Design Article
Simplifying design of industrial process-control systems with PLC evaluation boards (Part 1 of 2)
Colm Slattery, Derrick Hartmann, and Li Ke, Analog Devices Incorporated
8/24/2009 4:52 PM EDT
PLC evaluation board applications for industrial process-control
systems are diverse, ranging from simple traffic control to complex
electrical power grids, from environmental control systems to
oil-refinery process control. The intelligence of these automated
systems lies in their measurement and control units. The two most
common computer-based systems to control machines and processes,
dealing with the various analog and digital inputs and outputs, are
programmable logic controllers (PLCs) and distributed control systems
(DCSs). These systems comprise power supplies, central processor units
(CPUs), and a variety of analog-input, analog-output, digital-input,
and digital-output modules.
The standard communications protocols have existed for many years; the ranges of analog variables are dominated by 4 mA to 20 mA, 0 V to 5 V, 0 V to 10 V, +/-5 V, and +/-10 V. There has been much discussion about wireless solutions for next-generation systems, but designers still claim that 4 mA to 20 mA communications and control loops will continue to be used for many years. The criteria for the next generation of these systems will include higher performance, smaller size, better system diagnostics, higher levels of protection, and lower cost—all factors that will help manufacturers differentiate their equipment from that of their competitors.
This article examines the key performance requirements of process-control systems and the analog input/output modules they contain—and introduces an industrial process-control evaluation system that integrates these building blocks using the latest integrated-circuit technology. The article also looks at the challenges of designing a robust system that will withstand the electrical fast transients (EFTs), electrostatic discharges (ESDs), and voltage surges found in industrial environments—and present test data that verifies design robustness.
PLC Overview with Application Example
Figure 1 shows a basic process-control system building block. A process variable, such as flow rate or gas concentration, is monitored via the input module. The information is processed by the central control unit; and some action is taken by the output module, which, for example, drives an actuator.
Figure 1: Typical top-level PLC system
Figure 2 shows a typical industrial subsystem of this type. Here a CO2 gas sensor determines the concentration of gas accumulated in a protected area and transmits the information to a central control point. The control unit consists of an analog input module that conditions the 4 mA to 20 mA signal from the sensor, a central processing unit, and an analog output module that controls the required system variable. The current loop can handle large capacitive loads—often found on hundreds-of-meters long communications paths experienced in some industrial systems. The output of the sensor element, representing gas concentration levels, is transformed into a standard 4 mA to 20 mA signal, which is transmitted over the current loop. This simplified example shows a single 4 mA to 20 mA sensor output connected to a single-channel input module and a single 0 V to 10 V output. In practice, most modules have multiple channels and configurable ranges.
The resolution of input/output modules typically ranges from 12 to 16 bits, with 0.1% accuracy over the industrial temperature range. Input ranges can be as small as +/-10 mV for bridge transducers and as large as +/-10 V for actuator controllers—or 4 mA to 20 mA currents in process-control systems. Analog output voltage and current ranges typically include +/-5 V, +/-10 V, 0 V to 5 V, 0 V to 10 V, 4 mA to 20 mA, and 0 mA to 20 mA. Settling-time requirements for digital-to-analog converters (DACs) vary from 10 us to 10 ms, depending on the application and the circuit load.
Figure 2: Gas Sensor
The 4 mA to 20 mA range is mapped to
represent the normal gas
detection range; current values outside this range can be used to
provide
fault-diagnostic information, as shown in Table 1.
Table 1: Assigning currents outside the 4 mA to 20 mA output range.
PLC Evaluation System
The PLC evaluation system described here integrates all the stages needed to generate a complete input/output design. It contains four fully isolated ADC channels, an ARM7 microprocessor with RS-232 interface, and four fully isolated DAC output channels. The board is powered by a dc supply. Hardware-configurable input ranges include 0 V to 5 V, 0 V to 10 V, +/-5 V, +/-10 V, 4 mA to 20 mA, 0 mA to 20 mA, +/-20 mA, as well as thermocouple and RTD. Software-programmable output ranges include 0 V to 5 V, 0 V to 10 V, +/-5 V, +/-10 V, 4 mA to 20 mA, 0 mA to 20 mA, and 0 mA to 24 mA.
