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信息转换信息提取

Measured Medium

通信部分部分

Measured quantity

Primary sensing element

Variable conversion element

Variable manipulation element

Data presentation ,record, transmission element

Chapter 2 – Generalized Description of Measurement Systems

Ⅰ. Functional Elements


Basic components in a measurement system are shown below:

It is also important to mention that a power supply is an important element for the entire system.

Amplification and Conditioning


The General Measurement System (GMS)



a). Measured Medium( 被测对象 )

b). Measured Quantity ( 被测量 ) (P, T, Vel, Stress)

c). Primary Sensing Element ( 一次传感元件 ) : Element which receives ENERGY from the medium, producing a some

sort of output.

The medium will be PERTURBED (at least in principal)!

Ex: Heat transfer to / from a thermometer( 温度计 ) can change the medium temperature



I. Functional Elements (cont.)

d). Variable Conversion Element ( 信号变换部分 ): Converts primary variable sensed to some other variable.

Ex: current Voltage; Pressure Displacement; Strain Charge (Voltage)

e). Variable Manipulation Element ( 信号运算部分 ): Provides “gain” (amplifier, gear)

f). Data Transmission: (Wire, Fiber Optic, “Drive” shaft, Satellite)

h). Data Storage:(Computer, Storage Oscilloscope, Chart / Pen)

i). Data Presentation (CRT, oscilloscope, dial indicator)


Complete measurement system

Instrumentation is the science and technology of complete measurement systems with which physical quantities are measured so as to obtain data which can be transmitted to recording and display devices.


Example – Pressure Gauge( 压力计 )

Measured Medium / Quantity:

Primary Element:

Variable Conversion Elements:

Variable Manipulation (“Gain”)

Data Transmission:

Data Presentation:

Air Pressure

Piston( 活塞 )

Spring (F x)

Linkage( 连杆

机构 )

Piston Rod

Pointer/Scale

A.SZUDER 8

Exemple - Pressure gage

A.SZUDER 9

Exemple - Pressure thermometer


Transducers - The primary sensing element

The primary sensing element is that which first receives energy from the measured medium and produces an output depending in some way on the measured quantity (“measurand").

It is important to note that an instrument always extracts some energy from the measured medium. T

The measured quantity is always disturbed by the act of measure ment, which makes a perfect measurement theoretically impossible. Good instruments are designed to minimize this effect, but it is always present to some degree.


II. Transducers( 信号变换部分 )

A. Definition

“Any of varies substances or devices that convert input energy of one form into output energy of another.”

i.e. : mechanical energy mechanical energy

Ex: Linear Motion Rotational Motion


Transducers – cont.B. Types of Transducers

i. Passive( 无源 ): Output energy is supplied entirely (or almost entirely) from input.

(Examples: Thermometer( 温度计 ), barometer ( 气压计 ), thermocouple ( 热电偶 ), piezoelectric( 压电式传感器 ) )

ii. Active( 有源 ): Output energy is primarily supplied by auxiliary source.

Other Examples

Optical Diodes (light current

Thermistor( 热敏电阻 ) (T voltage)

Many others


III. Analogue / Digital Modes of Operation( 工作方式 )

Analogue: Continuous Variation in both “Signal” and Time Axes.

Ex: Analogue Oscilloscope, Strip Chart Recorder.

Digital: Signal is sampled in time (at finite frequency) and Voltage is “quantized” into “bits”

Example

Typical Resolution ranges between 8 bits (0 – 255) and 16 bits (0 – 65,546)

(10 – 12 is most common).


