Crab-drive steering modes?

[Dave Scheck]

Originally Posted by gorrilla

like our module is supported by a lazy-susan on the bottom, couldent we just turn the encoder sideways and rest it on the lazy-susan? kind-of like a friction wheel....

One problem with that is that if you ever need to remove a module you would need to remove the sensor as well. If you make your sensor part of the base, you never have to worry about it. I had to ruin the resolution of the picture to get it small enough to attach, but it should be good enough to show the concept. This is our base and single chain crab from this year. You can see the blue pot on the left side of the picture. We didn't use a second pot, but you can see where it would go on the right side. The pots were mounted to a small sheet metal piece that fit snugly in the recepticle (you can see it on the left). This allows us to quickly drop a new pot in if necessary. The shaft is keyed to prevent the pot from slipping, and the shaft runs down to a sprocket that the chain wraps around. The pot shafts were fitted with a collar that matched the key and fixed in place with a set screw. When we do 2 chain crab, the setup is identical.

[sgreco]

Originally Posted by gorrilla

Oh,thats cool

but couldent you just attach it to the module itself?

like our module is supported by a lazy-susan on the bottom, couldent we just turn the encoder sideways and rest it on the lazy-susan? kind-of like a friction wheel....

The best thing to do in my opinion is to attach the sensor to an idler sprocket that is along the chain line that is actuating it. I'm assuming you are using chain or belt. If you are using cable, it might be a little different...I've never dealt with cable, but I've heard through the grape vine that it works pretty well...

Like Aren said, the numbers are easier if you use the same sized sprocket, but you don't have to. The math isn't too complicated if the size is different though

Is this going to be co-axial, or is the motor going to be on the module?

[gorrilla]

Originally Posted by Sgreco27

The best thing to do in my opinion is to attach the sensor is to attach the sensor to an idler sprocket that is along the chain line that is actuating it. I'm assuming you are using chain or belt. If you are using cable, it might be a little different...I've never dealt with cable, but I've heard through the grape vine that it works pretty well...

Like Aren said, the numbers are easier if you use the same sized sprocket, but you don't have to. The math isn't too complicated if the size is different though

Is this going to be co-axial, or is the motor going to be on the module?

we are going to use the Gates-KIT-belting and pullies for steering,

its NonCo-axial,with a CIM in each module,and 2 globes for turning each set of wheels....


we thought about co-axial but we dont really have the machining tools to do it(or the experience) and we assumed it would be alot heavier(i dont know about this)

Dave-thanks for explaining that,I'd never really thought about that...hopefully we wont ever have to remove a module after we put it in(we tend to sacrifice weight for strength on our robots,06' escpecially)

[sgreco]

Originally Posted by gorrilla

we are going to use the Gates-KIT-belting and pullies for steering,

its NonCo-axial,with a CIM in each module,and 2 globes for turning each set of wheels....


we thought about co-axial but we dont really have the machining tools to do it(or the experience) and we assumed it would be alot heavier(i dont know about this)

The belt will work just fine for steering, my team used it this year and it worked fine. Just becareful though, if it slips, it can totally mess up you feedback sensors. My team found that you need a little more wrap around you sprockets with chain than you do with belt. My team also needed a spring tensioner to keep proper tension, but it these aren't really big problems.

Non-Coaxial isn't really much heavier than co-axial, the only thing is co-axial is typically much less efficient. So I'd say noncoax is a good call.

[Jared Russell]

Originally Posted by Sgreco27

... the only thing is co-axial is typically much less efficient. So I'd say noncoax is a good call.

Sorry for going off topic, but I have seen this statement on Chief Delphi a number of times from a number of different posters.

Maybe I'm missing something coming from my EE/CS background, but I don't really see why a coax crab would be appreciably more inefficient than a non-coax. Well meshed bevel gears have very similar published efficiency numbers to spur gears (~95-98%). And shafts rotating within each other shouldn't be an issue if ball bearings are used. Does anyone have any actual experience to the contrary?

Sure, there are potential issues with steering (as the wheel would rotate slightly even if the drive motor was held still during turning) and without being able to independently power the wheels some steering modes aren't as efficient (i.e. ackerman or warthog), but in terms of raw power transfer I have not been convinced of this perceived "inefficiency".

I'm looking forward to being enlightened.

[Dave Scheck]

Originally Posted by gorrilla

hopefully we wont ever have to remove a module after we put it in(we tend to sacrifice weight for strength on our robots,06' escpecially)

Based on experience, expect to replace modules at some point during the season. When we ship the robot, we remove the modules to protect them as well as to help prevent the robot from rolling around in the crate. We also replace them to perform routine maintenance. Most of all, you need to plan for the worst case. If you have module fail, you want to be able to replace it quickly. For us, it's 4 bolts to the mounting plate (you can see it in the bottom left of the picture) and 1 or 2 electrical connections. We've had to use that "feature" a few times in the heat of competition. One year we had our signage fall off and get sucked into the module gearing during a match. Without the quick replacement it would have made it tough to get to the next match reliably.

A big lesson that every engineer should take seriously is that it is critical to design for failure. What happens when it breaks? How will you be able to recover? This applies to all aspects of engineering. On the mechanical side, cars are designed to crash safely. On the software side, divide by zero exceptions are handled to prevent crashes.

[gorrilla]

Originally Posted by Dave Scheck

Based on experience, expect to replace modules at some point during the season. When we ship the robot, we remove the modules to protect them as well as to help prevent the robot from rolling around in the crate. We also replace them to perform routine maintenance. Most of all, you need to plan for the worst case. If you have module fail, you want to be able to replace it quickly. For us, it's 4 bolts to the mounting plate (you can see it in the bottom left of the picture) and 1 or 2 electrical connections. We've had to use that "feature" a few times in the heat of competition. One year we had our signage fall off and get sucked into the module gearing during a match. Without the quick replacement it would have made it tough to get to the next match reliably.

A big lesson that every engineer should take seriously is that it is critical to design for failure. What happens when it breaks? How will you be able to recover? This applies to all aspects of engineering. On the mechanical side, cars are designed to crash safely. On the software side, divide by zero exceptions are handled to prevent crashes.

we will make sure they can be replaced easily, Reliability and Simplicity is a big thing in the design of our swerve system,

of which I should have an Inventer rendering(of a single module) hopefully by this saturday....

my team is not very experienced with CAD

[NickE]

Originally Posted by Jared341

Maybe I'm missing something coming from my EE/CS background, but I don't really see why a coax crab would be appreciably more inefficient than a non-coax. Well meshed bevel gears have very similar published efficiency numbers to spur gears (~95-98%). And shafts rotating within each other shouldn't be an issue if ball bearings are used. Does anyone have any actual experience to the contrary?

Coaxial crab typically uses chain to connect the drive shafts on multiple modules to the gearboxes. Especially when used with tensioners that provide friction on the chain run, the chain is less efficient than spur gears.