ME 368
Laboratory 14 Characterization of a capacitive transducer
Laboratory 14
Characterization of a capacitive transducer
Equipment needed
 Mystery capacitor in the 0.5-5 nF range (its capacitance is known but it will not be revealed
until Thursday evening). All 6 mystery capacitors have the same capacitance (within 0.04%).
 Commercial capacitor that can be varied using small slotted screwdriver (to provide practice
measuring a variable capacitance test signal)
 Homemade linear slider capacitor with dedicated lead-wires attached (to provide another
variable capacitance test signal, to demonstrate a simple way to make a physical measurement
using a capacitor, and to reinforce capacitor physics)
 Resistance substitution box
 Capacitance substitution box
 LabVIEW / myDAQ
 Other standard lab bench equipment
Equipment not allowed
 Meter that measures capacitance
 Capacitance standards (e.g., a capacitor known to better than 2%)
Goals and Objectives
 Consider the use of capacitors for sensing
 Bring together many ME368 concepts: design, implement, and characterize the accuracy and
precision of a measurement system with limited guidance from the laboratory instructors
1. Contest
This lab will culminate in a contest in the week’s second meeting (Weds / Thurs) in which each group
will demonstrate capacitance measurements to the lab instructor and perhaps the other students in the
room. Your goal will be to demonstrate precise and accurate capacitance measurements, as well as to
demonstrate how you evaluated the accuracy and precision. Your system must display at least 1
capacitance measurement every second for at least 20 s, and produce an associated plot of capacitance
vs. time on a LabVIEW front panel.
Test (a): You will be given a linear variable slider capacitor on Weds / Thurs. While you vary the
capacitance of this capacitor for 20 s, you must demonstrate the ability of your LabVIEW program to
plot a capacitance that varies in time. No quantitative measure is related to this test, therefore there is
no associated contest for this part, but your lab instructor will still grade you on your demonstration.
Test (b): Based on one or more measurements of the capacitance of your mystery capacitor, evaluate
the precision interval [Farads] for your system, and demonstrate how you found the precision interval.
Best precision intervals will be logged on the whiteboard. However, a precision interval of zero will
not be accepted; if you get a precision interval of zero you should find a way to report a more realistic
precision interval.
Test (c): Provide your estimate of the mystery capacitance [Farads] with its accuracy interval. The
accuracy interval will come from hardcopy calculations that describe how you determined it, which
must be complete at the start of your lab’s 2nd meeting. These calculations should be based on
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ME 368
Laboratory 14 Characterization of a capacitive transducer
propagation of accuracy through the equation(s) that govern(s) your system. Based on measurements
demonstrated during your lab’s 2nd meeting, add to this hardcopy your measured mystery capacitance,
so you have the estimate of the mystery capacitance [Farads] with its accuracy interval clearly visible
on the front page of the hardcopy. Since the actual capacitance of the mystery capacitors will not be
revealed until Thursday evening, your lab instructor will not grade this part until Thursday evening at
the earliest.
You can use a different code and setup for each of the tests if you wish.
The lab instructor will figure each student’s grades for this lab based on the following rubric:
group’s effort in this lab……………………………………..……..…………………..…..10%
group’s creativity in this lab………………………..……………………..………………..10%
group’s demonstration of mastery of relevant lab skills…………………..……...…….…10%
group’s performance in test (a), varying capacitance………………………..…………….10%
group’s performance in test (b), precision interval..……….………………….....……….. 20%
group’s performance in test (c), capacitance value…….……………...…………………...20%
group’s performance in test (c), accuracy interval (prepared by start of 2nd meeting)…..20%
2. Hardware
For test (a), you will be given a “homemade” capacitor whose capacitance changes with the position of
a slider. It should be obvious to you by now that capacitors could form the basis for many types of
sensors. An example is the pressure sensor described at http://www.sensorland.com/HowPage018.html
3. Lab
Develop a technique of your choice to measure the capacitance of the mystery capacitor. You can use
any equipment you like, except equipment that directly measures capacitance. You may use Lab_14.vi
from the course website, a skeleton LabVIEW code that keeps track of time. After you get your code
working, consider that wires, substitution boxes, etc. can exhibit stray capacitance; accordingly, we
recommend you remove any test capacitors, measure the residual capacitance, and subtract this value
from future measurements.
4. Before your second lab meeting of the week
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Laboratory 14 Characterization of a capacitive transducer
You will have to do a propagation of accuracy calculation for this lab (which is good practice for the
final). If you don’t get through it before your 1st lab session of the week ends, you should do it before
your 2nd lab session of the week starts. You may do the propagation analysis in EES, but we
recommend you do it by hand as practice for the final exam. To simplify the analysis, identify any
variables that have negligible bias (show your work). Each variable you find that has negligible bias
represents one less partial derivative you need to compute. If you must compute challenging
derivatives, you may find an aid such as
http://www.quickmath.com/webMathematica3/quickmath/calculus/differentiate/basic.jsp
helpful. You will probably need to rely on lab equipment datasheets (available on the course website)
to get accuracy information.
5. The contest
Good luck and have fun! Test (b) winners: please save your winning results (screen capture OK if it is
clear, MS Excel file or similar is preferable) to the memory stick you can get from the instructor.
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