Key Factors in the Selection of Testing and Measuring Systems and Instruments
It is understandable that it is equally difficult for first-time users and experienced users to choose from the range of available testing and measurement hardware and software.
The progress of technology has accelerated the development of measurement and testing methods in an exponential manner, providing users with unimaginable powerful system functions.
Testing and measuring equipment span a wide range. For simple applications, it is relatively easy to select an appropriate structure to meet the needs and budget. As the complexity of applications increases, the selection of structures and related costs become more complex, and the wrong choice will be made, which will be more expensive. The right choice is more important.
The starting point is to know the level of signal or sensor output and the sensitivity required for measurement (defined as the minimum change that can be detected during measurement and expressed in units of measured value).
Accuracy, resolution, and measurement speed are important factors that engineers must consider when deciding how to collect and measure data. Another important factor is the environment. Whether the measurement is carried out in the factory with high electrical noise or in the laboratory with low electrical noise. In addition, the position of the sensor is far away and difficult to approach. In many applications, environmental factors may play a decisive role in the structure.
- Sensitivity: the minimum change of the measured signal that can be detected;
- Precision: the consistency between the measured value and the primary standard;
- Resolution: the smallest part of the observed signal;
- Measuring speed: maximum sampling rate;
- Bandwidth: the highest frequency signal component of the sampled signal;
- Data storage requirements; Number of channels to be measured;
- I/O number: analog and digital;
- Trigger: timing control and switch control;
- Immunity: normal noise rejection ratio and its analog rejection ratio;
- Signal regulation; Isolation;
- Network/bus protocol requirements, such as Ethernet and IEEE-488 (GPIB);
- Display; Easy to set and use;
- Calculation and analysis of collected data;
- Size, weight, and portability;
- Power demand; System integration problems;
- System cost and cost of each channel.
Selection of instrument structure
There are four main types of test equipment structures:
1. Independent instrument. It is a traditional instrument with many new and improved features, such as graphic display, key selection function, menu programming, etc. The portable digital multimeter with its own power supply is used for on-site measurement, but generally speaking, it does not have the performance, sensitivity, and accuracy of a desktop instrument.
2. Computer-connected instruments. It is a subset of independent instruments. When the quantity or type of measurement exceeds the capacity of independent instruments, a terminal display is required or flexible software control is desired, such instruments are used. Many instruments provide the ability to work independently, but also provide computer control mode for complex measurement and measurement systems. The external data communication bus connecting the instrument and PC controller can use one of several standard protocols.
3. Distributed instruments. It is currently provided by some manufacturers. This type of test system includes some independent instruments, which are connected together through the communication network. This structure is composed of some miniaturized instruments, which can be placed anywhere in the plant in principle, and the processed signals can be transmitted to the computer through the communication network. Many instruments meet the requirements of laboratory measurement grade. Because they are located near the test signal, the cable-induced noise is minimized, thus reducing the measurement error. The biggest advantage of the distributed instrument network layout is that it eliminates the cable connection from each test point to the PC and simplifies the installation.
There can be a small local display near the instrument, which can be used to read data and find faults. You can also rely on controlling the monitor on your computer. The data communication protocol used by distributed instruments is similar to that used by computer-connected instruments.
4. PC-based test instrument. The most important attributes here are the ability to measure speed and obtain large amounts of data. There are two basic configurations. The most common is that the analog test signal is connected to a PC plug-in board, which is located on a computer bus slot or a PC parallel port. The other configuration is composed of many boards installed in the chassis. The chassis installed in the rack is quite far from the PC The chassis contains the measurement board, multiplexer board, A/D conversion board, signal conditioning board, etc., which connects the processed digital signals with the PC The chassis system effectively expands the scale of the measurement system. The number of channels is much larger than the number of available board slots provided in the PC.
Check parametersOne of the important parameters determining the sensitivity of a digital instrument is resolution and sensitivity, where sensitivity is equal to the range divided by resolution. Therefore, for a specific instrument measuring range, the greater the resolution, the better the sensitivity.
The sampling rate (or measurement speed) is another parameter, but this is at the expense of sensitivity.
The expected instrument accuracy (the closeness of the measured value to the primary standard) shall also be considered. Accuracy is expressed in several ways, depending on the particular instrument. But it is represented by a percentage, PPM, or digit. Desktop-independent instruments provide the highest accuracy, sensitivity, and resolution. They are suitable as calibration sources for transferring standards.
For each basic type of instrument, some parameters discussed above.
Adaptability and noise suppression
Various additional signal conditioning amplifiers can be used to improve the signal level and input to PC-based test systems so that you can use this type of system in the case of low-level signals (such as signals from the low couple, strain gauge, etc.). When deciding what kind of hardware to use, it is often necessary to make some tradeoffs between the various attributes required.
These indicators reflect the highest or best value that each parameter of this type of instrument can usually reach, and step by step all these values can be reached at the same time.
Some measuring instruments have built-in soap sound suppression devices. They use some internal techniques, such as filtering, integration, and current inversion to reduce noise. However, there is no built-in noise suppression device in the PC-based insertion loss and PC-based external chassis systems. An alternative approach is based on PC test systems using averaging techniques provided by the software to reduce noise from measurement and/or signal conditioning add-ons such as filters. In short, the inherent disadvantage of PC is the bus with electrical noise, which limits the use of PC plug-in board for sensitive measurement.