Sensors: Getting into Position

What's 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 successful application. Link to related videos.

06/01/2005


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AT A GLANCE

 

  • Digital position sensing

  • Dual-encoder feedback underused

  • Creativity creates economy

  • Determining which technology

What's 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 successful 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, they're used in combination. Sensing elements are 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 many 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 inputs; 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 simulate 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 flexible 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-off 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 outgoing 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 make a second correction move,' he says.

Creative, economical

Sam Hammond, chief engineer, Innoventor, a St. Louis, MO-area system integrator, suggests that the application's purpose should guide selection of position sensors; measurements and feedback don't have to be complex. 'Creative implementations can 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 sensors 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 used. 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.

  • 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 operation; 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 wouldn't have worked because distance was 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 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 automation environments, 'the most significant factors even above speed and accuracy in customer's 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 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 interface 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 application 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, magnetostrictive 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 package. 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+Fuchs. '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 suppression. 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 require 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 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, magnetostrictive 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 ensure 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, and 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 environmental 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, 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-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 motion 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 require 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, accuracy—both static and dynamic—is important for end product quality, and system stability and repeatability (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 surpass 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 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 these prox devices are now available for as little as $20 and in much smaller form factors, down to 3 mm diameter. Some specialty models are also available with increased response frequencies up to 20 kHz. Where mounting difficulties and cost of an encoder are sometimes impractical, proximity switches provide an attractive alternative; many position control applications can benefit from increased performance, smaller package size, and lower purchase price and installation cost.'

Corbett concurs. 'Photoelectric sensors are getting smaller, more durable, and flexible, and are packed with more standard features than ever before. Some new photoelectrics are about half the size of conventional cylindrical housings and feature welded housings compared with standard glued housings. Such features are very desirable in manufacturing and machining applications where space is critical and durability is a must. And more flexible connectivity and mounting options—side mount or snout mount are available from the same product—allow users to adapt a standard sensor to their machine, rather than vice versa.'

Another simple innovation, Corbett says, is use of highly visible, 360-degree LEDs that clearly display status information from any point of view. 'Such enhanced LEDs indicate overload and marginal excess gain, in addition to power and output. Such sensors offer adjustable sensitivity as standard, but are available with optional tamperproof housings to prevent unauthorized adjustments.'



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Origin of paraphrase in title

A source for the often-paraphrased quote in the title follows, according to Control Engineering’s Jim Montague. ' 'Cause remember, no matter where you go—there you are.' That was stated by Buckaroo Banzai, the title character in 'Buckaroo Banzai Across the Eighth Dimension' (1984), written by Earl Mac Rauch. Peter Weller, of “RoboCop” fame, played the role.

Position sensing products
Balluff says its new GlobalProx line of sensors is right for two out of three general-purpose applications. Designed to address industry's need for global standardization and 'in stock' availability, the tubular-style sensors are offered in 2-wire and 3-wire dc, in sizes M8, M12, M18 and M30, with quick-disconnect or cable-out. They are ideal for non-contact presence sensing of metal objects in virtually any industry application for positioning, automation, control, or process monitoring. Three-wire dc models are available in standard or extended sensing ranges, with a medium-length housing and 360-degree-visible LED. Two-wire dc models offer extended range in a short housing, also with the 360-LED.

Banner Engineering’s World-Beam QS18 Glass Fiber Optic sensor is the company’s smallest unit of this type available in a QS18 package and an 18-mm threaded-barrel mount or side-hole mount. These sensors are said to be ideal for confined locations or for piping the fiber into wet, abrasive, corrosive, or high temperature environments up to 480 °C.

BEI Industrial Encoder Division ’s newest and largest-bore hollow shaft encoder is said to be ideal for large motor feedback applications. HS45 accepts motor shafts up to 1-5/8 in. dia (1-7/8 in. optional) and provides data for position and velocity control. This model incorporates precise feedback of an optical encoder with an easily mounted configuration to larger motors. There is no need to step down or alter most large motor shafts. HS45 encoder provides an option for dual electrically isolated outputs. Should one set of outputs sustain electrical damage, a quick change of connection enables a quick restart. Alternatively, dual output also can be used to feed two controllers from one encoder, to ensure speed and/or position synchronization without the cost of a second encoder.

Machine Control

This incremental encoder from BEI Industrial Encoder Div. incorporates precise feedback capabilities of an optical encoder with an easily mounted configuration to larger motors, providing data for position and velocity control.

Control Engineering video

Control Engineering Proximity Sensor Video: Carlo Gavazzi talks about capacitive sensors

 

Carlo Gavazzi Inc.’s CD46 Teach-Mode Tripleshield “Flat Pack” capacitive proximity sensor includes a thin-profile rectangular housing, expanding the Tripleshield line and making the CD46 “the most versatile and advanced capacitive sensor on the market,” the company says. The micro-processor-based sensors now feature a single pushbutton “teach mode” that allows the user to program the sensing range and output, eliminating the need for a potentiometer. Sensing range is up to 10 mm, and housing is 5.5 mm thick. List prices for the CD46 Series are $130.

