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Data Acquisition System and Signal Processing

作者：maoandwei　栏目：测控之家

Data Acquisition SYSTEM and Signal Processing
Data Acquisition SYSTEM and Signal Processing
People often MONITOR a handful of signals over extended periods of time, and then graph , analyze and PROCESS the resulting data. The need to MONITOR, record , analyze and PROCESS data arises in a wide range of applications, including the design-verification stage of PRODUCT development, environmental chamber MONITORing, component inspection, benchtop testing and PROCESS trouble-shooting.
This application note describes and PROCESSes the various methods and DEVICEs you can use to acquire, record and analyze data, from the simple pen-and-paper method to using todays sophisticated data acquisition SYSTEMs. It discusses the advantages and disadvantages of each method and provides a list of questions that will guide you in selecting the approach that best suits your needs.
Introduction
In geotechnical ENGINEERING, we sometime encounter some difficulties such as mo-nitoring INSTRUMENTs distributed in a large area, dangerous environment of working site that cause some difficulty for easy access. In this case, operators may adopt remote CONTROL, by which a large amount of measured data will be transmitted to a observation room where the data are to be collected, stored and PROCESSed.
The automatic data acquisition CONTROL SYSTEM is able to complete the tasks as regular automatic data MONITORing, acquisition and store, featuring high AUTOMATION, large data store capacity and reliable performance.
The SYSTEM is composed of acquisition CONTROL SYSTEM and DISPLAY SYSTEM, with the following features:
1. No. of Channels: 32 ( can be increased or decreased according to users real needs.)
2. Scanning duration: decided by user, fastest 32 points/second
3. Store capacity: 20G( may be increased or decreased)
4. DISPLAY:(a)Table of parameter(b)History tendency(c)Column graphics.
5. Function: real time MONITORing CONTROL,warning
6. Overall dimension: 50cm×50cm×72cm
Data acquisition SYSTEMs,as the NAME implies,are PRODUCTs and/or PROCESSes used to collect information to document or analyze some phenomenon.In the simplest form, a technician logging the temperature of an oven on a piece of paper is performing data acquisition.As TECHNOLOGY has progressed,this type of PROCESS has been simplified and made more accurate,versatile,and reliable through electronic equipment.Equipment ranges from simple recorders to so- phisticated COMPUTER SYSTEMs.Data acquisition PRODUCTs serve as a focal point in a SYSTEM, tying together a wide variety of PRODUCTs, such as sensors that indicate temperature, flow, level, or pressure. Some common data acquistion terms are shown below:
Data acquisition TECHNOLOGY has taken giant leaps forward over the last 30 to 40 years. For example, 40 years ago, in a typical college lab, apparatus for tracking the temperature rise in a crucible of sodiumtungsten- bronze consisted of a thermocouple, a BRIDGE, a lookup table, a pad of paper and a pencil.
Pencil and paper
The old pencil and paper approach is still viable for some situations, and it is inexpensive, readily available, quick and easy to get started. All you need to do is hook up a DIGITAL multimeter (DMM) and begin recording data by hand.
Unfortunately,this method is error-prone,tends to be slow and requires extensive manual analysis.In addition, it works ONLY for a SINGLE channel of data;while you can use multiple DMMs,the SYSTEM will quickly becomes bulky and awkward.Accuracy is dependent on the transcribers level of fastidi-
ousness and you may need to scale input manually. For example, if the DMM is not set up to handle temperature sensors, manual scaling will be required. Taking these limitations into account, this is often an acceptable method when you need to perform a quick experiment.
Strip chart recorder
Modern versions of the venerable strip chart recorder allow you to capture data from several inputs. They provide a permanent paper record of the data, and because this data is in graphical format, they allow you to easily spot trends. Once set up, most recorders have sufficient internal intelligence to run unattended — without the aid of either an operator or a COMPUTER. Drawbacks include a lack of flexibility and relatively low accuracy, which is often constrained to a few percentage points. You can typically perceive ONLY small changes in the pen plots. While recorders perform well when MONITORing a few channels over a LONG period of time, their VALUE can be limited. For example, they are unable to turn another DEVICE on or off. Other concerns include pen and paper maintenance, paper supply and data storage, all of which translate into paper overuse and waste. Still, recorders are fairly easy to set up and operate, and offer a permanent record of the data for quick and simple analysis.
Scanning DIGITAL multimeter
Some benchtop DMMs offer an optional scanning capability. A slot in the rear of the INSTRUMENT accepts a scanner card that can multiplex between multiple inputs, with 8 to 10 channels of mux being fairly common. DMM accuracy and the functionality inherent in the INSTRUMENTs front panel are retained. Flexibility is limited in that it is not possible to expand beyond the NUMBER of channels available in the expansion slot. An external PC usually handles data acquisition and analysis.
PC plug-in cards
PC plug-in cards are SINGLE-board measurement SYSTEMs that take advantage of the ISA or PCI-bus expansion slots in a PC. They often have reading rates as high as 100,000 readings per second. Counts of 8 to 16 channels are common, and acquired data is stored directly into the COMPUTER, where it can then be analyzed. Because the card is essentially PART of the COMPUTER, it is easy to set up tests. PC cards also are relatively inexpensive, in PART, because they rely on the host PC to provide POWER, the MECHANICAL enclosure and the user interface.
Data acquisition options
In the downside,PC plug-in cards often have ONLY 12 bits of resolution, so you cant perceive small variations with the input signal.Furthermore, the electrical environment inside a PC tends to be noisy, with high-speed clocks and bus noise radiated throughout.Often, this electrical interference limits the accuracy of the PC plug-in card to that of a handheld DMM .These cards also measure a fairly limited range of dc voltage. To measure other input signals,such as ac voltage,temperature or resistance,you may need some sort of external signal conditioning.Additional concerns include problematic calibration and overall SYSTEM cost, especially if you need to purchase additional signal conditioning accessories or a PC to accommodate the cards. Taking that into consideration, PC plug-in cards offer an attractive approach to data acquisition if your requirements fall within the capabilities and limitations of the card.
Data loggers
Data loggers are typically stand-alone INSTRUMENTs that, once they are setup, can measure,record and DISPLAY data without operator or COMPUTER intervention. They can handle multiple inputs, in some instances up to 120 channels. Accuracy rivals that found in standalone bench DMMs, with performance in the 22-bit, 0.004-percent accuracy range. Some data loggers have the ability to scale measurements, check results against user-defined limits, and OUTPUT signals for CONTROL.
One advantage of using data loggers is their built-in signal conditioning. Most are able to directly measure a NUMBER of different inputs without the need for additional signal conditioning accessories. One channel could be MONITORing a thermocouple, another a resistive temperature DEVICE (RTD) and still another could be looking at voltage.
Thermocouple reference compensation for accurate temperature measurement is typically built into the multiplexer cards. A data logg

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