Motion System Design

Vertical Hall sensing

The Hall effect is the production of a potential difference (namely, Hall voltage) across an electrical conductor in which an electric current flows in the presence of a magnetic field. One newer sensor choice called a vertical Hall sensor puts this phenomenon to good work by using magnetic Hall-effect elements that track a position marker (a permanent magnet) attached to a rotating shaft to be monitored. The direction of potential Hall difference is perpendicular to both the magnetic field and current; magnetic-field orientation is collected by the sensor's integrated circuit, which then outputs calculated shaft angle in analog form.

Because the rotating shaft (with its position marker) is separate from the sensor, vertical Hall systems are suitable for applications in which torque must be kept to a minimum — wind-direction trackers and dancer arms in wiring, textiles, and printing, for example. In the past, expensive sensors with special bearings to dampen torque to 0.002 or 0.003 Ncm were used. Now, vertical Hall-effect sensors are another option; they include no axial or radial forces that degrade accuracy or life, and no friction or mechanical parts to wear. In addition, the sensors are simpler to manufacture and less expensive than conventional low-torque angle sensors.

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Q & A

How do vertical Hall sensors differ from other Hall-effect varieties?

Vertical Hall sensors measure full 360° movement with a cross-shaped Hall element that produces both sinusoidal and cosinusoidal output signals.

By normalizing these two signals, a vertical Hall sensor's built-in microprocessor determines its position marker's magnetic-field orientation. In contrast, conventional Hall sensors use a semiconductor to measure the strength of a magnetic field perpendicular to the element itself: The position-marker rotation creates a sinusoidal output voltage on the element, to measure angles only to 180°. Vertical Hall sensors also eliminate aging of the position marker encountered in sensors with a single Hall element.

Reliable linkage of an angle sensor to a rotating element is not as simple as it seems, particularly where there are axial offsets between the rotating component and the sensor axis because of design features and manufacturing tolerances. High-resolution precision rotary sensors cannot tolerate linkage torsion: It degrades measurement results because axis twist generates erroneous angle data.

The working distance between magnet and measurement system and the permissible installation tolerance in the Z axis 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. Actual configurations represent a trade-off between magnet size and the possible axial offsets. Axial offsets in X, Y, and Z, however, do not change reproducibility within the limits of laws of physics.

Where are they used?

Vertical Hall-effect sensor-based systems can measure the position of valves sealed in nonmagnetic metal or plastic housings. (Here, the position marker is mounted on an internal shaft and the sensor is placed outside.) They also measure through pressurized housings without moving seals. Optional features such as analog output substantially reduce parts and cost. For example, 4 to 20 mA or 0 to 10-V outputs virtually eliminate the need for external signal conditioners to cover long wiring distances. The touchless sensors have no shaft, so drive shaft or bearing tolerance and play have no effect — eliminating the need for expensive mechanical coupling.

They are suitable in hazardous environments (sealable to IP69K) and exhibit no wear under vibration. Absolute linearity (to ±0.3%) and accuracy are maintained over 360° at any programmed electrical angle (with repeatability to ≤0.1°) so the sensors can be installed without any [error-prone] calibration and time-consuming adjustment.

With 14-bit resolution, they operate from -40 to 125° C; nonlinear, customized output curves are also possible. Software linearization provides the benefits of calibration and customization, and the ability to create custom output curves (or transfer functions) with different custom slopes, polarity, and deadbands — to simplify existing control applications and reduce implementation and maintenance.

Some of the sensors also include microchips with embedded programmable switches that can replace PLCs in providing signals to turn devices on or off at specific angles.

What if an application requires absolute position feedback over many turns?

One sensing design uses vertical Hall-effect sensors on spirals corresponding to each rotation. It leverages giant magnetoresistance in which thin films of magnetic and nonmagnetic layers (with the spiral's sensors) output absolute rotational position over each turn.

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