Most process control systems involve a structure of distributed equipment, with sensors and valves at the process equipment and the control calculations and displays located in a remote, centralized facility. Therefore, values of key variables must be communicated between the sensors, calculations and valves (or other final elements). Schematics of typical control systems with analog and digital control are given in Figure 4.0.1.
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Figure 4.0.1a Distributed control with analog control calculations.
Figure 4.0.1b. Distributed control with digital control calculations
Not all sensors and valves require signal transmission. Sensors with local displays and valves requiring manual operation have no signal to transmit. However, many (most) sensors and valves require signal transmission, so that personnel in a single location can manage the entire process. Reliable, accurate and rapid signal transmission is essential for excellent process control.
A schematic of the equipment in a control loop, which presents the most typical signal transmission, is given in Figure 4.02. The most common equipment in the loop is described in this paragraph, while some other possibilities are shown in the figure. The process variable is measured using a sensor applying technology presented in Section 1. Typically, the measured value is converted to a “signal” that can be transmitted. The signal can be electronic or digital, as will be covered in subsequent sections; this conversion is achieved at the location of the sensor. The signal is sent from the transmitter to the control room, where it can be employed for many purposes, such as display to control as shown in Figure 4.02. When the signal is used for control, the value of the controlled variable (signal from the sensor) is used by the controller to determine the value of the controller output. The controller output is transmitted to the final element, which is shown as a valve in Figure 4.02, but could be switching a motor on/off or other automated action. For a control valve, the stem position is affected by air pressure to a pneumatic actuator; therefore, the electrical signal from the controller must be converted to air pressure signal. This conversion is achieved at the valve. The valve stem moves the valve plug, changes the resistance to flow, and the flow rate changes.
Figure 4.0.2. Schematic of the key elements required for monitoring and control.
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New Technology for Signal Transmission initiates Revolution in Process Control
Learning the basics of signal transmission is becoming a greater challenge than it was in previous decades. The challenge results from the recent, rapid change in technology for control signal transmission. From the inception of process control until the 1990’s, each signal involved the value a single variable transmitted in only one direction by pneumatic, electronic, or hydraulic techniques; these are termed analog signals. This lack of two-way communication limits the capability of the system. For example, when the controller output value is sent to the valve (more correctly, to the i/p converter), no information can be returned by the same wire, so that the control system has no confirmation that the valve stem has moved.
Recently, new technology is being used for the signal transmission using digital communication, which has much greater flexibility for transmitting multiple variable values, communicating to both directions and performing calculations. Two-way communication and computation at any element in the system (not just the controller) provides the opportunity for diagnostic information to be communicated about the performance of the sensor and final element. Diagnostics can be used to schedule maintenance, when the maloperation is not too serious, such as a slow drift from good accuracy. When the fault prevents proper control, the system can immediately stop the operation of the control loop and alarm the operations personnel.
Since control equipment has a long lifetime, practicing engineers will encounter many examples of both analog and digital transmission technologies and therefore, must have a basic understanding of both.
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4.1 Transmission Issues
Signal transmission is an integral part of every feedback control loop. We must recall, “a chain is only as strong as its weakest link”. Therefore, excellence process control performance requires the signals to be transmitted between loop elements reliably, rapidly and accurately. To establish a basis for learning methods for signal transmission, we briefly review transmission issues in this section. The relative importance of each item depends on the specific application. For example, fast response is required for controlling a mechanical system with rapid process dynamics, while high reliability is required for a safety-critical application.
| Many methods for communication are in common use in society, e.g., radio and television transmissions. The following material is restricted to applications of signal transmission for automatic control in the process industries. |
The major issues in signal transmission for control are summarized in Table 4.1.1.
Table 4.1.1. Control Transmission Issues
ISSUE
COMMENTS· Accuracy and reproducibility - The signal transmission should be more accurate than the sensor and final element, so that no degradation results from the transmission. Here, accuracy can be taken to mean a difference in the signal value from its exact value.
Recall that the transmission occurs in the feedback loop, so that inaccuracy will affect the performance of feedback control.
Field calibration must be possible without removing the equipment or compromising the safety protection.· Noise sensitivity - The signal can be influenced by “noise”, including electrical signals from other devices. The system must be designed to reduce the effects of noise.
· Reliability - The failure of a signal transmission results in the loss of feedback control. For safety-critical signals, a backup (parallel) transmission path may be required.
