"Automatic Transmission"

Technical Technical Discussion
1

I’m sharing this with the community here because I need feedback on how to improve this design. Please, as you read this, keep in mind that it’s not entirely complete – but complete enough to be understandable. If I wait until I find the motivation to finish it before sharing it, it’ll never get done. Also, please keep in mind that this is very much a prototype and I’m sure that it could be improved – as I’ve already mentioned.

The two-speed shift-on-the-fly transmission has become a popular, almost required, feature of many robots. Similarly, coupling two or more motors together on a drivetrain has also become very popular and it offers many advantages over single motor drivetrains. Those benefits have been discussed in detail elsewhere.

There is, however, one large disadvantage to most of these motor coupling options. It is, of course, that they must obey the “law of averages” and we make compromises about the use of the motors. High torque motors sacrifice some of that available torque to achieve a higher speed. High speed motors are corraled a bit by the high torque motors. Both motors involved are often capable of providing a higher speed or higher torque individually – but the compromises made by coupling them together allow for more torque at a given speed.

In this thread, Ken Wittlief of 578 talked about a gearbox design that takes advantage of the unique speed/torque characteristics of each motor. What follows is my interpretation of that idea.

Of course, the overall concept behind this is to create something that makes good use of the unique characteristics of two motors while sacrificing as little as possible of those characteristics.

For our purposes, we’ll consider two motors.

  • The Drill w/ Gearbox in Low – this is our high torque motor with a free speed of 423 RPM.
  • The Chiaphua – this is our high speed motor with a free speed of 5,500 RPM.

When the robot’s moving slowly – probably because it’s pushing something – we’d like for it to have a lot of torque. For low-speed, high torque applications, the drill is ideal.

When there’s nothing in the way, we want our robots to get where they’re going as quickly as possible. For that, the high RPM of the Chiaphua motor is a real asset. That motors relatively high torque at high speeds is also a benefit.

So, the problem becomes finding a way of mechanically disconnecting one motor or the other from the drivetrain when it’s not needed. Ken discussed this some in the thread mentioned above, but I’m going to repeat a lot of that for completeness.

Why do we need to disconnect the motors? Well, so our robot doesn’t explode, of course! See, the drill just won’t spin any faster than 423 RPM. So, if we’re expecting to achieve a high speed, we need something that’s going to spin a bit faster than that. (Unless, of course, we use 16" wheels or some such thing. My work was all based on 8.5" wheels.)

We’re going to use the drill motor exclusively for low speed driving and the Chiaphua motor for exclusively high speed driving.

Car engines are always spinning – even when the car isn’t moving. There, a clutch is used to disengage the spinning parts (the engine) from the non-spinning parts (the wheels). Well, clutches are available off-the-shelf, but they’re prohibitively expensive. So much so, in fact, that many I’ve seen exceed the $400 per part limit FIRST applies to teams. So, that means we need to come up with some other method of “disconnecting” our motors. In fact, we need two.

Ken talked about a one way clutch. Well, I scoured the internet looking for information on a serviceable one-way clutch or one-way bearing. I found next to nothing. There’re a few things here or there for RC Helicopters, but not much more. I know this stuff exists, but I had to make do without it.

So, with that, we have my solution for the first “disconnect” – the mechanical disconnect. I called it a “spiral clutch.”

http://www.magenet.com/~imagination/auto_shift/spiralclutch2.jpg

Missing from this image is the compression spring, mounted to one side, that would force the two halves together. That ensures good contact.

It works pretty much as you’d expect. When the right shaft spins counterclockwise, the spiral engages and the two sides act as a single shaft. When it spins clockwise, the two halves slip against one another and power isn’t transferred to the left shaft.

What might be less obvious is that all that is required for this to work properly is “relative clockwise” motion; or, for that matter, what use it serves.

