Control signals - which option is best?

It is about 20 years since the industry "experts' stated that 4-20mA would disappear from the market, to be replaced by digital networks or fieldbus systems. While there is some truth that networked systems have taken a large portion of market share, 4-20mA is still alive and kicking. There are two main reasons for this: first, higher-priced intelligent sensors, made for specific networks are not always the most cost-effective options, nor are the "smart' features on offer from intelligent field devices always wanted or required.

Second, the process control industry has changed significantly, with much more automation being installed into smaller plants, like food processing, water treatment, remote monitoring, etc. It could be said that 4-20mA was the last truly universal signal, accepted by every system, every vendor, and every control component. This means that single control loops, small plants and transportable skid-type equipment will always have a need for the simple, low-cost option that 4-20mA offers.

Let's not forget just how versatile this "simple' signal is. It uses just two wires to transmit the signal, it can power the field transmitter with its own supply voltage and it can run for long distances on standard cable. Using the live zero (4mA instead of 0mA) it senses a cable break or instrument failure, it is not overly susceptible to noise, it is highly accurate and low-cost. The downside is that two wires can only carry one variable or instrument.

However, as the number of control loops increases, the costs start to multiply, as you need individual cables, glands, terminals and inputs for every element. As well as the hardware, the space it requires and, most importantly, labour to design, install and commission, make it uncompetitive versus fieldbus networks in applications with many sensors and feedback devices. It is different for every installation, but there is a point where the change from an analogue to a digital network becomes more cost effective.

By using fieldbus networks, you can now have thousands of inputs and outputs on a single cable. This reduces the infrastructure that you would have needed for a traditional system and can save up to 50 per cent of project costs, even though the actual hardware becomes more expensive. By using smart field equipment, the entire transmission of the signal is digitally accurate and generally there is configuration and often diagnostic data available. This methodology is suitable for large plant design, especially for heavy industry like petrochemical.

The hybrid system

The downside is the number of choices that a process designer has to make when designing these systems, the first being which network, as you cannot easily mix networks on site. While the most popular networks used are not proprietary, they are still not available from all vendors, which starts to limit choices in components, even though they may be better suited to the application. For these reasons, it is rare to see a site that utilises only digital networks; however, almost every larger site does use fieldbus technology. This leads us to the most common installation, the hybrid system.

A hybrid system incorporates both technologies. The 4-20mA is used in the field, which keeps the components generic and lower cost, and then the signal is fed into a field Input/Output (I/O) panel where these signals are clustered and then transmitted to the central control via fieldbus. Without doubt, this system is the lowest cost to implement and is simple to understand and maintain. The only downside for a network that is not fully digital is that you most likely won't get configuration or diagnostic information from the field instrument, but the upside is that the instrument itself will be considerably lower cost. In order to clearly show where each communication option may fit, let's look at an example.

Imagine a single flow control loop; say it is the water feed to another process that needs to be maintained at the same rate. We would choose a flow meter based on the application requirements, with 4-20mA output. To control the flow, we would choose a modulating control valve with 4-20mA input. But to close the loop, we need a Proportional Integral Derivative (PID) controller between these two instruments that can be configured to the application; this will need a 4-20mA input and output - done.

Now let's say there are 15 of these loops, all identical, and the plant manager wants to monitor and record the flows in the system (assume a PLC & SCADA layer). Using 4-20mA, we would now need to send 15 signals to the PLC, plus 15 signals out, needing 6-8 input cards and as many as 90 terminals, 30 glands, cable tray, panel space, power supplies, signal isolation, etc. Now this option starts to become more expensive.

The digital option

Let's try fully digital. We will choose a fieldbus protocol, such as Profibus. We will need to install a Profibus master at the PLC (unless it is incorporated) and then choose a flow meter that has Profibus, as well as a control valve that has Profibus. So now we have one slot in the PLC rack, one gland and one cable to all the devices. The PLC can now read the process signal and control the loop as well as log the data, via the 30 fieldbus nodes.

The only downside is that the field equipment is now more expensive, sometimes considerably, and specific to Profibus, so if the plant changed to Ethernet in 2 years, the field equipment would have to be discarded or a customised network-network gateway would need to be installed. This design is far more complex and may require highly qualified maintenance engineers.

A hybrid option would be to use the original lower-cost field equipment, but to wire them directly to a local (field) I/O panel that converts them to Profibus and transmits them to the PLC. You get the cost savings of the installation, with lower cost field instrumentation, even allowing for the I/O cluster hardware. For this example the hybrid design provides the most cost-effective option.

Brkert has systems available for each of the options above. The customer can choose the installation that suits them. But, what if this customer came to Brkert with this application at the design stage? The company would choose flow meters with 4-20mA output (8035) and feed these directly into the modulating valves (2301) with Profibus-enabled positioner and optional inbuilt PID controller (8693). This device would control the flows directly as well as transmit process, output, configuration and even diagnostic data to the PLC, via just 15 fieldbus nodes. This "Brkert' system, although being partly hybrid, would give all the functionality of the fully digital system, yet be around 30 per cent lower cost than the cheapest option above.

There is no perfect fit for all situations. There are many considerations that weigh into the correct selection of the site signal wiring, with cost being just one. Always seek competent advice and develop a tool that will estimate the total cost of the installation, including hardware, software and labour. Then consider whether it will be easy to commission and maintain, and whether you have the competence on site for fieldbus technology. As explained above, Brkert has systems for all options, so have no bias towards one or the other, just a commitment to delivering value and long-term reliability.

To learn more about control signal systems from Brkert, please visit www.burkert.co.uk.