Caster Mechanism

[mzitz2k]

I was wondering if a team has ever created a caster mechanism that raises and lowers one end of a robot. I know this is an easy mechanism to produce, but I would love to actuate it with just one window motor.

With a 4-wheel drive robot, the caster mechanism would have to lift 1/2 of the robot weight (assuming the weight is evenly distributed), which is 65 lbs = 289.5 Newtons. According to the specs the window motor is going to provide approximately 4.6 Nm of torque (this is 1/2 of the stall torque), which means the lever arm (off the motor) must be pretty small - or - one could gear the window motor down to achieve this actuation, but then speed of the caster actuation is lessened.

I would love to have two casters that work off of one mechanism allowing to lift the robot up and down off the ground.

Appreciate any tips!

[Veselin Kolev]

Yup, one team that used this type of caster mechanism was the Cheesy Poofs, team 254. I have a feeling that they used pneumatics though, not a window motor. They used the caster to alternate from 4 wheel tank drive and 2 wheel car drive. Why do you want to do it with just one window motor? Do you want this mechanism to use 2 casters to lift up the end of the robot? The Cheesy Poofs only used 1 caster. What I am going to assume is that you want to lift 2 wheels off the ground so you can steer differently. In this case, you only need to lift those wheels off the ground by like, a centimeter, so I think pneumatics is the best solution for this problem. To turn a lever with that much force will demand for some pretty thick shafts. You'll probably have to attach a sprocket to the window motor and gear it down a bit, and all this adds up to a lot of useless weight. You could do it with a window motor, but I strongly suggest doing it with pneumatics. All you need is a small cylinder with a thick bore, so it can give the 65 pounds of force. Stick a caster on the end of that, and you're done.

[RogerR]

my old team 312 had a castor mechanism that that started out being driven by the window motor, but in the end was pnuematically driven, because it was much simpler

http://www.firstrobotics.net/03galle...0312-1_JPG.htm

you can also see the optical sensors attached to it that we used during auto mode.
i believe that SPAM (180) had a similar device last year, and that Wildstang (111) used a retractable skid plate

[mzitz2k]

Thanks for all of your help and tips. The reason I would really enjoy accomplishing this mechanism with a window motor is so that we don't need to waste all of that weight on a compressor, accumulator, tubing, gauges, etc. that come with using pneumatics.

[Dan Richardson]

after getting all the gearing and everything that would be needed to lift the sides of the robot effectively and reliably.. it might be close to the same weight.. if its a few lbs lighter.. you might want to ask yourself is worth the effort?

[Gadget470]

As StudManDan pointed out, it will likely be close to the weight of a pneumatics rig being on the bot. However, it can be done with a single cylinder leaving (should rules stay the same) 3 availible for use. Don't rule out pneumatics on weight alone, straight-line actuation can be your friend.

[team222badbrad]

Our team the TIGERTRONS just did this our first time in the year 2003

In the past we found it very difficult or impossible to turn our robot in high gear with 4 wheel drive. This past years game we decided we needed to become very maneuverable so we had to change something...

So We created a pair of casters that were directly mouted to our stationary front axle. We then used a Globe Motor that was mounted to our frame. The Globe Motor also had a lead screw directly attached to the shaft.

The Globe then pivoted the castors on the stationary shaft. We also used a pair of limit switches to control the movement of the mechanism so that us drivers would not destroy it.

The only drawback was that is was rather slow. It took like 3-5 seconds to shift from 2 wheel drive to 4 wheel drive. Our front shaft also bent during one competition due to falling from verticle to horizontal on our castors that were down. We later fixed the problem by installing a solid 1/2 inch shaft instead of a hollow 1/2 inch shaft.

For pictures and more information please visit:
http://www.tahsroboticsteam.org/pictures.htm
http://www.tahsroboticsteam.org/robot2003.htm

I can try to get more detailed pictures of the mechanism if you want
just send me a pm or email at rigdonbrad@hotmail.com

[Matt Adams]

In the past we found it very difficult or impossible to turn our robot in high gear with 4 wheel drive.

I guess I'd just like to offer a few cents on this, because I have no doubt that this is common problem. There's a few different schools of thought. I'll offer my own. I'm sure others will chime in.

Pneumatics are an excellent but limited resource, often underused and underappreciated. I noticed that last year a number of teams had a "trick wheel" to aid in high-gear steering. I think the only reason that they could "get away with" using a pneumatic for this mechanism is since so few teams actually had any sort of complex manipulators that required penumatics.

Essentially, I think that using a pneumatic in this situation (unable to turn in high speed) isn't the best use of this resource. I'd rather try to properly design my high speed setting such that I could turn without this device. For better or worse, in almost every game that's been played so far, I'd rather just go 9 feet per second (faster than more than 70% of the competition) in high and have a spare pneumatic to use, rather than go 12 or 13 fps and need an addition device, which upon failure means that I have effectively neutralized my transmission. I know it depends on the game... but in principle, I think there's a more sound approach.

Just my pair of pennies...

Matt

[Stu Bloom]

Last year we wanted a very mobile robot and planned for a 2-wheel drive with casters. I introduced the idea of HDPE "glides" instead of casters for a couple of reasons. The rounded "skids" worked very well as they slid smoothly on the carpet as well as the ramp, and we avoided any resistance due to casters having to orient the correct way and/or fighting each other "directionally". They were also large enough to span over the grid in the ramp wire mesh, therefore eliminating the bouncing we would have seen from using casters. We also needed to raise and lower the back of our robot and used the pneumatic actuation system shown below:

http://us.f1f.yahoofs.com/bc/63e9492...58G8_AwWYPomxv

This arrangement worked great, and using the large pneumatic tires we also had a significant amount of traction.

[JessR]

In 2002 Team Paragon also used a castor option to add 2 wheel steering behavior to our tank tread drive, which of course was a bear to steer & overheated the motors when turning a lot.

We used a seat motor & cords pulling a couple of flaps with castor wheels mounted, one in each corner of the chassis. It worked great, outside the 4 second activation time - As noted above, worm drive motors llike the seat & window motors take about that much time to lift 60 lbs.

Depending on the game dynamics, I would lean to either using a small pneumatic cylinder & pre-charged onboard air tank(s) without the compressor, or a fisher price gearbox or similar driving a cam.
It depends on how often you'd have to drop down on all fours for shoving matches.

Jesse.

[Raul]

and that Wildstang (111) used a retractable skid plate

Yes, we used a skid plate to go into two-wheel drive for quick rotations of the robot. We used pneumatics for this for a simple reason - it can be actuated very quickly. The window motors can be geared properly to raise the robot, but the required gearing would make it move very slowly. If we used the window motors, we would have to wait too long to lower the skid and not be able to make very quick movements. Keep in mind that in last year's game, we had to raise our skid plate completely out of the way to allow for clearance while going over the ramp.

Raul

[Joe Ross]

In 2002, Team 340 did something similar. They used two wheel drive, with two ball casters on the far edge of the robot. When they grabbed a goal on the side with the wheels, they then actuated two pneumatic cylinders that lifted the casters off the ground and also lifted the opposite end of the goal. Thus, they had half off the goal transfering weight to the drive wheels with nothing dragging. It gave them a great turning radius.