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Logic Control for Robot Work Cells

by Bennett Brumson , Contributing Editor
Robotic Industries Association

Work cell control is an area of robotics that is changing rapidly due to the fast pace of change in technology. This is especially true of the evolution of controllers and their ability to monitor and manage elements within the work cell.

THERE ARE THREE MAJOR TYPES of control units available for robotic work cells. These include programmable logic controllers (PLCs), industrial personal computers (IPCs) and the straightforward robotic controller. Each has its strengths and weaknesses although advances in technology are blurring the distinctions among them.

The Old Standby
PLCs are a good choice for low cost applications which require digital IO. More expensive PLCs can also be used for analog IO, RS 232, reading encoders, and other more complex IO schemes. One advantage of PLCs is programming. Due to standards such as IEC 1131-3 it is easy to change from one PLC product to another as the languages are very similar.

'PLCs are more cost-effective, less expensive and simpler than IPCs, but are less versatile,' professes Michael Aulik. Aulik is flexible automation manager at Q Comp Technologies, Inc. of Greenville, WI.

Around since the 1970's, PLCs consist of a CPU, memory, as well as circuits to receive input and output data. PLCs analyze inputs and determine the phase of outputs and inputs. Using this information, the PLC will turn on or off appropriate outputs. The task that an operator needs performed is entered by use of a software interface. The late '70s and early '80s is when there was an evolution in PLC communication, giving them the ability to talk to other PLCs and to robots. PLCs contain relays, counters, timers and data storage areas.

'Festo makes PLCs that don't need a coordinated motion control, just discrete I/O. Our PLCs are optimized and are designed to operate continuously,' said Frank Latino, a product manger at Festo Corporation of Hauppauge, NY. 'There is less 'overhead' in the operating system of PLCs, so all its elements are used. IPCs are designed to run off of Windows, which has a lot of overhead. PLCs have less superfluous parts to perform its I/O control functions. Right now, there is still a price advantage to PLCs. PLCs generally afford a better cost-effective solution than do other types of controllers,' Latino continued. Festo is a maker of electronic interfaces with robotic controllers that connect to communication devices such as DeviceNet, TCP/IP, and Ethernet.

Latino went on to explain that a small PLC having eight inputs and four outputs would cost as little as $150. 'It depends on what you need. Unless there is a need for more complicated functionality, I think PLCs are a better solution. This is especially true if there is a low count of I/O,' he said.

'A PLC circulates and looks for signals, a signal activates another signal. A robot will not activate or deactivate a signal until it gets to that point in the program,' said Aulik.

New Kid on the Block
Industrial PCs are a good choice for more complex applications.

'The shortcoming of IPCs is they cannot circulate inputs and outputs constantly as do PLCs. IPCs have software timing issues and they don't circulate looking for signals through all inputs and outputs continuously like a PLC does. PLCs tend to work well with timing between it and the robot,' said Aulik. 'Both PLCs and IPCs can handle most applications, but the latter generally has a cost advantage. PLCs handle the timing issue a little bit better than IPCs but can not handle everything. On the downside, PLCs tend to have a monochrome graphic interface.'

While it is true that many stand alone industrial computers run windows based operating systems, many industrial PCs are SBCs, or single board computers. These are typically embedded in equipment and usually run an industrial real-time OS. These are much better at handling IO, and the operating systems are much more compact and stable. They do cost more than PLCs and are not as easy to program.

'IPCs are better than PLCs if there is a very high amount of I/O. Also, IPCs have a better operator interface. This is where IPCs have the advantage over PLCs,' asserted Festo's Frank Latino. 'There is certainly enough horsepower in an IPC to perform its functions in a typical I/O control. If complicated features or high I/O counts are needed then an IPC is justified, if price is less of a factor,' Latino added.

Michael Aulik concurred with Latino on IPCs interface.

'Because of their flexibility, IPCs can be programmed to be more user-friendly and have a more sophisticated interface,' Aulik said. Aulik said that in the near future, there will be more use of IPCs due to their increasing power but they will not supplant PLCs. 

'Robot users will tend to drift towards IPCs because of their flexibility and because they are more user-friendly. Robotic users can add a little bit of flair to what they are doing with IPCs. To distinguish themselves, controller makers might start to produce more IPCs,' Aulik said. 'For the time being, PLCs are too easy to use and too cost-effective to be wholly replaced by IPCs. Unless there is a major technological breakthrough that changes the cost dramatically, or new software that makes it very easy to program IPCs with little effort, I don't see a major change in the near future,' Aulik predicted.

Jeff Noruk, president of Servo-Robot Corp, amplifies on the pros and cons of IPCs and PLCs. 'The increased speed of IPCs helps them manage a work cell, especially for part inspection applications. IPCs will get faster, collect more information, do more analysis, and provide more complete summaries of data and other useful information,' he said. Servo-Robot Corp. is a U.S. subsidiary of Servo-Robot, Inc. of Saint Bruno, Quebec, Canada.