Because the equipment may be located outdoors, it must be physically rugged and be resistant to water and significant changes in temperature.
In a typical loop, the elements are connected in series. The reliability of a series of elements is the product of the reliabilities of each element.
The power supplies are important potential sources of failures that can affect many signals simultaneously.· Dynamics - Signal transmission is part of the feedback loop, and any delays degrade control. The transmission should be much faster than other elements in the loop.
Transmission by electronic analog or digital signal is much faster than the dynamics of a typical process element.· Distance - In large plants, signals can be transmitted several thousand meters.
Physical connections have distance limitations. For very long distances, telemetry is used; however, reliability is sacrificed, so that this method is normally restricted to monitoring, with control implemented locally.· Interoperability - We want to be able to use elements manufactured by different suppliers in the same control loop. For example, we want to use a sensor from supplier A, a controller from supplier B, and a valve from supplier C. To achieve this “interoperability”, international standards must exist for the signals being transmitted between elements, i.e., sensors, controllers, and valves.
Standards are easily achieved for analog signals, 4-20 mA (electronic) and 3-15psig (pneumatic).
At the present time, several competing standards exist for digital transmission.· Safety - Naturally, the signal must not compromise the safe operation of the system. Since power is used for the transmission, special considerations are required to prevent combustion or explosion.
The power supplied must be low or a dangerous event must be contained within a controlled environment (enclosure). In addition, a high voltage or current caused by a circuit fault must not be transmitted to a process area where a fuel is present.· Diagnostics and configuration - Ideally, the signal should be able to communicate several values, for example,- confirmation that the signal is being transmitted (live zero)
- confirmation that the signal was received (echo)
- configuration values required for sensors and final elements, e.g., sensor zero and span values.
More limited (analog) systems could provide many independent signals for every variable. However, this approach would be very costly because a separate cable would be required for each signal and is not used in practice.
Digital transmission can communicate many values related to each variable, e.g., process measurement.· Cost - Typically, several transmission methods will satisfy basic requirements, so that benefits and costs must be evaluated to determine the best choice.
Remember that the total cost includes costs of installation, documentation, plant operations, and maintenance over the life of the sensor.
See a reference on engineering economics to learn how to consider costs over time, using principles of the time value of money and profitability measures.
Key question: What should be transmitted?
A major issue in signal transmission is the variable(s) transmitted. In analog systems, this choice is limited to a single variable, so that the choice is obvious. The sensor sends its measured variable to the controller and the controller sends its calculated output to the valve.
With digital transmission, many variables can be transmitted essentially simultaneously. Therefore, we have the possibility for many variables. In addition, we can have two-way transmission, with some transmission in the directions opposite to the arrows in Figure 4.02. As we will see, benefits for digital transmission accrue to a large extent from the ability to transmit additional variables, not from simply duplicating the functions available with analog transmission.
Exercise 4.1.1. Consider the typical control loop in Figure 4.0.2, repeated below.
Suggest variables that can be transmitted in either direction between the following elements in a control system via digital transmission. In answering this question, you can assume that any variable or parameter can be transmitted without significant delay.
a. The sensor and control system.
b. The control system and the valve.
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4.2 Analog Signal Transmission
We have learned that feedback control consists of a loop with common elements, for example, a sensor, control calculation, and final element. Generally, the three elements are distinct physical entities and can be located at significant distances from each other. Therefore, transmission of signals, or information, between the elements is essential.
Figure 4.2.1. Typical analog signal transmission with analog control calculation.
Analog signal transmission was the only method for communication in process control from its inception until the later part of the twentieth century. By “analog”, we mean that the value of a physical variable is an “analogy” to the control system variable. For an example, consider the stirred tank process in Figure 4.2.1 in which the thermocouple measures the liquid temperature in the stirrer tank. When the temperature changes, the millivolt signal from the thermocouple also changes; therefore, the millivolt is an analog signal representing the liquid temperature. The relationship between the hot junction temperature and the millivolt signal for each thermocouple design (here, a J-type) can be found in standard references (Omega Themocouple, 2006). The millivolt is converted into a signal for transmission to a control system for display, recording, and control.
