FRC 95 The Grasshoppers 2019 Build Thread


#182

Good footage! Surprising amount of flex on the elevator during impact, I’d keep an eye on the angle brackets that mount it to the plywood base. I can’t tell from the pictures what kind of turnbuckles you are using, but the cheaper steel ones that we’ve used in the past typically have an open loop for the eye of the bolt. We’ve tacked them closed with a MIG welder to keep them from deforming when under a lot of load.


#183

I think we’ll do something similar with these. We have also done that in the past, it’s a great way to effectively increase the load capacity of those types of devices for little cost.


#184

Lost yesterday to weather. Only two days lost this year, which is below average. Most years we lose 3-7 days to weather. With no bag day next year it will be nice to be able to ‘make up’ those days.


#185

Your robot is in a great position right now even with the snow storms. You are further along than our team is currently. The robot looks great and I can’t wait to play with you at week 1.


#186

When you say “make up”, do you mean that without bag day you will schedule less days per week and therefore more days for extra meetings, or something else?


#187

We won’t lose up to 1/6th of our build time every year, we’ll consider those ‘days off’ and add extra time to the end of our build schedule to make up for them.


#188

Last night was full of… uh… a lot of good learning opportunities!

Our first climb attempt:

We thoroughly destroyed a winch guide plate and ripped the friction pads off of the bottom of our climbing jack.

We have adjusted the winch configuration to avoid this failure in the future. We are also testing some better ways of attaching the friction pads, likely bolts or rivets will win out.

After repairing/modifying everything that failed we found a new failure mode: buckling!

We are going to rebuild the skids with a wooden core to improve their buckling capacity and look for some tiny rubber wheels to more effectively cushion the HAB 3 impact.

These set-backs aside, Cecilia has every major mechanism installed and weighs in at 112.5lbs, so we have a healthy margin to add and rework components as needed.


#189

Better now than your first event! :slight_smile:

In addition to adding wood core, will you be adding skid plates using PTFE or UHMW or HDPE to the front?


#190

Yup! We have time to react to the problem now.

No. The fronts of the skids are covered with small delrin wheels, which have even less friction!

We worked out the engineering statics of this rather funny sort of climb and having the lowest possible friction at the skid-HAB interface is key to everything working with a good margin of safety.

Also, special shout-out to @Monochron and 4561 in helping to inspire this HAB 3 mechanism, even if we failed in our first attempt!


#191

Quite the pickle. Less friction means you can slide up. More friction would help support the robot and take some vertical load off the back legs.


#192

Is it possible to attach the two lifting legs together, which might help preventing some twisting. Additionally, have you thought of adding passive wheels to your wedge, that way the transition is smoother?


#193

As the robot slides up the HAB 3 wall any friction at the HAB-skid interface will tend to increase the load on the jack legs, not decrease it. Unless I’m missing an assumption that you’re making…

The legs could be secured together, but the twisting was a result of a friction pad failing. They are connected at the top of course.

See two posts above for passive wheels, unless you had something else in mind.


#194

Sorry. Before 7am, statics vs dynamics is hard to think about. I was thinking of a robot on stands leaning on the vertical face of HAB3. The friction force would be upward. While in motion, the friction would be downward, adding to the normal required by the stands… and a torque against the lift… and that would be bad. :grimacing:


#195

I see that you do have some wheels, but I’m thinking you might have more success with a larger wheel, and by reducing the twisting on the system in general.

As a more crazy suggestion, does the wedge need to have a straight profile? What would happen,I wonder, if the leading edge of the wedge were to curve closer to your front wheels like this:


#196

As drawn I think it would interfere with the bumpers. Definitely would be better for sliding onto level 3 though.


#197

We found a larger wheel to be a little more forgiving. Maybe just at the bottom but set back from front of the arms to stay tangent with the wall? It would give you something to roll over a little easier once you get above level 3.

Is the plan just to lift the rear of the robot high and fall forward until the drive wheels contact the platform?


#198

How would larger wheels help?

If the skid pads on the jack don’t fail, what twisting do we need to eliminate?

My response might sound harsh, but I want you to articulate WHY you’re making this suggestion. why is about the most important question to answer in any STEM field!

This is a big pet peeve of mine: I do not like the question ‘what if…?’ (Along with ‘why don’t you…?’) When presenting an idea one should think about what merits and faults the idea has, and communicate those. It shows that the idea is partly thought through and invites constructive feedback and a good conversation.

I’ll humor you in this case, by asking another question:

When looking at the HAB 3 layout sketch, does having a tapered skid as you have sketched enable an easier break-over of the HAB 3 platform?

We lift up, slide onto the lowest wheels of the skid, then roll forward onto the drive wheels.


#199

take back my larger statement… Wider is what I intended. We had issues with our narrow wheels binding on their axles, even if just ever so slightly. It caused a lot of tracking issues. Wider wheels seems to have increased the bearing surface on the axle to reduce the binding problem. We are also using UHMW for our front wheels in our lift system.


#200

I guess I should have explained myself better, sorry but I am work so I was just quickly making suggestions. Maybe next time I’ll wait to post until I have more time to explain myself

My “larger wheel” suggestion was meant to address two issues. I think a wider wheel will have more traction which could be advantageous to prevent twisting; whether that twisting comes from a piece failing (as was the case in the video), or from the driver or alliance partner causing you to not be alligned perfectly. Secondly it seems as the bottom roller is small enough that when misaligned, the bottom face of the wedge contacts the platform instead of the roller which adds friction where you don’t want it. In this case, either moving the roller to be mounted not off center or having a larger diameter roller could help.

As far as the curved wedge suggestion; I’m sure you’ve looked at the geometry over and over again so you have a better understanding of this than I do, but assuming there are no bumper clearance issues (huge assumption), a curved wedge with rollers along the surface (similarly to how you currently have it designed with a straight wedge) appears at least me to create a smoother transition from the wedge to your front wheels.


#201

It looked to me that the slippage occurred in part because one skid plate cleared the platform before the other, which is visible in the screenshot below. When only one of the skids is no longer contacting the platform wall, there is no horizontal normal force provided by that skid and the robot can rotate about its vertical axis and decrease the normal force on one of the rubber foot pads. I’d insert a complicated 3 dimensional free body diagram, but it’s hard to draw.

A more gradual transition off of the end of the skid pad might help alleviate this. Another option is to just lift a lot faster and the transition period over the one-skid-on-the-wall phase will be quicker. Since so much of this lift process requires traction of the rubber feet, I’d also suggest getting some HDPE sooner rather than later to test with.

Also, here is a prototype concept I was playing with earlier this year that is related to this one, but different.