When sensor signals are disturbed by an external power source, you can expect measurement errors and process malfunctions. This series will look at the mechanism and electrical principles behind electrical interference and point out some preventions and cures.
Interfering sources may include nearby power cables, motors, transformers, radio transmitters, variable-speed drives and discharge lighting. The offending sources can deliver leakage currents through imperfect insulation and capacitance. They also can spray magnetic and electric fields at signal wiring. Thermocouple and RTD signals are the most susceptible as they are at low DC millivolt levels.
Two common types of interference are leakage resistance and capacitance. Leakage resistance allows both AC and DC to enter signal circuits; capacitance allows AC to enter. The value of this current is capacitance times rate of change of voltage (i = C dv/dt or I = VwC for sinusoidal AC interference, where w is the frequency of the interfering voltage expressed in radians/sec instead of Hz). Your defenses include:
- High Insulation Resistance. Measure it and upgrade if it gives you trouble.
- Distance. Keep signal wires away from power cables, which should be in their own conduit or duct. This will minimize resistive and capacitive currents.
- Shielding. Keep signal wires inside a metal shield grounded at one point only. This diverts both resistive and capacitive currents harmlessly to ground (figure 1).
Magnetic InterferenceChanging magnetic fields from external equipment can link the signal wiring. This induces a voltage, which is the product of the mutual inductance (M) between the interfering source and the signal wires and the rate of change of the interfering current i (v = M x di/dt or V = IwM for sinusoidal currents). Conductive shielding is ineffective against it, and even magnetic shielding is of limited use. Again, separation of signal and power wiring is the best measure.
Using twisted pair signal wiring may help as it acts by successive twisted loops to cancel each other's pick up of magnetically induced voltages, reducing mutual inductance.
When the above measures have been completed, you still may be left with some unwanted voltage called series mode interference, which usually is AC but can occur in DC form (e.g., leakage currents in electrolytic tanks or a bare thermocouple clamped to a conductor carrying direct current). In the case of DC interference, prevention by good isolation is the only remedy because you cannot filter out DC from a DC signal.
AC interference can be further reduced by low-pass filters. The filters can be combinations of capacitors and inductors in the signal lines, or they can be digital filters built into the controller's electronics. Their purpose is to allow passage of the DC process signal while blocking or at least attenuating the interfering AC. Such filters increase the response time of the controller or indicator to changes in signal level. Because temperatures don't change quickly, this usually is not a problem. If you are scanning multiple inputs into a time-shared instrument, you may have to settle for a slow scanning speed to wait out the filter's settling time. Alternatively, you could use a filter on each sensor.
Modern controllers and indicators use an additional technique to deal with remaining AC superimposed on a DC signal. They digitize the average DC signal over a period that contains an equal number of canceling positive and negative half-cycles of the AC interference (usually 50 or 60 Hz).
In the next issue, I will focus on common mode definitions and isolation techniques related to electrical interference on control systems.