14 July 2008
The signals from these instruments are interconnected to each other and to sensors, transducers and output devices in the process loop. In any such measurement and control system there are several electrical interfacing problems that are likely to occur, all of which can be solved by incorporating the appropriate isolation between the signals.What are the Typical Symptoms Indicating the Need for Isolation?* Equipment works on the test bench but not in the plant.* Readings, received from a transmitter in the field, change noticeably when another, seemingly unrelated, piece of equipment is connected to the system.* Two pieces of equipment have different grounding requirements and cannot be interconnected directly.* The measurement into a receiving system varies unexpectedly when nearby electrical devices are operated.What are the Underlying Issues to be addressed?*Ground LoopsWhen the receiving system and the sensors and transducers are grounded at different points, substantial distances apart, there will almost certainly be at least several volts of potential difference between these two parts of the measurement circuit. If these two points are directly connected a ground loop will be created with the possibility of high currents flowing between the two ground points via the cabling, thus disturbing the measurement.*Common Mode NoiseOften the signals from the sensors and transducers are small voltages and susceptible to noise pickup from motors, variable speed drives and general switching spikes carried by the electrical reticulation system. If there is no isolation between input and output, the noise will produce unacceptable changes in the output current or voltage.*Safety of Plant and PersonnelFrequently signals must be measured or sent to pieces of equipment (e.g. variable speed drives) that may be floating at several hundreds of volts relative to ground. If signal isolators are used consistently throughout the plant, then a failure in the field or a human-induced problem such as an incorrect connection will not be carried through to the receiving system. What is important when selecting an Isolator?* Signal Powered, Output Loop Powered or Auxiliary powered.A signal powered isolator derives its power from the 4-20mA input signal, an output loop powered from the 4-20mA output signal and an auxiliary powered from the mains or a dc power source. * Linearity and RepeatabilityLinearity: The deviation of the input to output characteristic from the expected value anywhere between the maximum and minimum points. Repeatability: The output value should also settle back to within +0.05% of span or less of the last value when the same input signal is applied. * Power RequirementsThere is a range of supply voltages used in industrial applications from 24 to 240Vac or dc. Specifying the wrong supply voltage can be an expensive exercise so care should be taken and the supply availability checked. * Input and Output ConfigurabilityThe wider the range of signals types and measurement ranges that an isolator can handle on both input and output the easier it is to stock. This flexibility has to be tempered with the cost of the isolator and its basic function. Factors such as the method of setting the unit up and the technical expertise of the maintenance staff on site will affect the choice as well.* Choice of TechnologyUltimately the choice of micro-based versus analogue based type will depend on the preferences of the plant engineering staff. Some staff prefer to use multi-turn potentiometers for adjustment others prefer the computer configurable types with software that allows you to set all the parameters on the computer and then to download this complete configuration to the unit.
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