Bộ cách ly tín hiệu Dataforth

Bộ cách ly tín hiệu Dataforth

Bộ cách ly tín hiệu Dataforth

There are many pieces of equipment in both the field and control room that can interfere with measurement signals. This equipment and other sources of man-made and natural ‘noise’ are a part of the ‘electronically hazardous’ environment in which instrumentation exists. Since the equipment and natural events themselves cannot be eliminated, their effects upon the instruments must be understood and minimized or eliminated. By understanding the ways in which this noise gets into the instrument system, one can take rational steps to avoid problems in a new system or eliminate them in an existing one. Table 4 list the most common problems and possible solutions.

SOURCES OF ERROR
Each of the following influences can seriously degrade signals. Methods to avoid or minimize their effects are discussed in later sections. 

Capacitive Coupling
Any piece of plant equipment can develop an electric charge. So long as this charge does not change, it has little or no effect on an instrument system. However, all electrically powered equipment has a varying charge or voltage. It can vary in a smooth or erratic (transient) manner. When it does so, the equipment creates a changing electric field that can capacitively couple into the sensor, its signal conditioning equipment or its wiring. Man-made static discharges and lighting have been known severely damage instruments. 

Magnetic Coupling
An electric current produces a magnetic field. If a conductor moves through the magnetized field, a current is produced in the conductor. Similarly, if the electric current in a conductor varies, a current is induced in nearby stationary conductors. The resulting induced current can be a disturbing influence or can produce a voltage across the ends of the conductor, also producing an influence. Since most sensor wiring is fixed in place, varying currents are the usual cause of magnetic coupling in instrumentation systems. Often these current influences result from placing wires too close to power conductors. 

Table 3. Common Signal Types and Levels 

Table 4. Sources of Error and Possible Solutions 

Ground Loops
Ground is an elusive and often misunderstood electrical concept. Its very name implies that the soil we walk on is the place to which all currents and voltages are somehow referred. In an electric power distribution system, a rod driven into the earth or a buried metal pipe is ‘ground.’ 

Unfortunately, that is not the entire the story. The local ‘ground’ where you are now located can be several volts above or below that at the nearest building or structure. If there is a nearby lightning strike, that difference can rise to several hundreds or thousands of volts (Figure 23). Even in a home, different parts of the electrical grounding system can vary by several volts. This arises not only from the resistance of wiring but its inductance. If the currents change very rapidly, the voltage drops in the ground system will approach several hundred volts for short periods of time. For example, studies conducted by power companies have shown that the operation of an oil burner ignitor can produce transient differences up to 2000 volts routinely. Imagine the potential for similar voltage spikes in other industrial environments. 

The voltages themselves can obviously provide a great source of interference for a measurement loop, but the currents which cause them can also induce significant currents and voltages in the signal wires located nearby. Circulating currents in ground loops may also be periodic in addition to being transient events. Consider the case when a ground loop is formed with AC power lines such that a 50 or 60Hz signal is imposed on the gound loop. If the measurement signal ground is part of the ground loop, the unwanted AC signal will appear as an error voltage or perhaps as a common mode signal to the system inputs. Figure 24 shows a 10mV thermocouple signal which is corrupted by a 60Hz and a 180Hz (3rd harmonic) ground loop voltage. The original signal has been corrupted beyond recognition. Proper grounding practice and use of a signal conditioner with high common mode rejection is vital in preserving signal integrity (Figure 25). 

Over Voltage and Transients
Beside the transient voltages caused by ground potential differences, large voltages can appear in the signal wiring directly. This can arise from capacitively or magnetically induced sources, from accidental electrostatic discharges or from nearby high-voltage arcs such as welding. Some of the instrumentation installed in today’s industrial plant is wired in close proximity to power wiring. Although this is very poor instrument wiring practice, it does happen. Accidental connection of signal conditioner inputs to 110 or 240 VAC is not an uncommon event. 

Many manufacturers of industrial equipment provide input circuit protection to prevent damage from transient voltages and the misapplication of power line voltages. Such abnormal voltages will render the measurement useless during the time they exist, but will not damage the equipment if it has this protection. When specifying signal conditioning equipment, choose equipment that provides SWC (surge withstand capability) protection and line voltage protection. Compliance with ANSI/IEEE C37.90.1-1989 standards is a good indicator that the equipment supplier has provided adequate protection (Figure 26a, b). All Dataforth SCM5B products are designed to meet this specification. 

EMI and RFI
Electromagnetic interference (EMI) is a general term for induced errors signal loops are subjected to. It includes most of the sources discussed before. However, it has come to have a somewhat more restricted usage in the instrument business. It has become synonymous with the term RFI (radio frequency interference). The usual source of this particularly annoying disturbance is a radio transmitter. If it is from a nearby radio or television station, it can at least be diagnosed with some ease. More often it appears as a ‘random’ or intermittent change in the measurement signal. In this situation, the usual sources are CB transmitters or two-way radios used in the plant. The normal signal conditioner will not amplify these signals because they are too high in frequency. However, the input stages of some signal conditioners will rectify the RF voltage in a similar to the old ‘crystal’ receivers. The rectified voltage appears as a drift or sudden shift in the measurement signal (Figure 27). Two-way radio conversations between the field and control room locations can cause seemingly random changes in signal levels. Look for signal conditioning equipment that specifies EMI/RFI immunity. A typical specification might state “less than 0.5% shift for RF fields of 30V/M between 27 and 500MHz”. This RF field strength is roughly equivalent to a 5 watt transmitter at 3 feet or 1 meter. 

Aliasing
‘Aliasing’ becomes an issue when signal conditioning passes from the purely analog world to the digital world. This, of course, is the norm in today’s systems. The notion of aliasing is confusing to some users of data acquisition equipment. The need to prevent aliasing arises from the use of analog-to-digital conversion in the signal conditioning path.