Tuesday 2/3 Barrel Lifter Design

Ok, here’s a TLDR description of how the barrel lifter will be assembled, but I just want to get this written down. If you can help me with any egregious errors, subassemblies, encoder/speed control, that would be fantastic!

The mechanism is a four link driven by four (count ’em four) motors. These will be a left- and right-side hybrid pair of window motor + gearmotor 0914, just like on the conveyor driver. I like this combo because it will generate a peak stall torque of 50 ft-lbs ; 40 ft-lbs is the anticipated maximum (nearly 10 lb barrel on a nearly 4 ft arm). Unlike the conveyor belt incarnation of this motor hybrid, these pairs must have the motors pointed both inboard to satisfy the collapsibilty requirement.

With a full-throttle speed around 1 rev/sec, we will definitely need some very effective speed control, perhaps encoder and PID? Limit switches may be OK, but won’t prevent carnage if it’s going fast.

Niles says he can laser engrave some painted aluminum or mild steel sheets with all the motor mounting holes copied over from AndyMark CAD files. That’s great! These must have 6″ center-to-center between the motor shafts, as well as some extra surrounding metal to make reinforcing bends with the brake.

The motor shafts will be coupled directly to the 4-links, with window motor & aluminum AM hubs embedded inside the 1″ square tube to save space. The hubs will be end milled to form two flat “cheeks”, mating into a square hole cut into one face of the square tube. Some reinforcing plates/tubes may be desired, and these would engage the bolt holes in the AM hubs, forming a “sandwich” around the shoulder end of the 35″ long arm tube.

The motors and arms will be mounted on the fixed end of the 4-link, which is inclined 20º, like the conveyor. The bracket carrying the motors is fixed to a 2X2″ C-channel which passes into the core of the conveyor belt frame in “U” orientation. There is a similar inverted-U piece holding the motors on the other side, and the two C-channels are nested within each other and their drawer-like motion is limited by aluminum pieces bolted to the conveyor frame. The head of a small sheet metal screw serves as a detent to prevent the C-channels from being pulled too far out of the machine. Because they are firmly squeezed by the frame, it may be unnecessary to provide any means of locking these in the transport/deployed positions, but a lynch pin (with a “remove before flight” ribbon) would be snazzy.

The window motor is topmost and with its hub is 3.5″ long overall, so the square tube must fit inboard of the flange. This 3.5″ is exactly the space available outboard of the conveyor to the edge of the chassis. (The chassis is roughly an inch narrower that the legal limit.) The gearmotor is 6.5″ long, but will fit into the space inside the conveyor frame if some care is taken not to break off its wiring. Some acrylic will need to be removed.

The range of motion, with the topmost window motor axis located 30″ above the floor allows the arm to reach the floor ahead of any loaded tote, and the topmost outboard link goes no higher than 67″ from the floor. If the barrel-handling device is 14″ long and rotates about the lower outboard link axis (which is 61″ above the floor), the highest extent of the robot is never more than 75″ tall.

On the “bucket” end of the mechanism, there is a fork with two 14″ long barrel-grasping fingers held 20″ apart by a removable piece of split PVC. The fork will rotate around the lower link axis, or a point 2″ above on the moving link (which would get up to 77″).

Rotation of the fork causes the barrel to “flip” at the upper end of the arm’s travel. The flip is initiated by a pair of Bowden cables/tubes (a heavy duty bike shift cable) on both the right and left side of the device. The upper end of the cable winds around the circumference of a small wheel and the tube is fixed nearby on the lower link arm. The lower end of the cable is attached to a fixed point so that there is plenty of slack in the arms’ lower range of motion, but it becomes tight in the final inches of the arm’s swing. There should be a return spring on the Bowden cables or flipper arm

There can be a drawer slide between the wheels (near their circumferences), behind the flipper’s axis of rotation which makes sure the fingers travel through identical arcs. Installing a split PVC pipe over the telescoping axle with a couple of velcro bands will ensure that the bucket end does not change width in operation.

The bucket picker could have a hook that engages under the lip of the barrel, but it must also provide some support below the center of gravity of the barrel. The CG is located at the top of the inner green triangle of the barrel’s recycle emblem, 16″ above the ground.

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