Magnetic rotary sensors offer more benefits

Variohm Eurosensor, on behalf of Contelec, compares the complementary strengths and weaknesses of optical encoders and magnetic rotary sensors.

Established in 1960, Contelec in Biel, Switzerland, is part of the German Siedle Group, and employs more than 100 people.

Contelec specialises in high-quality sensors for angular and linear measurement.

Products range from low-cost, high-quality standard components to individual customised sensors, 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.

Around 80% of the company's products are exported.

The Contelec Engineering Division, provides services for engineering and production of complete sub-assemblies.

Contelec has been ISO 9001 certified since 1995 and invests heavily in research and development.

* Magnetic rotary sensors - 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, said Contelec.

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 - optical encoders include a rotating code disc and an optical ASIC.

Light emitted from an LED.

The light shines through a high precision code disc, normally manufactured from glass or plastics.

It produces 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, said Contelec.

The measurement disc tends to oscillate in the presence of heavy vibration, producing unstable output signals.

In extreme situations, the code disc can even break, resulting in the total failure of the measurement system.

* Contamination - 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.

Contelec does not recommend this type of technology 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.

* Magnetic absolute rotary sensors - magnetic absolute rotary sensors offer an attractive alternative.

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 deg C to +125 deg C.

* High degree of resolution and precision - magnetic absolute rotary sensors offer a high degree of resolution and precision even with extremely small device dimensions.

Systems are now available with a resolution of up to 14 bits and accuracy of up to 12 bits in housings no bigger than 13mm 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 deg.

Full resolution and precision are maintained even for angles less than 360 deg .

The implication is that, for example, a sensor programmed to a 180 deg 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 - when compared 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 that must be considered is that of external magnetic influence, magnetic fields of >1000A/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.

As with the Contelec Vert-X family of rotary sensors, it is not the field strength but the field direction that is measured, so this aspect is 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 subassemblies are available which can be integrated into customer applications.

These are much more susceptible to faults, since the glass, plastics or metal discs have to be installed by the customer with a high level of precision.

Contamination and other media can interfere with proper operation.

* Simplified installation - 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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