Joining 2 dif. motors

[JVN]

Originally Posted by Rob Colatutto

There was no option to slow down, the robot slowed itself down when the additional load of the goals was added to the robot. The drills couldn't fully handle the extra 362 pounds and so the motor slowed down and then the Atwood picked up for it, drawing less current than running the Atwood at a slower rpm. We also had a 'software shifter' which took the joystick feedback and divided it by 1.5 so the operator could switch the speed down when latching onto goals.

Matt and Rob are both on the same page with this one.

Basically it is possible to design a gearbox to emphasize different motors depending on the torque-load on the gearbox (which determines motor performance... speed, efficiency, current draw, etc).

This means: your gearbox is designed with 2 motors. When the robot is moving across the field unimpeded, the torque load on the motors is such that one motor is carrying most of the load, and the other is somewhat dragging within the gearbox. When the robot is under a high-load condition (like towing goals, or pushing against another robot), there is more torque loading on the motors, and now the other motor's output is greater.


I don't like this method.
What we use on 229, (and many other teams have used, with great success) is simply matching the free speeds of the motors. This means, that if the robot is opperating under a no-load condition, both motors will be spinning the same speed. Will this ever happen: No! But... it works out pretty well. Essentially you get a nice balance of torque output from each motor at all different load conditions. It's not perfect, but it works well.

We also couple this with a 2-speed shifter. This allows the designer to provide 2 different torque output conditions, depending on torque loading. (Need to tow a goal? Just downshift!)

*shrug* This is all a matter of preference. There has actually been quite a bit of debate on this subject within the past year (Tytus' differential combiner concept). I think for now we'll stick with matching free-speed, simply because it's easy, and it works well enough for our applications.

Good luck,

John

PS - As always, for more information about the theory and physics involved in this stuff, look for other posts on this forum (There are plenty!), contact your friendly neighborhood FIRST mentor/enginerd. (Or drop me an IM.)

[JVN]

Originally Posted by Matt Adams

This is incorrect.

Atwood at normal load (from 2003 spec sheet) is 4378 RPM.
Free speed is 5,500.

Free speed on Bosch is 19,670 RPM as Sarah stated for normal load speed.

Just wanted to make sure that the right data is out there.

Check it out here:
http://www.usfirst.org/robotics/2003/specsheets.htm


Matt

Advice to the rookies:
Don't sweat the numbers TOO much... (you still need to calculate!)
These values vary so much from motor to motor, and the specs aren't 100% accurate anyways...
Using a free speed of ~5,500 for the Chip, and ~20,000 for the Bosch is fine. Any more detail than that and you're wasting your time. (Motor performance will change over time as well).

Don't believe me? Perform some tests... you'll be surprised by what you find.

If you get a 90% speed match between combined motors, you're probably doing okay.

[Matt Adams]

Originally Posted by JVN

Advice to the rookies:
Don't sweat the numbers TOO much... (you still need to calculate!)
These values vary so much from motor to motor, and the specs aren't 100% accurate anyways...
Using a free speed of ~5,500 for the Chip, and ~20,000 for the Bosch is fine. Any more detail than that and you're wasting your time. (Motor performance will change over time as well).

Don't believe me? Perform some tests... you'll be surprised by what you find.

If you get a 90% speed match between combined motors, you're probably doing okay.

John is right on.

I just wanted to make sure that the right information is out there. You gotta be close, but it helps to start with the most accurate numbers available.

Matt

[Rob Colatutto]

In regards to the speeds on the new Bosch motor, the spec sheet on the FIRST site gives the numbers at 11v, not 12v or 13.2v when the battery is fully charged. So the 19670 works as a normal load speed at 12v. As for the Atwood speed that was my bad, I posted under her name and I didn't have my papers handy, thanks for catching it.

[K.Shaw]

thanx alot guys! all this info i feel so luved as for the fighting and arguing calm down lol anyway keep up the info this is really helping alot!

