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Posted on 10 March 2019

Angle Sensor Devices in On-Axis and Off-Axis Applications

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This article provides a basic under- standing of how Allegro’s precision Circular Vertical Hall based angle sensor integrated circuits are used in on-axis and off-axis applications.

Allegro MicroSystems, LLC

Hall based angle position sensor ICs are increasingly preferred over other Hall and non-Hall based sensor solutions for position monitoring applications. They are particularly sought after in automotive and industrial markets. Specific automotive applications for precision angle sensor ICs include, but are not limited to, wheel and motor position in electronic power steering and braking systems, transmission systems (PRNDL, clutch, inhibitor switch), windshield wipers, turbo charger exhaust gate valves, accelerator and brake pedals, and fuel tank level sensing. Similarly, in the industrial market, angle sensor ICs are sought after for motor control, valve, lever and joystick applications.

There are a number of key factors on why the demand for Hall based angle position sensor ICs has grown so rapidly in recent years. First, the level of accuracy available with Hall based angle sensor ICs has improved dramatically in recent years. Today’s Hall based angle sensor ICs can now measure angles from 0º to 360º, with an accuracy of less than 1º, or <0.3%. Both linear position Hall sensor ICs and some non-Hall based sensor ICs cannot match this level of accuracy performance.

Second, the measurement of an angular position provides system level cost savings over other sensor technologies. Instead of having to measure a long stroke displacement using one or more traditional linear position Hall sensor ICs and a complicated mechanical assembly, many applications today can instead use a single angle sensor and cheaper mechanical assembly to accomplish the same task. And again, with an angle sensor they can do so with better accuracy. In many cases a simple puck magnet attached to the end of a rotating shaft and placed in close proximity to an angle sensor is all that is required to obtain accurate position information of a mechanical component in a system.

Lastly, Hall based angle sensor ICs can accurately measure magnets rotating at very high rates of speed.

In this application note, I describe and demonstrate the use of Circular Vertical Hall (CVH) technology based angle sensor ICs in several onaxis (end-of-shaft) and off-axis (side-shaft) applications.

Circular Vertical Hall Technology Overview

Unlike other angle sensor technologies that employ orthogonal Hall plates (with or without concentrators) or magnetoresistive elements to measure target magnetic field amplitudes, CVH technology utilizes a circular Hall element with a ring of electrical contacts to measure the target magnetic field and produce a single channel front-end output. The front-end output’s phase relationship, after being digitally processed, is then compared relative to an internal reference signal to produce an angle measurement.

The actual CVH ring structure is monolithically and inseparably integrated into the silicon along with the back-end digital signal processing functions and interface circuitry. There are no unique processes required to fabricate a CVH based sensor IC. It is constructed by implanting X vertical contact elements (e.g. 64) in a ring shaped N-channel well. Due to the vertical contact elements, CVH based angle sensor ICs measure planar fields emanating out from an interface magnet.

Other competitive angle sensor technologies produce two front-end outputs from two separate Hall element or magnetoresistive sensor element channels. These front end signals are then processed through a CORDIC algorithm to produce an angular output. With this type of approach, performance and accuracy issues arise when the two channels are mismatched either in channel offset or sensitivity. Additionally, if either of these channel output signals saturate, then the output signal will exhibit large errors. Saturation can occur when there is air gap variation between the target magnet and the angle sensor IC.

CVH based angle sensor ICs on the other hand are much less affected by these types of channel mismatch issues. Since a single front-end channel is produced by the CVH ring there are no mismatch concerns. And since the CVH front-end channel’s phase, instead of amplitude, is compared to an internal reference signal, field level saturation is not a concern. Moreover, since the CVH based angle sensor doesn’t measure magnetic field amplitudes it provides high levels of air gap independence.

Shown in Figure 1 is a representation of a target magnet placed over a CVH ring structure.

Target Magnet Field Placed Over a CVH Ring Structure

Again, it is important to note that the CVH ring is actually part of the angle sensor silicon die.

With the magnet stationary over the CVH ring, the angle sensor digitally switches between groups of CVH ring contact elements to effectively create an array of miniature Hall plates.

In total, 64Miniature Hall plates, in this example, are dynamically constructed and measured during one full electronic rotation of the CVH ring. The result is a coarse sine wave that constitutes output voltage from each of the contact elements in the CVH ring.

The coarse sine wave is then passed through a series of filters to produce a smooth sine wave. The smooth sine wave is then processed through a zero crossing comparator. The phase of the “zero crossing point” is proportional to the phase or vector direction of the external magnetic field sensed by the angle sensor IC. Its “zero crossing point” is then compared to an internal reference signal’s “zero crossing” point. The phase difference between the two zero crossing points represents the angle measurement of the target magnet.

Due to the fact that the digital switching between the 64 contact elements can be performed at very high speeds, refresh rates as fast as 25 μs can be achieved. As a result, CVH based angle sensor ICs can produce highly accurate angle measurements even on target magnets rotating at very high RPM rates. The only issue with increasing target magnetic rotational rates is an increasing lag factor relative to where the target magnet is actually located over the CVH ring, and when the angle last measured is transmitted out of the device. However, with a steady rotational velocity or a rotational speed calculation algorithm in the system microprocessor that interpolates between at least 2 angle data points from the angle sensor IC, this lag factor can easily be accounted for at the system timing level.

