How to read Speedo at low speed

[davewanna]

I have built a controller of sorts for my car that does many things. The latest being control the automatic gearbox.

The problem I'm having is reading the speedo sensor. The sensor gives one pulse per tailshaft revolution, and with 33" tyres and a 4.3:1 ratio diff, that gives me roughly 61cm of travel per tailshaft revolution. (I think haha)

According to my calculations, sampling every 100ms will not give me either 0 or 1 pulse under 21kph.
I need to measure slower speeds than this.

What is the best way to measure this? I currently use pulsein to read the tacho input also, but this obviously takes less time and higher RPM, so I'm looking at changing that also.

The problem I have is the pic has to act quickly, the engine revs very quickly, and if it spends 1 second counting pulses, it will probably be over-revving by then.

I assume the best way to handle this is with interrupts, but have not used them yet, so would love to be pointed in the right direction. Any help would be appreciated.



The unit is based on a 16F877a running at 4mhz (I can run it faster if I have to) It also controls the air-conditioning & thermo fans in a climate control kind of way, aswell as having a 20x4 display that gives me info on engine stats and temperatures.

Thanks in advance.

Dave

[Acetronics2]

Hi,

The only way is to have " real " pulses ( otherwise ... a PLL Osc would do it !!! )

So, as a sensor ... you should try the Anti-lock brake sensors ( ABS wheel sensors ) that show teeth , like a gear pinion, but are designed to fit car's wheels ...

available from junked cars for little cost ...

And it allows you to add the anti-skid function .... Waaaaahhhh !!!

Alain

[davewanna]

Is it not possible to use the factory sensor? As it's already in the gearbox, and it is what the factory auto computer uses.

I have this setup on my desk at the moment all connected, and by running a piece of metal past the sensor I can read the pulses with the pic. Looking at the pulses on the oscilloscope gives me the indication that it would be ok to use.

The other issue about using optical sensors is that this is on a 4x4 which will see a lot of mud and water and the like.

[pedja089]

You can use tmr0 to count impulses from gear box, and some kind interupt to read impulese and reset timer. And you should use longer sampling time. So if you use interupt on evry 500ms, minimal speed is about 5KM/h.
you can use tmr2 with cristal to create interupt or some kind of pauses in progrm or something like that.

[l_gaminde]

Is it not possible to use the factory sensor? As it's already in the gearbox, and it is what the factory auto computer uses.

Let me ask why not, two units can read the same factory sensor!!

You said you had a sensor ( never said what kind of sensor ) you need more pulses how can you use this sensor and get more pulses? what metal are you sensing on the shaft ( hall effect sensor and have magnets ) or is it a gear tooth sensor or what. You could use a hall effect sensor and mount maginets on the shaft you could use 1 or 30 magnets and get the pulses you need.
if its a gear tooth sensor pull the plug on your rear end and see if you could mount a sensor to sense the teeth on the ring gear, gear tooth sensors are about 35 bucks.

now back to the factory setup Many of the PCM's have an output that can be used by a PIC, 2000 or 4000 Pulses Per Mile would this be enough.

[Darrel Taylor]

At low speeds, the time between pulses becomes more important than the Number of pulses per second.

That value is easiest to measure with the CCP module in "Capture" mode, using Timer1.

This way, the minimum refresh time for the speed value is only limited to the time between pulses, instead of a number of pulses across a fixed time base.

The low end can be extended by counting an Upper WORD for Timer1, giving a result with a resolution proportional to the circumference of the tires.

Not easy stuff, but PULSIN or COUNT won't do it.
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