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Variohm EuroSensor - optical encoders and magnetic rotary sensors


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Variohm EuroSensor
: 28 July, 2008  (Technical Article)
Complementary strengths and weaknesses of optical encoders and magnetic rotary sensors
Established in 1960, Contelec, with headquarters in Biel, Switzerland, is part of the German Siedle Group, and employs more than 100 people. Contelec specialises in the engineering and production of high-quality sensors for angular and linear measurement. The company’s offering ranges from low-cost, high-quality standard components to individual custom solutions, which can be supplied in small-run production batches. The range includes contact-free rotary position sensors (magnetic/Hall-effect) as well as potentiometers based on conductive plastic, wire or cermet technologies. With a strong international orientation, around 80% of the company’s products are exported.



A key part of the company is the Contelec Engineering Division, providing integral services for engineering and production of complete subassemblies. This makes Contelec a strategic solution partner for a wide range of customers, making a significant contribution to the successful development of new products. Contelec has been ISO 9001 certified since 1995 and has made its name in the top-end quality segment. The company invests heavily in research and development to ensure that the high innovation rhythm will be maintained into the future.



There are applications in which optical encoders are unchallenged. However, huge leaps in the development of magnetic rotary sensors have moved the boundaries a long way in favour of smaller, more robust and lower-cost angle sensors.



Whatever type of absolute rotary sensor – optical or magnetic – is employed, the essential characteristic of every contact-free, absolute measurement system is that the measured value is unique to only one angle position of the rotating axis. The absolute value (angle position) is indicated immediately the power is connected. Reference travel, such as is necessary on incremental systems is completely eliminated.



Precision measurements using optical techniques



As their name suggests, optical encoders use an optical measurement system. The Main elements of such a system are a rotating code disc and an optical ASIC. Light emitted from an LED shines through the high precision code disc – normally manufactured from glass or plastic – producing an optical, digital code pattern on the panel underneath the disc. This transmits information to the optical ASIC which, in turn, converts the angle information directly into electrical signals. In terms of resolution, precision and speed, absolute optical encoders are the unchallenged leaders. However, they do have disadvantages. The measurement disc tends to oscillate in the presence of heavy vibration, leading to unstable output signals. In extreme situations, the code disc can even break, resulting in the total failure of the measurement system.



Optical encoders are also susceptible to contamination by dust and dirt which results in a weakened or lost signal. They are also badly affected by low temperatures, temperature fluctuations and moisture. These factors lead to condensation forming on the code disc which, in turn, leads to measurement errors or even failures. This type of technology isn’t recommended for applications in really hostile environments. When the evaluation criteria are extended to take account of factors such as size and price – optical encoders face some further difficulties. For example, large code discs are necessary to achieve high levels of resolution, and this has a direct knock-on effect on the size of the angle sensor. On top of this, the complex procedures used in the production of optical measurement systems, combined with their comparatively high number of components, mean that they tend to be more expensive than alternative systems.



When ruggedness counts



Magnetic absolute rotary sensors offer an attractive alternative, their complimentary properties ensure that they are receiving wider acceptance. These sensors use a non-contacting, magnetic measurement process involving a magnet mounted on the rotating axis. The lines of the magnetic field impinge on a magnetic field sensor, which converts them to an analogue output signal (which is normally converted to a digital signal by a downstream A/D converter). Different sensors are used, depending on the measurement principle, e.g. magneto-resistive, giant magneto resistors and Hall sensors. Although magnetic rotary sensors are inferior to optical measurement systems in terms of resolution, speed and accuracy, they offer a viable alternative in a large number of applications, particularly due to the fact that they are considerably smaller, more rugged and less susceptible to environmental influences.



The compact rotary sensors are (virtually) unaffected by factors such as vibration, shock, dirt and condensation. They can also be used directly in temperatures from -40°C to +125°C.



Magnetic rotary sensors on the advance



Another important feature is that a high degree of resolution and precision is achievable even with extremely small dimensions. Systems are now available with an impressive resolution of up to 14 bits and accuracy of up to 12 bits in housings no bigger than 13 mm in diameter.

Other benefits of magnetic rotary sensors include the possibility of free parameterisation in terms of programming the rotary angle from zero to 360°. Full resolution and precision are maintained even for angles less than 360°.



This means, for example, that a sensor programmed to a 180° electrical angle has the full 14-bit resolution available – which corresponds to a 15-bit optical encoder. Even the angle of rotation and the index point can be individually configured. The parameters can be programmed at the time of calibration of the sensor during manufacture or by the customer during installation. The primary advantage of customer set-up is the ability to compensate for installation tolerance, reducing the specification levels and costs of parts provided by the customer. A further benefit of magnetic systems is the possibility of economically integrating two complete measurement systems in a single rotary sensor, creating fully redundant solutions.



Ageing of permanent magnets in magnetic sensors



In comparison with optical systems, magnetic rotary sensors have some disadvantages. These include their lower speed due to the time required for signal evaluation, in many cases, however, the operational speed of physical movement is limited, so that this factor is of little significance. A further point which must be considered is that of external magnetic influence, magnetic fields of >1000 A/m can cause inaccurate measurements. The effect of this can be considerably reduced by appropriate screening of the rotary sensor. An additional criterion in the evaluation of suitable technology is the ageing of the permanent magnets used in magnetic rotary sensors. On simple Hall systems, the resulting signal change over time influences the accuracy of the measurement.



If, however – as with the Contelec Vert-X family of rotary sensors – it is not the field strength but the field direction that is measured, this aspect is completely eliminated. The accuracy of measurement is not affected by the magnetic field and field strength changing over time or as a result of the magnet-temperature coefficient. Consequently, magnetic sensors are also suitable for applications in which a rugged, precision measurementsystem with a long life is required.

Greater versatility



Instead of simply being alternatives for existing solutions, innovative technologies tend to open up new functions and fields of application. This aspect is particularly the case with magnetic rotary sensors. Completely contact-free rotary sensors can be implemented with external magnets attached by the customer. Non-ferromagnetic material can even be interposed between the externally rotating magnet and the rotary sensor – further increasing the degree of versatility, for example by allowing the rotating magnet to be operated in environments with higher temperatures or pressures than completely encapsulated analytical electronic systems.

In the case of optical encoders, individual sub-assemblies are available which can be integrated into customer applications, but these are much more susceptible to faults, since the glass, plastic or metal discs have to be installed by the customer with a high level of precision, and contamination and other media can interfere with proper operation.



To make the installation of magnetic sensors as simple as possible, various types of magnet supports are available, with a range of attachment options. It must be noted, however, that the installation accuracy required by manufacturers is of paramount importance in achieving the specified measurement precision. This must therefore also be taken into account in evaluating the different systems. Vert-X rotary sensors are now available which permit installation tolerances of up to +/- 0.5 mm (0.5mm (xx, yy) and +/- 1mm (zz).

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