Use of CVH based Angle Sensor ICs in an On-Axis Application

On-axis angle sensing is the most common type of angular position measurement application. It involves measuring the displacement of a rotationally moving magnet that is commonly placed on the end of a shaft or underneath a gear (see Figure 2).

On-axis angle sensing

Compared to off-axis applications, on-axis angle measurements yield higher accuracy results and require less digital post processing such as harmonic linearization. However, the attachment of button magnets to a shaft (for example a motor shaft), typically used in onaxis applications, creates mechanical mounting and EPS Module cost challenges. Mounting a magnet on the end of a shaft often involves the use of non-ferromagnetic fixtures (i.e. brass) to hold the magnet in place. Despite the cost of the brass fixture, on-axis sensing is common in most motor position applications.

Figure 3 shows a typical on-axis Electronic Power Steering (EPS) application configuration.

Typical On-Axis EPS Application

In the EPS sensing module, along with two linear position sensor ICs for measuring handwheel torque, there resides a Hall based angle sensor IC for measuring steering wheel rotational position. The angle sensor IC is positioned underneath a puck magnet that is fastened into the underside of a spur gear. The top surface of the angle sensor IC and the puck magnet are separated by a small air gap with minimal tolerance variation. Typically the airgap is in the range of 1 mm to 3 mm. The smaller the air gap the stronger the magnetic field presented to the angle sensor IC, which results in better angle measurement and improved accuracy over a wider temperature range. As Figure 4 illustrates, the smaller the air gap, the stronger the field level presented to the surface of the angle sensor IC, and the more accurate the angle measurement.

Changes in Peak Angle Error Over Air Gap Variations Relative to a 1.76 mm Air Gap with 900 G Field

Use of CVH based Angle Sensor ICs in an Off-Axis Application

Off-axis angle sensing is another common type of angular position measurement application. It involves an angle sensor IC measuring the angular position of a magnetic field generated by a ring magnet that is attached around a shaft. As observed in Figure 5, the angle sensor IC is located adjacent to the shaft and ring magnet.

Location of Angle Sensor IC

As the shaft and ring magnet turn, the angle sensor IC measures the resulting angle position.

One major issue with off-axis angle sensing is that a wide variation in magnetic field orientation and strength is observed by the angle sensor IC as the magnet and shaft rotate. As a result, the angle sensor IC requires significantly more digital post processing of the raw angle measurement to achieve an accurate final angle measurement output.

The additional post processing adds cost and complexity to the angle sensor integrated circuit, and typically does not produce the same level of angle accuracy measurement as an on-axis angle measurement system. This said, mounting a ring magnet around a shaft is usually less complex and lower cost to produce than an on-axis angle measurement system.

A1332 Harmonic Linearization Results

As shown in Figure 6, the raw angle measurement (described in the figure as prelinearization or Pre Lin) observed by Allegro MicroSystems’ A1332 angle sensor positioned in an off-axis orientation, is non-linear and does not meet the required accuracy of many target applications. In this particular application the raw angle error measured is ≈ +10º to -14º. Post processing, however, of the raw angle measurement with such techniques as harmonic linearization can dramatically reduce the angle error output from the angle sensor IC.

A1332 Harmonic Linearization Results

As observed in Figure 7, the raw input angle error measurement, similar to what is shown in Figure 6 and indicated by the black line, is dramatically reduced after performing harmonic linearization on it. (see Post Lin curves).

The raw angle error observed by the magnetic sensor IC in an off-axis application can be reduced by increasing the strength and uniformity of the ring magnet. The field strength that the sensor IC sees can be further improved by positioning the angle sensor IC closer to the ring magnet, however mechanical tolerances of the system (e.g. run-out of the rotating shaft) can limit how close the angle sensor IC can be positioned next to the ring magnet.

Arc Magnets in Off-Axis Applications

Arc magnets can also be used in off-axis angle measurements, as shown in Figure 8 below, for “Short Stroke” applications.

Arc Magnet in Off-Axis Application

In short stroke applications the shaft does not rotate a full 360º. As depicted in Figure 8, the arc magnet is located on only one half of a circular plate that is attached to a rotating shaft. Since the application doesn’t require a full rotation of the shaft, the cost of the magnet and the overall sensor system can be reduced.

Short stroke off-axis angular sensor systems, however, usually require the same level of additional post signal processing as full stroke/ring magnet off-axis applications.

Conclusion

Hall based sensor integrated circuits have proven to be preferred sensing solutions for harsh automotive and industrial environments. They are rugged, highly reliable, contactless, and can effectively “see through” non-ferrous materials. As such, they are insensitive to dust, dirt and humidity often found in harsh environments.

Allegro MicroSystems’ CVH based angle sensor ICs offer all of the same benefits of their Hall based predecessor devices, but with improved accuracy and speed, and support for a wider array of harsh and demanding sensing applications. They also offer unmatched precision and repeatability, and are more immune to air gap variations that can cause magnetic field saturation in other Hall based ICs. Moreover, with integrated features, such as linearization, extensive on-chip diagnostics, and redundancy package options, Allegro’s MicroSystems’ angle sensor ICs are ideal for automotive and industrial safety critical applications.

Although CVH based angle sensor ICs are designed as standard off-the-shelf devices, magnetic sensor subassemblies and systems are complex and require careful magnetic and mechanical system design. This application note only touches upon the various factors that need to be considered when designing a magnetic based sensor system.

For more information please contact: Michael Doogue, Business Unit Director, Advanced Sensor Technologies: Wmdoogue@allegromicro.com

 

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