IV. Null( 零位法测量 )and Deflection Techniques( 偏移法测量 )

A. “Deflection Technique”: Input induces a measurable non-zero output.

( 从被测量中获得信号转换所需能量 Example: Pressure Rise of Hg Column (Manometer( 压力计 ))

Example: “Anything” which causes a motion

Example: Thermocouple (Temperature induces a voltage )

测量误差 )


Deflection instruments


IV. Null and Deflection TechniquesCont.

B. “Null” Technique:

Secondary input applied which “nulls” (drives to zero) the output (This can be VERY sensitive)

Examples

i. Double Pan Balance( 天平 )

ii. Wheatstone Bridge Circuit

( 不从信号源获得能量高精度测量 )


Null instruments


Wheatstone Bridge Null Circuit


（ 1 ）按测量手段：直接测量 (Direct measurement) 、间接测量(Indirect measurement)

（ 2 ）按测量值的获得方式：偏移法测量 (Deflection) 、零位法测量 (Null) 、差分式测量 (Differential)

（ 3 ）按传感器与被测对象是否直接接触：接触式测量、

非接触式测量（ 4 ）根据对象变化的特点：静态测量、动态测量

（ 5 ）根据闭环与否：开环测量与闭环测量

Summary of Methods of Measurement


开环测量：

反馈测量：

特点：简单、直观、明了；测量精度不高

特点：精度高；复杂、成本高、要求高

传感器输入量x

输出量 y

传感器输入量x

输出量 y放大

反响传感器


Considering of the requirement of measurement and the properties and characteristics of measured variables.

被测量的性质、特点和测量任务要求

Choices of Methods of Measurement

A.SZUDER 22

Measurement System

Conversion

A.SZUDER 23

The variable-conversion element

The output signal of the primary sensing element is some physical variable, such as displacement or voltage.

For the instrument to perform the desired function. it mav be necessary to convert this variable to another more suitable 'suitable while preserving the information content of the original signal.

An element that performs such a function is called a variable-conversion element.

Not every instrument includes a variable-conversion el ement, but some require several.

The “elements" are functional elements, not physical elements,

A.SZUDER 24

Conversion of resistance to voltage or current change

Use a bridge circuit

A.SZUDER 25

Signal Conditioning /Processing

The output of a transducer may have to be altered to make it suitable for transmission display recording

A.SZUDER 26

The variable-manipulation element An instrument may require that a signal represented by

some physical variable be manipulated in some way. Manipulation -- a change in numerical value according to

some definite rule but a preservation of the physical nature of the variable.

Thus an electronic amplifier accepts a small voltage signal as input and produces an output signal that is also a voltage but is some constant times the input. An element that performs such a function is called a variable-manipulation element,

A variable-manipulation element does net necessarily ,follow a variable-conversion element, but may precede it, appear elsewhere in the chain, or not appear at all.

A.SZUDER 27

Processes involved Signal level change ‑

amplification or reduction

Linearization Conversion of a

change in resistance to a variation in voltage or current

Filtering Impedance

matching A/D conversion

A.SZUDER 28

Signal conditioning

“Signal Conditioning” is the manipulation of the output of a sensor, probe, or transducer to perform one or more of these functions:•Amplification

•Attenuation•Filtering•Linearizing•Signal conversion

A.SZUDER 29

Signal Conditioning

Attenuation and Amplification

A.SZUDER 30

Signal Conditioning

To minimize input resolution error (quantization error), the input to the A/D converter should utilize the full range of the A/D converter (e.g. +/-10 volts).

If our input signal is in the range of +/-10 mV (a microphone signal), we would want to increase this signal to bring it into the range of our A/D… (amplification)

If our input is in the range of +/-100 V (power line signal), we would want to decrease this signal to bring it into the range of our A/D… (attentuation)

A.SZUDER 31

Signal Conditioning There are several ways that a signal can be

attenuated. We’ll talk about one of those tomorrow.

For a signal to be amplified, there needs to be an active source of power placed in the circuit. Such an active source might be an operational

amplifier (Op Amp).


V. Interfering and Modifying Inputs

In a perfect measurement system, the output is related to the input in a unique and time-invariant ( 时不变） manner.

Output = FD (Input) + “Baseline”( 附加值 )

(where FD is the “Instrument Response Function.)

If FD is known and if “Baseline” can be eliminated, then the detected output provides a unique measurement of the input.

