Recently my team is thinking about going from 4 motors on 2 gear boxes to 2 geared motors. Do think this is suffice for a good robot?
What are some of the more specifics on pros and cons of each?
Thanks
I’ve seen robots do it reasonably well. So it is sufficient.
I used the handy-dandy JVN design calculator to check, though. Without the extra motor, you increase the load on the remaining motor and halve your torque output without changing maximum speed. It still works, it just doesn’t push as well.
That’s good when you don’t have to, or can’t, push other robots. In 2009, additional torque was pretty much wasted on the slick floor; you’d just spin out the wheels. The weight of the motors was better used elsewhere that year.
Agreed.
I guess we’ll know in about a month, if it’s a good idea or not.
The max speed changes somewhat. There is drivetrain friction and rolling friction. When you put all that load on one motor instead of two, the motor slides further down the speed vs torque curve.
Additionally, the acceleration of the robot will be cut approximately in half. This may or may not be a big deal depending on the game and what you are trying to do, but it could potentially make the difference between getting away from a defensive robot and them staying in front of you.
For 2009, many teams went with two motors because the low friction meant that any additional torque beyond what two CIMs provide would likely be wasted. Also, if you have a lighter robot the drop in acceleration may be less noticeable.
The obvious advantage to using only two CIMs in drive is allowing them to be used elsewhere. However, this past year the large number of other relatively high power motors (FP, 4 Banebots) made this much less important.
Would a two motor drive do anything like significantly increase the risk of tripping a victor or jaguar?
Yes, you could also take into consideration that the more motors you use, the less overall current draw you use. And all of your electronics will last longer.
Possibly. The torque required to go from full forward to full reverse causes a current spike. With 2 motors that spike will be twice as high as with 4 motors. I have seen robots with high speed gearing trip the 40A breakers when doing this maneuver.
You can get these answers from the JVN calculator, too. The current draw does increase, though not necessarily to the point of tripping the breakers.
Sorry, yes, of course that’s true. The calculator is a little ideal, so in the theoretical version the maximum RPM (and max speed) shouldn’t be changed, it just takes longer to get there. In reality, though, there is speed loss, but it will probably be less dramatic than the torque loss.
We ran single CIM drives in 2006 and 2007.
From my experience with driving them, they were always grossly under-powered and rarely fast enough to catch another team while playing defense. If you run calculations on the drives you’ll see that the math will agree with this. By the time you’ve geared a single CIM drive down enough to ensure proper loading on the motor, it won’t be fast enough to catch much of anything.
IMO, running a single CIM drive in most games, Lunacy being the exception, is a good way to handicap yourself in competition.
Engineering and math aside, try it. Build two kitbots, one with 2 motors and one with 4, and bring them both to weight.
Because some teams drop the extra CIMs for weight, also test the two bots without extra weight to see if the weight reduction changes how our predictions play out. It’s possible that the lighter version may still be a viable option, especially for low-weight robots. (I think it will get back some of the lost acceleration, but none of the pushing ability.)
Depending on your wheels, and weight. The wheels may begin to slip at a certain point where two motors would not help with pushing power. Like lunacy. I think. Another thing to test for.
Emphasis mine.
I think you meant to say “the less per-motor/controller current draw you will potentially use” because with 4 cims (4x40A breakers) it is possible to draw twice as much total current as 2 cims (2x40A breakers).
That’s right. Last year’s rules prohibited splitting the output of a speed controller to 2 separate motors. What we did was split the PWM signal to 2 jaguars, each hooked up to their own 40-amp breaker, for a total of 4 jaguars and 4 CIM’s. It worked just fine.
So what you’re saying is that with the torque divided upon the 4 motors vs 2; has less potential to break the victors (because of the current load); but has more potential to drain the battery faster (again because 4 motors are able to draw twice as much current as 2 motors).
When accelerating you will draw less current with 2 drive motors, but at speed 4 will draw less then 2 overal. This is presuming that both senatios are using the same gear ratio. Accounting for the powers loss (with a lower gear ratio) will lower the draw. Check it out with the jvn calc.
Even at the same ratio, this really depends on the ratio. If you’re at the point where two motors just isn’t enough power to accelerate quickly, you’ll end up drawing more current (when averaged over time) total.
At speed with the same ratios and friction, your net current draw should be the same, you’ll just be going slightly faster.
It’s worth noting that the robots total coulombic consumption (number of electrons taken from the battery, i.e. energy) to reach top speed will be essentially the same with 4 cims as it will with 2 cims.