测控技术专业英语部分课文翻译.doc

Unit 1 Digital Signal Processing(DSP)Having heard a lot about digital signal processing (DSP) technology, you may have wanted to find out what can be done with DSP, investigate why DSP is preferred to analog circuitry for many types of operations, and discover how to learn enough to design your own DSP system. This article, the first of a series, is an opportunity to take a substantial first step towards finding answers to your question. This series is an introduction to DSP topics from the point of view of analog system designers seeking additional tools for handling analog signals. Designers reading this series can learn about the possibilities of DSP to deal with analog signals and where to find additional sources of information and assistance.1.1 What Is DSP ?In brief, DSPs are processors or microcomputers whose hardware, software, and instruction sets are optimized for high-speed numeric processing applicationsan essential for processing digital data representing analog signal in real time. What a DSP does is straightforward. When acting as a digital filter, for example, the DSP receives digital values based on samples of a signal, calculates the result of a filter function operating on these values, and provides digital values that present the filter output; it can also provide system control signal based on properties of this values. The DSPs high-speed arithmetic and logical hardware is programmed to rapidly execute algorithms modeling the filter transformation.The combination of design elements arithmetic operators, memory handling, instruction set, parallelism, data addressing that provide this ability forms the key difference between real-time signal and DSP calculation speed provides some background on just how special this combination is. The real-time signal comes to the DSP as a train of individual samples from an analog-to-digital converter (ADC). To do filtering in real-time, the DSP must complete all the calculations and operations required for processing each sample(usually updating a process involving many previous sample) before the next sample arrives. To perform high-order filtering of real-world signals having significant frequency content calls for really fast processors.1.2 Why use a DSP ?To get an idea of the type of calculations a DSP does and get an idea of how an analog circuit compares with a DSP system, one could compare the two systems in terms of a filter function. The familiar analog filter uses resistors, capacitors, inductors, amplifiers. It can be cheap and easy to assemble, but difficult to calibrate, modify, and maintain a difficulty that increases exponentially with filter order. For many purposes, one can more easily design, modify, and depend on filter using a DSP because the filter function on the DSP is software-based, flexible, and repeatable. Further, to create flexibly adjustable filters with higher-order response requires only software modifications, with no additional hardware unlike purely analog circuit.1.3 Sampling Real-World SignalsReal-world phenomena are analog the continuously changing energy levels of physical processes like sound, light, heat, electricity, magnetism. A transducer converts these levels into manageable electrical voltage and current signal, and an ADC samples and converts these signals to digital for processing. The conversion rate, or sampling frequency, of the ADC is critically important in digital processing of real-world signals.This sampling rate is determined by the amount of signal information that is needed for processing the signals adequately for a given application. In order for an ADC to provide enough samples to accurately describe the real-world signal, the sampling rate must be at least twice the highest-frequency component of the analog signal. For example, to accurately describe an audio signal containing frequencies up to 20 kHz, the ADC must sample the signal at a minimum of 40 kHz. Since arriving signal can easily contain component frequencies above 20 kHz (including noise), they must be removed before sampling by feeding the signal though a low-pass filter ahead of the ADC. This filter, known as an anti-aliasing filter, is intended to remove the frequencies above 20 kHz that could corrupt the converted signal.However, the anti-aliasing filter has a finite frequency roll off, so additional bandwidth must be provided for the filters transition band. For example, with an input signal bandwidth of 20 kHz, one might allow 2 to 4 kHz of extra bandwidth.1.4 Processing Real-World SignalsThe ADC sampling rate depends on the bandwidth of the analog signal being sampled. This sampling rate sets the pace at which samples are available for processing. Once the system bandwidth requirements have established the A/D converter sampling rate, the designer can begin to explore the speed requirements of the DSP processor.Processing speed at a required sample rate is influenced by algorithm complexity. As a rule, the DSP needs to finish all operations relation to the first sample before receiving the second sample. The time between samples is the time budget for the DSP to perform all processing tasks, For the audio example, a 48 kHz sampling rate corresponds to a 20.833 us sampling interval.Next consider the relation between the speed of the DSP and complexity of the algorithm (the software containing the transform or other set of numeric operations). Complex algorithms require more processing tasks. Because the time between samples is fixed, the higher complexity calls for faster processing. For example, suppose that the algorithm requires 50 processing operations to be performed between samples. Using the previous examples 48 kHz sampling rate (20.833 us sampling interval), one can calculate the minimum required DSP processor speed, in millions of operations per second (MOPS) as follows:DSP Speed=Operations/Sampling Interval=50/20.833 us=2.4 MOPSThus