Figure 3: Analog input/output module.
Output Module:
Table 2 highlights some key specifications of PLC output
modules. Since the true system accuracy lies within the measurement
channel (ADC), the control mechanism (DAC) requires only enough
resolution to tune the output. For high-end systems, 16-bit resolution
is required. This requirement is actually quite easy to satisfy using
standard digital-to-analog architectures. Accuracy is not crucial;
12-bit integral nonlinearity (INL) is generally adequate for high-end
systems.
Calibrated accuracy of 0.05% at 25C is easily achievable by overranging the output and trimming to achieve the desired value. Today's 16-bit DACs, such as the AD5066, offer 0.05 mV typical offset error and 0.01% typical gain error at 25C, eliminating the need for calibration in many cases. Total accuracy error of 0.15% sounds manageable but is actually quite aggressive when specified over temperature. A 30 ppm/C output drift can add 0.18% error over the industrial temperature range.
Table 2: Output module specifications.
Output modules may have current outputs, voltage outputs, or a combination. A classical solution that uses discrete components to implement a 4 mA to 20 mA loop is shown in Figure 4. The AD5660 16-bit nanoDAC converter provides a 0 V to 5 V output that sets the currrent through sense resistors, RS, and therefore, a through R1. This current is mirrored through R2.
Setting RS = 15 kohm, R1 = 3 kohm, R2 = 50 ohm and using a 5-V DAC will result in IR2 = 20 mA max.
Figure 4. Discrete 4 mA to 20 mA implementation.
This discrete design suffers from many drawbacks: Its high component
count engenders significant system complexity, board size, and cost.
Calculating total error is difficult, with multiple components adding
varying degrees of error with coefficients that can be of differing
polarities. The design does not provide short-circuit
detection/protection or any level of fault diagnostics. It does not
include a voltage output, which is required in many industrial control
modules. Adding any of these features would increase the design
complexity and the number of components. A better solution would be to
integrate all of the above on a single IC, such as the AD5412/AD5422
low-cost, high-precision, 12-/16-bit digital-to-analog converters. They
provide a solution that offers a fully integrated programmable current
source and programmable voltage output designed to meet the
requirements of industrial process-control applications.
Figure 5. AD5422 programmable voltage/current output.
The output current range is programmable to 4 mA to 20 mA, 0 mA to 20 mA, or 0 mA to 24 mA overrange function. A voltage output, available on a separate pin, can be configured to provide 0 V to 5 V, 0 V to 10 V, +/-5 V, or +/-10 V ranges, with a 10% overrange available on all ranges. Analog outputs are short-circuit protected, a critical feature in the event of miswired outputs—for example, when the user connects the output to ground instead of to the load. The AD5422 also has an open-circuit detection feature that monitors the current-output channel to ensure that no fault has occurred between the output and the load. In the event of an open circuit, the FAULT pin will go active, alerting the system controller. The AD5750 programmable current/voltage output driver features both short-circuit detection and protection.
Figure 6 shows the output module used in the PLC evaluation system. While earlier systems typically needed 500 V to 1 kV of isolation, today >2 kV is generally required. The ADuM1401 digital isolator uses iCoupler technology to provide the necessary isolation between the MCU and remote loads, or between the input/output module and the backplane. Three channels of the ADuM1401 communicate in one direction; the fourth channel communicates in the opposite direction, providing isolated data readback from the converters. For newer industrial designs, the ADuM3401 and other members of its family of digital isolators provide enhanced system-level ESD protection.
Figure 6. Output module block level.
The AD5422 generates its own logic supply (DVCC), which can be directly connected to the field side of the ADuM1401, eliminating the need to bring a logic supply across the isolation barrier. The AD5422 includes an internal sense resistor, but an external resistor (R1) can be used when lower drift is required. Because the sense resistor controls the output current, any drift of its resistance will affect the output. The typical temperature coefficient of the internal sense resistor is 10 ppm/C to 20 ppm/C, which could add 0.12% error over a 60C temperature range. In high-performance system applications, an external 2-ppm/C sense resistor could be used to keep drift to less than 0.016%.
