Machine, 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.
|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.
"A similar system with multiple, distributed, special-purpose controllers might need a motion controller to handle dispensing, a process loop controller to handle material curing, and a discrete I/O controller to handle board trafficking. All these different components would require their own software programming packages, and we would have to make all the devices communicate with each other," observes Baj.
PVA also finds that hybrid controllers make field service easier. "Whether it's our own service engineers in the field or our customer's personnel from the plant floor, knowing that there is only one point of intelligence in the machine to deal with and be familiar with really helps in software modifications and trouble shooting," reports Baj.
Although PVA found that hybrid control works best for their applications, they have encountered a few drawbacks to the hybrid approach. "Many hybrid controllers, including Galil, use I/O that operates on the 5-VDC TTL level. Because our machine logic uses 24 VDC, we use Opto 22 solid-state relays to provide interface and optical isolation," relates Baj. "This adds cost and requires special cabling between the controller and the I/O rack."
The second issue they have is with the control's programming software. "The Galil software is programmed in a top-down configuration such as C, in a style familiar to most programmers," concludes Baj. "The downside is that the software executes one line at a time, and if it needs to wait for something, the software can miss inputs such as toggling sensors. That is a big concern to us if we have an application where higher I/O speeds are required, or if the process is non-sequential."
Hybrids a Fit for Motion Control
Other machine builders echo many of the comments made by PVA about the advantages of hybrid motion controllers, and also add some concerns of their own. In business since 1905, R.A. Jones is best known for its line of high-speed, high-capacity production packaging machines (See Figure 2). R.A. Jones has also been performing a complete range of turnkey design and integration services for more than 25 years.
Hybrid control streamlines the architecture by removing the loop needed for handshaking between the motion controller and the PLC in this all-servo, intermittant or constant-motion stainless cartoner. Source: R.A. Jones
R.A. Jones sources hybrid motion controllers from a variety of automation suppliers. "Hybrid control streamlines the control architecture by removing the logic needed for handshaking between the motion controller and the discrete controller (PLC)," says Darren Elliott, chief engineer for R.A. Jones' electrical controls group. "Removing handshaking logic means less logic overall, quicker scan times, less hardware, and less wiring. Reliability is better because there are fewer points of failure, and startups are quicker because there is less debugging time and hybrid controllers are easier to troubleshoot." Elliott thinks it is also simpler to integrate advanced, time-critical functions such as registration when motion and I/O information are controlled uniformly into one central processor.
A lot of users think the main drawback of hybrid controllers is the proprietary programming software associated with each individual product. "Logic is not transportable from platform to platform for the exact same application," says Elliott. "This re-inventing the wheel, going from platform to platform, is not a very good use of our resources."
Top 10 reasons to use a hybrid controller instead of multiple special-purpose controllers
- Lower cost--one controller as opposed to multiple controllers.
- No software integration among controllers is required.
- No interwiring among controllers is needed.
- Smaller footprint.
- Only one controller programming software package to buy and pay to maintain.
- Only one control platform to program, support, and maintain.
- Faster logic execution because handshaking among controllers is not needed.
- Fewer points of failure, one controller as opposed to multiple devices.
- Lower power consumption and less heat generation as opposed to multiple controllers.
- Easier to standardize on one controller for many different OEM products.
Of course, PLCs and other types of controllers share the same problem. In the PLC world, vendors try to address this issue by adopting the IEC 61131 software standard. This standard defines five different programming languages: ladder diagram, structured text, instruction list, function block diagram, and sequential function chart.
Many software vendors already market Windows-based software packages that use these five languages. Applications are programmed on a PC and then downloaded to the desired target platform or control engine. A target can be anything from a PLCs and PCs to a Windows CE box.
A similar--and somewhat parallel--initiative is underway in the motion control arena. "Organizations such as OMAC have been trying to put pressure on technology suppliers to standardize programming languages, but have not succeeded to date," states Elliott. "This affects industrial OEMs such as our company because training and support of the proprietary technology for the life of the machines, as well as integration of machines that use different platforms, is a major problem."
Another machine builder that uses hybrid motion controllers to perform discrete, analog, and motion control is GP2 Technologies. Its book binding machines feature automatic setup/changeovers and basic operations that make hard cover book production runs of one to 1,000 feasible.
GP2 uses Galil hybrid controllers to control and interface to servos, air valves, and push-button inputs. "We use a hybrid for temperature control, motion control, and to interface a touchscreen," says Tom Porat, CEO of GP2. "This single platform handles all our automation control functions and is scalable without being overly complicated. It's an approach that allows small manufacturers with limited programming resources to quickly develop more sophisticated automation products than was previously possible."
