NI Mechatronics Machine Design Guide

With this comprehensive machine design guide learn step-by-step best practices for designing your next machine. Throughout the guide, in-depth application examples illustrate the approaches successful designers have implemented.

Choosing the Right Controller

Engineers and machine builders are under increasing pressure to differentiate their machines, provide new capabilities to decrease operational costs for their customers, and design and produce these systems quickly. While a deliberate mechatronics-based design process with emphasis placed on up-front design and simulation helps machine builders create sophisticated machines in less time, a critical decision is the selection of the embedded controller. Though time-to-market pressure makes it tempting to simply use the same controller installed in previous systems, selecting the appropriate controller with the right software support not only increases machine capabilities but also can reduce design time by helping engineers and machine builders reuse work done in the original design steps and by eliminating the need to rework control strategies.

For instance, in a steel rolling application, an initial simulation may show that placing the sensor used for gauge thickness close to the roller nip leads to faster press hydraulic control and more accurate steel component production. During component selection, designers find that a gamma sensor is necessary to get the required bandwidth for the control rate. However, the gamma sensor has a nonlinear output that requires computationally intensive algorithms to convert the signal to a thickness. By selecting a controller with the ability to run complex floating-point operations and still meet the cycle times of the machine, the designers are able to retain their original control strategy. Had they automatically selected the controller used on previous designs, they may have been forced to either add separate hardware to perform the signal processing or change their control strategy to allow for slower loop rates.

Traditional Options – PLCs, PCs, and Custom Hardware

Traditionally, designers have had three classes of control systems, with distinct advantages and disadvantages, to choose from for their designs: programmable logic controllers (PLCs), PCs, or custom hardware. PLCs and PCs share the same basic electrical components: a microprocessor and associated memory to run the control code, I/O modules to convert sensor signals to digital data, a bus to pass data between the modules and the processor, and communications ports to program the controller and pass data between networked devices. Where these devices differ is in the actual components used and the software to program these devices.

PLCs are specialized industrial computers first introduced in the 1960s to replace relay banks for digital control. Programmed with vendor-specific software, they have a fixed execution model with a reliable and easy-to-use scanning architecture where the control engineer is concerned only with the design of the control code because the input cycles, output cycles, and housekeeping cycles are all performed automatically. This makes them particularly popular in applications where nonprogrammers – such as end customer maintenance staffs who update and modify the control code – need access to the controllers. This rigidity also allows PLC designers to optimize their designs and select lower-cost processors and memory architectures. However, the strict software architecture can also make it inflexible for custom applications such as communications, data logging, or custom control algorithms.

PCs are general-purpose computers initially designed to handle a variety of non-real-time applications. Unlike a PLC, the typical PC uses rotating media and is not designed for rugged industrial use. On the positive side, however, PCs are based on physical and electrical standards that allow easy expansion and system upgrades. Compared to a PLC, a typical PC uses a more powerful floating-point processor and includes more memory, making it suitable for more intensive data processing applications. The software on a PC starts with a general-purpose operating system such as Microsoft Windows. Windows is capable of running many software programs, which provides great flexibility but makes the system less reliable and stable. PC applications are developed with a wide variety of common programming languages and tools, unlike the relatively restricted and specialized set of languages used on PLCs. The lack of a predefined software architecture and physical ruggedness in typical PCs makes PCs less usable for “standard” control applications. However, a PC is much more able to adapt and incorporate different types of control and processing functions for custom or complex applications. Also, the greater I/O and connectivity of a PC make it much easier to integrate disparate, even proprietary, devices into systems. Because of their powerful processing capacity, networking capability, and graphical interfaces, PCs play a key role for supervisory and advanced control, human machine interfacing, data logging, and enterprise communication.

Custom hardware, by definition, is specialized circuitry designed to meet a specific need. Because it is expressly designed for each application, it may use any hardware architecture. Typical custom hardware uses include providing add-on functionality to a larger system such as sensor-level signal processing, offloading high-speed control loops, or completely controlling high-volume applications such as embedded electronic control units (ECUs) used in automotive or custom circuitry in medical or consumer devices. Although in theory these devices could be designed to use almost any programming language, in general, the focus on high volume and low cost pushes most designers to use low-level programming tools and to write these applications in ANSI C or VHDL.