The AD5422 has an internal 10-ppm/C max voltage reference that can be enabled on all four output channels in the PLC evaluation system. Alternatively, the ADR445 ultralow-noise XFET voltage reference, with its 0.04% initial accuracy and 3 ppm/C, can be used on two output channels, allowing performance comparison and a choice of internal vs. external reference, depending on the total required system performance.
Input Module: The input module design specifications are similar to those of the output module. High resolution and low noise are generally important. In industrial applications, a differential input is required when measuring low-level signals from thermocouples, strain gages, and bridge-type pressure sensors to reject common-mode interference from motors, ac power lines, or other noise sources that inject noise into the analog inputs of the analog-to-digital converter (ADCs).
Sigma-delta ADCs are the most popular choice for input modules, as they provide high accuracy and resolution. In addition, internal programmable-gain amplifiers (PGAs) allow small input signals to be measured accurately. Figure 7 shows the input module design used in the evaluation system. The AD7793 3-channel, 24-bit sigma-delta ADC is configured to accommodate a large range of input signals, such as 4 mA to 20 mA, +/-10 V, as well as small signal inputs directly from sensors.
Figure 7. Input module design.
(Click on image to enlarge)
Care was taken to allow this universal input design to be easily adapted for RTD/thermocouple modules. As shown, two input terminal blocks are provided per input channel. One input allows for a direct connection to the AD7793. The user can program the internal PGA to provide analog gains up to 128. The second input allows the signal to be conditioned through the AD8220 JFET-input instrumentation amplifier. In this case, the input signal is attenuated, amplified, and level shifted to provide a single-ended input to the ADC. In addition to providing the level shifting function, the AD8220 also features very good common-mode rejection, important in applications having a wide dynamic range.
The low-power, high-performance AD7793 consumes <500 A, and the AD8220 consumes <750 A. This channel is designed to accept 4 mA to 20 mA, 0 V to 5 V, and 0 V to 10 V analog inputs. Other channels in the input module have been designed for bipolar operation to accept +/-5 V and +/-10 V input signals.
To measure a 4 mA to 20 mA input signal, a low-drift precision resistor can be switched (S4) into the circuit. In this design, its resistance is 250 ohm, but any value can be used as long as the generated voltage is within the input range of the AD8220.
S4 is left open when measuring a voltage.
Isolation is required for most input-module designs. Figure 7 shows how isolation was implemented on one channel of the PLC evaluation system. The ADuM5401 4-channel digital isolator uses isoPower6 technology to provide 2.5-kV rms signal and power isolation. In addition to providing four isolated signal channels, the ADuM5401 also contains an isolated dc-to-dc converter that provides a regulated 5-V, 500-mW output to power the analog circuitry of the input module.
Complete System: An overview of the complete system is shown in Figure 8. The ADuC7027 precision analog microcontroller is the main system controller. Featuring the ARM7TDMI core, its 32-bit architecture allows easy interface to 24-bit ADCs. It also supports a 16-bit thumb mode, which allows for greater code density if required. The ADuC7027 has 16 kB of on-board flash memory and allows interfacing to up to 512 kB external memory. The ADP3339 high-accuracy, low-dropout regulator (LDO) provides the regulated supply to the microcontroller.
Figure 8. System-level design.
(Click on image to enlarge)
Communication between the evaluation board and the PC is provided via the ADM3251E isolated RS-232 transceiver. The ADM3251E incorporates isoPower technology—making a separate isolated dc-to-dc converter unnecessary. It is ideally suited to operation in electrically harsh environments or where RS-232 cables are frequently plugged in or unplugged, as the RS-232 pins, Rx and Tx, are protected against electrostatic discharges of up to +/-15 kV.
Look for Part 2 next week including information on the evaluation system software, analog input and output protection, and IEC test results.
References
Authors
Colm Slattery is an applications engineer for Analog Devices Precision Converters group in Limerick, Ireland. He graduated from the University of Limerick with a bachelor's degree in engineering. He joined Analog Devices Digital-to-Analog Converter group as a test engineer and has spent three years working for Analog Devices in China. He can be reached via email at colm.slattery@analog.com.