[Joe Johnson]

Originally Posted by Matt Adams

Now with the above illustration.. don't think of motors as people rowing a boat, where if one is rowing harder the boat will turn. Think of the motors as a bicycle with two riders. If one works harder, that's fine. But if someone's legs just can't physically move fast enough, it's bad news![/font]


Matt[/color]

I generally agree with what Matt has to say, but I want to point out that the free speed of a motor is not like the speed of sound in which there is some major discontinuity in the behaviour of the motor. A motor does not have an opinion about what speed it spins at. In particular, it has not qualms about going faster than its free speed.

What I think folks need to keep in mind is that the speed/torque line does not end at the Y-intercept, nor the X-intercept. If you couple 2 motors together, they are going to run with some (fixed) relationship between their speeds. SO... at each given speed, the torques are going to ADD. They add even if you are opperating at a speed higher than the free speed -- BUT, the motor running faster than its free speed is providing NEGATIVE torque at that point.

As Matt said, this is not a 100% tragedy. The motor does not suddenly blow up like a it was in a cheap action movie. Just as Matt said, it hurts the effeciency (because, just like the speed/torque curve, the effeciency parabola does not end either -- you have NEGATIVE effeciencies for that one motor in this range -- which only makes sense, you are putting still putting positive electrical energy in and you are also adding work via the shaft i.e. getting negative energy out). If you opperate in this range very long, you will smoke your motor but that is more of a gradual thing not an explosive process.

Another thing to think about is that up to this point, we have only been comparing motors as if there is only ONE speed/torque curve. This is not true. There is a family of speed/torque curves based on the voltage applied to the motor. It is possible to give more or less voltage to one motor or the other to reduce some of these problems - it is not a perfect solution, but it can be a way to help the motors share the load a bit more effectively.

Joe J.

[Larry Barello]

One reason to gear the motors differently (or to electrically change the ration in S/W) is that the Bosch can't seem to dump heat as fast as the CIM.

My team choose 3.5:1 as the gearing ratio (output of bosch gear box in high speed vs shaft speed of CIM) and found that we needed to drive the Bosch at ~70% to keep the heating approximately equal over the match. I think this year we will go for 1:4 and dump the Bosch gearbox to better match the motors. I think our problem was not only too low a gear ratio but the added load of the Bosch gearbox. We kept it in high gear to minimize that, but it would be best to use a single pinion to mate the Bosch with the CIM.

Excellent PPT presentation, Matt. You are dead on. Nice to see someone who knows what they are doing and takes the time to go through the math.

[Norm M.]

Last year, our robot used a pair of the gearboxes on each side. (Four total). We chained the front to the rear, thus making a 4 wheel drive, skid steer robot.

We used the drill motors in the stock configuration (geared down 2:1) on the rear gearbox.

We used the FP motors on the front gearbox, with the large and small gear swapped (thus geared up 2:1).

This combination worked quite well. One thing to remember about 'pushin and shovin' is that the circuit breakers are quite often the limiting factor in how much power you can put into your robot. Thus, using the FP and Bosch gave us (70amp x 12v) watts of power available. A Chip and Bosch would have given us (80amp x 12v) watts. Of course, there are thermal constants and time constants associated with the breakers.

A good gearbox with the Chip and Bosch tied to together is a lot of power. But it takes some gearbox work to get there also!

Regards-
Norm Muzzy

[ttedrow]

All this talk about matching free speed or matching torque is great. It adds to the knowledge base for the students. But when it comes down to the practical application in the FIRST robots, we need to match the current draw for each motor. If one motor is drawing too much current and trips the breaker then what? You can design the most mechanical efficient drive train but you are still limited by the 40A circuit breakers (P=IE)

Lets keep the debate going. This is great stuff

Go Boiler!!!

[eugenebrooks]

[/font][/color]
Input welcome, could a EE out there explain back EMF for me a little better?
Matt[/color][/quote]

If one motor is over driven by the other, it becomes a generator,
in effect and saps power from the drive train to backfeed electrical
power in into the robots electrical system. This is easily demonstrated
by turning a motor by hand and watching the leds on robot control system.
You will see them light up.