A New Option – Programmable Automation Controllers

Today, the availability of high-performance PC components suitable for industrial environments makes it possible to create controllers with PLC ruggedness and PC architectures for performance and openness. Examples are processors with -40 to +85 °C operation and mass storage without moving parts. Additionally, the inclusion of real-time operating systems (RTOSs) such as Phar Lap from Ardence (formerly Venturcom) or VxWorks from Wind River provide reliability and determinism, which are often not available in a general-purpose operating system such as Windows. These RTOSs offer the capability to control all aspects of the control system, from the I/O read and write rates to the priority of individual threads on the controller. Individual vendors add abstractions and I/O read/write structures to make it simpler for engineers to build reliable control applications. The result is flexible software suited for multidomain applications that mix discrete, process, and motion custom control, as well as the capability to perform other tasks such as signal processing, data logging, and communication. Industry analyst Automation Research Corporation has coined a term to describe this new class of controller that combines the advantages of PLCs and PCs: programmable automation controller, or PAC.

Figure 1. Programmable automation controllers provide the reliability of the PLC and the functionality of the PC.

National Instruments manufactures PACs that run NI LabVIEW software. LabVIEW features an intuitive graphical programming style, similar to flowcharts, to provide the functionality of a full-featured programming language with an easy-to-use interface. With a background in measurements, National Instruments is extending PAC beyond simple I/O by incorporating higher-speed measurements and machine vision capabilities. Many industrial applications collect high-speed measurements for vibration or power quality applications. The collected data is used to monitor the condition of rotating machinery, determine maintenance schedules, identify motor wear, and adjust control algorithms. Normally collected using specialized data acquisition systems or stand-alone instrumentation, the data is incorporated into a control system using a communication bus. National Instruments PACs can take high-accuracy measurements at hundreds of thousands of samples per second and then directly process the data in real time on high-performance CPUs to perform sophisticated control, monitoring, and diagnostics.

Engineers also can incorporate vision into their control systems. Vision is an area of automation that has gained a lot of momentum in the last decade. In a manufacturing environment, engineers can use visual inspection to identify many flaws or mistakes that are difficult to detect using traditional measurement techniques. Common applications include part inspection for manufacturing or assembly verification such as checking for correct component placement on a circuit board, optical character recognition (OCR) to examine date codes or to sort products, and optical measurements to find flaws in products or to sort based on quality criteria. National Instruments PACs incorporate vision, high-speed measurements, logic, and motion control, eliminating the need for engineers to integrate dissimilar hardware and software platforms.

Modern PACs Eliminate the Need for Custom Hardware

Although PACs represent the latest in programmable controllers, the future for PACs hinges on the incorporation of embedded technology to eliminate the need for custom hardware. One way to eliminate this need is to use software to define hardware. Field-programmable gate arrays (FPGAs) are electronic components commonly used by electronics manufacturers to create custom chips that engineers can use to place intelligence in new devices. These devices consist of configurable logic blocks that perform a variety of functions, programmable interconnects that act as switches to connect the function blocks together, and I/O blocks that pass data in and out of the chip. Engineers use software to define the functionality of the configurable logic blocks and the way they are connected to each other and to the I/O. FPGAs are comparable to having a computer that literally rewires its internal circuitry to run a specific application.

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Figure 2. National Instruments PACs not only combine the features of PC and PLC platforms but also offer the performance and flexibility of custom hardware through programmable FGPAs.

FPGA technology has traditionally been available only to hardware designers who were proficient in low-level programming languages such as VHDL. However, controls engineers today can use LabVIEW FPGA to create custom control algorithms that are downloaded onto FPGA chips built into NI PACs. With this capability, engineers can incorporate sensor-level signal processing and time-critical control functions such as motion control and sensor health monitoring. Because the control code runs directly in silicon, engineers can quickly create applications that incorporate custom communication protocols or high-speed control loops including up to 1 MHz digital control loops and 200 kHz analog control loops.

National Instruments PACs

NI PACs offer the capability to add advanced I/O, advanced processing, and advanced communication to existing systems. This includes motion, vision, high-speed acquisition, and signals that require special signal conditioning such as electrical power measurements, strain, LVDT, and vibration. With FPGAs, PACs perform sophisticated analysis and high-speed control. PACs can also interface to the enterprise by directly logging information for insertion into SQL databases or hosting Web pages. National Instruments offers a number of PAC platforms based on LabVIEW including PXI and CompactRIO.