if all of the time between sample is available for operations to implement the algorithm, a processor with a performance level of 2.4 MOPS is required. Note that the two common rating for DSPs, based on operations per second (MOPS) and instructions per second (MIPS) , are not the same. A processor with a 10 MIPS rating that can perform 8 operations per instruction has basically the same performance as a faster processor with a 40 MIPS rating that can only perform 2 operations per instruction.Unit 2 Temperature MeasurementTemperature is the degree of hotness of one body, substance, or medium compared with another. When measuring temperature, we usually compare this degree of hotness to a fixed reference point, using temperature scales. The Kelvin thermodynamic scale uses absolute zero as its reference point. The Celsius (which used to be called centigrade) scale uses a point of reference based on the freezing-point of water (0 ) and the boiling point of water (100 ).Temperature measurement is important because, at different temperatures, substances have different physical properties and behave in different ways. For example, the temperature of a substance will affect its electrical properties, whether it is solid, liquid, or a gas, and it will also affect its volume. Small changes in body temperature can show whether a person or animal is ill.There are many different types of thermometer. The main types of thermometer we shall look at are ones which measure temperature by means of:l Liquid expansion.l Metal expansion.l Electrical resistance.l Thermoelectricity.l Heat radiation.Proximity of field lines: the force is stronger where the lines are closer together.2.1 Thermocouples are probably the most widely used and least understood of all temperature measuring devices. Thermocouples provide a simple and efficient means of temperature measurement by generating a voltage that is a function of temperature. All electrically conducting materials produce a thermal electromotive force (emf or voltage difference) as a unction of the temperature gradients within the material. This is called the Seebeck effect. The amount of the Seebeck effect depends on the chemical composition of the material used in the thermocouple. When two different materials are connected to create a T, a voltage is generated. This voltage is the difference of the voltage generated by the two materials. In principle, a thermocouple can be made from almost any two metals. In practice, several thermocouple types have become de facto standards because they possess desirable qualities, such as highly predictable output voltage and large voltage-to-temperature ratios. Some common thermocouple types are J, K, T, E, N28, N14, S, R, and B. In theory, the temperature can be inferred from such a voltage by consulting standard tables or using linearization algorithms. In practice, this voltage cannot be directly used, because the connection of the thermocouple wires to the measurement device constitutes a thermocouple providing another thermal emf that must be compensated for; cold junction compensation can be used for this purpose.2.1.1 Cold Junction Compensation.Maintaining an ice bath and an additional reference thermocouple for every thermocouple probe is not practical in most systems. If we know the temperature at the point where we connect to our thermocouple wires, (the reference junction), and if both connecting wires are at the same temperature, then the ice bath can be eliminated. In such a case, the opposing thermoelectric voltage generated by the nonzero temperature of the reference junction can simple be added to the thermocouple emf. This is cold junction compensation (CJC).CJC is an essential part of accurate thermocouple readings. CJC must be implemented in any system that has no ice-point reference junction. The technique works best if the CJC device is close to the terminal blocks that connect the external thermocouple, and if there are no temperature gradients in the region containing the CJC and terminals.2.1.2 Thermocouple LinearizationThermocouple voltage is proportional to, but not linearly proportional to, the temperature at the thermocouple connection. There are several techniques for thermocouple linearization. Analog techniques can provide a voltage proportional to temperature form the thermocouple input. In addition, a voltage measurement can be made with an ADC and the temperature looked up in a table, as in the above example. To speed this process, the look-up table can be stored in computer memory and the search performed with an algorithm. Thermocouple linearization can also be accomplished using a polynomial approximation to the temperature versus voltage curve.2.2 Additional ConcernsCare must be taken when using thermocouples to measure temperature. Sources of minor error can add up to highly inaccurate readings. Additional concerns include:2.2.1 Thermocouple AssemblyThermocouple are assembled via twisting wires together, soldering, or welding; if done improperly, all of these techniques can introduce errors in the temperature measurement.2.2.2 Twisting Wire TogetherThermocouple junctions should not be formed by twisting the wire together. This will produce a very poor thermocouple junction with large errors.2.2.3 SolderingFor low temperature work the thermocouple wires can be joined by soldering, however, soldered junction limit the maximum temperature that can be measured (usually less than 200 degree Celsius). Soldering thermocouple wires introduces a third metal. This should not introduce any appreciable error as long as both sides of the junction are the same temperature.2.2.4 WeldingWelding is the preferred method of connecting junctions. When welding thermocouple wires together, care must be taken to prevent any of the characteristic of the wire from charging as a result of the welding process. These concerns are complicated by the different composition