Control issues are encountered everywhere on the factory floor, and systems of PLCs, PACs, and sensors provide the solutions to those control problems
By Brian J. Hogan
By Patrick Waurzniak
Flexible factory operations and lights-out (untended) operations require rock-solid automated solutions on the factory floor. Coordinated operation of different pieces of equipment on the floor is critical. Modern automation requires distributed intelligence and sensors that can operate reliably under all conditions.
At Siemens Industry Inc. (Elk Grove Village, IL) the Simotion Programmable Automation Controller (PAC) is one of the company’s latest advances in machine controllers. Formerly, users required a PLC and various dedicated hardware components to properly monitor and control a machine. Today, according to Zuri Evans, product manager, Simotion, one Simotion PAC can handle the PLC logic, motion control, drive control, motion safety, robotic control, and PID control for elements like temperature, force or pressure regulation.
"I look at a PAC as being the next evolution of PLCs, because in the past, your PLC would be mainly focused on the logic. A lot of the other functions in the machine were either handled mechanically or with a dedicated piece of hardware. Those mechanical solutions are not very flexible, they’re very expensive to maintain, and having the specialized hardware is also very expensive. You must interface different systems together, so it becomes very complicated, especially as these applications become more demanding with regard to improved performance and better reliability."
PAC is an open term, and its definition varies. "But these flexible architectures that allow one controller to handle multiple tasks, instead of having dedicated controllers, constitute a big trend throughout the industry," observes Evans. "So I define a PAC as a single controller that can handle multiple tasks in addition to just logic."
The expanded capability of the PAC is partly the result of hardware improvements, such as the use of faster processors and expansion of memory to 70 MB, but also the use of compiled code rather than interpreted code. This innovation allows faster execution of user code, down to cyclic calls every 250 µsec.
"We’ve taken all the basic functions of a PLC, and have added more. So the PAC includes motion control, drive control, robotics, hydraulic controls, and temperature controls; we have integrated safety. We’ve got a full-blown Web server built-in, for maintenance and diagnostics. So it’s basically taken all this technology and it’s enabled it on a software level, instead of a hardware level."
Programming is done in an object-oriented environment, which allows the Simotion system to convert from running a third-party drive to running a Siemens Sinamics drive with no changes to user code. A customer is no longer required to handle direct hardware addresses—everything is referenced by a symbolic name. Hardware address allocations are handled automatically. As a result, it’s less difficult to re-use user code, because nothing is hard-coded. "Additionally, there are two components when you think about programming," says Evans. "There’s the actual language in which you develop your code, and how you execute that code." The user can employ cyclic events for PLC logic, sequential events for motion logic, timer events for communication, and data management, interrupt events to respond to high-speed inputs, and deterministic events (to 250 µsec) for adaptive position corrections.
Siemens continues to support ladder logic. "The languages that we have available are IEC, 61131-3 compliant, and that includes the ladder editor. Ladder is great for cyclic tasks, for example I/O logic, and mode management. We support PLCOpen motion control function blocks, so you can do motion control in ladder as well," Evans states. "We also have a graphical editor called motion control charts; it’s a flow-chart style of programming for sequential logic. Finally, we support structured text, which is a high-level text-based programming language for complex and modular code. Each of those languages has benefits."
Because the Siemens PAC can provide faster execution of user code, true real-time control becomes possible. The basic foundation of the PAC is for motion, so these are deterministic systems. Switching from an interpreted architecture to a compiled architecture is described by Evans as a major change. "With interpreted, you’re basically compiling the code during run time. With compiled, we actually compile the code ahead of time, down to the machine language," he explains. Because of this change, the PAC can execute user code much faster, and handle many more tasks. "It allows very fast execution of deterministic tasks," he says, "and so I can in essence call a program that is synchronized with my servocycle, my servo loop, for my position control."
The PAC can be quite accurate, but doesn’t have all of the interpolation function found on a CNC—for example, look-ahead capability. "So we can do robotic control and 3-D interpolated moves," Evans states, "but we can’t do those moves with the high speed and precision of a NC controller. But that’s interpolated motion. When we’re talking about synchronized motion between two axes, it’s accurate down to fractions of a micron. The accuracy really depends on your feedback, how accurate your encoder is."
At Rockwell Automation (Milwaukee), Mike Burrows, director, Integrated Architecture, says control system platforms have evolved over the years to help companies meet more of their needs. "The first programmable controllers were little more than electronic relay replacements," he observes, "but today’s PACs have become critical operational tools.
"By migrating from multiple proprietary programmable controllers to a single convergence-ready control system that communicates over EtherNet/IP," says Burrows, "design and development time, installation time, and wiring costs are reduced, and end users have a much easier time maintaining their system.
