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A guide to choosing the right optical or magnetic encoder

Supplier: Transtech Electronic Controls
12 May, 2017

Rapid progress in magnetic encoder development brings up this question more and more often.

Magnetic encoders are extremely durable and guarantee maximum reliability and service life. In addition, magnetic encoders are superior to their competitors with respect to compactness and price/performance ratio. 

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

While optical encoders are superior to magnetic encoders with respect to resolution and accuracy, their use is limited in the above-described application areas. Optical encoders contain a glass or plastic pulse disk. Under vibration conditions, this disk can oscillate and therefore can be scratched. Hard shocks can result in disk breakage and total encoder failure. Moreover, the proper function of optical encoders depends on highest 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. This automatically leads 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 5 mm per revolution and a typical optical encoder repeatability of 0,05° results in a theoretical position accuracy of 0,0006 mm. 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.

MAGRES magnetic encoders – robust and adequately precise

The MAGRES encoder family from Baumer detects the single-turn position magnetically rather than optically. At a repeatability of 0,3° a nd a spindle slope of 5 mm per revolution, a theoretical position accuracy of 0,004 mm is achieved which can be further increased by reducing the spindle slope. An additional highlight of the MAGRES encoders is their patented, magnetic multi-turn sensing. By using an innovative electronic gear, Baumer is able to provide contactless and wear-free multiturn 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. MAGRES encoders convince with outstanding durability, reliability and maximum lifetime. This is ensured by a minimum number of components while eliminating oscillation sensitive components which might be easily broken. MAGRES encoders are completely wear-free and unaffected by dust and dirt. Even demanding media can’t harm them. In addition, bearingless kit versions provide practically perfect 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.

The MAGRES BMMH 30

The latest and by far the most compact MAGRES encoder is the BMMH 30. With a compact 30 mm housing diameter, the MAGRES BMMH 30 sets the standard among the smallest absolute multi-turn encoders. It is available in hollow shaft, shaft or kit versions and can be equipped with connector or cable output in axial or radial orientation. Bearingless kit versions attain a protection class up to IP 69K. The MAGRES BMMH 30 completes the Baumer portfolio of magnetic absolute encoders which now consists of three broad product families with 30, 42 and 58 mm housing diameters.

The MAGRES BMMH 30 is also convincing due to its excellent economy and – thanks to its outstanding price/performance ratio – even recommends itself as a replacement for incremental encoders. Due to its compact size, it requires significantly less space than conventional encoders. Typical application areas are in the fields of machine building, laboratory automation, semiconductor industry and medical technology.

A wide variety of mounting options and comprehensive accessories guarantee that the MAGRES BMMH 30 can be quickly and easily installed. A modular system ensures a maximum level of flexibility with a large number of immediately available versions. And if some requirements are still not met, customer-specific versions can be created in the shortest possible time.

Wherever optical encoders reach their mechanical limits, magnetic encoders come into play. An impressive 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.

To found out more about or range of  Magnetic Encoder suited to your specific applications please call or email for a direct reply.