So as you may remember from my previous post, my Intro to Engineering Design team is competing in a Vex competition in early December. We’ve made a bit of progress since last time.

This doesn't look generic or anything.


Previously, the team wished to use a four bar linkage arm with a Vexplorer claw to grab game pieces and dump them in an arbitrarily sized basket. Since then, we have done a bit more development and thinking. There was some concern that building a four bar linkage with crossing links would be very difficult. Others expressed concern over the uncertainty of the linkage once it passed the “cross point”, a very valid concern. Ultimately, a much more conventional looking robot arm was decided on. The claw is mounted to a wrist powered by a single motor geared 7:1, and the shoulder joint will be powered by a single motor geared approximately 25:1. It looks like any arm bot you’ve ever seen. The arm will start “folded up” into the basket and then expand after the match begins running “behind” the robot for most of its use. This lets us reach over shelves, up to doors, etc. while still fitting in the sizing box.

I thought up that little cantilevered first stage right as we were building it 🙂

Calculations were made to ensure the arm could lift the heaviest load (full soda can) in “worst case” scenarios; with this we arrived at our gearing choices. We didn’t assume any counterweight or balancing on the arms, figuring that Vex gears have enough backlash to stop significant backdriving and as long as we can move the load it’s fine. We may add some latex later, we’ll see… but even the 7:1 reduction stopped the Vexplorer claw from backdriving with a soda can in the gripper.


Simple wins.

We learned a few things quickly with our tests. First, with an increase in tank tread traction the treads can negotiate the “first step” of the stairs on their own, eliminating the required in chassis “foot” assisting device we had planned. Which is great, since the second thing we learned is that the small Vex chain is truly not designed for the loads of a drivetrain, at all. We drove it into the first step of the stairs and in under half a second, both properly tensioned chains failed completely. We were using a design that mounted the motors internally, “within” the tracks, using chain to connect the motors to the tread sprockets. This ended quickly.

It would have looked super cool though.

One team member proposed using bike tires as tread material. I’m not sure if he was joking, but we tried it and our thrown together prototyped worked GREAT for what it was. It ran up the stairs once, then repeatedly failed because the rubber bands holding the tread on would jam up. Our plan was to fix this with a more permanent mounting solution, but then we discovered that since the tread “expands” around the corner of a sprocket, the tread could not be held in place with any kind of rigid attachment method. We really didn’t want to go with the foot, so we decided to try another idea…

Arm AND Drive!

We assembled the arm and then quickly tested it and found that the 25:1 shoulder reduction was a lot of torque for a relatively light arm (less weight than expected). Perhaps one crazy idea might just work…

Well damn.

With proper coordination the arm lifted the chassis off the ground easily. It even pulled the chassis forward onto the first step. With a little play it could pull the chassis further up the first step and nearly onto the second one. This was without any tank treads on the robot; with a drive base to move the bot forward during this operation we could easily get up the stairs. This saves us considerable time making custom treads, weight with a bogey wheel, and many other headaches in general.

When I first proposed this idea on Friday it was the “ah-ha!” moment in our design process. We combined two complex parts into one simple and light part with dual functionality, and all we have to do to have a “mechanically complete” robot is to get the high strength motors on order from Vex and install them in our drivetrain.