Sizing my DC Motor and Controls

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Sizing my DC Motor and Controls

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I have an unique application where I am using a DC motor and controller. I do not know what size motor to start with for this application or what type of controller I need. At this point this is theory and I need help putting together a prototype to test my theory. I need to attach my motor to a 7" pulley that has a half inch shaft size. On this pulley is 60' of 1/8" aircraft cable. The motor is mounted vertically and the pulley is positioned horizontally. The cable transitions from around the pulley horizontal, then vertically through a tube where it is attached to a 10 ounce object. The motor needs to be in neutral mode to start. The object is struck with great velocity which starts the motor and pulley turning. The encoder counts the revolutions to tell the motor when to engage. The idea is to start retracting before the object reaches the end of the 60" cable. The motor amperage is a factor because the weight of the object and the velocity are significant at the time I am asking the motor to start, so the motor amperage needs to be sized accordingly. The motor will spin at the desired rate to retrieve the 10 ounces back to the home position where the encoder will tell the motor to stop and go back into neutral. The process will be repeated thousands of times. I can be reached at 803-335-7799 if you have suggestions or find these type of project interesting. Thanks so much! Chris

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Re: Sizing my DC Motor and Controls

Post by adafruit_support_bill »

The cable transitions from around the pulley horizontal, then vertically through a tube
How does the horizontal cable make the transition to the vertical tube? Sounds like there may be a lot of friction there.
The motor needs to be in neutral mode to start. The object is struck with great velocity which starts the motor and pulley turning.
So you are going to either need a motor that can be easily back-driven, or a clutch.
On this pulley is 60' of 1/8" aircraft cable.
So your 10 ounce object is going to be pulling close to 30 ounces of steel cable - plus accelerating whatever mass there is in the pully & motor. Not to mention the drag from friction. And your motor is going to need to do all that in reverse.

What exactly is this system designed to do?

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Re: Sizing my DC Motor and Controls

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Thanks for the reply:
The transition from horizontal to vertical goes through 90 degree aluminum tubes just like on a bike brake. Cable is also about that size. The 60' of cable weighs less than 6 ounces. The question of needing a clutch is my issue I am afraid. When the motor starts pulling in reverse there will be very little resistance but once the slack hits, it will have a momentary spike I expect. After the initial impact it will just be pulling 1 pound back until it sees it is home. I have a patent on this but prefer not to put it out there until it's on the market. If I can perfect it. If you wanted to call I would be happy to share the design with you. 803-335-7799.

Thanks again,

Chris

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Re: Sizing my DC Motor and Controls

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The transition from horizontal to vertical goes through 90 degree aluminum tubes just like on a bike brake.
Steel on aluminum has a relatively high coefficient of friction. Brake cable housings typically have a low-friction liner such as polyethylene or Teflon which can reduce the friction by a factor of 10 or more. The radius of the bend will also be a factor. You could do a lot of scary math to come up with an estimate of the drag, but it is probably a lot more practical to just prototype it and measure directly.
Cable is also about that size. The 60' of cable weighs less than 6 ounces.
OK. Bike cable is substantially thinner than the 1/8" you mentioned earlier.
The question of needing a clutch is my issue I am afraid.
I suspect it will be necessary. DC motor have little torque at low speeds, so you would need a gear-train which would be difficult to back-drive. Steppers have very good low-speed torque, but significant 'cogging' resistance when unpowered.

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Re: Sizing my DC Motor and Controls

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I think I am good on the brake lining tube. It is lined with Teflon as you mentioned. When ordering the cable it calls it !/8 inch cable. What controls should I use and what size motor should I start with? I am fine with trial and error as well. Doesn't appear these motors or controls are extremely expensive. Where would the clutch mount. I am assuming between the pulley and the motor? If you are able to call me I would really appreciate it. I would love to get these on order so I can start putting my prototype together ASAP.

Thanks again!

Chris

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Re: Sizing my DC Motor and Controls

Post by adafruit_support_bill »

You'd want the clutch between the motor and the pulley. To spec the motor you would need some estimate of the maximum torque required. And the driver selection would depend on the motor requirements.

