AOE 3054: Final Exam
Single-ended MUX input:
A/D measures difference between signal and ground; same ground is used for all channels
T/F: A strain gage gives a voltage output that is linearly proportional to the strain.
True
T/F: For a normally distributed quantity, 95% of a set of samples lies within two standard deviations of the mean.
True
Precision:
a measure of how close a series of measurements are to one another (i.e., resolution with which a measurement may be made) (e.g., digital display resolution or ruler graduation)
Strain gauge:
a sensor whose resistance varies with applied force; It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured
What is the purpose of the engineering code of conduct?
a set of rules and guidelines that foster trust=> in the results, to work collaboratively, to receive proper credit, to make it more likely to get funded
Capacitor:
ability to store charge [Farads]; V=I/jwC; Z_C=1/jwC
When and why is the dynamic response of a measurement system important?
allows researcher to characterize how a structure will respond to a time-varying set of loads
Relationship between amplitude, peak-to-peak, and RMS voltage for a sinusoidal signal:
amplitude is half the peak-to-peak; V_peak2peak = V_rms * 2*sqrt(2)
Sample and hold circuit:
an analog device that samples (captures, takes) the voltage of a continuously varying analog signal and holds (locks, freezes) its value at a constant level for a specified minimum period of time
Multiplexer (MUX):
an analog switch that creates a composite signal containing short bursts of each of the signal channels to be measured
Response function of a dynamic structural system:
change in amplitude and phase at each frequency
Describe plagiarism:
copying someone else's work and passing it off as your own
A/D converters:
designed to measure (sample) a voltage signal and convert it to a number at regular intervals (sampling rate=no. of samples per second); produces a binary number with a fixed number of digits or "bits"
Repeatability:
difference between multiple measurements of the same quantity
What are primary measurements and how do you estimate uncertainty in them?
measurements that are not derived from any other/read directly from a gauge; uncertainty is equal to twice the standard deviation
Multimeter:
measures current, voltage, and resistance; used to check transducer properties/connections/measurement system outputs and inputs
Standard deviation (definition and equation):
measures the dispersion of a dataset relative to its mean; σ = √( (Σ(xi-mean)^2) / N )
Root-sum-square equation for combining uncertainties: *be able to apply it analytically, along with primary uncertainty estimates, to compute uncertainty in a result*
√( Σ σ(i)^2 )
Differential MUX input:
A/D measures difference between two signals (reduces noise, but halves number of channels)
What is broadening, and why does it occur?
sudden jumps resulting from implied periodicity introduce new frequencies not in original signal (can be fixed by smoothing)
Mean (definition and equation):
sum of numbers divided by the amount of numbers (i.e., the average)
What does a normal distribution look like and how is it used to compute probabilities given a set of experimental measurements?
symmetrically distributed, no skew (i.e., a bell curve); find area under the curve to compute probabilities
Natural frequency:
the frequency at which a system oscillates when disturbed
Gauge factor:
the ratio of relative change in electrical resistance R, to the mechanical strain ε
What is meant by uncertainty in the context of a statistical result (e.g., while computing the mean of a data set)?
the uncertainty in the mean value goes as the inverse square root of the number of samples; δ(y) = (2*σ)/(√N)
Aliasing:
this kind of error occurs when components of a signal are above the Nyquist frequency or one half the sampling rate
What is the effect of using an internal amplifier on an A/D converter?
to amplify the signal so it fills the full range
How does aliasing affect a spectrum, and how can this be prevented?
to avoid aliasing, sampling rate must be at least 2 times the highest frequency in signal
Describe the importance of life-long education:
to stay relevant, competitive, and up-to-date
What are the various types of pressure probes/transducers, and how are they used? What do they measure, and what limitations do they have?
transducers: manometer (good at larger pressures), diaphragm, piezo-electric probes: tap, standard, other devices
Oscilloscope:
used to display your measured signal to make sure it's right
What are the 3 parameters that govern the response of a single degree of freedom system?
effective mass (m), spring stiffness (k), viscous damping (b)
How do you calculate the resolution of an A/D converter from its bit count and range?
resolution = (voltage range) / (2^N) where N is the number of bits of the converter
Describe how to foster a productive work environment through integrity and respect:
respect your engagements (contact sponsors for usage/sharing of data, non-disclosure agreements, honor pledge) and treat colleagues/collaborators with respect (regardless of sex/race/etc., recognize contributions regardless where it originates)
What is the relationship between sampling rate and Nyquist frequency?
sampling frequency must be at least two times the highest frequency component of the signal
What limitations does an A/D converter place on the information that can be extracted from a sampled signal?
