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Re: Electric Vehicle C 2009
Posted: March 4th, 2009, 7:18 am
by fleet130
dudeincolorado wrote:Is it possible to build an EV with out gearboxes?
It’s possible, but besides decreasing the speed of the motor, the gearbox also multiplies its torque. Finding a motor with enough torque to reliably drive the vehicle without using a gearbox could be difficult.
Belt drive is one alternative to a gearbox. One can be made by connecting a pulley on the motor to another pulley on the drive axle/wheel with a rubber band or large o-ring. I have seen designs where the tire was removed from the wheel and the o-ring/rubber band placed around the wheel. The wheel served as a pulley and the o-ring served as both the tire and the drive belt. When used in this manner, the pulleys are actually a form of gear system. Using a smaller pulley on the motor than on the drive axle creates a mechanical advantage that reduces the vehicle's speed and increases the available torque. The mechanical advantage produced is equal to the ratio of the diameters of the motor pulley and the drive axle pulley. It can also be calculated using the radius or circumference of the pulleys.
MA = Diameter(Drive Pulley)/Diameter(Motor Pulley)
Or
MA = Radius(Drive Pulley)/Radius(Motor Pulley)
MA = Circumference(Drive Pulley)/Circumference(Motor Pulley)
I have always found it difficult to produce reliable results with this method, but I have seen others that were quite successful.
One of the most interesting gear boxes I've seen was made using gears salvaged from a $3 wind-up alarm clock. I haven't checked recently, but I think they may still be available at places like K-Mart & Target's.
Gearboxes can be salvaged from almost any battery-operated, motor driven device. Re-chargeable screwdrivers are an especially good source. The motor is designed to produce high torque from a low voltage (2, 3 or 4 rechargeable cells to provide 2.4, 3.6 or 4.8 volts). A high current source is necessary for getting the most torque they can provide. The higher internal resistance of heavy duty and alkaline cells may prevent them from sourcing the necessary current. You will probably need to use some form of secondary (rechargeable) cells.
A couple of other items I have seen used are: A rear axle assembly (including motor and battery box) from a $5 radio controlled car, a battery operated TOY screwdriver. In one case a whole battery operated car with one of the rear tires removed was mounted on the vehicle. A large rubber band was used as a belt to connect this wheel to the drive wheel of the vehicle.
You may be able to get more ideas by looking closely at the Battery Buggy photos in the photo gallery.
Re: Electric Vehicle C 2009
Posted: March 4th, 2009, 12:35 pm
by andrewwski
Ah yes - my bad for not reading closely.
With more batteries in series, there will be more voltage, and since resistance is going to be practically constant (adjusted slightly for the increased internal resistances), there will be more current drawn, and likewise more acceleration.
If the batteries were in parallel such a change would not be observed.
Increasing the voltage makes it go faster though - which may or may not be desirable. In my case, I was using a timer, so running slower was advantageous as it gave me greater precision.
Re: Electric Vehicle C 2009
Posted: March 4th, 2009, 1:29 pm
by fleet130
andrewwski wrote:Increasing the voltage makes it go faster though - which may or may not be desirable.
As you've observed, faster is not necessarily better in this event.
One solution is to increase the mechanical advantage of the drive system. You could increase the gear ratio, use smaller drive wheels or a number of other modifications. Increasing the mechanical advantage has a second benefit of increasing the torque.
Tricks and tips for getting the vehicle to go straight
Posted: March 4th, 2009, 9:35 pm
by rman
We are able to reliably go the exact distance and the exact time (whatever time is required) but we just don't seem to be able to get the vehicle to go to the same point every time. It seems to vary a little to the left and right with each launch. We have sights that allow us to aim very accurately but the vehicle seems to have an unpredicatable drift. We have tried different wheels, with and without rubber tires, we have tried slower and faster accelleration but it just doesn't seem to be able to go to the exact same spot every time. I would guess that the best we can consistently do is +- a few centimeters either side of aim point. The chassis is extremely stiff, we are using precision ball bearings, the wheels are aluminum on steel axles.
Has anyone had this problem and found a solution, or even know what the cause is?
Re: Electric Vehicle C 2009
Posted: March 5th, 2009, 4:15 am
by jazzy009
we had very similar problems. still do kinda. what is your base made out of? we changed the axles we were using as well as the tires and that seemed to help. you may want to check your axles to make sure they are straight, if they are even the slightest bit skewed, it will ruin your score.
Re: Electric Vehicle C 2009
Posted: March 5th, 2009, 5:46 am
by Balsa Man
If I understand your post correctly, you're saying you're getting unpredictable.....wander both left and right, on the order of a few centimeters; yes?