Imagine the right side is attached to the drill motor and the left is coupled to the Chiaphua motor as well as acting as an output to further gearing or straight to the ground. The drill starts at a standstill, spinning counterclockwise, and engaging the spiral clutch – which acts as a solid shaft and drives the robot.

Well, during that time, so that it doesn’t drag on the system, we also want the Chiaphua motor to be spinning – but we don’t want for it to see any load. To accomplish that, we have to employ our second “disconnect,” but this time it’s an electronic disconnect. By varying the voltage to the Chiaphua via PMW output, we can make it such that its free speed corresponds with the current drill motor speed while under load. This is our electronic clutch.

Once the drill has reached its highest RPM, the Chiaphua will still have plenty of its operating range left over. It can continue to accelerate to its top speed thanks to the clutch and “relative clockwise” motion.

Imagine that the left and right sides of the clutch each want to spin at 200 RPM thanks to their motors each outputting at 200 RPM. As I’ve mentioned, it’ll act as a solid shaft because there’s no clockwise motion. Imagine next that the left side speeds up to 1000 RPM while the right side remains at 200 RPM. There’s a “relative clockwise” motion of 800 RPM. The clutch will slip.

The images that follow show the completed gearbox – including the associated gearing for each motor. (Well, most of it. The drill output shaft is missing its 18 tooth, 1/4" pitch roller chain sprocket.)

The Chiaphua is geared down 15:1 over three stages when it meets the clutch shaft. The drill is geared down 2:1. I am probably going to revise these ratios – I can already see where I’ve made an error. Can you? :slight_smile: The top speed is 15 ft/s. The low speed torque output is ~57+Nm per side (depending on external gearing).

The gearboxes are big. Remember this is the first iteration. I sincerely hope I can make them 25-33% smaller. They measure approximately 10" x 10" x 4" each. They can be oriented vertically or horizontally.

http://www.magenet.com/~imagination/auto_shift/auto_shift_side.jpg
Side view.

http://www.magenet.com/~imagination/auto_shift/auto_shift_front2.jpg
Front view.

http://www.magenet.com/~imagination/auto_shift/auto_shift2.jpg
Isometric view.

That iso view is the oldest of the views. I assure you that the end plate now has holes and bearings in it.

So, that’s that. Feedback, comments, suggestions, etc. are encouraged. Please feel free to take any ideas from this thread and apply them to your own projects – but please, tell me. I’d like to follow along and see how it turns out.

Edit: Sorry about blowing up the margins. I’ll shrink those images and edit again as soon as I can.

2

Wow, that is so incredibly awesome, Ive been wondering what youve been designing for the last few months :). Very cool idea, like a CVT, but not as complex, and far more efficient, as well as being easier to build. You really explained the whole idea well also.

Congratulations on a great job!

Cory

3

Very interesting idea you have there.

4

Very interesting. Another method I’ve seen for coupling two motors whose speed needs to be independent of each other is the modified planetary used in the Toyota Prius. In the Prius, either the gas or the electric motor can run at any speed, and both will provide power to the wheels. “Gearing” is done by adjusting how much power comes from the lower RPM electric motor or the higher RPM gas motor.

The gearbox itself is almost identical to a standard planetary gearbox, except that the Sun and the Ring are both driven, one by the electric motor, and one by the gas motor. The Planet Carrier is then connected to the drive shaft. The advantage is that, if needed, both motors, each going at their optimum speed, could power the wheels at the same time.

Another idea that I came up with over the summer (while playing with mindstorms, of all things), is to use a standard differential, except that one wheel output is replaced by a second motor input. Mechanically, this would function identically to the Prius’ system, with all the same advantages. However, building a dual power planetary gearbox or a differential (I’m not sure if they are available cheaply off the shelf) may be just as complex as building a CVT.

5

*Originally posted by ahecht *
The gearbox itself is almost identical to a standard planetary gearbox, except that the Sun and the Ring are both driven, one by the electric motor, and one by the gas motor. The Planet Carrier is then connected to the drive shaft. The advantage is that, if needed, both motors, each going at their optimum speed, could power the wheels at the same time.

