### Author Topic: How to hold a load  (Read 1084 times)

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• Beginner
• Posts: 2
##### How to hold a load
« on: February 20, 2011, 10:49:10 AM »
So for a senior project our class is designing and building a R/C bomb disposal robot. From our statics we have found our 10lb load to need a torque of 33ft-lb to lift it. What we need to figure out is how that load will be held after we stop powering the 18v motor attached to the arm when it reaches the proper height. As far as I know, the instant we release the control stick on the R/C the arm will lower because of the 10lb load. Any ideas??  P.S. We are almost done Inventor drawing so this needs to be handled STAT if it will require a mechanical fix. Thank you.

#### rbtying

• Supreme Robot
• Posts: 452
##### Re: How to hold a load
« Reply #1 on: February 20, 2011, 11:47:07 AM »
At first glance, I'd suggest a worm gear setup - it requires no motor power to stay in one place.  You could also implement a position PID controller for the motor, although this will draw lots of power (not necessarily a good thing).

A way you could reduce the force would be to use a constant-force spring to reduce the load (acting in the opposite direction on the arm).  This allows your arm to be, relative to the motor, a tiny fraction of its weight, and easy to keep in position.

#### Cyntrox

• Jr. Member
• Posts: 13
##### Re: How to hold a load
« Reply #2 on: February 20, 2011, 01:02:05 PM »
You could apply partial power to the motor, using pulse width modulation (PWM). You would have to work out how much power to apply, though.

• Beginner
• Posts: 2
##### Re: How to hold a load
« Reply #3 on: February 20, 2011, 04:55:09 PM »
Thank you both very much. I will ask the others tomorrow. Great ideas!

#### TrickyNekro

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##### Re: How to hold a load
« Reply #4 on: February 21, 2011, 07:12:34 AM »
I really support the worm gear setup... But keep in mind that feedback will still be needed for position control.
For whom the interrupts toll...

P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

#### Soeren

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##### Re: How to hold a load
« Reply #5 on: February 22, 2011, 03:57:44 PM »
Hi,

As much as I second (or "thirth"?) a worm drive...
If the joy stick arm is spring loaded and return, so should the arm it controls, no matter the mechanical implementation, so the stick has to be changed a bit.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?