If the over driven motor is sapping power from the mechanical drive
train, it is taking up power that would otherwise move the robot forward,
hence the loss in efficiency.

[Matt Adams]

Originally Posted by eugenebrooks

If one motor is over driven by the other, it becomes a generator, in effect and saps power from the drive train to backfeed electrical
power in into the robots electrical system. This is easily demonstrated
by turning a motor by hand and watching the leds on robot control system.
You will see them light up.

If the over driven motor is sapping power from the mechanical drive
train, it is taking up power that would otherwise move the robot forward,
hence the loss in efficiency.

Since I asked this question, I've taken a class at Purdue that dove into DC motor information. What you're saying is mostly correct. At the time, I was specificly looking for how to find the back-emf constant, that is, relating the amount of power loss quanitatively to RPM. I've come to discover that these coefficients are usually listed within motor specs from the manufacturer, and is essentially the torque constant for most of the motors that we'd see in FIRST.

Thanks for answering the question, though.

Matt

[pryoplasm]

ok, a few things, i dont know the exact stuff for what my team did this year because i specialized on the electrical stuff rather than the mechanical side, but i can give a rough idea of what we did

we designed and made our own gearboxes, with different reductions for the drill and the CIMs. not sure what the exact ratio is. also, a heads up, unless the drills/CIMs are mounted in the same direction (ie. both facing left) then one will have to run in reverse, which isnt the same as the high speed for the forward drill.

also, if you only want to use 2 motors for the entire drivetrain, its possible for tankdrive. we did that, oh, maybe 3 years ago, the game with the ramp that could tip back and forth.., used 2 drills in what i can only incorrectly describe as a figure 8 looking thing, with 2 drills in the middle of each side, 2 chains going to the front and back wheels. we also had a sweet transmission that could pop a pin and slide around in neutral, but that belongs in a different thread.

hope this helped

[Ryan F.]

Heres the simple method we used that worked just fine. We used the drill motors with the FIRST supplied grearboxes, and just stuck those on the front. For the CIM's we designed a simple gearbox with a relatively simular output speed, and just chained the two drive shafts together. All the gearbox consisted of was a 12 tooth gear right on the CIM moror, that lead to a 60 tooth on an idler shaft which also contained a 16 tooth gear. That 16 tooth gear was then meshed with a 70 tooth gear on the output shaft. It turned out running well for us, but the CIM's might have been running a little too fast, so if I used a concept like this again, I would change the gears around a little to acquire a slower output speed.

Here's a crude diogram of out Gearbox

CIM MOTOR ---12 tooth gear
.......................60 tooth gear----------16 tooth gear
.................................................. ......70 tooth gear--wheel!

Simple is good.

[Matt Adams]

Originally Posted by rforystek

It turned out running well for us, but the CIM's might have been running a little too fast, so if I used a concept like this again, I would change the gears around a little to acquire a slower output speed.

Here's a crude diogram of out Gearbox

CIM MOTOR ---12 tooth gear
.......................60 tooth gear----------16 tooth gear
.................................................. ......70 tooth gear--wheel!

In general, I've found it much more efficient to combine a pair of motors, and then run this combined set out via some sprockets to the wheels. This ensures that one wheel won't ever try to run a bit faster than the others. If you try to run them individually, your motor torque and speed outputs vary as the speed of your robot changes. Fortunately, the Chips and Drills happen to have roughly the same trip points when matched at free speed, which is why I'd imagine a lot of people don't notice it.

Matt

[Gamer930]

What our team (Team 930) did was discussed on Thread 25012 It gives a lot of information from the people that actually designed it and picture. It also has very little custom parts.

That system work very well (Never had any problems with it).
Running both motors backwards they are almost the exact same RPM.
Going forward you have to make some programming adjustments to get the RPM close.