If I’m not mistaken, this is how the Thunderchicken’s CCT works. There’s a whitepaper on this site detailing it.

**

Another idea that I came up with over the summer (while playing with mindstorms, of all things), is to use a standard differential, except that one wheel output is replaced by a second motor input. Mechanically, this would function identically to the Prius’ system, with all the same advantages. However, building a dual power planetary gearbox or a differential (I’m not sure if they are available cheaply off the shelf) may be just as complex as building a CVT. **

This sounds a lot like what Tytus has been working on. There are several threads devoted to debating the benefits of that design.

There are several ways of combining motors – many of which compromise the motors involved in some ways. Continuously variable transmissions offer the same or greater benefit as the idea above, but I think they’re a bit more complex. So, of course, it’s a tradeoff based on what resources are available to you.

6

Would the spiral clutches wear out? It seams like repeated slippage and heavy spring tension wouldn’t do them good.

7

I don’t understand how the drill can power the robot in one direction until a certain speed and then the Chiaphua will take over propelling the robot in the same direction at a higher rate of speed.

I guess I’m just stuck on the idea of “relative clockwise” This is a very neat idea and I’m terribly frustrated that I don’t understand it. Would someone put arrows of rotation on the drawings to explain how it works at what stage in the movements or give me an extremely detailed description?

Thanks. I would really like to understand this concept as I can see that the mechanics are quite simple.

8

This is very cool. Has this been used on a bot before? It seems that this would have been perfect for either the 02 or 03 games because of the need for speed followed by the pushing match for the goal position or king of the hill.

Sanddrag, I would explain it as I understand it, however I may just confuse you more. I’ll leave it to the creator :).

9

M: What happens when you want to go backwards or turn? Would the clutch slip? Would you do this using only the CIM? Would the latter cause you to trip breakers?

Maybe I’m not understanding it totally. (Likely solution)

Great idea, though… It’s making me think! =-]

George

10

What about a differential? Except a backwards one. Say, take a standard diff, two output shafts (the axle) and one input shaft (or gear, from the engine/transmission)

Except here, you power one of the output shafts with the drill, and the other with the chip.

So, as long as they spin the same direction, you get power out of both of them, and you don’t need them to spin at the same speed or anything. you could even turn one off completely, and the thing would still turn.

I think this idea is worth looking into. you yessssss, I do indeed. The more I hink about this he more wonderful it sounds. I will post more after school (and after I’ve thought about it some more and maybe created some prelim drawings. )

This could be a winner, folks.

with this, you could get power from both motors, each going at their respective optimal speed . . . no need to use two motors only to use one at a time, you know?

11

*Originally posted by Frank(Aflak) *
**What about a differential? Except a backwards one. Say, take a standard diff, two output shafts (the axle) and one input shaft (or gear, from the engine/transmission)

Except here, you power one of the output shafts with the drill, and the other with the chip.

So, as long as they spin the same direction, you get power out of both of them, and you don’t need them to spin at the same speed or anything. you could even turn one off completely, and the thing would still turn.

I think this idea is worth looking into. you yessssss, I do indeed. The more I hink about this he more wonderful it sounds. I will post more after school (and after I’ve thought about it some more and maybe created some prelim drawings. )

This could be a winner, folks.

with this, you could get power from both motors, each going at their respective optimal speed . . . no need to use two motors only to use one at a time, you know? **

tytus was(is?) working on a gearbox using a differential, and there are already a few threads dedicated to discussing its advantages and drawbacks, located here, here, and here.

