Transcript 幻灯片 1 - 燕山大学教务在线

2 Sensors and transducers
2.1 Sensors and transducers
Sensor is an element which produces a signal relating to
the quantity being measured.（传感器）
Transducers are elements that when subject to some
physical change experience a related change.（变送器）
Sensors are transducers.
2.2 Performance terminology
Range The range of a transducer is the limits between
which the input can vary.
Error Error is the difference between the result of the
measurement and the true value of the quantity being
measured.
Error=measured value-true value
Accuracy Accuracy is the extent to which the value
indicated by a measurement system might be wrong.
(percentage of full range output or full-scale deflection)
Sensibility The sensibility is the relationship indicating how
much output you get per unit input.
Hysteresis error The hysteresis error is the maximum
difference in output for increasing and decreasing values.
Non-linearity error The error occurs as a results of the assumption
that the transducer has a linear relationship between the input and
output. (Percentage of the full range output)
Repeatability The repeatability is the ability of the transducer
to give the same output for repeated applications of the same
input value. (Percentage of the full range output)
Reproducibility The reproducibility of a transducer is its ability
to give the same output when used to measure a constant
input and is measured on a number of occasions. (Percentage
of the full range output)
Stability The stability of a transducer is its ability to give the
same output when used to measure a constant input over a
period of time.(The term drift is often used to describe the
change in output that occurs over time.)
Dead band(Dead space) The dead band of a transducer is
the range of input values for which there is no output.
2.2.1 Static and dynamic characteristics
Static characteristics are the values given when steadystate condition occur, i.e. the values given when the
transducer has settled down after having received some input.
(The terminology defined above refers to such a state)
Dynamic characteristics refer to the behavior between the
time that the input value changes and the time that the value
given by the transducer settles down to the steady-state value.
Response time (output corresponding to the value of input , 95%)
Time constant is the 63.2% response time.
Rise time is the time taken for the output to rise to some specified
percentage of the steady-state output.(from 10% to 95% of output)
Settling time is the time taken for the output to settle to within some
percentage, e.g. 2%, of the steady-state value.
2.3 Example of sensors and selection
2.3.1 Displacement
Potentiometer
V0/Vs=R23/R13
Consider the effect of the a load
RL , there is
Error=xVs – VL=VsRP(x2-x3)/RL
Strain-gauged element
ΔR
R
 G
Capacitive element
C
 r 0 A
d
Capacitance
Permittivity of the dielectric
Permittivity of free space
Area of overlap
Plate separation
Differential transformers
Optical encoders
2.3.2 Velocity
Incremental encoders
Tachogenerator
2.3.3 Force
Strain gauge load cell
2.3.4 Fluid pressure
2.3.5 Liquid flow
2.3.6 Liquid level
2.3.7 Temperature
2.3.7 Temperature
Bimetallic strips
Resistance temperature
detectors (RTDs)
Thermistors
Thermocouple
Law of intermediate temperature,
Et ,0  Et ,I  EI ,0
For example, a type E thermocouple, the following is data
from standard tables.
Temp.(oC) 0
e.m.f(mV) 0
20 200
1.192 13.419
What will be the thermoelectric e.m.f with a cold junction at
20oC?
E200,20  E200,0  E20,0  13.4191.192  12.227mV
2.3.8 Proximity
Microswitch, Magnetic reed switch, Inductive proximity
switch,
Eddy current proximity switch, Photoelectric proximity
switches.
2.4 Selection of sensors
1 Identify the nature of the measurement required.
2 Identify the nature of the output required from the sensor.
3 Identify possible sensors.
Homework: page 40, problems 4,5,8,13
2.4 Signal conditioning
2.4.1 Interfacing
Interface is the term used for the item that is used to
make connections between devices, and/or between
devices and ports.
The interface often contains signal conditioning and
protection.
2.4.2 Signal-conditioning processes
1 There are protections to prevent damage to the next
element as a result of high current or voltage.
2 Getting the signal into the right type of signal.
3 Getting the level of the signal right.
4 Eliminating or reducing noise.
5 Signal manipulation.
2.5 Digital signal and filtering
The term filtering is used to describe the process of removing a
certain band of frequencies from a signal and permitting others to
be transmitted.
Pass band. Stop band.
Cut-off frequency (boundary between stopping and passing)
Passive filter made up using only resistors, capacitors and
inductors.
Active filter is the filter which also involves an operational
amplifier.
Low-pass filters are very commonly used as part of signal
conditioning.
The cut-off frequency of low-pass filter is generally selected as
40Hz.
Digital signal
Analogue-to-digital conversion unit composes of a sample
and hold unit and an analogue to digital converter.
The resolution of an analogue-to-digital converter with the
word length of n is VFS / 2n
2.5.1 Analogue-to-digital converters
Aliasing—Under-sampling
Successive approximations method is the most commonly used method.
This involves selecting the most significant bit that is less than the
analogue value, then adding successive lesser bits for which the total
does not exceed the analogue value. The conversion time is n / f
Flash AD converter
2.5.2 Digital-to-analogue converters
A simple form of digital-to-analogue converter uses a
summing amplifier to form the weighted sum of all the
non-zero bits in the input word. It is called weightedresistor network.
A more commonly used digital-to-analogue converter uses
R-2R ladder network.
2.6 Modulation
conversion the d.c. signals to alternating signals can prevent the drift
of operational amplifier while it amplifies the low-level d.c. signals, and in
addition can assist in the elimination of external interference from the
signal.
Pulse amplitude modulation. (PAM) (for d.c. signal)
Pulse width modulation. (PWM) (for d.c. signal)
Amplitude modulation.(for a.c. signal)
Phase modulation. (for a.c. signal)
Frequency modulation. (for a.c. signal)
Homework: page66-67, problem 1, 2, 3, 6, 8, 9, 11