How do electronic tachometers work?

Question

No dumb answers please. Here's the deal, my bike has an electronic sending unit on the front wheel that provided 23 pulses per mile. Those pulses provide signals for the mechanical odometer display, the trip computer and the digital speedometer. Here's the problem, the odometer is very accurate, the mileage and mph the trip computer displays is very accurate but the speedometer reads 10% fast. The speedometer isn't defective, all of this model came from the factory this way. Is there any way to correct the displayed speed without affecting the accuracy of the other units? What principle is used to count the pulses and convert them into a readout?

The bike (motorcycle) is an '84 Kawasaki Voyager. I know the odometer is accurate to within 1% by comparing to known distances (mile markers on interstate). I know the true ground speed by timing the time required to cover a known distance (60 mph=1 mile in 1 minute). The trip computer provides an active display of the distance covered since the last start up or reset of the distance, the average speed since the last startup or reset as well as a number of other functions including stopwatch, mpg, fuel left in the tank, ect. All 10 computer functions use the inputs of distance, time and current fuel consumption. The speedometer display is seperate from the computer display.

First of all, sorry about the word tachometer, I meant speedometer. But I'm assuming they both work on the same principle of counting electrical pulses whether it's from the signal generator on the wheel or pulses from the ignition coil.

I appreciate all the wise information from you folks. Since the speed display is in LCD numerals, I have no needle or spring I can calibrate like I've done on mechanical meters.

The 23 pulses is per wheel revolution which comes out to around 17,421 pulses per mile. Since the trip computer (a seperate unit) uses that number of pulses to correctly calculate speed and distance, it must be the time part of the dash's speedometer that's incorrect. Incidently, 20 years ago motorcycle speedos were universially incorrect, always reading fast. A good way to slow the rider down I guess and make the bike seem faster than it really was.

Answer 1

People sure like to make things complicated by their display of mostly useless and incorrect knowledge.
No matter if you have a cable running a tiny generator or sixteen supercomputers, in the end it comes down to generating a DC current that moves a mechanical needle. The two things that need to be done for accuracy are to have the starting point of the needle in the right place, and to have the tension on the spring that returns the needle to zero right.

Answer 2

Since I have no idea what bike you have I can only guess at what you will need to do.

There is probably a small computer chip (Atmel or other brand) which is programmed with code which interprets the pusles for the speedometer, odometer, and trip calculator.

You would need to change the programming on this chip to tell the speedometer to react to more pulses for a specific reading.

The chip is most likely programmed in assembly language (or a proprietary offshoot of assembly language). And re-programming may or may not be possible.

Of course you will need a chip reader and burner (you can make them easily), and a programme to read hexidecimal (such as WinHex).

Answer 3

10% is a substantial error on a electronic speedo.

B.K.,,Frankly I have ZERO specific knowledge of your bike.
So I have No Idea how'd you go about achieving a modification to correct the speedo.

Perhaps You can check it out and see if there's any practical way to do it?

Ok,,all the functions Except Speed are the results of a COUNTER.
It simply tallies the impulses as a gross quantity ,,accumulated totals,etc.
Which is why,,,"One is accurate,,ALL are accurate",,Except for Speed.

Speedo is a DIFFERENT function.
One of it's operations is as a counter,,,but it compares that to a TIMER.
The Relationship between TIME and Input Pulse is caluculated to yield a SPEED number result.

Part of your multi-function Instrumentaion just COUNTS the pulses,
but the SPEEDO section TIMES the Pulses.

Here's where I go Clueless,,,
Moving the Impulse Trigger/Pickup Assy Farther AWAY from Axle would cause the Speedo to "Slow Down".

The Larger radius results in a Larger Circumference being Timed Point-to-point.

That make any sense?

Simplified Example:
Imagine your wheel 's RPM at 10mph.
Now picture the Valve Stem as a refference point.
Let's start from 12:00 High,,Valve Stem at Top of Wheel.

On a Stopwatch,,,measure the TIME it takes that valve stem to make 1 Full Revolution at 10mph.

Then on a radial Line from Valve Stem down to Axle Centerline,,,
Measure the time it takes to make 1 full revolution at Midway Point between Axle and Rim.

Then Again,,,but maybe at only 1" from Axle.

Same 10MPH Speed,,,,
But as Diameter/Radius gets Smaller towrds center of Wheel,,
The Time to make 1 Revolution DECREASES.

At wheel center,,the "timing Mark" comes back around Very Quickly,,measured on a Clock

At halfway point between Axle and Tire,,,it's a Much Longer Distance to travel,,,,so it takes More TIME.

At Outer Diameter of RIM,,,it's slower yet.

So to a Time Based instrument sensing Rotational Speed,,,
The Farther away from Axle center/The Larger the Radius,,,
The SLOWER the Instrument will Read.

.........................
The Trigger Wheel needs to be a Larger Diameter.

The Effect is same as putting Oversized tires and seeing Speedo's indicated speed drop.

HOW to actually achive that within Kaw's speedo pick-up assy,,,,???
Some creative butchery ,,at Minimum,,would be required.

Or a dismantling>parts salvage>fabrication operation of a factory unit.

It would be tricky and almost certainly require some machining work.

Me personally,,,If I Did go to any such length I'd make it "radius Adjustable" to be able to dial in accurate speed readings.


Only Other alternative would be to tear into "Black Box" somewhere and find what oscillator establishes the processors Clock Speed,,,and add /swap a Resistor to slow down the clock,,,,or swap the crystal.

All that stuff is surely VERY Micro-sized,surfacemount components which is near impossible to deal with on a "do it yourself,at home" level


A R/C network in series with Speedo Trigger Output would Delay the Signal,,,
but since it would not change pulse-to-pulse interval it would not alter Speedo reading

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You GOTTA be getting more than 23 pulses per mile,huh?

I'm sure it's gotta be a simple magnetic pickup/reluctor wheel arangement???

That's the "normal" device to produce such signals.
23 impulses per mile would have to be TERRIBLY underdriven ,,,in that little speedo "drive" case there's no room for such reduction,,,,except maybe wristwatch-sized.

Wouldn't be too much trouble to fab a bolt-on reluctor wheel and an adjustable mount pickup coil to replace the speedo drive.

The radius of it,,thus the swept circumference is what sets calibration.
Farther from Axle the Pickup Sense Point gets,,the Slower the Speedo Reads.

No Facts,,All & Only Imagination

(I dont think Yahoo allows us to say,,"Just tossing out some bullshitt" :)

Maybe you'll get some ideas or stumble upon a workable solution,,,????


Good Luck ,,and take care

..............................................
Or,,just do it the Uncomplicated way,,,and skip the useless Electronics considerations.

Go Bend the Needle around to correct position,,,and adjust the Spring.

Yeah,,that's the ones.
The needle and spring in your DIGITAL LCD DISPLAY Speedo

:)

Answer 4

Tachometers display engine RPM.
Your speedo and Odo are calibrated together. How do you know either is accurate? I'm pretty sure the odometer is using the same information as your speedo. So they're both wrong or they're both right.
Is this a bicycle? Never heard of a motorcycle using this sort of instrument.

Answer 5

tire size will determine exactly how far the bike travels on each rotation

Answer 6

What does this have to do with a tachometer?