Efficiency and availability of production facilities depend on the reliable transmission of signals between field devices and the process controller. Interference in the signal circuit may cause related processes to run incorrectly.
It is the task of the galvanic isolation function on signal conditioners to prevent this problem. These conditioners may offer additional functions such as signal splitting or signal conversion.
Ensuring safe and efficient process control is a must when it comes to initiating a seamless workflow with a high degree of production stability. This process control is closely linked with error-free transmission of measurement and control data between the sensors in the field and the control level, or between the controller and the actuators. Interference in the signal path between the field and control level can cause processes to be controlled incorrectly due to distorted signals.
Fig. 1: If galvanic isolation is not present, an equalising current can flow via the grounding shared by field devices and the controller; it is this current that distorts the signal current.
One potential cause for interference of this kind is the equalizing currents created as a result of ground loops. These currents are caused by differences in potential between the grounded field and controller sides. They lead to deviating measuring and controller signals and can affect communication to such an extent that safe process control is no longer an option. Similar problems occur when high-performance components are grounded near signal lines or cause interfering signals from frequency converters in motor load changes.
Interference can be fed directly into the signal path as well. Sensors are often installed in pumps, motors, and fans to monitor temperatures, frequencies, or oscillations. If an isolation problem occurs on these devices, high voltages can find their way into the measuring circuit and cause a surge on the signal line. If the total of the signal current and fault current is between 4 mA and 20 mA, the fault will not be detected and displayed as a malfunction, but can still have serious consequences.
Signal conditioners are used exclusively in safe areas. They do not have a barrier function designed to prevent unacceptably high energy in hazardous areas.
The use of signal conditioners is particularly helpful if
Plants are large and have long signal paths.
Powerful components are grounded in the vicinity of the signal lines.
The signal line environment is placed under significant electromagnetic charge by other signals or by motors with frequency converters.
No single-channel input isolators are present in the respective controllers.
Signal conditioning is more than galvanic isolation
Galvanic isolation as offered by signal conditioners can prevent interference of this kind, or at the very least protect the process control system. The signal conditioner separates the loop into two circuits, which are connected to each other via a transformer.
Other tasks performed by these interface modules include converting signals. Conversion is needed if field devices cannot deliver the standard signals that the controller needs to process the signal.
Fig. 2: Galvanic isolation prevents equalizing currents in ground loops and also protects controllers against other interference.
This situation exists with many small PLCs. Even though they are cost-effective, they often have only voltage inputs as standard. This is why a signal transformer is needed if only current signals are provided from the field. In addition, the analogue inputs are usually purely passiv — a transmitter power supply with current output or voltage output via a signal conditioner is therefore very beneficial. Also, small PLCs often allow only a few signal types on the input side. You will generally not find temperature inputs for all commercially available temperature sensors.
Signal conditioners offer short-circuit protection for each individual current circuit, since the conditions switch corresponding outputs on the control side into the safe mode. Measuring instruments on other channels are not affected by a failure.
Often, field signals need to be shared by various systems, such as control systems, emergency shutdown (ESD) systems, or data acquisition systems. If this is the case, several systems must not be connected in series to the output side. If the connection to one of the systems were to be interrupted, none of the other systems would receive any more signals either. The galvanically isolated outputs on a signal splitter guarantee reliable distribution of the process signals to the various systems. This avoids the problem of high resistive loads in a series connection.
Eliminating counter currents plays a role when viewed against safety and availability. If field devices such as 4-wire transmitters with active current output and a control system attempt to supply current to the signal circuit, the result is a nonfunctioning current circuit. With signal conditioners, it is usually possible to handle all combinations of current sinks and current/voltage sources.
Safety first – isolation quality
Fig. 3: DIN rail mounting, isolation quality up to 3 KV/300 V (test/working voltage).
The basic functions of signal conditioners have varying technical differences, depending on the supplier. It is plain to see that with over 15 years of experience, Pepperl+Fuchs reflects users’ priorities.
The main customer need is good isolation, to ensure the device reliably fulfils its core tasks. The isolation quality of a circuit is influenced by many parameters. Influencing factors range from circuit design, such as the spacing between the isolation plugs, to the quality of components used. The need for electric strength increases proportionally, since users increasingly face high, steep pulses. The test voltages and maximum permissible working voltages are relevant in this regard; today they are typically anything up to 3 KV (test voltage) or 300 V (working voltage) and thereby ensure protection against contact on the controller side.
A crucial factor for users when choosing devices is often whether the manufacturer has much experience in developing and manufacturing isolated barriers for hazardous areas. After all, it is precisely in the hazardous area that good isolation quality is essential; the products used here must meet stringent standards as well as passing various external tests. For logistical reasons, the higher-value circuit concepts and components for the isolated barriers for signal conditioners are then applied, ensuring that users receive extremely reliable products.
Every degree counts
When selecting signal conditioners, it is important to look for low self-heating and a relatively high permissible operating temperature—for switch cabinet builders, literally every degree counts. Today’s standard operates at an ambient temperature of up to 70 °C. However, the requirements for the lower temperature limit are by no means unlimited. Ambient temperatures lower than – 20 °C are not practical in switch cabinets.
The measure of all things
According to users, suppliers often underestimate the importance of the dimensions and mechanical properties of isolators. A module width of as little as 6 mm is possible today and is offered by various companies—even for dual-channel types with a packing density of 3 mm/channel. However, the overall height is also a big factor to consider. Experience has shown that in virtually pure signal conditioner applications—i.e., without or with a small number of isolated barriers—you can find products from multiple suppliers. When replacing or expanding, new signal conditioners must fit into the existing switch cabinet grid. From the users’ perspective, noncritical functions such as the display or removable terminals are actually counterproductive when viewed against this background—both functions require extra space.
The mechanical stability of the housing is important in the context of mounting and handling—something that manufacturers often underestimate but is repeatedly requested by customers. With this in mind, Pepperl+Fuchs carries out Elmech tests in a separate, independent testing laboratory. The purpose of these tests is to examine the mechanical properties of the devices according to the specified standards. The tests include subjecting prematurely aged devices to mechanical and climatic loads and then monitoring compliance with the relevant specification.
Summary
The use of signal conditioners in safe areas effectively prevents interference in the signal path between the field and control level, thereby increasing machine and plant availability. Interference of this kind may be undetected equalizing currents, but could equally be dangerous contact voltages in specific fault situations on the field side. Signal lines in large plants or near grounded, powerful devices are particularly sensitive to interference. The increasing electromagnetic load of plants and the related risk of unwanted couplings can be effectively counteracted with galvanic isolation provided by signal conditioners. Furthermore, signal conditioners offer key additional functions such as signal splitting or signal conversion. The latter is increasingly gaining importance in areas where there are larger numbers of small PLCS that can process only a limited range of input signals. In addition to the basic functions, users want signal conditioners that are practical and technical in design: first-class isolation quality, maximum permissible ambient temperature, minimum height and width of the module, and robust mechanisms can all be found at the top of users’ wish lists.
Contact Mark Bracco, Pepperl & Fuchs, Tel 087 985-0797, mbracco@za.pepperl-fuchs.com
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