pic: 3946 off-season strafe module, oblique view

Oblique view of strafe module to show construction.

Is there a reason the piston is so close to the pivot point? Would moving it closer to the wheel provide more down force on the H wheel?

Yes, and that’s exactly why. A 2-inch diameter bore cylinder exerts a force of 157-188 pounds when pressurized with 50-60 psi. We only wanted about 30-40 pounds to be carried by the strafe wheel, so we applied it with a short lever arm. As noted on the preceding image, we were building with parts already in stock. If we’d had a 3/4" or 1" bore cylinder with a 2-4" stroke, we’d have mounted it much farther from the pivot.


We thought of using a lower pressure, but after using three separate regulators in 2013, we decided we did not want that maintenance headache again.

And yes, the stock used for the arm and gearbox mount is 2" x 1" x 1/8" c-channel, left over pieces from our Recycle Rush lift frame. The torque I’m most worried about is on that hinge, but it’s an exterior door hinge, so it should be able to handle 30 or 40 pounds of thrust at a 14" moment arm.

Edit: In case I misread the earlier question, the cylinder body is mounted in a piece of 2" x 2" x 1/8" aluminum angle (whatever HD had in stock; I think it’s 6061).

A much better solution: Mount the cylinder away from the pivot and use a regulator to lower the pressure in your 2 inch diameter cylinder until you get the desired amount of weight carried by your strafe wheel.

Also, I feel sorry for the bending moments in that bar stock supporting the nose of your cylinder. I recommend putting a 2x4 or something on there to better support the load. Edit, could be angle stock? If so, no problemo


I can’t imagine that that hinge does well with the side load it’s getting when the module is down either. Plus the side loads on the cylinder when the module is driving are a little worrisome.

I’d me more concerned with the sideloads the wheel produces on that hinge when driving. I could be wrong, though. With just a CIM reduced by 12 times in the toughbox onto a 3 inch radius wheel, that is potentially 100 lbf on the end of that arm, on a ~12 inch lever arm? That’s a good bit of torque on that hinge. Plus, there’s going to be a lot of backlash. I’d reduce the length of the arm and/or add side bearing supports to take the sideloads.

By nose mounting the cylinder while also mounting it to the pivot block, you’ll be putting a bending moment on the cylinder shaft when you actuate it, which isn’t best practice. Other strafe modules that nose mount their cylinders just use the cylinder to push on their pivot module rather than rigidly mounting the end of the cylinder to the module in order to avoid this moment. You could also pivot mount the top of the cylinder to avoid this.

I’m also concerned about the moment and side load placed on that hinge by the overhanging gearbox and wheel.

Look a bit closer - there will be no significant bending moment on the cylinder shaft unless the arm is practically ripped off the hinge. The pivot block pushes on a (loosely held) round bar which rides in oversize slots on the arm. So while the cylinder body is rigidly mounted to the chassis, the rod is coupled to the arm so as to only transmit force in the vertical direction.

By keeping the load carried by the wheel down to about 30 or 40 pounds, we are also keeping the strafe force down to that same value, as the static CoF of the hard plastic rollers is about 1.0 according to AndyMark. We were planning to keep a close eye on the hinge when we start driving and add some side bearing supports if needed.
If there’s most of an inch or more of backlash, we’ll definitely add side bearing supports. This is still much less effective backlash than a torque-actuated strafe module.

I think the valid point posters have made is you can improve this design a lot.

You’re on the right track. I’d encourage you to fail faster!


PS. “Fail Faster” is Golden Rule of Robots #4.

 If this had been for competition* we would have purchased a cylinder of more appropriate size, eliminating the need for the long arm.  If we don't need our Bimba coupons for the robot this year, this mid-size cylinder will be on our wish list.

Speed to testing was definitely one of our goals on this. Unfortunately for this project, the availability of the control system and programmers disappeared a few days into the process due to other team priorities. (Good reason, though: we are running workshops for two rookie teams plus our own rookies in java programming for FRC.)

What are Golden Rules 1 through 3 (and 5 through however many for that matter)?

    • or if our sponsors’ recent generosity had come a month earlier - many thanks to NASA Stennis and all of our sponsors!

Thanks for sharing your prototype! Can’t wait to see how it works - because that, after all, is the purpose of a prototype - not to be fully engineered and competition-ready.

I finally have an update worth reporting on this one!

I had hoped to use this as a hook to teach the controls and programming first and second years about pneumatics, which we did not use in 2015. The program and wiring teams did not get on board with this project before build season was upon us. Early during build 2016, the cylinder was removed, the hinged wheel tied off, and a very crudely built control board was added to this so it could serve as the programmers’ testbed. I figured the strafe project was history.

Then, in the aftermath of Red Stick Rumble (our only post-season event of the 2016 season), it was time to disassemble or re-purpose the Stronghold practice and test bed robots (the competition robot is being retained as a demo robot at least until build season). Christian, our HP* and bumper guy this year who also did a bunch of controls work, saw the motor, gearbox and wheel on the hinged arm, and asked (probably rhetorically, possibly sarcastically) what this strange mess was for - in my hearing. I ignored the tone, but walked over and explained the plan. He looked at me, and the robot, and at me, and the robot, and said “That would be so cool!” (or something very much like that).

In about four hours of build spanning two sessions, the removed pieces were located (or purchased) and replaced, the control board was centered, a hole was drilled in the control board for the cylinder to hang through, and the battery was tied down (not competition ready, but enough for a test drive). The roboRIO was still loaded with the Stronghold code, but they wired it so that the arm raise-lower became the strafe raise-lower and that the ball intake roller became the strafe motor. (This bothers me as I write this, because I realize that it means we had a CIM controlled by a spike.)

I would like to say that it worked the first try, but that would be a slight exaggeration. The robot was quite light, and the pneumatic working pressure was a bit too high, and the two wheels not pictured here came right of the floor and the frame stuck up about sixty degrees above horizontal like some over-the-top low-rider trick. Once the air pressure was dialed down, it worked almost exactly as designed. I did not note what pressure was needed, but Christian obviously did, because they did the trick and driving several times that evening always according to intentions.

I shot some cell phone video, but it’s too long to e-mail to myself. If I can get it off the phone onto the network, I’ll post a youtube link.

The strafe wheel made a significant racket as it slipped (should have been on a motor controller rather than a spike), and we new have skid marks on the shop floor, but we did achieve my primary goal for this project: To move H-drive from the “we failed to make this work in 2015” column to the “we successfully prototyped this drive in 2016” column.

To Corsetto: We didn’t fail fast, but we did finally learn from this one outside of build season! I don’t think that anyone came away with the thought that this was good enough for competition, but we spent less than $100 (excluding about 60 man-hours of our time over many months) on this and learned a bit of the art of the possible. That is time and money very well spent, in my book.

*For those of you at Bayou and/or Red Stick, Christian is the drive team member who wore our green 'fro wig this year, and who led our cheers from the stands with the gonfalon (standard).

The video.