June 2013
I want to build a 'drive by wire' system for a project that I have been working on. I have enough electronics knowledge that I feel I could build the circuit by hand if I could get help on the circuit diagram and parts needed.
I want to operate the throttle on a 14 HP engine via a servo with the gas pedal hooked to a potentiometer, so that when the 'gas' pedal is pushed, it will open the throttle on the engine and vice versa (close when 'gas' pedal is released). The servo would need to return to the same position as the pedal potentiometer.
The circuit needs to run on a 10-15 volt DC automotive voltage, and be turned on and off with the ignition switch. This would have to be bullet proof because of the vibration caused by the engine and terrain. The servo must be able to hold the throttle position for as long as the driver has his foot on the gas pedal in any position, and then return to the zero position when the foot pedal is released (return to the zero position). Can someone help me accomplish this?
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I think the simplest way would be to buy this servo adjuster (about $20), remove the pot and add your own wiring to your throttle control pot. With a low cost RC servo connected to the carburetor you should be good to go. Just do a search on the internet to find sources (even Ebay).
These RC servos are for 5V only, so you need to convert any 12 volts to 5V before using them.
Of course another way is to use a microcomputer like the Picaxe. This puppy has analog inputs (for the pot) and servo output. With a microcomputer you could add your own software to do other things, like keep the rpm constant with load changes. Again, it would be 5 volts only.
Also keep in mind a failure of the system may leave you without throttle control. Since you didn't state what this is for, be safe.
Any further questions or help you may contact me directly at: [email protected]
A stepper motor should work and there are already stepper motors being used in cars under the hood. You will need:
Digi-Key sells an encoder with 16 different pulse counts per revolution, set with tiny switches, for 26 bucks, part number AMT103-V. US Digital sells a chip that will convert the encoder output into pulses that a stepper driver can use. Part # LFLS7184, $3.50. Interinar sells a stepper driver board for $36.00. Stepper motors are available surplus everywhere.
You will have to have an idea which stepper you need if you decide to go this route. You will also need a 5 volt power supply for the encoder and the 7184 chip. US Digital also sells encoders but none that have 16 different resolutions, as far as I know. I have used all the devices mentioned, bought from the companies mentioned. I have used them for a similar application, so I know they will work. The encoder mentioned is a robust device that will be better over time than a cheap pot to measure the foot pedal position. Since the encoder has so many different resolutions it should be fairly easy to find a ratio between the foot pedal position and the throttle opening.
You could also use an RC servo to open the throttle. These servos need a pulse train of a particular width to know how much to move. The pulses must be constant, because when the pulse train stops the servo returns to the starting position. So, you need some kind of servo controller that will use a pot connected to the foot pedal to tell it how wide to make the pulse it sends to the servo.
The stepper solution suggested above, pretty much just requires connecting the different parts with wires according to the info in the data sheets. But, you can make your own servo controller from a 555 timer, some fixed resistors, some variable resistors (AKA pots), and some capacitors. You will need to solder all this together in an enclosure. This link: pcbheaven.com/wikipages/How_RC_Servos_Works/ will explain how RC servos work and has a sample circuit that you can build to control the RC servo.
One advantage of the RC servo is that it is an absolute positioning system. A certain resistance measured at the foot pedal results in a certain throttle position. The stepper system described above does NOT work that way. The throttle must be at the zero position when power is removed. If the throttle is at half way when power is removed, then the pedal will start to move the throttle from the half way position when power is restored, unless some type of mechanism always returns the throttle to the fully closed position, when power is removed.