12

Kevin A wrote:
**

Would the spiral clutches wear out? It seams like repeated slippage and heavy spring tension wouldn’t do them good.**

I have no way of answering that, unfortunately. I’m hoping that I can have a set made and tested once I’m happy with the overall design. Admittedly, I’m really poor at choosing the right material for a job, so I’m not even going to venture a guess as to the best one for this application. In the drawings, the parts are shown as delrin – mostly because of its low coefficient of friction. I’m unsure of the wear it’d see or if it’d have a significant effect. If the wear, even if it were bad, was consistent and even, the clutch should still work – a bit like car brakes.

Sanddrag wrote:
**

I don’t understand how the drill can power the robot in one direction until a certain speed and then the Chiaphua will take over propelling the robot in the same direction at a higher rate of speed.**

In physics, one of the first things you learn about is relative motion. I’m not sure if you’ve taken physics, so I’ll briefly explain.

If you’re standing beside a railroad track, not moving, and a train goes by at 50 mph – well, it looks like the train is passing at 50 mph, right?
Now, if you’re in another train, heading in the opposite direction of the first, but also at 50 mph – well, then that first train looks like it’s traveling at 100 mph.
Think of things the opposite way as well. If you’re on a train that’s running parallel to the first at 50 mph, it appears as if the other train isn’t moving at all. If the first train speed up to 60 mph, it would appear to us as if it’s going 10 mph. See?

Imagine both sides of the clutch are now spinning at 300 RPM. It appears as if they’re not moving. Speed one side up to 1000 RPM and then it appears as if it’s spinning 700 RPM when your frame of reference is moving at 300. I’m not sure if this is helping.

The real point is that “relative clockwise” motion never requires the motors to spin clockwise. It just requires that the left end spin faster than the right end such that the clutch slips. After a certain point, you could actually turn the drill off altogether.

generalbrando wrote:
**

Has this been used on a bot before?**

To my knowledge, no. My knowledge of things FIRST is fairly limited, though, as I’ve only been around for 6 years. If someone knows of a robot that’s used something similar, please let me know. I would like to see their implementation. I know mine needs improvement.

George wrote:
**

M: What happens when you want to go backwards or turn? Would the clutch slip? Would you do this using only the CIM? Would the latter cause you to trip breakers?**

Without adding more complication, going in reverse would rely solely on the CIM motors. The error I mentioned in my first post, in fact, had to do with this very item. I need to double and triple check my gearing to make sure it doesn’t strain the system too much while going in reverse. It is possible to make a design that uses both motors in forward and reverse, but it adds an additional level of complexity that may or may not be worth the hassle. In short, it’d involved two clutches and a shifting dog.

Turning is an interesting issue, I think, and something I’m still investigating. It seems that the gearboxes would behave very differently depending on how the robot turned. If it were to steer like a car, the gearboxes, together, could be used as a “differential” and the speed differences between the motors do not seem like they’d pose any real problem – much like many robots. With a zero turning radius, however, things get a little stickier. For that, one side is going in reverse – using the Chiaphua motor – while the other goes forward using whichever motor is most appropriate for the speed at which it’s turning. It may be that there’s an easy solution, a programming solution, or that a zero turning radius is impossible to achieve. Again, I’m just not sure.

Frank wrote:
**

What about a differential? Except a backwards one. Say, take a standard diff, two output shafts (the axle) and one input shaft (or gear, from the engine/transmission)

Except here, you power one of the output shafts with the drill, and the other with the chip.
…
with this, you could get power from both motors, each going at their respective optimal speed . . . no need to use two motors only to use one at a time, you know?**

The differential idea, as I mentioned above, is Tytus’ kingdom.

I agree that there are better solutions if you want to couple two motors together and run them all the time. You’ll get more power at a given speed that way. Most of those solutions rely on averages – as does the differential. This solution does not.
This design is explicitly for maintaining separation of your motors and taking advantage of the unique characteristics of each. The additional performance range possible via this solution means that your robot could simultaneously have the ability to output more torque and more speed than any other robot – especially middle-of-the-road, average robots using a two motor drivetrain.

