How to Improve PLC-Based System Performance

Engineers and scientists have used programmable logic controllers (PLCs) for decades to automate the world around us, and they will continue to use them for the foreseeable future. PLCs are built for discrete control applications; they are a useful and dominant tool for industrial applications, but as industrial machines and factories grow in complexity, it is difficult if not impossible to do it all with PLCs. Today’s automation systems overextend the capabilities of PLCs, leaving engineers in the industrial machine industry with the need to integrate more advanced I/O, processing, and control within their automation systems. New programmable automation controller (PAC) hardware systems can easily be integrated with PLCs to add more advanced functionality into industrial machines and improve efficiency, making PACs an ideal add-on solution to PLC-based systems.

 Improving Machine Efficiency

Integrated Industrial Systems (I²S), a privately owned U.S. OEM that has specialized for decades in producing state-of-the-art metal rolling mill equipment and control systems, offers an excellent example of how to improve an existing PLC-based system. I²S has used PLCs for years to automate and control the rolling mills it produces. In recent years, the company has been trying to update its rolling mill control system to improve efficiency and quality. In order to update the system and improve its machines, I²S needed a solution that would provide more precise analog input resolution to interface with its gamma gauge sensors, as well as advanced signal processing in order to take the analog signal from the sensor and convert it into a highly accurate thickness measurement that the PLCs could use within the rolling mill control loop.

 To save time and money, I²S first tried to implement the advanced analog measurements and processing within the existing PLC system. When the PLCs did not provide the precise analog I/O and signal processing required, I²S turned to National Instruments CompactRIO, a reconfigurable embedded PAC system.

 Adding Advanced I/O and Advanced Signal Processing to an Existing PLC System

NI CompactRIO has an embedded FPGA chip and real-time processor that you can program with built-in National Instruments LabVIEW function blocks. CompactRIO also has more than 30 analog and digital I/O modules that contain built-in signal conditioning (including anti-aliasing, isolation, ADCs, and DACs), high-speed timing (up to 800 kHz for analog I/O and up to 30 MHz for digital I/O), and high resolution (up to 24-bit ADCs) for connection to any industrial sensor or actuator.

[+] Enlarge Image

Figure 1. You can customize timing for each I/O module using NI LabVIEW tools with the FPGA in CompactRIO.

I²S connected CompactRIO analog input modules to the gamma-based thickness sensors to provide the high-speed timing and resolution needed to take the necessary precise measurements. Because each I/O module is connected directly to the FPGA, I²S engineers can easily customize CompactRIO analog I/O rates using LabVIEW FPGA.

After they acquire analog data from the gamma sensors, the CompactRIO real-time processor uses built-in LabVIEW Real-Time floating-point function blocks to convert the data from the sensors to an accurate thickness measurement. CompactRIO performs all of the I/O and signal processing in the FPGA and real-time processor and transmits a high-accuracy thickness measurement to connected PLCs without slowing down the existing PLC control loop rates.

Integrating PACs into an Existing PLC System

Figure 2. You can easily integrate PACs with PLCs, human machine interfaces (HMIs), and enterprise systems.

There are three fundamental methods of connecting PACs into existing PLC architectures:

1. Basic analog and digital I/O – Analog and digital data can be outputted from the PAC into a PLC. This is how I2S transfers the processed data from the CompactRIO PAC to the PLCs running the rolling mill control loop.
   
2. Industrial networks – A majority of PAC products support industrial protocols such as DeviceNet, Profibus, and CANopen along with Ethernet-based protocols like TCP/IP, UDP, and Modbus TCP/IP. I2S uses Ethernet protocols to transfer data between the CompactRIO PACs and to interface the PACs and PLCs to the networked HMI.
   
3. OPC connectivity – PACs can also act as OLE for process control (OPC) clients or servers and send and receive networked data to PLCs or other PACs using OPC tags. The OPC standard provides a generic set of routines that gives automation systems from different vendors a way to interface easily.

At I²S, each rolling mill contains three networked CompactRIO systems. Two CompactRIO systems connected to the gamma-based sensors perform the analog input measurements and the advanced processing to calculate the accurate thickness measurement. The third CompactRIO system takes the thickness measurements from the other two systems and performs an analog output measurement to the PLCs that control the rolling mills.

The processed data is transmitted between networked CompactRIO systems over the Ethernet in less than 20 millisecond intervals. The acquisition, processing, and transmission of the CompactRIO measurements all happen fast enough to inject accurate thickness measurements into the PLC control loop without decreasing control loop speed. Using LAN-based CompactRIO systems with a 10/100 Mbps Ethernet port, I2S can easily connect to networked Allen-Bradley PLCs and the HMI system via a standard TCP/IP protocol.

Conclusion

Engineers and scientists will continue to use PLCs to automate the world around us for years to come, but with the advancement of machines and the need to increase automation efficiency, PLCs often cannot do everything alone. PAC technology provides a great complement to PLCs and adds the high-performance I/O and processing that traditional PLCs cannot provide. With the variety of methods available to connect PACs into existing PLC architectures, engineers now have an easy way to boost their PLC-based automation systems.

Learn how to connect LabVIEW and PACs to any PLC. 

This article first appeared in the January 16, 2007 issue of NI News.

11 ratings | 4.18 out of 5

Read in English Chinese (Traditional) French | Print

Reader Comments | Submit a comment »

Excellent work for the people to know.
It is great to know bout the PLC based system. thanks
- Russel, pharmaceuticals. chemist971@yahoo.com - May 1, 2007