Bridgeport CNC + EMC2, which output hardware?

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19 Jul 2011 01:51 #11627 by arsenix
I am planning on a conversion to EMC2 for my Bridgeport R2E4 CNC. It has Boss9 control which although it is fairly functional is finicky (hardware wise). The good thing is that the machine is already equipped with excellent servos and encoders as well as a host of power supplies, isolation componentry etc most of which can be reused. The servos are brushed and run on ~100V/10A.

I plan to rip out the BSP2, FDMC and video boards and replace with a PC/EMC2. My main question is how to do the PC to servo interface. From poking around the Mesa cards seem like one of the better options. The 7i20 should be able to do what I need.

From there though I need to decide how to drive the servos. The cheapest/simplest option (in theory) is to use the existing analog servo amps and just buy an analog output card for the Mesa (+-10V). I'm skeptical about this though since even apart from the terrible whining the amps put out I'm not sure how long they will last. I could also go with the analog output cards and buy surplus industrial servo amps (ebay etc) although it seems difficult to get a matched set unless you buy new which is very expensive. My last option is to buy some PWM output cards (like the 7i29 from Mesa or the Pico Systems PWM card) and drive the servos directly from the Mesa card with no analog output card. This may or may not be cheaper than the industrial amp route depending on how lucky I get on ebay.

Is there a preference amongst the EMC2 users between these options? Will I have any issues doing direct PWM control or is the +-10V velocity control a more tried solution?

Thanks!


James

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19 Jul 2011 02:31 #11628 by 1:1
I am a beginner - so take what I say with a grain of salt...

But check out what PWM scheme your drives expect - as it turns out mine didn't use PWM and Dir - they used %5 d/c as full backwards - %50 as stop - and %95 as full forward ...

So any DC source could pretty much read as 'go flat out' NOW - I think thats why they specified the offset of %5 so it might have worked out anyway, but I just didn't want any headaches so went with a 7i33 PWM (from the supported anything I/O cards) to analog interface ...

My gear would eventually be operating around people so the less dangerous situations to think about the better ;)

Your gear probably wont be as finnicky as mine was with PWM

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19 Jul 2011 08:02 #11630 by andypugh
arsenix wrote:

From there though I need to decide how to drive the servos. The cheapest/simplest option (in theory) is to use the existing analog servo amps and just buy an analog output card for the Mesa (+-10V). I'm skeptical about this though since even apart from the terrible whining the amps put out I'm not sure how long they will last.


Why not keep them for the time being (they might last several more decades). You can always swap them for something else if they do die, either any other analogue input servo amp, or one of the direct-attachment cards if you with Mesa. Keep an eye out for eBay bargains in the interim, while you don't actually need the amps.
There is a chance that the whining is the servo position loop tuning, and will disappear when you bring the loop inside EMC2.

There are a lot of successful Pico converted Bridgeports out there, Jon@pico has one himself.

The _cheapest_ solution is to hook up the existing servo amps direct to the parport through a simple PWM/voltage converter. I am not _entirely_ sure how to do PWM to +/-10V, but an up/down PWM configuration would probably help.

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22 Jul 2011 05:22 #11723 by jmelson
andypugh wrote:


There are a lot of successful Pico converted Bridgeports out there, Jon@pico has one himself.

[/quote]
I just want to point out that email is wrong, my email is This email address is being protected from spambots. You need JavaScript enabled to view it.

If your servo amps take in a PWM signal, then the Pico Systems PWM controller could also be used.
It needs a slight variation in the HAL file settings to accommodate your scheme where 50% duty cycle
means "don't move".

Jon

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26 Jul 2011 11:29 #11866 by andypugh
elvistaylor wrote:

How to use such PWM signal in edm machine controller..


That rather depends on what input signal type the controller wants.

0-10V + direction is a bit simpler than -10v to +10v.

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29 Jul 2011 10:23 #11983 by andypugh
elvistaylor wrote:

I need that like 0-10V + direction signal controller to control my edm machine..


That's relatively easy. Depending on how good you are at electronics, you can either build your own PWM to voltage conversion circuit, or buy a commercial interface card.

This card will do what you want: homanndesigns.com/store/index.php?main_p...ath=1&products_id=38

If the drive can be configured to work with 0-5V then the minimal solution is just a capacitor between 0V and input on the drive, and a resistor from the PWM output pin to the drive input. That is probably a bit _too_ minimal really, though.

This circuit might work, but I have never actually tried it.
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