Eaton Corp. iProx sensor can be programmed for a wide scope of challenges. Flexibility, adaptability, and sensing capability allow it to replace numerous sensors and reduce design engineering time and inventories. Programming takes minutes from any PC with a few mouse clicks. Just affix the magnetic remote programmer to the face of the sensor and the interface cable to the PC serial port. Place a target in front of the iProx to teach it exactly what it should see. Variables that can be altered include sensing distance, operating modes, background suppression of metals, and sensing band. Factory can preconfigure; reprogramming is possible later as needs change.

Machine Control

Carlo Gavazzi CD46 Tripleshield Flat Pack capactitive proximity sensors have a remote-teach wire for programming from a remote location.

ifm efector GMR technology provides high sensitivity to magnetic targets, using Giant Magnetoresistive (GMR) technology to detect magnets through all non-magnetizable materials, including stainless steel, non-ferrous metal, aluminum, plastic, wood, and glass. They offer high switching frequencies of 5,000 Hz for fast processes and extremely long sensing ranges up to 70 mm. Sensors are polarity independent and provide reliable position detection regardless of magnet orientation. They reportedly offer excellent repeatability and stable operating characteristics on a wide temperature range of–13 to 158 °F. Tubular-style sensors are 316 stainless steel quick disconnect models in M8, M12, and M18 diameter housings. A miniature rectangular model can retrofit V3-style mechanical switches. For washdown applications, ifm offers an M12 and M18 stainless-steel housings with FDA-approvedPEEK material sensing face.

Keyence offers advice in a “Non-contact Expert Laser Measurement Guide,” explaining techniques for measuring length and width using two reflective sensor heads, diameter using two X-Y sensor heads, and position and stroke using a two-axis measurement method, among other information, in a four-page PDF document .

MTS Sensors ’ R-Series sensor, the latest in the Temposonics family of magnetostrictive position sensors, is now offered with standard Fieldbus outputs. R-Series is also now available in flexible (RF) sensing element application housings, in addition to hydraulic (RH) and profile (RP) housings, and TempoGuard II high-pressure housing (HPH) is now approved for hazardous environments with the R-Series. R-Series Fieldbus output sensors can be directly interfaced to CANbus, DeviceNet, and Profibus systems. R-Series CANbus and Profibus products also offer multiple magnet sensing with up to 15 positions and five velocities simultaneously. This enables customers to replace multiple limit switches or linear sensors along a common axis, thereby providing more useful feedback at a reduced cost. R-Series sensors with MTS Sensors’ patented RF design is capable of sensing complex curvilinear motion and provides customers with a convenient packaging and installation envelope for long stroke sensors. RF design can be coiled to a bend ra-dius of 8 in. (200 mm) that can significantly reduce shipping costs for long sensors. Finally, Temposonics TempoGuard II explosion-proof housing is UL, cUL and ATEX approved for use with all outputs of new R-Series and G-Series sensors. This high-pressure housing (HPH) not only provides a waterproof package, but is approved for operation in Class I, Division I, Groups A, B, C and D areas. List pricing for the R-Series sensor starts at $622.

Pepperl+Fuchs introduces V18 Series photoelectric sensors. These rugged sensors feature an 18- mm diameter cylindrical metal housing with straight or right-angle sensing configurations. They are reportedly an ideal sensing option for thread detection, parts detection, and weld-seam detection applications. V18 family offers visible red and Class 1 laser light sources with adjustable focus and sensitivity settings. They have automatic cross talk/mutual interference protection and high immunity to ambient light sources.

Turck ’s R10 (rod style) version of the EZ-track line of programmable, non-contact linear displacement transducers (LDTs)—with an environmental rating of IP67, a continuous pressure rating 5,000 psi,, and a shock rating of 2,000 G–can monitor position of hydraulic cylinders. It is said to be ideal in heavy-duty applications, such as lumber mills, steel mills, stamping, and injection molding plants. R10 transducer uses magnetostrictive technology to monitor a magnet’s position along the active stroke of the rod without causing any wear on sensor parts. The sensors are 13.5 to 30 V dc input units with a variety of analog outputs, including voltage, current, and differential, which can be programmed over the active stroke length of the sensor. With 16-bit resolution (0.001 in.), R10 LDT can achieve repeatability to 0.001% of full stroke. Standard R10 housing material is anodized aluminum; however, it is also available in stainless steel for use in food processing or caustic applications. The sensing rod is always stainless steel and is available in stroke lengths from 2 to 180 in. R10 LDT also comes with a standard 5-pin M12 Turck “euro-fast” quick disconnect, which offers the customer higher performance, lower cost, and a faster, more reliable installation.

