The following text discusses results from the electronic step testing and the beginning of velocity step testing in the shock tube. It should be kept in mind that frequency response is always measured as the time from the beginning of the event to the minimum (positive inflection) of the 'bucket' that immediately follows the response. This report is not a complete account of the results from square wave testing. Some data is still in the process of being analyzed and efforts are being made to fit the data to both Freymuth's third order theory and modelled responses from SPICE circuit simulation software

Moen, Michael J.

English

NASA Technical Reports Server

NASA-CR-190623
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Testing Of The Anemometer Circuit:
Data Report
Michael J. Moen
March 15, 1992
(NASA-CR-190623) TESTING OF THE N92-3_I06
ANEMOMETER CIRCUIT: OAT_ REPORT
Semiannual Status Report, period
ending 31 Dec. 19ql (Purdue Univ.) Unclas
42 p
G3135 0109251
https://ntrs.nasa.gov/search.jsp?R=19920024862 2020-03-17T09:58:37+00:00Z
something different when we plot the actual frequency of the response versus the
overheat. What results in Figure 32 is a fairly linear curve for each sensor. Figure 33
shows a linear curve fit for each point set. It should be kept in mind that the data only
looks linear within the range of tested overheats. The important result is that if the
relationship between the overheat and response frequency is truly linear, then we have
not yet seen an upper limit for the frequency response of the sensor with the existing
architecture. This might suggest that the best way to get a high frequency response is to
turn up the overheat even higher, but then we risk burning out the sensor. An upper
materials limit of the sensor was stated as an overheat of 2.5. This overheat saw a
significant degradation of the film sensor. It is also important to note that not only does
the smallest sensor have the highest frequency response, it also has a greater increase of
frequency response with increasing overheat. Towards the end of the research, higher
overheats will be tested when it is no longer such a crucial issue to burn out the sensor.
Though voltage step testing is just about finished, more analysis will be made to
determine how response amplitude increases with increasing overheat and sensor size,
and also as to how response frequency changes with identical overheats for different
sensor sizes.
Velocity SteD Test Results
Finally, in Figures 34-37 we show some results for velocity steps in the shock
tube. If the curves are overlaid, it can be seen that the response shape is similar, which is
encouraging for initial results. In addition, the peak amplitude does increase with
increasing velocity step. As testing with the velocity step (shock tube) continues, similar
analysis to what is being done with the voltage step testing will be made in order to
confirm assumptions made based on voltage step testing alone. More tests are underway
for the effects of changing the overheat and sensor sizes in the velocity step test.
Thefollowing textdiscussesresultsfrom theelectronicsteptestingandthebeginningof
velocity steptestingin theshocktube. It shouldbekept in mind thatfrequencyresponse
is alwaysmeasuredasthetime from thebeginningof theeventto theminimum (positive
inflection) of the "bucket"that immediatelyfollows theresponse.This reportis nota
completeaccountof theresultsfrom squarewavetesting. Somedatais still in the
processof beinganalyzedandeffortsarebeingmadeto fit thedatato bothFreymuth's
third ordertheoryandmodelledresponsesfrom SPICEcircuit simulationsoftware.
Voltage Step Test Results
Voltage step testing is initially performed using two anemometers. These
anemometers are referred to simply as Anemometer I (AI) and Anemometer II (All).
Figure 1 shows a schematic of the architecture of AI. This architecture is chosen for
historical reasons and has been shown to work well in various applications. Figure 2
shows a schematic of the architecture of AII. The AII architecture incorporates changes
that were suggested by LaRC. These changes are (1) input isolation resistors on the op
amp, (2) variable inductor on the ground side of the control resistor, (3) trim capacitor to
vary the op amp rolloff and (4) resistor in series with the base of the transistor. Figure 3
shows what happens when the square wave test is applied to AI and AII using identical
overheats and sensor sizes. AII exhibits a faster response than AI by about 2 I.tsec, but
this is before the inductor has been tuned for optimal response. Although a plot is not
included, it can be shown that the variable inductor improves the response time by 2-3
_sec for the smallest sensor on the highest tested overheat (1.7). In general the noise
level is higher for AII than for AI. This might be due stray capacitance introduced
through the use of sockets for the IC's and more components than AI. However, the most
desirable feature of AII is the stability of the circuit. AI has a tendency to become
unstable at high overheats due to switching between negative and positive feedback.
This renders it useless for testing.
Initially, AI and AII were tested at low overheats and a series of square wave tests
were applied with varying amplitudes for all three sensors. Figure 4 shows and example
of one sensor tested at one overheat while varying the input voltage where input voltage
is measured from the function generator rather than on the board. It was determined that
if theresponseswerenormalizedby dividing theresponsedataof anyoneconfiguration
(sameanemometer,sensor,overheatwith varyingvoltage)by thepeakmV response
valuethenthecurveswouldcollapseneatlyon top of eachother (seeFigure5). Plotting
thesenormalizationfactorsagainstthe inputsquarewaveamplitudeproducesavery
linearcurve(seeFigure6). For example,All normalizationfactorsfor the0.005x
0.0005inch sensorat anoverheatof 1.19werelinearly regressedandproducedanR
squaredvalueof 0.999729.ThecloserR squaredis to 1.0,thecloserthe actualdatais to
beinglinear. After thisresultof linearnormalizationfactorswasseenidentically for
severalconfigurations(noconfigurationsdeviated)it wasdeterminedthat responsesof
anyconfigurationcould becomparedregardlessof thesquarewaveamplitudeaslongas
thedatais normalized. This becomesahandyresultin lateranalysis.As anexample,
Figure7 showsthesquarewaveresponsefor thethreedifferent sensorsizesall testedat
onesquarewavevoltageandoverheat.Theactualamplitudeof theresponseto an
identicalsquarewaveincreaseswith increasingsensorsize. To geta betterpictureof the
comparativeresponsetime, thecurvesarenormalized(seeFigure 8) with no lossin the
datafor responsetime. Figures9-13showthesamecycle for AII at theoverheatof 1.19
with the0.005x 0.0005inch sensor.Theinput voltageis increased,thecurvesare
normalized,thenall threesensorsizesarecompareat oneoverheatin both the
normalizedandnon-normalizedformats. As anaside,Figure 14showswhathappensto
theanemometeresponseastheoperatingpoint reachesthecurrentlimit for the
anemometer.Caremustbetakento provideenoughleewayfor currentfluctuationswhen
settingthecurrent limiting resistorin theanemometer.Figures15-19showthesame
cycleasbefore,but this timefor AI at anoverheatof 1.34with the0.005x 0.0005inch
sensor.Figures20-24showthesamecycle for AII at anoverheatof 1.34with the0.005
x 0.0005in sensor.Figures25-27showthenormalizedresponsesfor different sensor
sizesat a singleoverheat.In eachcase,thesmallestsensorhasthefastestresponseandit
beginsto appearthat theremaybesomescalingfactorwhich relatesthefrequency
responsebetweenthethreesensorsizes.Looking at Figures28-30,theresponsefor a
singlesensorsizeis shownat five different overheats.At first glance,it appearsthat the
responseincreaseswith increasingoverheat,buteachhigheroverheatseesa
progressivelylesssignificantincreasein theresponse.As we look at thedatawith a little
moredetail, resultsindicatethat thismaynotbe thecase.
Table 1catalogstheresponsetimeof eachof thethreesensorsat five overheats.
This datais plottedin Figure31 for All. Onceagain,thedataseemsto indicatethatwe
areapproachingsomelimiting valuefor thefrequencyresponse.However,we see
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Testing of the anemometer circuit: Data report

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