If you log on to any industrial control discussion forum, you find threads about the advantages and disadvantages of PC-based control and programmable logic controller (PLC) control. Recently, you may have also seen some threads discussing PACs and asked the question, "What the heck is a PAC?" To understand PACs, you need to review the history of industrial control.
In the 1960s, engineers achieved industry control using large banks of mechanical relays. These systems were complicated, hard to modify, and prone to failure. In the late 1960s, Bedford Associates proposed a new system called Modular Digital Controller (MODICON), which used a CPU to perform digital logic and interface with digital inputs and outputs. Think of this system as the first “virtual instrumentation” for industrial applications. The MODICON 084 was the first PLC. The new PLCs efficiently performed digital operations and digital control and became very popular by mid-1970. Early PLCs used bit slice-based CPUs, such as the AMD 2901, and were limited to digital control. To make them reliable and simple to program, PLCs employed rigid control architectures and simple instruction sets. Engineers programmed the majority of PLCs using ladder logic, a language created to mimic the original relay diagrams of the 1960s.
The "80-20" Rule to Meet Application Needs
PLCs evolved to incorporate analog I/O, communication over networks, and new programming standards such as IEC 61131-3. However, engineers create 80 percent of industrial applications with digital I/O, a few analog I/O points, and simple programming techniques. Experts from ARC, VDC, and PLCS.net estimate that:
- 80 percent of PLCs are used in small applications (1 to 128 I/O)
- 78 percent of PLC I/O is digital
- 80 percent of PLC application challenges are solved with a set of 20 ladder-logic instructions
This is why some PLCs still use the original AMD 2901 CPU and why companies such as Keyence only offer ladder-logic programming.
So if 80 percent of applications incorporate simple digital and analog control, then the engineers creating the other 20 percent of the applications must push the boundaries of PLCs. In the 1980s and 1990s, these "20 percenters" were the ones who considered PCs for industrial control to obtain unparalleled flexibility, highly productive software, and advanced hardware. However, PC-based industrial control had weaknesses:
- Stability -- Often, a general-purpose operating system was not stable enough and lines were forced to handle system crashes and unplanned rebooting.
- Reliability -- With rotating magnetic hard drives and nonindustrially hardened components such as power supplies, PCs were more prone to failure.
- Unfamiliar programming environment -- Plant operators needed to override a system when it went down. Using ladder logic, they knew how to manually force a coil and patch code to quickly override a system. But with PC systems, operators needed to learn new tools.
Building a Better Controller
With no clear PC or PLC solution, engineers with complex applications often worked closely with control vendors to develop new products. These lead users requested the combination of advanced functionality and reliability and helped guide product development for PLC and PC control companies, such as Rockwell, Siemens, GE Fanuc, Beckhoff, and National Instruments. The resulting new controllers, designed to address the "20 percent" applications, combine the best PLC features with the best PC features. Industry analyst ARC named these devices programmable automation controllers, or PACs. In their "Programmable Logic Controllers Worldwide Outlook" study, ARC identified five main PAC characteristics:
- Multidomain functionality, at least two of logic, motion, PID control, drives, and process on a single platform
- Single multidiscipline development platform incorporating common tagging and a single database for access to all parameters and functions
- Software tools that allow the design by process flow across several machines or process units, together with IEC 61131-3, user guidance, and data management
- Open, modular architectures that mirror industry applications from machine layouts in factories to unit operations in process plants
- Employment of de facto standards for network interfaces, languages, etc., such as TCP/IP, OPC and XML, and SQL queries
National Instruments PACs
National Instruments PAC hardware targets are based on NI LabVIEW technology, including LabVIEW Real-Time and LabVIEW FPGA. With LabVIEW Real-Time and LabVIEW FPGA, engineers can program custom measurement and control systems using LabVIEW and deploy them on reliable embedded targets running a real-time operating system or embedded in silicon. The PAC hardware targets are designed for applications requiring:
- Graphics -- Because a LabVIEW programmer natively builds a user interface, you can easily incorporate graphics and an HMI for control systems.
- Measurements (high-speed data acquisition, vision, and motion) -- National Instruments has a strong history in high-speed I/O, including vision acquisition, so you can incorporate measurements such as vibration or machine vision into your standard control systems.
- Processing capabilities -- In some applications, you need specialized control algorithms, advanced signal processing, or data logging. Using LabVIEW, you can incorporate custom control code built using NI or third-party tools, implement signal processing such as joint time-frequency analysis, or log data locally or remotely.
- Platforms -- With LabVIEW, you can create code that runs a variety of platforms including a PC, embedded controller, FPGA chip, or handheld PDA.
- Communication -- LabVIEW makes it easy for you to pass data to the enterprise with tools like OPC and SQL.
National Instruments offers four PAC hardware targets:
- PXI improves on an industrial PC with a real-time OS, standards for cooling, optional nonspinning solid-state hard drives, and intermodule synchronization. The PXI standard requires all chassis to provide air flow for 25 W of cooling per module slot, which ensures operation without overheating or shortened life even when using high-power relay or high-speed PXI or CompactPCI cards. PXI also provides tight synchronization among different modules, so engineers can design motion, vision, and I/O systems for high-speed control applications such as those found in packaging and semiconductor handling.
- Compact FieldPoint uses industrially rated parts to achieve high shock and vibration, to handle a wide temperature range from -40 ºC to 70 ºC, and to achieve Class 1 Division II and Lloyd's certifications. It also uses conductive cooling instead of spinning fans to increase reliability by eliminating moving parts. Compact FieldPoint systems incorporate PC functionality through a floating-point processor running a real-time OS, CompactFlash drives for data logging, and an Ethernet port for communications.
- Compact Vision System is a rugged controller designed specifically for machine vision applications. It uses IEEE Standard 1394 FireWire interfaces to communicate with up to 16 cameras in a vision application and run a high-speed Intel processor for fast image analysis. Compact Vision System also uses no moving parts and conductive cooling, so you can mount the system close to the machine. It offers 29 built-in digital I/O lines that you can control either from LabVIEW Real-Time or directly from an embedded FPGA using LabVIEW FPGA.
- CompactRIO is a new reconfigurable embedded system based on LabVIEW FPGA and LabVIEW Real-Time technology. The CompactRIO system uses an FPGA chip with up to 3 million gates to control modular digital and analog I/O. The FPGA chip can run embedded code in silicon for digital control loops up to 1 MHz and analog loops up to 150 kHz. The FPGA can pass information back to a floating-point processor running LabVIEW Real-Time for advanced computation, data logging, and communication. With a metal case and conductive cooling, this controller is ideal for harsh environments.
Industrial engineers who need to create the "20 percent" applications are pushing the boundaries of controller technology, and PAC manufacturers are responding by providing a selection of hardware targets that combines the best functionality of the PC with the reliability of the PLC. New tools, such as LabVIEW Real-Time, that make it easier to program real-time OSs, FPGAs, and DSPs promise a breadth of new options for industrial engineers.
Todd Walter
Industrial Measurement and Control Product Manager
[email protected]
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