of the wires being joined. Commercially manufactured thermocouples are typically welded using a capacitance-discharge technique that ensures uniformity.2.2.5 Decalibration of Thermocouple WireDecalibration is another serious fault condition. Decalibration is particularly troublesome because it can result in an erroneous temperature reading that appears to be correct. Decalibration occurs when the physical makeup of the thermocouple wire is altered in such a way that the wire no longer meets NIST specifications. This can occur for a variety of reasons including, temperature extremes, coldworking of the metal, stress placed on the cable during installation, vibration, or temperature gradients.2.2.6 Insulation Resistance FailureTemperature extremes can also introduce error because the insulation resistance of the thermocouple will often decrease exponentially as the temperature increases. This can lead to two types of errors: leakage resistance with an open thermocouple and leakage resistance with small thermocouple wire.2.2.7 Leakage Resistance with an Open ThermocoupleIn high temperature applications, the insulation resistance can degrade to the point where the leakage resistance R will complete the circuit and give an erroneous reading.2.2.8 Leakage Resistance with Small Thermocouple WireIn high temperature applications using small thermocouple wire, the insulation R can degrade to the point where a virtual junction T1 is created and the circuit output voltage will be proportional to T1 instead of T2. Furthermore, high temperatures can cause impurities and chemicals within the thermocouple wire insulation to diffuse into the thermocouple metal, changing the characteristic of the thermocouple wire. This causes the temperature-voltage dependence to device from published values. When thermocouples are used at high temperatures, the atmospheric effects can be minimized by choosing the proper protective insulation. Due to the range of thermocouple choices, thermocouple quality is an issue. Select the thermocouple that meets your application criteria.Unit 3 Serial Communication3.1 Asynchronous CommunicationMost PC serial devices such as mice, keyboards and modems are asynchronous. Asynchronous communication requires nothing more than a transmitter, a receiver and a wire. It is thus the simplest of serial communication protocols, and the least expensive to implement. As the name implies, asynchronous communication is performed between two (or more) device which operate on independence clocks. Therefore, even if the two clocks agree for a time, there is no guarantee that they will continue to agree over extended periods, and thus there is no guarantee that when point A begins transmitting, point B will begin receiving, or that Point B will continue to sample at the rate Point A transmits. See the diagram below for an illustration of what happens when transmission clocks differ significantly.To combat this timing problem, asynchronous communication requires additional bits to be added around actual data in order to maintain signal integrity. Asynchronously transmitted data is preceded with a start bit which indicates to the receiver that a word (a chunk of data broken up into individual bits) is about to begin. To avoid confusion with other bits, the start bit is twice the size of any other bit in the transmission. The end of a word is followed by a stop bit, which tells the receiver that the word has come to an end, that is should begin looking for the next start bit, and that any bits it receives before getting the start bit should be ignored. To ensure data integrity, a parity bit is often added between the last bit of data and the stop bit. The parity bit makes sure that the data received is composed of the same number of bits in the same order in which they were sent.3.2 Upgraded UARTs For Increased PerformanceAt the heart of every asynchronous serial system is the Universal Asynchronous Receiver/Transmitter or UART. The UART is responsible for implementing the asynchronous communication process described above as both a transmitter and a receiver (both encoding and decoding data frames). The UART not only control the transfer of data, but the speed at which communication takes place. However, the first UARTs could only handle one byte of information at a time, which meant that the computer needed to immediately process any transmission or risk losing data as the next byte of information pushed its way onto the UART. Not only does this make for unreliable and slow communication, it can slow down the entire system.Improved UARTs, such as the 16750 UARTs, increase communication speed and lower system overhead by offering 64-byte FIFOs (first in first out buffers). With the 64-byte FIFO buffer, the UART can store enough information that the data stream need not be suspended while the computer is busy. This is particularly helpful in heavy multi-tasking operating systems such as Windows 95/98/Me/NT/2000 and OS/2.3.3 Synchronous CommunicationAs its name implies, synchronous communication takes place between a transmitter and a receiver operating on synchronized clocks. In a synchronous system, the communication partners have a short conversation before data exchange begins. In this conversation, they align their clocks and agree upon the parameters of the data transfer, including the time interval between bits of data. Any data that falls outside these parameters will be assumed to be either in error or a placeholder used to maintain synchronization. (Synchronous lines must remain constantly active in order to maintain synchronization, thus the need for placeholders between valid data.) Once each side knows what to expect of the other, and knows how to indicate to the other whether what was expected was received, then communication of any length can commence.The theory behind asynchronous and synchronous communication is essentially the same: Point B nee