Analog and Digital Transducers
As mentioned previously, considerable experience has been accumulated with analog transducers, signal conditioning, A/D converters etc. , and it is natural that the majority of current systems tend to u the techniques. However, there are a number of measuring techniques that are esntially digit in nature, and which when ud as parate measuring instruments require some intrgral digtal circuitry, such as frequency counters and timing circuits, to provide an indicator output. This type of transducer, if coupled to a computer, does not necessarily require the same amount of equipment since much of the processing done by the integral circuitry could be programmed and by the computer.
Collins classifies the signals handled in control and instrumentation systems as follows:
volatile
(1) Analog, in which the parameter of the system to be measured although initially derived in an analog form by a nsor is converted to an electrical analog, either by design or inherent in the methods adopted;
(2) Coded-digital, in which a parallel digital sigal is generated, each bit radix-weighted according to some predetermined code. The are referred to in this bood as direct digital transducers;
(3) Digital, in which a function, such as mean rate of a repetitive signal, is a measure of the parameter being measured. The are subquently referred to as frequency-domain transducers.
Some analog transducers are particularly suited to conversion to digital outputs using special techniques. The most popular of the are synchros, and similar devices, which produce a modulated output of a carrier frequency. For ordinary analog u, this output has to be demodulated to provide a signal who magnitude and sign reprent any displacement of the transducer’s moving element. Although it is possible to u a conventional A/D technique to produce a digital output while providing a high accuracy and resolution, and at a faster rate than is possible in the A/D converter method.
Direct digital transducers are, in fact, few and far between, since there do not em to be any natural phenomena in which some detectable characteristic changes in discrete intervals as a result of a change of pressure, or change of tempreature etc.. The are many advantages in using direct digital transducers in ordinary instrumentation systems, even if computers are not ud in the complete installation.
The advantages are:
推广英文
(1)The ea of generating, manipulating and storing digital signals, as punched tape, magnetic tape etc. ;
(2)The need for high measurement accuracy and discrimination;
skyfall什么意思(3)The relative immunity of a high-level digital signal to external disturbances (noi);
(4)Ergonomic advantages in simplified data prentation(e. g. digital readout avoids interpretation errors in reading scales or graphs).
The most active development in direct digital transducers has been in shaft encoders, which are ud extensively in machine tools and aircraft systems. High resolution and accuracies can be obtained, and the devices may be mechanically coupled to provide a direct digital output of any parameter which gives ri to a measurable physical displacement. The usual displacement of the systems is that the inertia of the instrument and encoder often limit the speed of respon and therefore the operating frequencies.
Frequency domain transducers have a special part to play in online systems with only few variables to be measured, since the computer can act as part of an A/D conversion system and u its own re
gisters and clock for counting puls or measuring pul width. In designing such systems, consideration must be given to the computer time required to access and process the trransducer output.
Data Line Isolation Theory
baronWhen it comes to protect data lines from electrical transients, surge suppression is often the first thing that leaps to mind. The concept of surge suppression is intuitive and there are a large variety of devices on the market to choo from. Models are available to protect every-thing from your computer to answering machine as well as tho rial devices found in RS-232, RS-422 and RS-485 systems.
Unfortunately, in most rial communications systems,surge suppression is not the best choice. The result of most storm and inductively induced surges is to cau a difference in ground potential between points in a xommunications system. The more physical area covered by the system, the more likely tho differences in ground potential will exist.
The water analogy helps explain this. Instead of phenomenon water in a pipe, we’ll thi nk a little bigger and u waves on the ocean. Ask anyone what the elevation of the ocean is, and you will get
an answer of zero－so common that we call it a
level. While the average ocean elevation is zero, we know that tides and waves can cau large short-term changes in the actual height of the water. This is very similar to earth ground. The effect of a large amount of current dumped into the earth can be visualized in the same way, as a wave propa-gating outwards from the origin. Until this energy dissipates, the voltage level of the earth will vary greatly between two locations.
Adding a twist to the ocean analogy, what is the best way to protect a boat from high waves? We could lash the boat to a fixed dock, forcing the boat to remain at one elevation. This will protect against small waves, but this solution obviously has limitation. While a little rough, this comparison isn’t far off from what a typical surge suppressor is trying to accom-plish. Attempting to clamp a surge of energy to a level safe for the local equipment requires that the clamping device be able to completely absorb or redirect transient energy.
