Since there is some interest in Tachs at the moment , I
thought I might mention some information that some on the list may not be
aware of and perhaps save TACFRUSTATION.
There are three common methods of deriving rotational
information (RPM) from an internal combustion engine.
1.Frequency to DC conversion – This is probably the
simplest (and cheapest) method in which you simply use a capacitor and resistor
of the right values to integrate (sum) the voltages of a series of
pulses. The faster the pulses arrive (higher rpm of the engine) the less
time the RC circuit has to discharge and a voltmeter is used to show this
rising voltage calibrated as your engine RPM. The down side is Frequency
to DC conversion is probably the least accurate although for many uses acceptable
and it is slower to react to changes.
2. Pulse counting - as it sounds, you simply
count the pulses per unit time and if for example your had 1 pulse per 360 deg
revolution and counted 100 pulses in one second you would know your rpm was 60(sec/min)
/( 1/100 sec/rev) = 6000 rev/min or 6000 rpm.
The disadvantage of pulse counting are measurement speed and resolution
at low rpm. Very low rpm could take several minutes to count – not really
a problem at our engine speeds {:>)
3. Period Measurement – offers the ultimate in measurement
speed and resolution. With microchip clocks able to measure millionths of
a second, the accuracy possible is far greater than any real use to our
application. The downside is increased complexity and cost – but there
is more flexibility (if designed in) to handle a greater diversity of types of
inputs.
So all tachs are not created equal and it can be frustrating not having the
documentation of what your particular tach needs to register properly.
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://www.andersonee.com
http://www.dmack.net/mazda/index.html
http://www.flyrotary.com/
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.rotaryaviation.com/Rotorhead%20Truth.htm