Maintaining Good Signal Integrity
How many times have you been on the telephone with an instrumentation vendor trying to explain an intermittent problem? Their answer is always that it is the other instrument’s problem. You could spend several hours on the telephone back and forth with different vendors trying to track down a gremlin that no one can identify, yet there is proof on your chart recorder of some phantom noise or inaccurate control. The instrumentation vendors may be correct. The problem is probably originating somewhere else, but it’s not in the other vendor’s instrument.
The need to accurately measure and control process equipment is a requirement in today’s automated facilities. Instrumentation is the brains of modern automation plants, and its wiring and sensors are the nerves that connect the remote areas of your processes to the brain. It controls and monitors the operation of the equipment in your plant. In many cases, the signals that are being monitored are in the millivolt range. To get the most out of the control signals that are being read from the process, signal integrity must be maintained from the process all the way to the instrumentation.
Many times shop managers don’t think twice about spending top dollar for quality instrumentation to run their processes, but little thought is given to the wiring carrying the signal from the process to those instruments. On new installations, the cost of wiring can be a significant portion of the instrumentation package.
If proper installation of the wiring is ignored, any gains one was trying to receive by purchasing new instrumentation can be marginalized by unwanted noise on the signal wires. On existing installations where performance may have been compromised by noise, upgrading instruments may mask the problem to some extent if the instrument is of good quality, but it could be a disaster if a budget model is selected. Maintaining the integrity of the control signals should be a primary goal of any instrumentation installation.
The Industrial Measurement Environment
Instrumentation is often located in the field next to the oven or at least somewhere on the shop floor. What makes this area a problem is the harsh electrical environment that equipment located there is exposed to. Unfortunately, this is the area where process variable measurements must be made. The process sensors and associated wiring are most likely near heavy electrical equipment, motor contactors or even exposed to static discharge from welding equipment. Field wiring can often run several hundreds of feet, increasing the likelihood that electrical interference could affect the signal.
The control room is typically much kinder to control signals, but the high concentration of computers, two-way radios and other electronic equipment does provide an opportunity to degrade the quality of your signals. There are many pieces of equipment and natural phenomenon that can interfere with measurement signals. This equipment creates an electronic hazard for the instrumentation that exists in the environment and the signals that pass through it.
Since this environmental hazard typically cannot be eliminated, the effects it has on instrumentation must be understood so it can be isolated and its effects can be removed or at least minimized. It is well understood how audio signals in high-fidelity equipment must be cared for to achieve the best sound quality. Process instrumentation is no different. By understanding the ways that noise affects your control system, you can take the necessary steps to avoid these problems.
Some Methods to Ensure Signal Quality
The easiest way to ensure good signal integrity is to employ the use of signal conditioners. Signal conditioners play an important part in instrumentation by ensuring the signals measured by sensors in the field are transmitted to the control instruments representing exactly the same conditions as were measured in the process. The long distances a signal has to travel between the process and the control instrumentation offers ample opportunity for electromagnetic pollution to degrade the control signal.
Signal conditioners provide two primary functions: maintaining signal integrity and signal isolation. Even though a single instrument can provide both functions, there are distinct and individual functions that each one provides. The most common use of a signal conditioner is signal conversion, and the most common conversion is to change any process signal from its original form and transmit it as 4-20 milliamps. A 4-20 mA current loop is a very robust carrier signal that is virtually impervious to noise. It is the preferred method of signal transmission over long distances. Why does a 4-20 mA loop have such good noise immunity? It has to do with the transmitter.
All of the resistive components in a circuit drop voltage in proportion to their percentage of total resistance in the circuit. So the more resistance loop the component has, the more voltage it will drop in the circuit – Ohm’s law. Every current transmitter has some output resistance that it contributes to the circuit. In proportion to the loop resistance, a transmitter’s input impedance can almost be considered as infinite.
A transmitter’s typical output resistance may be 3-5 MΩ, whereas the loop resistor is fixed at 250 Ω. Under normal conditions, the transmitter is acting as a current source, and its resistance is not taken into account when dropping voltage. When an additional voltage source is introduced to the loop (e.g., unexpected noise), the voltage is dropped across each of the resistors in the loop in proportion to its size.
If we remember the superposition theorem, the 5 MΩ resistance of the transmitter is no longer a current source but a resistive load on the circuit with respect to the noise. The signal conditioner becomes a passive component in the circuit and absorbs most of the noise voltage with its 5 MΩ load, whereas the 250 Ω resistor on the receiving device receives only a tiny fraction. For example, if the noise voltage source induced a voltage of 100 volts, Ohm’s law would dictate that the signal conditioner would absorb 99.995 volts, whereas the control instrument will receive 5 millivolts. This is 1/200th of the 1 volt minimum that can be measured across the 250 Ω resistor on a 4-20 mA loop. There is no better way to eliminate noise from your process than by converting your transmission signals to 4-20 mA.