sampling, accuracy, and can only sample one channel
Transducer/sensor:
senses the quantity you want to measure and transforms it (usually) to an electrical signal (usually a voltage)
Describe processes to make ethical decisions:
seven-step guide to ethical decision making: 1. state the problem 2. check all facts/list assumptions 3. identify relevant factors 4. develop multiple options 5. test each option 6. make a decision 7. review
What is flow-visualization, and what is it used for?
smoke, oil-flow, tufts; used for LDV, PIV, Hot-wire velocity/turbulence measurements; used for mean surface pressure measurements (with taps); pitot-static traverses to look at wake and flow patterns
What is the approximate relationship between uncertainty interval and standard deviation, and where does it come from?
uncertainty interval is equal to 2 times the standard deviation; for determining uncertainty in primary measurements
What kind of circuit is used to operate a strain gauge?
wheatstone bridge circuit
Describe conflict of interest (financial or otherwise):
when a personal interest could compromise judgment, decisions, or actions in the workplace
How do you compute a spectrum?
with amplitude, phase, and frequency
Error:
(same as accuracy) the difference between value indicated by measurement system and true value
Current vs. voltage (AC and DC):
Current: flow rate of charge through conductor Voltage: potential difference that causes current to flow (relative measurement)
Names given to devices containing A/D and/or D/C converters:
DAQ cards aka DAQ devices aka Data Acquisition Systems aka Digitizers
Input vs. output resistance:
V2/V1 = Ri / (Ro+Ri) => therefore, a good instrument is one with high input impedance and low output impedance
Digital vs. analog signals:
digital: physical property is represented as a series of numbers in a computer (most useful form for signal) analog: physical property exists as a voltage signal in a wire (output of almost all sensor systems)
Describe how to ensure the safety and legality of you and your team's activities:
ensure safety of everyone involved (research personnel, end-users) and contact funding agency/organization or government or legal counsel for legality matters
What can be reduced by calibration?
error (improves accuracy)
Uncertainty:
estimated error/accuracy (i.e., errors you don't know about)
What errors and limitations does a Pitot-static probe have?
flow must be aligned with probe and steady; yaw or pitch of more than 35° causes serious errors; turbulence may increase or decrease measured value depending on scale; cheap ($100); cannot measure fluctuations; robust; dynamic response is poor regardless of transducer used
Linearity in a dynamic structural system:
for small deformations, structures tend to respond linearly to dynamics loads=> this means that the response of the structure is the sum of its response to the individual frequencies in the load
Function generator:
generates simple waveforms; used to provide fluctuating inputs for experiments or for checking hardware; used to trigger events
Nyquist frequency:
highest permissible frequency
Uncertainty interval: (95% condition, various representations)
how big the margin of doubt is; symmetrical band around a measurement chosen so that there is a 95% probability of the true value lying within it; 95% condition also called "20:1 odds"
Accuracy:
how close a measurement is to the true value (i.e., difference between value indicated by measurement system and true value)
Inductor:
inertia of a current flow [Henrys]; V=I*jwL; Z_L=jwL
Static flexibility:
inverse of the spring stiffness
How is a Pitot-static probe used to measure velocity, and how do you calculate that velocity from a pressure reading in inches of water?
measures difference between stagnation and static pressure in subsonic flow; p0 - p = ½*ρ*U^2
Counter:
measures the frequency of simple periodic voltage signals; used with function generator testing dynamic response of experimental set ups or measurement hardware
What is a window function, and how does it reduce broadening?
multiplies record by a smooth function that is zero, or nearly so at the start and end points of the record
What is the governing differential equation of a single degree of freedom system, and how is it solved (in principle)?
mẍ + bẋ + kx = f(t) => since governing equation is linear, we can add solutions together to create new solutions (alt. approach: laplace transforms)
Amplifier:
often unassuming, large amount of variety; increases voltage/current/power of signal; transforms output impedance
Phase spectrum:
phase as a function of frequency
Describe how one can be honest and objective in the context of engineering work:
present (all) the facts and disclose all references; explain missing data and limitations; do not falsify or fabricate
Power supply:
provides fixed DC voltages; used to provide reference voltages (to compare with output of sensor)
Limitations of a D/A converter:
range and resolution; produces one channel of voltage signal (need multiple D/As to produce multiple channels
Dynamic flexibility:
ratio of the amplitude of the displacement to that of the force
Calibration:
relationship between the output of a measurement system and the quantity it senses
Resistor:
resistance to current flow [Ohms]; V=I*R; Z_R=R
