Team #405
 

Hi guys, I'm new to the forums but this is my 2nd year mentoring. I don't see anybody else from our team on here so figured I'd post a couple of pics of where we're at with our robot. We're going with a car type driving style and rear wheel drive with differential. Here are some pics showing the robot being built up...The arm is now attached and should be finalized in the next couple of days, then it's all software and practice. So far, weight (without battery) is 51lbs, projected to be at around 80-85lbs when complete. Speed is around 14-16ft/s...

Re: Team #405
 

Re: Team #405
 

wow it's set up just like a go-kart, i really like it, should turn really well.

Re: Team #405
 

cool pics
next time please resize them

Re: Team #405
 

What type of servoes are you using?
Looks fast.

Re: Team #405
 

Cool... I've been generally opposed to usage of car style steering, or at least in my teams case, but your drive train is built so well it's amazing. Do you have any issues using those servos to steer?

Re: Team #405
 

I would strongly suggest that you look into other methods of steering than those servos. HS-322's have problems holding a 4 pound R/C car in a straight line...so I know for certain that they will not be able to turn your front wheels on carpet. I would strongly suggest that you replace the servos with a motor such as the van door motor or a window motor.

Re: Team #405
 

That drivetrain looks super nice but I would echo the concern of the other people about the servos for steering. Definitely switch them out for a different motor because I don;t see those servos being able to steer for very long.

Re: Team #405
 

We're using the Hi-tech servos, when we first put it together we got too much flex in the steering so we had to switch from the 'arms' shown in the week3 pictures to the circular mounts shown in the last photos and add some corner gussets to make sure the servos stayed in place. We put about 100lbs of weight on the front end to make sure we wouldn't have an issue when loaded up and the wheels were able to steer (though on tile instead of carpet and a bit labored). Once it's moving the servos don't have to work hard, the only issue is when stationary and so far it looks like they'll be more than strong enough.

All of the parts for the drivetrain except for the rear axles and some of the plates on the back end were made using a band saw and drill press or hand tools, we just spent a lot of time discussing design before building it and it came together pretty well. The rear diff came from a gas powered RC car, the rest of the stuff was either standard kit stuff or cut from aluminum.

The whole front and rear assemblies each can come off by removing 4 screws each for easy maintainence, some of the students are working on an improved and lightened design for the front and rear that may end up in the crate if they get it done in time.

We're going to do a bunch more driving tomorrow (on carpet) so I'll try to get some video of how the steering works and see if I can post it. I already have some video but it's just some low speed testing stuff, though the driver did floor it once. It appears to be on the verge of too fast, but we'll see :).

FYI, in the last pic you can see the beginnings of a different steering mechanism in case the servos don't cut it, we'll find out more tomorrow.

Re: Team #405
 

Looks awesome! It's built like a racecar (and I like the strategic batt. placement)

I would highly suggest switching the servos to a globe motor(s) with a differential (what we're doing).

Also, why did you decide not to do an Ackermann front steering?

Re: Team #405
 

I would also HIGHLY recommend the Globe motor. The HS-422 Hitec servoes only have a torque of 42 OZ/in :ahh:

Re: Team #405
 

I think the way the servos are actually doing the steering there's not a lot of pressure on them. As I said, we have an alternate steering mechanism in the works just in case we have a problem.

In regards to why we didn't do Ackermann steering...the alternate steering method incorporates it, though from what I can gather it won't make a big difference at higher speeds and we are going to run a little toe out so the car doesn't dart around which should make the inside wheel turn tighter. If we want to incorporate Ackermann steering in the current design, all we have to do is move the tie rod pivot points in a little, but it may stress the servos a little more as then they'd be fighting each other a little bit at max lock. We'll see what the students come up with. I'm heading over to the school now, I'll try to get some videos.

Re: Team #405
 

What are the dimensions for your robot? Because it looks really small in the pics.

Re: Team #405
 

Don't use sevos period. Our team 2083 built a car robot last year and used servos. They held up fine in regionals, but at worlds they started to fail like crazy. Every match one would go and we would have to replace it. DON'T USE SERVOS!!!

Re: Team #405
 

It's about 26" wide and 32" long, but should be right around 36" long once the bumper mounts are put on.

We removed the servos and went with the globe motor but won't have it running until Monday or Tuesday as it'll need some software work. We found that the servos didn't provide enough turning radius and when we increased the pivot, they'd start to lose power so we figured we wouldn't risk it and made some changes. Before removing them, we did do some speed testing and went about 35' in 3 secs from a dead stop. We may gear a little higher to prevent wheel spin as it seems to have a little too much torque right now.

Re: Team #405
 

Wow, that looks really nice.
Good luck!

Re: Team #405
 

you have some messy electrical work >.<

Re: Team #405
 

Whats the turning radius of those wheels? Seems short imo...Then again, it looks very fast. Good luck.:yikes:

Re: Team #405
 

For your stationary turning issues, I'd like to point you to this thread:
http://www.chiefdelphi.com/forums/sh...hlight=current

Note the posts by Dave Lavery on the total current available to the servos. It's only 1 amp :ahh:. When you're stationary, you have evidence that your servos have trouble manipulating the steering. I'd be weary about this, as the mechanical stresses combined with current pull could be the cause of servos burning out. I'd have plenty of spares if I were you.

Also, I'd recommend putting some sort of support going from the main drive base to right next to the front wheels, or enclose the whole front. At the moment, a high speed off-centered frontal crash will knock your steering out of alignment, regardless of where you mount a front bumper to the current design.

Otherwise, this looks like a fantastic drive train!

Re: Team #405
 

dang, this suckers a tank, 4 cims on 1 axle, holy crap lol, got some serious torquage goin on here

Re: Team #405
 

Thanks for all the comments, we switched to a globe motor on the front with a pot and now have around 55 degrees of steering lock (on the inside wheel, it's ackerman so around 40 deg on the outside) and no issues with power. The wheels won't be exposed, we're going to run bumpers all the way around and nothing is going to be attached to the front or rear end so that they aren't stressed in an accident and are easy to remove for repairs if need be. The robot is pretty much finished now (except for the bumpers), I have a couple of pics of it, see below:



Total weight as shown is 84lbs. We figure we might get it up to 90lbs with bumper mounting brackets and the cage around the front and rear wheels, so still 30lbs under weight.