Team Update #18

[wireties]

Our minibot is not particularly fast but is getting better. During Week 1 at Alamo, it tripped the sensor every time. But at the next event it will be lighter and faster (a design which we built before we shipped but had no time to test due to weather-related delays). After some complaints on Thursday, the few teams with really light and fast minibots got the referees to manually judge the minibot race. It seemed fair to us, those light minibots are great machines.

I want to point out a common engineering practice. If my company had to build a minibot that triggered the sensor EVERY time in order to get PAID, we would build something with a 2:1 margin for error - something that would under nominal conditions exert 8N of force.

And I think that minibots reversing direction too quickly may play a bigger role that we think. We may add a spring in the lever that depresses the switch in our newer design.

[pfreivald]

Originally Posted by wireties

And I think that minibots reversing direction too quickly may play a bigger role that we think.

That's a great point. Our minibot just turns off when it hits the top. (And usually comes back down because of the impact with the plate -- meaning that the plate is providing the impulse to reverse the momentum). If some teams are actually reversing their wheel direction before the impact is finished, this could have a dramatic effect on the overall force (and the maximum force) with which they are hitting the plate.

[jspatz1]

"Week 2 results indicated that this change was successful."

Hmmm. Like many others who have commented, our 1.2 sec. minibot smacked the target with a substantial impact in Week 2 (my calculations say in excess of 100 N), and yet did not result in a trigger light a couple of times. Clearly with all the antecdotal evidence there is still a phenomenon occuring which can miss an impact of adequate force. Any mechanical action has both a time and force component, and perhaps the rules should have specified both. Like any control system the field system has a controller with a finite scan rate, and any control system is capable of missing a signal if it is short enough. I am a little nervous about a statement that essentially says "we have determined the system is now perfect" if that means no provision will ever be made for correcting a result when it is clearly wrong. I'm grateful we are not playing again until Week 6 to give some time for this issue to get sorted out. Let's hope no Week 3 teams experience any problems which effect their results.

[colt527]

2 observations:

1.) Design the mini-bot so that it can't hit the bolt first. Honestly, I think the bolt pattern was the cause of most of the times it didn't trigger at Pittsburgh. We personally checked every single contact point on the towers before each day and they all triggered with minimal force.

2.) Make sure there is a delay between when you hit the top and when you start reversing the wheels.

I think this is a necessary rule for the Championships. Mini bot races are very soon going to get too close to call and we need to perfect the tower designs and also for the teams to perfect their mini-bot designs so they do not hit the bolts and provide the necessary amount of force. I think thats the only path to reliable calls at the championships.

[pfreivald]

Forgive my tired brain, but is the bolt pattern in the field drawings?

[Cory]

Originally Posted by colt527

2.) Make sure there is a delay between when you hit the top and when you start reversing the wheels.

I think this is a necessary rule for the Championships. Mini bot races are very soon going to get too close to call and we need to perfect the tower designs and also for the teams to perfect their mini-bot designs so they do not hit the bolts and provide the necessary amount of force. I think thats the only path to reliable calls at the championships.

Where in the rules does it say that you have to provide a given impulse? All it says is 4 newtons, not 4 newtons for 2 seconds, half a second, etc.

Teams made their minibots to FIRST's rules. They should fix their (still) not fully functional system, or put measures in place to ensure that minibots conforming to their published rules actually get scored.

Instead they just say "our system is perfect, so if you don't trigger it, even if you blast the darn plate off the top of the pole entirely, you still don't get the points". If the system is proven to have any flaws then FIRST ought to recognize that it is possible for mistakes to be made.

Anyone seriously arguing whether a 2.5 lb minibot going 6-10 ft/s will impart 4 N of force or not is just being a FIRST apologist. This is simple physics and some efficiency losses due to friction, etc aren't going to change much.

Perhaps we should take some video of our official competition towers being repeatedly triggered by our minibot to silence the doubters?

[BrendanB]

Originally Posted by Cory

Where in the rules does it say that you have to provide a given impulse? All it says is 4 newtons, not 4 newtons for 2 seconds, half a second, etc.

Teams made their minibots to FIRST's rules. They should fix their (still) not fully functional system, or put measures in place to ensure that minibots conforming to their published rules actually get scored.

Instead they just say "our system is perfect, so if you don't trigger it, even if you blast the darn plate off the top of the pole entirely, you still don't get the points". If the system is proven to have any flaws then FIRST ought to recognize that it is possible for mistakes to be made.

Anyone seriously arguing whether a 2.5 lb minibot going 6-10 ft/s will impart 4 N of force or not is just being a FIRST apologist. This is simple physics and some efficiency losses due to friction, etc aren't going to change much.

Perhaps we should take some video of our official competition towers being repeatedly triggered by our minibot to silence the doubters?

I agree! With all the minibot rules the time to change how the race is drawn and towers triggered is not week 3! I would expect FIRST to do the opposite and put refs in place of the electronic or at least watch.

Grrrr.

[Ether]

Originally Posted by pfreivald

Anyone want to poke holes in my analysis?

