T&T: : Advice on Equalizing & Tachometer?

[Arild Jensen]

> -----Original Message-----
> From: Dave Cooper

> Any resolution better than 1 revolution would seem to be overkill in any
> mechanical controlled engine. I can see the need for much greater accuracy
> of where the engine is in the cycle for valve or injector timing on ECU
> engines and so the need to count teeth in the ring gear, etc.
>
> However, for a tach intended to be read by us humans 1 rpm should be
> accurate enough.



REPLY
Its a question of how the display averages out the pulses and how often it
updated the readings.

Humans tend to distrust a display that flickers or jumps around from number
to number.
The solution is to maintain a rolling average.  If you only have on pulse
per rotation and you need at least ten pulses, it takes longer to calculate
the rolling average compared to  when you have 2, 4, 8, or ten pulses per
revolution.
People are accustomed to tachometers from gasoline engine days that had an
almost instantaneous response to a blip on the throttle. If there is a
perceptible delay, its regarded with suspicion. Likewise a tach that
measures the time from one pulse to the next at one pulse per revolution.
The cost of building a precision quartz timing circuit is far more than the
counters circuits now being used.

We had a similar problem in early Loran and GPS displays. If they updated
too quickly and the display numbers changed so fast it looked like
flickering; people thought the instrument was in error.

Arild