Derrick Hartmann is an applications engineer in the Digital-to-Analog Converter group at Analog Devices in Limerick, Ireland. Derrick joined ADI in 2008 after graduating with a bachelor's degree in engineering from the University of Limerick. He can be reached via email at derrick.hartmann@analog.com.
Li Ke joined Analog Devices in 2007 and is an applications engineer with the Precision Converters product line, located in Shanghai, China. Previously, he spent four years as an R&D engineer with the Chemical Analysis group at Agilent Technologies. Li received a master's degree in biomedical engineering in 2003 and a bachelor's degree in electric engineering in 1999, both from Xi'an Jiaotong University. He has been a professional member of the Chinese Institute of Electronics since 2005. He can be reached at li.ke@analog.com.
The standard communications protocols have existed for many years; the ranges of analog variables are dominated by 4 mA to 20 mA, 0 V to 5 V, 0 V to 10 V, +/-5 V, and +/-10 V. There has been much discussion about wireless solutions for next-generation systems, but designers still claim that 4 mA to 20 mA communications and control loops will continue to be used for many years. The criteria for the next generation of these systems will include higher performance, smaller size, better system diagnostics, higher levels of protection, and lower cost—all factors that will help manufacturers differentiate their equipment from that of their competitors.
This article examines the key performance requirements of process-control systems and the analog input/output modules they contain—and introduces an industrial process-control evaluation system that integrates these building blocks using the latest integrated-circuit technology. The article also looks at the challenges of designing a robust system that will withstand the electrical fast transients (EFTs), electrostatic discharges (ESDs), and voltage surges found in industrial environments—and present test data that verifies design robustness.
PLC Overview with Application Example
Figure 1 shows a basic process-control system building block. A process variable, such as flow rate or gas concentration, is monitored via the input module. The information is processed by the central control unit; and some action is taken by the output module, which, for example, drives an actuator.
Figure 1: Typical top-level PLC system
Figure 2 shows a typical industrial subsystem of this type. Here a CO2 gas sensor determines the concentration of gas accumulated in a protected area and transmits the information to a central control point. The control unit consists of an analog input module that conditions the 4 mA to 20 mA signal from the sensor, a central processing unit, and an analog output module that controls the required system variable. The current loop can handle large capacitive loads—often found on hundreds-of-meters long communications paths experienced in some industrial systems. The output of the sensor element, representing gas concentration levels, is transformed into a standard 4 mA to 20 mA signal, which is transmitted over the current loop. This simplified example shows a single 4 mA to 20 mA sensor output connected to a single-channel input module and a single 0 V to 10 V output. In practice, most modules have multiple channels and configurable ranges.
The resolution of input/output modules typically ranges from 12 to 16 bits, with 0.1% accuracy over the industrial temperature range. Input ranges can be as small as +/-10 mV for bridge transducers and as large as +/-10 V for actuator controllers—or 4 mA to 20 mA currents in process-control systems. Analog output voltage and current ranges typically include +/-5 V, +/-10 V, 0 V to 5 V, 0 V to 10 V, 4 mA to 20 mA, and 0 mA to 20 mA. Settling-time requirements for digital-to-analog converters (DACs) vary from 10 us to 10 ms, depending on the application and the circuit load.
Figure 2: Gas Sensor
Table 1: Assigning currents outside the 4 mA to 20 mA output range.
PLC Evaluation System
The PLC evaluation system described here integrates all the stages needed to generate a complete input/output design. It contains four fully isolated ADC channels, an ARM7 microprocessor with RS-232 interface, and four fully isolated DAC output channels. The board is powered by a dc supply. Hardware-configurable input ranges include 0 V to 5 V, 0 V to 10 V, +/-5 V, +/-10 V, 4 mA to 20 mA, 0 mA to 20 mA, +/-20 mA, as well as thermocouple and RTD. Software-programmable output ranges include 0 V to 5 V, 0 V to 10 V, +/-5 V, +/-10 V, 4 mA to 20 mA, 0 mA to 20 mA, and 0 mA to 24 mA.
Figure 3: Analog input/output module.
