Which Positioning Sensor Is Right for My Application?

How to make sense of the many options available for ensuring accuracy

By Mike Bacidore

Our metal stamping machines need to be able to run a variety of parts through the press. Any errors in the coil-change process can be costly. The manual changeover itself is time-consuming, but a damaged press means expensive repairs and lost production. We need continuous position sensing, but our machines typically operate in harsh environments, and sometimes vibration and magnetic fields can be an issue. There are so many potential positioning-sensor solutions. What’s the difference? Any advice on where to start?

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  • <p>Too few details to make a recommendation. Do you need to sense to an accuracy of microns, millimeters, or do you just need to detect that something is present? My first choice for sensing metallic objects is usually an inductive prox sensor. They do not require adjustment, are extremely durable, and are available in 'weld field immune' models for cases where high magnetic fields are present. The diameter of the sensor is roughly proportional to the sensing range, with an effective range of about 25-30% of the sensor diameter. Some 'high sensitivity' models improve this range somewhat. These sensors are fairly repeatable, but their apparent sensitivity does change as the object to be detected is nearer or further away, but still within the sensing range. Note that the sensitivity is dependant also on what metal is sensed. The range is longest with ferrous metals and shorter for nonmagnetic metals like copper and aluminum. Optical sensors can work well also and they will detect nonmetallic (even clear) materials, but dirt can be a problem in industrial environments. Through-beam sensors give the most accurate and repeatable edge detection, but fixed-field types (which use sensor and emitter in the same enclosure and which have a fixed cut-off distance beyond which objects are not detected) are generally easier to use and to keep aligned. And then there is the old time mechanical limit switch. Reasonably repeatable and durable industrial versions are available that will stand up to a lot of abuse, and which have lifetimes measured in the tens of millions of cycles. At the high end of cost are laser based optical sensors that sense distances to micron resolution and can accept an input which can change the distance setpoint 'on the fly' where that type of action is necessary.</p>

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  • <p>Would recommend using a position sensor that is not based on a magnetostrictive technology for this application to due your concerns for shock and vibration tolerance as well as noise immunity. Either potentiometric (track and wiper) or inductive technology sensors would be better choices. One such sensor is Novotechnik’s LS1 Series. Without knowing your requirements for stroke length and accuracy, these sensors have worked well in applications similar to yours and have the following specifications: Programmable to optimize the sensor for your application, Stroke lengths up to 200 mm, linearity of &lt; ±0.15%, life of &gt;100 million movements, voltage or current output, sealed to IP 40, withstands shock up to 100 g and vibration up to 20 g. The LS1 Series has been tested to meet or exceed the following noise immunity standards: ESD EN 61000-4-2, EN 61000-4-3, EN 61000-4-4 and EN 61000-4-6 for continuous, radiated, burst and disturbances induced by RF fields respectively. With this sensor series you can typically measure within ±0.15 mm for a 100 mm stroke length. If you would like to discuss your specific application and/or you need a sensor with a longer stroke length, Novotechnik engineers would be glad to make further recommendations.</p>

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  • <p>Hello Mike! Great topic - thank you. You're right, there are a lot of position sensors out there and choosing one can be daunting. The reason is that there is an even bigger universe of applications and mounting conditions to consider, and there is no single universal sensor that will work in every situation.</p> <p>Your statement about harsh environments is an important one. A suitable sensor needs to be physically robust. Most industrial sensors anticipate shock, vibration, temperature swings, and the presence of water and/or oil. What many do not anticipate is the danger of physical impact. In that case, special brackets and guards are available to protect and "bunker" the sensor.</p> <p>Regarding the best technology or physical sensing principle, it depends on the measuring range and whether or not the sensor needs to detect without any mechanical contact, i.e. at a distance. For short range (generally less than 12 mm or 0.5") measurement of moving metal (ideally, steel) targets, analog inductive sensors are a great choice. Be wary of extremely strong magnetic fields, however; these can saturate the sensor coil core and cause it to lock its position. On the upside, inductive sensors are extremely hardy and are relatively inexpensive compared to alternatives.</p> <p>For longer ranges (a few inches to a few meters), analog photoelectric distance sensors (e.g. infrared, visible red, laser) are a good option but keep in mind they are subject to dirt, oil, mist, vapor, and unintended beam breaking caused by people, tools, equipment, or materials. This can cause unexpected and possibly dangerous machine reactions if not anticipated by the programming.</p> <p>Similar to photoelectric sensors but more robust physically and more hardened against dirty environments are the analog ultrasonic position sensors. They are still subject to unintended beam breaking and successful application can be a bit trickier. Usually an ultrasonic is selected when a photoelectric solution will not work.</p> <p>The position sensors discussed above may all work fine within their measuring performance envelopes, but for more precision position sensing jobs they may lack sufficient resolution (smallest detectable change) and accuracy (deviation from ideal measurement). This is where magnetostrictive linear position sensors come into the picture. These devices are available for internal installation in a hydraulic cylinder, or as externally mounted versions for attaching directly to the machine. Sensor lengths are dependent on the needed measuring range and are typically available between a couple of inches up to about 300 inches (more than 7 meters). The position resolution can be in the range of ± 1 μm with typical accuracy to ± 30 μm. This type of sensor is inherently an absolute positioning system, so that the sensor reports its position upon power-up with no homing. It is non-contact and wear-free, so service life expectancy is very long. This is contrasted with linear potentiometric or linear "pots", which have a sliding contact element that is subject to wear and deterioration, particularly in dirty and/or high-vibration environments. In particular, a linear pot can burn through its rated cycles in a stationary position due to machine vibration, resulting in a bad position reading at that spot. Linear pots can also become rather expensive in longer lengths relative to equivalent-stroke magnetostrictive sensors.</p> <p>Magnetostrictive linear position sensors use a very strong permanent magnet as a position marker, and are largely unaffected by all but the strongest magnetic fields. The magnet can be "captive" which means it slides on a low-friction rail and is moved by an operating rod attached to the moving member of the machine, or the magnet can be "floating" above the sensor on a bracket that moves with the machine. Automatic gain control compensates for variation in the magnet-to-sensor distance. Depending on the mechanical layout and restrictions in your situation, one or the other set up would become fairly obvious. Magnetostrictive linear position sensors have been around for a few decades and have been developed to a high degree of reliability and application flexibility. Shock tolerance is in the range of 150g and continuous vibration to 20g. Just about every conceivable electrical interface and housing variation is available.</p> <p>If the machine "ways" or guide rails are fairly stable and precise, an emerging option is the magnetic tape linear encoder. This consists of a precision-coded magnetic tape that is glued to the machine or installed in an aluminum track. A sensor head then floats over the tape or rides a sliding sled back and forth. The biggest caveat is the sensor-to-tape gap. Generally: the more precise the system, the tighter the gap tolerance. Some gaps are in the range of 0.3 mm, 2.0 mm, up to 6.0 mm or more. If possible, install the system so the sensor is stationary (no cable flex) and the tape moves with the machine. Otherwise, the flexing sensor cable must be managed (not impossible but easier to avoid it if you can). The position resolution for magnetic linear encoders can be in the range of 1 μm with typical accuracy to ± 20 μm for precision systems, and resolution of 10 μm with ± 0.4 mm accuracy for "big gap" systems. Magnetic linear encoders are generally incremental quadrature interface but there are absolute digital versions recently appearing in the market.</p>

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