One obvious omission is that you did not account for the mass of the plate. The plate must be violently accelerated in the short amount of contact time it has with the minibot in order to move it enough to trip the sensor(s).

How much does a 12" diameter 0.25" thick polycarbonate plate weigh? My quick calculation says it's more than 4 Newtons (please check my math). So obviously 4 Newtons is not enough force to move the plate at all, let alone violently accelerate it. So where did the 2-4 Newton number in the manual come from?

The specification of what exactly is required of the minibot in order to trip the sensors is inadequate to allow accurate analysis. What exactly is being sensed? Proximity? Rate of change? Dwell time? Some combination? Does anyone know?

[colt527]

I am not apologizing for FIRST, I am just advocating pragmatism from the teams. The bolt pattern was definitely not obvious and I don't remember seeing it anywhere in the official rules. They are in the official field drawings in sections 37, 38 and 69 of the Game Field Elements but I know most teams (including my own) don't check those.

Believe me I sympathize with any team who's bot successfully climbs the pole but doesn't get the points for it. I know that it must be immensely frustrating. I want to eliminate that from ever happening. Thats why I think that with a few small minibot modifications, this will become simply a non-issue and the tower results will match everyones expectations.

I know there were cases where the minibots reached the top of the tower but it didn't trigger. I am just being honest and did not see any of those cases where it was completely obvious that the bolt was not hit and that it delivered enough force and still did not trigger.

Originally Posted by Cory

Where in the rules does it say that you have to provide a given impulse? All it says is 4 newtons, not 4 newtons for 2 seconds, half a second, etc.

Teams made their minibots to FIRST's rules. They should fix their (still) not fully functional system, or put measures in place to ensure that minibots conforming to their published rules actually get scored.

Instead they just say "our system is perfect, so if you don't trigger it, even if you blast the darn plate off the top of the pole entirely, you still don't get the points". If the system is proven to have any flaws then FIRST ought to recognize that it is possible for mistakes to be made.

Anyone seriously arguing whether a 2.5 lb minibot going 6-10 ft/s will impart 4 N of force or not is just being a FIRST apologist. This is simple physics and some efficiency losses due to friction, etc aren't going to change much.

Perhaps we should take some video of our official competition towers being repeatedly triggered by our minibot to silence the doubters?

[Dave Flowerday]

Originally Posted by Cory

Where in the rules does it say that you have to provide a given impulse? All it says is 4 newtons, not 4 newtons for 2 seconds, half a second, etc.

Teams made their minibots to FIRST's rules. They should fix their (still) not fully functional system, or put measures in place to ensure that minibots conforming to their published rules actually get scored.

Cory, I agree with what you're saying. However, I've heard that, due the false triggering seen early on in week 1, the field hardware now looks to see that the towers remain triggered for 75 milliseconds before a hit is registered. I can imagine that really fast minibots hit so hard that they bounce away and aren't making contact for 75ms.

What's annoying is that this means that the rules have changed, as you suggested. The de-facto rule now appears to be 4 newtons for 75 milliseconds.

[Ether]

Originally Posted by Dave Flowerday

The de-facto rule now appears to be 4 newtons for 75 milliseconds.

Please see my earlier post about "4 Newtons".

[MrForbes]

my math agrees with yours. And I know the off switch on our minibot takes much less force than that to actuate.

Interesting.....the rules have been that the tower sensor must be triggered for it to count. Just because teams got away without triggering the first week or two, doesn't mean the rule doesn't apply.

I wonder how many slow heavy minibots will win races now?

[JesseK]

Assuming the minibot stays on the pole after impact, the trick is to make the cut-off switches either slow (not a quick flip) or require more than 4N of force to push. Either will increase contact time with the top plate. Squishy surgical tubing comes to mind.

Design is iterative, right?

[coldfusion1279]

Originally Posted by Cory

Anyone seriously arguing whether a 2.5 lb minibot going 6-10 ft/s will impart 4 N of force or not is just being a FIRST apologist. This is simple physics and some efficiency losses due to friction, etc aren't going to change much.

I would appreciate a more explicit mathematical write up as to why you are so convinced.

Most importantly, I want you to account for the drift in the motors due to momentum after the limit switch shuts down the motors at the top of the pole. Do you know, for a fact, the robot has enough speed/momentum to continue to carry itself upwards despite non-powered motors and gravity contributing to rapid deceleration? As pointed out in this thread, the non-powered motors might even stop the robot on a dime...

(For the record, I think there should be enough force too, but I think is not good enough in engineering, and FIRST hasn't given me good enough reason not to trust them)

[AlexD744]

I don't know about the towers at other regionals, but at Florida, for our 3 lb/1.6 second minibot, the tower triggered every time. Granted we didn't get our minibot and deployment working until our last qualifier, so it only had 4 goes at it, but it triggered it four out of four times. And for the most part, the towers there were registering minibots, both slow and fast.

However, being on the drive team leaves me with little time to watch matches, so I am not sure if every other team had this success rate, feel free to correct me if you minibot experienced a false negative at Florida.