Vance Jaqua (sp?) wrote:

We were faced with a similar problem in experimentally measuring the
total impulse of very short thrust pulses of attitude control rockets.
We ended up using an integrating voltmeter "gated" to the time of
interest. Virtually all digital voltmeters generate a frequency
based on voltage and counts that over a time (I think).
"Gating" such a circuit over the port open time would give a pretty
good measure of total charge - and that is what you want to base the
injected fuel on.
Just my nickel - butting in
Vance

Paul Lamar replied:

No I appreciate the advice Vance. No substituted for your extensive
experience in these matters. I will give it some serious thought.

Let's see if I understand this. These voltmeters generate an
increasing signal frequency as a function of the voltage they measure.
Is that a correct statement? Basically it sounds like a voltage to
frequency converter.

Yes.  In the electronics industry (where I work), they are called
"voltage controlled oscillators" or "VCO".

Gate the frequency generator on during the time the port is open and
read the total count. I did something similar when I was measuring
speed with my computerized automotive road test system. No software
math of any kind required.

Sounds great. The question is how do we get the number into
Tracy's existing computer. Tracy's computer is currently designed
to take an analog voltage from .5 to 4.5 volts as a measure of
the intake manifold pressure.

I will have to think about this some more.

Relatively easy.  You need four functional blocks:
    a VCO to "sing" the pressure transducer's voltage output
    a resettable counter to accumulate the VCO ticks
    a DAC to convert the accumulated ticks into 0.5V to 4.5V
    a enable/reset circuit to enable the VCO between Tio and Tic

Of course, the EC2 needs to sample the voltage output by this circuit
sometime between Tic and Tio (note the order there: accumulate from Tio
to Tic, and then report to EC2 between Tic and Tio).

Russell Kent

Some of that counter stuff is built into the PIC17C752 CPU Tracy is using
in the form of a counter timer. See this attachment and the message
that just crossed in the email.

Paul Lamar