Mechanical engineering is not our forte. But it sounds like you would probably need a fairly high-torque stepper and a driver to match. We have one fairly high-torque NEMA-23 stepper, but we don't have a driver yet that will get optimal performance from it. www.pololu.com carries a much wider selection of steppers and drivers.

For a clutch, I'd check McMaster Carr (mcmaster.com), Stock-Drive (sdp-si.com) or Misumi (misumi-ec.com)

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Re: Sizing my DC Motor and Controls

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Thanks so much. I will check with the sources you suggested and order the stepper from you. What is your name?

Thanks again, I will definitely let you know how this turns out.

Chris

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Re: Sizing my DC Motor and Controls

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I ordered two different clutch bearings they were less than $10 each. I ordered two power supplies. One for the motor and the other for the drive/controller. I want to use the H-Bridge motor driver and the feather M-4 but not sure how to order them or how much they cost. Also, The NEMA-23 stepper motor needs to have an encoder on it. Is that an option?
So for a recap. I need a motor with an encoder, H-Bridge driver, and the feather M-4 all to work together. Once I have these 3 units I can put it all together. Please give me the right part numbers to order these 3 pieces if you can. Would this motor be big enough in your opinion? Geared DC Motor with Magnetic Encoder Outputs - 7 VDC 1:20 Ratio PID: 4416

XiKe 1 Pack CSK8PP Bearings 8x22x9mm, One Way Bearing with Keyway Sprag or Clutch Freewheel Backstop.
Bstinay Bearing Steel CSK8PP 8x22x9mm One Way Dual Keyway Bearing Clutch Sprag
I have ordered (2) adjustable power supplies. SHNITPWR 60W Universal Power Supply DC 3V 4V 4.5V 5V 6V 7V 7.5V 8V 9V 10V 11V 12V Adjustable Variable Power Adapter 100V-240V AC to DC Converter 1A 2A 2.5A 3A 4A 5A with 14 Tips & Polarity Converter.

Thanks so much for your help!
Chris

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Re: Sizing my DC Motor and Controls

Post by adafruit_support_bill »

So for a recap. I need a motor with an encoder
We do not have an encoder option for that motor. Larger industrial suppliers like AutomationDirect typically stock motors with integrated encoders. You can also find add-on encoder modules such as this one: https://www.cnc4pc.com/capacitive-encod ... cable.html
H-Bridge driver,
A bipolar stepper motor needs 2 H-bridges - one for each winding. And a high-torque stepper generally requires a current limiting "chopper" circuit on each bridge. There are plenty of stepper driver modules that incorporate the 2 bridges and current limiting into a single circuit. www.pololu.com carries a good selection of them. https://www.pololu.com/category/120/ste ... or-drivers
Would this motor be big enough in your opinion? Geared DC Motor with Magnetic Encoder Outputs - 7 VDC 1:20 Ratio PID: 4416
It is not clear what the torque requirements of your project are. But the stall torque on that motor is only 1kg/cm. That is about 1/10th of the torque of the NEMA23 and I doubt that it would be sufficient.

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Re: Sizing my DC Motor and Controls

Post by Return2Home »

Awesome feedback, thank you. I am obviously out of my league here and you are so much more knowledgeable. Can you please put together a parts list of what you feel I need to order so I have a starting place? Once I learn more about what it is I am dealing with I can try to tweak the various components until I find the perfect combination. Your input is invaluable to me and if it is not perfect no big deal but having a starting place is key for me right now. If you would be so kind as to just list part numbers and manufacturers you feel would work, I would be most appreciative. I have two variable power supplies and the clutch.

Thanks a million

Chris

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Re: Sizing my DC Motor and Controls

Post by adafruit_support_bill »

Hard to put together a specific BOM since what you need to get started are not products that we sell here at Adafruit. For prototyping a project like yours, I'd probably start with something like the Pololu TIC controller: https://www.pololu.com/product/3138
This will drive a pretty beefy stepper and has inputs for a quadrature encoder. It also has multiple control interfaces, so you can drive it from your computer for easier prototyping in the early phases and transition to a dedicated microcontroller for a final product.

The folks over at Pololu are pretty helpful and should be able to recommend a suitable motor, encoder & power supply combination that would work with that.

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Re: Sizing my DC Motor and Controls

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Awesome. I will get with them right away. If there is any way I can use your products I will.

Thanks so much!

Chris

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