Simplify your machine control

 By Dan Hebert, Senior Technical Editor, PE

achine, skid-mounted process, and robot builders often need discrete, analog, and motion control functionality for the systems they build. Until recently, this meant figuring out how to integrate several task-specific controllers to get the job done. The means toward that end have changed--there are hybrid controllers available now that perform all those control functions within one integrated platform.

We define hybrid controllers as capable of at least two of three general types of control: discrete (on/off), analog (process), and motion. Hybrid controllers differ from special-purpose controllers in that they only perform one type of control. Typically, special-purpose controllers include small PLCs (discrete), single or multi-loop PID controllers (analog), and motion-only controllers.

Many machine builders recall a time not long ago when all controllers were special-purpose. After all, PLCs were not invented until well into the 1960s, and vendors spent decades refining and perfecting on/off control with PLCs. It was not until the '90s that most PLC vendors began to add process control and motion control functions to PLCs.

Single and multi-loop controllers were in use before PLCs, but the addition of discrete control functions did not follow until the 1990s, perhaps in response to the encroachment of PLCs upon their turf.

Dedicated motion controllers have been used as long as loop controllers, but the addition of other control functions is a fairly recent phenomenon. Motion control vendors first added discrete control, and now some motion controllers can also execute analog process control functions.

Virtually All Vendors Do It
At this point in the market's evolution, virtually every PLC vendor offers hybrid control technology capable of discrete, analog, and motion control. Most loop control suppliers offer products that also can perform discrete control, but motion control is not addressed with this class of products. Motion controllers that perform both discrete and motion control are now common, and it also is possible to find motion controllers with some level of analog process control capability.

Given the market environment, controller selection is now driven by the specific characteristics of each application, not by the product's characteristics. An examination of various applications reveals that industrial OEMs are deciding what controller best suits their particular needs.

Hybrids in Motion
Motion control coupled with discrete and/or analog control is one of the main applications for hybrid controllers. Two primary types of products compete in this market space: motion controllers with discrete and analog I/O and logic, and PLCs with motion control capabilities.

						FIGURE 1
						A hybrid controller performs underfill dispensing, heating and trafficking tasks in this PVA dispensing unit. Source: PVA

PLCs with motion control add-on or plug-in boards would seem to be the best of both worlds, but this is not always the case. One such company is PVA, Halfmoon, N.Y., which makes multi-axis robotic dispensing equipment (See Figure 1). Before settling on hybrid control, they relied on a combination of PLC and motion controllers to accomplish machine functionality. The PLC worked well for I/O control, but PVA ran into problems trying to integrate motion with the PLC. "Open-loop motion was easy when accuracy was not important," says Joseph Baj, PVA controls engineer. "Closed-loop motion was much more of a headache."

PVA also tried using motion modules attached to the PLC and standalone motion modules. "With motion modules attached to the PLC, trying to debug and determine why the motors weren't moving or why error lights lit up was not as apparent or as easy as it should have been," adds Baj. "Trying to troubleshoot problems in the shop or out in the field was difficult."

Compounding PVA's control issues was the fact that some motion modules that weren't PLC-based had communication problems with the PLC. "Proprietary communication protocols were an obvious problem, and 'standard' protocols always seemed to have some kind of twist," continues Baj. "Often, there was a communication problem, either written into the software or with keeping the PLC and motion controller synched together."

To eliminate these problems, PVA selected a hybrid controller from PVA that features up to eight axes of motion control, 64 points of digital I/O, and eight analog inputs.

PVA's 5000 line of machines offer customers a wide functional range and can manage a variety of dispense and apply operations (motion control), material curing via a controlled heat process (analog control), and circuit board trafficking through the work area (discrete control). Regardless of the machine's ultimate application, all control functions are executed with a single Galil hybrid controller.