1. ME 311 Mechanical Measurements Instrumentation II 5/6/03

2. ME 311 Mechanical Measurements
Announcements
The attendance sheet is being circulated. Please sign it before you leave!
Your final project presentation is postponed to next Wednesday (5/14/03).
Any general question regarding your project presentation?
We are meeting for “canned” lab tomorrow according to our original 9th week schedule.
If you have specific question, don’t hesitate to contact your professors! We are here to help you.
Instrumentation II
ME 311 Mechanical Measurements

3. ME 311 Mechanical Measurements
Road Map of Lecture 8
Recap from Lecture 7
Measurement of solid-mechanical quantities
Strain
Force
Acceleration
Measurement of thermodynamic quantities
Pressure
Temperature
Measurement of fluid system
Flow rate
Flow velocity
Instrumentation II
ME 311 Mechanical Measurements

4. ME 311 Mechanical Measurements
Recap from Lecture 7
In general, how many stages are there in a measurement system (from measurand to recorded data)? Can you name them?
sensor/transducer system
signal conditioning system
data acquisition system
What is the most distinctive characteristic of a transducer?
It transforms one physical variable to another.
Important questions related to a sensor/transducer system:
operating principle
range (detectable minima and maxima)
resolution (highest precision in the recorded signal)
sensitivity (change in measured signal per change in measurand)
bandwidth (frequency range over which the signal is accurately tracked)
Instrumentation II
ME 311 Mechanical Measurements

5. Recap from Lecture 7 (Cont’d)
Every instrument is a dynamical system which responds to the change in the measurand within the bandwidth of the instrument.
It has its own amplitude modulation and phase lag (response time) characteristics.
No instant (phase lag) and exact (amplitude modulation) data transmission
Instrument calibration is an important procedure in performing an experiment. Do NOT overlook it.
Instrumentation II
ME 311 Mechanical Measurements

6. Measurement of Solid-Mechanical Quantities
In solid-mechanical systems, the concepts of force, acceleration, deformation (strain) and stress are inter-related. They form the repeating theme in this section.
Strain
Strain gage
Force
Load cells
Acceleration
Piezoelectric
Instrumentation II
ME 311 Mechanical Measurements

7. ME 311 Mechanical Measurements
Measurement of Strain
When a force is applied to a structure, the structure changes in its dimension and are said to be strained. Devices to measure these small changes in dimensions are called strain gages.
Common device: the electrical resistance strain gage
A simple strain gage example (strain in one dimension):
The strain in the wire is approximately the same as the strain in the lower surface of the beam.
The stretching of the wire will cause its electrical resistance to change so that the wire is a detector of strain.
Instrumentation II
ME 311 Mechanical Measurements

8. Measurement of Strain (Cont’d)
It is difficult to measure stress directly. But a strain gage can be used to measure strain and then the material properties can be used to determine the stress. What is the material property called?
Rather than in straight wire form, it is more common to have foil strain gage in the following form:
Apart from metallic strain gage, semi-conductor strain gages are common. They have higher gage factor but they are not as ductile.
Additional issue: strain in multi-dimensions in complicated situations.
Instrumentation II
ME 311 Mechanical Measurements

9. Strain Gage Signal Conditioning
Since the change in resistance is usually small, simply measuring the gage resistance before and after the load will not work well.
Instead, we should measure the change in resistance rather than the resistance itself.
Since the signal is small, we need a conditioning system to measure it. For example, the Wheatstone bridge:
If the bridge is initially balanced, the output voltage, V0, is proportional to the change in gage resistance.
Instrumentation II
ME 311 Mechanical Measurements

10. ME 311 Mechanical Measurements
Measurement of Force
The most common force-measuring devices are strain gage load cells.
In the hollow cylinder type, two of the gages measure axial compression while the other two measure tensile diametral strain.
In both cases, the four gages form the Wheatstone bridge.
(Cantilever beam)
(Hollow cylinder)
Instrumentation II
ME 311 Mechanical Measurements

11. ME 311 Mechanical Measurements
More on Load Cells
The force-deformation relationship is used again in the load cell application.
Commercial load cells are available with ranges from a few ounces to several hundred thousand pounds.
However, the frequency response of load cells are not usually specified because the mass and flexibility of the instrument control the dynamic response.
Instrumentation II
ME 311 Mechanical Measurements