Again, thank you everyone for taking the time to read through that very long post and ask questions. You’re raising issues that I haven’t thought of yet or don’t have satisfactory answers to and I really appreciate that. I understand that the design in its first iteration isn’t close to perfect and so I’m seeking your feedback so I (and others) can make improvements before testing and using it – if it’s worth using at all.

13

M, this is a great design. You have any desire to come to school at U of A? Our team could use someone with your talents to teach our kids part design techniques :wink:

That being said, I do have one question. You said that this works like a simplified version of a car clutch right? If so, does this mean that both motors are spinning constantly? The reason I ask is because I know several teams last year had battery drainage issues with high usage of both the drills and the chip. Let me know your thoughts :slight_smile:

14

*Originally posted by WakeZero *
**M, this is a great design. You have any desire to come to school at U of A? Our team could use someone with your talents to teach our kids part design techniques :wink:

That being said, I do have one question. You said that this works like a simplified version of a car clutch right? If so, does this mean that both motors are spinning constantly? The reason I ask is because I know several teams last year had battery drainage issues with high usage of both the drills and the chip. Let me know your thoughts :slight_smile: **

If I had a nickel for everytime someone told me to come join their team . . . :slight_smile:

As for your second question – the only motor that’s spinning constantly is the Chiaphua motor. Even then, it’s spinning at low voltage with a low current draw when the robots at the low end of its operating range. The drill can be safely turned off entirely once the robot’s moved beyond the useful speed of the drill motor.

15

Threads like this should outnumber the pointless threads by 2:1"

M,

Great idea, but I will get back to you on comments I am proving some technical questions out on paper.

Ahecht & M,

The Thunderchicken CCT, while it does manipulate the advantages of a planetary gearset, is nothing like the Toyota Prius Mechanism. On howstuffworks.com it describes how the Prius’ gearbox works, but it uses 2 electric motors and a gas engine. One electric motor acts as a generator. Toyota uses the planetary to couple the 2 electric motors and the engine together, but the main electric motor is attached directly to the driveshaft. There are several modes of operation, but the one that gets them fuel efficiency attaches the output of the engine to “help” the electric motor. The other electric motor spins to regulate the speed of the engine.

It is much more complicated than the ThunderChicken CCT and has 2 separate patents filed: 5,914,575 and 6,067,801. We had to reference these patents to make sure we did not patent the same idea.

-Paul

16

Originally posted by M. Krass ***
The differential idea, as I mentioned above, is Tytus’ kingdom.*

bzzt… wrong answer.

The differential idea (aka Planetary gearset) was the Thunderchicken’s invention (and patented) long before Tytus even thought of it.

Such an idea would not work unless the stall torques of each motor were the same. The only way to overcome this fact is to put a anti-backdrive mechanism on the weaker motor, hence the worm gear on the 2002 CCT.

I’ve considered many similar designs to your clutch. Also, I would like to note that a simple search at McMaster Carr would yeld clutch bearings (aka ratchet bearings, one-way bearings). I would know because I’ve looked at them before for this purpose.

The bad side is: propulsion in reverse. You could only do this using the high-speed, low torque motor. Everybody knows that robots often get pinned. A high-torque reverse is a nice thing to have…

17

*Originally posted by Jnadke *
bzzt… wrong answer.

In any case, my point is that the differential design, whoever it is developed by, is different and is the subject of its own threads and discussion. Why it does or does not work isn’t relevant to this thread. Thanks.

**

I’ve considered many similar designs to your clutch. Also, I would like to note that a simple search at McMaster Carr would yeld clutch bearings (aka ratchet bearings, one-way bearings). I would know because I’ve looked at them before for this purpose.
**

I’ve looked at what’s available through McMaster-Carr in the past and either ruled it out or flat out forgot about them when I began this design last week. I’ll look again and see if those products are applicable to this concept. If they are, that’s a great thing because it greatly reduces the machining complexity required and brings the design within reach of most teams.