The basic equipment in a loop is shown in Figures 4.2.1 and 4.2.2. The measured value from the sensor is converted to the appropriate signal (physical variable and value) by a transmitter. The converted signal is sent to the control system. The controller can be analog (continuous) or digital (discrete), and naturally, the conversion of the signal for the controller must be appropriate for the type of controller. The controller provides an output that must be converted to a signal for transmission to the final element. The figures show an additional conversion that provides pneumatic pressure to the actuator of a control valve. This design is appropriate for electronic analog signals being transmitted to a pneumatic valve, which is typical in the process industries. However, we recognize that other situations occur, for example, the final element could be speed of an electric motor, which would not require conversion to a continuous pneumatic signal.
Figure 4.2.2. Typical analog signal transmission with digital control calculation.
In the process industries, the following analog signals are in use.
· Physical position (connecting rod)
· Hydraulic pressure
· Pneumatic (air) pressure (3-15 psig)
· Electronic (4-20 mA DC)
In this section, we will restrict the discussion to electronic signals, except for the signal to pneumatic control valves. The other three signals have limitations in distance, but find application in specialized, simple control equipment. The standard electronic analog signal used internationally is 4-20 mA (milliamp) to ensure interoperability.
The use of an analog signal to represent a sensor value involves the concept of a scaled variable that establishes the relationship between a specific value of milliamps and the control system variable. To understand, consider the example given in the following and shown in Figures 4.2.1 and 4.2.2.
Temperature = 145 °C
Temperature sensor/transmitter range = 100 to 200 °C
Therefore, the transmitter zero = 100 °C and the span is 200-100=100 °C
Sensor signal range = 4 to 20 mA
We note that the sensor/transmitter provides a valid indication of temperature within its range. If the temperature exceeds the range, the reported value remains at the appropriate limit, either low or high. The use of a limited range improves the accuracy of the measurement and signal transmission. The transmitter provides the value of milliamps (mA) for transmission, according to the following calculation of the scaled variable, which represents a linear relationship between process variable and signal value.
 | (4.2.1) |
 | (4.2.2) |
with
PV = the process variable (engineering units)
PVscaled = the process variable (scaled variables in % of span)
Zero = the low value of the range of the process variable (engineering units)
Span = (high - low) values of the process variable range (engineering units)
Signalsensor = transmitted signal in milliamps
Applying this concept to the temperature example using equation (4.2.1)
 | (4.2.3) |
Also, the value of PVscaled is 45%.
Equations (4.2.1) and (4.2.2) are valid when the process variable has a value within the range of the sensor. The signal is never less than 4 mA nor greater than 20 mA. Therefore, the engineer must take care when specifying sensor ranges; ranges that are too small to achieve improved accuracy will fail to provide a useful valve during disturbances. Also, care is needed interpreting a signal at either limit of the sensor range.
In many controllers, including all analog (continuous controllers), the calculations are performed in scaled variables, i.e., PVscaled. See Marlin (2000) Chapter 12 or other references for further discussion of the use of scaled variables.
Note that the electronic analog signal contains no information about the process variable type (whether it is temperature, pressure, etc.), variable identification (heat exchanger outlet, reactor bed, etc.) or the transmitter range. Therefore, thorough documentation, calibration and verification are required when installing control equipment so that a signal can be correctly interpreted where it is received.
The transmitter generally is faster and more accurate than other elements in the loop, and the signal transmission is essentially instantaneous and without error. Generally, the sensor and transmitter are purchased as a single unit from the instrument supplier. Therefore, dynamics and accuracy specifications are typically provided for the integrated sensor and transmitter. Typical values are given in Section 2 on sensors.
The control calculation can be analog or digital. If digital, the signal is converted from a current to a digital representation by an analog to digital (A/D) converter. Sampling rates can be over 100,000 samples per second, which is much higher than required for most process applications. However, control of high-speed machinery requires very high sampling rates. The A/D accuracy depends on the equipment design and the number of bits in the digital (binary) representation of the number. The accuracy is determined by the last bit, because signal change smaller in magnitude than the smallest bit does not change the value of the binary number. The accuracy is approximately 1 in 2n-1, where n is the number of bits in the binary number and is usually between 10 and 13. Typically, commercial process control A/D equipment has an accuracy between 1:1024 to 1:8192 (expressed as inaccuracy in fraction of sensor span) and a conversion time much faster than process elements in the loop (0.1 ms) (Liptak, 1999). Because of the speed and accuracy, the A/D conversion has little effect on a typical feedback loop controlling pressure, level, temperature, etc.
