Effective Protection against Faults by Using Signal Conditioners

Related Vendors

TURCK GmbH

Donadon SDD Srl

ProMinent GmbH

Pepperl+Fuchs SE

Decoupling the Field and Control Side

Using galvanic isolation, signal conditioners provide effective protection against interference that impairs signal quality between the field and the process control system. They prevent measurement and control errors that occur due to equalizing currents in ground loops. Ground loops can interrupt communication between field and control levels to such an extent that safe monitoring and control of processes is no longer guaranteed.

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There are several causes of equalizing currents in ground loops: Differing ground potentials can occur in the area surrounding many high-performance devices. The conditions of the potential can change when the input cards of the controller are changed. Galvanic isolation prevents ground loops caused by electromagnetic interference.

Interfering signals can be fed directly into the signal path, e.g., if there is faulty insulation in mains-powered devices such as pumps, motors, or fans. In this case, high voltages can enter the measuring circuit and cause a surge. In the event of a fault, there is a risk of fatal injury for operating personnel if there are no protective devices in place. Signal conditioners protect plants and personnel against this type of line-related risk.

Additional Functions at Interface Level

When using standard signals, no expensive input cards are required in the controller. An important additional function is therefore to convert sensor measuring signals into standard signals — frequently 0/4 mA … 20 mA — for further processing at the control level.

A high level of signal integrity in a field circuit is ensured only when the field circuit is protected against short circuits. Signal conditioners not only provide an individual power supply to each field device, but also offer short circuit protection for each current circuit. If a line fault occurs, the signal conditioner switches the outputs on the control side to a safe state. Additional channels with connected measuring devices are therefore not affected by the failure.

Measured values are frequently sent to emergency shutdown systems and data collection systems, as well as control systems. Versions with a splitting function that supply the measured signal to parallel, galvanically isolated outputs are available in this case. The splitting function ensures that information is transmitted reliably to other systems, even when one channel experiences a fault. This technology circumvents the disadvantages of serial wiring, in which downstream systems receive incorrect information or no information at all when an error occurs.

It must be noted that any existing galvanic isolation in the process control system cannot replace a signal conditioner. This usually concerns multichannel solutions located between I/O cards and processing units. They are therefore unable to protect the input cards against interference. Another advantage of using signal conditioners is that if a fault occurs in the controller, the separation of the signal circuit means that you can temporarily switch to manual control.

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