After returning from IRI everything I was thinking about can be summed up into one simple question. How are some teams pushing so hard and for so long with their robots with out tripping breakers?
All you have to do is watch the finals. For about half of the match it is just a pushing/shoving frenzy. Then when the match is over the refs/announcer tell us about how the motors smell and are smoking. I understand that the top teams are using multiple motors and low gear ratios. But even so that does not allow you to get into a pushing stalemate and not pop breakers.
*Originally posted by Dave_222 *
**We had a problem with poping breakers when we had a 15:60 but once we bumped back up to 18:60 we were fine. (We only use 2 drills) **
I think you mean the other way around.
Also, low gearing will prevent you from tripping breakers during heavy load such as a pushing stalemate.
*Originally posted by sanddrag *
**I think you mean the other way around.
Also, low gearing will prevent you from tripping breakers during heavy load such as a pushing stalemate. **
Not necessarily.
Low gearing means that the robot is capable of outputting more torque at its wheels. A high coefficient of friction between your robot and the carpet helps to transmit that torque to the floor, moving your robot.
If the robot is geared so that it can push really hard and your wheels don’t slip on the carpet at a certain threshold, you can draw enough current to pop breakers.
Remember that motors draw the most current at stall – i.e., when they’re exerting the most torque possible. Gearing lower gives a robot the ability to exert more force, but it doesn’t do it without an expense.
Similarly, a high gearing trades torque for speed. However, highly geared robots often pop breakers, too. The reason for the popped breakers is the same in both cases. The motor reaches stall (or something over 40A) before the wheels slip.
The original question was about pushing power without tripping. A few things to consider: multiple motors help, high mechanical power motors help (I.E. drill and Chips), gear ratio vs. wheel diameter is everything.
Basically, in order to guarantee that your breakers will not trip you have to do a couple of things. I will keep this post short, but you should download my white paper on drive systems located here: Drive Sizing
You use the coefficient of friction between the surface you are driving on and your wheels as a fuse. You design your gear ratio and wheel diameter based on your motors working at 40 amps (for the drill and chip). Basically, take your weight * Mu * Wheel Radius and that is the torque at your wheels. Divide that number by the torque at the motor at 40 amps and that is the gear ratio you need to “slip before you trip”. My paper takes into account efficiency, but this gives you the general idea.
One other thing. It is possible to get magic smoke without tripping. Breakers trip at 40 amps, but if you do not put enough voltage into the system (only push your joystick forward very slightly), then your stall current will be less than 40 amps. DC motors do not like being at stall because they use moving air as a coolant and if at stall too long, then the smoke will be released.
I know this is a simple answer, but look at my white paper and you will get all the detail you need.
I don’t think you guys are understanding my question. I know all about getting the max pushing power with gear ratio/motor combinations… What I want to know is how teams are able to get into pushing stalemates for the last 10-15 seconds of a match. No matter what your max torque, gear ratio, or amount of motors are you are not able to have them stalled like that without popping breakers.
Paul: I just wanted to tell you that that is a great presentation with lots of great info!
My team didn’t end up going to IRI (Team 176), but the similar thing was happening at Battlecry @ WPI. We use 2 Chiapua (sp?) motors with 2-speed transmissions and we never had a problem with tripping breakers during shoving matches (I can’t remember what the low gear ratio was, maybe 60:18). Our motors burned my hand, however, after 12 straight matches (with maybe 30 seconds in between each).
*Originally posted by rlowerr_1 *
**What I want to know is how teams are able to get into pushing stalemates for the last 10-15 seconds of a match. No matter what your max torque, gear ratio, or amount of motors are you are not able to have them stalled like that without popping breakers. **
Our robot this year (Like some others) Used the Drills & Chips in a 2-speed with Blue Supergrip treads. **In low we had enough power in the drivetrain to spin the tracks against an immovable object without tripping the breakers for somewhere around 30 seconds. **I think what Paul is telling you is the same thing. The teams that can push continously for an extended period of time are usually spinning their wheels or tracks such that the motors are not stalled out. They may even have some sort of clutch to keep the motors from stalling (47). I don’t think there are any teams out there that can go up to an immovable object and stall the motors (edit: At full voltage) for 15 seconds without tripping the breakers or at least burning up the motors.
