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IndustryArena Forum > CNC Electronics > Servo Motors / Drives > Constant Torque Servo Choice and Control
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  1. #1
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    Constant Torque Servo Choice and Control

    I am an aerospace engineering student, so industrial servo's aren't really my strongest area. I am currently working on a gravity simulator project that requires a servo to apply a constant torque on a cable attached to a rover below the gantry while it slides along an x-axis. The rover will be about 40 or 50 lbs (could be scaled down if necessary), and for simplicity sake I would attach a 2 inch diameter rotor so that the maximum continuous torque I would need would be 40-50 lb-in. Ultimately, the goal is to maximize the torque resolution to within 0.01 lb-in but we aren't sure if thats feasible yet.

    We are probably going to be getting some servo drives from Advanced Motion Controls through their University Outreach Program. The list of available drives is attached.

    I found several servos on Ebay that meet the torque requirements for the project listed below, but I'm not sure which would be the best for maintaining constant torque. I think all of these can be driven with one of the drives listed at the bottom for most of these I think I would end up using the B30A40.

    So the AMC drives are just dumb drives from what I have read here on the forum. I just want to run these drives in constant torque mode, so what is the easiest/cheapest way to do that. I have read that I can use EMC2 but need 2 serial ports (one for pwm and another for the +-10 vdc signal), I'm not sure how difficult this is to implement, any experience out there? I saw a post on here where the encoder output was translated back to the drive as hall sensor output, would that be possible here to use servo's without hall sensors? Alternatively, is there is a simple plug and play controller available to do what we need instead of using EMC2? Any help would be greatly appreciated.

    ALLEN-BRADLEY 1326AB-B420H-21 AC Brushless
    -Max cont output power: 2.08 KW
    -Max cont rms amperes: 5.46 amps
    -Max cont torque: 42 lb-in
    -Resistance: 3.150 Ohms
    *Not sure what the voltage is on this motor or if it has Hall sensors

    Pacific R46GENA-TS-NS-VS-00 DC Brushless
    -RL 3.7 OHMS
    -Ke 128 V/KRPM
    -Ics 6.7 Amps
    -Tcs 64 LB-IN
    -360VDC
    *The TS seems to indicate that it doesn't have Hall sensors, I'm not sure what Tachsyn is though.

    Allen-Bradley BulletTN 8500 AC Servo
    -Rated Output - 0.85 KW
    -Rated Torque - 55 kg-cm / 47.7 lb-in.
    -Rated Current - 6.2 A (RMS)
    -Rated Speed - 1500 RPM
    *Don't know voltage or if it has Hall sensors

    AEG Modicon brushless servo motor cgp31-tb
    -feedback: resolver 1/1
    -rated current ic stall: 19.0
    -motor torque: 225lb-in/25.4nm
    -poles: 8
    *Dont even know if this is AC or DC

  2. #2
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    I get that I asked a lot of questions here in one post. No responses was kind of rough, but I get it, I don't know if I would have responded to me either. Anyways, after a semester of bugging tech support engineers I figured out some of these issues. I am going to be posting a build log soon in one of my other posts.

    So the servos I ended up using were some leftovers AMC had from some old kits they used to sell, they were great enough to donate these to the university. Really great people over there. One of the local distributors suggested that we simply supply a constant voltage and skip connecting the servo to the computer and that worked well enough.

    The project is finished, but they have asked me to upgrade the system using the servos we got to replace the stepper motor on the carriage and getting a bigger torque motor that can test some bigger rovers and possibly even people.

    The bigger servo brings up one of the questions that I had before that never got answered. The 1326ab series servos have a resolver that can be included, but I still don't know if a hall effect track is included on them. The spec sheet says commutation information is provided, I would think that means hall effect, is that correct? I am going to use a b30a40ac drive from AMC to run the servo. Any help this time around would be greatly appreciated.

  3. #3
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    resolvers are analog devices and absolute information comes from them so NO halls are every provided with or on them - just not required. amc does not accept resolvers so that 'option' is useless for their drives; if u have the ab motors with resolvers and want to use the amc drives u will have to replace the resolvers with encoders with hall channels for the absolute info. or use a different drive such as Kollmorgen AKD than accepts resolver or encoders natively and will run anyone's motor.

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    Thank you for a great answer. I really appreciate it. I was thinking it was possible I would have to replace the resolver with an encoder but really wasn't sure. The analog output problem makes perfect sense now. I was thinking of getting an encoder from Renco, Al mentions them often in his posts. There are some cheap ones on ebay right now, does anyone know if there is anything specific I need to consider other than shaft size before picking one of these up. Its either that or pick up a slightly less powerful Allen Bradley MPL-a454 that has a hall effect output track. I think I'm leaning in that direction if an encoder swap is going to present any challenges.

  5. #5
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    Quote Originally Posted by wilcorp70 View Post
    I get that I asked a lot of questions here in one post. No responses was kind of rough, but I get it, I don't know if I would have responded to me either.
    I missed the OP or I would have responded, but it is a pity you have the resolver/motor combo, as AMC are nice simple drives.
    If you picked up the motors off ebay, it would have been wise to ask about the resolver issue first.
    As to the controller question, it does not appear you need a CNC application, I would look at a Galil controller, it is very easy to program with the native commands, and if you pick up one of the surplus DMC-1500 stand alone versions, you can use a simple Maple Systems keypad/display for data entry.
    They integrate nice with AMC drives.
    BTW, I did a recent evaluation of CUI programmable encoders for DCBL motors and was not all that impressed as the results were mixed.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

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    Sorry, I didn't make it clear in my last post, I don't have the 1326's, I was going to try one more time here to get an answer before pulling the trigger. Lots more help this time, which is nice (it's good to know I just got overlooked and it wasn't just a horrible newbie post thing).

    As for the controller, Al, you are right on about this part of the build not being CNC related (although I guess you could consider it a very awkward Z-axis). Since I am replacing the steppers that used to move the thing around with the servos I got from AMC though, I have decided to go with a Mesa 5i25 and 7i77 combo to control all the servos at once. I know there are some Galil controllers to do this, but the Mesa option seemed much less expensive unless there is something I am missing.

    Reading my first post here is a little painful now, as I knew nothing about servo control. Now I just know a little more than nothing. Thanks again for the replies.

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