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How to Choose and Use Angle Sensors for Power Steering, Motors and Robotics

By Majeed Ahmad

Contributed By Digi-Key's North American Editors

As factories and vehicles become more automated, accurate and low-latency sensing of motor shaft speed and position is critical for process control, system reliability, and safety. To address these needs, designers require angular rotation sensors that are fast and precise, with the flexibility to address magnetic field variations and axial misalignment.

Complicating the issue for designers are ever-present cost and time pressures, as well as the nature of the operating environments for industrial and automotive applications, which can be challenging in terms of chemicals and oils, as well as temperatures and EMI. Other considerations include wear and tear and ever-changing configurations, which require a degree of flexibility within the sensing device.

This article describes the role of angle sensors and shows how position sensing features such as speed and low latency can be customized using specific combinations of magnetic input and sensor element. Sample sensor solutions from AKM Semiconductor, Infineon Technologies, and Monolithic Power Systems are then introduced, and their implementation discussed.

The role of angle sensors

Angle sensors are used to sense motor shaft position and speed variations for steering angle sensing for automobiles and high-precision control in robotic systems. They determine the absolute angular position of a diametrically magnetized cylinder on a rotating shaft by detecting the orientation of an applied magnetic field and measuring its sine and cosine components. As the shaft may be rotating at high speed, it’s critical that the data from the sensor be acquired and processed quickly, with minimal latency.

One of four magnetic technologies are typically used: Hall effect, anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) (Figure 1). When using any of these technologies, designers must first determine a suitable distance from the magnet surface to the sensor based on specific parameters such as magnetic properties, sensor specification, and assembly tolerances.

Diagram of Hall effect, AMR, GMR, TMR magnetic technologiesFigure 1: When a magnet is rotated on a TMR sensor, the resistance of the sensing element changes with the rotation angle. (Image source: Digi-Key Electronics)

This air gap must be consistent with parameters such as magnet size and remanence, also known as residual magnetization. Designers must also ensure that air gap variations don’t result in magnetic fields that are either too low or too high. This requires careful consideration of the appropriate magnet for the application’s air gap (Figure 2).

Diagram of three magnet-to-sensor positionsFigure 2: Designers can choose a magnet-to-sensor position based on design considerations such as the required level of immunity to external field disturbance and air gap tolerance. (Image source: Monolithic Power Systems)

That said, angle sensors can support a wide range of spatial configurations and magnetic field strengths, including both off-axis or side-shaft mounting and end-of-shaft configurations. To help accommodate variations, on-chip non-volatile memory is used to store configuration parameters such as reference zero angle position, ABZ encoder settings, and phase information for the motor windings.

Next, the device’s ability to detect various magnetic field strengths allows developers to customize the angle sensor for specific functions like diagnostics and axial movement sensing. The availability of programmable magnetic field strength thresholds also facilitates the implementation of a push or pull button function outputted as two logic signals.

However, while features like speed, low latency and resolution depend on application requirements, safety is at the heart of angle sensor designs. The compliance to functional safety standards further affirms the commitment for accuracy and reliability-conscious automotive and industrial design environments.

Meeting functional safety requirements

The angle sensors used in automotive applications require a high degree of precision, down to 0.1˚, to help ensure compliance with the ISO 26262 functional safety standard in the face of a highly demanding operating environment. The applications for these sensors include position measurement in brushless DC (BLDC) motors for pumps, wipers, brakes, valves, flaps, pedals, and steering angle. The accuracy of 0.1˚ applies across the entire temperature range and product lifecycle. Moreover, at low magnetic flux densities, between 10 millitesla (mT) and 20 mT, where the angle error significantly increases, angle sensors serving automotive and industrial designs must still achieve angle errors as low as 0.2°.

Additionally, angle sensors should be easily integrated into safety-critical designs such as electric power steering (EPS) systems, which are crucial for autonomous features like automated parking and lane keeping.

To address ease of use, Infineon’s XENSIV TLE5109 and TLE5014 angle sensors are available in both single and dual die versions, and integrate both the sensing and logic elements on a single chip (Figure 3). Dual die versions are more suitable for ASIL-D safety applications.

Diagram of dual die angle sensorFigure 3: The side view (left) of a dual die angle sensor (right) for safety-critical applications that uses top-bottom placement to shrink space and save on cost by using an inexpensive ferrite magnet. (Image source: Infineon Technologies)

The TLE5109A16E2210XUMA1 is part of a line that comprises high-precision AMR fast analog angle sensors with an error angle of 0.1°. Though AMR-based angle sensors are designed for 180° angle measurement, they are also applicable for 360° measurement in motors with an even number of pole pairs because the AMR sensing element actually measures the double angle, sine and cosine (Figure 4). Their small angle error also makes them suitable for a broad array of magnetic fields, with flux densities ranging from 10 mT to more than 500 mT.

Diagram of AMR-based angle sensorFigure 4: The AMR-based angle sensor is designed for 180˚ angle measurement, but it can be used to measure through the full 360˚ because it measures both sine and cosine angles. (Image source: Infineon Technologies)

The TLE5109 angle sensors operate from 3.3 volt or 5 volt supplies. Other features include a short start-up time of between 40 microseconds (µs) and 70 µs to ensure minimal latency and support for speeds of more than 30,000 revolutions per minute.