"On one end of the spectrum, we’re designing very simple machines optimized around a PLC in what we call a connected automation environment," he says. At the other extreme is what Rockwell Automation refers to as its Integrated Architecture class of controls. Burrows says there are five trends happening in the architecture class of controllers.
The first is expanding capabilities. "We’re expanding current capabilities and adding more types of control functionality into our multidisciplined control platform. This eliminates the need for dedicated controllers for motion, safety, process, and other automation disciplines. The second trend is providing scalability—right-sizing—to deliver all the values of the architecture-class system, including multidisciplined control and high performance, for both large and small systems," he states.
The third trend is the use of information. "Any machine that requires or manipulates information will use architecture-class systems," says Burrows. "If there’s value to knowing more about how the machine operates, or knowing how it connects into a plant, or how one production line talks to another to run more effectively, that’s what we call information-enabled. What’s happening now is that manufacturers require real-time information. From equipment diagnostics to supply-chain status reports, achieving productivity gains and meeting regulatory demands requires access to accurate, real-time information.
"The more information the users can pull out of the machinery, the more they can focus on how to reduce downtime, increase the production rate, and improve quality."
The fourth trend involves the network infrastructure, and how secure remote access is changing the automation world. "There are big concerns around secure access," observes Burrows. He cites the Stuxnet virus as the type of threat that worries manufacturers. Stuxnet is a Windows-specific computer worm first discovered in June 2010. According to security experts, it was written to attack SCADA systems used to control and monitor industrial processes, and it can reprogram PLCs and hide the changes it made.
In response to these issues, says Burrows: "We drive security in our platforms. We’re Cisco’s [San Jose, CA] only manufacturing alliance partner for automation. Our approach to Ethernet, and the industrial protocol we support called EtherNet/IP, is a core part of how we provide system integrity, and that’s co-developed with Cisco. EtherNet/IP is the only industrial protocol that uses standard Ethernet TCP/IP technology. By using standard Ethernet and IP network technology, EtherNet/IP enables the convergence of the plant-floor and the business enterprise. This provides greater access to manufacturing data for making more accurate and informed decisions, while optimizing internal assets and resources."
The fifth trend Burrows sees is knowledge integration, the intellectual property involved in running a production line. "Our customers don’t design automation products, they don’t design servomotors, they make their product. So the fifth trend is how we make the design environment capable of housing the users’ intellectual property, and how they manufacture their product. We sell tools that allow them to focus on how to make their product and less on how to program the system.
"We are continuing to evolve the PAC to push it to the next level," Burrows concludes.
Technology advancements continue to be a driving force with PAC controllers, according to Bill Black, controllers product manager, GE Intelligent Platforms (Charlottesville, VA). The company is in the process of releasing multiple controllers based on the recent release of the Intel Atom 1.1-GHz processor. "It enables us to provide more than three times the performance of current controllers, and at the same time reduce the physical footprint for the customer. The Atom processor runs cooler than its Celeron predecessor, and at a faster clock rate, improving machine productivity."
In addition to hardware development, software remains critical to PLCs and PACs, Black states. "Software plays a major role in application development productivity and downtime reduction. Advances in hardware technologies enable the software tools to be possible, compared to the limitations of a PLC. Our PACSystems controllers allow the integration of logic, process control, advanced motion, and other tools running at the same time on one CPU. This wasn’t possible years ago due to the inefficiencies of the microprocessor’s performance and the limitations of memory and cost."
Secure remote access is, of course, a concern at GE Intelligent Platforms, but Black appears to feel that the matter is under control. GE released a statement that the Stuxnet virus was a directed, specific attack, and that GE hardware is not at risk from this worm. "We recommend that users should execute proper IT practices, including firewall, Microsoft updates, etc., to best protect themselves from potential attack," explains Black. "In parallel, we’re looking at security techniques that can be implemented on hardware and software in the future that would enable additional protection from virus attacks."
The Charlottesville company is open to many approaches to machine-to-machine communication on the floor. Black remarks: "It has always been our position to give the customer options on how to integrate the controllers with other devices. GE supports serial, various bus networks, such as DeviceNet and Profibus, and a wide range of Ethernet protocols such as SRTP, EGD and Modbus TCP. The GE IP CMX network provides a high-speed deterministic 2.1-GB fiber optic Reflective Memory network that enables 128 MB of shared data to be transferred between controllers and other devices in 450 nsec. CMX technology is well-suited for high-speed deterministic systems, and it provides performance that can’t be accomplished with any other off-the-shelf networks.
