(Issue 4, 2005)
PLCs Evolving Toward Motion Control
by Jeff Payne,
AutomationDirect Product Manager
PLC, I/O and PC-Controls Group
In our fast paced and economically stressed industrial world, flexibility is becoming a higher priority for those who are specifying automation systems. Users need multi-functional control, simple connectivity and easy access to process data, all wrapped up in a nice neat package.
Today's Programmable Logic Controllers (PLCs) have to be much more than the relay logic replacement they were designed to be in the late 1960's.
As the PLC has evolved, we have seen its intelligence grow too. In the early 1970's, the PLC became capable of communicating to other devices. The introduction of MODBUS communications protocol by Modicon was the beginning of many new responsibilities for the PLC because MODBUS allowed communications among PLCs over standard cabling. This allowed the PLC to be placed in closer proximity to the real world devices and communicate back to the system controls in the main panel.
The communications craze has spread far and wide. In the past 30 years we have seen literally hundreds of proprietary and standard protocols developed, each with their own unique advantages. It is fast becoming the responsibility of the PLC to be the information superstar of the process communications conglomeration.
Today's PLCs have to be data compilers and information gateways. They must interface to bar code scanners, printers, temperature and analog sensors, and more. They need multiple protocol support to be able to connect with other devices in the process. Furthermore, they need all of these capabilities while remaining simple to program, easy to edit and allowing you to remain within your budget.
By expanding the controls beyond discrete I/O, we have opened the door for the PLC to perform much more than the basic sequencing functions of earlier models. With high resolution analog modules, most PLCs are capable of computing complex PID algorithms, including Auto-Tune capabilities. The PLC is now well equipped to replace stand-alone process controllers in many applications.
Whether it is reading a temperature, pulsing a heater element with a time-proportioning control output, or reading a 4-20mA pressure transducer for modulating a control value to adjust the hydraulic pressure in a system, today's PLCs are up to the task.
One of the most recent responsibilities the PLC has been tasked with is simple motion control. There are many process applications that require accurate control at a fast pace, without expecting exact precision and blazing speed. These are applications where the PLC works well. Many nano and micro PLCs are now available with high speed counting capabilities and high frequency pulse outputs built into the controller, making them a viable solution for open-loop control.
Despite being a relatively simple, less than ideal form of control, open-loop control alternatives are still a growing part of industrial applications. What makes this different from other types of motion control is that the controller does not know the position of the output device during the control sequence. So why even consider this as an option when designing your process application? Simple – cost.
This type of control is no stranger to the process world, but in the past has required an expensive option module, and at times has been restricted to the more sophisticated control platforms in order to achieve system requirements.
Positioning applications are probably the most common open-loop processes because a stepper is a position-based no-feedback device (it will drive at full force to get to commanded position or fail trying) vs. a servo that closes its position loop, and will vary its output torque to get into/stay at commanded position.
Consider the following applications –
• indexing tables or conveyors
• x/y tables (plotter/cutter)
These are only a few examples of what you can easily accomplish with a low cost PLC and stepper system.
So how is the PLC accomplishing these new market requirements? The modern PLC has incorporated many types of Commercial off the Shelf (COTS) technology in its CPU. Taking advantage of the latest technology gives the PLC a faster, more powerful processor and more available memory at less cost.
These advances in technology have allowed the PLC to expand its portfolio and take on new tasks without sacrificing the rugged nature of its design. It is better equipped to handle communications, data manipulation and high speed motion, without giving up the rugged and reliable performance we expect from our industrial control equipment.
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