采用英文
Instead of lashing the boat to a fixed dock let the dock float. Now the boat can ri and fall with the ocean swells (until we hit the end of our floating dock’s posts).
委员会英文
Instead of fighting nature, we’re simply moving along with it. This is our data line isola-tion solution.
according是什么意思Isolation is not a new idea. It has always been implemented in telephone and Ethernet equipment. For asynchronous data applications such as many RS-232, RS-422 and RS-485 systems, optical isolators are the most common isolation elements. With isolation, two different grounds (better thought of as reference voltages) can exist on opposite sides of the isolation element without any current flowing through the element. With an optical isolator, this is performed with an LED and a photonsitive transistor. Only light pass between the two elements.
愿意的英文Another benefit of optical isolation is that it is not dependent on installation quality. Thpical surge suppressors ud in data line protection u special diodes to shunt excess energy to ground. The installer must provide an extremely low imprdance ground connection to handle this energy, which can be thousands of amps at frequencies into the tens of megahertz. A small impedance in the ground connection, such as in 1.8m (6ft) of 18 gauge wire, can cau a voltage drop of hundreds of volts －enough voltage to damage most equipments. Isolation, on the other hand, does not require an additional ground connection, making it innsitive to installation quality.
Isolation is not a perfect solution. An additional isolated power supply is required to support the circuitry. This supply may be built in as an isolated DC-DC converter
or external. Simple surge suppressors require no power source. Isolation voltages are limited as well, usually ranging from 500V to 4000V. In some cas, applying both surge suppression and isolation is an effective solution.
When choosing data line protection for a system it is important to consider all available options. There are pros and cons to both surge suppression and optical isolation, however isolation is a more effective solution for most systems. If in doubt, choo isolation.
模拟与数字转换器
前面我们已经提到，人们在模拟转换器、信号调节器和A/D 转换器等的使用上已经积累了大量的经验。因此，目前大部分的系统自然都采用这些方法时，需要用到一些积分电路，如频率计数和计时电路等来提供指示输出。另外，如果把这种转换器和电脑相连的话，就可以省去一些器材；因为很多由积分电路执行的工作可以由计算机程序代为执行。avena
柯林斯把在控制和测量系统中处理的信号分为以下几类：
（1）模拟式。尽管系统的被测量参数最初通过传感器得到是模拟信号，然后通过设计或采用原有的方法将模拟形式的信号转换成电模拟信号。
（2）数字码式。产生的信号是并行的数字信号，每一位的基数权重由预先编定的号码系统决定。在本书中这些仪器称作直接数字转换器。
（3）数字式。其中的函数测量参数时用到的量度标准，如对重复信号取平均值。这些仪器在后来称为频域转换器。
特别地，一些模拟转换器适合用一些特别的技术来把模拟量转化成数字输出。其中最通用的方法是同步法和相似仪器的方法，即产生载波频率的调制输出的方法。在用作普通的模拟量输出仪器时，输出量必须经过解调。解调后输出的是直流信号，直流信号的大小和方向描述了转换器运动元件的偏移。虽然使用传统的A/D转换技术可以用来产生数字信号，在提供高精度时采用这些新技术将同步输出直接变为数字输出，比用A/D转换方法更快。aeroplane什么意思
直接数字转换器实际上用得很少，因为在自然现象中很少有那种由温度变化、压力变化等因素作用而产生的可测量的离散的变化量。在普通的仪器系统中使用直接数字转换器有如下优点（即使在完成安装时不使用计算机）：（1）容易产生、处理和存储信号，如打孔带、磁带等；
（2）高精度和高分辨率的需要；
（3）高阶数字信号对外部噪声的抗干扰性；
（4）在简化数据描述时的人机工程学优势（例如，数字读出器能避免读刻度或图表时的判读错误）。
在直接数字转换器中最能起作用的发展是轴编码器。轴编码器在机床和飞行系统中被广泛应用。利用这些设备能达到很高的精度和分辨率，而且这些设备能进行机动连接，给出任何可测量物理偏移的直接数字输出。这类系统通常和缺点是仪器的惯性及编码器限制了相应的速度，因而也限制了操作频率。
频域转换器在线系统（测量量较少时）有着特殊的地位。因为计算机能担当A/D转换系统的部分工作，能用它自己的寄存器和时钟来计算脉冲宽度。在这种系统和设计中必须考虑到计算机存取和处理转换器输出所需的时间。
