Use Sensors Intelligently

Intelligent sensing for discrete manufacturing is finally coming of age. With cost of embedded controller chips on a continual decline and performance ever increasing, manufacturers can apply them economically in smaller packages. Meanwhile, advances in vision system software have reduced complexity to the point where it no longer requires a Ph.




  • Machine control

  • Bar-code readers

  • Machine vision

  • Photoelectric sensors

  • Proximity sensors

IEEE 1451 aims to simplify sensor connectivity

Intelligent sensing for discrete manufacturing is finally coming of age. With cost of embedded controller chips on a continual decline and performance ever increasing, manufacturers can apply them economically in smaller packages. Meanwhile, advances in vision system software have reduced complexity to the point where it no longer requires a Ph. D. in vision to set up a working application.

Demands placed on manufacturing for information gathering and handling require sensors that can do more than simply report an 'on' or 'off' condition. For example, 2D bar coding enables placing information about the product being manufactured on the product itself. Now the control engineer must find a sensor to read the new bar-code format and send the information to a controller or data server in real time while living in a sometimes-hostile environment.

Sometimes it isn't enough for the proximity or photoelectric switch to just detect presence. If a reliable analog output were available, it could supply distance information, too.

Vision system price/performance improvements mean that the goal of 100% inspection can be a reality. Technology improvements are enabling 3D vision with ability for critical spatial inspections. Meanwhile, on the lower cost end of the spectrum, color capabilities expand usefulness of these 'vision sensors.' Higher inspection speeds and Ethernet connectivity are other benefits.

Standards have played an important role enabling interoperability of open systems components, and standards are beginning to do the same in sensor technology and applications now. IEEE 1451 defines standard interfaces and data sheets providing many benefits.

Whatever the application, it will pay to remember that discrete sensors may not be so 'discrete' today.

Industrial environments can be tough on sensors. Sometimes proximity sensors are used for ladders or pallet stops. There's not much a supplier can do to overcome those problems, but help is on the way for handling the problems dust and oil mist have for optical sensors.

Karl Klinger, ifm efector (Exton, Pa.) product manager, states that an intelligent sensor can adjust to its environment and continually regulate sensitivity to maximize performance. 'Sensors acquire intelligence through microprocessor-based technology. The microprocessor is the brain of a sensor that teaches the unit to learn the characteristics of an application. This microprocessor-based 'teach function' enables the sensor to process large amounts of data very quickly. It's this capability that provides the user with a sensor that is easy to use, setup, and adjust.'

This intelligent sensor from Banner verifies lot and date code
information on cap and plunger of a syringe assembly system.

Alerts operator for action

Mr. Klinger reports that the most troublesome condition for photoelectrics is accumulation of dust and dirt on the lens, causing a gradual degradation of light seen by the receiver. The microprocessor can continually monitor light value and compare it to the 'normal' value. When the actual value reaches a threshold, the microprocessor sends a signal to alert an operator to clean the lens.

Another problem for photoelectric sensors is detecting irregularly shaped objects or objects with shiny backgrounds. Charles Strobel, tech support at Keyence (Oakland, N.J.), says two new technologies enable more reliable detection. 'APR (automatic power reinforcement) circuit automatically adjusts the amount of light emitted according to the color and angle of the target. This reflected light is then focused on the PSD (position-sensitive detector) forming a beam spot. The beam spot moves on the detector according to the distance between the sensor and work piece. A change in work piece color, for example, from white to black, reduces light quantity but does not alter the position of the beam spot on the PSD.'

Some early intelligence involved 'teach' functions. This enabled photoelectric set up by training on the normal background, pushing a button, then training the device on the target and pushing the button again. This function was a giant step from potentiometer set up. Microprocessor-enabled switches, standard networking, and laptop computers suggested further advances to come.

Brian Libby, sensor product manager for Siemens Energy & Automation (Batavia, Ill.), says, 'Some sensors offer the flexibility to program parameters either from a remote workstation, via the process image, or at the sensor's teach-in button. Parameter programming can be replicated systematically to multiple sensors thus eliminating the time-consuming task of programming individual sensors on a line. Advantages include elimination of wiring three-, four-, or five-wire sensors, plus the costly set up of each sensor. Ability to hot-swap a failed sensor is an added bonus. Another built-in benefit is system diagnostics. We can automatically detect wire breakage, short-circuit, mis-adjustment, and module/sensor failure and send a signal for each channel.'

