Author Topic: Post-WPI to-do 3 of 3 (new arm)  (Read 229 times)

Louis L

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Post-WPI to-do 3 of 3 (new arm)
« on: March 06, 2018, 09:51:50 PM »
Use this to list what needs to be done to create a new arm.
« Last Edit: March 06, 2018, 09:54:42 PM by Louis L »

Louis L

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Re: Post-WPI to-do 3 of 3 (new arm)
« Reply #1 on: March 06, 2018, 11:51:27 PM »
Description
  • Create a new arm that will replace the existing one starting from the top shoulder of the current design. The tower remains the same. The extension, gripper/claw and wrist are replaced.
  • The intake is no longer necessary. It may be retained but since we removed it from the bag, we need to make sure the combined weight of the new arm and intake is not more than 30 pounds. If more than 30 pounds, we just don't use the intake. If we keep the intake, some changed may be necessary either in hardware or software to make sure it doesn't physically restrict the operation of the new arm.
  • The new arm is just a length of material (aluminum) that pivots starting at the former shoulder joint (as before) and holds a new claw at the end. The new claw is essentially an ice lifter mechanism.
  • The ends of both claw segments is just a round rubber (or similar material) nub that, when pressed against a Cube, presses into the nylon cover of the Cube and locks into the space between the Cube's rubbed walls.
  • The claw is closed by force from a Vulcan constant force spring. It is opened by a Bosch seat motor operating a cam. Sensors (switches) tells the RoboRio when the arm is open or closed. The claw is opened and closed by a motor and push-rod assembly. When closed, there is no chance of the claw opening and dropping the Cube.
  • Should the sensors fail, we'll obviously have issues but because the motor just turns in a circle, it won't burn out. This is a much safer design. The motor will turn only within a range of motion to open and close the claws. Limit switches will limit the rotation. The entire unit will be protected from external damage.
  • For autonomous, the cam placed is in the closed position, meaning the spring is forcing the claw closed. A drive team member manually opens the claw and inserts a Cube. a pin is used to detach one side of the claw to allow a Cube to be placed in its starting position. The pin is then reinserted.
  • A second joint (elbow) further up the new arm folds the arm upwards to shorten it. This is used to keep the arm inside the frame perimeter at the beginning of the match as well as allowing it to reach the highest positions on the Scale.
  • The gap in the bumper is no longer needed. A new bumper segment will plug the gap.
Performance
  • Can grab a Cube without an active intake.
  • Can grab a Cube in various orientations. It can't do every orientation but that's not necessary.
  • Can grab a Cube at various distances and heights and off-centered alignments due to the width of the claw's opening. Obviously, there is still the 16 inch restriction at play.
  • Can reach up to the upper Scale positions
  • Can reach the climber height if the arm is used as a Climber
Software Impact
  • Cube grab and release changes. It's just a motor rotation.
  • Arm folding has 2 (or maybe 3) positions. That's it; no fancy positioning.
  • Overall arm movement uses distinct steps - grab Cube, fold arm, raise arm, unfold arm.
  • No PIDs needed
  • Open and Close claw just has a sensor.
  • Autonomous remains the same in terms of navigation. The Cube drop will change.
Benefits
  • Revolves most (if not all) of the deficiencies of the current design. Just about everything that is wrong is gone. This is important. This is not a BandAid placed over a defective design; this is a replacement that is simple, light and better performing.
  • Easy to build and install. Unused electrical parts can stay in place; or can be removed if time allows. Unused software can stay as well - they just won't be used.
  • The new assembly is much lighter (no extension, one less motor) meaning we can add a Climber. If we remove the intake, the whole robot is even lighter.
  • We can also add protective covers to the rest of the robot.
  • It can't be worse than what we have right now!
Development Cycle
  • 1 week to assemble. Software can develop in parallel.
  • CAD model (done).
  • Proof-of-concept for claw (done).
  • Build wooden platform to hold new arm. Attach test-bed and arm to this platform. This can test the new claw and elbow joint to fold the arm.
  • To test the new arm with the tower is more difficult unless we build a replica of the tower's chain drive system. If we do that we can test things further. But we know that the tower can handle this type of movement.
6 Hr Open Bag Time Estimate
  • 1 hr to replace the arm, connect wires, clean up the old cables (tuck them safely away; no shorts).
  • 0.5 hr of additional needed cleanup like a hardstop of the arm, protective polycarb, etc.
  • 3 hrs of software integration and testing - just the arm.
  • 1.5 hrs for whatever else we want (other stuff to do, practice, etc.)
Other Side Effects as a result of weight reduction
  • We can add a Climber.
  • We should add a hard stop to the arm.
  • We should add protective polycard to various parts of the chassis to protect against falling objects.
« Last Edit: March 16, 2018, 02:37:06 PM by Louis L »

Louis L

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Re: Post-WPI to-do 3 of 3 (new arm)
« Reply #2 on: March 16, 2018, 02:27:46 PM »
Update - after trying with a wooden mock-up, we found that using a spring has a drawback: when the added mass of the Cube is taken into account, horizontal movement of the arm (such as when the robot turns) shifts the force applied to the Cube by the 2 pincers. The forces are no longer even due to the robot's movement and this causes the Cube to fall out. One solution is to add strength to the spring. Another is just to lock down the pincer. We decided to replace the spring with a motor that will effectively close the pincer to a fixed position and keep it there.

Louis L

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Re: Post-WPI to-do 3 of 3 (new arm)
« Reply #3 on: March 26, 2018, 10:50:53 AM »
Post RIDE update.
This arm worked for the most part. The most glaring problem was that it sometimes refused to release the Cube. We didn't see this happen until about halfway through the matches. We believe this happens when the Cube's fabric starts to stretch from use and when we grab the Cube where the handle lies. The circular guard plate we use to keep the nubs from digging too deeply into the Cube is getting caught on the plastic lattice behind the nylon fabric.

We tried to fix this by enlarging the guard a little bit (we have to be careful how large to make this to keep within the 16 inch rule). Finally we removed the plate entirely. This seemed to work but we would need extensive testing to prove it really is fixed. We'll also need Cubes that are beat-up to simulate the real game pieces.