Effectiveness of Casters

[Max Lobovsky]

Originally Posted by KenWittlief

Try it on ANY bot - feedback is the most powerful tool an engineer has at their disposal - see them F111s flying around, they would be completely unstable, unable to fly AT ALL without fly-by-wire closed loop feedback controlling its motion. And guess what? the same type of yaw rate sensors they use are available to you in the suppliers catalogs.

I believe you meant the F-117, a small diamond shaped, stealth bomber with no conventional tail developed in the 80's, not the F-111, a large swingwing conventional bomber from the 60's and 70's.

Regardless, I agree with Gary, castors will always align with one direction and this requires them to turn and through your robot off course. A feedback system could align it in the end, but it will take time and the robot wont be in the same place you expected it.

[KenWittlief]

the time it takes is one fourtieth of a second for the feed back loop to see that 'something' is trying to push the robot sideways, when its not suppose to be going sideways, and the SW corrects the power to the motor to force it to do what the driver wants.

This works to stablize castors, it also works if you hit a ball, if a bot hits you, you encounter a change in resistance (like driving on carpet or on the smooth surface)

and it worked last year going up the ramp at an angle.

The PID loop doesnt care whats trying to push the bot off course, it senses the slightest change in yaw rate, and applies an immediate correction.

We used it last year - the bot steered like it was a large servo.

[minic@HYPER69]

All I have to say is DONT DO IT! Casters are like commiting suicide!

[Spikey]

It all depends on what kind of competition you are competing in. In 2000 we built a casterbot that won 2 regionals, and in 2002 we built a caster bot that finished in the top 10 in both regionals, casters can be valuable, it all depends on the competition playing field.

[Ryan M.]

If you're thinking about getting onto the platform at all your casters will have to be incredibly strong. Even the small steps will have a tendency to snag on them and the casters will very likely be ripped off.

[Pat Roche]

Our team tested a prototype drive train/robot at River Rage and I drove all the matches and I can say from experience you will have trouble controlling it with two wheel drive and casters. Remember though that casters arent bad when used in combonation with tracks/tread or 4/6 wheel drive. We competed well with a caster to support or robot with tracks last season.

Also remember to acount for what you actually using them for. They do add weight to the bot.

Just some thoughts to dwell upon,

-Pat

[ShadowKnight]

other people have brought up valid concerns that could lead to certain teams deciding to not utilize casters in this year's game. As far as a robot that intends to climb the stairs, casters WILL become a hangup and possibly could cost the team points and time in the pits try to fix their messed up bot because the casters broke off. Casters also are bad bad bad for driving if used as 2. 1 is MUCH better because of how the drive train pivots. so...I personally see casters as a design flaw that will leave a team hanging on a hangup. Having driven bots with 2 casters, I can tell you that they are crazy bad on anything but carpet and are not that easy to control with precision...

[Gary Dillard]

Originally Posted by KenWittlief

the time it takes is one fourtieth of a second for the feed back loop to see that 'something' is trying to push the robot sideways, when its not suppose to be going sideways, and the SW corrects the power to the motor to force it to do what the driver wants.

This works to stablize castors, it also works if you hit a ball, if a bot hits you, you encounter a change in resistance (like driving on carpet or on the smooth surface)

and it worked last year going up the ramp at an angle.

The PID loop doesnt care whats trying to push the bot off course, it senses the slightest change in yaw rate, and applies an immediate correction.

We used it last year - the bot steered like it was a large servo.

At 10 fps, that's 120 ips, I've travelled 3 inches in 1/40th of a second. That's about 120 degrees of rotation on a 3 inch caster - it has already spun around and pushed me off position by then.

If you want to drive all over the field at part power because you're shutting down one motor or the other all the time to compensate for instability in your drive system, please do. We'd love to have you as an opponent.

Spikey, I'm impressed that you won 2 regionals with casters, but I'm betting it's not because of the casters right? You just focused your resources on other parts of the robot rather than on the drive system, so you had a robot that played all aspects of the game and didn't just drive.

The question of this thread is, what do you think of casters. My answer - not much.

[ShadowKnight]

liek alavinus posted, our team is team is using skids...the only thing that 4-wheel drive is good for is added traction...for manuevering stuff liek the steps, it'd be great...6 could work but I'm not entriely sure that many are needed. I'm not entirely sure that you would WANT to get onto the stairs though because if I read your team number correctly, you're a a rookie team...having been on a rookie team myself and knowing a number of people who were rookies at one time...I would focus on a driving box...maybe something that herds balls...and use 1 caster not 2 if your team decides to use casters, though I like many others do not recomend it.

But like I was saying, our team is using skids...so the front of our robot will just slide along the surface of the carpet. you guys might want to look into that...

[Spikey]

Originally Posted by Gary Dillard

Spikey, I'm impressed that you won 2 regionals with casters, but I'm betting it's not because of the casters right? You just focused your resources on other parts of the robot rather than on the drive system, so you had a robot that played all aspects of the game and didn't just drive.

The question of this thread is, what do you think of casters. My answer - not much.

Our robot that year was very fast and maneuverable, we could fly across to the other side of the playing field and steal our opponents black ball, then quickly collect our own giving us a huge advantage. Also we had a very effective scissor jack to deposit balls, and to hang on the bar. Casters actually ended up as a last resort for our team because our 4 wheel drive destroyed the carpet, after we slapped the casters on we went from being dead last to winning two regionals. However this year will not be a good year for casters because they do not do well at all on the polycarbonate(u need traction on there) and the difficulty they add when you have to climb stairs(getting caught on them). This year is a bad year for casters.

[briholton]

Originally Posted by KenWittlief

where are all the REAL engineers in this thread? ...