**

The bad side is: propulsion in reverse. You could only do this using the high-speed, low torque motor. Everybody knows that robots often get pinned. A high-torque reverse is a nice thing to have… **

Agreed. But, remember, this is just a gearbox. There’s nothing stopping anyone from attaching it to something fancy like a swerve drive or something. Additionally, if the products available through McMaster-Carr are viable (I’m concerned about the torque loads they’re designed for), creating a similar system that works in forward and reverse becomes a bit less prohibitive. It’s still possible in either case as I already mentioned, however. This is just the starting point and there are many improvements that could be made upon this concept.

Paul – I am very interested in what you come up with and your suggestions.

I’ll admit that my understanding of motors and gearboxes and things is a bit rudimentary. There could be large swaths of important information that I’ve failed to account for.

Also, forgive me for my bad comparison of your CCT to the Prius’ arrangement. Where planetary gearsets are concerned, the moment someone talks about locking the ring gear in place, my mind shuts down. I’ve never been able to sufficiently imagine how that’s accomplished. I’ll go read through your whitepaper again. :slight_smile:

Edit:
It seems like part #2489K14 from McMaster-Carr (www.mcmaster.com) would do the trick. The only mitigating factor might be how high above 40A the current can spike before the breaker trips – if it’s too high, above about 70A – it’ll exceed the specifications of the clutch bearing. This all assumes my drill gearing of 1:2 as well. Anything lower than that would also exceed the specifications of the clutch bearing.

Additionally, it seems like you can reverse the orientation of the clutch bearing on one side – eliminating the usual need of having to run your drill motors in opposite directions and compensate for the difference in programming.

I’ll work on a second iteration that includes this clutch bearing. The first problem I see is finding a way of mating the clutch bearing to a gear – either by welding it (they seem to be plastic, partly) or by press fit. I really have no idea at all if a press fit is capable of transmitting 25-35Nm of torque.

18

Has anyone done some rough calculations on the power output of this transmission. I know from the past post that the main advantages were the lack of having to shift.

It would seem to me that you would have higher power available to you in both high and low at the output shaft using a shifter with the motors individually geared to match free speeds and all driving the output. From my first breeze over this I think everyone is saying that the Chip is overdriving the drill to go faster than the drill can when in high and the chip is essentially ideling in low. In the 45 shifter both motors are chipping in in both low and high. Shifting with the 45 design gives all of the power you could want in low (Can spin the treads!) and it is fast in high, and the gearbox is tiny by comparison.

I may be missing the whole point though??? Wouldn’t be the first time…

19

I’ll try to explain it one more time:

The Thunderchicken CCT is nothing like Tytus’ differential idea. Please read the thread discussing the Tytus gearbox for the explanation. Tytus is using a differential with the motors connected where the wheels (on a car) would normally be. The output is where the input (on a car) would normally be. The Thunderchicken CCT uses a planetary (not a differential) and places one motor on the input and one on the ring gear.

JNadke,

The motors will not overrun each other on the CCT when both motors are powered. The reason we put the worm gear was to not overrun a motor with no power to it. During low speed mode, only one motor was running (much like what M. Krass is trying to do) and the ring gear motor was turned off. We decided to use a worm gear to HELP not backdrive the unpowered motor. In the end, we had to pulse the motor ‘on’ every once in a while in low speed mode in order to stop the occasional back driving. Both motors were turned on in high speed mode with the SAME torque available as high torque mode, but much more power consumed (aka battery life).

Matt R.,

I agree with your initial thoughts. I am doing some equations to show why combining motors at a compromised speed is still better than one motor optimized.

I hope this clears it up,

Paul

20

*Originally posted by M. Krass *
Agreed. But, remember, this is just a gearbox. There’s nothing stopping anyone from attaching it to something fancy like a swerve drive or something.

Like Swerve^2 :rolleyes: