Optical encoders and magnetic rotary sensors

As the exclusive UK distributor for Contelec, Variohm Eurosensor presents the following technical note on optical encoders and magnetic rotary sensors.

A common feature of contactless sensors for angle measurement is that the rotating shaft is mounted on bearings in the sensor body and, consequently, is subject to a certain degree of wear.

Non-contact rotary sensors completely eliminate this weakness.

Although the contactless rotary sensors are based on a non-contact measurement method, they are not the same as non-contact sensors.

On contactless sensors, there is no direct contact between the magnets located on the rotating axis and the actual measurement system, for which reason this can be referred to as a contactless, non-wearing measurement method.

This is in contrast to potentiometers, the wipers of which are in physical contact with the conductive coating (for example conductive plastic) and, at best, can be termed low-wear.

Yet even contactless rotary sensors are not totally immune to mechanical wear effects.

This is due to the fact that the rotating shaft is mounted on bearings in the sensor body and, particularly in the case of poor concentricity or when subject to axial forces, is prone to a certain degree of mechanical wear.

A remedy to this problem is offered by rotary sensors termed non-contact, in which the permanent magnets are attached by the customer to the rotating object - usually a shaft.

This eliminates any direct mechanical linkage between the axis and the measurement system, so that axially and radially acting forces cause no wear and the life of the sensor is not impaired.

Because of their architecture, noncontact rotary sensors are used in cases where the out-of roundness of the rotating object exceeds a specific tolerance range or if physical contact between the rotary sensor and the object is not possible.

An example of this is in applications in which rotating components are screened by a protective body, so that there is no direct contact with the sensor unit.

The transmissive measurement in the non-contact rotary sensors elegantly bridges the spacing.

Irrespective of whether contactless or non-contact angle sensors are used, a key issue in the evaluation of the suitable technology is the ageing of the permanent magnets and the resulting signal deviation.

This is a factor that needs to be taken into account with simple Hall systems.

However, it is quite easy to eliminate this shortcoming.

If, instead of measuring the field strength, it is the direction of the magnetic field that is measured, the measurement method is based on a stable value.

In measurement systems of this kind, the field direction of the permanent magnet is converted by the sensor into angle information and output via analogue or digital interfaces.

This method ensures that measurement precision remains at a high level, totally unsusceptible to the ageing-related reduction of the magnetic field strength or a changing magnetic field strength as a result of the temperature coefficients of the magnet.

Non-contact rotary sensors are ideally suited for implementing tailor-made customer solutions, which can differ in aspects such as the body form, magnetic transducer, distance between the permanent magnet and the measurement system as well as the permitted tolerances.

The Vert-X exMag family of rotary sensors are suitable for applications in ultra-confined spaces and heavy-duty applications.

Standard components include body diameters from 13 to 37mm, resolution of up to 14-bit, fully moulded-on bodies, IP69K-specified versions or solutions for the temperature range between -40C and +125C.

A range of connection technologies and output signals significantly adds to the user-defined individualisation possibilities.

Available options include analogue output - with various voltage and current values - and digital interfaces such as PWM, SPI or SSI.

Physical connections can be by stranded wire, straight cable or pin-connector packages.

Other advantages of magnetic rotary sensors include the possibility of free programming of the rotary angle from 0 to 360.

A remarkable feature here is that the full resolution and precision is available even at angles of less than 360.

This means that a sensor programmed to a 180 degree electrical angle offers the full 14-bit resolution, thereby corresponding to a 15-bit optical 360-encoder.

Even the direction of rotation and the index point can be individually configured, allowing customer parameters to be programmed in the calibration of the sensor during manufacture.

An additional system-related advantage of the magnetic systems is the possibility of integrating two complete measurement systems into a single rotary sensor, thereby implementing completely redundant solutions.

These are available both as contactless and non-contact rotary sensors, further strengthening the position of systems for non-contact angle measurement.

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