Calibrated accuracy of 0.05% at 25C is easily achievable by overranging the output and trimming to achieve the desired value. Today's 16-bit DACs, such as the AD5066, offer 0.05 mV typical offset error and 0.01% typical gain error at 25C, eliminating the need for calibration in many cases. Total accuracy error of 0.15% sounds manageable but is actually quite aggressive when specified over temperature. A 30 ppm/C output drift can add 0.18% error over the industrial temperature range.
Table 2: Output module specifications.
Output modules may have current outputs, voltage outputs, or a combination. A classical solution that uses discrete components to implement a 4 mA to 20 mA loop is shown in Figure 4. The AD5660 16-bit nanoDAC converter provides a 0 V to 5 V output that sets the currrent through sense resistors, RS, and therefore, a through R1. This current is mirrored through R2.
Setting RS = 15 kohm, R1 = 3 kohm, R2 = 50 ohm and using a 5-V DAC will result in IR2 = 20 mA max.
Figure 4. Discrete 4 mA to 20 mA implementation.
Figure 5. AD5422 programmable voltage/current output.
The output current range is programmable to 4 mA to 20 mA, 0 mA to 20 mA, or 0 mA to 24 mA overrange function. A voltage output, available on a separate pin, can be configured to provide 0 V to 5 V, 0 V to 10 V, +/-5 V, or +/-10 V ranges, with a 10% overrange available on all ranges. Analog outputs are short-circuit protected, a critical feature in the event of miswired outputs—for example, when the user connects the output to ground instead of to the load. The AD5422 also has an open-circuit detection feature that monitors the current-output channel to ensure that no fault has occurred between the output and the load. In the event of an open circuit, the FAULT pin will go active, alerting the system controller. The AD5750 programmable current/voltage output driver features both short-circuit detection and protection.
Figure 6 shows the output module used in the PLC evaluation system. While earlier systems typically needed 500 V to 1 kV of isolation, today >2 kV is generally required. The ADuM1401 digital isolator uses iCoupler technology to provide the necessary isolation between the MCU and remote loads, or between the input/output module and the backplane. Three channels of the ADuM1401 communicate in one direction; the fourth channel communicates in the opposite direction, providing isolated data readback from the converters. For newer industrial designs, the ADuM3401 and other members of its family of digital isolators provide enhanced system-level ESD protection.
Figure 6. Output module block level.
The AD5422 generates its own logic supply (DVCC), which can be directly connected to the field side of the ADuM1401, eliminating the need to bring a logic supply across the isolation barrier. The AD5422 includes an internal sense resistor, but an external resistor (R1) can be used when lower drift is required. Because the sense resistor controls the output current, any drift of its resistance will affect the output. The typical temperature coefficient of the internal sense resistor is 10 ppm/C to 20 ppm/C, which could add 0.12% error over a 60C temperature range. In high-performance system applications, an external 2-ppm/C sense resistor could be used to keep drift to less than 0.016%.
The AD5422 has an internal 10-ppm/C max voltage reference that can be enabled on all four output channels in the PLC evaluation system. Alternatively, the ADR445 ultralow-noise XFET voltage reference, with its 0.04% initial accuracy and 3 ppm/C, can be used on two output channels, allowing performance comparison and a choice of internal vs. external reference, depending on the total required system performance.
Input Module: The input module design specifications are similar to those of the output module. High resolution and low noise are generally important. In industrial applications, a differential input is required when measuring low-level signals from thermocouples, strain gages, and bridge-type pressure sensors to reject common-mode interference from motors, ac power lines, or other noise sources that inject noise into the analog inputs of the analog-to-digital converter (ADCs).
Sigma-delta ADCs are the most popular choice for input modules, as they provide high accuracy and resolution. In addition, internal programmable-gain amplifiers (PGAs) allow small input signals to be measured accurately. Figure 7 shows the input module design used in the evaluation system. The AD7793 3-channel, 24-bit sigma-delta ADC is configured to accommodate a large range of input signals, such as 4 mA to 20 mA, +/-10 V, as well as small signal inputs directly from sensors.
Figure 7. Input module design.
