Speed Data Logger
I had been toying with the idea of reading the Rovers speed
transducer with a computer for a
while, and when the Trax'99 day
came up (driving on the racetrack at Silverstone) it prompted me into
action. After the useful advice from several Rover list members
(Thanks Andrew, Tony and Nigel) I set about a few trials.
DISCLAIMER - if you try any of this and it kills your car
electrics don't blame me. I know the risk I am taking, make sure you
do too ... I am in no way implying that any of the circuitry shown
will not cause damage to your car.
Hardware Setup
I have an old 486-66 portable
running in the car, with a parallel port connection using an
opto-isolator connected to the speed transducer. The exact circuit
is -
This basically draws the select pin (13) to ground through the
570 Ohm resistor and the opto-isolator activates every time the
speed transducer sends a pulse. This switches the 5v signal (from a
data line,
I couldn't get a 5v signal from the parallel port any
other way) to the select pin and lets it go high again. This works
reliably within the 4000 pulses per mile specification of the speed
transducer, and the parallel port detects the level change. I built
the whole circuit in the hood of the parallel connector I took apart
to use to connect to the PC.
Software
I set about writing some simple code in C to
poll the port as fast as it could and record the time for
level
changes in the select pin. Apart from problems with accurate timing
it was fairly straight-forward
to implement. I ended up using a
variety of different timers and timing the pulses in blocks of 50
until I found a relatively reliable method. I am still working on
this too, but the current versions which I tested today can be
downloaded here. Instructions are inside
the archive.
And here is the graph generated for a 2 mile
stretch of my drive to work this morning -
The start is a slow crawl onto a roundabout, then a bit of power until I caught someone up. Slowing down to the next roundabout and then about 40 along the next stretch.
Even better though are the graphs recorded from Silverstone during
Trax'99.
They are not actually exact laps, but 3 mile segments whereas the circuit
is approx 2.5 miles.
