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Position sensing in vehicles: how new magnetic encoder technology helps ensure accurate, fault-free sensor outputs

At the same time, the standards for electromagnetic compatibility (EMC) in vehicles are becoming more stringent, and the development of a new generation of low-carbon diesel and gasoline engines requires better sensor performance with higher resolution and low-power operation. For instance, in steering applications automotive designers are looking for components that draw ultra-low stand-by current. There is even a move towards 'measurement on demand' to reduce power consumption in control electronics.

Sensors also need to adapt to new digital interfaces such as SENT and PSI5, which will replace traditional analogue outputs.

All of these factors in combination are accelerating the elimination of traditional contacting sensor solutions, and their replacement by contactless sensors.

austriamicrosystems has pioneered the development of integrated Hall encoder solutions, with its AS5xxx series optimised for automotive applications. This article describes how robust performance in challenging automotive conditions is facilitated by the use of a patented differential sensor technology.

Immunity to stray magnetic fields

Fig. : contactless magnetic encoder

Figure 1 shows a magnetic sensor, AS5163, with a simple diametrical (two-pole) magnetised magnet. The sensor IC measures the vertical field components, Bz, and calculates an absolute angle value.

The differential principle implemented by the AS5xxx devices is designed to eliminate the effect of environmental factors such as stray magnetic fields, vibration and misalignment.

Fig. : differential principle involves the use of four Hall elements. For full resolution, click here.

This differential principle entails the use of four Hall elements inside the IC – traditionally, magnetic position sensors use only Hall element. These four elements, equally spaced underneath a diametrically magnetised rotating magnet (see Figure 2), generate four sinusoidal waveforms, each phase shifted by 90° from its neighbour. This can be expressed in the formula:

 H1 = â * sin(α)                          

H2 = â * sin(α+90°) = â * cos(α)           [1]

H3 = â * sin(α+180°) = â * -sin(α)

H4 = â * sin(α+270°) = â * -cos(α)

where

â...peak amplitude

α...magnet rotational angle relative to the sensor

By using differential amplification of the opposing sets of sensors: 

H1 – H3 and

H2 – H4 =

â * sin(α) – [-â * sin(α)] and                  [2]

â * cos(α) – [-â * cos(α)]

two 90° phase-shifted signals with double amplitude are generated:

2* â * sin(α) and

2* â * cos(α)

These two analogue signals are then digitised by Analogue-to-Digital Converters (ADCs) before being processed further in the digital domain. A CORDIC (co-ordinate rotation digital computer) inside the AS5xxx device transforms the sine and cosine data into angle and magnitude information, using the following formulae:

 

where

A= measured angle

α = magnet rotational angle relative to the sensor

â = peak amplitude

Through dividing

2*â*sin(α) by 2*â*cos(α),

the term 2*â cancels and the remaining expression is

This reveals the major advantage of the differential principle: it is independent of signal amplitude, and consequently independent of temperature variations, as well as of variations due to the size of the gap between the sensor IC and the magnet. It also cancels out magnetic offsets caused by stray magnetic fields.

Fig. : cross-section through the magnet and sensor. For full resolution, click here.

Figure 3 shows a cross-section of a typical sensor-magnet implementation. If the distribution of the vertical magnetic field (to which the sensor is sensitive) is plotted along the diameter of the magnet, the maxima appear at the poles near the edges of the magnet. The strength of the magnetic field at the neutral zone in the centre of the magnet is zero; magnetic field strength increases in a relatively linear fashion as the radius increases.

As long as all the Hall elements are within this linear range, the differential signals [2] will remain unchanged, independent of the horizontal position of the magnet. Temperature compensation, required for linear Hall sensors, is not necessary in the AS5xxx. The chips automatically correct for changing magnetic field strengths over the full operating range of -40°C to +150°C.

It is interesting to compare magnetic encoders that use the differential principle to other Hall encoder solutions that use ferro-magnetic field concentrators. In the case of the AS5xxx, compression measurements show no change of angle output in the presence of homogeneous stray fields up to 20.000 A/m (25 mT). Static and dynamic stray fields were applied to the sensor system (see Figure 4). The position information (X,Y,Z) produced by the sensor was unaffected; the sensor is robust by design.