Signal integrity can be affected by wiring and shielding practices. Wiring your process using twisted-pair cabling provides a significant level of protection from noise that may be coming from outside of the process. In this type of wiring, a pair of conductors in the same circuit is twisted together for the purpose of canceling out external electromagnetic interference. Since noise that is picked up by the wire along its path to the control panel affects both wires equally but oppositely, a good control device is able to reject the noise and recover the signal fully. This is called common mode rejection.
It is tempting to run all of your wiring through a single conduit. The best words of advice for this would be, “Don’t do it!” This practice is an open invitation to added troubles and sorrows.
The magnetic coupling or induced voltages caused by the proximity of high voltage and currents next to your signal wires can induce some very high voltages on your signal wires. These voltages at a minimum could interfere with your process and at a maximum damage the inputs on your instrumentation. Most signal conditioners have input circuitry designed to prevent damage due to these high transients. So while your chart recorder is recording chaos, at least you won’t have to worry about replacing the entire recorder.
Ground loops are the most common noise problems in large-scale electrical systems and stem from poor grounding practices. It is a major misconception that earth ground at one location contains the same voltage potential as earth ground at any other location. Ground is an often misunderstood electrical concept. The ground potential where you are currently located can be several volts above or below the ground potential of another part of your building. A nearby lightning strike could cause this potential to jump several hundred to thousands of volts.
If you have instrumentation grounded in separate locations in your facility where these ground potentials do exist, you could very easily experience a ground loop if your system is not employing some type of isolation. The noise caused by a ground loop can wreak havoc on your system just by being there. There is also the issue of the induced voltages they create as they travel near and on signal wires. A common sign that a ground loop exists is the presence of induced 60-Hertz power-line noise on the circuit, which can be easily measured with a voltmeter.
One way to eliminate ground loops is to employ a single-ground concept where no ground loops can be created. All circuit grounds are returned to a common point. This is often done through bonding and the use of bus bars to carry all the ground signals to a single point. This may not be practical in many facilities due to the physical size of the operation. This is where the use of a signal conditioner designed for isolation comes into play.
An isolator is one of the easiest ways to eliminate a ground-loop problem. In its simplest terms, isolation interrupts current flow between the different potentials in the grounds. This is accomplished by inserting some type of electrical or electronic device, such as a transformer or a signal conditioner, to break the physical connection between the two grounds. While the isolator interrupts the ground-loop circuit, it allows the desired control signal to be passed through unencumbered. Isolation is a very cost-effective method for removing noise and transients due to ground loops.
One good lightning strike in proximity to your process could do considerable damage to your process equipment. The discharge from an indirect strike could put several thousand volts and amps on your power and signal wires. Typically, the further the sensor and instrument are apart and the longer the wiring run is connecting the two, the greater the chance of experiencing capacitive coupling from a lightning strike, and the higher the voltages and currents are.
In situations where the electronic equipment narrowly escapes destruction, its circuits’ operation may become intermittent or its performance may begin to deteriorate. A lightning arrester is a type of signal conditioner that is used to divert induced voltages that may be large enough to damage instrumentation. When properly installed, lightning arresters redirect damaging surges to ground where they belong. With the capability to withstand 10,000 volts and up to 5,000 amps, you will not be left with the unpleasant task of replacing equipment after a storm.
At the heart of it all, an arrester is a switch that diverts high voltages and currents to a path to ground. There are many technologies that make up this type of switch, but a very common one is a metal oxide varistor (MOV). An MOV is a semiconductor that is sensitive to voltage. At normal operating voltages, the MOV acts as an insulator and will not conduct current. At higher threshold voltages, it responds like a conductor. This threshold voltage is determined by the manufacturer by the doping process.
Modern instrumentation uses a wide variety of sensor inputs with varying voltage input levels. The use of a general-purpose lightning arrester would provide some protection but is not optimal. Arresters are designed to work specifically with the particular input signals that are needed, with their specific voltage requirements taken into consideration. The operating requirements for protecting an instrument’s power circuits would be different than one protecting its thermocouple input or its Ethernet IP connection. Selecting the proper arrester ensures signal integrity is maintained as well as solid protection for the instrument.
You have spent a lot of money on equipment to provide the best results for your customers. The probes, sensors and instrumentation designed to operate your processes for peak performance can be undermined by the smallest amount of interference.
Having and implementing a strategy to deal with environmental noise is the difference between a good process and an exceptional one, and it is something that should be required of anyone installing systems into your facility. Signal conditioners are the last step that allows an operator to provide the very best in quality and squeeze every bit of performance out of a system into which you are heavily invested.
For more information: Contact Clayton Wilson, Yokogawa Corporation of America, 2 Dart Rd., Newnan, GA 30265; tel: 678-423-2524; fax: 770-251-6427; e-mail: firstname.lastname@example.org; web: www.us.yokogawa.com