I’m building a chess-playing robot. The board might be as wide as 22’’. The pieces are standard sized (I’d guess 4-5oz). I would like to use a pre-built arm for simplicity.
How far (ballpark) could I extend the reach of the AL5D without stripping the servos? Is there anything I need to know before doing this? Is there a better option?
The AL5D can reach the entirety of an 11" chess board. You can increase the range by substituting the 4.5" tube in the forearm with a 6" tube, and you can also put a 1" spacer in between the two “C” brackets that make up the bicep.
Interesting part.
lynxmotion.com/Product.aspx?productID=711&CategoryID=89
Not sure where it’s used! I get the spacer aspect, more holes, presumably for a 45 deg. rotation?
The bore appears to be bigger then the 8mm, perhaps as much as .5"?
Just curious. I’m looking for something to work over the hex nut of the LPA.
Alan KM6VV
No specific purpose, just thought it could be useful. The ID is in fact 0.50" lol Wanted to reduce the weight to a minimum.
So is 22’’ out of the question? Will I need to look for a bigger arm/more powerful servos?
I’m not a chess guru or anything, but there has to be more options for the size of the chess board, than there are for larger hobby level robotic arms… Right? I’m pretty sure the AL5D is the largest 3DOF planar robotic arm available.
When sizing the arms reach you must define the angle of the gripper as it relates to the ground plane. Do you need the gripper to be completely vertical? If so the arm with modifications could to about a 13" x 13" board. Slightly larger is the gripper can be at an angle.
There is an upcoming article in the next issue of Robot Magazine illustrating how Steve Norris made a chess arm by rotating the board and using a sliding table for the standard unmodified AL5D arm. I think it would have worked perfectly without the sliding table, but that’s how he did it.
I should have clarified: this is for a competition. The rules say the board will be ~22’’ wide. The arm will be on a mobile base (iRobot Create). I really want to avoid having to navigate around the board in order to make a move.
If you were to place the arm mid-point along an edge this would require a reach of roughly 27". You should be able to estimate the torque required at the base servo if you know the weight of the gripper mechanism and have an approximate weight for 24" or so of tubing. Personally I would recommend putting the elbow servo back at the base and using a linkage to operate it so you avoid having to lift the weight of the elbow servo.
Another idea along those lines is to put an ASB-06 / HUB-08 on the back side of the elbow servo and add a counterweight to it. This should allow a much longer tube on the front. This is standard .5" OD .04" wall thickness aluminum tubing. 6" is the longest we carry though.
As a last resort, some of the metal suppliers stock this tubing in 3’ or 6’ lengths. I think I found some at:
You’ll have to cut it to length (hacksaw) and carefully file it square on the ends. Then cross drill a hole for the screw securing it to the flanges (a V-block works good for that).
Alan KM6VV
If you need a longer arm, you will need to do some thinking and make one specifically for the competition that has a longer reach. Lots of possible ways. Without any details of the competition rules, it would be hard to give suggestions.
Just thinking out loud, but since the chess pieces are all about the same height, you don’t need much of a Z (up/down) travel. This suggest to me that a Rho-Theta (pivot and extension) is all that’s really needed to get over any piece on a board. A counter weight on the other side of the pivot balances the arm. The pivot can be the rotary base. A continuous rotation servo and a long leadscrew moves a carriage along the arm. A second, shorter leadscrew moves the gripper up/down to move the chess piece. Some gearing and a multi-turn pot controls the arm leadscrew. For the short leadscrew, a 10-turn pot might suffice. Attach a gripper operated by a simple R/C servo, and you can pick up the piece. Arm length would be about 26", but doesn’t matter as it’s firmly supported on the rotary base, doesn’t move in or out, and is simply counterbalanced.
Alan KM6VV
