7 Ways to Boost Your PLC System [July 2012]

July 18, 2012 12:16 pm

For decades, PLCs have controlled most production and automation through reliable, discrete logic and straightforward analog I/O. While most industrial applications are well-served by these capabilities, today’s industrial machines continue to push the performance boundaries of what traditional PLCs can provide. This article explores different technologies that can integrate with PLC-based automation systems to increase efficiency, reduce energy and maintenance costs, and stay competitive with existing equipment
1. High – Performance Analog MeasurementsWe live in an analog world and often find ourselves in need of high-performance analog measurements as we push the limits of automation technology. Can programmable logic controllers (PLCs) keep up?
New Technology for High-Performance Analog MeasurementsImproving Your Control ApplicationIt has been said that one can control only that which he/she can measure. As control applications become more and more sophisticated, there is a need to turn to higher-speed and higher-quality measurements. Unfortunately over the years, due to cost considerations and broad-based market needs, the PLC has become more and more tailored to general control applications with limited analog I/O and subkilohertz loop rates. There are a few axioms that all automation engineers know: • Outputs are only as good as inputs• The faster the loop can be closed, the more precise and efficient its control.
However, due to the generalised functionality of PLCs, getting high-speed, high-quality measurements, such as dynamic signal analysis, high-precision voltage, and current measurements, is often challenging without using specialised PLC hardware.
NI CompactRIO programmable automation controllers (PACs) address these challenges by incorporating a flexible, high-performance real-time system and a highly flexible and reliable user-programmable FPGA in measurement-class NI C Series modules.  NI offers more than 50 NI C Series modules. A variety of I/O types is available, including ±80 mV thermocouple inputs, ±10 V simultaneous-sampling analog inputs/outputs, 24 V industrial digital I/O with up to 1 A current drive, differential/TTL digital inputs with 5 V regulated supply output for encoders, and 250 Vrms universal digital inputs.
Because the modules contain built-in signal conditioning for extended voltage ranges or industrial signal types, one can usually make wiring connections directly from the CompactRIO module to his sensors/actuators. In most cases, the CompactRIO modules provide isolation from channel-to-earth ground.
CompactRIO modules connect directly to reconfigurable I/O (RIO) FPGA devices to create high-performance embedded systems that deliver the optimisation and flexibility of a custom electrical circuit completely dedicated to your input/output application. The RIO FPGA hardware provides unlimited options for timing, triggering, synchronisation, and sensor-level signal processing and decision making.
2. Advanced Analysis and ControlIn a typical PLC – based system, an input value is often compared to a set point for a binary result that turns on a light, opens a hatch, starts the conveyor, and so on. This level of functionality works well in most straightforward applications. However, more advanced control systems require additional analysis through signal processing and advanced decision making to calculate meaningful data before adjusting the output values.
Different Approaches for Signal Processing and Advanced ControlThousands of engineers and scientists rely on National Instruments hardware and software for their test, measurement, and control applications. The NI LabVIEW graphical programming platform provides powerful programming tools and hundreds of analysis function that can be used to filter and process data and extract the required information for analysis tasks or as feedback for advanced control algorithms. Depending on the task requirements, analysis can be incorporated into application in different ways.Inline versus Offline AnalysisInline analysis implies analysing the data within the same application used to acquire it. This is generally the case when dealing with applications where one needs to make decisions during run time, and the results have direct consequences on the process – typically through changing parameters or executing actions. By measuring and analysing certain aspects of the signals, the application can be made to adapt to the circumstances and enable the appropriate execution parameters. Although this is only one example, there are thousands of applications where a certain degree of intelligence – the ability to make decisions based on various conditions – and adaptability are required, which can be achieved only by adding analysis algorithms to the application. While standard PLCs are well-suited for discrete (on/off) control, they lack the processing power and functionality needed to perform high-speed analog measurements and analysis. By adding PACs based on LabVIEW and using industrial protocols to connect them to a PLC, those capabilities can be easily added to existing systems. While PACs offer the processing power and the fast and precise hardware I/O modules, LabVIEW delivers analysis and mathematical routines that natively work together with data acquisition functions and display capabilities, so one can easily build them into any application.
In addition, LabVIEW offers analysis routines for point-by-point execution. These routines are designed specifically to meet inline analysis needs in real-time applications. Point-by-point analysis is essential when dealing with control processes featuring high-speed, deterministic, point-by-point data acquisition. Anytime resources are dedicated to real-time data acquisition, point-by-point analysis becomes a necessity because acquisition rates and control loops are increased by orders of magnitude. Point-by-point analysis is streamlined and stable because it ties directly into the acquisition and analysis process. With streamlined, stable, point-by-point analysis, the acquisition and analysis process can move closer to the point of control in field-programmable gate array (FPGA) chips, embedded controllers, or dedicated CPUs executing a real-time OS.
