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I require your help regarding a problem I’m encountering.

I am a beginner in robot modelling. I am designing a robotic arm for agricultural use and I am having issues with the motors. I am using Maxon motors however the calculated load torque (including arm weight is larger than stall torque of all the motors within the Maxon catalog).

The robot arm is approximately 8 kgs and is lifting a weight of 12 grams (reach=1 m).

Does anyone know why there may be an issue and if there are any possible solutions?

Will adding another motor in the base half the loads that are applied to each joint? Or should I use gears to increase the output torque?

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  • $\begingroup$ You should also consider whether or not you can counterbalance the weight of the arm to reduce the required torque. $\endgroup$
    – SteveO
    Commented Nov 4, 2019 at 13:55

4 Answers 4

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Use gears, adding second one will increase the torque, but it's better to have 1 motor instead of 2. You can also add a gt2 pulley that increases torque decreasing speed. Replacing this motor with a stronger one will also help. If your motor's max speed is too high, I recommend gears or a pulley, so speed won't be too high.

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  • $\begingroup$ Thank yo so much for replying! I finally used gears to increase the torque. Another question i have when selecting a motor, how do i choose the watt in each motor? for example how do i know if i should your EC-i-40 70W or 100W? For the selection i just checked 1.the maximum calculate torque(including gears conversion) does not exceed the stall torque. 2. I selected a speed constant Kn>(nl+Δn/ΔΜ)/(Applied voltage) $\endgroup$
    – el.T
    Commented Nov 9, 2019 at 1:25
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If I understand correctly, your motors do not have enough torque for the given mass distribution in the robot.

There are a few suggestions to handle the issue

  • Use two motors at joints closed to the base that observe the maximum torque. (I would suggest doing some calculations about maximum torque requirements doubling up the motors). However, there would be challenges with synchronizing the two motors
  • You can use gears or pulleys or belt mechanisms and shift the motors towards the base of the robot, but they would have slippage and backlash issues. They might also result in speed reduction.
  • If you have electronics or other mountings increasing the weight of the links, you could shift them closer to the base to reduce torque requirements.
  • The best approach would be to use new motors with sufficient torques assumed to be efficient at no more than 80% of the catalog rating.
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  • $\begingroup$ Thank yo so much for replying! I finally used gears to increase the torque. Another question i have when selecting a motor, how do i choose the watt in each motor? for example how do i know if i should your EC-i-40 70W or 100W? For the selection i just checked 1.the maximum calculate torque(including gears conversion) does not exceed the stall torque. 2. I selected a speed constant Kn>(nl+Δn/ΔΜ)/(Applied voltage) $\endgroup$
    – el.T
    Commented Nov 9, 2019 at 1:28
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The robot arm is approximately 8 kgs and is lifting a weight of 12 grams (reach=1 m).

Another solution would be to design the arm to be much lighter. 12 grams does not require an 8kg arm to hold it one metre away. For example, a 30mm carbon fibre tube one metre long weighs 200g, adding a couple of pivots and rods for actuation should still be under half a kilo.

Also, the highest rating for a maxon gearmotor in this list is 120Nm, which would lift your original arm, but the smaller ones are cheaper.

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  • $\begingroup$ Thank yo so much for replying! I finally used gears to increase the torque. Another question i have when selecting a motor, how do i choose the watt in each motor? for example how do i know if i should your EC-i-40 70W or 100W? For the selection i just checked 1.the maximum calculate torque(including gears conversion) does not exceed the stall torque. 2. I selected a speed constant Kn>(nl+Δn/ΔΜ)/(Applied voltage) $\endgroup$
    – el.T
    Commented Nov 9, 2019 at 1:28
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You almost always need some type of transmission between the motor and the joint. This is often a set of gears or a timing belt with 2 different sized toothed wheels. In some case it maybe a planetary gear of cycloidal transmission.

Adding a transmission allows you to use smaller motor and to get more precision on your encoder feedback. The downside is that it makes the joint slower.

Also keep in mind that for agricultural use, the arm will probably need to be sealed so that it can be washed off to keep it clean.

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