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09/16/2010
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Temperature Scales
In industrial applications many physical quantities
must be measured and controlled. For many processes,
temperature measurement is critical.
F
You are probably familiar with the two
principle temperature scales in use –
Fahrenheit and Celsius. Recall that…
On the Fahrenheit scale, water freezes at
32o; on the Celsius scale, it freezes at 0o.
On the Fahrenheit scale, water boils at
212o; on the Celsius scale, it boils at 100o.
C
212
100
32
0
Temperature Scales
Most industrial applications use the Celsius scale,
which is more common in most countries. The
conversion formulas between Fahrenheit and Celsius
are:
9
F  C  32
5
and
C
5
 F  32 
9
Temperature Measurement - Thermocouple
There are a number of temperature sensors available.
The choice of which one to use depends on the
specific requirements (range, threshold, accuracy,
resolution, cost, etc.)
A common industrial temperature sensor with a very large range
is the thermocouple. In general, thermocouples can be used from
about 250oC (418oF) to 2000oC (3632oF) but any given
thermocouple has a smaller range.
Metal A
Metal B
Heat
+
VAB

Temperature Measurement - Thermocouple
A thermocouple measurement may use a reference
thermocouple that is kept at a known temperature. This
avoids errors due to unwanted thermocouple junctions.
Measuring
thermocouple
+
Copper (Cu)
VT

Constantan
Cu
Cu
Av
VREF

+
Reference
thermocouple in
controlled
environment
Av(VT – VREF)
Temperature Measurement - Thermocouple
Another method to avoid unwanted thermocouple
junctions is to use a compensation circuit as shown in
the green box.
The output is only
a function of VT.
The unwanted thermocouple
voltage is canceled by Vc.
Copper (Cu)
+
Measuring
thermocouple
VT

Constantan
Cu
 +
+
Vc

Cu
Av
Ic
An unwanted thermocouple voltage
is created between the Constantan
and the Copper junction.
+V
The current source generates a
voltage in R that matches the
unwanted thermocouple junction.
AvVT
Temperature Measurement - Thermocouple
An RTD is a resistance temperature device in which the
resistance increases with temperature. RTDs tend to be
expensive, but are more accurate than other sensors.
A three-wire bridge connection cancels wire resistance of the leads
by putting each lead from the RTD in a different leg of the bridge.
Wire
resistance
R1
+
R3
VOUT

R2
Upper leg
RA
The wire resistance of the third wire
has little effect on VOUT because the
current in this lead is almost zero.
RTD
RB
Lower leg
Temperature Measurement - Thermocouple
Thermistors are formed from certain metal oxides with a
large negative resistance coefficient. Although they are
not as accurate as RTDs, they are small and inexpensive,
so have application in thermostats and other non-critical
systems.
A simple thermistor may be formed as a small bead, such as the
ones illustrated here. Because of their small size, they can respond
quickly to a temperature change.
Temperature Measurement - IC
IC temperature sensors are based on the changes in
breakdown voltage that occur in zener diodes with
temperature. They have limited range, but are small,
linear, and inexpensive.
A typical IC temperature sensor is the LM135, LM235, and LM335.
The LM335 can measure temperatures from 40oC to 100oC. It is
available in a surface-mounted package or the TO-92 package,
common with small signal transistors.
TO-92 package
Strain Measurements
Strain is the deformation of a material due to a force
acting on it. Strain can be induced along a material when
a force is applied to it. The basis of most electronic scales
is measuring the strain due to compressing or stretching a
F
solid material.
F
When a force is applied as shown, the
material compresses a small amount. This
can change the resistance of a strain gauge,
which is bonded to the material. The
resistance change is extremely small, but
can be measured with sensitive instruments.
Strain gauge resistance changes
with the deformation.
DL
L
Strain Measurements
The strain gauge is constructed from a resistive wire
(typically about 350 W), which changes by a few mW
when it is stretched or compressed. The basis of the
resistance change is the wire resistance equation you
studied in Chapter 2:
L
R
A
Compressing the strain gauge causes the length to decrease and
the area to increase, thus making the resistance smaller.
terminals
Resistive material
Strain Measurements
The gauge factor for a strain gauge is the ratio of the
change in resistance to the nominal resistance divided by
the change in length divided by the nominal length.
Gauge factors for metallic gauges are typically about 2.
DR
R
GF 
DL
L
The fractional change in length (DL/L) is called strain (e). Strain
is such a small quantity it is usually expressed in parts per
million (designated as me).
Strain Measurements
The most common method for measuring the resistance
change of a strain gauge is using a Wheatstone bridge
because of its great sensitivity. A three wire bridge circuit
+V
is illustrated.
R1
In automated scales, the bridge
circuit is generally automated,
so that the readout is directly in
weight units. Frequently, in this
application, all four legs of the
bridge contain active gauges.
R3
R2
Strain
gauge
RG
+