Getting positioned behind plug-and-play sensors
Nathan Luck is responsible for developing automated test solutions for various MTS products, in-cluding materials testing equipment and automation components like Temposonics magnetostrictive position sensors. 'The ultimate goal for production test engineering is to integrate and automate the entire assembly and test process to reduce labor costs and increase productivity. What makes this especially challenging is that products are becoming increasingly complex with a wider range of options, each of which must be tested,' says Luck.

MTS Systems Test Group develops highly flexible test equipment that depends largely on sophisticated software routines running on in-house PC-based controllers, he says. With the wide variety of sensors and actuators required, Luck faces challenges in properly interfacing elements to the host controller.

Machine Control

Visual diagnostics with dual-color LEDs are standard on MTS Sensors’ Temposonics G-Series and R-Series sensors. These tell if power is out of the recommended operating range, and if the sensing magnet or the controller interrogation signal is missing.

Plug and play devices and 'smart' sensors have potential to make many of these headaches a thing of the past. Because analog devices remain prevalent, few new solutions have been offered in recent years to ease connectivity challenges. This is the motivation behind the IEEE 1451 initiative, a multi-layered set of standards to provide a means for the device to communicate with the host interface/controller to pass sensor information to enable plug-and-play operation. The issues with IEEE 1451 are that the standards have been geared primarily toward embedded devices, and that development and adoption of the standards has progressed slowly, Luck suggests.

In the meantime, MTS Sensors has developed a new type of analog magnetostrictive position sensor that can address many solutions typically found with a smart sensor. It’s a hybrid device with digital electronics that handle programming, communication, and all parameter variations and analog electronics necessary to handle the core position sensing. This simplifies sensor design and allows for numerous interface options. For example, visual diagnostics use dual-color LEDs, which are now standard on MTS Sensors’ Temposonics G-Series and R-Series sensors. These basic visual indicators tell the operator if power is out of the recommended operating range, and if the sensing magnet or controller interrogation signal is missing. Such capabilities can save an engineer an hour or two in troubleshooting compared to a system that only provides an error indication of 'sensor bad.' In most cases, the reality is that wiring, power or something besides the sensor is not quite right, Luck says. Visual diagnostics can save hours of troubleshooting and system downtime.

Along with visual diagnostics, serial communication enables PC-based diagnostics and programming. This is accomplished using a very basic set of ASCII commands over a standard serial in-terface (RS-485 or RS-422 depending on sensor type). Using these commands, end-users can view factory sensor parameters like serial number, model number, date of manufacture, firmware revision, etc. They can also reprogram sensor parameters, such as setpoint values and output range. At a lower level, factory programmability provides engineers extra knobs to tweak the product for specific uses.

'With the new G-Series product, the amount of flexibility and adaptability for a given test fixture is maximized; it's all in the software now. Besides that, the simple serial interface allows me all the programmability and diagnostics I need to troubleshoot, fix, and move on,' says Luck. 'As a result, our setup time has essentially been eliminated and overall test time has been cut in half. Furthermore, 90% of troubleshooting requirements are addressed by LEDs and serial diagnostics. Such diagnostic capabilities have made a serious contribution to our overall productivity.'


Online Extra

AGV uses 3 position-sensing technologies
An automated guided vehicle (AGV), built by Innoventor on a wheelchair base, uses three-position sensing technologies to move around and test wireless technologies: laser distance sensor, ultrasonic distance sensors, and encoders.
( To see a thumbnail of the cover image of the AGV and its sensors, click here. )

The AGV uses a Sick Inc. Nav 200 laser scanner and transputer navigation computer to look at its surroundings two-dimensionally with a rotational movement of 360 degrees. In the process, it can detect 3-28 reflectors on each of up to 40 “layers” taught into the system. Ultrasonic distance sensors in the bumper area recognize objects to avoid collisions. Each SensComp Mini-A sensor has an electrostatic transducer and supporting circuitry for an analog output over the sensing range of 6 in. to 40 ft. Encoders monitor drive wheels to help calculate dead-reckoned position. The AutomationDirect Light Duty Hollow Shaft Incremental Encoder, with 38-mm diameter body, has 2,500 pulses/revolution, line driver (differential) output, and 5 V dc supply.

Four other sensing technologies, unrelated to the AGV, also have images in the printed article.

  • Banner Engineering’s World-Beam Expert QS18E glass fiber-optic proximity sensors offer easy pushbutton teach-mode setup and a 360-degree visible multi-function LED indicator.

  • MTS Sensors’ R-Series sensor has a flexible (RF) sensing element; the device is network enabled and can replace multiple limit switches or linear sensors along a common axis, providing more useful feedback at lower cost.

  • Pepperl+Fuchs V18 Series photoelectric sensors in an 18-mm diameter cylindrical metal housing offer visible red and Class 1 laser light sources with adjustable focus and sensitivity settings. Automatic cross talk/mutual interference protection help make them highly immune to ambient light sources.

  • Turck’s R10 (rod style) version of the EZ-track line of programmable, non-contact linear displacement transducers (LDTs) uses magnetostrictive technology to monitor a magnet’s position along the active stroke of the rod without causing wear on sensor parts.