Remote configuration a plus

One of the most attractive features of embedded intelligence according to Pepperl+Fuchs (Twinsburg, O.) manager of industrial bus systems, Gary Frigyes, 'is the ability to connect multiple sensors along a single cable and remotely configure and diagnose them. Remote configuration of sensors includes options such as target alignment, mode of operation selection, switching frequency, and switch point settings. Diagnostics include coil monitoring, target stability, sensor health, and weak signal indication that warn of potential problems before a complete failure occurs.'

Open networks have been available for some time now. Although implementation of these technologies seems to be only now gaining momentum, benefits extend beyond those mentioned by Mr. Frigyes.

Rockwell Automation (Milwaukee, Wis.) incorporates DeviceNet into many of its sensors. Network-enabled sensors reduce start- up costs due to plug-and-play ability of preconfigured devices. Wiring costs are often a fraction of standard parallel wiring systems because of one-cable wiring with quick connect terminations.

Another approach is to look at major applications and develop specialized intelligent solutions. Matt Sims, photoelectric sensor product manager at Eaton/Cutler-Hammer (Pittsburgh, Pa.), reveals, 'We're using embedded smarts in a photoelectric product to handle logic that would otherwise be managed in a remotely located PLC. This is a practical application where a smart device meets the needs of a customer by reducing wiring and PLC I/O costs while simplifying system installation. Our conveyor control system targets material handling and conveying applications by bundling sensors with other components to supply a zero pressure accumulation solution.'

Analog proximity switch

A good, but unfriendly, application for inductive proximity switches is in stamping presses and dies. A prox is a good 'go - no go' sensor for part-present detection. But what if there are problems other than just part present? Like having the correct part, or the part inserted only partially by the feeding process? The solution could be with a prox, but an analog output would work better than a myriad of small standard switches.

Henry Menke, Balluff (Florence, Ky.) product manager, discusses its analog inductive proximity switch. 'The analog signal can be analyzed to yield a lot of information, for instance part profile or part ID. An analog prox can be combined with today's less expensive analog PLC modules for an overall lower-cost solution. Setpoints programmable from the PLC are an attractive feature for someone who must make frequent changeovers.'

Floyd Schneider, Banner Engineering (Minneapolis, Minn.) vp, discusses a technology for photoelectric sensors that adds laser measurement capability to traditional part-present applications. ' 'Time of Flight' sensors are good for such applications as metal forming, die protection, and crane and gantry control,' he states.

The technology involves a short electrical pulse driving a laser diode. Part of the light is scattered to an internal photodiode sensor while the main beam leaves, then reenters, after reflecting from the target. Circuitry calculates the time differential and converts it to an analog signal proportional to the user-defined window limits.

Developers have attacked vision system constraints from all sides. Cameras are getting smarter and smaller, standard communications are reducing cabling problems, while software advances ease set up and configuration and make previously exotic applications accessible to many users.

Color, connectivity in vision

Advances in the 'vision sensor' market include color recognition and high-speed connectivity. Omron Electronics (Schaumburg, Ill.) has a color vision solution. Cognex (Natick, Mass.) and DVT (Norcross, Ga.) announced color capability for self-contained vision sensors during summer with shipments expected this fall.

Chuck Gillingham, senior industry marketing manager at Cognex, says, 'There are two technologies that people can expect to see in vision sensors. One is color recognition. This will help in applications like part sorting on a conveyor, where a high-end system may be too expensive or 'overkill.' The second is increased communication support. Our Vision Area Network is built on Ethernet to allow communications among sensors and other devices and systems. Families of application software provide templates for users developing applications in specific industry areas like packaging.'

Michael Williams, DVT market communications manager, points to the ability of color sensors to detect features that may be invisible to gray-scale analysis. Further, building on Ethernet connectivity, web-enabled diagnostics add power to a user's toolbox.

Mark Sippel, vision product marketing manager for Omron Electronics, concurs with the importance of communications in the new generation of vision systems and adds, 'Also look for high-speed processing for image capture and measurement leading to much more sophisticated measurement and inspection capabilities. Optical character recognition (OCR) is an emerging application for vision sensors inspecting shipping containers.'

Vision Area Network from Cognex enables coordination among many vision sensors
and the overall controls of this bottling and packaging line.

Vision system benefits

Matt Allen, Microscan (Renton, Wa.) product manager, describes vision-system benefits, 'Today's drive for just-in-time inventory fulfillment and build-to-order manufacturing practices has made information one of a company's most important assets. Vision systems are the enabling technology behind automation of a lot of manufacturing processes. As a data-capture device, vision systems have been used in high-speed sorting, product inspection, and work-in-progress tracking.'

Adds Microscan applications engineering manager, Bob Taplett, 'I think we can safely say that vision technology will evolv e into the data-capture system of the future. Vision-based systems will eventually replace bar-code scanners largely because of the additional tasks they can accomplish.'

