1. ME 311 Mechanical Measurements Instrumentation I 4/29/03

2. ME 311 Mechanical Measurements
Announcements
The attendance sheet is being circulated. Please sign it before you leave!
We are not meeting for lab tomorrow. Time to finish up testing and prepare for oral presentation.
How are your projects coming along? How many groups have finished their testing? Any general question?
If you have specific question, don’t hesitate to contact your professors! We are here to help you.
Instrumentation I
ME 311 Mechanical Measurements

3. ME 311 Mechanical Measurements
Road Map of Lecture 7
Overview of a measurement system:
sensor/transducer
signal conditioning (will be covered in greater details in ME 421)
data acquisition (will be covered in greater details in ME 421)
Focus on the sensor/transducer system in this course
How does it work? (operating principle)
Issues on range, resolution, sensitivity
System characteristics
Need for calibration
Terminology with system characteristics
Homework assignment (as a group)
Instrumentation I
ME 311 Mechanical Measurements

4. Overview of a Measurement System
e.g. pressure, temperature,
light intensity
Measurand
Sensor/Transducer System
Stage 1
e.g. pressure gauge, thermocouple,
photo-cell
Signal Conditioning System
Stage 2
e.g. amplification, band-pass filtering
Data Acquisition System
Stage 3
e.g. display, record, storage
Reduced data for analysis/presentation
measurement system
Instrumentation I
ME 311 Mechanical Measurements

5. ME 311 Mechanical Measurements
Basic Terminology
The measurand is a physical quantity, property, or condition that is being measured.
A sensor is the portion of a measurement system that responds directly to the measurand.
In the general sense, a transducer is a device which transforms one physical variable into another.
In most cases, the physical variable is transformed into an electrical signal.
Instrumentation I
ME 311 Mechanical Measurements

6. Advantages of Electrical Transducers
Amplification or attenuation can be obtained easily
Inertial effects are minimized
Frictional effects are minimized (no moving part)
Remote indication of recording is feasible
Commonly susceptible to miniaturization
Majority of output/recording devices require electrical input
Instrumentation I
ME 311 Mechanical Measurements

7. The Sensor/Transducer System
The fundamental function of a sensor/transducer system is to sense the desired input, excluding all others, and provide analogous output. (Example: microphone for sound detection)
In reality, the sensor/transducer system always measures both the quantity of interest and some “noise”.
Can you name a few examples of transducer which you worked with in the first four labs and your project? What is the variable transformation in each case?
thermocouple (from temperature difference to potential difference)
thermistor (from temperature difference to resistance difference)
piezoelectric transducer (from stress/deformation to potential difference)
strain gauge (from strain/deformation to change in resistance)
photo-cell (from light intensity to change in resistance)
manometer (from pressure difference to height difference between fluid columns)
Instrumentation I
ME 311 Mechanical Measurements

8. ME 311 Mechanical Measurements
Issues to Consider
Knowledge of the measurand:
static or dynamic (mean versus fluctuations)
time scale
amplitude scale
frequencies present (How large is the range?)
magnitudes
Knowledge of the sensor:
operating principle (Does it imply any limitation on what can be measured?)
resolution (How does the resolution compare with the magnitude of the measurand?)
sensitivity
settling time
bandwidth (Is it large enough to capture the variation in the measurand?)
Instrumentation I
ME 311 Mechanical Measurements

9. Characteristics of a Sensor/Transducer System
Viewing from a “black-box” perspective, every instrumentation is a dynamical system which has its own frequency response to the change in the measurand (amplitude and phase characteristics)
always has a lag time, no perfect (instant, exact) data transmission
recall the plotting device in Professor Olson’s lab (Does anyone recall the highest frequency which can be tracked?)
the measurement system may be of first or higher order ranging from simple to complex response
Instrumentation I
ME 311 Mechanical Measurements

10. More Vocabulary on System Characteristics
Time scales:
time constant (for 1st order system)
rise time
settling time
Frequency response
Transient response
Bandwidth
Accuracy, range
Overshoot
Instrumentation I
ME 311 Mechanical Measurements

11. ME 311 Mechanical Measurements
Need for Calibration
One of the most overlooked procedures in conducting an experiment. (Did you do it for your project?)
Rather than just accepting the reading of an instrument, calibration firmly establishes the accuracy of the instruments.
Sometimes the manufacturer’s specifications may not be taken at face value.
It involves a comparison of the instrument with either
a primary standard (may be difficult to obtain),
a secondary standard with a higher and known accuracy than the instrument to be calibrated,
a known input source.
Instrumentation I
ME 311 Mechanical Measurements

12. Many Types of Sensor/Transducer System
Acceleration
piezoelectric accelerometer
strain gage accelerometer
Force & Torque
shaft torque sensor
dynamometer
Pressure transducer
Temperature
thermocouple
resistance temperature detector (RTD)
Flow rate
rotameter
turbine flowmeter
Instrumentation I
ME 311 Mechanical Measurements

13. ME 311 Mechanical Measurements
Homework Assignment
Each group chooses one of the sensor/transducer systems to research on.
Address the following key questions:
What is the operating principle?
Range, sensitivity, resolution (based on the chosen model)
Any required accessories (signal conditioning)?
This homework is an attempt to get you involved in our second lecture on “Instrumentation”. It will be due at the beginning of the next lecture.
Instrumentation I
ME 311 Mechanical Measurements

14. ME 311 Mechanical Measurements
Suggested References
T.G. Beckwith, R.D. Marangoni, and J.H. Lienhard V, Mechanical Measurements, Addison-Wesley.
R.S Figliola & D.E. Beasley, Theory and Design for Mechanical Measurements, Wiley.
J.P. Holman, Experimental Methods for Engineers, McGraw Hill.
A.J. Wheeler & A.R. Ganji, Introduction to Engineering Experimentation, Prentice Hall.
SENSOTEC
OMEGA
DYNOmite
Instrumentation I
ME 311 Mechanical Measurements