子信息工程(中英文对照)数据仓库-正确选择数据采集系统-学位论文.doc

Selecting the Right Data Acquisition System

Engineers often must monitor a handful of signals over extended periods of time, and then

graph and analyze the resulting data. The need to monitor, record and analyze 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 the various methods and devices you can use to acquire,

record and analyze data, from the simple pen-and-paper method to using today's

sophisticated data acquisition systems. It discusses the advantages and disadvantages of

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 user's real needs.)

2. Scanning duration: decided by user, fastest 32 points/second

3. Store capacity: 20G( may be increased or decreased)

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 sophisticated 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.

Today's college students are much more likely to use an automated process and analyze the

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

transcriber's level of fastidiousness 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

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

instrument's 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

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.

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 logger's built-in intelligence helps you set up the test

routine and specify the parameters of each channel. Once you have completed the setup,

data loggers can run as standalone devices, much like a recorder. They store data locally in

internal memory, which can accommodate 50,000 readings or more.

PC connectivity makes it easy to transfer data to your computer for in-depth analysis. Most

data loggers are designed for flexibility and simple configuration and operation, and many

provide the option of remote site operation via battery packs or other methods. Depending on

the A/D converter technique used, certain data loggers take readings at a relatively slow rate,

especially compared to many PC plug-in cards. Still, reading speeds of 250 readings/second

They are used in automated test applications for gathering data and for controlling and routing

signals in other parts of the test setup. Front end performance can be very high, with speed

and accuracy rivaling the best standalone instruments. Data acquisition front ends are

implemented in a number of formats, including VXI versions, such as the Agilent E1419A

multifunction measurement and control VXI module, and proprietary card cages.. Although

front-end cost has been decreasing, these systems can be fairly expensive, and unless you

require the high performance they provide, you may find their price to be prohibitive. On the

plus side, they do offer considerable flexibility and measurement capability.

Data Logger Applications

A good, low-cost data logger with moderate channel count (20 - 60 channels) and a relatively

• Battery testing

• Building and computer room monitoring

• Process monitoring, evaluation and troubleshooting No single data acquisition system works

for all applications. Answering the following questions may help you decide which will best

meet your needs:

1. Does the system match my application?

What is the measurement resolution, accuracy and noise performance? How fast does it scan?

What transducers and measurement functions are supported? Is it upgradeable or

expandable to meet future needs? How portable is it? Can it operate as a standalone

instrument?

application packages are supported?

Conclusion

Data acquisition can range from pencil, paper and a measuring device, to a highly

sophisticated system of hardware instrumentation and software analysis tools. The first step

for users contemplating the purchase of a data acquisition device or system is to determine

the tasks at hand and the desired output, and then select the type and scope of equipment