2 Replies Latest reply on Nov 10, 2017 10:51 AM by agribot

# Motor Torque Calculations for Robot

Hi

Can you recommend an on-line web site that provides a clear and useful motor torque calculation tool ?, suitable for use by people working to define and select motors for use in medium size robots ?

So far I've found the following sites, but .. be ware .. they seem to provide very different results !

http://www.robotshop.com/blog/en/drive-motor-sizing-tool-9698

http://www.societyofrobots.com/RMF_calculator.shtml

and then there's an equation at this site;-  https://www.instructables.com/id/Complete-Motor-Guide-for-Robotics/

Different information, different results !

Do you know of a tool that you think provides clear and valid results ? Have you used such a tool and what was the actual result when you used the motor, compared to the calculations ?

• ###### Re: Motor Torque Calculations for Robot

Hi Robert,

Your brain is the best tool for providing clear and valid results.  All on-line calculators are just using some known formulas that you can find in any good physics book.  The problem with on-line calculators is that they know nothing about the materials and manufacturing quality of the parts they are simulating.  Because of this, they "guess" or estimate efficiencies, environmental factors, and friction values.  Unless you have actual empirical data for the motors, gears, bearings, and couplings you intend to use in a system, an on-line calculator is only going to give you an estimate of actual values.  Even the data sheets for the motors and servos have average values for a given series of parts that will have a +_ error for the parts you purchase.

If you have a DC motor and want to generate a motor curve:

1) Put a measured voltage on it and measure RPM - this is Max RPM for this voltage

2) Measure the current with no load on the shaft - this is the "No Load current"

3) Stop the motor by applying a torque at the shaft - don't hold it for very long or the windings will overheat.

4) Measure the current - this is the "Stall current"

5) Measure the torque required to stop the motor - this is the hardest part but can sometimes be done with lever arms and small weights - this is "Maximum torque"

6) Make a graph using a straight line from No Load Current to Stall current, vs 0 to Maximum Torque, vs Max Speed to Stall (0) speed

*** note *** If you change the voltage, the curves will also change

D.C. Motor Torque/Speed Curve Tutorial:::Understanding Motor Characteristics

The temperature of the environment and the motor windings will also affect the motor curves because the resistance and the friction numbers change.

The more you understand the motor physics, the better you will be able to use the tools to estimate the motor properties and behavior.  An on-line calculator will never give you the exact numbers that you will see in the final application, unless it takes into account all the possible variables in the system and the operating environment.

Clear as mud?

Scott

6 of 6 people found this helpful
• ###### Re: Motor Torque Calculations for Robot

I think that at least some of the torque calculation tools must be relevant and useful - they can't all be completely orders-of-magnitude wrong !. It's just that it would be valuable for people to share their experience with them, as well as making tests as you rightly suggest.

I'll probably do some sort of tests similar to what you describe and it might be interesting to compare results with prior calculations / specifications ( if clear data is available ).

Thanks for now,

Agribot