Gearing That Cannot Overcome Static Friction

[Bill_Hancoc]

Originally Posted by Cuog

Out of curiosity why would one try this? I'm pretty sure that in a FIRST field a transmission that will stall motors when standing still would be very bad if someone ran into you or the driver wasn't paying attention not to mention that would be amazingly fast and again possibly dangerous to the robot and others.

I agree...what would be the purpose?? Seems kinda overkill and to be usefull you would need a shift on the fly to even need i say move

[Veselin Kolev]

Oh its pretty hard to make a robot not move because its so geared up. Stall torque is so high that it creeps a bit and then kills your breakers. Ive tried it, direct driving 4 CIMS to six inch wheels. It's actually pretty funny to see, large buzzing noise from the motors vibrating at close to stall. Then the magic smoke comes, and you hope you didnt use competition CIMs.

It is a lot easier to stall out a gas engine because the torque curve tops out at close to max RPM, while an electric torque curve tops out at zero RPM.

With that said, anyone who gears up a robot so much that it doesnt move from standstill will probably never get their robot going fast enough to maintain any kind of speed at the highest gear.

[Madison]

I'm specifically considering something like a 7- or 8-speed transmission wherein the third gear or so would be geared so as to overcome static friction and get the machine moving under unstressed conditions. The lower gears would, of course, offer more pushing force and each would be able to start the machine moving as well, while the higher gears would help the robot to accelerate once it's already moving.

Assuming, for the sake of argument, that the torque needed to overcome rolling friction in a typical drivetrain is significantly, measurably smaller than that needed to overcome static friction, it is possible to gear a robot such that it accelerates to a usefully greater speed than is possible from a standstill with a single speed. I was curious if anyone had gone so far as to implement such a system, but since this thread was quiet for so long, it may just be that the typical difference in resistance is not significant enough to warrant such examination.

As an aside, and forgive me a moment if this seems bitter, but I've noticed that often, rather than simply answer the question asked -- even if they don't agree with the implementation -- people offer an unsolicited alternative or outright tell people that something can't be done. Surely, we've seen enough new and interesting implementations of technology in FIRST that we won't keep limiting our imagination to the tried and true.

[Wetzel]

I don't know if this is useful information or not, but I can start my car in 3 gears out of 5. The lower gears will accelerate the car much faster. Just a suggestion that maybe gear 3 out of 8 is too low to pass the "no move" threshold.

I'm going to assume that you have started to work on the CAD already and have some crazy gearbox/shifter mechanism halfway done and are just trying to figure out the gear ratios. I'd really like to see how small and light you've made your 8 speed gearbox.

Wetzel

[KenWittlief]

the deciding factor (as Don Rotolo alluded to but did not completely explain) is the inertia of the whole robot.

If you look at the current vs RPM specs for the motors, the stall current with 2 or more motors can be more than 150A.

The real question is how fast can you accelerate your whole robot, so the motors will get up to an RPM that will not trip the breakers?

If the drive train is geared way up it will take several seconds for the robot to get from 0 to 1mph, and by then your breakers will have already tripped.

Even if your drivetrain can overcome the static friction, if you cant get the motor current down below 40A within a second or so, the breakers will trip - thats the limiting factor. It all boils down to F=MA, the force being the torque of the motors, and the Acceleration needed to get them motors up over their 40A part of the current/rpm curve, within a second or so.

If you use the static friction of the drive train as the controlling limit, the robot may be able to move, but it may not be able to accelerate fast enough to keep the breakers from tripping.

I cant give you a specific motor torque, because it depends on the weight (mass) of the robot, and the resistance of the drivetrain (narrow hard rubber wheels vs tank treads....)

[Dick Linn]

M. Krass, Take this all with a grain of salt, as I'm not an engineer, just a wannabee. I guess the operative word here is "useful" in terms of what you are trying to accomplish in the game. The only way you are going to get the robot going faster (top speed) is by putting more energy into it (such as some flywheel effect) or accelerating it to a higher speed within a certain limited time period (getting up to top speed within the confines of the game). Ultimately, you have only so much power so the beast can go only so fast. That's just physics. You can alter the rate of acceleration to get to top speed more quickly or gear it such that it operates at maximum power or efficiency at the point you want it. Correct me if I'm wrong, but you cannot make it truly go faster with a fixed energy source (not counting reducing friction or weight). If you want speed, look at your power curve and weight and figure out what kind of top gear you can pull. The lower gears just help you get there in a reasonable amount of time, not to mention being able to push the competition around

[KenWittlief]

Originally Posted by Dick Linn

... The only way you are going to get the robot going faster (top speed) is by putting more energy into it (such as some flywheel effect) or accelerating it to a higher speed within a certain limited time period (getting up to top speed within the confines of the game). Ultimately, you have only so much power so the beast can go only so fast. ...

To understand the reason for using a multispeed transmission on a FIRST robot, you need to look at the torque vs RPM curve for the motors they give us.

like this one for the Chiaphua motors: http://www2.usfirst.org/2004comp/Specs/Chiaphua.jpg

Unlike gas engines, which have their max HP at a high RPM, the electric motors have the max torque at zero RPM. HP is a function of torque and RPM -Im not sure off the top of my head where the max HP point is on the curve

but, to get the most power from the motors you want to keep them at the max HP RPM point - therefore, to get the most HP at different speeds, you need a multi speed transmission.

If you have a transmission that shifts at the right points on the curve, your robot will have the best acceleration, pushing power, and top speed possible from what we are given to work with.

[Tristan Lall]

Originally Posted by KenWittlief

If you have a transmission that shifts at the right points on the curve, your robot will have the best acceleration, pushing power, and top speed possible from what we are given to work with.

