1、What is a smart sensorOne of the biggest advances in automation has been the development and spread of smart sensors. But what exactly is a smart sensor? Experts from six sensor manufacturers define this term. A good working smart sensor definition comes from Tom Griffiths, product manager, Honeywel
2、l Industrial Measurement and Control. Smart sensors, he says, are sensors and instrument packages that are microprocessor driven and include features such as communication capability and on-board diagnostics that provide information to a monitoring system and/or operator to increase operational effi
3、ciency and reduce maintenance costs.No failure to communicate The benefit of the smart sensor, says Bill Black, controllers product manager at GE Fanuc Automation, is the wealth of information that can be gathered from the process to reduce downtime and improve quality. David Edeal, Temposonics prod
4、uct manager, MTS Sensors, expands on that: The basic premise of distributed intelligence, he says, is that complete knowledge of a system, subsystem, or components state at the right place and time enables the ability to make optimal process control decisions. Adds John Keating, product marketing ma
5、nager for the Checker machine vision unit at Cognex, For a (machine vision) sensor to really be smart, it should not require the user to understand machine vision.A smart sensor must communicate. At the most basic level, an intelligent sensor has the ability to communicate information beyond the bas
6、ic feedback signals that are derived from its application. says Edeal. This can be a HART signal superimposed on a standard 4-20 mA process output, a bus system, or wireless arrangement. A growing factor in this area is IEEE 1451, a family of smart transducer interface standards intended to give plu
7、g-and-play functionality to sensors from different makers.Diagnose, program Smart sensors can self-monitor for any aspect of their operation, including photo eye dirty, out of tolerance, or failed switch, says GE Fanucs Black. Add to this, says Helge Hornis, intelligent systems manager, Pepperl+Fuch
8、s, coil monitoring functions, target out of range, or target too close. It may also compensate for changes in operating conditions. A smart sensor, says Dan Armentrout, strategic creative director, Omron Electronics LLC, must monitor itself and its surroundings and then make a decision to compensate
9、 for the changes automatically or alert someone for needed attention. Many smart sensors can be re-ranged in the field, offering settable parameters that allow users to substitute several standard sensors, says Hornis. For example, typically sensors are ordered to be normally open (NO) or normally c
10、losed (NC). An intelligent sensor can be configured to be either one of these kinds.Intelligent sensors have numerous advantages. As the cost of embedded computing power continues to decrease, smart devices will be used in more applications. Internal diagnostics alone can recover the investment quic
11、kly by helping avoid costly downtime.Sensors: Getting into PositionAs the saying goes, No matter where you go, there you are. Still, most applications require a bit more precision and repeatability than that, so heres advice on how to select and locate position sensors.The article contains online ex
12、tra material.Whats the right position sensor for a particular application? It depends on required precision, repeatability, speed, budget, connectivity, conditions, and location, among other factors. You can bet that taking the right measurement is the first step to closing the loop on any successfu
13、l application.Sensor technologies that can detect position are nearly as diverse as applications in providing feedback for machine control and other uses. Spatial possibilities are linear, area, rotational, and three-dimensional. In some applications, theyre used in combination. Sensing elements are
14、 equally diverse. Ken Brey, technical director, DMC Inc., a Chicago-based system integrator, outlined some the following position-sensing options.Think digitally For digital position feedback: Incremental encoders are supported by all motion controllers; come in rotary and linear varieties and in ma
15、ny resolutions; are simulated by many other devices; and require a homing process to reference the machine to a physical marker, and when power is turned off. Absolute encoders are natively supported by fewer motion controllers; can be used by all controllers that have sufficient available digital i
16、nputs; report a complete position within their range (typically one revolution); and do not require homing. Resolvers are more immune to high-level noise in welding applications; come standard on some larger motors; simulate incremental encoders when used with appropriate servo amps; and can simulat
17、e absolute encoders with some servo amps. Dual-encoder feedback, generally under-used, is natively supported by most motion controllers; uses one encoder attached to the motor and another attached directly to the load; and is beneficial when the mechanical connection between motor and load is flexib
18、le or can slip. Vision systems , used widely for inspection, can also be used for position feedback. Such systems locate objects in multiple dimensions, typically X, Y, and rotation; frequently find parts on a conveyor; and are increasing in speed and simplicity. A metal rolling, stamping, and cut-o
19、ff application provides an example of dual-encoder feedback use, Brey says. It required rapid and accurate indexing of material through a roll mill for a stamping process. The roll mill creates an inconsistent amount of material stretch and roller slip, Brey explains. By using the encoder on the out
20、going material as position feedback and the motor resolver as velocity feedback in a dual-loop configuration, the system was tuned stable and a single index move provided an accurate index length. It was much faster and more accurate than making a primary move, measuring the error, then having to ma
21、ke a second correction move, he says.Creative, economical Sam Hammond, chief engineer, Innoventor, a St. Louis, MO-area system integrator, suggests that the applications purpose should guide selection of position sensors; measurements and feedback dont have to be complex. Creative implementations ca
22、n provide simple, economical solutions, he says. For instance, for sequencing, proximity sensors serve well in many instances.Recent sensor applications include the AGV mentioned in lead image and the following. In a machine to apply the top seals to tea containers, proximity and through-beam sensor
23、s locate incoming packages. National Instruments vision system images are processed to find location of a bar code on a pre-applied label, and then give appropriate motor commands to achieve the desired position (rotation) setting to apply one of 125 label types. Two types of position sensors were u
24、sed. One was a simple inductive proximity sensor, used to monitor machine status to ensure various motion components were in the right position for motion to occur. The camera also served as a position sensor, chosen because of its multi purpose use, feature location, and ability to read bar codes.