....you close the loop on steering by having the SW look at what the driver is commanding the bot to do

....and use a PID closed loop algorythm to MAKE the bot do what its told!

Hey Ken, can a REAL engineer point me in a direction to learn how to use the yaw rate sensor? I'm just a dumb physicist. I'm curious to its use and have seen it for a number of years now and always just piled it up in the dunno pile. Cuz, after all this talk, I have to give up on my casters..... darn it, had some really nice ones ordered too! hmm, perhaps for the goal.

[maxgebhardt]

We used castors our rookie year and had less qualifing points than a robot that didn't even move... they are awful. just imagine pushing a 130lb shopping cart at the supermarket. those castors are also tempermental and jam easily. if anyone on you team still likes castors, then take them to pathmark and you jump in a cart and tell them to push you for 2 minutes, and see how much excess energy it takes.

[KenWittlief]

I dont know if I can give enough info in a post to explain how to use them if your are not familiar with PID control loops.

The idea behind it is simple. If you are using single joystick control then the X axis is indicating how fast your driver wants to turn

if you just feed this into the single joystick equation that comes with the default code - then the stick postition is mapped directly to a PWM output, which ends up telling the motor how much force to apply (you can think of this as how much voltage is on the motor)

problem is - there is friction in your drivetrain that will resist movement - so putting a small pwm command on the motor will not be enough to make it move - once you have moved the stick enough to make it move, you have overcome the static friction and now you are dealing with kinetic friction, which is less, so once the motor moves a little, that pwm command is enough to make it move noticeably more

to make matters worse, the left and right drivetrains will not have exactly the same amount of static friction, so one will start sooner than the other. If you are telling the bot to go straight, it will goto one side a little.

The yaw rate sensor detects how fast the robot is turning - if you mount it with its bolt holes up and down.

the way a PID control loop works is, instead of taking the signal from the joystick and driving the motors pwm with that, you take the joystick X axis signal and you subtract the yaw rate sensor signal.

so if the driver is commanding a slight turn, and the yaw rate sensor says "im not seeing any turning going on here!" you end up with an error signal - the error is the difference between what the driver wants, and what the robot is doing.

the simplist form of closing the loop would be to use the error signal to drive the motors - replace the joystick X axis signal with the error signal and feed it into the one joystick equation. You have to adjust the error signal for 127 being zero, something like this:

yaw_error = (((2127 + p1_x - yaw) Min 2000 Max 2254)-2000)
yaw_command =(((2063 + yaw_error/2) Min 2000 Max 2254) -2000)

BTW - this is in basic from last year - wont take much to change it to C

In PID lingo this is a Proportional loop. The output to the motors is proportional to the difference between the commanded value and the actual (the feedback) value. how responsive the system will be depends on the gain. in the code example above, the error is divided by 2, so the gain is 0.5

thats not a bad place to start with a machine that is using two wheels and castors, esp if your weight is balanced towards the two drive wheels. if you try the code and the bot is sluggish, you can increase the gain ( and you have to adjust the 2063 number so that a yaw error of 127 (zero) stays zero

if you have a bot with 4 wheels, you might need more gain, like 2 maybe - in that case you would adjust the 2063 down to 1873.

if you crank the gain up too high, what will happen is, when the loop corrects itself it will over correct, and overshoot the turn rate - then the yaw rate sensor will say "you went too far" and try to correct it back, which will overshoot again, and the bot will twist back and forth. If the gain is just slightly too high, the bot will respond like its nervious and twitchy - if its way too high the bot will swing back and forth all by itself.

one thing you have to be carefull here - make sure your yaw rate sensor is mounted in the right orientation - for the code above we had our sensor mounted upside down, the holes were upward (simply because that was a convient place to put it on our frame). Also the sensor has to be solidly attached to the frame. If you tie wrap it on, and it can move around, it will go nuts on you.

so what are the I and D parts of PID? I is intergral - if you move the stick a little bit, the error signal might not be enough to get the motors to move - so to add an intergral part you take part of the error signal and you sum it, or accumulate it over many cycles - so if the driver is telling it to move, and its not moving, the error signal keeps increasing until it does.

The D is differential. sometimes you want the bot to respond quickly to changes in the joystick position, so you can either keep track of where the joystick was last time, and subtract where it is now, to tell when it has been moved, and apply a gain to that signal to sort of give the motors a 'kick' when the joystick has been moved - or you can use D on the error signal in the same way.

For our bot last year, all we needed was the P part of the PID loop - the equation I listed above. We ended up with the best response with the gain at 2, but I would try it at lower values and work your way up.

If you want to learn more about PID control loops, try searching the web - lots of companies make equipement that is used professionally, and I have seen some excellent websites that explain the concept better than I have here.

PID loops can also be used for thing like postioning an arm on the bot, or using pots to tell where your wheels are pointing if you use crab steering, or your front wheels turn left and right to steer.

hope this has been useful.

[KenWittlief]

I broused through a few web sites, if you search yahoo on

PID control feedback

you will get many links

this one gives a good overall explaintion without going into the math too much:

http://www.netrino.com/Publications/Glossary/PID.html

I found others that use fequency domain equations, if you can get past that, they are very informative - ie, skip over the math you dont understand and look at the graphs and you will get some insite.

[KenWittlief]

if you dont like castors then the next best thing is to have steerable wheels, like a car or wagon.

4 wheel drive with tank/skid steer is hidious - the amount of power and energy you waste dragging a drivewheel sideway is tremendous - in this years game, as other have pointed out, if you dont want to use castors, use a couple blocks of nylon instead, and have most of your weight balanced over your drive wheels.