by Steve Meyer, Contributing Editor
As multicore processor solutions become less expensive and easier to use, unique architectures provide important benefits for control system users.
Industrial control systems emerged in the 1960s as automotive manufacturers applied relays to the control of machinery in the plant. In spite of the very primitive level of control, just being able to turn things on and off—automatically, reliably and safely—produced great gains in productivity. However, each new gain in productivity brings about demand for increasing functionality.
Relay logic as a form of electrical control is still in use, and uses the common electrical documentation of the ladder diagram. As PLC technology became widespread, Ladder Logic came to be a recognized language to describe what the control system was hard-wired to do. Unfortunately, as relay control systems became more complex, the interconnect requirements did, too. This complexity led to large cabinets full of wiring that were expensive, hard to troubleshoot and almost impossible to re-use for other applications.
Programmable Logic Controllers
Enter the Programmable Logic Controller (PLC). Dick Morley’s introduction of the Modicon 084 PLC in 1970 changed the way controls would be done for decades to come. Programmable controllers were just that—programmable. They offered a lower cost, configurable alternative to hard-wired relay logic with comparable reliability and repeatability.
PLCs as “relay replacers” were limited to controlling devices within a defined radius based on running discrete wire. In the early days, they were primarily limited to discrete inputs and outputs of various voltages and gradually expanded to controlling analog signals. Because the PLC processor and electronics were proprietary, networks between controllers were proprietary too. Programming terminals were bulky CRT based systems that, like the PLC hardware itself, reflected the same approach to an industrially rugged system.
Ladder logic symbols directly copied from the electrical documentation of relay systems could be programmed on custom-built CRT terminals to create control systems more quickly and with greater complexity than relays. The compelling performance of the PLC gradually gained wide acceptance, turning the new technology into a multi-billion dollar industry. The predominance of ladder logic as a descriptive language for control emerged as a standard under the European Directive IEC 61131.
Programmable Automation Controllers
Integration of PLCs with external Information Technologies networks, the need to exchange information with intelligent sub-systems, and the incorporation of motion control requirements drove suppliers to look for ways to expand sales. More powerful processors from the semiconductor industry gave control system suppliers new choices in how to address emerging requirements.
While there does not appear to be a precise specification, control vendors began to adopt the term Programmable Automation Controller, or PAC, during the 1990s for anything that looked like a PLC but had more going on. A major element in the PAC and large PLC system is a multiprocessor approach to host asynchronous services while ensuring that the main control program is not compromised. Complex peripherals like bar code scanners, components using RS232, and network interface modules became plug-in options added to traditional PLCs.
PACs also brought other enhancements. Multiple programming languages like C/C+, flowcharting and state machine offer certain advantages to control engineers where ladder diagrams would be limited. The growing adoption of robots and vision systems as end-of-line packaging equipment and CNC load/unload systems similarly required increased capability that the PAC approach supports.
Personal computers and control
The personal computer exploded into widespread usage in the 1980s and portable models, later replaced by laptops, were the preferred hardware for programming industrial control systems. Where the emergence of the lower cost, higher performance computer was an advantage for programming control system hardware, the operating systems and hardware did not have the stability or reliability of PLCs. Thus, the PC was not contemplated as an alternative to the PLC initially. Rigid, logical flow of I/O and “real time” operation is difficult to program in processors that are designed for flexibility. Commissioning a complex control system often meant having two or three portable PCs connected to a new piece of production machinery in order to troubleshoot control programs.
The personal computers with real time operating system options made their way into industrial applications so that “soft” PLC programs reached the point of being certified as equal to PLC hardware. Lower hardware costs from high volume “consumer” chipsets have translated to industrial solutions and it is not uncommon to see a “brick” style PC as the heart of machine control.
Next Generation Controller
The complexity of today’s industrial control ecosystem of discrete, analog and intelligent systems is placing demands for control system performance in the same way that the Internet of Things is taxing communications bandwidth. Today’s PLC may have multiple processors communicating with smart peripherals, like vision systems, with application code executing discrete input-output control, and performing precision motion tasks while updating plant networks with production status, all at the same time. As suppliers compete to provide more capability, users are also evaluating issues like hardware cost, ease of programming and commissioning time in their decision making process. Enter the Next Generation Controller: Compact RIO.
In National Instruments’ conception of the Next Generation Controller, a general-purpose processor is combined with a Field Programmable Gate Array (FPGA) in a tightly architected system to maximize the ability to measure and control. System resources are configurable to deliver the maximum performance in user applications.
National Instruments’ has partnered with Xilinx over the past decade to create best-in-class measurement and test systems based on the FPGA. The recent increase in speed and performance of the FPGA enables integration of high speed, parallel signal processing with plenty of bandwidth for precision motor control, all in the same platform. With the LabVIEW programming environment, the FPGA/Processor combination can be optimized for each user requirement.
Xilinx invented the FPGA in 1984 and has been the leading supplier and innovator of FPGA technology. Unlike some technology companies, Xilinx has worked outside its FPGA boundaries creating development systems for the Power PC processor. Understanding the relative strengths and weaknesses of processors versus gate arrays led to the inevitable question: How do we get the best of both worlds? That question brought about a series of unique combinations of the two technologies.
Higher integration has led to a System-on-Chip version of the processor and FPGA, which is implemented in the National Instrument 9068 version of Single Board RIO. A 667 MHz dual-core ARM Cortex A9 processor is combined with an Artex-7 FPGA in a multicore processor architecture with support for external communications using CAN, Ethernet, USB, Gigabyte Ethernet and PCIe. This provides high-speed communications for external systems as well as high-speed I/O to and from the real world. The SoC also has parallel pipeline DSP capabilities for doing advanced math functions without slowing down the system.
The Xilinx Zynq architecture creates a platform that gives the user the ultimate freedom to tailor the hardware to a unique requirement and apply the maximum computing power to the tasks required.
In the NI Single BoardRIO family of products, the FPGA + Processor architecture and hierarchical programming provide a technology core similar to the multiprocessor solution of PACs. Add to that National Instruments’ extensive suite of input/output and signal conditioning products and you have the best of both worlds, off-the-shelf solutions and open customization as needed.
Easier Integration, Lower Costs
Control system integration usually represents a huge amount of development time. National Instruments took the extraordinary step of creating enhancements to NI LabVIEW that allow the user to program the FPGA directly through the LabVIEW FPGA Module. Control programmers don’t have to learn the complex programming language of the FPGA, they can program in LabVIEW and it’s done. In addition, there are features in the LabVIEW development suite that allow the user to assign tasks to specific cores and priorities. This ensures that critical functions like waveform capture are not interrupted by lower priority activities like sending data files over Ethernet.
For OEMs with unique control system requirements, the hardware cost for the Single Board RIO family is comparable to the cost of a brick style industrial computer. The additional benefit of custom hardware integration and accelerated development make a compelling argument for this solution on next generation requirement. Single Board RIO also minimizes physical packaging and interconnect needs to a small footprint. Hardware packaging, development and testing are all made easier and quicker.