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If you can’t stand the heat, get out of the kitchen. This applies to encoders as well. In many applications encoders are exposed to extreme vibrations and shocks. Contamination by dust or exposure to liquids also happens frequently. On the other hand, machine designers are always concerned with optimizing the space requirements of their equipment. Therefore, the installation size of the components used is of increasing importance and compact encoders with versatile connection options are demanded.
Moreover, an increasing number of applications require an absolute output signal. The actual absolute angular position is available at any time immediately after switching on the encoder. In this case, reference travel as found with incremental encoders is no longer required.
Optical encoders - sometimes too accurate
Optical encoders are superior to magnetic encoders with respect to resolution and accuracy, but their use is limited. Optical encoders contain a glass or plastic pulse disk. Under vibration conditions, this disk can oscillate and be scratched. Hard shocks can result in disk breakage and total encoder failure. Moreover, the proper function of optical encoders depends on high cleanliness inside the housing. Condensation on the optics results in immediate measuring errors or encoder failure. This might occur due to moisture or temperature fluctuations, for example in cold storage facilities or outdoors. Dust or liquid intrusions also have significantly negative effects on encoder
function and will sooner or later result in total encoder failure. In general, the protection class for optical encoders is limited since hermetic encapsulation is technically impossible due to the bearing based encoder design.
High angular resolution can be obtained only with large diameter pulse disks, leading to larger diameters of encoder housings. The production for optical encoders is elaborate and requires a large number of components.
Most manufacturers rely on standard position measurement components with resolutions in the area of 8’192 positions per revolution and 65’536 revolutions. Due to the high encoder repeatability and the limited number of revolutions, spindle with large slopes are applied in most applications. As an example, a spindle slope of 5mm per revolution and a typical optical encoder repeatability of 0.05° results in a theoretical position accuracy of 0,0006mm. This position accuracy is generally over-specified and can usually not be achieved in the equipment because of mechanical tolerances. However, it must be paid for.
Magnetic encoders - robust and adequately precise
The Magres encoder family detects the singleturn position magnetically rather than optically. At a repeatability of 0.3° and a spindle slope of 5mm per revolution, a theoretical position accuracy of 0.004mm is achieved which can be further increased by reducing the spindle slope. By using an innovative electronic gear, they are able to provide contactless and wear-free multi-turn sensing with a minimum number of components and a virtually unlimited number of revolutions. No moving gears or optical components are required, just a microcontroller, a high-performance lithium cell and magnetic sensing elements. They are needed to count and maintain the multi-turn value in power-off state.
Durability, reliability and maximum lifetime are ensured by a minimum number of components while eliminating oscillation sensitive components which might be easily broken. The encoders are completely wear-free and unaffected by dust and dirt. Even demanding media can’t harm them. In addition, bearingless kit versions provide protection against all types of contamination thanks to their hermetically sealed design. This efficiently avoids machine downtimes and repairs. And for Magres encoders, the combination of high resolution and small size is much easier to achieve than for optical encoders since any dependence on
the pulse disk size is eliminated.
Success for magnetic encoders
Wherever optical encoders reach their mechanical limits, magnetic encoders come into play. A Magres success story comes from a well-known textile firm: This firm had used conventional optical encoders in a 200 unit machine park for manufacturing of denim textiles. The air in the manufacturing plant is permanently filled with fine textile dust. The atmosphere is also very hot and – because of the sturdiness of the material – there is a high level of machine and encoder vibration. The average
encoder lifetime in this twenty-four-seven application was only a few months and regular failures lead to simultaneous downtimes of several machines, in turn resulting in serious production gaps. One individual spent all his working time simply on repairs. Since all encoders were replaced by Magres encoders there has not been a single failure for years.
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