linear actuator

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

"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

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

"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

Take it into a motorcycle shop and see if they have something that will
about work. Or, wind your own.

-- Gordon

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

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

     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:

  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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