Variohm non-contact angle sensors open new applications
27 October 2009
Conventional angle sensors are not suitable for all applications despite their refined development, but Variohms' non-contact rotary sensors with offset magnetic transducers represent a virtually perfect alternative.
In contrast to conventional low torque angle sensors, which require an intricate manufacturing process and expensive materials, another attractive feature of non-contact rotary sensors is their price. The simpler makeup leads to a significant reduction of production costs – and consequently to considerably lower prices.Countering interference fields
In applications requiring a low torque level of the angle sensor, the earth’s magnetic field comes into play. This acts on the permanent magnet, trying to align this in parallel with the north-south magnetic poles (identical to the principle of a compass, in which the compass needle aligns to the earth’s magnetic field), and by doing so imposes a torque on the magnet. However, in the case of small permanent magnets with simple dipole magnetisation, this torque is in the order of magnitude of 0.0002 Ncm and, as a consequence, is generally non-critical. A considerably greater influence can be exerted by static or dynamic magnetic fields generated by external sources.
Motors, live coils or permanent magnets positioned nearby can interfere with sensors working on the magnetic measurement principle. It is therefore important to screen the sensors from magnetic interference fields. The screening is performed by surrounding the magnet and sensor with a material that conducts magnetic flux. To ensure that the performance of the sensor is not impaired, this material has to have special properties. The use of a simple magnetic steel sheet is frequently insufficient for this purpose.
The precision of the sensor can also be influenced by electrical interference fields. These can be screened by an electrically conductive body, though this does not eliminate line-linked interference that penetrates into the sensor through the cables. Protection against interference of this kind is primarily provided by suitable electrical circuit elements. Appropriate EMC tests must be carried out to ensure that the sensors are suitable for the required conditions.
Overcoming existing limits
Some benefits that make noncontact and contactless rotary sensors so valuable are shared by optical encoders. However, in the latter case, these advantages are frequently eliminated by major disadvantages, such as size, price, possible fogging and breakage of the code disk. By contrast, the latest generation angle sensors achieve specifications that would be inconceivable with optical systems. These include miniature sensors with weight-optimised magnets and magnet carriers with impressively low external diameters of just 13 mm, resolutions of up to 14 bit and precision of up to 10 bit. Solutions are also available that are particularly suited to heavy-duty applications, which are unsusceptible to intense vibrations and impacts. A key feature here is that, in addition to standard products, custom solutions with individualised performance features and designs are obtainable. They differ in aspects such as body shape, magnetic transducer, distance between the permanent magnet and the measurement system and permitted tolerances. Solutions with body diameters from 13 mm to 37 mm, resolutions of up to 14 bit, IP69K-specified versions and versions for the temperature range between -40ºC and +125ºC are available as both standard components and custom designs.
In addition, a range of different connection technologies and output signals make an important contribution to application orientated individualisation. Available options include analogue outputs, with various voltage and current levels, and digital interfaces, such as PWM, SPI or SSI. Connection 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 0º to 360º. A remarkable feature here is that the full resolution and precision are available even at angles less than 360º. Some solutions even allow individual configuration of the direction of rotation and the index point.
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