Hi all,
I know this may be a topic that has reached the over-discussed level, but as much as I’ve seen it discussed why you don’t have to use 2 CIM motors to power a launcher, I haven’t seen a good write-up explaining what may be the results if you do so to your drive train. If a robot is, say, 90lbs - what problems would be encountered by only having 2 CIMs on the drive train?
Thanks.
When the robot is moved with 4 CIM motors, the load is distributed among them. However, with 2 CIM motors, the load on each motor is essentially doubled. That can mean a few things. Mainly more current, faster cooking, lower acceleration, and less pushing power. In some cases it can mean worse turning. Now if you use an fp or banebots 775-18V in your drivetrain to supplement, you are using a motor that heats up faster for an application where it is used constantly and under high loads. Use the fps or banebots in the shooter and they will not be under high loads, and you can use the more durable CIMs for the drivetrain.
For reference:
CIM max power, 337 watts
FP max power, 291 watts
banebots RS-775-18V, ~ 275 watts
They are all about the same power, it’s mostly about weight distribution and durability under constant loads that determines which is best for each application. Use them wisely.
There is nothing wrong with using only two CIMs in a drive train. I’m willing to bet that a large percentage of rookie teams do it every year (like us last year). If you don’t have a need for pushing it works just fine. With a light robot, we were at 87 lbs last year, we did well enough to make it to the finals in both of our regionals. Never once cooked a motor, in fact we still use those motors on our old robot.
We have four this year simply because we plan to cross the barrier using six wheel drive and use bigger heavier wheels and tires.
I’m just remembering getting pushed around in Archimedes With our mecanums and Id rather that not happen. Our lead tech mentor says all will be well with only two, and I tend to trust him in these decisions but as a Coach, I’m still a bit nervous.
It’s all about competitive advantage/disadvantage. With this years game we are allowed an exorbitant amount of powerful motors, more than ever! This means you will be competing against teams who have harnessed that power. If you don’t do so yourself you are at a disadvantage. That’s just the way I see it anyway.
Cims are the most electrically durable motors available, when a motor is Loaded it uses more electricity, heats up, and generates more torque. Because the Cims are nearly impossible to break with shock loads, stalling, and aggressive heating due to heavy use, they are best suited to go in the drive-train, an arm without any balance, or any other application where other motors would not fare as well.
The short answer is power. More watts to the wheels means better performance.
Kind of like using a V-12 in a race car when a 4-cylinder certainly would get the car to move. Not fatal, but it may be a disadvantage, depending on your strategy. (e.g., if your strategy doesn’t include moving around a lot, then it might be OK).
If you are using a CIM on your shooter, you may want to consider using 2 other, smaller, motors instead, like two Fischer-Price connected by gears to a single shaft. For example.
Quoted for truth.
To expound further, the claim that a 2-CIM system inherently will get pushed around is a generalization rather than a fact. You could gear any system down to the point that it could bulldoze anything else on the field, but you sacrifice speed. You could make it fast, but you sacrifice torque. Since you generally want your drivetrain to be “traction limited” (e.g, your wheels slip before your motors stall), then you have to use slicker wheels, and you get pushed around.
Put another way, pushing ability is not a function of the number of motors you have. It’s a function of your robot mass, the friction of your wheels on the carpet, and the ability of your drivetrain to deliver torque to those wheels. Using four CIMs instead of two doubles your power, which means you accelerate more quickly and gives you twice the torque for a given speed. But it doesn’t directly increase your pushing ability.
If you’re not comfortable with the physics behind this, I suggest you look at JVN’s work on CD-Media.
Also, when the CIM’s first came out you were only allowed to use 2 for the drive, so they are capable.