Measurement Systems 2(测量系统)

2.3 Analog and digital modes of operation
Discuss: Why digital mode can reject noise better than analog mode? 讨论：为什么数字模式比模拟模式有更好 的噪声抑制能力？
2.3 Analog and digital modes of operation
Explanations:
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. An element that performs this “translation” function is called a data-presentation element.
Chapter 2
Generalized Configurations and Functional Descriptions of Measuring Instruments
Lecturer: Hou-Neng Wang
Chapter 2
Generalized Configurations and Functional Descriptions of Measuring Instruments 测量仪器的通用结构和功能描述
Explanations:
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.
Explanations:
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 may be necessary to convert this variable to another more suitable variable while preserving the information content of the original signal. It should be noted that not every instrument includes a variable-conversion element, but some require several.
Example1 pressure gage（压力表）
Example1 pressure gage
Explanations:
The primary sensing element is the piston, which also serves the function of variable conversion since it converts the fluid pressure to a resultant force on the piston face. Force is transmitted by the piston rod to the spring, which converts force to a proportional displacement.
2.4 Null and deflection methods
Another useful classification separates devices by their operation on null or a deflection principle.
Pressure thermometer
2.2 Active and passive transducers
A component whose output energy is supplied entirely or almost entirely by its input signal is commonly called a Passive transducers. 无源变送器：是指该器件的所有输 出能量来完全自于输入信号。
Nomenclatures（术语）
Calibrated scale 标定刻度 Field effect transistor 场效应管 Auxiliary power 辅助电源 Servomechanism 伺服机构(系统), 自动控制装 置 High-gain feedback 高增益反馈 Plunger 活塞
Explanations:
This displacement if the piston rod is magnified by the linkage to give a larger pointer displacement. The pointer and scale indicate the pressure, thus serving as datapresentation elements.
For analog signals, the precise value of the quantity (voltage, rotation angle, etc.) carrying the information is significant.
However, digital signals are basically of a binary (on/off) nature, and variations in numerical value are associated with the changes in the logical state ("true/false") of some combination of "switches".
Physical objects
abstract
Common theory
Practice in reality
Explanations:
The primary sensing element is that which first receives energy from the measured medium and produces an output depending in some way on the measures quantity. It is important to note that an instrument always extracts some energy from the measures medium. 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 loading effect”, but it is always present to some degree.
2.1 Functional Elements of an Instrument 仪表功能模块
A general method in scientific research: abstract common character, then a model, which does not rely on any real physical parameters, can illustrate the object
有源变送器：工作时需要外加电源,输入信 号仅提供非常微小的能量输入。
2.3 Analog and digital modes of operation
It is possible further to classify how the basic functions may be performed by turning attention to the analog or digital nature of the signals that represent the information.
Explanations:
By “manipulation”, we mean specifically 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.
Thus whether the voltage is 3 or 4V is of no consequence. The same result is produced, and so the system is quite tolerant of spurious "noise" voltages which might contaminate the information signals.