Altec Integrated Solutions uses hybrid hydraulic motion controllers in their rotary veneer clippers. These machines are used to clip a moving web or ribbon of veneer into sheets. The veneer is clipped with a rotary knife that rotates 180? in approximately 60 msec.
The hybrid controls rollers that guide the travel of the veneer ribbon, and it also controls the rotary knife. The size of each veneer segment is specified by an optimizing computer that communicates with the controller via digital I/O lines. The sequence of machine operation is controlled by a 25-step event sequence stored inside the controller. "With digital I/O, in combination with the event step editor, machine sequencing can be performed along with motion control by a Delta hydraulic motion controller," reports Bill Long, the president of Altec. "This results in a significant cost savings." Many motion controllers feature discrete and analog control, and some come supplied with built-in high-level communications protocols such as Ethernet.
Calypso Waterjet Systems uses such a controller in its turn-key waterjet cutting systems used by the machine tool and fabrication industries.
"We use Galil's Ethernet motion controller with circular interpolation and the ability to compensate for mechanical inaccuracies and backlash control," says Richard Carey, a senior engineer with Calypso. "To accomplish precise waterjet cuts, Calypso runs a large amount of code through the motion controller, most of which is devoted to a streaming interpreter that allows Calypso to make use of Galil's powerful trip points. These are needed to perform the coordination of single-axis and vector motion, all of which requires very complex tool sequencing and process."
Carey believes hybrid control allows Calypso to accomplish all control functions at a fraction of the cost of multiple controller systems, PLC controllers, or modular systems. "The card-level Galil controller is about half the price and half the size of box-level Ethernet controllers, it measures just 4.25x10.75 in.," he adds.
Hybrid Control Connects
PLCs were originally conceived as replacements for relays, timers, and drum sequencers--they were strictly discrete controllers. Analog I/O eventually was added, but only rudimentary operations such as alarming, setpoint monitoring, and basic signal conditioning were available.
Not until about 15 years ago was true process loop control--in the form of the PID algorithm--added to the PLC. Even then, this functionality only was available on high-priced, high-end PLCs. For industrial OEM applications with discrete and process control needs, it often was cheaper to pair a low-end PLC with a single-loop controller as opposed to using a high-end PLC.
In similar fashion, digital communications beyond RS-232 and RS-422 were not available on low-cost PLCs. Industrial OEMs that needed standard communication protocols such as Ethernet, or with a need to perform loop control, were not able to use low-end PLCs.
Many industrial OEMs were forced to create their own proprietary control platforms because the market did not provide a low-cost controller with discrete control, analog control, and Ethernet communication capabilities.
This state of affairs has changed dramatically in the past few years, and now low-end and low-cost PLCs are available with analog I/O, process control capabilities, and Ethernet communications. These low-cost, high-performance hybrid PLCs can be extremely attractive options for many industrial machine applications.
Hybrid controller drawbacks
- Often optimized for the main function, so it may be difficult to execute secondary functions.
- Motion controllers usually feature less I/O and interface choices than PLCs and PCs.
- Highly proprietary programming languages are often employed.
- All eggs in one basket, entire system fails if controller goes down.
- High-end functionality in multiple control areas may require special-purpose controllers.
BOC Edwards designs and builds gas delivery systems for the semiconductor industry. Its control systems perform discrete control along with monitoring and controlling of gas pressures and flow rates. The control system also interfaces with toxic-gas leak detectors and flame detectors.
A hybrid controller allows this skid-mounted collator to communicate with remote I/O via a digital network. Source: Eascan Industrial Controls & Automation
BOC Edwards was using a custom-designed controller based on the Intel 8088 processor, but wanted to deploy hybrid PLC control. "We found a low-cost PLC from AutomationDirect with all of the capabilities we needed," says Jeff Sapach, technical development manager with BOC Edwards. "Our systems require 16 analog inputs (some thermocouple), 32 digital inputs, and 32 digital outputs. We also need to interface with facility monitoring systems via an OPC interface or via our own proprietary communications protocol using RS-422." To BOC Edwards, being able to do all of this with one PLC is a clear benefit.
Previous application examples have shown how motion controllers can be used in hybrid motion/discrete applications, but PLCs and PCs can also be used in this role. This was the case for Eascan Industrial Controls & Automation when they selected a control system for a skid-mounted collator (See Figure 3). The hybrid controller for the collator is Rockwell Automation's SoftLogix, which is a "soft PLC" running on a PC. The hybrid communicates with a decentralized I/O network via DeviceNet. "One hybrid controller with distributed I/O is more cost effective than multiple controllers, and having just one controller makes the system simpler," says Phil Bernardin, control system programmer/designer with Eascan. "As far as machine operation goes, everything comes into one brain, and there's no need to do any synchronization between controllers because everything is performed sequentially by the soft engine."