Data communications in industrial environments can present special problems. Lines must be run over long distances, often ranging from thousands of feet to several miles, indoors and outdoors, from the field to the control room, and from building to building. These data communication systems must be able to function in the presence of electrical transients, and noise, ground loops, and surges from nearby lightning strikes.
Electromagnetic interference (EMI) occurs when devices either affect each other unintentionally or are affected by natural causes. While advances in technology stimulate system upgrades, improve performance, and perhaps even lower costs, new technology can also create new potential for EMI.
Elimination of interference in a system can be difficult because universal solutions to noise problems do not exist. However, given today’s proliferation of electronics and continually increasing circuit speeds, electromagnetic interference is on the rise, making the need for increased and more effective protection of signal integrity even more critical.
Three basic elements are required for a noise problem: 1) a noise source to generate the noise, 2) a receiving device that is affected by the noise, and 3) a coupling channel between the source and the receptor. Signal and power conductors are the simplest means of interconnecting different elements of an electronic system, and it is not uncommon for these lines to be hundreds or thousands of feet long. As they wind their way from source to destination, the lines often pass through high electric and magnetic fields, which can severely distort the intended signals. Other threats to signal integrity include interference caused by ground loops and differences in ground potentials. Signal and power wiring can be a conductive path for noise, but can also be sources and receptors of noise.
To effectively eliminate or minimize interference problems caused by electric fields, magnetic fields, and ground loops, one of the three elements necessary for a noise problem must be minimized, diverted, or eliminated. In most cases, the element over which a designer has the most control is the coupling path.
Capacitive or Electric Coupling
Wires and cables possess parasitic capacitance to each other, to ground, and to other pieces of equipment, and these capacitances are coupling paths for electric fields. Any piece of plant equipment or wiring can develop an electric charge, or potential. If this charge changes, the changing electric field that results can couple capacitively to other equipment or wiring, creating noise.
An easy and effective way to minimize capacitively coupled interference is to use cable shielding. The shield is a Gaussian or equi-potential surface on which electric fields can terminate and return to ground without affecting the internal conductors.
Shielding is only effective against electric fields if it provides a low impedance path to ground; a floating shield provides no protection against interference. When the shield in Figure 1 is grounded, capacitance Cs2 disappears and parasitic capacitances Cp and Cs1 are grounded, thus nearly eliminating the path between the noise source and the conductor. There can still be a small capacitance along this path due to imperfections in the shield, holes in a braided shield, or the length of conductor extending beyond the shield, so careful attention is necessary to avoid “leaky” shields.
As the capacitance between two conductors is inversely proportional to the distance between them, another way to reduce capacitive coupling is to simply increase the distance between the victim cable and the source cable. In any case, it is always a good idea to route “noisy” cables such as power input wiring, motor control wiring, and relay control wiring separately from “quiet” cables such as analog I/O lines, digital I/O lines, and LAN connections.
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.
The data communication platform most suitable for a specific application must be determined through careful consideration of data transfer rate, data security, redundancy, location, environmental conditions, and installation and maintenance costs. Industrial applications that employ signal conditioners to measure parameters including temperature, pressure, rpm, voltage, and current run the gamut from wind turbines, to garbage truck transmissions, to detection of human heartbeats or hazardous materials in shipping containers.
Sensors in wind turbine controllers, for example, measure many parameters, including generator voltage and current, frequency, shaft rotational speed, wind direction, vibration, hydraulic pressure, and component temperatures. This harsh environment with its electrical noise, wide temperature variation, and high vibration requires rugged signal conditioners for sensor interface.
In the automotive, truck, and helicopter industries, to give another example, research and development efforts to improve transmissions require high performance, isolated signal conditioners to preserve the integrity of signals from frequency, pressure, and temperature sensors (see Figure 4).
This article was written by John Lehman, Engineering Manager at Dataforth Corporation, Tuscon, AZ. For more information, Click Here .