DIGITAL DATA VIA ANALOGUE SIGNAL FORMS
ACDC – A COMPLEMENTARY ALTERNATIVE TO HART FOR EXISTING WIRING
It is a contradiction of sorts; most fi eld devices on the ends of 4-20 mA current loops have become so smart that the transmission systems connecting them cannot handle the additional information.
In a world in which economic success increasingly depends on data-driven process optimization and process automation, not being able to access all necessary information is no longer an option. But to rip and replace an existing and functioning infrastructure, just to achieve the necessary bandwidth for data transmission, makes
Illustration of limit ranges 4-20mA EN What was the reason for 4-20 mA in the fi rst place? CC33 stainless steel neither economic nor ecological sense.
Round about 95 percent of the gas detection systems installed in industry plants communicate on the “last mile” via 4-20 mA systems. The connection between controllers and transmitters is still analogue, while the data exchange with other components of the infrastructure is done digitally.
For more than three decades, the means of choice to solve this problem has been the HART communication protocol (Highway Addressable Remote Transducer). It still is the best solution for customers who would benefi t from putting together the optimum solution from the list of more than 1,500 registered products, but it is often too sophisticated for less complex applications.
One has to keep in mind that gas detection systems consist of fi nely tuned and tested components. Although it may occasionally be necessary to integrate a third-party pressure or temperature sensor into a solution if the manufacturer does not offer one, it makes no sense at all to put together such a complex solution using transmitters and controllers from several vendors.
Thus the real challenge – to transmit digital data quickly and cost-effectively via analogue 4-20 mA lines – is unsolved even with HART. It is time for a new approach. It’s time for an Analogue Carrier for Digital Communication
Why use a 4-20 mA solution these days anyway?
If data and information ultimately have to be available in digital form in order to be processed, why not transmit them digitally?
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1) The forerunners of electronic instruments were pneumatic solutions that worked on the principle of the baffl e nozzle. 3-15 psi with a ratio of 1:5 was chosen as it is the most linear part on the curve for the movement of the baffl e and the resulting backpressure in the nozzle.
2) An intermediate step had been 10-50 mA devices. The fi rst analogue electronic instruments used magnetic amplifi ers. 10 mA offset zero (then called live zero) was chosen as this is the lowest value at which instruments based on magnetic amplifi ers could operate. While maintaining the ratio of 1:5, 10-50 mA was selected as the signal.
3) With the introduction of the transistor, it became possible to develop devices that required less current. Since semiconductor devices require 3 mA current to operate, the new standard had to be above 3 mA.
The exact reasons that led to the choice of 4-20 mA are unfortunately lost. It was likely a combination of a desire to work with integer values, to consume as little power as possible, and the tendency to maintain the familiar 1:5 ratio. Only 4-20 mA or 5-25 mA were practical, and with multiples of 2 it is simply easier to calculate.
For every task in the fi eld of gas detection, devices with analogue and digital interfaces are available. GfG customers can choose between 4-20 mA and Modbus/RTU versions for almost all transmitters.
Even more advanced would be solutions that rely on Industrial Ethernet for communication and offer data transmission in near real time. However, this is a speed level that would be wasted in the fi eld of gas detection systems, where T90 reaction times are measured in seconds, often in the double-digit range.
A glimpse into history There are a number of good reasons why 4-20 mA became the industry standard (ISA SP50, originally published in 1966), mainly because the concept is simple, reliable and cost effective.
It can be operated over long distances with minimal signal loss and a varying load impedance or supply voltage has no signifi cant effect on the signal as long as the recommended component
limits are not exceeded. The fact that an offset-zero measurement verifi es that the sensor is electrically functional was also a fi rst type of remote diagnosis. This offered considerable advantages compared to the pneumatic 3-15 psi control signals used to date.
Why not use HART in modern gas detectors?
Over the past decades, many clever minds have invested a great deal of time and effort in the development of the HART protocol and the corresponding devices. It is the perfect solution for many applications, but it was never intended to simply transfer additional data at low cost and high speed.
In many cases, the only reason for implementing HART is the fact that there is simply no alternative if you needed a digital communication protocol to send and receive information between
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