Cruise control mod?

So I have this aftermarket cruise control in my car that uses the speed signal from the car to set the speed. It does not use magnets on the driveshaft. The cruise control only starts working above 35 mph. Every cruise control I have ever used, factory or aftermarket, only worked from about 35mph and above. But I do a lot of driving below 35 mph and want to use the cruise control for it. So I'm hoping that if I measure the signal from the car with an oscilloscope it wil show either a chain of pulses or a voltage that changes with the speed. I looked online and found a couple frequency doubling circuits that look like they would work. I'm sure that the extra pulses the circuit would make must be evenly spaced between the pulses from the car for my scheme to work. And at least one of the circuits I found appears to do just that. If it is a rising voltage that I need to modify I'm not sure how I would do that. What if the rise with speed isn't linear? Anyway,any thoughts or advice? Any circuits that would work? Maybe a chip to do all the work? Thanks, Eric

Reply to
etpm
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It makes perfect sense to have it not operate until that speed or there abouts.

Or is there some ulterior motive at work?

Jamie

Reply to
Jamie

It's non-linear... dphi/dt. Count the pulse rate, not the average amplitude. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

I drive a lot of roads here on the island at about 25 mph. I would like to use cruise control. That's the only motive. So any help? Eric

Reply to
etpm

I'm sure I answered this, yet I can't see anything.

Look for a PLL frequency doubling circuit. Expect that the PLL may add enough delay to mess up the system response. Try, and see.

If you really want to get into it, you could always keep the servo mechanism and put in your own electronics.

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Tim Wescott 
Control system and signal processing consulting 
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Reply to
Tim Wescott

I've had success, both over (:-) and under range, by initially setting a mid-value, then using either the Accel or Coast buttons to get the speed I want...

For instance, my Q45 won't set above 75, but I can Accel it up to my desired 100 >:-}

BTW, most speed sensors are wheel-based. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Is it an option to use engine rpm rather than the speedometer pickup as the controlling signal? Possibly better suited for lower speeds.

30 to 35 mph is where the torque converters usually lock up so using the speedo pickup presents hunting problems at lower speeds.
Reply to
Oppie

Oppie expressed precisely : See BOTTOM Posting

If you use engine rpm then you have to take the gear selection into account. lol

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John G
Reply to
John G

There is that... One can always (if so equipped) put the transmission into manual to hold a particular gear.

I had a car with a speed control problem. Never quite fixed it so just learned to live with it...

2001 Saturn LW300 (3 liter, 24 valve V6, 4 speed "rocket") - going along on level ground with the speed control set, when approaching a hill and speed begins to fall off, it would downshift *two* gears and red line the tach! Very unnerving when automatic systems do that. Saturn "fixed it" briefly but it happened again about six months later. Finally just turned off the speed control when on anything but flat road...
Reply to
Oppie

I don't know if I can use engine RPM. But since the car has an automatic transmission I can see the RPM approach causing all sorts of hunting problems if the transmission shifts. I suppose I could put the transmission selector in second for low speed but that would prevent using the cruise control for anything but low speeds. Also, since the transmission doesn't hunt when I use the gas pedal to hold it at 25 I don't see how it would with a cxruise control. Next time I drive the car I will pay attention to what speed the transmission decides to shift. Eric

Reply to
etpm

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