Pressure Measurements
Pressure is force per area. The defining equation for
pressure is
F
P
A
In the English system, the typical pressure unit is the psi (pound
per square inch). In the metric system, pressure is measured in
newtons per square meter, which defines the pascal (Pa). The
Pa is a very small unit, so kilo and mega prefixes are common.
Pressure Measurements
There are three ways to specify a pressure measurement.
These are
Absolute pressure: pressure measured with respect to a vacuum.
Gauge pressure: pressure measured with respect to the atmosphere.
Differential pressure: the difference between two pressures.
vacuum
atmosphere
open
pressure
pressure
pressure-1 pressure-2
Pressure Measurements
When you have a flat tire, the
pressure is said to be zero.
What type of pressure measurement is this?
This is gauge pressure because the remaining air in
the tire has the same pressure as the atmosphere.
Pressure Measurements
There are many types of pressure transducers. One
type uses a diaphragm which is instrumented with
a strain gauge.
When pressure is applied to the gauge, it
expands and elongates the strain gauge,
causing the resistance to increase.
pressure
Motion
Displacement transducers can detect the position of
an object using either contacting or noncontacting
methods. A widely used contacting transducer is the
LVDT (linear variable differential transformer).
The output of the LVDT is
proportional to the position of a
movable core that is inside a
differential transformer. When
the core is centered, the output
is zero. The output is positive or
negative when the core moves.
Moveable core
ac
Demodulator
Vout
Motion
Velocity is the rate of change of displacement, so
velocity measurements can use an LVDT to find the
position as a function of time and calculate velocity.
Another measurement of motion is acceleration.
Acceleration is the rate of change of velocity.
Acceleration measurements
are important in fields such
as destructive testing or
vibration analysis. A basic
accelerometer is shown.
V
mass
Vout
dashpot
Sample and Hold Circuit
A sample-and-hold circuit is used in instrumentation
applications to retain a voltage from a transducer to
allow time for conversion to digital form.
A basic sample-and hold circuit consists of an analog switch, a
storage capacitor and a high impedance amplifier that prevents
the capacitor from discharging too quickly.
JFET switch
+

