Re: Linkage Design Problem
 

Here's a simpler solution to the problem that a friend of mine came up with.



In this design the force is constant and tangential.

Here's the formula and derivations you'll need to determine the stroke length, sprocket diameter and angle of rotation.

Let the angle of rotation be theta
Let the sprocket diameter be d
Let the stroke length be L

Arc Length = (theta)x(d/2)*(pi/180)
- from the diagram we know that Arc Length = L
Therefore:
L = (pi)(theta)(d)/360
OR
d = 360L/(pi)(theta)
OR
theta = 360L/(pi)(d)

This solution is simple, elegant and definitely doable within the constraints we see in the FRC.

Re: Linkage Design Problem
 

I was toying around with ideas very similar to these earlier in the season, before deciding that they would not be strategically worth the added weight, complexity, real estate, cost, and complexity in a typical FRC game.
A standard scrub steering system can escape most situations that a system like this would be useful for (and many of the situations it could not, this wouldn't be able to either, such as the areas immediately in front of the corner goals this year). A system like this grants a limited amount of increased ability, for a cost almost equivalent to that of a "normal" sweve system. The one area it saves a great amount of complexity would be in the programming (but the load may be passed along to the drive crew, depending on how you decide to control the system).
Also, to get greater usage from the system would demand an on-board compressor, especially if you use pnuematics elsewhere on your robot. And that compressor also drains the battery, which was one of the reasons you decided to use pistons over motors (lesser battery drain).

Re: Linkage Design Problem
 

When I began considering this design, the idea was not to gain full swerve capability, but to offer some swerve mobility to a defensive base. The problem is that holonomics have little pushing ability, and Crab/swerve drives have inherent mechanical weakness that do not lend them to really, really aggressive driving. But that's not what they're designed to do in the first place. This more or less bridges the gap, and creates a more agile base with fully defensive capabilities. The costs are high- not in complexity, but in space and weight. The trade off is some mobility for robustness.

Re: Linkage Design Problem
 

While obviously being more agile than a standard scrub steering system, I fail to see how it is more robust or better defensively than a more traditional crab or swerve system.
Drive system styles are no more or less powerful than eachother, just more or less efficient. That is, you can create a holonomic system that has more speed and/or pushing power than any other drive system. But it recquires more motors and/or current than it would with a different system. A 4-wheeled holonomic system get's 50% of the efficiency of a scrub steering system when driving in a straight line (but has the equal efficiency when spinning in place, as all 4 motors can be fully powered and applying the same % of their power to the final vector of movement as a scrub system would). A swerve system applies the force of it's drive motors identically to how a scrub system would, but it losses 100% of the power of its steering motor(s) (or pistons in this case). Assuming a "traditional" crab system has 5 identical motors, it would have a theoretical 80% efficiency.
In short, your system is less efficient in terms of driving power than a scrub system, but posseses added agility. Depending on the game, I do not beleive that the added agility is worth the price of weight and space (and complexity) for the limited increase in agility it gives. But that cannot really be decided until the new game is revealed.

Re: Linkage Design Problem
 

there is something missing from your analysis. With a scrub drive (skid steer?) system, if you have high traction wheels you need to apply just about full power on all four motors (wheels) to make the robot turn.

but with a swerve drive (steering) you only need a little bit of power to point the wheels in the direction you want to go, the drive motors to the wheels dont need that full-power-scotty to turn

and depending on whether your wheels are driven independantly, you could angle them 45º to each other, and have a sit-and-spin mode for quick turning, that would take very little power on the wheel drive motors.

The effeciency of the drive/steering system isnt a function of the number of motors driving or steering, its how much power they take to drive and to turn that counts.

Re: Linkage Design Problem
 

Sean,
I am going to throw in with Ken here. We use two globe motors for steering control when we use crab. Each with a stall current of what, 23 amps. (don't quote me, I'm on vacation and too lazy to go look it up.) However, a four motor drive with no steering provision will run each motor to stall in a turn on carpet. Use Chalupas and that will be 129 amps per motor or on a fully charged battery 500 amps. I have discussed this phenomena on many occasions, even the most efficient transmission design will still stall during turns. The effect, and this is critical, is a 500 amp induced voltage drop across the internal resistance of the battery. At 11 milliohms 500 amps will produce a 5.5v drop if you neglect all other system losses. 12-5.5=6.5 volts available for the RC which is 2.5 volts below the point that the RC goes into protection and all drive to PWM outputs stop. At that point efficiency is zero, no power out.

That is not to say you are seeing in your estimate the mechanical losses of four transmission for crab where you might have two in a traditional tank drive. Many crab systems, do not efficiently (de)couple the structure from the robot so that movement causes the drive to move, which changes friction with the floor, adds friction in the bearings, etc.

Re: Linkage Design Problem
 

You are correct that if you are comparing every system with identical transmission losses, motors, etc., that you output the same amount of power from each equally. Each system has losses in transmission to the floor, holonomics especially, and you can possible correct for holonomic losses by increasing motor power in the system. But this inefficiency at providing physical translation makes them somewhat resigned to extreme mobility applications, where there inefficiencies can be tolerated for the advantages they have. But to try and make up for the inefficiencies to compete defensively eats up power resources, something that could be avoided by not making a holonomic defensive base.

Swerve drives are a whole different animal. They *can* use high traction wheels, translating to high power to floor transmission, but this results in slower or less efficient module turning. Many crab/swerve drives choose to simply trade off traction for better mobility. Power to floor is directly related to gearbox efficiencies and coefficient of friction. Where crabs fall behind in the defensive game is not pushing force as much, but strength of the module. You can build a module to stand up to a completely destructive defense, but generally speaking, you still put a lot of load onto the pivot of a crab module because it's so long. The pivot is up top, and the only thing keeping the bottom's of crab modules in place to reduce loads up top is a teflon ring. Tolerance and use wears the ability of this to reduce the loads incurred on the top pivot. It's not a huge deal, but still a weakness, especially with very violent defense.

In terms of driving efficiency, the design is no less efficient than skid steer (unless you consider the power required to charge the cylinders before driving). In fact, even with the power required to move the modules, it's probably more efficient than a long-ways skid steer, because you can switch to wide-ways and avoid high turning current.

Ultimately, it all depends on the game. And it helps if you already intend to, or could use pneumatics. But it's kind of like shifting gear boxes- you get speed, mobility, and power, but is it worth the weight, the worry, and the pnuematics?

Re: Linkage Design Problem
 

i'm curious as to what these inherent weaknesses are. swerve drives, like any other, can be made as robust (or flimsy) as you like. the cost is usually weight, simplicity or both.

i'm also confident that you can achive what you want by motors. we (312) used 2" wide roughtop covered wheels, and initially turned the modules at ~60 RPM. we had to tone it down a bit because we had issues with stability when our high CG robot changed directions suddenly.