"Ethernet continues to be a major connectivity network on the plant floor, and GE IP will be expanding our offering in 2011 to include Profinet and DNP 3.0. And at the end of 2011, GE IP will also be introducing a 1-GB Ethernet module that will support both copper and fiber. The Ethernet module will address the performance limitations of current controller offerings."
GE Intelligent Platforms will continue to support ladder logic. Conversion tools from GE enable a PLC application programmed in ladder logic or C to be converted to a PAC controller. The company also offers programming languages such as ladder logic, Function Block Diagram, Structured Text, C, motion, and process blocks. "In addition," observes Black, "GE IP offers a very powerful User Defined Function Blocks tool that enables the customer to create their own unique function block with any of the languages supported."
Unless they run open-loop, control systems require feedback to the PLC or PAC from sensors located on the factory floor. Operations are driven by the demand for flexibility and untended operation. One type of sensor technology widely used in production operations these days is RFID. Tom Rosenberg of Balluff Inc. (Florence, KY), says approximately two-thirds of Balluff’s products are sensors and RFID, the rest are RFID and network-type systems. "RFID is probably our biggest product outside of sensors, and the network systems. We have some really high-end expandable Ethernet I/O that’s working pretty well.
"If someone is building something in a flexible manufacturing environment line, they must tell the automated line what type of product they’re building. That information’s typically held in some sort of build sheet or build information. Those data are put onto the RFID tag."
Briggs & Stratton (Milwaukee) is one of Balluff’s customers. "You can imagine how many types of engines they need to put together," says Rosenberg. "So when an engine gets put on a line, they have to give the line all the information about it: is it a horizontal shaft, vertical shaft? Is it configured for a lawnmower? All these different parameters are loaded into the build information. So now it starts its journey down the manufacturing line, and at each station it comes to, the station then reads the RFID tag, understands what they’re supposed to be making, and then they’re able to go forward and do those operations, if need be."
RFID is used a good deal by robotics vendors. If a robot is carrying a vision inspection system, for example, whoever is integrating the line has to get the right images from the robot, and then test it against the part that they’re supposed to be making. Remember, they can be making multiple parts, so if their vision system snaps a picture of a part, they have to figure out what part it’s supposed to be, match it up, and then make a pass-or-fail decision. Typically that’s done in a PLC somewhere else."
Encoders and linear scales provide feedback for all types of PLC and PAC-based control systems. According to Chris Weber, Machine Tool Div., Heidenhain Corp. (Schaumburg, IL), the company makes literally thousands of types of encoders and scales. "Design trends are moving toward serial communication interface technology and absolute positioning technology. There are many benefits. Absolute positioning makes it unnecessary to reference a system. Also, you can eliminate the use of devices like limit switches, and prevent overtravel and crashing."
According to Weber, serial communication is important to new sensor technology from Heidenhain. "With serial communication, our best technology is a bidirectional serial interface called EnDat, which provides a very high-resolution encoder interface. We do interpolation in the interface; there’s no requirement for interpolation from the subsequent electronics."
One benefit of serial communication is more resistance to noise or electronic interference. The technology is more robust than standard incremental 1 Vpp (peak to peak) or TTL, according to Weber. "With serial communication, we also have the ability to use much longer runs of smaller-diameter cable and much smaller connection technology, which makes routing cables easier. Serial systems can also use Profibus, and daisy chain devices back to the controller.
"Part of the initial problem with serial communication was speed. It could not be used for real-time applications," says Weber. "But a 14-meg clock frequency beats the problem. Depending on the device, you can have up to 29-bit resolution on a measuring device using EnDat."
When confronted with the increased operating speeds of today’s PACs and PLCs, Weber says: "Speed is good for us. PLCs can handle more channels and more feedback devices can be plugged into them. As they increase capability, we’re already set. The majority of our feedback devices are intended for CNCs, so we’re more than capable of handling the PLCs as well. We can provide them the position information they need."
Another advantage of today’s scale technology involves safety on the shop floor. "I can see the concept of integrated functional safety of systems. It typically involves redundant systems within the application. So, for example, we offer feedback devices that are inherently safe because they are scanning twice within the system," says Weber. "There is never a missing position value. And within EnDat we do position checks within the system, so there are no erroneous or missing data. If we detect a problem, a message is output [warning or alarm] to the PLC by the feedback device."
The whole concept behind functional safety, according to Weber, is that the operator can’t defeat the safety system. Sensors such as those made by Heidenhain will output the required information irrespective of what the operator does and send it to the supervising PAC or PLC. Data output from these sensors will be functionally safe. ME
This article was first published in the January 2011 edition of Manufacturing Engineering magazine. Click here for PDF.