2 motor drive VS 4 motor drive
 

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

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

Agreed.

Re: 2 motor drive VS 4 motor drive
 

I guess we'll know in about a month, if it's a good idea or not.

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

Would a two motor drive do anything like significantly increase the risk of tripping a victor or jaguar?

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

Engineering and math aside, try it. Build two kitbots, one with 2 motors and one with 4, and bring them both to weight.

Re: 2 motor drive VS 4 motor drive
 

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.)

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

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).

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

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).

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

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.

Re: 2 motor drive VS 4 motor drive
 

Not likely. The motors are far less efficient operating near stall, which the 2-motor setup will spend much more time in.

The magnitude of this difference will depend greatly on the gear ratio.

Re: 2 motor drive VS 4 motor drive
 

I was assuming that the 4-motor drive would accelerate about twice as quickly as a 2-motor drive, spending about half the time in the 'near stall' regime but drawing twice the current over that time, resulting in the same number of A*s (electrons) being used.

Is this a poor assumption to make?

Re: 2 motor drive VS 4 motor drive
 

I modeled a drivetrain with a 10:1 ratio, 3-stages of reduction at 90% efficiency, 150 lbs, 4" wheels and 2/4 CIMs. Timestep of .01 seconds, calculated colloumb use per timestep by multiplying timestep by current draw. This was done by modifying JVN's 2004 design calculator, under drive modeling (so that anyone can repeat these results).

With 2 Cims Total
-5.4 Amps per CIM at steady state (fully accellerated).
-1.41 seconds to reach 9 fps (9 fps was picked as a value just below it's steady state speed).
-45.4 C used per side in this 1.41 seconds, 90.8 Total.

With 4 Cims Total
- 8.1 Amp per side at steady state (4.05 per CIM). This difference is likely from the motors not having zero current draw at free speed.
-.58 seconds to reach 9 fps.
- 43 C used per side in this .58 seconds.

I was a bit surprised by these results, the difference in energy use exist, but isn't huge. I imagine this difference would be larger the higher the gearing.

I'm curious how well this data matches the real world.

Re: 2 motor drive VS 4 motor drive
 

JVN's 2011 calculator has a "Speed Loss Constant" of around 81%.

Is that not used in the 2004 calculator?

Re: 2 motor drive VS 4 motor drive
 

I used .73, representing the 3 stages of gearing at 90% each.

Re: 2 motor drive VS 4 motor drive
 

OK, I've not looked at the 2004 version.

The 2011 version has a "Speed Loss Constant" and a "Drivetrain Efficiency". (see attachment)

The "Drivetrain Efficiency" affects the Wheel Stall Torque but not the Max Speed.

The "Speed Loss Constant" affects the Max Speed but not the Wheel Stall Torque.


With a 10:1 reduction and 4" wheels, the CIM's free speed of 5310 rpm corresponds to a vehicle speed of 9.27 fps. The purpose of the "Speed Loss Constant" in the 2011 version appears to be to allow the user to de-rate the vehicle max speed calculation to better match reality.

Re: 2 motor drive VS 4 motor drive
 

Is there an inefficiency associated with the CIM's output gear to the first stage gear? That is the only difference in efficiency between the two that I can think of at 7:15 in the morning :D

I am surprised at how much longer it took the 2-CIM drive train to reach 9ft/s, though it makes sense when one factors in drive-train loss.

Re: 2 motor drive VS 4 motor drive
 

Hard question to answer in a vacuum. 4 motors will give you almost twice the power. The three questions are do you need it? can you use it? Would a limited resource be better used some where else? The answer to those questions is why we have have the competition.