+
RG
CH
Control
input

Holding
capacitor
Sample and Hold Circuit
The output of the sample-and-hold circuit tracks the
input when the control input is high, and holds the input
voltage when the control input is low.
For the input and control waveforms shown, sketch the output.
S
H
S
H
S
H
S
H
S
H
S
H
Sample and Hold Circuit
Specifications for sample-and-hold circuits:
Aperture time – the time for the analog switch to fully open after the
control voltage switches from the sample level to the hold level.
Aperture jitter – the uncertainty in the aperture time.
Acquisition time – the time required for the device to reach its final
value when the control voltage switches from its hold level to its
sample level.
Droop – the change in voltage from the sampled value during the hold
interval because of charge leaking off the hold capacitor.
Feedthrough – the component of the output voltage that follows the
output voltage after the analog switch is opened.
Analog-to-digital Conversion
Analog-to-digital conversion is the process of converting
the output of the sample-and-hold circuit to a series of
binary codes that represent the amplitude of the analog
input at each of the sample time.
The output of the sampleand-hold circuit is a series of
pulses that represent the
original input. The ADC
converts each pulse into a
group of binary pulses that
represent a binary number.
S/H
output
ADC
output
0100 0101 ………. 1100 1010 …
Analog-to-digital Conversion
Three specifications for analog to digital conversion circuits are:
Resolution – the number of bits involved in an analog to digital
conversion.
Conversion time – the time it takes for an ADC to convert an analog
voltage sample to a digital code.
Quantization error – the
change in value of an analog
signal during the conversion
process as shown in the figure.
Notice that the analog voltage
changes during the conversion
process leading to the error.
V
Value that is actually converted
Value that
should have
been converted
DV
(Quantization error)
t
Start
End
Nyquist Rate
Harry Nyquist, working for AT and T, studied
information theory and published an important paper
entitled Certain topics in Telegraph Transmission
Theory. In his paper he showed that an analog signal
should be sampled at a frequency that is at least
twice the highest frequency of the signal in order to
be able to adequately reconstruct the signal. This rate
is known as the Nyquist rate or Nyquist frequency.
The human ear can detect frequencies as high as 20 kHz.
What is the Nyquist frequency for this upper limit? 40 kHz
(Actual digitizing would be done a little faster than this – about 44 kHz.)
Power Control
Electronic control of power is generally accomplished
using a class of devices known as thyristors. A
thyristor is a semiconductor switch composed of layers
of alternating pnpn material.
(A)
Anode (A)
One important type of thyristor
is the SCR, which acts like a
diode that is turned on by a
Gate (G)
control signal. The control
signal is applied to the gate.
p
n
p
n
Cathode (K)
(G)
(K)
The SCR
Normally the SCR conducts by forward-biasing the anode
to cathode and applying a positive pulse of current to the
gate. The gate then loses control.
IF
Conduction continues as long
Forwardconduction
as the anode to cathode
region
remains forward-biased.
The characteristic curve
IG2IG1> >IG1IG0IG = 0
changes when a current pulse
IH2
is applied to the gate. As gate VR
VF
V
V
V
current increases, the forward
BR(F2)BR(F1)BR(F0)
breakover voltage decreases,
Reverseblocking
which triggers the SCR.
I
region
The holding current, IH, is the minimum anode
current to keep the SCR in conduction.
R
Selected Key Terms
Thermocouple
Transducer
RTD
Thermistor
A type of temperature transducer formed by
the junction of two dissimilar metals that
produces a voltage proportional to
temperature.
A device that converts a physical parameter
into an electrical quantity.
Resistance temperature detector.
A type of temperature transducer in which
resistance is inversely proportional to
temperature.
Selected Key Terms
Strain gauge A transducer formed by a resistive material in
which lengthening or shortening due to stress
produces a proportional change in resistance.
Quantization The determination of a value for an analog quantity.
Thyristor
SCR
A class of four-layer (pnpn) semiconductor switching
devices.
Silicon controlled rectifier; a type of three terminal
thyristor.
Quiz
1. The number of Fahrenheit degrees between the
normal freezing point and boiling point of water is
a. 100.
b. 180.
c. 212.
d. 473.
Quiz
2. A thermocouple is a temperature transducer that
a. has a resistance change that is
proportional to temperature.
b. has a resistance change that is inversely
proportional to temperature.
c. is constructed from two dissimilar
metals connected together.
d. is constructed from a semiconductor
material.
Quiz
3. A sensitive and inexpensive temperature transducer
that is a good choice for a home thermostat is
a. an RTD.
b. a thermocouple.
c. a strain gauge.
d. a thermistor.
Quiz
4. The response of a strain gauge to a strain (DL/L) is
a. a small resistance change.
b. a voltage that is proportional to the strain.
c. a voltage that is inversely proportional to the
strain.
d. none of the above.
Quiz
5. Assume a pressure gauge reads the pressure of a gas
relative to the atmosphere. This is an example of
a. absolute pressure.
b. gauge pressure.
c. differential pressure.
d. none of the above.
Quiz
6. An example of a transducer that has an output voltage
that is proportional to displacement is
a. an LVDT.
b. an RTD.
c. a Wheatstone bridge.
d. a strain gauge.
Quiz
7. The circuit illustrated below is a
a. demodulator.
b. analog-to-digital converter.
c. power control circuit.
d. sample-and-hold circuit.
JFET switch
+

+
RG
CH
Control
input

Holding
capacitor
Quiz
8. The term high resolution with an ADC means that it
a. is very fast.
b. is able to digitize with very small errors.
c. uses a large number of bits.
d. all of the above.
Quiz
9. Assume a measuring system is required to measure
mechanical vibrations that are no greater than 500 Hz.
The Nyquist frequency is
a. 250 Hz.
b. 500 Hz.
c. 750 Hz.
d. 1.0 kHz.
Quiz
10. Once an SCR is conducting, it continues until the
a. gate signal is zero.
b. gate signal is made negative.
c. holding current is exceeded.
d. none of the above.
Quiz
Answers:
1. b
6. a
2. c
7. d
3. d
8. c
4. a
9. d
5. b
10. d