### Author Topic: Calculating required servo torque  (Read 4063 times)

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#### rexxy

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##### Calculating required servo torque
« on: July 05, 2011, 05:09:38 AM »
Hi All,

Can someone please explain how you would go about calculating the required servo torque for the following structure considering the mass of the arm acts purely in a vertical direction and the servo torque purely in an horizontal direction.

thanks!

#### corrado33

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##### Re: Calculating required servo torque
« Reply #1 on: July 05, 2011, 02:55:04 PM »

#### rexxy

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##### Re: Calculating required servo torque
« Reply #2 on: July 05, 2011, 09:13:36 PM »
yea i've looked at all the tutorials however from what I can see unless i'm missing something is that they all assume the arm is moving in a vertical motion, not a horizontal motion. If the arm is moving vertically then no doubt the weight has a significant affect on the required torque on the motor however if the arm is moving purely horizontally then how does the weight of the arm affect the required output torque of the servo?

#### waltr

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##### Re: Calculating required servo torque
« Reply #3 on: July 06, 2011, 10:10:17 AM »
The weight of the arm does not affect the torque requirement, the MASS of the arm does. Also the friction of the pivot bearings.
The Mass of the arm needs to be accelerated and that requires torque. Once the arm is moving no torque is required to continue motion at a constant speed except to counter the bearing friction.

So this becomes a calculation of dynamics not statics. Do you have a first year college Physics book? If not your local library should.
Look up the chapter on "Dynamics of Rotational Motion".

#### rexxy

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##### Re: Calculating required servo torque
« Reply #4 on: July 06, 2011, 10:04:16 PM »
ahhh yes thanks.

So the required torque is given by

T = (m)(r^2)(angular acceleration) + pivot bearing friction.

However am i right in saying given that since the servo doesnt really have an acceleration requirement (can move as slow as i want - within reason) and calculating the pivot bearing friction cant be done (since im trying to select a servo, not see if one I already have will work) basically makes this calculation useless?

thanks

#### waltr

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##### Re: Calculating required servo torque
« Reply #5 on: July 07, 2011, 07:43:49 AM »
Not really. If that arm is small (less than 10's of cm) and the Mass is low (less than a few 100g) then any of the standard size servos would work (this is just a feel from experience with servos). But if the arm length and mass are larger then it would be a good idea to run the calculation to ensure that the servo will actually move the arm in a reasonable time.

A servo Does have an acceleration (there will be an acceleration any time something goes from zero speed to X speed). The acceleration may be very low but it is not non-existent. You can calculate the average 'angular acceleration' by the equation: aa = (Vx - V0)/time to use with the required torque equation.

The bearing friction and even the total torque needed could be measured with a pull scale or pulley weight system. The directly measured value will be the Force at some distance from the pivot.
Is that enough of a hint?
« Last Edit: July 07, 2011, 07:45:55 AM by waltr »

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