Motor/controller question for a gondola/nacelle contraption

Hi everyone,

I’ve been researching the last few days for some parts to my idea, and I think the Roboclaw/37D combination might be what I need.

I want to build a contraption with two motors which are meant to lift a dynamic load varying from ~2kg to ~7kg similar to the spidercams that can be seen in football arenas - Just in only two dimensions.
sketch overview
sketch detail

For simplicity the whole mechanical and electronic part is meant to be part of the load (weight already included in the estimated load).
The motors should wind two nylon lines onto reels with a radius of ~6cm which would result in a torque of ~42kg*cm, which is, as I understand, too much for the 37D motor series at my desired speed/gear ratio.
But, I guess, the upper end of the load is negotiable by not moving the contraption “as far upwards” and also, by using lighter batteries.

I want to use an ESP8266 (Or a RasPi) to make the controller spin the motors with a (fairly) constant/controllable speed. The Battery would provide ~18V

  1. As the controller basically reads the encoder, is it possible to get the “revolution count” via USB/Serial to let my code apply corrections for possible deviation?
  2. How detailed can I control the speed, if exact position is not as important as reproducability?
  3. Up to which rpm can a Roboclaw controller achieve to keep the speed contant while the load varies (as I understand, the motor should be operated max to 75% of the stall torque)?
  4. As I understand, I can define a current limit to the motors and also get a readout of the currently used current, so if I’d set it up to a maximum of 4A (or any other recommended value) operation at this value would just result in a reduced speed?

Also please let me know anything you might consider relevant.


Almost forgot one question:
5) Are the pinouts of the 37D motors compatible with the appropriate Roboclaw controller, so there’s no need to re-wire/solder?

Hello, Andi.

Your torque requirement, 42kg•cm is significantly higher than what we would expect any of our 37D gearmotors to be able to handle, enough so that you should probably be considering more powerful motors. As stated on the product pages for all of our 37D gearmotors, the recommended upper limit for continuously applied loads is 10 kg-cm (150 oz-in), and the recommended upper limit for instantaneous torque is 25 kg-cm (350 oz-in).

Many of your questions about using the RoboClaw are answered in the RoboClaw’s user manual, which is available on the RoboClaw product pages under the “Resources” tab, so I strongly recommend you consult that for more detailed information. Here are some quick answers to get you started though:

  1. You can get velocity or position feedback from the RoboClaw using packet serial mode, but you will likely need to do a conversion to turn the feedback into whatever units you want to use.
  2. If you use the basic drive commands, you can set the speed with a integer value from 0-127. The accuracy/repeatability of that will depend on how consistent the load is on your motor and how well you tune your PID constants.
  3. As with the previous question, this likely depends on how consistent the load is on your motor and how well you tune your PID constants.
  4. If your load is such that the motor is drawing near the current limit you configure and you proceed to increase the load, the motor’s speed will decrease. We generally recommend running brushed DC gearmotors such that the continuous current draw does not exceed about 25% of the stall current.
  5. The connector on the 37D gearmotor with encoder is not directly compatible with the RoboClaw pinouts. Also, the pinouts on different RoboClaws are not identical across the different versions (most notably the single channel RoboClaws are much different from the dual channel controllers). You can find the pinout for each RoboClaw in its unique datasheet (not the user manual) which are available here. (That link is also available on each RoboClaw’s product pages under the “Resources” tab.

- Patrick

Hi there,

thanks a lot for your feedback.

I’ve researched a bit for more powerful motors but realized using these will also result in more weight requiring even higher torqe… (even building an “elevator” can be rocketscience? :wink: )
On the other hand I very much appreciate your (pololu’s) approach getting most components out of one hand and giving support to your community.

So - I re-evaluated my requirements and decided I can go with lower speed (by modifying the functionality of my payload)

So, looking closer at the 37D series, I have another thing that confuses me:
The note here says

The recommended upper limit for continuously applied loads is 10 kg-cm

But the stall torque for the 12V variants range from 0.5 to 49 kg-cm. What does the recommended torque apply to?
For me it seems the “recommended upper limit” should be more a percentage of the listed stall torque?

Sorry for asking these questions. My technical background lies more in programming than in mechanics/engineering.
From what I can see so far, using two 37D motors (with encoder) should work with the 2x7A Motor Controller? Because in that case the only thing I’d need to figure out is the best gear ratio capable to carry my weight regardless of speed…

The "recommended upper limit for continuously applied loads " value you are talking about has to do with the limitation of how much mechanical stress the gears in the gearbox can handle, so it is not directly related to the gearmotor’s theoretical stall torque. Low gear ratios won’t be able to get there, so it’s generally not as much of a concern for them. High gear ratios can surpass this threshold easily, so you need to be especially careful with those.

There is a relevant consideration that scales with the stall torque, and I mentioned it in my previous post: you generally want to keep the continuously applied loads to 25% or less of the stall torque, which keeps the operating current below 25% of the stall current. This constraint protects the motors and windings from overheating.

Driving two 37D gearmotors with the RoboClaw 2x7A should be fine.

- Patrick