Re: 2 motor drive VS 4 motor drive
 

An important consideration here that everyone seems to be ignoring is the loss that occurs due to internal resistance. The internal resistance that the manufacturer reports for the battery is .011 ohms. Based on my experience with these batteries, this number is very low. As a battery ages, the internal resistance gets higher. In addition, there are other resistive losses such as the battery connection terminals, the Anderson connector, the PD connections, the PD itself, the breaker connections, the breaker itself, the wires, the motor controller the.... But you get the point. All this can add up up to a significant number. What this means is that when you go from a two motor drive to a four motor drive, you do NOT double the available torque. The current draw goes up, but doesn't double because the voltage goes down. In 2010, we had a three motor per side drive (2 CIMs and one FP). After the season, we did some testing and found that the current draw caused the voltage to drop so far that we had more available torque without the FP motor than with it. We also tried adding another battery (two in parallel) to reduce the drop in voltage. The difference was incredible. I almost think two batteries with a two motor drivetrain would out-perform one battery and a four motor drivetrain. (so far, not legal in competition)

We know for certain that we can't draw full current because even with our three per side drivetrain, we never tripped a 120 amp breaker. With all those motors stalled, we should been pulling ~750 amps. We definitely stalled the motors on more than one occasion, but never tripped a breaker (the battery voltage dropped down below 7 volts at times though)

If you want to run some numbers, decide on a number for total resistance for the system, .035 to .045 ohms is not a bad guess. Multiply this number by the current draw to get the voltage drop. Subtract that number from the starting battery voltage (~12.6V for a charged battery) to get the voltage available to the motor.

Unfortunately, the motor curves are all shown at 12V so it is difficult to estimate the real torque available when the voltage drops to 8V or so.

Try some calculations, then run some experiments. Tell us what you found.

Re: 2 motor drive VS 4 motor drive
 

I don't mean to toot my own horn per-se, but there is a tool available that shows the tradeoffs for 4-vs-2 motors with respect to top speed and current draw.

Attached are screenshots of how I'd design a 2-motor drive and the respective 4-motor equivalent (all else equal). It uses the 2011 KOP transmission with 6" skyway wheels. The exact #'s are always subject to verification (efficiency measurements, exact motor specs, weight, etc), yet it does show a meaningful difference between the two.

Note that the difference in max speed will be noticeable if the robot ever has to traverse the whole field. Other than that, if the 'zones of play' this year are small, 2 motors might not be that big of a deal if you avoid pushing matches.

edit -- I'd use a 12:34 sprocket ratio instead of a 12:28. Single-speed geared for ~10 ft/s.

Re: 2 motor drive VS 4 motor drive
 

There's a simple-to-use motor calculator here which handles different voltages easily.

You can even enter a desired operating point (speed and torque) and it will tell you the voltage.

Re: 2 motor drive VS 4 motor drive
 

Thanks to James for pointing out that I hadn't actually uploaded the version of the tool that generated those screenshots. It's up as "Version 3.0" now.

http://www.chiefdelphi.com/media/papers/2469

Re: 2 motor drive VS 4 motor drive
 

it all depends upon the application and drivetrain. we use mechanum drive, so we use 4 independent motors and gearboxes. we have used mechanum every year but '09, and we honestly have no problem w/ pushing power or inclines. we also have not had any chatter problems, although, in 2010, crossing the bumps took it's toll on the bot. we even managed to drive surprisingly well on 3 motors when a gearbox stripped out a pinion.
if your not gonna be pushing a whole lot, 2 motors will work fine. we use 2 motor RWD tank drive on our t-shirt cannon that is probably 50% heavier than a competition robot, and it works well.

Re: 2 motor drive VS 4 motor drive
 

For students interested in the math, attached are the equations of motion¹ and plots² of distance, velocity, and acceleration vs time, for the 2 and the 4 CIM vehicles, at 73% and 100% drivetrain efficiency, assuming:

- assume constant 12V supply to the motors (not a good assumption, agreed)

- "speed loss constant" 100% (e.g. vehicle top speed corresponds to motor free speed. may not be a good assumption)

- motive force varies linearly with vehicle speed

- wheels are not traction limited

With the given assumptions, the equations of motion can be solved analytically to obtain a closed-form solution (no need for numerical integration).

¹ http://maxima.sourceforge.net/

² units converted to feet for plotting