Analysis of key points of data collection technology (IV)

"The challenge of the measurement and control system lies not only in greater data throughput, but also in low power consumption, stability and deployability of the system..."

—— "Global Data Collection Technology Development Trend Report"

Recently, a concept is quietly changing.

That is, engineers' demand for the stability of data acquisition systems has gradually reached the highest peak. Looking at the entire test and measurement industry, although equipment cost is still one of the factors to be considered, we can see that the overall reliability of the system, long-term working stability and adaptability to harsh environments have gradually become key indicators for system selection and decision-making; basically, for data acquisition systems with a large number of channels, there are almost no low-end data acquisition equipment manufacturers.

This phenomenon is not groundless. First, with the development of technology, the sampling rate and number of channels of data acquisition equipment have been greatly improved, and the pressure brought to the whole system by this improvement is not linear but exponential, so the error rate of the whole system will also increase exponentially (imagine the difference between wiring 8 channels and 80 channels), and the low-probability events that could be ignored before are now almost inevitable; secondly, the working environment of the data acquisition system is much more complicated than before. The previous systems were basically static and independent and placed in the cabinets in the laboratory, connected to various sensors on the object under test (the real test site) through long cables; and now with the distributed architecture and the wiring principle of placing the data acquisition equipment as close to the sensor as possible, the data acquisition system will be placed in an unmanned outdoor area (such as tropical rainforest environmental monitoring) or work directly at the test point where the object under test is located (such as an aircraft engine test bench that produces high temperature and corrosive gases); finally, engineers have also been updated in their awareness of the value of data. In an extremely expensive test (such as a destructive collision safety test of an automobile), if there are some problems (unnecessary delays or leaks) in the data acquisition system, it is very likely to cause the complete failure of this test.

Although the above three current situations are caused by the continuous development of the application field of data acquisition systems, which is a good thing, ironically, due to the two-sided nature of things, this in turn poses a greater challenge to engineers. Therefore, in response to this development trend, a series of requirements for the stability of the new generation of data acquisition systems are being integrated invisibly; referring to the use of "ultra-clear" in the television industry to represent the next stage of development of "high definition", here we will temporarily name this ideal system as the "ultra-stable system".

Although there is no clear standard yet, we can be sure that the following factors will be the main considerations for achieving an "extremely stable system":

Protective

The various hardware devices that carry out the actual data collection work are the core of the entire multi-channel data acquisition system. If they fail under certain sudden changes in the working environment (such as excessive temperature), the entire system will inevitably be paralyzed. Therefore, it is easy to think of trying to improve the protection of these hardware devices.

The so-called protection, for electronic equipment, is mainly the "three protections" in the usual sense, namely dustproof, waterproof and vibration-proof. For the first two, the more commonly used internationally is the IP (Ingress Protection) level, which consists of two numbers. The first number indicates dustproof; the second number indicates waterproof. The larger the number, the better the protection function. For example, products that pass the IP67 standard mean that they can completely prevent dust from entering and can prevent water from intruding when immersed in water. For vibration-proof, the main thing to look at is how much reciprocating motion and impact the device can withstand when it is in working state. In addition, there are specific drop tests from how many meters, etc.

Observing the development of mobile phones, it is not difficult to find that people in the past would not have thought that touch screens are now an indispensable component of mobile phones, and the flip-top phones that were popular at the time have now disappeared. Similarly, not long ago, having three-proof features would be considered a sign of high-end equipment. Generally, such products would only be used in military-grade fields and were extremely expensive. Now, data acquisition equipment with IP ratings has fully entered the industrial field, with moderate prices, and even some data acquisition equipment manufacturers provide product series that are defaulted to three-proof.

Determinism

Many multi-channel data acquisition applications require the system to have a high degree of determinism. The scope we are going to discuss here is actually unrelated to hardware, but is a software-level issue.

Determinism refers to whether a task can be completed consistently within a known length of time in software. For example, if the execution time of a device changes or jitters more when it executes the same task multiple times or cyclically, or if it is affected by external interactions (such as mouse operations or opening a new folder, etc.), then its determinism will be worse. From this perspective, general operating systems such as Windows 7 or Mac OSX are actually completely non-deterministic, while real-time operating systems (such as VxWorks, uC-OS/2, etc.) can achieve determinism by very accurately controlling the timing of your application tasks, thereby ensuring that key applications are executed consistently and in a fixed time. Therefore, in the data acquisition system architecture, we generally place the actual acquisition operations that require high determinism in the real-time system of the lower computer, and place system configuration, emergency stop, display and other functions in the upper computer.

Of course, in addition to using real-time systems, some manufacturers think further for engineers. NI creatively combines real-time systems with FPGAs and proposes a reconfigurable I/O, or RIO, system architecture, as shown in the following figure:

Figure: Reconfigurable I/O architecture

Due to space constraints, we will not describe it in detail here. The key point of this architecture is the introduction of FPGA, which can help the processor share intensive tasks and has extremely high throughput. In addition, its deterministic decisions and operations can be directly connected to hardware I/O to achieve more high-performance access, such as interlocking, conditional triggering, timing, synchronization, etc. The control rate can reach the nanosecond level at the fastest. Compared with a single real-time operating system, such an architecture will have more plasticity and imagination.

Data redundancy

As a multi-channel data acquisition system that works stably, the most important thing is that the collected data is error-free, without missing points, and without extra points. Especially in the case of long-term data collection (such as long-term monitoring for 3-4 consecutive months), it is actually very difficult to meet such stringent conditions. At this time, the use of redundancy is a simple and effective idea.
The concept of redundancy is not new, and usually refers to increasing the reliability of the system through multiple backups. For example, in engineering projects, engineers sometimes provide UPS backup power for certain key equipment, so that when a sudden power outage occurs, the equipment can continue to work normally for a period of time without affecting the entire system. This is the simplest application of redundancy.

In summary, engineers must consider the stability of the overall system before building a multi-channel data acquisition system. We believe that the "extremely stable system" will develop into a specification and standard in the near future, and the protection, determinism, and redundant design discussed above are very likely to become the "standard configuration" of various test equipment.