By W. Jarrett Campbell
W. Jarrett Campbell, Ph.D.
Requirements for higher performing, more flexible and modular packaging machinery are driving engineers to rethink the traditional, monolithic control architectures represented by the high-powered programmable logic controller (PLC) that handles all control action on equipment. Engineering focus is shifting to distributed architectures where large portions of control responsibility are integrated into dedicated task-specific devices throughout the machine with higher-level control systems in charge of overall machine control.
The key feature of an embedded control system is that it is built for specific tasks. It performs a small subset of the overall requirements, and so needs only those features related to the task, which reduces board and device space requirements. You can encapsulate the hardware and associated firmware inside the device the control manages.
Embedded controls are characterized by configurable, or even programmable, logic capability integrated into a device developed for a specific control action. These devices may be as simple as a smart relay or as complex as a dedicated motion controller that includes logic, motion, and human machine interface (HMI) functions. Examples of embedded control devices include variable frequency drives (VFDs) with a built-in logic controller, and configurable safety controllers that combine multiple application-specific safety relays in one device.
Generation 3 machines may look like a step
back to monolithic control architectures. However, closer inspection
reveals that these specialized motion controllers are event-driven,
rather than scan-driven. They are a distributed control system
contained in a single hardware device, which reduces wiring, network
latency issues and application code development.
The right choice?
Just because embedded control devices are available, it does not mean they are the right choice for your machine design. Anyone with experience in control architectures will acknowledge the art, perfected through trial-and-error, as well as the precise science in control system design. A quick study of system requirements will indicate whether or not a machine is a likely candidate for devices with integrated control.
Do you have extreme performance requirements?
The number one reason to use an embedded control system is to meet extreme machine performance requirements. These include extremely fast or precise control, which often need a powerful logic controller or specific algorithm. Traditionally, these requirements have been met through PLCs with fast microprocessors.
However, usually only a small portion of a machine’s overall control requires this level of performance. The majority of a machine could easily be controlled with a less expensive PLC, while a dedicated processor manages the response times needed by the few extreme tasks. If this is the case, it is an ideal scenario for the use of an embedded controller.
Embedded architecture can lower the overall control cost for the machine. It can reduce network traffic since there is little need for large amounts of real-time data to leave the integrated control device. It can also reduce diagnostics and troubleshooting costs as critical functions are isolated within the embedded device.
Can the control task be separated?
A second requirement for embedded systems is the ability to confine a function to a specific device. Embedded systems should wholly encapsulate control responsibility into a functional unit that can be organized as modules if necessary. Such an arrangement simplifies the system to meet the extreme performance requirements.
This organization does not necessarily mean isolation. The control system can exchange information through inputs, outputs, and machine states with other control system components. However, the information must be clearly defined to reduce the affect changes in the internal embedded system or in the higher-level control system may have on other components within the control architecture. A key benefit is the ability to “plug-and-play” different embedded systems within a common control architecture.
“Plug-and-play” design is attractive as it lets you size controllers to meet specific requirements at a minimum cost. Should a moderate level performing machine need to be upgraded, an embedded control system allows the upgrade with little to no effect on the overall control system design. This flexibility reduces testing, commission problems and start-up costs for machine upgrades and revisions.
The overall design architecture can remain common across machines. Additionally, many integrated controllers can be purchased with or without an embedded controller, allowing use of the same control device over multiple systems for economies of scale in support, inventory, training and supplier management costs.
Can the design be replicated?
Embedded control systems are generally used in systems that can be replicated. High volume production does not constrain the use of embedded systems. The greatest value in their use comes when you can tune the embedded control to a specific task and replicate that in many machines and designs. One interesting benefit of embedded control systems is the ability to encapsulate aspects of the machine control that are proprietary, while still giving the end user the ability to monitor the embedded system and modify its performance through inputs and outputs. While the customers can neither modify nor glean the details of the actual control algorithm, they can manage the controller’s performance by modifiying setpoints and batch recipes downloaded from a higher level control system.
Meeting the need for flexibility
Packaging machines today must be flexible and modular, a perfect opportunity for embedded controls. The most common embedded control system seen in the packaging industry is the dedicated motion controller. These controllers are necessary to drive the precision, coordination and speed needed by newer packaging equipment, commonly known as “Generation 2” machines, as well as control robotic packaging lines.
A typical embedded motion controller coordinates the position, velocity and acceleration of various machine elements through one or more axes of motion. In Generation 2 machines, the motion controller generally coexists and interacts with a higher-level control system such as a PLC, which is responsible for the overall coordination of all machine functions.
Recently, the packaging industry took another step in the use of embedded systems with the introduction of Generation 3 machines. Here, the PLC function is embedded in the dedicated motion controller. Initially, this may look like a step backwards towards the generic, monolithic control architectures that embedded controllers were meant to replace. Closer inspection, however, reveals that these specialized motion controllers are event-driven, rather than scan-driven. This design lets them internally distribute control responsibility in such a manner that they could be considered a distributed control system contained in a single hardware device. These third generation controllers reduce wiring, network latency issues, and application code development through the reuse of code, and a modular and flexible design.
Positional control with a variable frequency drive
At the opposite end are packaging machines with motion and control requirements sufficiently simple that an embedded control system can eliminate the need for the supervisory controller (PLC). In this case, a variable frequency drive with embedded controller manages simple motion operations as well as overall machine control. The embedded controller also aids the drive, enabling it to self-diagnose problems and readjust continuously.
A VFD with embedded controller was successfully used in a case erector machine. Its tasks include loading a flat case template, erecting it, taping it, and ejecting it, while ensuring sufficient motion control to prevent jams and clear them should they occur. A variable frequency drive with an embedded controller and an optional encoder board allowed the engineer to install the integrated controller on the drive and eliminate the PLC previously used in his control system. The change saved approximately 25% in overall control system cost while reducing the total size of the machine’s control panel footprint by 15%.
Advances in the automation and control industry let packaging machinery OEMs simplify diagnostics, integration and commissioning while increasing the flexibility of their machines. Through the use of embedded controllers, machine designers can push control functions to the lowest level possible and free up bus network traffic and computing resources at higher-level control systems.
Additionally, embedded controllers protect proprietary algorithms and implementations, and yet still allow some flexibility and custom design. A final benefit of embedded systems is the inventory, training and support savings that can be realized by using control devices in both the integrated control and non-controller versions, depending upon the machine’s requirements.
A variable frequency drive with embedded controller considerably reduces the size of control cabinets.
Paving the Way for Generation
“3” Packaging Machinery.
ARC Advisory Council. October 2002.
American Variable Speed Drives Markets.”
Frost & Sullivan report
A635-17. October 31, 2000.
Web: www.squared.com (Schneider Electric USA)
Web: www.schneider-electric.com (worldwide)
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Filed Under: Packaging, Motion control • motor controls
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