'In the short term, there will be more compatibility with other companies' equipment. Information will become easier to read, by use of simple green, yellow, and red status lights. There can be an IPC communicating with a master IPC, keeping track of data throughout a whole factory.' Noruk foresees this in the next three to five years.

The idea of a more completely integrated control system has champions in many sectors.

'If there are specific information needs, such as process monitoring, data-base applications, or a lot of information flow going through other computers, IPCs are best at networking them,' said Gary Zywiol. Zywiol is vice president of product development at FANUC Robotics North America, Inc., of Rochester Hills, MI.
 
IPCs are a major player in electronics assembly applications because of their precision.

'Cimetrix software is for assembly applications that have complex motion requiring vision and accuracy,' said Steven Sorensen, chief technology officer at Cimetrix. 'We haven't done much in heavy industry, where six-axis articulated robots are used for welding. More traditional heavy industries tend to use PLCs rather than IPCs as controllers. The electronics industry is very 'PC-centric,'' reflected Sorensen. Cimetrix of Salt Lake City, UT, is a manufacturer of software for PC-based controllers and communication software for the electronics industry.

Control Master
Robotic controllers are the master component used by robot users to successfully exercise direction over a work cell, and offer more than just robot control functionality.

Zywiol explained that controllers can do a lot of things besides runningthe robot itself, including capabilities beyond motion control. For instance, controllers can do all of the tooling I/O, manage fixtures and act as a communication device. He went on to explain that a controller can have links to networks in a manufacturing plant that gives it a great ability to carry information when linked to PCs. The robotic controller itself generally does everything done in a cell, he maintains.

Robot controllers are starting to use industrial PCs. Many of the issues such as safety and IO scan times are not issues with the PC hardware, which is very reliable and problem free, but with the software. By switching traditional operating systems such as Windows to faster, more robust real time operating systems, these problems are solved. The next step will be for robot programming languages to adopt standards such as IEC 1131-3, just as PLCs have.

Zywiol remarked that in some instances, 'There is PLC functionality within a robotic controller. Or, the robot controller is taking on the tasks done by PLC's in the past. A company that does things well will take advantage of the strengths of the PLC, IPC, and robot controller platforms. For example, in the PC world, the strength is information flow, its data base applications, the ability to communicate between computers, and other intelligent devices. We frequently use PCs when there is a need to move large amounts of information.'

Zywiol also mentioned the downside of IPCs.

'The disadvantages of IPCs are their reliability and predictability. IPCs are still not as reliable as a dedicated motion control system in those areas. There is guaranteed performance with a robotic controller or a PLC,' said Zywiol. 'When you get into motion control, there are issues like safety. You must guarantee the safety of the customer. The robot's load can be up to 400 kg and moving at high speeds. By using robotic controllers for motion control, you get very high performance and guaranteed safety,' he said.

As controllers and other robotic electronics get more sophisticated, there is a potential difficulty in servicing these components. Industrial Control Repair (ICR), a Warren, MI-based firm specializing in repair and replacement parts of robotic cell equipment, including controllers, shares its perspective.

'Controllers are similar to cars. As they get more complex, there is less ability to do self-service, so there is a need for specialized technicians. Controllers are more difficult to troubleshoot and to dismantle to gain access to internal parts where the problem may be,' observed Glenn Dantes. Dantes is vice president of ICR.

'Adding options on a robot intimidates some users. Those new to robotics want a controller that is older, simpler, easier to operate and maintain,' added Dennis Edney. Edney is Dantes' colleague at ICR and is the firm's robotic specialist. 'Controllers mirror computers in that they are getting faster, have more memory, and are getting smaller. They take up one-third less space than they did in the 1980's. Newer controllers have circuit boards that make them simpler to use,' he said.

It is somewhat ironic that the move toward smaller size in controllers, an attribute coveted by customers, has presented problems for repair and maintenance procedures. But some experts foresee an Internet-based solution.

'Controllers are more compact so they are more difficult to access. Many controllers have more features and can do more complex tasks. They are more software-intensive than ever before. There will be an increase in the trend of robot makers looking at the manufacturing process of their robotic customers over the Internet to do remote diagnostics,' said Edney.

Dantes agreed with his associate's analysis. 'New systems are more costly and complex. There could be a trend towards older, simpler systems. New isn't necessarily better, especially for first time users of robotics.'

Controlling the Future
As technology advances making robotic control systems more complicated, there will be less of a do-it-yourself aspect to their maintenance and repair. Although its day is far from over, there will be an inevitable replacement of the ubiquitous PLC with the ever-increasing power of IPCs and robotic controllers.

Automated Imaging Association Motion Control Association