Signal isolators, converters and interfaces - Part 2

By using the right signal interface instruments, in the right ways, potential problems can be easily avoided, well before they boil over.

In Part 1 of this article we covered some of the basics of signal isolation and conversion. In this part, we will go beyond the basics to look at some more advanced applications.

Beyond the basics

Area isolation (divert and protect signals) It is quite commonplace to share process signals between two different systems. It could be two control systems, one emergency shutdown system (ESD) and one control system, one DCS and a data acquisition system, and other numerous combinations. Generally it is unacceptable to create one series loop between the transmitter and two systems. Why? You would not want a series loop if you had to disconnect the input at one system for maintenance purposes, because both systems would lose the signal. One solution to sharing a variable with two systems is to use a single signal isolator (Figure 1). One system is declared the primary system and it powers the transmitter.

Figure 1: A signal isolator can be used to share a process variable with two different systems.

In Figure 1, the primary system is an ESD. The output loop-powered isolator (with a passive input) isolates the primary loop from the secondary loop. The DCS is the secondary loop. Maintenance can disconnect the input to the DCS without impacting the signal going to the ESD.

This architecture is very common, but there is a weakness to this design: if you had to disconnect the input to the ESD, then the DCS also loses the signal. Disconnecting the ESD input removes power from the transmitter. Fortunately, there is an alternative solution using a ‘splitter’.

Split a signal

A signal splitter is a 4-wire signal isolator/converter that takes one signal input and provides two or more identical outputs proportional to the input. All inputs and outputs, and power, are isolated from one another. In Figure 2, the transmitter is provided a 24 V power supply, called ‘transmitter excitation’, by the splitter. The advantage of the splitter approach (versus area isolation) is that you can disconnect either control system for maintenance without affecting the signal going to the other system. Some of the popular splitter applications (other than area isolation) include custody transfer and isolating validated systems from non-validated systems in the biopharmaceutical market.

Figure 2: A signal splitter takes one signal input and provides two or more identical outputs proportional to the input.

Solving bucking power supplies

Some DCS manufacturers offer lower cost 4-20 mA input cards. However, there is a significant trade-off - the card must power all the loops. This is not a problem when all inputs are from loop-powered transmitters. But if you have 4-wire magmeters, or other line/mains-powered transmitters, both sides of the loop are trying to source the 4-20 mA. The result is either too little current or no current at all. A simple output loop-powered isolator solves this problem. It can operate with powered inputs from both sides, thus restoring normal operations to the loop (Figure 3).

Figure 3: An output loop-powered signal isolator eliminates bucking power supplies.

Boost a signal

Loops rarely start out overburdened, but, over time, devices get added to the point where the drive capacity of the transmitter is exceeded.

Nominally, a 2-wire (loop-powered) transmitter, powered with 24 V, will drive into 600 Ω. If your receiving device is still using a 250 Ω input impedance, it does not take much to overburden the loop. A signal isolator solves the problem by providing a way to add more power - called drive capability - to an overburdened loop. In Figure 4, if you exclude the isolator from the loop, there is an overall impedance of 615 ohms, plus wire impedance. That exceeds the 600 Ω drive of a loop-powered transmitter. With the isolator added to the loop, the input loop is satisfied with only 415 Ω plus wire. The output loop is also satisfied with only 250 Ω of load. The PLC’s 24 V powers the isolator, which in turn provides another 600 Ω of load capacity.

Figure 4: A signal isolator adds more power, or drive capability, to an overburdened printer.

Pass or block a HART signal

By choosing the proper isolator, you can allow the HART signal to ‘pass’ to the output side of the isolator and on to the receiving device. Alternatively, you can block the HART signal from going beyond the primary loop to the receiving device.

Passing the HART signal: Select a signal isolator that allows the HART signal to pass when you want a technician to be able to access a transmitter’s process and diagnostic information via the HART signal, using a HART handheld, from any termination point on the loop (Figure 5). To pass the HART signal you need an isolator specifically designed for that purpose. A ‘hole’ has to be created in the filtering to allow 1200 and 2200 Hz signals to pass through.

Blocking the HART signal: There are a couple of reasons why you may not want to pass the HART signal to the isolator output. For one, you might have an older receiving device with insufficient noise rejection on its inputs, and the HART signal causes interference with the analog measurement. Another reason might be that you have a process signal that has to go to a DCS and also to a PLC and you do not want the instrument technicians reconfiguring the field transmitter. Furthermore, blocking the HART signal makes it impossible for a technician to make unauthorised changes to the HART transmitter using a handheld communicator or from a HART-based control system. An isolator with good common mode rejection is generally all you need to block the HART from the output loop.

Figure 5: A HART isolator allows the HART digital signal to pass through the isolator.

Multi-channel instruments save costs

Multi-channel signal isolators/converters combine multiple analog signal channels into a single instrument to substantially save panel space and instrument costs.

Multi-channel instruments can be used in most of the applications of traditional signal isolators and converters at a very low cost per point. This would include taking one signal and, in the case of a 4-channel unit, splitting one signal into four, and sending it to four different receivers.