The TLE5014C16XUMA1 is one of a line of GMR sensors that can be programmed to accommodate a wide range of applications by storing the required configuration in on-board EEPROM (Figure 5). These sensors boost flexibility and ease of use by also offering a choice of interfaces that includes PWM, SENT, SPC, and SPI.

Diagram of pre-configured and pre-calibrated Infineon TLE5014 angle sensorsFigure 5: The pre-configured and pre-calibrated TLE5014 angle sensors have the flexibility to be programmed to adapt to any application using the on-board EEPROM. (Image source: Infineon Technologies)

The TLE5014 angle sensors typically draw 25 milliamps (mA) from supply voltages of up to 26 volts (absolute maximum) and meet ISO 26262 ASIL-C for the single die and ISO 26262 ASIL-D for the dual die versions.

Key performance parameters

To fully realize the ability of angle sensors to reduce audible noise and optimize motor smoothness and torque, designers should carefully consider the key parameters: accuracy, speed, latency, axial misalignment, and magnet drift.

For example, high-accuracy readouts are crucial for automotive and industrial environments, despite harsh environmental conditions. That makes factors like thermal stability and air gap tolerance vital in an angle sensor's ability to meet accuracy objectives without adding cost and complexity to the system design.

To meet such requirements at minimal cost, Monolithic Power Systems’ MagAlpha magnetic position sensors—MA302GQ-P, MA702GQ-P/Z, and MA730GQ-Z—can be mounted on the edge of the board for both end-of-shaft and side-shaft (off-axis) configurations. For speed, the contactless sensing and 12-bit resolution absolute angle encoder allow MA302 sensors to provide accurate angle measurement from 0 rpm to 60,000 rpm. The MagAlpha MA730GQ-Z features 14-bit resolution and provides digital readouts over the SPI link (Figure 6).

Diagram of Monolithic Power Systems contactless MagAlpha MA730GQ-ZFigure 6: The contactless MagAlpha MA730GQ-Z features 14-bit resolution and provides digital readouts over the SPI link. (Image source: Monolithic Power Systems)

However, for slow operations like human-machine interface (HMI) or manual controls where the rotating speed remains below 200 rpm, the company offers the MagAlpha MA800, a digital magnetic sensor designed to replace analog potentiometers or rotary switches. It’s used with a diametrically magnetized cylinder of 2 millimeters (mm) to 8 mm, and its magnet configurations and shapes are flexible.

The MA800 has lower resolution (8 bits) but does feature on-chip non-volatile memory and programmable magnetic field strength thresholds. These make it suitable for applications that require the implementation of push-button readouts via register bits as well as output signals.

Zero latency angle sensors

The AK7451 is a 12-bit angle sensor that detects rotation speed and angles by measuring the intensity of a magnetic field. It features a combination of magnets operating parallel to the IC surface while offering tracking speeds of up to 20,000 rpm. After detecting the magnetic field vector parallel to the IC surface, it outputs the absolute angular position of the magnet, and subsequently, the relative angular position.

The AK7451 employs the tracking servo system architecture to ensure zero latency rotation angle sensing. The zero-latency angle sensor can output up to eight-pole UVW winding phases (Figure 7), which significantly improves its versatility, allowing it to serve a broad array of motor drive and encoder applications.

Diagram of AKM Semiconductor AK7451Figure 7: AK7451 enables designers to program 16 ABZ output resolution settings and eight UVW output pulse number settings via EEPROM. (Image source: AKM Semiconductor)

Also, the expansion of the ABZ phase output resolution setting from four types to 16 types enhances motor control usability. It also allows AK7451 angle sensors to facilitate rotor position detection in DC brushless motor-driven operation without Hall IC installation.

Here, it’s worth mentioning that for some position sensing applications, latency is not a critical issue. In electric power steering (EPS) hand wheel angle sensing, for example, a new angle value is requested every millisecond (ms). Also, it’s important to distinguish between errors caused by sensor IC and magnetic input, allowing the angle sensor IC to be used to compensate for the errors related to magnetic input.

Conclusion

While greater accuracy and smaller form factors largely drive the feature set in angle sensors for automotive and industrial applications, compliance with functional safety standards sums up the overall value proposition of these high-precision devices. However, to fully leverage their capabilities, designers need to carefully consider specific application requirements to get clarity on performance parameters such as appropriate air gap, magnetic field strength, rotation speed, and angle error.

As shown, once these requirements are established, there is a wide variety of contactless sensors available that provide the necessary accuracy, speed, and programmable flexibility to meet them.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Majeed Ahmad

Majeed Ahmad is an electronics engineer with more than 20 years of experience in B2B technology media. He is former Editor-in-Chief of EE Times Asia, a sister publication of EE Times.

Majeed has authored six books on electronics. He is also a frequent contributor to electronics design publications, including All About Circuits, Electronic Products and Embedded Computing Design.

About this publisher

Digi-Key's North American Editors