Inductive or Magnetic Coupling
When a cable carries current, a magnetic field is generated. The direction of the magnetic field for current flowing in a long, straight wire can be visualized using the right-hand rule. With the right-hand thumb pointed in the direction of the current flow, the fingers will show the direction of magnetic field lines. Lenz’s Law states that currents can be made to flow in conductors by moving them through a magnetic field. Similarly, a changing magnetic field will induce currents in a stationary conductor that is in the field. Since most wiring is fixed in place, varying currents are the usual cause of magnetic coupling — again, creating noise.
Figure 2 shows magnetic coupling between wires. As mutual inductance is only defined between separate current loops, there is none between the common ground conductor and conductors 1 or 2. Mutual inductance provides the coupling path for magnetic coupling, which is more difficult to reduce than capacitive coupling because magnetic fields can penetrate conductive shields.
The simplest way to reduce magnetically induced interference is to use twisted pair wires. Twisting the wires forces them close together, reducing the loop area and therefore the induced voltage. Since the currents are flowing in minimum loop areas, magnetic field generation is reduced. The effectiveness of twisted pair wires increases with the number of twists per unit length, but benefits are only realized in balanced circuits where the currents in the two wires are equal and opposite.
Twisted pairs also reduce capacitive coupling at frequencies below approximately 1 MHz. Each lead has an equal capacitance to a noise source, causing equal and opposite charges to appear along the leads. This results in a net induced charge of zero and, ideally, no capacitively coupled noise.
Eliminating Noise Through Isolation
Ground systems in industrial environments carry signal and power return currents, form references for analog and digital circuits, bleed off charge buildup, and protect people and equipment from faults and lightning. Ground loops exist when there are multiple current return paths or multiple connections to “earth ground.” Current flowing in a ground loop generates a noise voltage in the circuit. The most obvious way to eliminate the loop is to break the connection between the transducer and ground, or between the receiver and ground. When this is not possible, isolation of the two circuits is a universal way to break the loop. Isolation prevents ground loop currents from flowing and rejects ground voltage differences.
An effective method of isolation uses signal conditioners based on transformers or optical couplers, shown in Figure 3. The common-mode noise voltage appears across the isolation device internal to the signal conditioner, and noise coupling is reduced to the parasitic capacitance across the isolation barrier.
Signal conditioners also provide signal amplification to reduce signal-to-noise ratios, level translation, transducer non-linearity correction, and signal filtering. These features both preserve sigvnal integrity and relax receiver requirements. Signal conditioners are often rated to withstand transient events, thus providing a level of protection for the host system against harsh industrial environments and costly damage in fault situations.
Data Communication Platforms
While there are many data communication platforms available today, some are more suitable for particular applications because of their intrinsic qualities. RS-485 has been the industrial standard for many years now. It offers inherent noise immunity, multidrop capability, and communication over distances approaching one mile without repeaters. Ethernet has rapidly gained acceptance in industrial applications, and has become known for its low-cost hardware. Data transfer rates up to 1000 Mbps have reduced early concerns over determinism, and Ethernet is inherently isolated.
Developed for connecting computer peripherals over short distances, USB 2.0 is a widely available serial interface with a data transfer rate of 480 Mbps. USB was not designed for industrial environments, but does find wide usage in laboratory applications and as a temporary connection to systems for configuration or data transfer.