Best type of ancilliary motor



  • I'll need to drive a small ancillary motor motor for something I'm working on. Low torque, slow speed, DC voltage. I'll need to be able to change the motor speed during testing and it may or may not need to be variable in use. Given those very sketchy details, from a Duet point of view, what sort of motor would be the easiest to drive/interface. Continuous rotation servo? Small stepper like a Nema8? Some other - if so what?

    Thanks in advance.


  • administrators

    @deckingman Hi Ian, as you have a Duex5 in your setup then you might be able to use a continuous rotation servo on a servo output with little trouble. depends if you can find a hobby type one that has sufficient torque for your application.



  • @t3p3tony said in Best type of ancilliary motor:

    @deckingman Hi Ian, as you have a Duex5 in your setup then you might be able to use a continuous rotation servo on a servo output with little trouble. depends if you can find a hobby type one that has sufficient torque for your application.

    Thanks Tony. Yes, I forgot to mention that I have a Duex 5. Thinking ahead, will Gen 3 Duets also support servos?

    I'm guessing that if I used a small stepper motor (Nema 8 or some such) that it would need to be mapped to an axis for Duet to control it? Actually, that's a thought - could be treated as an extruder and mapped to a tool - no, that would bugger up mixing ratios - never mind (sorry, thinking out loud).


  • administrators

    @deckingman The plan is to support small hobby servos in the Duet 3 as we do on the Duet 2. Nothing is finalised yet but I don't expect that part of the plan to change.



  • @t3p3tony said in Best type of ancilliary motor:

    @deckingman The plan is to support small hobby servos in the Duet 3 as we do on the Duet 2. Nothing is finalised yet but I don't expect that part of the plan to change.

    Cheers Tony. Remind me what sort of servo rating (current\voltage) are we looking at?


  • administrators

    The servo I test with is this one https://www.electronicoscaldas.com/datasheet/MG996R_Tower-Pro.pdf. Its running current is OK for the Duet, but the stall current is rather high. It is not continuous rotation.

    One issue I have found with this servo is that when it stops moving, it pumps a large amount of energy into the +5v rail, which was increasing the voltage to around 8V - more than the rating of the 3.3v regulator. This is likely to happen for any servo of a similar size. The 1.04 revision of the Duet has an extra 220uF capacitor on the 5v rail to help absorb this, but older Duets don't, so when using them it is advisable to add your own 220uF capacitor between the +5V and ground servo wires.



  • @dc42 Excellent. Thank you. Can you tell me what the current limit for servos on the Duex board is? Can't seem to find it anywhere on the Wiki.

    In view of your comment about pumping energy back into the 5V rail, would a geared 12v motor connected to a fan output (with Vfan jumper set to 12v) be a better choice? (I can see the current limit for that is 2A)


  • administrators

    @deckingman said in Best type of ancilliary motor:

    In view of your comment about pumping energy back into the 5V rail, would a geared 12v motor connected to a fan output (with Vfan jumper set to 12v) be a better choice? (I can see the current limit for that is 2A)

    DC motors tend to have high starting and stall currents (more than the fan mosfet peak current rating of around 4.4A), so I advise against connecting them to fan outputs. A DC motor plus flyback diode connected to a spare heater output would be better.



  • @dc42 Thanks again. So given a free choice, which would you do? A servo with capacitor or a geared DC motor with fly back motor connected to a spare heater output? Or is there nothing to choose between those options?


  • administrators

    Unless whatever it is you want to move is very lightweight, I suggest a DC motor.



  • @dc42 said in Best type of ancilliary motor:

    Unless whatever it is you want to move is very lightweight, I suggest a DC motor.

    Excellent. Brushless I assume?



  • Ian how about a brushless RC Motor driven with an ESC Which in turn is controlled by a servo output (The esc takes in the DC Supply and then drives the motor.

    Would think you would need a low KV one mind (KV being the RPM per Volt supplied rating)

    Motors

    ESC's

    ESC Are rated useing a S figure which means the number of Lipo Cells it is designed to operate with.

    HTH And that I am not trying to teach you to suck egg's

    Doug

    ps there are Brushed versions of those as well



  • @dougal1957 said in Best type of ancilliary motor:

    Ian how about a brushless RC Motor driven with an ESC Which in turn is controlled by a servo output (The esc takes in the DC Supply and then drives the motor.

    Would think you would need a low KV one mind (KV being the RPM per Volt supplied rating)

    Motors

    ESC's

    ESC Are rated useing a S figure which means the number of Lipo Cells it is designed to operate with.

    HTH And that I am not trying to teach you to suck egg's

    Doug

    ps there are Brushed versions of those as well

    Thanks Doug - you're not teaching me to suck eggs. I know next to nothing about motors, hence my asking for advice here. I'd never heard of an ESC (but I'm guessing it stands for Electronic Speed Control?)

    For the particular application I have in mind, the motor (or at least the output shaft) would need to rotate slowly. Possibly down to 60 rpm, maybe even less or it could be up to 300 rpm but I doubt much faster. From what I can tell, those RC motors and ESCs would be designed for much higher speeds yes?



  • Yep normally but some are quite slow it is a matter of selection



  • For low force at 60-300 RPM, I'd probably just find the cheapest NEMA 17 I can find off Amazon. Just about any NEMA 17 will work fine up to 300 RPM, and you know the Duet can run a stepper. Or, wait, do you need perpetual rotation? Does RRF have an RPM-based stepper motion command?

    If you don't want use a regular stepper, I'd go with a cheap surplus DC gearmotor.



  • @rcarlyle said in Best type of ancilliary motor:

    For low force at 60-300 RPM, I'd probably just find the cheapest NEMA 17 I can find off Amazon. Just about any NEMA 17 will work fine up to 300 RPM, and you know the Duet can run a stepper. Or, wait, do you need perpetual rotation? Does RRF have an RPM-based stepper motion command?

    If you don't want use a regular stepper, I'd go with a cheap surplus DC gearmotor.

    Thanks. I wondered about a small stepper (as per my OP when I mentioned a Nema 8). Yes I do want continuous rotation while printing (perpetual is a bit much to ask for ☺ ). I already use the 10 drivers that are on the Duet and Duex5 and I have plans for two more added to the ConnLCD connector so that's my maximum quota of stepper drives taken care of.

    The main reason for starting this thread was to find out what would be the best choice from a Duet point of view, given that I might want to vary the speed. It seems like a geared DC motor is the consensus.

    Thanks for the input.



  • @deckingman If you want approximate RPM rather than position, then yeah, I'd do DC motor (with diode) on a heater output. RPM will drop a bit with increase in load but if you have a gearbox it shouldn't drop much. Redefine the heater output as a fan and control with fan PWM settings and you're in business, I think.



  • @rcarlyle Thanks. Out of curiosity, if load increased to the point where the motor stalled, I'm assuming that would be a bad thing and likely burn out the motor. Is that a correct assumption?



  • @deckingman typically yes, cheap DC motors will overheat and burn out if held stalled for more than a few moments. Less of an issue if you’re giving it lower voltage to regulate down speed already (eg 20% PWM). You can also get gearboxes with clutch / slip type mechanisms but I wouldn’t know offhand where to order such a thing off the shelf.

    Could probably design yourself a printed clutch with some washers and a spring or something.



  • @rcarlyle Thanks. Thought so. I can make a clutch, that's no problem.


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