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Figure 3. LabVIEW and PXI

PXI features a modular and rugged Eurocard form factor based on CompactPCI to deliver a modular, compact, and rugged industrial system. A PXI system is controlled by an embedded controller with a high-performance multigigahertz processor. It offers easy access to cabling with connectors on the front of PXI modules. The PXI platform offers a broad range of measurement modules and connectivity to field devices using CAN, DeviceNet, RS232, RS485, Modbus, and FOUNDATION fieldbus.

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Figure 4. LabVIEW and CompactRIO

CompactRIO is an FPGA-based reconfigurable control and acquisition system designed for applications that require a high degree of customization and high-speed control. The architecture combines a real-time embedded processor for complex algorithms and custom calculations with a reconfigurable I/O (RIO) FPGA core. The CompactRIO platform accommodates up to eight analog or digital I/O modules manufactured by either National Instruments or other companies. It is ideal for complex and high-speed applications such as machine control and, with FPGA, is a good option for applications that normally require custom hardware development.

Software Reuse

One barrier that prevents machine designers from selecting the appropriate controller for a specific application is the learning curve associated with mastering new software and the ability to reuse algorithms and designs among projects. This is especially problematic when incorporating advanced functionality into machines such as machine condition monitoring, custom motion control, or enterprise integration. For efficiency, machine shops must standardize on a core set of software tools than can cover all of their application needs such as logic, process, motion, HMI, vibration, vision, IT integration, communications, and logging. To address this problem, National Instruments has invested more than 1,000 man-years of development in LabVIEW and its integration with NI PAC platforms. LabVIEW is designed to be open, allowing connectivity to databases, third-party PLCs, and serial devices and encouraging code reuse from programming and design languages such as ANSI C and The MathWorks, Inc. Simulink® environment. Every PAC National Instruments sells is programmed with LabVIEW, so designers can easily scale between platforms based on their application requirements without the need to learn new tools.

Figure 5. With a common programming language designers have the freedom to choose the right platform for the application, whether CompactRIO for high speed control of die-casting press machines at EUROelectronics or PXI for distributed control of 108 collimators for particle research at CERN.   

System integrator Captronic Systems builds turnkey systems for control and measurement in a variety of industries. Because each system the company designs has unique needs, it standardized on LabVIEW to provide the flexibility to tackle disparate applications without incurring the cost of learning new tools. These applications require different controller platforms, and Captronic is able to use the same software but select the appropriate platform for each application, whether balancing rotors for power generation turbines, performing online inspection of 5,000 ton presses, or conducting glow plug endurance tests. For instance, Captronic was contracted to build a control system for TIG welding. In this application, different objects are welded based on a recipe specific to the shape, size, and material composition of the object to be welded. Various safety interlocks are incorporated because welding current can be as high as 200 A. An operator interface is provided on a Windows PC and welding profiles can be imported from AutoCAD. This system performs motion control to position the weld object and to control the arc distance using the weld voltage level as feedback. Because of the computationally intensive algorithms and the precision motion control, Captronic chose to implement this system using a National Instruments PXI programmable automation controller programmed with LabVIEW. For another job, Captronic was hired to create the controller for the engine in an underwater crawler. This application required embedded control and logging but also needed a very rugged platform that could withstand wide temperature variations and high shock and vibration. Captronic was able to continue to use LabVIEW for the development of the underwater crawler control but, in this case, ran its control code on a CompactRIO PAC.

Additionally, because of the openness of LabVIEW, designers who require customization beyond that available from an NI PAC can still reuse their software investment. For example, a major Japanese automotive manufacturer used LabVIEW and CompactRIO to develop a control system in its R&D center. With CompactRIO, the manufacturer could rapidly iterate on designs and control strategies; however, the mechanical form factor did not meet the company’s needs for the next stage of development. LabVIEW has the ability to target arbitrary 32-bit processors, so the manufacturer could have taken its development work and built custom hardware to meet its packaging needs. But because CompactRIO met the existing control and electrical requirements, it was ideal to simply redesign the form factor. This customer opted to work with a custom electrical design company, Flextronics (now Prevas), who licensed the rights from National Instruments to develop custom layouts of the CompactRIO platform. This helped the manufacturer quickly transition its application directly from the CompactRIO PAC to a custom layout.

Simulink^®is a registered trademark of The MathWorks, Inc.

Download the Complete NI Machine Design Guide

Additional chapters include:

• Understanding customer requirements and conceptualizing design ideas
• Following the Mechatronics-integrated design approach
• Lowering design risk and increasing machine productivity while prototyping the machine
• Deploying and maintaining the machine

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