(Click on image to enlarge)
Care was taken to allow this universal input design to be easily adapted for RTD/thermocouple modules. As shown, two input terminal blocks are provided per input channel. One input allows for a direct connection to the AD7793. The user can program the internal PGA to provide analog gains up to 128. The second input allows the signal to be conditioned through the AD8220 JFET-input instrumentation amplifier. In this case, the input signal is attenuated, amplified, and level shifted to provide a single-ended input to the ADC. In addition to providing the level shifting function, the AD8220 also features very good common-mode rejection, important in applications having a wide dynamic range.
The low-power, high-performance AD7793 consumes <500 A, and the AD8220 consumes <750 A. This channel is designed to accept 4 mA to 20 mA, 0 V to 5 V, and 0 V to 10 V analog inputs. Other channels in the input module have been designed for bipolar operation to accept +/-5 V and +/-10 V input signals.
To measure a 4 mA to 20 mA input signal, a low-drift precision resistor can be switched (S4) into the circuit. In this design, its resistance is 250 ohm, but any value can be used as long as the generated voltage is within the input range of the AD8220.
S4 is left open when measuring a voltage.
Isolation is required for most input-module designs. Figure 7 shows how isolation was implemented on one channel of the PLC evaluation system. The ADuM5401 4-channel digital isolator uses isoPower6 technology to provide 2.5-kV rms signal and power isolation. In addition to providing four isolated signal channels, the ADuM5401 also contains an isolated dc-to-dc converter that provides a regulated 5-V, 500-mW output to power the analog circuitry of the input module.
Complete System: An overview of the complete system is shown in Figure 8. The ADuC7027 precision analog microcontroller is the main system controller. Featuring the ARM7TDMI core, its 32-bit architecture allows easy interface to 24-bit ADCs. It also supports a 16-bit thumb mode, which allows for greater code density if required. The ADuC7027 has 16 kB of on-board flash memory and allows interfacing to up to 512 kB external memory. The ADP3339 high-accuracy, low-dropout regulator (LDO) provides the regulated supply to the microcontroller.
Figure 8. System-level design.
(Click on image to enlarge)
Communication between the evaluation board and the PC is provided via the ADM3251E isolated RS-232 transceiver. The ADM3251E incorporates isoPower technology—making a separate isolated dc-to-dc converter unnecessary. It is ideally suited to operation in electrically harsh environments or where RS-232 cables are frequently plugged in or unplugged, as the RS-232 pins, Rx and Tx, are protected against electrostatic discharges of up to +/-15 kV.
Look for Part 2 next week including information on the evaluation system software, analog input and output protection, and IEC test results.
References
- http://en.wikipedia.org/wiki/Programmable_logic_controller
- http://en.wikipedia.org/wiki/Distributed_control_system
- www.analog.com/en/digital-to-analog-converters/products/evaluation-boardstools/CU_eb_PLC_DEMO_SYSTEM/resources/fca.html
- Information on all ADI components can be found at www.analog.com
- www.analog.com/en/interface/digital-isolators/products/CU_over_iCoupler_Digital_Isolation/fca.html
- www.analog.com/en/interface/digital-isolators/products/overview/CU_over_isoPower_Isolated_dc-to-dc_Power/resources/fca.html
- www.analog.com/en/analog-microcontrollers/products/index.html
- www.ni.com/labview
Authors
Colm Slattery is an applications engineer for Analog Devices Precision Converters group in Limerick, Ireland. He graduated from the University of Limerick with a bachelor's degree in engineering. He joined Analog Devices Digital-to-Analog Converter group as a test engineer and has spent three years working for Analog Devices in China. He can be reached via email at colm.slattery@analog.com.
Derrick Hartmann is an applications engineer in the Digital-to-Analog Converter group at Analog Devices in Limerick, Ireland. Derrick joined ADI in 2008 after graduating with a bachelor's degree in engineering from the University of Limerick. He can be reached via email at derrick.hartmann@analog.com.
Li Ke joined Analog Devices in 2007 and is an applications engineer with the Precision Converters product line, located in Shanghai, China. Previously, he spent four years as an R&D engineer with the Chemical Analysis group at Agilent Technologies. Li received a master's degree in biomedical engineering in 2003 and a bachelor's degree in electric engineering in 1999, both from Xi'an Jiaotong University. He has been a professional member of the Chinese Institute of Electronics since 2005. He can be reached at li.ke@analog.com.
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