Day 7: Commitments and Conundrums
Sorry that this post came out on Day 8…
With our drivetrain out for welding, the Design Committee formally met to commit to an arm design. With a short discussion about complexity, ease of build, and ease of scoring, the committee unanimously decided to go for the single jointed arm design. All subsystems except the gripper were formally decided on – my arm gearbox is currently planned for production soon. Details are being firmed up on the sensitive joints of the arm over the weekend – but we’re all on the same page and concept for sure now.
Then the discussion turned to the gripper. Since day 1, everyone has been in favor of a roller claw. The design is cool, effective, and proven useful. However, we wanted to make sure we were making a smart decision, so the Design Committee drafted a Weighted Objective Table. We started talking about features independent of any design that a claw would need. After brainstorming some specifications, we then went piece by piece and weighted them based on our perceived importance. Here’s what we came up with.
Criteria ranked by priority, 3 is highest.
From there, we went through the chart with four manipulator concepts. First was a hinged-jaw roller claw designed to articulate around the contours of tubes. Second was a simpler rigid roller claw similar to the popular 2007 design. Third was a wide pneumatic pincher, also like 2007. Fourth was an “inside tube” grabber that we didn’t really take that seriously. One by one we ranked and agreed upon how each design would perform each specification, only assigning a value with the consensus of everyone. After about 20 minutes of methodical categorization, we summed up all the values and came up with this surprising result:
Items ranked 1-5, 5 highest.
This result was quite surprising. Everyone thought a roller claw would be flat out better, but it looked like a pinching claw had a significant lead on our WOT. We double checked every number and we agreed with each individual assigned value, as we had spent a long time working out all the values. This unexpected result got us seriously thinking about our direction. After twenty minutes of discussion, we reached the following conclusions:
- As long as we commit to a design and refine it, the roller claw or pincher claw choice probably won’t make or break our season.
- The hinged roller is unacceptably worse than the piston pinch in nearly every category.
- Since we’re getting roller claw parts from (team name removed), we can spend the week or so until those parts get here building a pinch mechanism, testing them side by side by the end of next week, and deciding with more information on hand.
Overall, this semi-quantitative look at the engineering design tradeoffs in various manipulators was definitely an eye-opening experience and helped change our direction from here on out. Battle of the Manipulators starts today!
As for the minibot – we smoked our motors. Less than half a second of “stalling” them, kind of sort of, and poof. The Tetrix parts are annoying… I’m thinking we’ll end up being conservative and direct driving the wheels with the motors instead of overgearing. Marginally slower, but we need reliability. Luckily, one of our brightest students is heading up that subgroup so I don’t have much to worry about. I’ll put up pictures later…
-Chris
