I’m designing a new powered robot cart for my team and was curious as to what I should use to power it? Its going to be running on 4cims a window motor and possibly some rgb light strips. I know that the standard robot batteries don’t last very long. Would it be worth while hooking up two batteries in parellel or getting some other type of battery?
Yeah, no reason why not to run them in parallel. If you do though, make it so you do not modify the batteries away from FRC regulations, in case you suddenly need another battery!. plug the batteries into the cart and only then have the wires splice together.
There are a few other considerations you should have, too. Just to bring them to your attention:
Make sure there’s a NEUTRAL mode so that the queuing staff or field reset can move your cart when you aren’t driving it. Better, so that YOU can move it when the power fails!
SAFETY. SAFETY. SAFETY. Make sure it’s a slow speed, can go over cable protectors without tipping or tipping the robot, has padding, dead-man switches, flashing lights while moving… Basically, make sure that you can be safe while moving through a crowd. That is, unless you like the safety inspectors removing your cart from the venue.
Batteries: Instead of running two batteries in parallel, which can be rather interesting to set up correctly so they’ll work together, go with a 12v deep cycle battery. One of those should last longer than even two batteries in parallel.
There is a reason not to run them in parallel. I’ll let the EEs like Al get into greater detail on why, or let you look at the other thread on batteries in parallel, but from what I understand, you pretty much have to charge them as a pair and discharge them as a pair, after balancing the two batteries. No suddenly pulling a battery out to use on the robot, no swapping, just use as a pair on the cart–unless you really want to go through the setup hassle again.
I would go for Marine batteries because of their ability to deeply discharge. However, if you aren’t continuously driving the cart for fun or like the robot, a robot battery should be fine. However, if you are willing to have it similar to my driver station power, using a HUGE marine battery, it is up to you. my project is at: http://devstuff.no-ip.biz:890/?page_id=58
Something to consider if you are already going to have power on your cart is to have the ability to charge your driver station while waiting in line. I know from experience this can be a life saver with close turnaround matches, especially if you have an older laptop with a dying battery.
It is a pretty simple project, just splice one of these into your setup so it gets 12V, then plug the laptop AC adapter into one of these. Connect the 2 together and you are set. Alternatively if you have a universal laptop adapter you can keep with the cart that is even better. (Also, a place to rest your OI on the cart is nice)
(I also have to mention, when designing carts people always forget about wheels, pretty much the bigger the better with the huge cable covers FIRST uses.)
Are you building this cart for moving the robot to queuing? If so, I would highly recommend not using any method of propulsion other than humans. It’s less complicated, ultimately safer, and will get you to the field no slower than a motorized system. Realistically, the robot cart should have space to carry a robot, space for the bare minimum tools (some sockets, wrenches, tape, pliers), an extra battery or two, and space to store your driver station, if required. The addition of an FRC battery and an inverter will help provide a few extra hours of juice for when you’re out testing at the practice field or in queuing.
These are the only rules and guidance I have come across from the Administrative Manual:
4.2.2 Robot Carts
To protect team members from muscle strains and other injuries as they transport the robot between the Pits and the competition area, we strongly recommend team members use a cart. Please keep the following in mind:
•Carts must remain in the team Pit area when not in use for robot transportation;
•All carts should fit through a standard 30-inch door;
•Wheels on the cart must not damage site flooring;
•Do not add music or other sound devices to the cart; and
•Put your team number on your cart so it can be identified by field personnel.
NOTE: Carts must be safe. They must be easy to control and maneuver, and pose no risk to bystanders. Carts identified as unsafe by Safety Advisors must be made safe before they will be allowed to be used.
If you do decide to use batteries, you should use the sealed, non-spillable type.
As I work at a battery store, I can back up the notion of a marine deep cycle battery for running motors and the AC inverter, but if you’re charging batteries off of the cart’s power supply *and *doing all that, FRC batts aren’t gonna do the job but I’m not certain a marine battery will. The marine battery will be good for a couple hours at a competition,
as its reserve capacity will only last for about an hour and a half – two hours if you’re lucky – before you have to recharge. You would need to take serious power saving actions like turning it off when you aren’t using it and keeping the motor controllers set at coast at all times before you could really get the most out of it.
My Driver station idea is similar to this robot cart and I came up with the idea of having an onboard mcu monitoring all the charge levels and everything. This will have relays to be able to turn on and off things like inverters. I thought of this idea just while writing this:
Measure, using a multimeter how much current the inverter draws in standby (no load). Have the MCU power down the inverter if it doesn’t notice a load for a couple minutes and a button to reactivate it. That will feed you with more hours. Also, plug in the robot cart when at the pits or near a power plug. That will make sure that you can get the longes runtime, and that will also give the battery some rest. Also, if a team is ready to spend a thousand dollars, check this out: http://www.howellenergy.en.alibaba.com/product/779395247-214095807/LiFePO4_12V_200Ah_Battery_Pack.html
Agreed. However, if you’re set on using a battery, I’d agree with the previous posters and say that a marine deep-cycle battery would be best. It can last for a very long time (depending on usage), and it’s nearly bombproof.
I would advise against using an FRC battery to drive the cart–if you end up needing an emergency battery you don’t want to have to use a half empty one.
I think it is safe to have motors as long as there is a speed and torque cap. Maybe, you can have two modes programmed, a “high” speed one for long-distance travel outside, where no one is, and a “slow” speed for indoors.
Also, for the controls, maybe you can place pressure sensors on a handlebar so that the motors will only trigger if it thinks your are trying hard to push the cart. I think that a Window motor will just be perfect because it is high torque, low speed. Also, make sure it isn’t too slow, as that would render it as more-or-less useless because it would take to long to transport the cart in this way. I think it would be wise to stay away from CIMs because they are high speed low torque so you would need to add money and weight to gear them down. Also, if there are motors, you should probably have a physical “emergency” switch to break the circuit if the cart goes out of control. Could save a few lives if the cart is heavy
We are already have many things planned for safety measures. The cart will not only be moving the robot but the operator will also be riding on it. We plan to have a pressure plate where the operator stands and if there is mot more than say 75 pounds on it, the cart wont start. We will also have a main kill switch for the drive lights and horn to help ensure it is safe. The only other thing i can think of off the top of my head is that we will have the ability to disengage the drive motors from the wheels so we can move it manually if needed.
As for the motors, we have the machining capability to make the gearbox and I don’t see the added weight being an issue so I think we will be planning on working with the cims still. Also the cart is going to be weighing upwards of 150lbs without robot or operator so a couple of window motors would not cut it. We would need at least 6 if not 8 just to make it move.
Riding on the battery cart is quite a new concept! Just make sure it is stable and maneuverable. Something like mechanum can help because it will get you around obstacles easily. My battery cart is designed to be small, so this heavy cart of lead, if it has motors, will be controlled by pressure sensors, only triggered it the cart thinks you are pushing to actually move it, not just leaning on the cart :D. But currently, our team doesn’t approve using motors as it could be dangerous!