New Machine Build- Back EMF - Drive Sizing - Limit Swith Placement - 1125 oz-in Servos

[Plugger]

Hi,

I'll put some background that is relevent, but also a little to help the person who is still plugging along.... I could be wrong any place along the way, so feel free to comment!

I am designing a metalworking gantry-type CNC. It will be used to cut aluminum plates from 1/4" to 1/2" thick or more in multiple passes of course. The gantry will weigh about 165 lbs.

I prefer to err on the side of safety, so I want to use 1125 oz-in servos (Kelinginc.net) with about a 1.77:1 to 2:1 sprocket ratio and ballscrews
with a .2 inch/rev pitch. There are dual ballscrews on the gantry axis to prevent racking inaccuracies. I know most people use higher sprocket
ratios, but max motor rpm is only 3200, and I believe this will give me cutting power in the 100-150 IPM range -- at least 100 IPM no sweat.
Rapids will only be about 300 IPM, but that's okay with me. For now my spindle is only a 3.25 hp Porter Cable router, but if (when!!) my
manufacturing business grows, I would like to replace it with an industrial spindle. So, the machine is being built with a future upgrade in
mind. Also, for those speeds and hp range, Gates PowerGrip GT2 belts are recommended for sprockets of no less than 2.2 inch in diameter. So, a
2:1 ratio means the other sprocket is over 4 inches in diameter already. I'd rather avoid the larger sprockets if possible.

Lastly, I prefer to use the same motor, sprocket, belt and ballscrew pitch for all axis so that on-hand spares are cheaper. I am debating about
that for the Z-axis though if only to save money now.


My question:
For the larger size servo motors like this 1125 oz-in for example, it requires 90 VDC supply, 7.8 Amp contiunous and a peak of >40 Amp. I calculated: motor Kt x max rpm = 44 peak Amps. I believe, most amplifier drives on the do-it-yourselfer web sites and even more expensive ones like Copley Motion only seem to go up to 20, 30, 36 and even 40 Amps. (Copley does have another line of drives, but wow the price has got to be up there!) In any case, that's a ton of amps and the cost is going up, but I am concerned about Back EMF pushing too much current through the amplifier drives when it decellerates quickly. If I use a 40 Amp drive, I would probably set a current limit to 38 Amps. I would never peak out at more than about 95% on either the motor or drive, but I prefer not to get a drastically undersized drive like 20 Amps for example. I have seen these under powered ready-made kits on several web sites. Also, I want to avoid the need for a series resistor on the motor power feed.

Since rapid decellerations, generate high Back EMFs, which push excessive current through bypass diodes in the power semiconductors, I want to get
a properly sized drive. (This assumes they all use diodes and not some other protection circuitry.) I am waiting for a price quote now, but this should be a driver capable of supplying 36-40 peak Amps. I assume, with this motor of 44 peak Amps, I should use a drive with at least 44 Amps or more. Within reasonable cost, I can only find 40 Amps max.

So, if my axis, end-travel limit switch is only an inch away from the end, is that a practical distance for a runaway servo to stop from like 300 IPM if the encoder fails. I can also put springs or possibly rubber bumpers in there if that will help, but I don't want to. I don't know. Maybe this is not even a practical problem. Can I just use MACH3 to decellerate the axis once a limit switch is tripped if the encoder has failed, or do configuration restrictions usually mean the limit just turns the drive off? Lastly, I don't want to damage the ballscrews with a crash into the end that could happen on a bad, bad day over the years to come...

Thanks in advance for any thoughts!

[Al_The_Man]

I use Advanced Motion A-M-C, drives and the larger drives have auto shunt regulator internal.
For the lower current versions there is add on shunt regulator you can fit as an option.
http://www.a-m-c.com/content/prods/d...egulators.html
These should work with practically any drive.
Al.

[Plugger]

Thanks! I'll check that out!

Here are some images for a little show and tell....




(The one below now has two ballscrews on the gantry axis.)