linear actuator

Hi,
I'm seeking for suggestions on how to automate the throttle cable of my ATV. I tried using a small gearmotor I had home where the cable rolls around the
axis (on a pulley) of the motor. It was a total failure. The motor is too weak to pull the cable. I've measured the force necessary to pull the cable up to its limit and it is about 18 pounds (80N?)
For brakes I have already bought firgelly linear actuators and some cable potentiometers (to close the loop), but that solution costs about $350. Do you have any other idea?
Cheers
Padu
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Padu wrote:

I would bet a great deal of the force required for the throttle is due to a heavy spring on the carburetor or on the handlebar lever of the ATV, which you'd want with a human rider. You don't need it if you are using a motor in a push-pull situation.
I think you are better off removing the spring, or replacing it with one that requires less force, and trying the motor again. This is going to be your cheapest method.
OTOH, if the force is needed because of friction in the cable, remove the cable, and attach the motor to the throttle plate lever on the carburetor. With any luck, a cheap hobby servo ought to be able to move that.
-- Gordon
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"Gordon McComb"

That's a great idea, even more because I've already spent some time designing and fabricating the bracket to mount the little motor.

The carburator of this [kid's] ATV is a bit different from the ones I've seen before. Well, I'm not even sure if what I've seen was the carburator, but the cable goes inside a cylindric chamber where it lifts a small piece of plastic (through a spring, that I will replace by a weaker one now) and allowing gasoline to go through. It doesn't look like a venturi in any way.
I guess replacing the spring will solve my problem... I'll try. A million thanks!!!
Cheers
Padu
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Padu wrote:

It's a carb slide by the sound of it. The principle is the same, and there is a venturi effect going on, unless it's a fuel injected system. There's air going through something and causing gas to get sucked in at a metered rate.
Just know that whatever you put in has to have a fail-safe. I'd still have a spring in there that at the least if the power to the motor goes the spring will return the throttle closed. If I were doing this, I'd probably use a good RC servo and connect it to a microcontroller that has a hardware watchdog. If the watchdog detects a fault the control pulse to the servo stops, and the spring will return the throttle closed. You can have other fail-safes, of course, but IMO this one would be the mandatory first-responder.
-- Gordon
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"Gordon McComb"

Well, I've extracted the spring from the carb today. It is 2" long, OD is 0.354" and the wire thickness is around 0.025". It is pretty strong. I've been looking at mcmaster and small parts inc website looking for a weaker spring, but so far for that type of spec (length and diameter) they only have ones that are as thick or thickier...
do you know any place that sells weaker springs?
Cheers
Padu
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Padu wrote:

Take it into a motorcycle shop and see if they have something that will about work. Or, wind your own.
-- Gordon
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Try Muscle Wire for the throttle. You can control contraction and expansion very nicely by adding or subtracting electrical energy. Stretch five inches taught, give it a one volt shot and it will contract .X, and it's proportional all the way. Double it up if you need more power. No motors required. It's an ideal solution to throttle control.
Wayne

ATV.
the
cable
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Wayne Lundberg wrote:

Wayne, I gotta say IMO this isn't a good application for Muscle Wire (nitinol). First, it would be a Bad Idea to use it near the carb, because the heating of the wire could cause ignition of gas fumes. Second, you'd need a bunch of the strands to amount to the same 100 oz in or so of a common hobby servo or small motor. And third, they "relax" by cooling. If they cool slowly -- which you'd expect around a hot gasoline engine -- they don't relax quickly enough. At best that means an unresponsive throttle.
-- Gordon
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We had to do this for our DARPA Grand Challenge vehicle. Here's what we did:
First, we had a Y-cable for the throttle fabricated. One end went to the carburator, one went to the accelerator pedal, and one went to our box.
Our box was built from a cruise control unit designed to pull on a throttle cable. We removed the motor and electronics and replaced it with a a servomotor and encoder to drive the leadscrew. The cruise control unit also had an electromagnet which had to be energized before the unit could pull on the throttle cable, and the electromagnet was tied to the emergency stop system. So if anything happened to deenergize the electromagnet, the throttle snapped to idle.
(The emergency stop system, incidentally, had not only big red buttons and an emergency RF link that had to stay up, but a hardware stall timer. The computers had to toggle a control signal every 100ms to keep the emergency stop system armed. If 120ms went by without a toggle, everything shut down.)
This was surprisngly complicated to get working right, but eventually it worked very well.
                John Nagle
Padu wrote:

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Carbs use vac cans to open the throttle . You open a port to allow the manifold to be connected to the can , but thru a venturi ( using vent' and NOT manifold vacum ). If engine is idling , then very little air will rush by a venturi , and this applies very little vac to the vac can , so throttle does not open fully . But it does open quickly .
S.U. carbs made a big mistake in hooking the fuel needle to the piston . Its much easier to control fuel without jets and needles ! Use one normal throttle blade and another that takes in only air , no fuel . Now just make the fuel "blade" , close against the idle mixture holes and NOT rotate on a shaft . What little air enters MUST go past only the fuel entrance holes . The fuel holes have a 2ndary air hole that can be blocked to richen . so that as you open the big throttle , you can bypass air , so it does not suck fuel as fast thru the "jets"
1) tiny air bypass on the tiny fuel hole to richen . 2) to oppose this rich , is a large "air only" throttle blade .
This gives you a complete range . So theres no need for accel pumps , idle mixture adjusters , power valves
Since it has a air only big blade , the other "fuel" blade can be huge , as they need for racing .
Look at all the dumb mistakes in carbs ! Like a shaft to carry the blade , so you must put fuel holes on both sides , and the stupid venturis way to far from the air flow ! And the idle mixture , without an air control ! All you need is an air control to "modulate" the tiny amount of idle mixture !
Accell pumps arent needed cause you simply close the air only in my method and it gets way rich , for the high vacum on the tiny fuel hole . ------------------------------- They want more H.P. , but search WIKI Atkison cycle . It has low H.P. and high economy , so just make it big size and get more econmy . Like an aircraft engine , where you cant rev it high , for the propellor speed limit . The exhaust valve is way late , for lower heat loss , but the intake is closed way late for lower "real" compression and less fuel and air intake ! Now just add heat regeneration and expect 70 miles per gallon . This is not a trick , a low compression engine is NOT less heat effecient than a high compression engine .
Have you noticed some MiniVans had the US made engines and had far lower compression ! 145 PSI . While Hondas use 200 to 280 psi ! Even the other MiniVans had 180 ! Yet the ultra low compression , did get great milage .
You can get higher power from higher compression , but it is NOT better efficiency ! Diesels dont work at low compression .
.

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