Joining 2 dif. motors
 

Team 237 would like to know how to join the Chiaphua and drill motors into one for the drive system. Can anyone help us on this feat and tell us in exact detail how you can do this? Pictures, diagrams, anything that can help us out post it here or email it to me at kevin185@optonline.net . Thanks alot! :ahh:

p.s. even joining any other 2 motors post it here also thanks :cool:

Re: Joining 2 dif. motors
 

Are you saying use a chiaphua and a drill for each of the 4 wheels? or like 2 sets of that in the back and FP's in the front?

If you can describe exactly what you are trying to do I will look through all my downloaded stuff and archives to see if I can find something like it.

-Greg The Great

Re: Joining 2 dif. motors
 

ok cool umm we are trying to use tank steering with a drill and chiaphua motor on each side of the robot. meaning 1 chiaphua and drill motor on one side and aanother set on the other side.

Some Comprehensive Theory
 

Oh boy.. do I have something for you.

As part of the FIRST college course that I taught this semester, I have some rather comprehensize data on combining both the chips and drills... from a number crunching standpoint, it's very complete.

There aren't any sprocket sizes, CAD drawings, or the like.. but theory wise.. this is (fundamentally) very strong material. I will be publishing a white paper along these lines soon on www.boilerinvasion.org.

Here's the PowerPoint. If you have questions, please ask. If someone has corrections, comments, or concerns, please ask.

http://web.ics.purdue.edu/~mjadams/motorsinfirst.ppt

Matt

Some well.. less Comprehensive Theory
 

I avn't read the whitepaper listed above, it might say what i'm about to say but anyway... Don't match the moters at free speed. I have seen many teams do this. This is the quickest way i know to mismatch them. Match them under some typical load. Further matching can be accomplished through software.

Re: Joining 2 dif. motors
 

thanks for the info guys keep replying to this post! we need everyones input :)

Re: Joining 2 dif. motors
 

Search... there are 1000000000000000000 threads on this topic (well, not that many, but there are certainly more than I care to count) download a white paper such as the Technokat's transmission, or Team 116's, or Team 716's They all offer complete prints and hints/tips etc.

Re: Some well.. less Comprehensive Theory
 

Quote:

Originally Posted by Rickertsen2 I avn't read the whitepaper listed above, it might say what i'm about to say but anyway... Don't match the moters at free speed. I have seen many teams do this. This is the quickest way i know to mismatch them. Match them under some typical load. Further matching can be accomplished through software.

From the best of my understanding, nothing is wrong with matching at free speeds. If you don't, at higher RPMs you will have back EMF occur, reducing efficiency if you have one motor trying to go faster than the other's free speed.

Take a look at the following figure for a Drill in low gear and Atwood coupled together at the drill's free speed:




Let's say that you have 200 in-lbs of torque applied to the shaft. In this case, the motors coupled at free speed will spin at about 225 RPM. The atwood motor will be contributing more torque to the system than the drill. I do not see a problem with this. I admit that there's uneven load sharing between the motors... but there is nothing fundamentally wrong with this from a mechanics standpoint.

I think that some people believe that if motors aren't sharing equal load, then there's a "problem" or the motors are "fighting" each other. This is not the case, and isn't "bad." You don't have an even current draw from each motor.. but your motors aren't being ruined, and your efficiency is constant. They're pretty happy. One is working harder, but they don't get jealous of each other! The only time problems arise is if there's a situation with little torque, and they're coupled at a non-free speed. One motor (higher free speed) says, "Woo hoo! let's go really really fast!" and the other says, "But my max speed is slower than yours. I'll try to do what I can to keep up, but we won't be able to go as fast as you combined!"

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!

Now, you could optimize your motors such that they're intersecting at a low torque that your robot could never achieve due to friction, slightly below the free speed. This could help have the amount of torque (and current) distributed between the motors be more even. But having the load sharing imperfect is not a "bad" thing if you're careful about analyzing the situation to make sure the load isn't so uneven that a breaker on one motor will trip before the other can effectively contribute.

Let's take a look at this:




NOTICE: As you can see, the free speed of a combined motors in low torque conditions (to the left of their intersection point) I made to be the geometric mean of the two speeds. This is not completely correct, (neither is the fact that these show all linear relationships between torque and RPM, this is simplified). There are coefficients that determines the ratio between the two motors free speed which sets what the RPM the motors will have at low torque values. However, the RPM in low torque conditions is NOT the fastest motor's speed, nor is it the slowest. The reason for this is unique to DC motors, and refers to the back-EMF, which occurs when a DC motor is spinning faster above it's free speed. You run this risk of this occuring if you do not match them at free speed.

However, will you ever have motors truly reach free speed in a robot? Nope. So can you adjust the ratios by a little so that they're share the toque more evenly? Of course. Even matching free speeds is impossible since the free speed varies from motor to motor of the same type.

However, keep in mind that if you have low torque conditions that yield RPMs above the slower motor's free speed, you loose efficiency.

And this is going to bring up one heck of a hail storm... I know it.

Please, let's keep this at a technical level, and be as specific if you can. Disagree with facts. Graphs help out. Hack away at mine if you wish. Just keep it legit and not hunches. We'll settle this.


