RC robot for a competition

[SciOlyStudent]

I am making an RC robot for a competition called Science Olympiad.  The robot has to be able to pick up objects and move them into a box.  I am pretty sure the different objects are a stack of index cards, a few 9v batteries and tennis balls.  I'm not good at guessing the torque and power and whatever I need in a servo to do this, so are there any suggestions for servos to use?  I'm going for low cost and fair quality.  And by the way, is there any way to hook up two servos to one channel so that you push forward on one control and they both go forward?

[SomeSaba]

the two servo thing is called a y-harness, assuming u want then to receive same PWM

You can use the robot arm calculator to find the torque u need for your servo
 http://www.societyofrobots.com/calculator.shtml

y-harness: http://www.graysonhobby.com/catalog/images/y-harness.gif

[SciOlyStudent]

Thanks you, but I am an idiot and need someone to translate for me .  Let's say I have a servo that has 47 oz/in of torque, how much weight can my arm lift if it is 7 in long and weighs around 6oz?  And by the way, can a servo spin in the opposite direction (if pushing up on joystick=forward, does down=backwards)?  If not, is there any way to make it so?  Thanks for any help!

[cooldog]

Thanks you, but I am an idiot and need someone to translate for me .  Let's say I have a servo that has 47 oz/in of torque, how much weight can my arm lift if it is 7 in long and weighs around 6oz?  And by the way, can a servo spin in the opposite direction (if pushing up on joystick=forward, does down=backwards)?  If not, is there any way to make it so?  Thanks for any help!

yes all servos go both ways

[bulkhead]

assume the 6oz acts half way along the arm, so the torque is 3.5in * 6oz = 21oz* in

47 oz*in - 21oz*in = 26oz*in

26 oz*in/7 in = ~ 4 oz

This is assuming worst case scenario, where the arm is perpendicular to gravity.

Cheap standard servos start at around $10 for ~50 oz*in.  A decent servo is the Hitec HS-645mg, which has 100+ something oz*in torque, and metal gears, for around $30.  Blue bird and Cirrus (identical) make a similar servo that costs around $20, but the quality isn't the same as the Hitec.

You can consider using levers/springs in order to help lift objects.

Check some of these videos out:

http://video.google.com/videoplay?docid=6982090482546045111

They might not be the best designs but theres much that can be learned...

[Admin]

And by the way, is there any way to hook up two servos to one channel so that you push forward on one control and they both go forward?

as SomeSaba said, yes you can, and you need the y-harness . . . but Im assuming it will be two servos driving the same joint right? if so, you will need to reverse the direction of one of them by using something called a servo programmer.

you may find this robot helpful for you:
http://www.societyofrobots.com/robot_asme.shtml

and to understand the math on torque and forces, read this:
http://www.societyofrobots.com/mechanics_statics.shtml

[SciOlyStudent]

New question time!  If I had a conveyor belt type structure on my robot, what would be the easiest way to make it continuously spin without having to do it myself (so I can drive it around while the belt is moving)?  If possible, I would like to avoid any circuitry.

[SomeSaba]

i guess just hook up the motor that spins the belt straight to the battery, or give it its own battery and pop a switch on it and voila!

[cooldog]

a idea get a r/c car or make the $50 robot r/c

next get this claw
http://www.pololu.com/products/joinmax/0093/

the hook up a mod servo to a string to lift and lower the claw

like a crane

vola

(there may be complictions but this is the basic idea)

you need a 4ch remote if you make the $50 robot with this mod
1ch- left side
1ch- right side
1ch- claw
1ch-lift