12. Measurement of Acceleration
Acceleration measurement is common in vibration analysis and impact testing.
Two common types of accelerometers:
Piezoelectric
Strain gage
Choice of particular type depends on the targeted frequency range.
Piezoelectric accelerometers can respond to much higher frequency than the strain gage accelerometers (inertial effects). However, they are also more expensive.
Instrumentation II
ME 311 Mechanical Measurements

13. Piezoelectric Sensors
Piezoelectric materials are capable of generating an electric charge when it is deformed. For example, quartz crystal.
Two configurations of piezoelectric materials under compression:
(longitudinal)
(transverse)
Instrumentation II
ME 311 Mechanical Measurements

14. Piezoelectric Sensor Signal Conditioning
With either longitudinal or transverse loading, the generated charge is proportional to the applied force.
The charge generated by the piezoelectric element must be sensed in a manner which does not dissipate the charge. The sensor device is called a charge amplifier.
Instrumentation II
ME 311 Mechanical Measurements

15. Piezoelectric Accelerometer
Schematic:
Mechanism: an initial force between the mass and sensor is obtained with a preloading spring sleeve. As the housing of the accelerometer is subject to an acceleration, the force exerted by the mass on the quartz crystal is altered. This generates a charge on the crystal which can be sensed with a charge amplifier.
Instrumentation II
ME 311 Mechanical Measurements

16. Strain Gage Accelerometer
Schematic:
Mechanism: As the base is accelerated, the force accelerating the mass is transmitted through the cantilever beam. The bending of this beam is sensed with strain gages.
Usable upper frequency limits of a few hundred Hz (considerably lower than that of piezoelectric transducers.)
Instrumentation II
ME 311 Mechanical Measurements

17. ME 311 Mechanical Measurements
Pressure Transducer
Schematic:
Mechanism: The test pressure is applied to one side of the diaphragm, a reference pressure to the other side, and the deflection of the diaphragm is sensed with strain gages.
Normally, a Wheatstone bridge signal conditioner is built into the transducer.
Instrumentation II
ME 311 Mechanical Measurements

18. Measurement of Temperature
Two traditional electric output devices for measuring temperature are thermocouple and resistance temperature detector (RTD).
Semi-conductor devices have applications at moderate temperatures.
Pyrometer, a non-contact device, is appropriate at high temperatures.
Instrumentation II
ME 311 Mechanical Measurements

19. ME 311 Mechanical Measurements
Thermocouple
Schematic:
Mechanism: When any two metals are connected together, a voltage is developed which is a function of the temperature of the junction. This junction, when used as a temperature sensor, is called a thermocouple. The physics is governed by Seebeck effect in detail.
Instrumentation II
ME 311 Mechanical Measurements

20. Resistance Temperature Detector
Schematic:
Idea: A normal characteristic of metals is that the electrical resistance is a function of the metal temperature. As a result, a length of metal wire combined with a resistance-measuring device is a temperature-measuring system. Such temperature sensors are called Resistance Temperature Detectors (RTD).
Instrumentation II
ME 311 Mechanical Measurements

21. Thermocouple Versus RTD
RTD tends to have more linear output response.
RTD tends to be more stable than thermocouple (characteristics are less likely to change over time).
RTD is generally larger than thermocouple physically suffering poorer spatial resolution.
RTD has slower transient response.
RTD is generally more expensive.
Instrumentation II
ME 311 Mechanical Measurements

22. ME 311 Mechanical Measurements
Turbine Flow Meter
Schematic:
Turbine flow meter may be used to measure the flow rate of clean liquids.
Mechanism: The fluid turns a turbine wheel as it passes through the meter. The turbine rotates and each time a tip passes the magnetic pickup, as electrical pulse is generated. The output signal is a frequency that is proportional to the flow rate.
Flow
Instrumentation II
ME 311 Mechanical Measurements

23. Differential Pressure Transducer
Schematic:
A differential-pressure transducer measures the difference between total and static pressure for a flow velocity measurement. Recall the wind tunnel experiment!
flow
transducer
voltmeter
Instrumentation II
ME 311 Mechanical Measurements