Back during the low times of 2020 I picked up a half-finished Clausing 5937 CNC lathe conversion. Now that it's come up to temperature in my garage for the past year and a half, I figured it's about time to finish it and start making some parts.
The problem I'm facing right now is while I got servos, ballscrews, and a power supply with the project, I didn't get any drives or control electronics.
I'm planning to run linuxcnc as my controller, and I believe a Mesa 7i97 will provide all the I/O I need with encoders connected directly to the mesa cards and the analog out connected to the yet-to-be-selected servo drives.
Motors I have are Electrocraft brushed DC motors P/N 643-003-0132, and 0644-06-011. I can find some old docs referencing these P/Ns directly, but only a peak current is provided, no continuous.
Electrocraft has some legacy documentation for motor parts 643 and 644 (assuming these might be updated versions of the same models?) with similar specs and a continuous current rating is provided, though there are 3 ratings provided for each motor and I can't find an explanation of why.
I've attached PDF files of the two docs that I found in case some other unfortunate soul finds themselves searching these part numbers so they don't have to go through what I have.
Based on this info, I think my current ratings are 40/7.7 for the 643, and 24/4.8 for the 644 (which seems odd since it's bigger but I'll roll with it for now)
Now the crux of the issue is this: how do I size a servo drive for these, assuming the motors are correctly sized for my machine? Should I aim to have enough current to max out the motors? Or should I try to undershoot so that I don't risk overcurrent from bonehead mistakes? Do I even need a servo drive if I'm only using it in current mode?
By the time you are done with power supply, drivers, tuning, etc. and you will have old technology, if it were me, I would just go with some good used AC (brushless) servos from allen bradley, yaskawa, dmm, clearpath, or the like. You will have a decent software tuning and set up program, and drives that match the motors. Just my two cents. There are some good, digital brush drives, but its harder to set them up and they cost around 300 a drive. Machdrives makes some, Copley also has some decent digital brush drives. You could also go with geckoes I guess, but I've never had any success with them.
I totally agree with you, that is definitely the sensible thing to do. When I do eventually give up on these motors, that's almost certainly the direction I'm going.
I'm not enamored with these old motors, but the brackets are already made and I'm not presently equipped to remake them, so I'm going to try and use them as long as I can do so without digging myself too deep a hole.
On that note, I've gone ahead and ordered a used pair of AMC 12A8E to get me running. I saw the gecko drives but I don't like the idea of using step/direction to control a dc motor. It feels like a step backwards to me but that may be my inexperience showing.
When those show up, I think the only electronics I'll have left to acquire would be the mesa card itself, unless there is a more preferred solution available.
Hi,
if the old servos are standard, that is to say made to NEMA standards then modern AC servos which are made to the same standards will be exactly the same size
and bolt straight in.
I favor Delta servos but being Taiwanese they are metric sizes and while similar to NEMA sizes differ just enough that they are not bolt in replacements..
Those are industry standard drives,and I guess you have tach's already on the motors? The mesa will have to handle the encoders. Its more than I would be comfortable figuring out, but thats the way it was done for years. Keep us up to speed on how it goes, and good luck!
Motors have encoders on the back. My planned control loop is encoders into the mesa card, +-10v analog from the mesa into the drives, drives in current mode to the motors.
And yes I believe the motors are nema 42, so replacements are available if/when I decide these suck.
Help Selecting a Servo Drive
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