Just this past weekend we went to a competition and did really well… except for our power. The issues that we had had were centered around 2 things: the batteries and the circuit breakers.
For the first time ever we ran into problems with our batteries holding a charge. normally I wouldn’t think anything about it since they are several years old, except EVERYONE of our batteries stopped holding a charge at the same time. Does anyone know what could cause this to happen to all of our batteries at once? Also seeing as we now have about 15 shot batteries, are non-vex batteries legal in competitions?
Our second issue had to do with the circuit breakers. We are using a power expander and have our 4 drive motors going to it. About our 2 or 3 match we started to have our circuit breaks tripping. It started when we got stuck on top of a bean bag but after it the first time that it tripped it started to. Would this problem be able to be solved by splitting up our drive motors so that they are spread over 3 different circuit breakers or has anyone found any other solutions to this problem?
I’m not sure about your battery problem, could you elaborate on the problem, any events/details leading up to their failure?
As for non-VEX batteries, they are definitely illegal.
For your breaker problem, you’re probably going to want to distribute your motors better over the three breakers. Usually drive motors are the most stressed subsystem of the robot (drive, lift, intake) since you can get stuck on a sack or get into a minor pushing match with another robot.
After the drive train, the lift system is probably the second most stressed subsystem, but once again, this requires an individual assessment in terms of your lift ratio/your desired capacity.
The point is, you probably don’t want your most stressed subsystem all on one breaker (your power expander) but instead split them 2-2 over two breakers. In the most common configuration of 4-4-2 (drive motors - lift motors - intake motors) we like to have the motors distributed over the breakers as such:
2 drive, 1 lift
2 drive, 1 lift
2 lift, 2 intake
You did not state how many of which types of motors you have on your robot. If you are using many 393 motors, you could be asking too much from the system overall no matter what. 269 motors are not free either. Everyone takes some power. Seeing as you mention low battery life you could be asking for a lot of amps too. A power expander will help with additional capacity but it has a circuit breaker too.
Search around for some of the threads looking at power management. Also slew rate control might stop sudden spikes of power requests that could be creating tripped conditions. (seach jpearman’s posts on this)
But knowing how much power you are using and doing something about it are two different things. You will then have to decide if you are at max amperage out which motors get cut from more power? Drive? Lift? Grabber? Oh the perils of this choice! Otherwise you can be stresssing those poor circuit breakers into tripping.
You need a Star Trek Scotty control: “She can’t handle any more Captain!” Krik would have busted the Enterprise every episode/movie if he did not have Scotty to stop him from overlaoding the warp core. Same deal here.
We are currently using 9 motors, all of which are 393 motors geared for high torque. We have 4 for our drive, 2 for the lift, 2 for intake, and 1 for unfolding our intake chain and raising it so that the sacks slide out of our ramp into the trough.
As for the battery failure, we had 3 teams using the batteries and about half way through the competition some of the batteries stopped working. we would plug them in to the chargers and it would start to blink indicating that the batteries we charged but then when we would plug it into the robot we would get about 30 seconds and then it would die. By the end most of the batteries couldn’t even get the light on the power expander to even light up. the We were having to borrow batteries from our alliance team but they did not have enough batteries to be able to provide batteries for us after every match.
If the issue really is that the robot just can’t handle 9 high torque motors then is there any other option other than swapping a few motors for 269 motors? We are already using almost all aluminum to reduce the weight so that there isn’t as much stress on the motors.
It sounds like you discharged them too much, have you tried charging them since the competition. Which batteries are they? 2000mAh or 3000mAh ? When you plugged them into the robot what color was the “robot” light on the cortex, green, yellow or red ?
It should not be a problem running 9 or 10 393 motors using two of the 3000mAh batteries. Using 269 motors will not help much, you are still trying to do the same amount of work with them.
When we plugged the batteries in most of the time both the light on the microcontroller and the light on the expander would start out being green and most of the time during the match or right after the match started the light would turn yellow and then there was only about 10-30 seconds before it turned red and finally we would lose power. Like I said before, by the end of the competition the batteries could not even get the expander light to turn on.
We have charged the batteries since but we have not run our robot yet. My school has a robotics class that make basic vex robots like the clawbot and protobot and they used the batteries. At least 2 batteries caused the PIC microcontrollers to start buzzing, indicating that the battery is extremely low. As soon as we get a chance to test them I will post the findings. We are likely going to take today as a rest day, tomorrow most of my team can’t make it, so likely Wednesday will be the first day we can really push the motors to see if they do it again.
We also have a scrimmage this Saturday so that will be the best time for us to stress test them since there is less teams then normal meaning matches are more often.
With this in mind do you have any suggestions as to how my team should approach this problem? Based on what people have said it seems like the only thing we can really do is maybe more torque gearing. We currently have a 1:1.3 gear ratio for our drive. Would it help if we dropped down to a 1:1 ratio?
Yes, increasing the ratio on your drive train would help, though my experiences with four motors on the torque setting around that gear ratio have been positive. You shouldn’t be having any problems. I would suggest looking at excess friction in your drive by removing the motors and spinning the wheels/any axles that you have to see if they are grinding against anything. Make sure to use bearings wherever the axles pass through metal.
This could be your problem. On the vex chargers, blinking does not indicate done, but instead indicates something has gone wrong with the charging. I thought it meant done my first year, which caused all sorts of problems.
We were plagued with battery troubles for the first two years because of old batteries. Before worlds last year we got all new batteries and chargers. This has fixed all of our problems, and I would highly recommend investing in new batteries and smart chargers if you can afford it.
So would you say it would be best to go to a 1:1 or gear for torque? The only reason that we have the slight gear ratio is because there are a few teams here in colorado that have extremely fast robots that, like us, are great at picking up the sacks in bulk so we added the gearing so that we would be able to keep up with them a little better
I would say that you should mess around with the breaker distribution and check for unnecessary friction before you go to the last ditch effort of gearing your robot for more torque. In my own opinion, and I’m sure there are other experienced teams out there as well that agree, 4 393 motors on torque setting at a 1:1.3 ratio should not be having problems unless you have excessive friction in your drive or an overly heavy robot (upwards of 20 or 25 lbs).
It could be that you think the batteries are charged but in reality they are only partially charged. To know for sure, we use a battery tester from Radio Shack that has a connector on the side that fits the connector on the VEX batteries. It’s discontinued by Radio Shack but you can still find it on Amazon.