For offline applications, typically there is no need to obtain the results in a real-time fashion in order to make decisions on the process. Offline analysis applications require only that sufficient computational resources are available. The main intent of such applications is to identify the cause and effect variables have on a process by correlating multiple data sets. These applications generally require importing data from custom binary or ASCII files and commercial databases such as Oracle, Access, and other QL/ODBC-enabled databases. Once the data is imported into LabVIEW, one can perform hundreds of analysis routines, manipulate the data, and arrange it in a specific format for reporting purposes. LabVIEW provides functions to access any type of file format and database, seamlessly connect to powerful reporting tools such as NI DIAdem software and the LabVIEW Report Generation Toolkit for Microsoft Office, and execute the latest data-sharing technologies such as XML, Web-enabled data presentation, and ActiveX.
Common Analysis and Advanced Control Algorithms in LabVIEWFast fourier transform: The fast fourier transform (FFT) and the power spectrum are powerful tools for analysing and measuring signals. FFTs are useful for measuring the frequency content of stationary or transient signals. They produce the average frequency content of a signal over the entire time that the signal is acquired.Time-frequency analysis: This technique can reveal information that is not immediately obvious with standard frequency analysis tools such as a FFT-based spectrum. Engineers usually implement time-frequency analysis algorithms to analyse time-varying signals whose frequency components evolve over time. Some common time-varying signals include biosignal, sound and vibration, and seismic signals.Wavelet analysis: Wavelets are oscillatory and compact signals that have zero-mean and limit width in both the time and frequency domains. Wavelet analysis algorithms represent a signal by wavelets and are ideal for the following:•  Detecting discontinuities, spikes, sharp  peaks/valleys, edges, and other transients  in signals or images•  Compressing signals/images•  Reducing noise or removing trends.Sound and vibration: Sound and vibration analysis works in a variety of applications including acoustic measurements, environmental noise monitoring, machine condition monitoring, and rotating machinery evaluation.Image analysis: Image processing and analysis can be used to enhance images, check for presence, locate features, identify objects, and measure parts.Curve fitting: Curve fitting is the process of finding a function or model that matches a series of data points and possibly other constraints. The process of curve fitting can be very important for modelling, predicting, and calibrating test and measurement equipment.System identification: System identification algorithms enable accurate plant modelling. The advantage of LabVIEW intuitive data acquisition tools can be taken to stimulate and acquire data from the plant and then automatically identify a dynamic system model. System identification models can be converted to state-space, transfer function, or pole-zero-gain form for control system analysis and design.Control design and simulation: With LabVIEW control design and simulation tools, one can analyse open-loop model behaviour, design closed-loop controllers, simulate online and offline systems, and conduct physical implementations. Models can be created from first principles using transfer function, state-space, or zero-pole-gain representation. With time and frequency analysis tools, such as time step response or Bode plot, one can interactively analyse open and closed loop behaviour and deploy algorithms to real-time hardware using built-in functions and the LabVIEW Real-Time Module.
3. Custom Field-Programmable Gate Array (FPGA)Have you ever had a control loop that just wouldn’t run fast enough or a custom digital interface to an actuator or sensor that just wasn’t possible with your traditional PLC? The innovative FPGA chip at the heart of NI PACs such as CompactRIO can meet both of these needs and much more.
FPGAs are Programmable Chips with the Reliability and Performance of Custom HardwarePLCs are a mainstay for industrial process control and automation applications. They are low-cost, reliable, easy to use, and have been proven with years of successful operation. Their discrete analog and digital I/O features and ability to close control loops in the hundreds-of-hertz range meet many application needs. Unfortunately, not all applications easily fit into these constraints. Many new approaches to machine building have necessitated higher-performance controllers with innovative architectures. NI CompactRIO is a PAC that combines the real-time computing power of a computer with the reliability and flexibility of a FPGA. The FPGA portion of the RIO architecture enables three core benefits over traditional control systems: High-performance parallel processing, custom hardware flexibility, and hardware logic reliability.Running advanced algorithms, such as field-oriented control (FOC) for brushless DC motors, can reduce power consumption and increase the life of components. These control algorithm advances are making machines more efficient, but often the algorithms need too much computational power to run on a PLC. For example, an FOC controller must continuously compute the vector control algorithm at a rate of 10 to 100 kHz. In parallel with the control algorithm, additional intellectual property (IP) blocks such as the high-speed PWM outputs need to execute without affecting the timing of the control algorithm. With their inherent parallel execution, FPGAs are able to perform control algorithms with loop rates up to hundreds of kilohertz, with room left over to handle multiple axis control algorithms, data communication for a human machine interface (HMI), or interaction with a host microprocessor. Moreover, with the reconfigurable nature of FPGAs, the control algorithm can be adjusted whenever required.Advanced, high-speed control algorithms are not the only reason to consider adding an FPGA-based PAC to PLC system. The programmable FPGA can also be used to implement custom logic for timing or triggering or communication protocols to talk to nearly any sensor or actuator. For example, with rotating machinery, the health of bearings, gears and other mechanical components can be determined by monitoring and analysing the amplitude and frequency components of machine vibration. Because the FPGA on CompactRIO is strategically placed between the I/O modules and the high-level real-time controller, it can enable data reduction and reduce the processing load on the processor by resampling, filtering, or pre-processing the I/O data as it is acquired. The FPGA can also fill the gap between the PAC and any sensor that needs a custom digital communication protocol.

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