The result of a control calculation is sent to the final element. If the control calculations are performed via digital computation, the result must be converted using a digital to analog (D/A) converter. Typically, commercial process control D/A equipment has an accuracy of about 1:1024 (expressed as inaccuracy in fraction of output span, 0-100%) and a conversion time much faster than other elements in the loop (Liptak, 1999). Again, the D/A conversion has little effect on a typical feedback loop controlling pressure, level, temperature, etc.
Typically, the controller output in percent is transmitted to a throttling control valve, i.e., a valve whose opening is adjusted as a continuous variable to determine the flow. Often, the value transmitted is the result of the famous PID calculation. The signal transmitted is the same percentage of the range of the electronic signal. Recalling that the electronic signal is 4-20 mA, the conversion from controller output to current is given in the following linear relationship.
 | (4.2.4) |
with
MV = controller output in % (0-100)
Zero signal = 4 milliamp (live zero)
Signalvalve = signal from controller to valve (i/p converter) in mA (4-20)
For example, if the controller output (MV) were 63%, the signal transmitted would be 14.08 milliamp.
The electronic signal is converted to pneumatic because most control valves employ air pressure to provide the force needed to move the stem position. Using air eliminates the need for electric motors, which could be sources of hazardous power and equipment faults. Naturally, a reliable source of dry compressed air must be provided and distributed throughout the process to every valve actuator. Again, an international standard signal has been agreed; for pneumatic signals, the standard is 3-15 psig. When applying this standard to the example above, the input to the i/p converter would be 14.08 mA and the output would be 11.56 psig. Ideally, the valve stem position for the signal of 11.56 psig would be 63%; the actual position would not be exactly 63% because of calibration inaccuracies and friction.
Exercise 4.2.1 Calculate all signals with units for the system in Figure 4.2.3. You do not have to determine the binary values for the digital numbers, and my assume that all equipment functions perfectly (which does not happen in real life!).
Thermocouple type: K
Temperature transmitter range: 150-400 °C
Set point (SP): 245 °C
Controller tuning: 
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Figure 4.2.3. Loop schematic for Exercise 4.2.1.
Exercise 4.2.2 The electronic signal from the sensor/transmitter to the controller has a value of 1.3 mA. What can you conclude from this value? What is the proper action to be taken by the controller? Can this action be automated?
Exercise 4.2.3 Typical values are given for measured variables in Figure 4.2.4.
a. Propose initial values for sensor ranges for the measured variables.
b. Specify additional information that you would need before you were confident that the values you estimated in part (a) were appropriate for the plant.  |
Figure 4.2.4. Distillation sensors with design values of selected variables.
Exercise 4.2.4 For each of the following sensors, describe the variable measured by the sensor. In addition, define the calculations required to determine the process variable used for control from the signal from the sensor.
a. Orifice flow sensor
b. Thermocouple
c. Level by pressure difference
d. Pressure 4.3 Digital Transmission
Electronic analog transmission has been employed successfully for many decades, so that little benefit would be gained from replacing the same functions via digital communication. Therefore, digital communication has been developed to provide additional capabilities at reasonable cost. In this section, we will restrict the discussion to digital communication linking elements in the real-time control loop. The enhanced transmission must be complemented with increased capabilities in the loop elements, i.e., the sensor and final element. These will be “smart”, that is, they will have memory, programming, and computing capabilities. This design for digital transmission provides distributed computation, which is the true advantage for replacing analog with digital transmission.
The term fieldbus refers to the use of digital transmission and distributed computing for real-time process control. Several system designs exist for fieldbus, and only some features common to most designs are introduced here. |
A typical fieldbus structure is shown in Figure 4.3.1. One important difference from analog transmission is immediately apparent. In analog transmission, individual cables link each sensor and final element to the controller. The multitude of cables is very expensive but has the advantage of limited effect from a single cable fault. On the other hand, the fieldbus structure has one (or a few) cable for the data transmission for all sensors and final elements. This design is much less costly to purchase and install but has the disadvantage of greater effects from a cable fault.
Figure 4.3.1. A typical fieldbus control system structure highlighted with dark blue lines.
The major advantages of fieldbus require that sensors and final elements have digital computing capabilities. With computing at key elements in the control loop, much more information is available and can be provided for improved control. To take advantage of much of this information, we must broaden our view of the control loop, which traditionally involves one-way communication from sensor to controller to value. Now, we seek advantages from two-way communication and calculations at all loop elements. A few examples are given in Table 4.3.1.