If that is what you're seeing, then it has to be either-
-variations in the floor you're running on causing wander
-something loose; something that can (and does) move (e.g. axle(s) not tight in bearings/bushings, play in whatever setup you have for steering adjustment, or,
-you're not starting with exactly the same starting alignment- its not that the vehicle is wandering, its that your starting alignment is slightly different, so that the outcome (ending up a bit left or right of centerline)is different. With this year's wheelbase, a 1/2mm difference (assuming front wheels in the same place, back end left or right a hair) will compound to 2 cm at 5m. If you're using a system with some kind of sights to align to target, and you're getting left and right error, its imprecision in the sighting system.
If nothing is loose, you should be able to line the vehicle up against something- a piece of wood along the side, run; and run again, and both those runs should get you the same left/right end point.
Len Joeris
Fort Collins, CO
Re: Electric Vehicle C 2009
Posted: March 6th, 2009, 1:48 pm
by rman
We are using a high powered rifle scope that is adjustable to 1/4 MOA (minute of angle, a minute of angle is about 1 inch at 100 meters so we can adjust to well under a mm at 10 meters). It is mounted on rifle bases on solid plastic blocks which are bolted to the chassis. The chassis is a carbon composite and extremely stiff. We are using precision ball bearings mounted in aluminum blocks and precision axles that fit perfectly in the bearings (tight fit, zero play). The wheels are aluminum and are extremely flat (so flat that if we stack them together they actually stick to each other). We have shaft collars that are tight up against the bearings so there is no possibility of the axle sliding sideways through the bearings. The scope seems to have virtually zero paralax error (the aim point doesn't change if you move your eye relative to the scope). The one thing that we do seem to have some trouble with is getting hubs that run absolutely true (no runout).
Has anyone seen this sort of problem caused by a slight runout (wobble) of the wheels? I am not sure but it seemed as if the vehicle ran a bit more consistently when we removed the rubber covering over the wheels. When we ran on bare metal wheels the vehicle seemed to go a little more consistently, although I am not absolutely sure about this because we also changed the front to rear weight distribution at the same time.
Is a few cm left and right a reasonable amount of inconsistency or is it possible to expect better?
Re: Electric Vehicle C 2009
Posted: March 6th, 2009, 4:26 pm
by fleet130
You should be looking for something with some slop/play or that is not aligned straight. If two wheels on the same axle are trying to go in a slightly different directions (even if its only for part of their revolution), random variations in traction as the vehicle travels can cause unpredictable "drift". My vote for the culprit is: "The one thing that we do seem to have some trouble with is getting hubs that run absolutely true (no runout)".
Another thing to look for is slight variations in the track surface. If your vehicle is running parallel to cracks in the floor, they can "capture" it and make it change direction. It's amazing how much change a small a crack can cause. If running on a floor with cracks, the vehicle should cross them at an angle as close to 90 degrees as possible.
Good Luck!
Re: Electric Vehicle C 2009
Posted: March 6th, 2009, 9:55 pm
by rman
fleet130 wrote:You should be looking for something with some slop/play or that is not aligned straight. If two wheels on the same axle are trying to go in a slightly different directions (even if its only for part of their revolution), random variations in traction as the vehicle travels can cause unpredictable "drift". My vote for the culprit is: "The one thing that we do seem to have some trouble with is getting hubs that run absolutely true (no runout)".
Another thing to look for is slight variations in the track surface. If your vehicle is running parallel to cracks in the floor, they can "capture" it and make it change direction. It's amazing how much change a small a crack can cause. If running on a floor with cracks, the vehicle should cross them at an angle as close to 90 degrees as possible.
Good Luck!
I hope you are right about the wheel hubs because that is about the last thing we can think of to work on. Two more questions though. You mentioned crossing cracks inthe flool at 90 degree angles, and that certainly seems like it would cause the least direction change but what do you do if the track you have to run on in competition has cracks, spaces between boards or other imperfections that aren't at a 90 degree angle to the direction of travel. I have some vague memory of the track at Wichita being almost a sort of parquet floor with cracks running in two different directions. We were doing wheeled vehicle and were very concerned about just the thing you were writing about (that the wheel would get "captured" by the track). I don't think it ended up having much effect on our vehicle though (or we were just lucky). My other question is how consistently can one expect a vehicle to run? There is going to be some random error in direction or else everyone would be going absolutely straight down the track. Is a typical good vehicle able to repeat the aim point within a mm or cm or what? Are people getting scores of 199.9 at Nationals?
Re: Electric Vehicle C 2009
Posted: March 6th, 2009, 10:11 pm
by nejanimb
I believe the winning score last year was 199.8 last year. Less, actually, I think. Either way, no - no one got a 199.9 at nationals.
If the floor is your problem, then that's something you should be able to handle when adjusting for the second run. Randomness, on the other hand, would be a problem.