Simplifying drive-based and controller-based automation
The digital era has been referred to by some as a revolution. However, the long history of manufacturing automation reveals a technological evolution. Within the context of the continually evolving landscape, the requirements for electric motor drive speed control, precision, safety, scalability, and efficiency remain constant. They are hallmarks of innovative machines and robotics.
The rate of adoption for industrial control and automation has been extraordinary, with nearly every factory now leveraging motion control robotics and machine technologies to gain efficiency. Projections show the global robotics industry expanding to more than $200 billion by 2020 (Source: MarketsandMarkets, 2016). Dynamic global markets and supply chain models with shorter production cycles require greater agility to reduce machine development time and turnkey system integration.
Machines with parametric programming, self-optimization, and motion control systems using built-in intelligence and software modules are making automation more flexible and efficient. A recent study (Quest Technomarketing, Germany) reports that half of all mechatronic engineers now rely on modular, intelligent machines.
The best innovations simplify work. Managing complexity is a top priority for machine builders, integrators, and operators. Comprehensive tools exist to overcome complexities of automation and motion control. Agile and scalable drive technologies power efficient motion control and enable precise speed control, safety, diagnostics, and maintenance. Human-machine interface (HMI) systems, network connectivity, and other advanced features give machine builders the freedom to design, commission, program, and connect machines more quickly.
Machine control topography dictates how a motor or motors move an axis or multiple axes. In every application, there will be variations in motion control requirements. Choosing between a controller-based versus drive-based machine control is a key design decision. Essentially, the choice comes down to whether speed and positioning of each axis are controlled by the drive or a main controller making those decisions. While the choice may be obvious for some applications, there may be multiple options. No litmus test exists. Therefore, it is important to carefully consider the strengths and limitations of both centralized and decentralized automation control.
A typical controller-based automation system would consist of a main controller and motor drive components connected via a real-time fieldbus. In this scheme, the intelligence resides in the main controller, which constantly conveys, to multiple drives, precisely what position to be in at any given time.
A main motion controller typically controls multiple drives at a 1-millisecond scan rate, whereas a drive-based scheme is normally given logical information at a much slower rate by a master programmable logic controller (PLC). Smaller packaging or food and beverage processing machines are generally controller-based because the drives require relatively low power for the predominantly high dynamic axes. As a machine exceeds four controller-based axes, the cost per axis decreases. Therefore, controller-based machines can have a cost advantage with multiple axes.
A powerful controller-based scheme also offers greater flexibility for running multiple axes applications-from four axes up to an entire factory with 100 axes or more with multiple controllers. In most cases, controller-based applications run between four and 20 axes using a single controller. Robotic and machine applications with three or more axes can run off of a main controller to provide precise coordinated movement. Coordinated motion for robotics using more than two axes may be controller-based to assure precision in a packaging operation.
For example, a delta type robot might have three outer axes and a fourth axis for picking up products from an incoming belt conveyor and organizing them in some fashion on an outgoing belt conveyor. The main controller would operate all six axes simultaneously for smooth product flow (see Figure 1). In another scenario, a controller-based scheme can operate a four-axis rolling metal machine producing metal downspouts. The single machine can perform all tasks needed to roll and shape flat metal before turning the spout multiple times and positioning it precisely to create offset "S"-shaped crimping at the top and "C"-shaped crimping at the bottom. In another example, a controller-based 10-axis machine could be used to manufacture plastic irrigation pipe, where pipe material is fed repeatedly-indexing and positioning within an area containing blades on either side to cut drainage holes into the material at specified intervals.
Centralized versus decentralized drive-based automation
Unlike controller-based drives that cannot operate without controller direction, a drive-based scheme provides intelligence within the drive itself. Both centralized and decentralized drives with built-in intelligence can run independently with internal controls or via digital controls or other inputs.
A centralized drive-based scheme would be a likely choice for synchronous applications, such as winding, camming, positioning/indexing, and electronic gearing. Drives with enhanced built-in intelligence are capable of making complex calculations and logic-based decisions, as well as communicating from drive-to-drive to perform synchronous functions. In these cases, the physical proximity of the motors to the main control cabinet offers an advantage because all the controls and power distribution are in one central location and can be easily monitored and maintained.
