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Topic: CHALLENGE QUESTION ONE
Replies: 6   Last Post: Jul 27, 1996 12:54 AM

 Messages: [ Previous | Next ]
 Helmut Forren Posts: 2 Registered: 12/12/04
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(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.

Note: If you are familiar with this field and don't like my use of
the term "potentiometric bridge sensor", I apologize!

Date Subject Author
7/19/96 Helmut Forren
7/19/96 Frank Miles
7/19/96 Ilias Kastanas
7/19/96 Kent Farnsworth
7/22/96 Don Yuniskis
7/27/96 Gordon Smith
7/27/96 Don Yuniskis