just call me: Bernard
 
Georgetown Texas
United States of America
Mike,

Thanks for the heads up.
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  #13  
Old Sun 06 January 2008, 12:22
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by Richards View Post
There's really no such thing as too much capacitance.
....except for the safety aspects. You don't wan't too much stored energy when you switch off to put your fingers in there.

Also, the inrush current can get to be excessive.

Big caps make me nervous - I have been covered in oily paper with ringing ears too often. (twice)
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  #14  
Old Sun 06 January 2008, 15:21
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Gerald,
You brought up a good point. Capacitors can fail and they can hold a charge for a long long time. (I use bleeder resistors to automatically drain a capacitor when the power is off.) It is also common to put a low resistance, high wattage resistor in series with the positive wire connector to slow the inrush current. That is something that I don't normally do. And, it's also important to know that capacitors have a useful working life of only about two years (depending on how hard they're worked and how hot they get. Some electronic sales outlets have data sheets for their power capacitors that give the expected life along with working voltage and working temperature.
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  #15  
Old Sun 06 January 2008, 18:25
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Gerald, Mike,
Inrush current is a concern, but there are devices available to limit the inrush current to a respectable level. We use them in some of the military electronics we do at work. On some of our assemblies we have quite a bit of capacitance.
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  #16  
Old Sun 06 January 2008, 19:18
BernardR
Just call me: Bernard
 
Georgetown Texas
United States of America
Using fairly large servos with a big supply I took the precaution of using movs, they're cheap and good insurance.

There's some information here: http://morley.eng.ua.edu/G332b.pdf
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