>HERE IS A CHALLENGE DESIGN QUESTION.
>ANYONE WITH A GOOD WORKABLE SOLUTION WILL BE WELL REWARDED.
>PLEASE RESPOND DIRECTLY TO ME VIA EMAIL, AND PERHAPS ALSO WITH A POST
>
>I have a potentiometric bridge type sensor (** see definition below),
>a microprocessor, and possibly some 555 type timers. How can I
>use RC timing techniques to successfully measure the sensor value.
>
>Obviously, the combination of a microprocessor and a 555 timer may
>be used to determine an unknown R when in a circuit with a known C.
>For example, the 555 can be wired with the R and C into a multivibrator
>circuit and the microprocessor can be used to time the circuit's
>period. From there, the unknown R may be calculated. As a result,
>a resistive temperature sensor such as a thermistor may be used in
>combination with the microprocessor, 555, and known C in order to
>measure temperature.
>
>With a potentiometric bridge type pressure sensor, however, you can't
>(and don't want to have to) get at the resistive elements independently.
>I don't necessarily need to know the exact value of all four resistors.
>I only need to know enough to calculate the pressure that's exerted
>on the sensor. To do this, the appropriate ratios of the left and
>right half bridge is sufficient.
>
>In case you're not familiar with this
>field, such a sensor is modeled by 4 resistors connected as if they
>were the four sides of a diamond. Typically, the sensor is wired as
>follows. The bottom point (common to R3 and R4) is connected
>to ground. The top point (common to R1 and R2) is connected
>to a known voltage reference (the excitation voltage, VI). Meanwhile,
>the left point VL (common to R1 and R3) and the right point VR (common
>to
>R2 and R4) are the outputs. The difference in voltage between the
>outputs is indicative of the pressure, e.g. P=k(VR-VL). Obviously,
>VL=VI(R3/(R1+R3)) and VR=VI(R4/(R2+R4)).
>Thus, (VR-VL)=VI( R4/(R2+R4) - R3/(R1+R3) )
>
>For the purpose of this question, note that it's only necessary to know
>particular ratios involving the resistors, not the actual values of the
>resistors themselves.
Why bother even trying to find out the resistor values? Your equation
P=k(VR-VL) is sufficient. Simply run these voltages to an op-amp then
compare with a ramp generated by another op-amp. The time it takes the
comparitor to switch from the beginning of the cycle is proprotional
to the input voltage. Monitor this time with your processor. Like
this:
R
|-----~~-|
| | Comparitor
| |\ |
R | | \ | |\
VR -~~-*-|- \ | | \
| >--*-------|- \
VL -~~---|+ / | >------ To processor
R | / *---|+ /
|/ | | /
Op-amp | |/
|
|----------------|
| Ramp Generator |
|----------------|
What's my reward?
Brian Rose
Date: Fri, 19 Jul 1996 16:49:30 GMT
Original Subject: CHALLENGE QUESTION ONE