Accuracy, resolution, and measurement speed are some of the important factors that engineers must consider when deciding how to collect and measure data. Another important factor is the environment. The measurement is carried out in a plant with high electrical noise or in a laboratory with small electrical noise. In addition, the location of the sensor is far apart and difficult to access. In many applications, environmental factors can play a decisive role in the structure.
1. Sensitivity: the smallest change of the measured signal that can be detected;
2. Accuracy: the degree of agreement between the measured value and the primary standard;
3. Resolution: the smallest part of the observed signal;
4. Measurement speed: maximum sampling rate;
5. Bandwidth: the highest frequency signal component of the sampled signal;
6. Data storage requirements; the number of channels to be measured;
7. I/O number: analog and digital;
8. Trigger: timing control and switch control;
9. Anti-interference degree: normal noise suppression ratio and its analog suppression ratio;
10. Signal conditioning; isolation;
11. Network/bus protocol requirements such as Ethernet and IEEE-488 (GPIB);
12. Display; easy to set up and use;
13. Calculation and analysis of the data collected;
14. Size, weight and portability;
15. Power requirements; system integration issues;
16. System cost and cost per channel.
After considering these factors and indicating your needs, you can consider the correct system structure for your application.
Instrument structure selectionThere are four main types of test equipment structures:
1. Independent instrument. The most accurate and sensitive stand-alone instrument is a benchtop instrument. They are traditional instruments with many new and improved features such as graphic display, button selection, menu programming and more. Portable digital multimeters with self-contained power supplies for on-site measurements, but in general, they do not have the performance, sensitivity and precision of benchtop instruments.
2. Computer connected instruments. It is a subset of independent instruments. Use this type of instrument when the quantity of the quantity being measured exceeds the capacity of the stand-alone instrument, when a terminal display is required or if flexible software control is desired. Many instruments offer independent work capabilities and computer control modes for complex measurement and measurement systems. An external data communication bus that connects the instrument to the PC controller can use one of several standard protocols.
3. Distributed instruments. Currently available from some manufacturers. This type of test system consists of a number of separate instruments that are connected together via a communication network. This structure consists of a number of miniaturized instruments that can in principle be placed anywhere in the plant and transmit the fully processed signals to the computer via a communication network. Many instruments meet the requirements of laboratory measurement levels. Because they are located near the test signal, the cable induced noise is minimized, thus reducing measurement errors. The biggest advantage of the distributed instrument network layout is that it eliminates the cable connection from each test point to the PC, simplifying installation.
There can be a small local display near the instrument that can be used to read data and find faults. You can also rely solely on controlling the display on your computer. The data communication protocol used by distributed instruments is similar to the protocol used by computer-connected instruments.
4. PC-based test equipment. The main attributes here are the ability to measure speed and get a lot of data. There are two basic configurations. The most common is that the analog test signal is connected to a PC plug-in board that is located in a computer bus socket or on a PC parallel port. The other configuration consists of a number of boards housed in the chassis, and the chassis-mounted chassis is at a considerable distance from the PC. The chassis contains a measurement board multiplexer A/D converter board and a signal conditioning board to network the fully processed digital signal with the PC. The chassis system effectively scales the measurement system by a much larger number of channels than is available in a number of available board slots in a PC.
Check parameters
An important parameter that determines the sensitivity of a digital instrument is resolution and sensitivity, where sensitivity is equal to the range divided by the resolution. Therefore, for a particular instrument measurement range, the greater the resolution, the better the sensitivity.
The sampling rate (or measurement speed) is another parameter, but at the expense of sensitivity.
Also consider the expected instrument accuracy (the proximity of the measured value to the primary standard). Accuracy is expressed in several ways, depending on the particular instrument. However, representative is expressed in terms of percentage, PPM or number of digits. Bench-top stand-alone instruments provide the highest accuracy, sensitivity and resolution. They are suitable as a calibration source for passing standards.
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