25、A progressive-die stamping machine operates in closed loop. A linear output proximity sensor provides control feedback for optimizing die operation; a servo motor adjusts die position in the bend stage. A linear proximity sensor was selected to give a dimensional readout from the metal stamping oper
26、ation; data are used in a closed-loop control system. Part inspection uses a laser distance measurement device to determine surface flatness. Sensor measures deviation in return beams, indicating different surface attributes to 10 microns in size. An encoder wouldnt have worked because distance was
27、more than a meter. Laser measurement was the technology chosen because it had very high spatial resolution, did not require surface contact, and had a very high distance resolution. An automotive key and lock assembly system uses a proximity sensor for detecting a cap in the ready position. A laser
28、profile sensor applied with a robot measures the key profile.What to use, where? Sensor manufacturers agree that matching advantages inherent to certain position sensing technologies can help various applications.David Edeal, product marketing manager, MTS Sensors Div., says, for harsh factory autom
29、ation environments, the most significant factors even above speed and accuracy in customers minds are product durability and reliability. Therefore, products with inherently non-contact sensing technologies (inductive, magnetostrictive, laser, etc.) have a significant advantage over those that rely
30、on physical contact (resistive, cable extension, etc.)Other important factors, Edeal says, are product range of use and application flexibility. In other words, technologies that can accommodate significant variations in stroke range, environmental conditions, and can provide a wide range of interfa
31、ce options are of great value to customers who would prefer to avoid sourcing a large variety of sensor types. All technologies are inherently limited with respect to these requirements, which is why there are so many options.Edeal suggest that higher cost of fitting some technologies to a certain a
32、pplication creates a limitation, such as with linear variable differential transformers. For example, LVDTs with stroke lengths longer than 12 inches are rare because of the larger product envelope (about twice the stroke length) and higher material and manufacturing costs. On the other hand, magnet
33、ostrictive sensing technology has always required conditioning electronics. With the advent of microelectronics and the use of ASICs, we have progressed to a point where, today, a wide range of programmable output types (such as analog, encoder, and fieldbus) are available in the same compact packag
34、e. Key for sensor manufacturers is to push the envelope to extend the range of use (advantages) while minimizing the limitations (disadvantages) of their technologies.Listen to your app Different sensor types offer distinct advantages for various uses, agrees Tom Corbett, product manager, Pepperl+Fu
35、chs. Sometimes the application itself is the deciding factor on which mode of sensing is required. For example, a machine surface or conveyor belt within the sensing area could mean the difference between using a standard diffused mode sensor, and using a diffused mode sensor with background suppres
36、sion. While standard diffused mode models are not able to ignore such background objects, background suppression models evaluate light differently to differentiate between the target surface and background surfaces. Similarly, Corbett continues, a shiny target in a retro-reflective application may r
37、equire use of a polarized retro-reflective model sensor. Whereas a standard retro-reflective sensor could falsely trigger when presented with a shiny target, a polarized retro-reflective model uses a polarizing filter to distinguish the shiny target from the reflector.MTS Edeal says, Each technology
38、 has ideal applications, which tend to magnify its advantages and minimize its disadvantages. For example, in the wood products industry, where high precision; varied stroke ranges; and immunity to high shock and vibration, electromagnetic interference, and temperature fluxuations are critical, magn
39、etostrictive position sensors are the primary linear feedback option. Likewise, rotary optical encoders are an ideal fit for motor feedback because of their packaging, response speed, accuracy, durability, and noise immunity. When applied correctly, linear position sensors can help designers to ensu
40、re optimum machine productivity over the long haul.Thinking broadly first, then more narrowly, is often the best way to design sensors into a system. Edeal says, Sensor specifications should be developed by starting from the machine/system-level requirements and working back toward the subsystem, an
41、d finally component level. This is typically done, but what often happens is that some system-level specifications are not properly or completely translated back to component requirements (not that this is a trivial undertaking). For example, how machine operation might create unique or additional e
42、nvironmental challenges (temperature, vibration, etc.) may not be clear without in-depth analysis or past experience. This can result in an under-specified sensor in the worst situation or alternatively an over-specified product where conservative estimates are applied.Open or closed Early in design
43、 those involved need to decide if the architecture will be open-loop or closed-loop. Paul Ruland, product manager, AutomationDirect, says, Cost and performance are generally the two main criteria used to decide between open-loop or closed-loop control in electromechanical positioning systems. Open-
44、loop controls, such as stepping systems, can often be extremely reliable and accurate when properly sized for the system. The burden of tuning a closed-loop system prior to operation is not required here, which inherently makes it easy to apply. Both types can usually be controlled by the same motio
45、n controller. A NEMA 23 stepping motor with micro-stepping drive is now available for as little as $188, compared to an equivalent servo system at about $700.Edeal suggests, Control systems are created to automate processes and there are many good examples of high-performance control systems that re
46、quire little if any feedback. However, where structural system (plant) or input (demand or disturbance) changes occur, feedback is necessary to manage unanticipated changes. On the process side, accuracyboth static and dynamicis important for end product quality, and system stability and repeatabili
47、ty (robustness) are important for machine productivity. For example, Edeal says, in a machining or injection molding application, the tool, mold or ram position feedback is critical to the final dimension of the fabricated part. With rare exceptions, dimensional accuracy of the part will never surpa
48、ss that of the position sensor. Similarly, bandwidth (response speed) of the sensor may, along with response limitations of the actuators, limit production rates. Finally, a sensor that is only accurate over a narrow range of operating conditions will not be sufficient in these types of environments
49、 where high shock and vibration and dramatic temperature variations are common.The latest What are the latest position sensing technologies to apply to manufacturing and machining processes and why?Ruland says, Some of the latest developments in positioning technologies for manufacturing applications can be found in even the simplest of devices, such as new lower-cost proximity switches. Many of th
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