On the other hand, if you're slow to shift, or more precisely, if you can't go through a large spread of ratios quickly, you'll be spending too much time in neutral to be useful.

This is a bit of a problem with the DeWalt gearboxes, it seems. While the NBD actually shifts quite nicely (once the extra parts like the detent leaf spring and the clutch rod are removed), it takes too much time for a servo to work the action (and still have enough torque to move smoothly). It's also why a 6-speed based on (for example) 222's design might be a little tricky, because the locking grooves would be spending a disproportionate amount of their time in between gears.

If anyone's seen Volkswagen's new DSG gearbox, it addresses this issue by having two separate power paths, and alternating between them. This means that the left side is driving while the right is shifting (or already in gear and waiting to be engaged). When an upshift is commanded by the driver or engine computer, it just switches power paths. (If you wanted a downshift, and it's ready for an upshift, then there's an additional step, but it's still pretty fast for an automatic.)

It seems to me that to use these extra-high gears, you'd need to ensure that you shifted quickly in and out of them; they'd theoretically be useful for the high-speed dash at the end of a match, but only if you could accelerate quickly enough while running through the lower gears. The thing is, for a moderately efficient drivetrain and a 120 to 130 pound robot, you'd have to be gearing for something like 30 or 40 fps in order to be unable to move appreciably from a standing start (due to stall overcurrent). This means that in order to go that fast, you're probably going through so many ratios that the delay in shifting could represent a whole lot of coasting—in other words, you could have done the same job at a lower top speed, but equivalent average speed, with simpler gearing.

Also, 40 fps represents corner-to-corner dashes of less than 3 seconds, assuming prompt acceleration in low gear and fast shifts. Does the driver want to deal with that? I was quite happy to knock Sterilite boxes (and other robots) around on a wide-open field at 12 fps in 2003, but I'm told that gearing the 2004 robot for 16 fps in top gear made for a challenging test of skill for the driver. (This predates the current interpretations of the high-speed ramming rules. Things, especially in autonomous mode, were much more flexible at that point.)

[KenWittlief]

we dont know what the game will be this year, but high speed may not be what you need.

Remember a few years back, teams pulling two mobile goal platforms around with one robot - geared down to speeds like 6 inches per second, and lifting the goals in the air to get extra traction? Maybe thats what they are thinking - several gears to get very low speeds with super pushing power, and everything in between up to a normal 10 or 12 F/S?

Making a transmission that shifts quickly is part of the challenge.

One other thing Ive noticed - Ive yet to see a FIRST transmission that has a clutch, like a car does. Im sure they didnt have torque converters either, so how they get away with shifting under power, with no clutch, is beyond me!

I can do it with my car, by matching the engine rpms while I upshift - but that has taken years of practice to accomplish without mashing the gears.

[Andrew Blair]

Originally Posted by KenWittlief

we dont know what the game will be this year, but high speed may not be what you need.

Remember a few years back, teams pulling two mobile goal platforms around with one robot - geared down to speeds like 6 inches per second, and lifting the goals in the air to get extra traction? Maybe thats what they are thinking - several gears to get very low speeds with super pushing power, and everything in between up to a normal 10 or 12 F/S?

Making a transmission that shifts quickly is part of the challenge.

One other thing Ive noticed - Ive yet to see a FIRST transmission that has a clutch, like a car does. Im sure they didnt have torque converters either, so how they get away with shifting under power, with no clutch, is beyond me!

I can do it with my car, by matching the engine rpms while I upshift - but that has taken years of practice to accomplish without mashing the gears.

Actually Ken, CIA had a clutched transmission a few years back. There was a double sided rubber disk that acted as a clutch, and it engaged one of two transmission stages. It was very smooth. No grindy noises at all.

As far as 40 ft/sec goes, I would say it could only realistically be used on a field like this past years: flat and open. But it would be wild to watch.

As far as keeping it out of neutral, perhaps a sacrifice to a finer tooth gear? you could more easily aquire .25" wide gears, and 222's ball shifter could be used effectively. especially if it was activated by a motor & lead screw, or other fast actuator. Perhaps you could even use helical? With respect to gearboxes we've had in the past, even though they were built badly, with awful tolerances and extreme loads, Boston Gear 1/4", I think 24 or 28 DP helical's stood up wonderfully.

An 1/8" or 3/16" ball could travel 1/4" quite quickly, and if you put some sort of lateral spring allowance into the shifter, the balls would *pop* between gears.

[Dick Linn]

Originally Posted by KenWittlief

... HP is a function of torque and RPM -Im not sure off the top of my head where the max HP point is on the curve...

Max power is right at half the maximum RPM on these DC motors. Power is Torque x RPM. In the attached picture, power is "inside the box" if you will. At any other point on the chart, the resulting box will be rectangular and thus have less area (less power). Maximum efficiency is at somewhat higher RPM. Ideally, you want to have the motor working at somewhere between 20% to 50% of maximum torque to keep the motor happy.

[Ken, are you still using globe motors soaked in liquid hydrogen? ]

[Madison]

Originally Posted by Tristan Lall

The thing is, for a moderately efficient drivetrain and a 120 to 130 pound robot, you'd have to be gearing for something like 30 or 40 fps in order to be unable to move appreciably from a standing start (due to stall overcurrent).

This is what I've been curious about and why I was interested in learning if anyone had any practical knowledge about how greatly the torque required to start moving varied from that required to keep moving for a typical drivetrain.

I'm trying to adapt a COTS product for FIRST use and so I've just been poking around to see what range of performance I can get from it to see if it's worth the investment.