# Robotic Arm Torque

Discussion in 'The Projects Forum' started by x1222, Jun 18, 2012.

1. ### x1222 Thread Starter Member

Oct 22, 2011
31
0
So I want to make a simple robotic arm for a summer project, with a 40cm lever arm with the same number joints as a human arm from shoulder to wrist. I'm going to use several hobby servos, I'm not sure there are any better motors used for arms?

The problems I'm going over right now are calculating the torque, and reducing servo speed.

I know how to calculate static torque, but there's no acceleration given in servo speeds, so how do you guys calculate the torque when the arm has to suddenly stop? Use a good estimate?

For servo speed reduction, is software or gears a better route? I've tried using software in the past to slow down servos, but they ended up jerky. Although, I didn't spend long trying. If I use gears to reduce the speed, say 2:1 ratio, I'm guessing it will also limit my range of motion for my servos as well?

Last edited: Jun 18, 2012
2. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
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You will need quite a bit of headroom over your static torque figures, I would aim for a few times the static torque.

There are a few robot arms of a similar size on the internet that people have made from RC servos. I suggest you google for "RC servo robot arm" or something like that and look at how they have used larger and smaller servos in the right places and whatever counterbalances and linkages etc needed to get the torques in the right range.

Here's one;

3. ### x1222 Thread Starter Member

Oct 22, 2011
31
0
Few times? That was alot more than expected. I feel dumb for not thinking about a counter balance.

Do you need specific servo's to sync them up? I would think using two servo's like that in the diagrams base, the servos would oppose one another.

4. ### Skeebopstop Active Member

Jan 9, 2009
358
3
Not sure I understand the question, but torque is related via a constant to the current. Regardless of whether it is static or moving, ignoring complex effects, this is true. Therefore generally closed loop control allows you to ignore many other parts of the system and it is only a tuning exersize of basic pid loops. Servos are used to reduce jerk as they are smoother than most

5. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
They've used two standard servos at the shoulder instead of one large one, it's a clever idea. If they are the same brand and type they will match pretty well.