Robot Arm C

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Re: Robot Arm C

Post by sachleen » January 20th, 2012, 1:52 pm

OldSpice wrote:That arm in the video didn't use VEX. They appeared to be using just plain old hobby servos and possibly a homemade controller. The controller is obviously wired which eliminates the need for a transmitter and receiver.
3 hobby servos are probably around 20 to 40 dollars depending on which type you get and the controller was probably around 20 bucks. I'd say Illusionist was probably right in assuming it was around 65 bucks.
The gears at the top and the claw are both from the VEXPlorer kit. The remote control can be seen on the right side of the video and it looks awfully similar to the VEXPlorer remote. The control box on the left has a switch on it which controls the direction of the motor that rotates the entire arm. That is custom.

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Re: Robot Arm C

Post by OldSpice » January 20th, 2012, 2:23 pm

sachleen wrote:
OldSpice wrote:That arm in the video didn't use VEX. They appeared to be using just plain old hobby servos and possibly a homemade controller. The controller is obviously wired which eliminates the need for a transmitter and receiver.
3 hobby servos are probably around 20 to 40 dollars depending on which type you get and the controller was probably around 20 bucks. I'd say Illusionist was probably right in assuming it was around 65 bucks.
The gears at the top and the claw are both from the VEXPlorer kit. The remote control can be seen on the right side of the video and it looks awfully similar to the VEXPlorer remote. The control box on the left has a switch on it which controls the direction of the motor that rotates the entire arm. That is custom.
Oh wow, I stand corrected. I'm used to VEX being all green and when I saw that the claw was red I assumed it was from a different manufacturer.
My apologies sir.
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Re: Robot Arm C

Post by illusionist » January 20th, 2012, 2:36 pm

If the robot is within the 30 x 30 box at the beginning of the run, but the back end of the robot exits the back edge of the 30 x 30 square during a run, is that a violation? I couldn't find any mention of it in the rules.
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Re: Robot Arm C

Post by sachleen » January 20th, 2012, 3:01 pm

illusionist wrote:If the robot is within the 30 x 30 box at the beginning of the run, but the back end of the robot exits the back edge of the 30 x 30 square during a run, is that a violation? I couldn't find any mention of it in the rules.
I think 6.c.v addresses that.

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Re: Robot Arm C

Post by illusionist » January 20th, 2012, 3:18 pm

sachleen wrote:
illusionist wrote:If the robot is within the 30 x 30 box at the beginning of the run, but the back end of the robot exits the back edge of the 30 x 30 square during a run, is that a violation? I couldn't find any mention of it in the rules.
I think 6.c.v addresses that.
How did I miss that...
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Re: Robot Arm C

Post by NinjaChicken » January 20th, 2012, 6:55 pm

Gah. I don't know how readily available money is to other teams, but anything we do comes out of pocket so this event is going to be our bane :(
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Re: Robot Arm C

Post by chalker » January 20th, 2012, 8:08 pm

harryk wrote: For instance my shoulder and elbow servos are 1/4 scale and output 343 ozin which is still not enough to lift a battery, though I can accomplish that through the use of some load balancing springs
This is a REALLY good tidbit I haven't seen publicly mentioned before and thus I wanted to call out so it didn't get lost. With the use of load balancing springs (or even a simple load balancing pulley / counterweight system), you can essentially cancel out all the weight of the arm structure / servos. Meaning your servos only have to have enough torque to lift the D batteries (should you choose to go for those points).

To give you a rough 'back of the envelope' calculation on the necessary torque:
D battery weight = ~150 g
Distance from a motor in the center of the arm square to the bonus jug: ~65 cm
Torque required to lift a D battery at the end of a 64 cm arm = .956 Nm
Torque output by the servos cited above = 343 ozin = 2.42 Nm

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Re: Robot Arm C

Post by chalker7 » January 20th, 2012, 8:14 pm

chalker wrote:
harryk wrote: For instance my shoulder and elbow servos are 1/4 scale and output 343 ozin which is still not enough to lift a battery, though I can accomplish that through the use of some load balancing springs
This is a REALLY good tidbit I haven't seen publicly mentioned before and thus I wanted to call out so it didn't get lost. With the use of load balancing springs (or even a simple load balancing pulley / counterweight system), you can essentially cancel out all the weight of the arm structure / servos. Meaning your servos only have to have enough torque to lift the D batteries (should you choose to go for those points).

To give you a rough 'back of the envelope' calculation on the necessary torque:
D battery weight = ~150 g
Distance from a motor in the center of the arm square to the bonus jug: ~65 cm
Torque required to lift a D battery at the end of a 64 cm arm = .956 Nm
Torque output by the servos cited above = 343 ozin = 2.42 Nm
I definitely agree, but one note is that you still need to make the frame/structure of your robot strong enough to handle the stresses of a balancing spring (which I imagine will be easier and cheaper than finding strong servos.)
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Re: Robot Arm C

Post by ichaelm » January 21st, 2012, 7:12 am

I just want to say that personally, I've had bad experiences with using normal hobby servos to handle loads like this. Even if you balance the torque with counterweights or springs, you still have the weight of your arm resting on the servo horn, unless you use an axle/bearing system to ensure that the servo is only used to provide torque, not to support the arm. That's what I recommend.

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Re: Robot Arm C

Post by chalker » January 21st, 2012, 7:38 am

ichaelm wrote:I just want to say that personally, I've had bad experiences with using normal hobby servos to handle loads like this. Even if you balance the torque with counterweights or springs, you still have the weight of your arm resting on the servo horn, unless you use an axle/bearing system to ensure that the servo is only used to provide torque, not to support the arm. That's what I recommend.

Good point, and actually not that hard to design. If you look at model RC airplanes that is inherent in the design of all the control surfaces. Here's a random image I just found using google showing such a setup: http://bfishersc.files.wordpress.com/20 ... 1024&h=332

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