Safe converter

Dipl.-Ing. Wilfried Grote

In industrial plants and systems, the transmission of analog measuring and control signals almost always demands that the signals to be processed are electrically isolated. This is necessary for safety reasons as well as to achieve the optimum signal quality. Measuring hazardous high voltages or different ground potentials – for instance in plant sections that lie far apart – does not allow measuring signals to be directly connected with other devices or modules. In cases such as these, electrical isolation of the signals to be transmitted is mandatory.

Phoenix Contact offers the appropriate electrical isolation modules precisely for these types of applications. Depending on the particular type, the measuring signals are either amplified or converted into standard normalized signals (e.g. 0…10 V, 4…20 mA), where voltages from ±50 mV to ±100 V and currents from ±1 to ±100 mA can be transmitted, filtered or converted with a high degree of accuracy.

In machinery construction, frequently, special signals that are not in the standard range from 0…20 mA or 0…10 V must be processed. Generally, universal isolation converters are used as there are no stand-ard devices available for these special signals. Furthermore, time-critical control processes are often encountered in special machinery construction, which require high limit frequencies and associated fast response times. Although the universal isolation converters presently available on the market already provide a high degree of performance, there are requirements that they cannot address. These include applications that require a wide range power supply, short step response times (high limit frequencies) as well as special approvals, such as functional safety according to IEC 61508.

For this reason, Phoenix has developed a universal isolation converter, which secures and increases the transmission quality and therefore the quality of control circuits as a result of its precise conversion, isolation, filtering or adaptation. New technical functions, fast response times and additional approvals make it possible to use this device in a wide range of applications in process engineering. The wide range power supply allows the device to be used with all line supply systems worldwide.

The universal MACX-MCR-UI-UI-UP analog isolation converter covers more than 1600 signal combinations in the area of instrumentation and control technology. Neither software tools nor an adapter cable is required. The combination options, which can be read from a configuration table, are simply set using a DIP switch. The switches are accessible from the front, which means that the device configuration can be identified and changed if necessary – even when the devices are installed and lined up. Users save time when selecting devices and can also reduce their spare parts inventory thanks to the high degree of signal flexibility.

On the input side, unipolar and bipolar signals from ±50 mV up to ±100 V, ±1 up to ±100 mA as well as the typical 1…5 V, 2…10 V and 4…20 mA live-zero signals are available. On the output side, in addition to the live-zero signals, users can also select between unipolar and bipolar analog signals in the range ±20 mA or ±10 V respectively. The input and output signal ranges can be freely combined with one another.

Pure analog isolation converters are frequently compared with processor-based, programmable signal converters. However, signal converters cannot achieve the short response times of analog isolation converters as a result of the signal processing times. This is why they are not suitable for closed-loop control processes that are critical from a time perspective. Not only this, signal converters require additional service tools such as programming software and adapters.

Applications that are critical from a time perspective – for instance monitoring frequency converters in drive systems or in special machinery construction – demand high limit frequencies. Welding currents are generally measured using shunt resistors in automatic welding machines. The voltages that are generated here in the mV range must be electrically isolated and also converted to standard signals. Fast control operations such as these demand short response times and therefore high limit frequencies of the isolation converter.

Furthermore, for unipolar output signals, there is an inverse output characteristic option, for example, to convert from 0…50 mV to 20…0 mA. In addition to the active current or voltage output, there is also a passive current output that can be used to directly drive active PLC cards.

Analog isolation converters are predominantly available as an enclosure for rail mounting. As a consequence, the modules can be flexibly mounted next to one another regardless of the manufacturer in line with the application requirements. The MACX Analog series of isolators set themselves apart as a result of an enclosure that is just 12.5 mm wide. When compared to typical widths ranging from 16 to 22.5 mm, a space saving of up to 45% is obtained on the mounting rail in the control cabinet.

To achieve this low-profile design, the power loss was minimized using patented transformer and switching technology. At the same time, the users do not have to cut corners when it comes to performance. Analog isolation converters transmit signals in a temperature range from -20 to +70 °C, are precise regardless of mounting position, drive high loads and offer integrated, protective separation for a 2.5 KV test voltage applied between the supply voltage input and output. Interference, which for example can come from the line supply system, can be excluded in this way and in turn the plant or system reliability increased.

Error-free mounting and pre-wiring is facilitated by the pluggable and coded connection terminals, either available in screw or spring-cage technology. In addition to the domestic approvals such as UL/CUL, the analog isolation converters have also been approved for use in Ex-Zone 2 according to EN 60079-15 (type of protection n). Approvals under the new IECEx scheme make it possible to directly enter many additional international markets.

The series of MACX devices has been developed for use in safety-relevant circuits in compliance with EN 61508 and achieves here Safety Integrity Level SIL 2. As a consequence, it covers most of the requirements in process engineering. Especially in this area, more and more applications require that the complete signal chain is evaluated from the aspect of functional safety. For instance, the probability of a signal chain going into a dangerous state when a fault situation develops has to be taken into account. A SIL certification of the components used in the signal chain ensures that the user has parameters available to him that are necessary for the overall evaluation.

Online-Info www.cpp-net.com/2309433