信息转换

1. Chapter 2 – Generalized Description of Measurement Systems Ⅰ. Functional Elements Variable conversion element Variable manipulation element Data presentation ,record, transmission element Measured Medium 信息提取 信息转换 部分 通信部分 Primary sensing element Measured quantity cary_wang@sohu.com

2. Basic components in a measurement system are shown below: Amplification and Conditioning It is also important to mention that a power supply is an important element for the entire system. cary_wang@sohu.com

3. The General Measurement System (GMS) cary_wang@sohu.com

4. Chapter 2 – Generalized Description of Measurement Systems 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 cary_wang@sohu.com

5. Chapter 2 – Generalized Description of Measurement Systems • 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) cary_wang@sohu.com

6. 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. cary_wang@sohu.com

7. 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 cary_wang@sohu.com

8. Exemple - Pressure gage A.SZUDER

9. Exemple - Pressure thermometer A.SZUDER

10. Transducers - The primary sensing element • The primary sensing elementis 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 measure­ment, which makes a perfect measurement theoretically impossible. Good instruments are designed to minimize this effect, but it is always present to some degree. cary_wang@sohu.com

11. II. Transducers(信号变换部分) • 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 cary_wang@sohu.com

12. Transducers – cont. • 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 cary_wang@sohu.com

13. 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). cary_wang@sohu.com

14. 测量误差) IV. Null(零位法测量)and Deflection Techniques(偏移法测量) • “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 ) cary_wang@sohu.com

15. Deflection instruments cary_wang@sohu.com

16. 高精度测量) Null and Deflection TechniquesCont. • “Null” Technique: • Secondary input applied which “nulls” (drives to zero) the output (This can be VERY sensitive) (不从信号源获得能量 • Examples • Double Pan Balance(天平) • Wheatstone Bridge Circuit cary_wang@sohu.com

17. Null instruments cary_wang@sohu.com

18. Wheatstone Bridge Null Circuit cary_wang@sohu.com

19. Summary of Methods of Measurement （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 cary_wang@sohu.com

20. 传感器 输入量x 输出量y 传感器 放大 输入量x 输出量y 反响传感器 开环测量： 特点：简单、直观、明了； 测量精度不高 反馈测量： 特点：精度高；复杂、成本高、要求高 cary_wang@sohu.com

21. Choices of Methods of Measurement 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 cary_wang@sohu.com

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 el­ement, 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. 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 Processes involved 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). A.SZUDER

32. 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) cary_wang@sohu.com

33. A. Types of Spurious Inputs • 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! cary_wang@sohu.com

34. Types of Spurious Inputs (cont.) • Modifying: Input which changes (modifies) FD (Inst Response Function). • We can use the Strain Gauge Example Again! (MS 2.6) • Some Modifying Inputs • Temperature: Changes GF (and perhaps Ra, Rb. and Rc) • Eb: Drop in battery voltage cary_wang@sohu.com

35. B. Methods of Correction for Spurious Inputs • Inherent Sensitivity: Attempt to minimize response to undesired inputs • Ex: Thermal insulation; low thermal response; Electrical shielding, etc. • 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. • Filtering: In many cases the frequency of the undesired input is very different from that of the desired variable to be measured. • Opposing Input Cancellation: It is often possible to design an instrument such that spurious inputs cancel one another. cary_wang@sohu.com

36. 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 cary_wang@sohu.com

37. Example of Low Pass Filtering Desired signal has frequency components in the range DC - ~ 2 Hz Noise is at 60 Hz. We need to “Pass” Signal and Filter Noise MS Fig 2.14 e cary_wang@sohu.com

38. Low Pass Filtering – Second Example MS Fig 2.14 a cary_wang@sohu.com

39. 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 cary_wang@sohu.com

40. Ex: Method of Opposing Inputs(输入抵消法) Pitot Probe for Velocity Measurement (阻滞点) (MS eqn 7.1) cary_wang@sohu.com

41. Pitot Tube – Method of Opposing Inputs Chapter 7 – ME 4th Edition cary_wang@sohu.com

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 ahuman 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. Display and Recording Data • 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. 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