Table 4.3.1 Typical communication for analog and digital transmission.
Traditional, analog | Enhanced, digital fieldbus | |
Sensor to controller | Signal representing the measured value sent to the controller | To controller · Measured value · Diagnostic from sensor To sensor · Configuration of sensors (e.g., zero and span values) Calculations at sensor · Filtering measurement · Linearization · Correction for process environment (e.g., orifice for fluid temperature and pressure) which can require the use of several sensors |
Controller to valve | Output of controller calculation sent to the valve (i/p converter) | To valve (to the i/p converter) · Output of controller · Configuration of valve (max/min openings, characteristic, etc.) To controller · position of stem position of valve · diagnostic from valve Calculations at valve · Modification of relationship between control signal and stem position to modify characteristic |
In the traditional analog system, the sensor and valve are passive elements and all decision-making ability resides in the controller. In the digital system, key loop elements send and receive information and perform calculations in real time.
Thus, the fieldbus includes a change from a “controller-centric” distributed digital control system (DCS) design to Field Control System (FCS), in which all key components are actively involved in computation and data storage. |
1. Configuration - A large effort is required to configure (specify parameters like sensor range and valve characteristic) and verify data for elements of the loop. With Fieldbus, configuration can be prepared prior to plant construction and can be loaded and checked quickly. The savings in time and personnel costs can be substantial.
2. Calculations - Many calculations can be performed by the local processors to improve the performance of the elements.
· Sensor nonlinearities can be corrected, e.g., thermocouple conversions from millivolt to temperature.
· Several sensors can be combined to determine a more accurate value of a variable, e.g., density correction for a flow sensor.
· A desired inherent valve characteristic can be programmed into a valve.
3. Multidrop - The fieldbus can connect many elements using the same cable, rather than using individual cables for each signal as required by analog transmission. Again, saving can be substantial.
4. Two-way communication - Any element can send and receive information, and any element can communicate with any other element on the fieldbus.
The distributed computing available in fieldbus makes possible the distribution of the controller calculations. For example, the element performing the controller (e.g., PID) calculation could be physically located at the sensor or valve. However, most plants desire control information to be available at a centralized location, the control room; therefore, the controllers are usually located in this control room.
In fieldbus designs, all elements (sensor, controller and final element) exchange information via digital transmission. We desire to purchase the best elements available from different suppliers.
Therefore, international standards are essential to ensure equipment from different suppliers will function in a network; this is termed interoperability. |
Industry began to develop these standards in 1985, initial fieldbus systems were placed in operation in the 1990’s, and standards and systems continue to evolve.
4.4 Comparison of Transmission Technologies
Electronic analog transmission has been the standard for several decades. As a motivation for changing, we require potential improvements for the digital transmission and smart instrument technology, and potential advantages for digital systems exist in performance and cost. A summary of some key advantages for each technology is given in Table 4.4.1.
Table 4.4.1. Major advantages of analog and digital signal transmission.
Technology | Advantages |
Standard analog transmission | · Lower level of technology requires less skill to install and maintain · Lower impact on plant operation of a single cable failure · Lower installed cost for very small systems |
Fieldbus transmission with “smart” sensors and valves | · Reduced cost for installation reduces cost and can shorten total project time · Sensor and valve diagnostics result in - reduced routine maintenance - faster trouble shooting - solution of incipient failures before they adversely affect the process · Higher accuracy for some sensors that use multiple measurements and non-linear calculations |
Some recent provides experience on the savings for fieldbus over standard analog equipment. Cost reductions for fieldbus equipment were reported to be (Baltus, 2004),
· Wiring - 50%
· Commissioning (checkout and calibration) - 90%
· Space in the control house for instrumentation - 85%
· Maintenace - 50%
Cost advantages have been reported for projects involving many instruments. For example, the instrumentation cost for an acetic acid plant was reduced 31% by using fieldbus technology rather than conventional analog technology (ARC, 2005). It is important to recognize that while the equipment cost was higher for fieldbus, substantial savings were realized in cabling, wiring, calibration and programming.
4.4 Resources for further study
More information is available on industrial signal transmission is available in the following printed resources; pneumatic (Andrew and Williams, vol. 2, 1980; Buckley, 1975; Harriott, 1964; While, 1979), electronic (Wright, 1995; Morrison, 1977), and digital (Linder, 1990; ISA, 1993).
Links to WWW Resources
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