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Emerald Group Publishing Limited
Copyright © 2010, Emerald Group Publishing Limited
New applications opened up by non-contact angle sensors
Article Type: Mini features From: Sensor Review, Volume 30, Issue 1
Whether for axial offsets, excessive torque or the necessity for transmissive measurements, conventional angle sensors – despite their refined state of development – are not suitable for all applications. It is therefore hardly surprising that non-contact rotary sensors with offset magnetic transducers represent a virtually perfect alternative in a large number of fields, with impressive benefits.
Reliable linkage of an angle sensor to the self-rotating element is – one would have thought – an established and unproblematic issue. That is not always the case, as is clearly illustrated in applications such as those where there are axial offsets between the customer’s rotating component and the sensor axis, attributable to design features and manufacturing tolerances. High resolution, precision rotary sensors tolerate no torsion of the linkage, and when this is in evidence, it has a negative influence on the measured results, since a twist of the axis generates erroneous angle data. Despite this fact, there are still a host of so-called “over-engineered designs” that can be encountered in practical applications, in which there is rigid linkage between the sensor axis and the rotating element supplied by the customer. Systems of this kind can lead to a number of problems, including increasing mechanical wear of the sensor bearings, resulting from transverse forces, and even possible fracturing of the sensor axis.
New degrees of freedom
A remedy for these situations is offered by state-of-the-art non-contact angle sensors, in which there is no direct contact between the rotating magnet and the actual measurement system. On non-contact rotary sensors, the customer normally secures the permanent magnet to the rotating object (shaft), so that the axis and the measurement system are completely separated. As a consequence, there are no axially or radially acting forces, which could cause increased wear and adversely affect the life of the sensor. The working distance between the magnet and measurement system and the permissible installation tolerance in Z can be optimized by choosing a suitable magnet. Depending on the size of the magnet, a range of axial offset distances is possible in the XY direction, while maintaining constant linearity. The actual configuration represents a compromise between the magnet size and the possible axial offsets. Axial offsets in XY and Z, however, do not in any way change the reproducibility.
Non-contact rotary sensors are based on a magnetic, contactless sensor principle. They are accordingly not only free of wear and extremely robust in withstanding external influences, but also demonstrate impressive long life, precision and resolution. They are available both as standard components and as customer-specific designs and, depending on the version, have tolerances of several millimetres in terms of axial offsets. They also boast convincing performance features, such as maximum 14-bit resolution and 10-bit precision at a linearity of 0.3 percent.
Transmissive and torque-free
Non-contact rotary sensors are not just suitable for applications with axial offsets (XYZ tolerances). They are also ideal for other tasks that are very difficult – or even completely impossible – to implement with conventional rotary sensors. These include transmissive measurements, in which it is necessary to measure the angle through liquid or solid materials.
A further area can be found in applications in which minimum torque is essential – for example in devices for measuring wind direction. Hitherto, solutions of this kind have required extremely expensive sensors and bearing systems to achieve torque levels as low as 0.002-0.003 N cm. This is an application that is virtually made for non-contact sensors, since the mechanical separation of the sensor and permanent magnet ensures that there is no torque (no rotary axis bearings – and consequently no friction). The new sensors even allow affordable implementation of redundant systems without essentially increasing torque. With existing rotary sensors with jewel bearings, it has not been possible to improve on torque levels of 0.004-0.006 N cm. Another deficiency of conventional low-torque systems is their inadequate seal tightness for a large number of applications. In many cases, the maximum seal tightness class is IP60, since at any higher values, the O-ring would effect an excessive increase in the torque. Non-contact rotary sensors, by contrast, allow the production of extremely seal-tight sensors, because the only part of the sensor that needs sealing is the one on which there are no moving components.
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 N cm 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 non-contact 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-optimized 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 to 37 mm, resolutions of up to 14 bit, IP69K-specified versions and versions for the temperature range between −40°C and +125°C 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 0° to 360°. A remarkable feature here is that the full resolution and precision are available even at angles less than 360°. Some solutions even allow individual configuration of the direction of rotation and the index point.