Drive-based control is often preferable to operate larger machinery requiring higher horsepower, such as printing and other converting applications requiring multiple steps. Electronic gearing must occur between printing units, so it is critical that the drives are able to communicate to run at the correct speeds relative to each other.
Synchronous device-based control is a common choice for processing continuous materials, such as paper, film, foil, or textiles. There still may be a PLC communicating basic start-stop and speed control. Intelligence in a drive-based system can even migrate between the drive and a PLC. However, it takes drives with built-in intelligence providing logic and the drive-to-drive communications to run synchronously. In electronic gearing, a master drive conveys its position to all other drives, which follow the master drive’s position.
Other applications also can benefit from built-in drive intelligence. Winding and unwinding by calendering or corrugating machines require constant tension control and line speed. Excess tension or too much slack can cause defects and even material deformation. Tension control requires slight incremental changes in the speed of material winding and unwinding—accounting for changes in the decreasing diameter of the rolled material on both ends. A drive with built-in intelligence uses an internal drive calculator to continuously track in real time the speed versus diameter to adjust the winding and unwinding speeds accordingly.
Camming is another application requiring precise coordination between axes. For a device to cut accurately, the surface material and cutting device must be traveling at exactly the same speed when they contact. Otherwise, some materials will crinkle or tear. Achieving precise coordination between a rotary cutting rod knife and material, for example, both rotating at variable speeds, can be tricky. For example, when the circumference of the knife roll is larger than the cut length, the cutter would generally need to speed up while rotating around and slow down in the cutting zone to match the speed of material while making the cut. The logic and coordination must be constantly occurring between drives in the drive-based or controller-based architecture.
In some cases, machine size may necessitate use of decentralized drive-based control. Long motor cables from a central control cabinet can be eliminated by bringing power to the decentralized drives in a daisy chain, drive-to-drive fashion or by feeding power from a source other than a central control cabinet. Decentralized drive-based inverters can allow for motor-proximity installation. Decentralized inverters can enable even large and complex machines to be more clearly structured, which can be particularly beneficial in applications in the automotive, intralogistics, and other industries.
Control integration tools
Application software tools exist that provide modular, ready-made motion control functions using customizable standardized interfaces. Standard machine tasks, such as cross-cutting and winding, and complete robotics modules can be quickly implemented. Robotic applications, such as pick-and-place movements, are programmed by simple parameter settings without requiring knowledge of robotics, which substantially reduces demands on engineering and design resources.
Solutions built on parameterized programming technology greatly simplify motion control and machine development from concept to deployment. Parameterization programming allows easier commissioning than traditional programming. Replacing complex programming with uniform machine-configuration software tools significantly reduces engineering time and technical requirements and eliminates redundancies that can drive up costs. Bringing a smart drive online no longer requires special training, thanks to modular components and engineering tools. That gives machine builders the freedom to focus on elements unique to their projects—differentiators to make their products more competitive.
Modular automation systems for device-based control can greatly simplify machine integration and automation. The increasing individualization of machine control brings challenges to reconcile design lead-times and set-up cycles with productivity. Short setup times can be achieved when machines operate as easily as possible, which requires simple operating concepts for machine operators (see Figure 2).
The manufacturing industry stands to gain enormous benefits from advances in automation. Developing an end-to-end automation solution requires a holistic and motion-centric approach. Ultimately, the machine tasks dictate the drive and control architecture. In terms of drive control complexity, the motto should be "no more and no less than is needed." This starts with knowing your options.
Choices made during the planning phase will influence performance throughout a machine’s lifecycle. The design process needs to begin by examining the intended motion control tasks, forming initial ideas, and taking strategic steps to select the right tools to develop an intelligent and sustainable concept aligned with the application tasks. There is no one-size-fits-all motor drive control scheme to fit every machine. However, advanced technologies have yielded more intelligent and powerful solutions, which translate into better choices.
Craig Dahlquist is an application engineer at Lenze Americas. He has worked for Lenze since 2003.
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