Input welcome, could a EE out there explain back EMF for me a little better?
Matt

Re: Some well.. less Comprehensive Theory
 

In fact, sometimes you will want to do something similar to this. For 2002 my team designed the gearbox so the bosch would be moving the robot almost to the point where the atwood was not adding any power under normal movement to keep current draw down. Then when we had goals or if we were pushing another robot we'd slow down about .5ft/s but then the torque would increase greatly while not drawing too much extra current. I wouldn't suggest doing something with that big of a difference in normal load rpm's of the shafts but if you wanted to keep current draw down on normal conditions you could do something similar. Also, I would suggest combining the two motors together as soon as possible in the gearbox to use less gears and keep weight down.

Re: Joining 2 dif. motors
 

bosch 307 Watts @37.4A
CIM 300 Watts @40 (estamate)

300+300=600
600W=one gearbox

600+600=1200
1200W 2gearboxes

1 HorsePower=746 Watts

1200/746.7=1.6

1.6 Horsepower :)

Re: Some well.. less Comprehensive Theory
 

Rob-

How did you "slow down"? Via the joystick or by some sort of a shifting device? I'd have to do some thinking about it, but initially I'd say that that just "slowing down" via a joystick wouldn't let one motor kick in more torque than the other since the amount of torque needed would be constant at any moving speed, and presumably, the joystick would control both motor's input voltage.

Matt

Re: Joining 2 dif. motors
 

Sorry if i repeat what someone else is saying....im short of time and didnt read the entire post...but you can simply use the drill motor alone(no clutch) and gear the two into a 1:1 ratio. From this point you can decide upon the rest of your gearing, etc.

If my memory is correct:
Drill no clutch:about 19000
Chip:about 5000
so its close to a 4 to 1 ratio
this means that the gear on the chip needs to be 4x bigger that the drill for a 1:1 ratio.

Hope this was helpful


-Pat

Re: Joining 2 dif. motors
 

Bosch normal load speed is 19670 and the Atwood normal load speed is 4967

Re: Some well.. less Comprehensive Theory
 

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.

Re: Joining 2 dif. motors
 

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

Re: Some well.. less Comprehensive Theory
 

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.)

Re: Joining 2 dif. motors
 

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.

Re: Joining 2 dif. motors
 

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

Re: Joining 2 dif. motors
 

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.

Re: Joining 2 dif. motors
 

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! :yikes:

Re: Some well.. less Comprehensive Theory
 

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.

Re: Joining 2 dif. motors
 

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.

Re: Joining 2 dif. motors
 

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

Re: Joining 2 dif. motors
 

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!!! :)

Re: Some well.. less Comprehensive Theory
 

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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.

Re: Some well.. less Comprehensive Theory
 

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

Re: Joining 2 dif. motors
 

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

Re: Joining 2 dif. motors
 

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.

Re: Joining 2 dif. motors
 

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

Re: Joining 2 dif. motors
 

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.

Re: Joining 2 dif. motors
 

What we did this year was to link the drills with out gearboxes to a main shaft from the Cims, This was done at 1:3.3333 (a 9 tooth on the drill and a 30 on the Shaft connected to the Cim) This was then reduced twice 1:4.5 to the 14 inch wheels. This gave us 1:67.5 if you start at the drill motor. Checking the current with wheels up and runnning full tilt, we had almost the same current at the Cims as the drills. Also we installed the drill motors facing the same way, the Cims oposide and as luck would have it the drills were in reverse, with forward for the bot being backward for the drill motors. This was left this way to help keep the bot from doing wheelies so easly and to make towing the mobile goal backward the most powerfull. We never had the weight left over to make a decated holder for towing so it didn't happen very often. With the exception of the time our arm was extended and dropped into the moble goal. One of the drills would run a little hoter than the other, but I suspect this was due to being dropped duriong he build and having a little more friction on that side do to chain. It may also be do to the not having room to mount the Cims facing the same way.

Re: Some Comprehensive Theory
 

I noted in your PPT workup that you mention back EMF. I have tried to get this concept across to several teams that this problem means that one motor is actually reducing the power available to drive the robot since that translates to a "braking" effect on the power train. I have not measured this effect but what does it do to the drivers on the motor controllers. These devices are current sources and not current sinks unless I have my theory wrong. Anyway thanks for the great work up. I just need more time to translate this for my students. LRU

Re: Some Comprehensive Theory
 

I don't quite follow what you meant in your original post but I'll try to respond as best I can. I'm not quite sure what you mean by the one motor reducing power available to the power train.

The back EMF generated by a motor is caused by the spinning magnet field of the motor. The back EMF is proporitional to the rotational velocity of the motor. The higher the back EMF, the less current that the motor is drawing.

I think what you're worried about for the speed controllers is driving a current back into them. The speed controllers which are based around an H-bridge basically just use transistor switches. They do not use transistors as current sources. When a current is driven back through them, they will pass it much like they'd pass a current the other direction (particularly since they're based on FETs). This just means that the current is going back to the battery. There's no problem with sourcing current into the speed controllers.

Matt