Drexel dragonThe Math ForumDonate to the Math Forum



Search All of the Math Forum:

Views expressed in these public forums are not endorsed by Drexel University or The Math Forum.


Math Forum » Discussions » sci.math.* » sci.math

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

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
Helmut Forren

Posts: 2
Registered: 12/12/04
CHALLENGE QUESTION ONE
Posted: Jul 19, 1996 2:31 AM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply



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!







Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© Drexel University 1994-2014. All Rights Reserved.
The Math Forum is a research and educational enterprise of the Drexel University School of Education.