Do nearby metal objects affect Zettlex sensors?

The IncOder™ and LinTran™ product ranges are unaffected by nearby metal objects.

Nearby metal objects need only be considered for OEM or unpackaged sensors – in other words, those that will be located in a host mechanical structure. You will see on some of our data sheets for OEM or unpackaged sensors a metal ‘keep out’ zone.

Metal objects – or more specifically conductive objects – will only influence Zettlex sensors if they are in close proximity to the sensor’s windings. These windings are on the main faces of the sensor’s antenna (stator) or target (rotor). The magnetic properties of such materials has no influence – the dominant effect relates to their conductivity.

Conductive materials may influence the sensor’s field when they are in close proximity to the sensor’s windings because they may provide a flux path for the sensor’s field. This may affect the sensor’s measurement performance.

To most intents and purposes when we talk of metal objects we are also talking about conductive objects. In other words, objects made from aluminium, steel, stainless steel, brass, copper, cast iron etc. The conductivity of materials such as polymer, glass, water, potting compound or ceramic has no effect.

Metal objects around the periphery or external edges of a Zettlex sensor’s windings have little or no effect. Accordingly, metal shafts through the centre of a rotary sensor or a metal housing around the periphery of a rotary, linear or curvi-linear sensor have little or no influence.

The main point for consideration is metal objects close behind the sensor’s antenna (stator) or rotor (target).

For a rotary sensor whose windings have an inner radius r and outer radius R, metal objects need only be considered if they are closer than (R-r)/2 to the rear faces of the stator or rotor. These are typically the dimensions of the metal keep out zone shown on some Zettlex data sheets. Metal objects can encroach within this area either if they are lower conductivity (e.g. stainless steel); have a small cross-sectional area relative to the area of the windings (e.g. a small diameter dowel or pin) or are of constant planar aspect (e.g. a planar sheet of steel or copper). In such cases it is usually permissible for the metal objects to approach within (R-r)/4 of the rear faces of the stator or rotor windings without affecting measurement performance.

For a linear or curvi-linear sensor whose windings have length L (along the measurement axis) and width t (across the measurement axis), metal objects need only be considered if they are closer than t/2 to the rear faces of the antenna or target. These are typically the dimensions of the metal keep out zone shown on some of our data sheets. Metal objects can encroach within this area either if they are lower conductivity (e.g. stainless steel); have a small cross-sectional area relative to the area of the windings (e.g. a small diameter dowel or pin) or are of constant planar aspect (e.g. a planar sheet of copper or steel). In such cases it is usually permissible for the objects to approach within t/4 of the rear faces of the antenna or target windings.

Metal objects outside the dimensions stated above have no influence on sensor performance.

In some instances it is not feasible for metal objects to sit outside such exclusion zones. In such instances, please contact Zettlex – we may be able to modify a sensor design so that it will cope with very close metal objects. This has successfully been accomplished on numerous occasions previously when space has been of paramount importance.

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