# Motor coupling

I’ve seen a number of designs where motors are linked together, and while I can make a guess as to why, I wanted to check on whether these reasons are correct, and whether these configurations represent good practice in reality.

1. 4-wheel drive where 2 motors on each side are geared or chained together, 1:1 ratio. I’m guessing that the reason for this is to compensate for the slight differences in motor speeds so that the 2 wheels on say, the left-hand side, will move at exactly the same speed for greater efficiency.

2. 2 motors geared together on a lift arm, again 1:1 ratio, using a Y-splitter. Obvious answer: more torque. However, based on the power consideration in this discussion, I’m wondering whether the current limit on the port will negate this advantage if the motors are “maxed out”, which is why you would want a second motor in the first place.

I don’t have a very thorough knowledge of basic mechanics, and if anyone knows of a “For Dummies”-type text that covers basic things like this, I’d appreciate a recommendation.

I would not create your first situation for the reason you gave. If the motors want to turn at slightly different speeds I don’t see an important difference between letting them fight by trying to propel the robot (through the wheels) at different speeds and letting them fight by trying to rotate a chain or gears at different speeds.

However, if one wheel gets lifted into the air during a pushing contest (or loses traction for any other reason), it would be helpful for that wheel’s motor’s output to get used by one of the other wheels. Linking both motors to both wheels can accomplish that.

FYI - Once you start linking the wheels together you can also start thinking about mounting the motors somewhere other than on the wheels’ axles, if putting them elsewherer place simplifies your design in some important way.

Blake

Hopefully I’ve got this part right: The motors themselves have a 1A cut off limit and the controller has a 4A total cut off limit. The motors pull peak current when stalled, so pretty much any other time they are fine.

Having 2 motors (or more) is sort of like gearing. 2 of them together provide the same approximate max rpm with approximately twice the torque. So you could double the rpm with gearing (halving the torque) and have twice the speed of 1 motor with the same torque as 1 motor.

I would wager that the main reason to tie 2 motors together to power a single side is to have twice the available torque available to either wheel. A secondary benefit is that they’ll go the same speed. If they weren’t tied together, they would still go the same speed (technically they are connected by the ground), but consider the situation where only one wheel encounters something it needs to overcome in some way. With 1 motor, it might stall out. If it were geared to the second motor, the extra torque from the second motor might allow it to overcome what ever it encountered.

The second situation should be reasonably okay on its own, each motor could pull ~1A (for 2A total) before anything happened. Of course, you couldn’t have 8 Y cables with 16 motors all pulling 2A (actually, you could probably only have ~2 Y cables and 4 motors) because you’d trip the 4A max.

Great point!

By amazing coincidence, I just uploaded this picture of a torque-improving solution: https://vexforum.com/gallery/showimage.php?i=4381. In this case, they did it to improve acceleration and to use *less *energy.

How does it use less energy? 3 motors running at less than full power is better than 2 motors (or 1?) running at full power or close to?

On most robots in most conditions, the wheels that the motors are driving will both be on the ground and therefore should be going at the same speed, which will cause the motors to go at the same speed, regardless of whether or not they are chained or geared together.

Chaining or gearing the motors together does help you though if the two wheels require a different torque to turn. This could happen if one of the wheels where lifted off of the ground or the center of gravity of the robot is closer to one wheel than the other. By connecting the motors, you allow the drive system to supply more torque to the wheel than the stall torque of one motors (~6.5 in-lbs).

According to this thread, the only current limit is on all of the ports combined (not on the single ports). Therefore, having two motors would give you more torque than one, unless you where using so much current through other motors on your robot that you drew more than 4 amps.

My team only succeeds at drawing more than 4 amps and tripping the resettable fuse on the microcontroller when we are running all of the motors on our robot with considerable torque (e.g. lifting an arm full of balls while pushing into something like another robot or the field wall). I would suggest rigorously testing out your robot to make sure that you don’t draw too much current. If you do, I would suggest gearing down or lightening some aspect of your robot.

Most teams put 2 or 4 motors on their robot’s arm (1 or 2 on each side of the arm), but gear it much lower than 1:1 (e.g. 1:5 or 1:7).

Yes. Think of it this way – a heavy robot with high gearing (like the link shown with 4" wheels geared at .5-1) will not reach full speed meaning the motors are close to their high-drain, low-rpm speed. You can see this in a robot that accelerates slowly and doesn’t reach full potential speed. Adding an additional motor can get the robot to the maximum speed possible faster and with less current draw than with fewer motors that are really working hard. That’s the theory, anyway. The team hasn’t tested this robot yet. Last time they tried it with 4-motor drive and it was sluggish and clearly not capable of full speed.

[FONT=Verdana]Folks,

Lest we forget one of our least favorite:( topics in engineering school, thermodynamics, let’s consider at least the first and second laws:

1. You can’t win.
2. ``````You can't break even.
``````

In the present context, this means that not only can’t you get more mechanical power out than you put electrical power in, you can’t even get as much mechanical power out as you put electrical power in. All we can do is minimize (within the constraints of the system) the difference between electrical power in and mechanical power out. (Put another way: All we can do is maximize the efficiency, which is defined as the ratio of mechanical power out to electrical power in. For a real system, efficiency is always less than 1.)

There are a few things we need to keep in mind about the power provided by a rotating motor. Because the power is the product of the torque [/FONT][FONT=Verdana]and the rotational speed:

1. When a motor is stalled, it provides no mechanical power. (Torque is high, but the rotational speed is zero.)
2. ``````When a motor is free running, it provides no mechanical power (Rotational speed is high, but the torque is zero.)
``````
3. ``````Maximum mechanical power is achieved at approximately the midpoint of the torque-speed curve.
``````

Furthermore,
4) The maximum efficiency (mechanical power out/electrical power in) is achieved at a speed below the midpoint of the torque-speed curve.

For a good, basic explanation (math optional, there are pictures;)), see:
For a more detailed analysis, see:
http://szabol0.tripod.com/WorkshopParamEM/Paper11_Manolea.pdf
For my alma mater’s take on this, see:
[[/FONT]

Thank you, Eric, for giving a more detailed explanation. “Full potential speed” was a sloppy way of saying “as fast as I think the motors should be turning” I suppose. When I help students work on robots I tell them to figure that VEX motors have a useful RPM of 50, which is half of their rated maximum of 105(ish) RPM.

The 6-motor drive was something we tried last year when some of our very light Elevation robots were sluggish in turning and lacked power to climb onto the platform. In a straight line top speed was not visibly faster, and hand-clocked sprints across the field were only slightly shorter, but the acceleration and turning rate of the robots was vastly improved. Adding two more motors to the chassis in the picture linked above was an effort to improve both of those attributes in a heavier chassis. For what it’s worth, the students involved don’t think this robot will perform well with the gearing in the pictures, but they can change that by changing chain sprockets without having to dismount motors and move bearings.

You’re welcome! I (perhaps perversely) enjoy writing such things.

That’s probably a good design point. It’s pretty close to maximum power (nominally 105/2 = 52.5) and a bit shifted toward maximum efficiency.

That’s not surprising. Turning is, after all, acceleration perpendicular to the current direction of travel, acceleration requires more torque than does maintaining speed, and increased torque at a given speed is what running motors in parallel gets you.

Eric