Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: Evan3846 on December 05, 2011, 10:56:30 AM

Title: Robotic Arm Design
Post by: Evan3846 on December 05, 2011, 10:56:30 AM

Hello All,

I'm a student and am in the process of designing a robotic arm for a research company. I am planning on using the Axon II MCU but am having difficulty finding motors. The arm needs to have 3 DOFs and the main 'shoulder' joint motor needs to have a torque of around 10Nm. The only problem is that it needs to be able to hold the position for around 30 seconds, so my question is what would be better servos or stepper motors and have you got any reccomendations on what ones to use?

Many Thanks,
Evan

Title: Re: Robotic Arm Design
Post by: Soeren on December 05, 2011, 12:44:36 PM

Hi,

Quote from: Evan3846 on December 05, 2011, 10:56:30 AM

[...] the main 'shoulder' joint motor needs to have a torque of around 10Nm. The only problem is that it needs to be able to hold the position for around 30 seconds, so my question is what would be better servos or stepper motors and have you got any reccomendations on what ones to use?

You're not talking about R/C servos (hobby servos) I hope?
10Nm equals ~102 kgf-cm or ~1416 ozf-in.

I'd rather do it with a DC motor, sufficiently geared down of course.
How fast should it move?

Title: Re: Robotic Arm Design
Post by: Evan3846 on December 06, 2011, 04:51:07 AM

The servo I was looking at was http://www.robotshop.com/ProductInfo.aspx?pc=RB-Ins-01 (http://www.robotshop.com/ProductInfo.aspx?pc=RB-Ins-01) which could do 11.3Nm. The arm doesn't have to move fast at all, it just needs to be able to hold that weight.

Many Thanks

Title: Re: Robotic Arm Design
Post by: Soeren on December 06, 2011, 09:39:29 AM

Hi,

Quote from: Evan3846 on December 06, 2011, 04:51:07 AM

The servo I was looking at was http://www.robotshop.com/ProductInfo.aspx?pc=RB-Ins-01 (http://www.robotshop.com/ProductInfo.aspx?pc=RB-Ins-01) which could do 11.3Nm. The arm doesn't have to move fast at all, it just needs to be able to hold that weight.

OK, that's not a typical servo - extremely slow (90° in 1.5 seconds or the equivalent of 10 RPM) and outrageously expensive at $290  :(

Unless you have someone paying the bills, perhaps you might wanna consider something like this motor (http://www.robotmarketplace.com/products/RP-MAGNUM775.html) at less than a third ($90 to be specific) and with a speed of 720 RPM, which equates to 90° in ~20.8ms (although this is the running speed, it will be a little slower in start-stop conditions). The torque is higher as well.

Title: Re: Robotic Arm Design
Post by: Evan3846 on December 06, 2011, 10:16:28 AM

Ah yes I did think it was a tad expensive and bulky. That motor looks perfect for the job!  :) 

Thank you very much for the advice

Title: Re: Robotic Arm Design
Post by: Admin on December 07, 2011, 08:26:18 PM

90° in 1.5 seconds for the lifting base section of a robot arm isn't slow :P

Anyway, before you decide what is cheaper, make sure you price out a quadrature encoder + motor driver for that motor. ;D