Author Topic: RMF Calculator - "motor characteristics" question about torque and speed fields  (Read 1840 times)

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Offline RoboRookieTopic starter

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Hello,
In the Motor Characteristics section there are two inputs for the motor parameters. Can you please clarify which parameters I should put in these fields? Specifically:
  • Torque field - Should I use the stall torque here?
  • Speed field - Should this value match the "Motor Rotation Speed" value in the "RMF Results section"? I assume it should, since according to the calculator, this is the rpm I need to run my motors at in order to achieve my desired velocity. Obviously I'd need to make sure that this speed is reasonably under the no-load speed (about 83% of no-load speed? See question below).
Since this PMDC motor doesn't specify nominal speed (no datasheet that I'm aware of), can I assume it's about 83% of the no-load speed? I came up with this value by comparing nominal and no-load speeds for this motor: http://www.robotmarketplace.com/products/images/0-COPAL60.pdf.

Link to motor the actual motor I'm considering: http://www.pololu.com/product/1101

Offline mklrobo

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 :) Hello!
In order to help you, I will need more info;
> Motor Characterists section - what is this, a web link, datasheet, etc?
> type of motor - PMDC yes, but can you be more specific? (Maybe I could scrape up the datasheet)
what type of application? Keep me posted....   ;)

Offline Schlayer

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   This calculator confused me with absurd underestimates for motor RMF at first. I've found a good method for a getting a realistic result is to take your no-load speed and reduce it by roughly 70-80% (your 83% measure may be a good guess for a lightweight wheel) and then for torque, do NOT use the motor's stall torque. You actually want to input the torque that your wheel will have before any external gearing. The motor you posted lists only rpm and stall torque values for the motor plus gearbox; that should be fine as long as you don't accidentally use the no load speed for the motor without gearbox and the stall torque with the gearbox. Just be consistent.
   In your case, just use the geared speed and then calculate the wheel torque based on the geared stall torque. The table on that page says with a 100:1 box, you have 320 RPM    and 30 oz-in. The wheel torque will be 30oz-in * (wheel diameter / shaft diameter). then enter .8*320rpm (or whatever percent you choose) for the speed, and you should get a fair estimate of RMF. You need to figure out yourself what wheel you are using and the schematic on that motor page tells you the shaft diameter is 2.5mm.
   I first discovered that my own values were incredibly off when I tried some calculations with a motor that I knew was used for a specific purpose but that the calculator told me has 1/10th the RMF necessary for it. Using the correction I've outlined above I got a new value of twice what was needed instead, and with a motor 100W more powerful than the typical one used for said purpose, it seemed like a very reasonable result.
   Best of luck, I hope this helps!

 


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