Vision has become quite a broad category of products from lower-priced 'sensors,' which are constantly becoming more sophisticated, to higher-end systems that entail image capture boards for PCs, fast cameras, and sophisticated software.

Pierantonio Boriero, product line manager for Matrox Imaging (Dorval, Quebec, Canada), discusses trends in these higher-end systems. 'The trend is for more sophisticated, yet easier-to-use, tools. An example is geometric pattern-recognition software. Using gray-scale analysis was good in controlled environments, but difficult to implement in manufacturing. Geometric pattern recognition is tolerant to more hostile environments. Plus the software is designed for users who don't have a Ph.D. in image processing.

'Trends in cameras are high-speed serial interfaces like IEEE 1394 (Firewire), CameraLink, and, maybe, USB 2.0. Use of these standards will simplify interfaces with the system by eliminating custom cables.'

Vision in 3D

Two companies have introduced 3D systems, albeit in differing versions. Cognitens (Ramat HaSharon, Israel) features a three-camera arrangement in a fixture with a system that can be a real-time metrology instrument used in applications like door alignment.

Meanwhile, Braintech (North Vancouver, British Columbia, Canada) has developed a system offering 3D output from a single camera system initially used in materials handling operations.

Both of these technologies are sophisticated yet available for most control engineers designing solutions for more difficult applications. For a more detailed discussion of these new technologies and applications, see this article online at , under CE issues, Oct. '01.

Jack Tedesco, vice president of RJS (Irvine, Calif.), defends bar-code solutions for automated data entry. 'A bar code is the world's least expensive and most robust way of entering data,' he contends. 'Because of this, the bar code is increasingly becoming the point of data entry into a system, and for a variety of reasons there simply is no backup option. Online bar-code verifiers offer real-time, on-the-fly print process error detection to keep the system up.'

Intelligence in sensors, like in humans, comes in many guises. The challenge for control engineers is to find the right sensor and apply it intelligently.

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IEEE 1451 aims to simplify sensor connectivity

While the technology to make sensors smarter continues to proliferate, the work of IEEE 1451 standards groups is attempting to bring some degree of order to the fragmented world of smart transducers. By doing so, the standards effort initiated in 1994 by IEEE and National Institute of Standards and Technology (NIST, Washington, D.C.) hopes to ease integration and adoption of smart transducer technology.

Simply stated, the goal of IEEE 1451 is to simplify transducer (sensor and actuator) connectivity, whether to instruments or networks. The participating companies are working to accomplish this goal through definition of a set of common interfaces for smart transducers, including mechanisms for plug-and-play sensor operation.

Recognizing that smart sensors will inevitably take a number of different forms with different levels of integration, IEEE 1451 standards define a set of complementary interfaces designed to work together or independently.

Progress continues in developing the two newest proposed standards aimed at analog sensors and high-speed distributed applications.

IEEE P1451.4 working group is finishing a proposed standard defining how analog transducers can inherit self-describing capabilities for simplified plug-and-play operation. The standard defines a mixed-mode interface that retains the traditional analog sensor signal, but adds a low-cost digital link to access the Transducer Electronic Data Sheet (TEDS) embedded in the sensor for self-identification. These plug-and-play sensors, while maintaining compatibility with legacy analog systems, bring the universal benefits of easier system setup and maintenance, improved sensor bookkeeping, and improved data integrity. In fact, the concepts and technology of IEEE P1451.4 are in use today in measurement applications involving very large numbers of sensors, such as structural testing. Work also continues in IEEE P1451.3 working group on the definition of a digital multidrop transducer bus for linking large numbers of physically separated sensors. The P1451.3 specification will support TEDS, high-bandwidth sensors (up to several hundred kHz), and time-synchronization on the bus.

IEEE P1451.3 and P1451.4 are the newest proposed members of the IEEE 1451 family of Smart Transducer Interface Standards, two of which are accepted and published. IEEE Std 1451.1-1999 'Network Capable Application Processor Information (NCAP) Model' defines a common object model, with interface specifications, of a networked smart transducer.

IEEE Std 1451.2-1997 'Transducer to Microprocessor Communication Protocol and TEDS Formats' defines a digital point-to-point interface to connect a smart transducer module with digital output to a microprocessor-based network adapter. IEEE 1451.2 was also the first standard to introduce the concept of the TEDS. The concept of the embedded, self-identifying TEDS is arguably the most popular component of the standards and is a key element throughout the family of IEEE 1451 standards.

David Potter, National Instruments director of emerging markets and vice chair of IEEE P1451.4 working group.

More information about IEEE 1451 can be found at the NIST website See also Control Engineering Online Extra from Endevco.

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