The 'gold standard' for industrial automation analog signals——4~20 mA

Why does industrial automation need a "gold standard"? Before the 1920s, the instruments and meters of industrial automation were still "independent and independent", with each instrument and meter doing its own thing and having little connection with each other. Most of them were field instruments installed on site, and operators needed to go to the site to check the readings of the instruments. Even if there is a centralized display of several instruments in the operating room, it is just a change of installation location for the on-site instruments.

If the pressure gauge needs to be moved from the site to the operating room, it is nothing more than extending the pressure tapping pipe of the pressure gauge to the operating room. This kind of thing is definitely not done now, but when I first started working in self-control design in the 1960s, I also did this thing: laying the pressure tapping pipe to the operating room, and selecting a pressure gauge with an edge, because the edge of the pressure gauge has holes that can be directly fixed on the front of the instrument panel. Of course, what is referred to here are instruments that integrate detection sensors and parameter displays, such as spring tube pressure gauges, diaphragm pressure gauges, U-shaped tube pressure gauges, glass liquid thermometers, bimetallic thermometers, rotor flow meters, mechanical water meters, glass tube water level gauges, and so on.

But with the emergence of sensors that output live signals, detecting a parameter with just a sensor is not enough. A display is also needed, which involves "contact signals", that is, what type of electrical signal the sensor outputs and how wide the signal range is. Therefore, the input signal of the display that matches this sensor must be able to receive this signal, and the signal range must also meet the requirements. The electrical signal output of sensors comes in various forms, such as voltage, resistance, inductance, capacitance, and pulse, and each has a different range. The scale used for display is also constantly changing, making the design, production, sales, and use of displays too complex. It may require hundreds or thousands of specifications and models to meet the requirements for measuring voltage alone. I am a self-control designer, and even in the 1970s and 1980s, I was still struggling with this matter. To design a temperature detection system, a temperature sensor and a display were needed. For example, the temperature sensor used was a platinum rhodium platinum thermocouple with a scale of S and a temperature range of 0-1300 ℃. Therefore, the display had to correspond to this, and it had to be matched with the platinum rhodium platinum thermocouple with a scale of S and a temperature range of 0-1300 ℃, without any mistakes. This is really frustrating for the manufacturers of monitors. They have to prepare hundreds or thousands of specifications of monitors for users to choose from, and even customize a monitor for each user!

This is just a thermocouple. There are also thermal resistors, infrared thermometers, resistive remote pressure gauges, metal tube electric remote rotor flow meters, with a variety of signal types, measurement parameters, and measurement ranges. Therefore, there may be thousands of detailed models and specifications for this display. And the key is that not many units of each specific model and specification of product can be sold, and it cannot form a batch. Do you think this is troublesome!

People are still smart. If we keep doing things like this, then some people will come forward and call for us to establish rules and standards, unify this signal, and create a 'universal language'. That is to say, no matter what sensor you are, if you want to send out the measurement signal, the category and range of the measurement signal must be the same for everyone to use. The production of the receiving measurement signal display instrument becomes simple. At least the signal category and measurement range are the same. Under the technical conditions at that time, only the measurement parameter range is different, which is just a problem of display scale. Different users only need to use a special scale to order.

The issue raised here is the unified standard communication signal mechanism adopted between instruments in industrial control systems, which is the problem of signal system. It is used to ensure the compatibility and interconnection capability of different instruments.

Of course, things need to be done step by step, so many instrument manufacturers have put forward some suggestions and practices. For example, in terms of electric instruments, Foxboro proposes 10~50mA, Taylor proposes 1~5mA, Honeywell proposes 4~20mA, and of course, there are also electrical signals such as 0~10V and 0~10mA. Pneumatic instruments were popular in the 1950s and 1960s due to their natural explosion-proof performance. Their pneumatic signal range was also very wide at the beginning, such as 2~14, 3~15, 3~18, 5~20, 5~25, 6~30, 3~27psi, and so on.

After years of accumulation, the advantages and disadvantages of various signal systems have been fully demonstrated. Pneumatic signals have been selected for 20~100kPa, while in the fierce competition of electric signals, 4~20mA stands out and becomes the final choice for electric signal systems. But in the late 1980s, due to the development of electronic technology, the explosion-proof performance and accuracy of electric instruments improved, and pneumatic instruments gradually emerged on the historical stage. Currently, it is difficult to see his trace.

Industrial control personnel refer to 4-20mA as the "gold standard" and "universal language" of industrial automation analog signals. It is speculated that the earliest introduction of 4-20mA by automation companies may have been in the mid to late 1950s, and it has been almost seventy years since then. These seventy years have proven the vitality of 4-20mA. Although the signal system of automation instruments has transitioned from analog signals to digital signals, the use of hundreds of millions of transmitters still works in analog signal mode in industrial control scenarios, and is far from being completely eliminated!