If your robot is continously tripping the breakers in a pushing match, you need a better gear ratio, more power, or less grip to keep you in the game under power.
Believe me, I understand your question. Many teams during those stalemates are spinning their tires, many teams are at stall under VERY LOW voltage which WILL NOT trip breakers, many teams apply a brake on their wheels when in position. What you were seeing at IRI was a combination of those things. I was very close to the action at IRI and saw the stalemates you were talking about. Some teams were tripping their 40 amp breakers and some teams were spinning their tires.
The bottom line is that current draw at stall is directly proportional to voltage input and at around 4 volts input, the drill motor can be at stall without tripping the 40 amp breaker.
Some teams aren’t actually pushing at all, but sitting on some sort of anchor while the other bot pops its breakers trying to move the anchor.
Most ‘anchors’ are just pads of some high friction material on a piston that pops up and down. If you take the weight off the wheels, it becomes very difficult to move that bot.
Some teams will also ‘precool’ their breakers just before the match with some cool in a can. I don’t know how much extra current draw this gives you. Its an on going debate on if thats a good idea or not.
I remember with us that we were pretty much always pushing robots in the final matches, just to keep the top of the ramp. We’d sit there and push while our alliance partner cleared boxes out and then we’d make room for them at the top. We were even successful at pushing two robots at once often times with no breakers popping, just motors getting hot. We had rubber pneumatic tires that worked really well on the HDPE, carpet and mesh, not to mention our robot weighed in at 129.5 lbs.
Ryan,
I think you can see from the above posts that there are variety of reasons for these robots being able to push and not trip breakers. This is some of what I have observed over the past few years working from the electrical side… Teams that are able to push will never have large tires (4-6" seem to be the average). Custom gearboxes or special transmissions are used. The software is written so that the motors are not driven at max input power or software backs down after a predetermined period of sustained pushing. The tires are not extremely sticky so that under extreme pushing the tires slip. Or my personal favorite, the drivers are so well practiced that they know their machines and don’t push them to the limit.
No teams that are able to last a prolonged pushing session would be able to last through a four minute match or longer. All of the variables (heat, voltage, friction, breaker temperature) will combine to trip breakers, reset the controller or set off the main.
Our robot was designed to have maximum speed and power, without EVER tripping a breaker. And we never did. Save the first round of the FIRST National finals. Oi! (We went through every round of Curie plus the two in the semifinals of the Nationals. Way too much for that machine).
How did we avoid tripping a breaker through all of that? Did we have small tires? Nope, 8 3/4 now, nearly 9 inches with extra layer of rubber with our special tread cut not yet ripped off. Cut back on programming? Only on one side to make the bot go straight - even then, very minimal.
Transmissions? A one speed gearbox? Custom gearbox? Definately! 4:1 gear ratio from the drill gearbox to each of three wheels, powered by an atwood geared down to the drill’s speed and then put into the same 4:1 system. They work together.
But what keeps us for tripping? From wasting battery power? From simply overheating those motors or, in this case, the breaker and thus tripping it?
Simple. Rarely move your bot.
The 25 trade secret we designed in 2002 and built upon this year for our braking system. Our brakes, which I will be glad to show you, consists of two servos, one on each side. When activated, the entire gear system is dead. It is quite literally impossible for any robot in FIRST to turn our wheels. It’s possible to move us, as it’s been done before, but we are nothing but a dead weight with an amazing amount of traction due to tires.
Why does this help?
It allows us to conserve battery strength, keep motors cool, and even keeps the entire system, including the breaker, in tip top condition. We tripped the breaker in Finals because it was the 9th match in a row without stopping for our dear Evil Machine.