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Servo Throttle Control


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Hello Everybody. I am looking for a good solution to a problem i have. Also, please correct me if what i am already doing can be done better. Basically, I have a throttle for a 8 cylinder gas engine that is cable driven. I want to convert it to servo driven. So what i have come up with so far is to hook the throttle up to a pot and use a pwm servo control circuit fed into an h bridge then on to my 12VDC motor. I was going to use either a power window or wiper motor. The next thing i want to do is read the RPM (I've seen the circuit on this site to do that and output a piezo and LED.) I know it would be best with a microprocessor circuit, an LCD screen and some buttons(which is what i would like to do), but i want to program a RPM in and have the circuits start throttle up and stop when it hits that RPM. The best I can think of is to use the circuit on this site but instead of driving an led, drive a relay and shut down the H bridge but i wanted a better approach to it. Any ideas???? Thanks alot!!!!!

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Mbedford,
Welcome to our forum.
I've seen and heard that way to make a fast, smokin' tires launch. You're waiting at the line with the engine revving like crazy, bouncing off the rev-limiter, then the lights change and you pop your clutch, or slam it into Drive. Ouch. Right?
Since you want to key-in your rev-limit, it sounds like a good job for a microcomputer chip.

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Hello Guys, thanks alot for the welcome and replies. It is actually for my boat. Right now there is a throttle lever that is cable driven back to the motor. We are un-happy with cable performance and being the kind of person that i am, won't sleep until i come up with a better way. So i am going to drive a pot with the throttle lever, then drive the circuits, then the servo (converted power window motor) That part I have, But i was thinking how cool it would be to build a sort of "cruise control" since i am going with a servo with a nice display and some buttons to program in an rpm, hit a "go" button and have the boat accelerate up to that RPM and hold it there. I will check into some microcomputer chips, do you have any suggestions to start with off the top of your head. The good news is that there is already an RPM gauge nearby that i can tap off of to get my signal for RPM. The other thing i wanted to know besides which chip to check on is the best way to interface this so that when it reaches RPM, how to hold the motor at that spot. I have read alot that using the "brake" feature in an H bridge is not good (having both A and B inputs pulled high) is this true? Any ideas would be greatly appreciated!!!!!! Thanks

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Mbedford,
Cruise control for your boat? Keep the pot and electronics dry.
You could keep it very simple by using only the throttle lever/pot with feedback from the RPM gauge, to set the RPMs. If you slowly move the lever, then the revs will slowly follow. But if you jerk the lever forward, then the speed control will probably give full power until the boat catches up and the revs rise, or until your propeller cavitates. Your boat might even take-off into a spectacular loop!
You won't need a brake since the speed control is dynamic with feedback.
You will need proper gain selection and filtering to keep the circuit from "hunting" around your set RPM.

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thanks ante. Hello mbedford. Here are some of my thoughts as well:

I am assuming this a larger size boat with some type of transmission between the engine and propeller? Some have load shifting capabilty, in which case, RPM of the engine is not necessarily the RPM of the propeller. On this system, your servo control could send the throttle going up and down in some situations. In some other situations, it would appear to be going too fast or too slow. Even with a direct drive, wind and water currents will be your worst enemy. A true speed control would have to work from a device that would read the speed of the boat. Does your boat have an instrument that tells you how many knots you are going? If so, this could be your feedback circuit. A micro would work well with such an input to determine the target throttle position. Also, you might note that you do not need a strong control motor if you control the lever at the carburetor. At this point there is only the resistance of a small spring. A smaller stepper without the H bridge would suffice.
As ante has mentioned, step #1 is to start thinking of everything that can go wrong and build safety into the circuit from the start.
Have you used microcontrollers? What programming language are you familiar with? It sounds like a very interesting project.

MP

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MP,

With a stepper linked to the throttle the carburettor return-spring will shut the barrels in case of a power-loss or on emergency stop. And I can see even more advantages with the stepper! It will reduce component count and size. Brilliant, why didn

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Hey everybody, thanks for all the replies, i appreciate the help!!! To answer questions, no i am not that experienced in microprocessor design. I am mostly experienced in analog circuit design, that is why i am kinda lost. I agree, it should be speed based at times as well as RPM based. I have both an RPM gauge and a speedometer nearby but a speedometer on a boat uses a pitot that is in the water and the air pressure it generates to calculate speed. I don't know how we could interface with that. Would a stepper be able to move quickly as well as accurately. Sometimes on a boat you punch the throttle quick (like with skiers, wakeboarders, etc...) and others you want to hold a steady speed. I don't mind the stepper idea as long as it will do the job well. The reason it sounds funny to have cruise control on a boat is because they all kinda have that already, you set the throttle and because it is cable driven, for the most part it will stay at that RPM. But i am trying to acheive two things, one get away from cables and two, if it is going to be microprocessor driven, there are some other advantages i would like to tap into. For example, we do alot of watersports, in wakeboarding, you have to punch the throttle to pull the guy out, then maintain the speed they like. So we have learned to watch the RPM gauge when you punch him out (so you can duplicate it as best as possible) then watch the speed after he is up to maintain it. Well it would be nice if we could program in an RPM and a speed. Then we could hit a "go" button to activate it, have the boat throttle up to that RPM and once it hits it, switch over and maintain the speed. I'm sure this can be done but i don't know how difficult it would be since i am not experienced in micros. Does any one have any ideas or suggestions??? Would anybody be interested in making a little extra money by selecting a micro that would do this and draw me up a schematic. I know how to program but only in visual basic, i know that it usually has to be c. So if there is programming involved, i may need somebody to do that too. Let me know!!! Thanks.

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I agree. A stepper will be faster than a window motor. If it is still not quick enough for a punch, you could add an actuator to punch the throttle.
mbedford, if visual basic is your language of choice, you are in luck. You can program AVR microcontrollers in a visual basic type IDE. There is a free version of the program (Bascom AVR) at www.mcselec.com in the downloads section of the site.
Also, there is a sample of using this language with a stepper controller at this website: http://home.planet.nl/~meurs274/
The AT90S2313 chip will do everything you need and is very inexpensive.
Hope this spurs some ideas!

MP

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MP, Thank you very much for the help, yes it does spur some ideas. Everybody else, thank you for all the help as well, this forum is great and it takes good people to keep it working. I will do some research and see if i can get this thing going myself thanks to your suggestion MP, but since i am inexperienced with micros, if there is still anybody out there that would like to draw up a schematic for my project, i would be happy to pay. Thanks!!!!

Mike

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Hello everybody!!! I spent hours tonight doing research on the project of mine. I am very satisfied with the Atmel AVR's. I made a decision to pay the extra 3.00 per chip and have the two extra banks with the AT90S8515 chip. But I am pulling my hair out because i am not that experienced with micros, and there is a whole other side to just drawing up the schematic to get it working electrically, there is also programming involved and this is just too much for a newbie like me. It is probably very quick and easy for an experienced micro engineer. So here is what i need, I would like somebody who has a general idea on what i am looking to do off of past replies (and i will give more details to you) to draw me a schematic for this Atmel micro to do what i want to do and write me some code, when done, i will pay via Paypal. I understand sharing advice on forums for free, but i understand that something like this will cost and i am fine with that. I don't have too much to spend as this is just a personal/hobby project and it comes out of my pocket, but i will take care of whoever can help. Like i said, i don't expect that this it too time consuming of a project for an experienced person. Anybody interested???? I would appreciate it!!!!!!

Thanks,
Mike

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