Custom linearisation

Microprocessor-based isolators/converters offer some solutions that are not found in analog isolators. Custom linearisation is one. This provides the ability to plot custom curves that can very simply address applications like linearising tank level to volume in a non-linear tank, square root extraction, signal limiting, characterising pH to reagent demand and valve linearisation. Some manufacturers offer curve capabilities of up to 128 points.

For example, because of an odd-shaped storage tank, the output from a level transmitter doesn’t correspond to volume because of the non-linear relationship. The strapping data is available for the tank, but the PLC is overloaded and can’t perform the linearisation function. The solution: a microprocessor-based PC-programmable isolator/converter (Figure 6). Simply enter the strapping data into the instrument’s PC configuration software custom linearisation table, and the output will be linearised and scaled to provide a 4-20 mA output proportional to tank volume.

Figure 6: Microprocessor-based isolators/converters provide the ability to plot custom linearisation curves.

Environmental considerations

Hazardous area isolators

Equipment location, lack of wiring and process flammables may be reasons that necessitate isolators/converters be installed in hazardous areas within a plant. When such needs arise, technicians have to determine whether the area is classified as NEC Class I, Division 1 (IEC Zone 0/1) or Division 2 (IEC Zone 2). Today, isolators come in all flavours to accommodate such installations. Intrinsically safe, non-incendive and explosion-proof are all options that one can now choose. One requirement that often arises is the need for an explosion-proof signal converter/isolator with a local indication for remote operators.

RFI/EMI protection

The effects of RFI and EMI can cause unpredictable and non-repeatable degradation in instrument performance and accuracy, and may even lead to complete instrument malfunction or failure. When this unwanted noise finds its way into your measurement circuits it can result in off-spec product, reduced process efficiency, plant shutdowns and, sometimes, dangerous safety hazards.

Some common sources of interference are: mobile and stationary radios and handhelds, static discharge, large solenoids or relays, AC and DC motors and welders. Because RFI can squeeze through even the smallest of cracks, an isolator housing should be designed so there are no openings, uncovered mounting holes or exposed PCB areas. In addition, the use of solid aluminium cases that help block stray RFI/EMI is a recommended feature.

To further mitigate the effects of noise, PCBs are carefully designed with noise in mind. To harden the designs, look to instruments that utilise low pass filters, a terminal strip common ground plane to the low side of the filters, strategically placed LC filters and the use of full spectrum ceramic RFI/EMI. In fact, some of the filtering guarantees less than 0.1% error at radiated noise levels of 50 V/m between 20 and 1000 MHz.

VFD noise

A problem that has cropped up in the last ten years or so is noise from variable frequency drives. Before the days of VFDs, output signals from PLCs or DCSs fed directly into AC or DC driven motors. However, after discovering the inefficiencies of AC and DC motors as compared to VFDs, plants everywhere started replacing traditional motors in plants. Shortly thereafter, engineers and operators at these plants starting seeing a new set of problems: noise on the output cards going to the VFDs. By their very nature, VFDs are a terrific source of common mode noise that can be directly coupled onto surrounding electronics and even onto its own input signal coming from the control system.

How do you solve this problem? Install a 4-wire (line/mains-powered) isolator to filter out the unwanted common mode noise (Figure 7). Rather than the input to the isolator originating in the field, the control system now supplies the input to the isolator, and it drives the output to the VFD. Now the isolator simultaneously isolates and shields the control system from the noise. Ambient temperature effects

Figure 7: A 4-wire (line/mains-powered) signal isolator prevents unwanted VFD noise from disturbing the output signal from the control system.

One of the many considerations that must be examined when choosing instrumentation is the amount of heat the electronics will be exposed to over its installation lifetime. Though field instruments often possess sufficient ambient temperature operating specifications, many DIN rail mounted instrument suppliers assume that their equipment will be installed in a climate-controlled environment. As such, you will often see DIN rail instruments with a top-end ambient operating temperature rating of 60°C. While heat is the major culprit of failure of electronic components, colder climates can also present a myriad of challenges. Some isolators and converters are suitable for installation in areas that see low temperatures of -40°C and high temperatures of 85°C.

Versatile workhorses

The ability to depend on accurate monitoring and control signals is literally priceless. Inaccuracies can lead to process inefficiencies, process upsets and even very costly plant shutdowns. Much worse, inaccuracies can lead to dangerous safety conditions for plant personnel.

Isolators and signal conditioners are indispensable tools that enhance measurement accuracy and protect signals from damaging conditions, thereby saving money.

With the extensive array of isolators available, selecting the correct isolator or the right combination of isolator features might be a bit daunting at first. However, if you take the time to add the right instruments to your process, signal isolators, converters and interfaces can help improve the efficiency and throughput of your process.

When you need to isolate, convert, share, split, boost, protect, step down, linearise or digitise process signals, look to the versatile workhorses of the process instrumentation world - signal isolators, converters and interfaces.