Fig. : AS5163 Sensor and magnet in a Helmholtz coil to analyze the impact of magnetic stray fields

Hall encoder solutions using ferro-magnetic field concentrators are very sensitive to static and dynamic stray fields. Several degrees of angle error can be observed at low magnetic field strength amplitudes of 500 – 1000 A/m. To avoid these effects, a sensor with field concentrator needs expensive magnetic shielding materials close to the sensor.

Fig. : comparison of AS5163 outputs with outputs from a sensor using a field concentrator, at 20Hz 1000 A/m. Upper ray: AS5163 with differential principle; lower ray: Encoder Solution with field concentrator.

Figure 5 shows the impact of a dynamic magnetic stray field (20 Hz) 1000 A/m (1.26 mT) on the sensors. The main magnet was located in the Helmholtz coil as well and placed at a certain position. The sensor was programmed for 360 degree, full-turn mode with an output range of 0.5V – 4.5V.

The oscillation represents the 20Hz stray field disturber. The angle error is around 2-3° in this case.

The differential principle works at any angle. The impact on sensors using a field concentrator, however, is strongly affected by the magnet's position. An analysis of sensor outputs over 10 different angles is shown in Figure 6.

Fig. : accuracy of angle output in the presence of stray magnetic field. For full resolution, please click here.

Robust protection and diagnostic functions

Magnetic disturbance is not the only environmental factor to affect position sensor outputs; electric disturbances are also found. Overvoltage pulses or micro-interruptions on the supply can occur at any time. The AS516x is able to overcome these without any damage and restarts without a reset as soon as the disturbance has disappeared. In addition, the AS516x is able to survive overvoltage pulses up to +27V. Damage resulting from faulty connection is also prevented, because the device offers -18V reverse-polarity protection.

To avoid permanent damage from a short circuit on the output, the AS5163 has an intelligent detection circuit. On detecting a short circuit at the output, the device switches off the output and restarts again after a delay. Periodically the device tries to switch on the output at reduced power.

In addition, a permanent short circuit at high ambient temperature cannot damage the device.

A watchdog function and internal and external diagnostics make the sensor solution suitable for safety-critical applications such as pedals.

In case of an internal failure of the die, the output is forced into the failure band. An overvoltage event forces the output to switch off and the load condition pulls the output into the failure band. Protection against external faults such as broken VDD and GND is also provided by the use of a failure band, and the AS516x family benefits from 4kV Electro-Static Discharge (ESD) protection.

austriamicrosystems also supplies a fully redundant sensor with 2 independent outputs, for safety-critical applications.

Robust communication from the sensor

Many traditional sensors deliver their output as an analogue signal. The vehicle's Electronic Control Unit (ECU) must then digitise it with an ADC before it can use it. This has many disadvantages. For instance, electro-magnetic events and other sources of noise can corrupt the signal. This requires the use of analogue filters in the ECU to compensate. Error handling is also difficult and expensive. To achieve small failure bands in the range of 1% is difficult, and it is challenging to handle the leakage effects caused by broken VDD or GND over all operating conditions.

Digital single-wire communication eliminates these problems. New automotive digital interfaces, such as SENT J2716, are becoming more widely supported, and the new AS5165 is compliant with the SENT J2716 standard. The PSI5 interface is another option. This interface is also robust and has many valuable features. Bi-directional communication is handled by current and voltage modulation on the supply line. The sensor has only two connections, which helps to reduce costs.

Conclusion

Magnetic Hall encoders are available from austriamicrosystems as rotary and linear variants: they are equally robust, and offer highly accurate and precise position information under the harsh environmental conditions experienced by vehicles. As austriamicrosystems continues to invest in improvements to its range of magnetic encoders, design engineers can expect to see a continued commitment to the production of robust devices that operate successfully in safety-critical applications.

Marcel Urban joined austriamicrosystems in 2000 as a design engineer working encoder products. Since 2008, he has been a product manager responsible for automotive magnetic encoders.

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