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CHALLENGE QUESTION ONE
Posted:
Jul 19, 1996 2:31 AM


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.
************************************************************************** Potentiometric bridge sensor:
I'll try to draw a picture. Don't know if tabs or proportional spacing will mess it up!
(Excitation Voltage "VI") + / R1 R2 / (Left Output VL) + + (Right Output VR) \ / R3 R4 \ / + (Ground)
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(VRVL). Obviously, VL=VI(R3/(R1+R3)) and VR=VI(R4/(R2+R4)). Thus, (VRVL)=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.
Note: If you are familiar with this field and don't like my use of the term "potentiometric bridge sensor", I apologize!