BUT THIS NEVER HAPPENS

(All we can do is minimize “spurious”( 假性 ) inputs)


A. Types of Spurious Inputsi. Interfering: Input to which instrument is undesirably sensitive.

Ex: 60 Hz (or other frequency such as “RF”) electrical noise.

(RF stands for “Radio Frequency” - ~ MHz [FM Radio].

Ex: Strain Gauge

Rg Strain

60 Hz interferes with Wheatstone Bridge Measurement

Interfering Input Limits Ability to Remove Baseline!


Types of Spurious Inputs (cont.)ii. Modifying: Input which changes (modifies) FD (Inst Response Function).

We can use the Strain Gauge Example Again!

Some Modifying Inputsa. Temperature: Changes GF (and perhaps Ra, Rb. and Rc)

b. Eb: Drop in battery voltage

(MS 2.6)


B. Methods of Correction for Spurious Inputsi. Inherent Sensitivity: Attempt to minimize response to

undesired inputs

Ex: Thermal insulation; low thermal response; Electrical shielding, etc.

ii. Calculated Correction: In some cases the response of a system to a spurious input (in particular temperature) can be determined in advance. If T is then measured, a known correction can be applied.

iii. Filtering: In many cases the frequency of the undesired input is very different from that of the desired variable to be measured.

iv. Opposing Input Cancellation: It is often possible to design an instrument such that spurious inputs cancel one another.


Filtering

A filter is a frequency selective network that passes certain frequencies of an input signal and attenuates others

Mechanical filtering separates particles of varying size - e.g. sieving

Electrical filtering - size of particles frequency or duration

Remove unwanted variations Leave wanted variations


Example of Low Pass FilteringDesired signal has frequency components in the range DC - ~ 2 HzNoise is at 60 Hz.We need to “Pass” Signal and Filter Noise

MS Fig 2.14 e


Low Pass Filtering – Second Example

MS Fig 2.14 a


High Pass Filtering(Using “Amplitude Modulation” )

Noise (such as thermally induced “drift” in baseline), occurs at low frequency.

Signal is “chopped” (or “Amplitude Modulated”).

Output is High Pass Filtered – Removing Low Frequency Baseline Drift!

Fig 2.14 g

斩波器频率


Ex: Method of Opposing Inputs( 输入抵消法 )

Pitot Probe for Velocity

Measurement

(MS eqn 7.1)

(阻滞点 )


Pitot Tube – Method of Opposing Inputs

Chapter 7 – ME 4th Edition

A.SZUDER 42

The data-transmission element

When the functional elements of an instrument are actually physically separated, it becomes necessary to transmit the data from one to another.

An element performing this function is called a data-transmission element.

It may be as simple as a shaft and bearing assembly or as complicated as a telemetry system for transmitting signals from satellites to ground equipment by radio

A.SZUDER 43

The data-presentation element. If the information about the measured quantity is to be

communicated to a human being for monitoring, control, or analysis purposes, it must be put into a form recognizable by one of the human senses.

The element that performs this “translation" function - data-presentation element.

This function includes the simple indication of a pointer moving over a scale and the recording of a pen moving over a chart,

Indication and recording also may be performed in discrete increments (rather than smoothly),

While the majority of instruments communicate with people through the visual sense, the use of other senses such as hearing and touch is certainly conceivable.

A.SZUDER 44

Display Systems Analogue meter

Simplest system - pointer and scale. Use for at a glance readings Use for monitoring limits - use colours Use for tuning to maximum or minimum preset limits. Cheaper than digital - but less so all the time as

electronics become cheaper. The input signal may need to be amplified or reduced

to give a direct reading Can follow the direction of changes easily

A.SZUDER 45

Display Systems Digital meter

Best when numerical value required Compare the use of a vernier calliper with a similar

device with a digital display. Don't assume that resolution = accuracy The input signal may need to be amplified or

reduced to give a direct reading Note: Analogue and digital direct reading meters are

suitable only for signals varying in times > 0.1s - why?

