Continuing my discussion of the effects of electrical interference on your control system, Part 2 deals with series and common mode definitions and isolation techniques.
Common mode voltage means that both signal terminals are elevated in voltage relative to ground. For example, a thermocouple could be welded directly to the live end of an AC heater. The AC voltage relative to ground is called the common mode voltage.
Instruments usually are designed to handle sensors connected to sources that are up to 250 V above ground. This elevation can either be by process requirements or by accidental contact with bare electric heaters. In the presence of common mode, some resistive or capacitive leakage current to ground always occurs from each signal wire, and they are not necessarily equal. These currents pass through two thermocouple conductors, which also are unequal in resistance. The difference in IR drops in these conductors can produce a series mode interference component, which can be dealt with via the methods discussed previously.
Controller manufacturers design their devices to minimize the effect of interference and specify a series mode rejection ratio, which is defined as the value of the series mode interference voltage divided by the error in indication that it causes (measured in volts). You will see numbers in the order of 103. In this example, 1 V of series mode causes only 1 mV of error.
Likewise, the common mode rejection ratio is the value of the common mode voltage divided by the error in indication that it causes (expressed in volts). You will see numbers in the order of 108. In this example, 100 V common mode causes 1 microvolt of error.
Instead of publishing these simple numerical ratios, it has long been the custom in manufacturers' specifications to convert them to decibels. For example, 103 converts to 60 dB, and 108 converts to 160 dB. To convert this weird unit back again so you can work with it, divide the decibel number by 20 and use it as the exponent of 10.
Minimizing Exposure of Vulnerable SignalsIn a thermocouple circuit, one or two millivolts of interference is all it takes to spoil your work or equipment. On the up side, the low source resistance of thermocouples makes the signal more robust against interference -- that is, until you start adding cable length and resistance to reach a distant control panel. At this stage, you also are presenting a receiving antenna to interfering sources.
Sensor types that deliver larger voltage signals are correspondingly less vulnerable. However, some can have source resistances of several kilohms or even megohms, and for this reason they present an easy target for interfering sources.
Signal Converters. There are times when you have done all you can with wiring layouts, twisting and screening, and you still want to protect vulnerable signals and long wire runs. In these cases, you should insert a signal converter close to or even on the sensor (figure 1). The output of a typical thermocouple converter is commonly a robust 4 to 20 mA signal that is both linearized and cold-junction compensated. You can transmit its clean, isolated output via cheap copper cable, and the signal will arrive unchanged at the controller or indicator.
Distributed Controllers. Digital control and data acquisition systems are a few steps up from signal converters. Each sensor signal enters its own controller just as it would enter a signal converter, at or near to the source of measurement on the process (figure 2). Instead of a DC output, digital information derived in the controller is carried on a common bus to the control station or human-machine interface (HMI), fast and without degradation. These signals can represent a process variable, one or more setpoints, alarm condition, control parameters and controller output. Local display and local heat output can be optional.
Poor Quality AC Power SuppliesIn the present industrial environment, you can expect severely nonlinear and fast-changing electrical loads such as high current arcs, variable-speed AC drives, switching power supplies and phase-angle SCRs. Instead of the docile, smoothly changing sine waveforms we usually take for granted, nonlinear and fast-changing electrical loads cause distorted waveforms on your power line. When you power your instrumentation from such supplies, you may encounter process upsets caused by corruption of the internal digital circuitry. Some instrument manufacturers severely torture-test their products as part of the design process and ensure they are virtually immune to bad power quality and electromagnetic interference.
If you are experiencing process interruptions, you may suspect your power quality or see a distorted and spiky waveform on your oscilloscope. If this occurs, consider using a voltage stabilizer or power conditioner capable of cleaning up the waveform (that is, rendering it sinusoidal). Devices can range from effective to useless, so be sure to read specifications and ask questions. These problems lead distributed control equipment manufacturers to use a stable and clean low voltage DC supply to power the equipment.