A.SZUDER 46

Display and Recording Data

Decided by the purpose of the measurement. If the measurement is used by the operator to

check if the value is within specified bounds, a simple visual display is required.

If the measurement is to enable a value to be set at a specified value, a digital display is best.

A.SZUDER 47

For 100% inspection of components on a production line, data may be printed on printer displayed on a VDU stored in a computer plotted on a plotter recorded by a computer in data files.

Display and Recording Data

A.SZUDER 48

Meter reading

Range - minimum and maximum reading Resolution - smallest change in reading

which is detectable Sensitivity - how many volts required to

produce a given output

A.SZUDER 49

Meter characteristics Size and clarity of display Dynamic response - response time etc. Digital or analogue? Accuracy - may have linearity, hysteresis,

drift, temperature errors Cost

A.SZUDER 50

Measurement Systems

Data recording

A.SZUDER 51

The data recording

Although data storage in the form of pen/ink recording is often employed, some applications require a distinct data storage/playback function which can easily re-create the stored data upon command.

The magnetic tape recorder/reproducer is the classical example here. However, many recent instruments digitize the electric signals and store them in a computer like digital memory.

A.SZUDER 52

Data Recording

Chart recorder - plots a signal against time Various types

pen and paper pressure stylus and pressure sensitive paper UV recorder ultra‑violet light deflected by mirror

onto light sensitive paper. Fibre optic recorders ‑ fibre optics direct light from

a fluorescent screen onto light sensitive paper.

A.SZUDER 53

Data Recording

Performance of chart recorders The main difference in the speed at which they write -

defined in terms of frequency response

A.SZUDER 54

Data Recording Magnetic recording

The same process as is used for recording audio and video signals.

Standard audio-tape recording used to be used for data storage with computers (early BBCs, SPECTRUMs)

Can be used for very fast data storage ‑ up to 10MHz. The maximum frequency recordable is related to the

cost of the system. Can be used to store data at high speed and replay at

much lower speeds.

A.SZUDER 55

Computers for recording and display

The signal can be downloaded into a computer

It can be displayed and recorded in a variety of forms - very flexible

A.SZUDER 56

Hard Drive Cantilever

The read-write head is at the end of a cantilever. This control problem is a remarkable feat of engineering.

A.SZUDER 57

Hard Drives

A.SZUDER 58


Output may be recorded/displayed using VDU Plotter Printer Magnetic media ‑ e.g. floppy disks, hard disks,

tapes. Computer memory Optical media (CD ROM)

A.SZUDER 59


The maximum frequency signal which can be read directly into a computer is limited.

PCs have a maximum data acquisition frequency of about 100MHZ.

Bigger computers may be faster. Computer speeds are increasing all the

time.

Sensor network

Would like: simplified access

easy to configure & integrate in applications standardised interface

no more stovepipes for each new sensor! remote, in-situ, fixed, mobile

E.g. Air quality monitoring network

Images from http://www.airquality.co.uk

Example: air quality

O&M• Feature-of-interest• observed Property• Procedure• result

Air quality• Urban monitoring station• CO2

• FTIR• 450 ppm

Image courtesy http://www.atmosfera.unam.mx

Sensor Observation Service ‘Observation Offering’

collection of related sensor observations

Allows queries against O&M model GetCapabilities → service metadata (incl. Observation

offerings) DescribeSensor → sensor metadata (Procedure) GetObservation → Observation (collection) GetResult → result only GetFeatureOfInterest → observed Feature-of-interest

Sensor Observation Service GetObservation

offering eventTime: time period(s) of observations procedure: which sensor system(s) observedProperty: observed phenomenon featureOfInterest: observed feature result: for filtering results


Summary

Measurement and Measurement systems

Engineering Measurement and Laboratory measurement

New Measurement Systems

Course’s Objectives


Assignment( 课后作业）

Review Ch2 (P13-38)

Problem 2.1

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