In the end market, significant progress has been made in replacing AC motors or high-efficiency mechanical pumps with brushless DC motors (or simply BLDC) technology.
Compared to AC motors, the advantages of using BLDC motors include higher power and thermal efficiency, smaller size, and more reliable performance. Furthermore, because BLDC uses electronic commutation instead of traditional mechanical commutation, it makes it easier to control torque and speed parameters across the application speed range, and also enables more complex controls such as maintaining torque or speed limits. Due to these advantages, BLDC is finding its way into more and more existing and emerging applications.
Angle sensor in BLDC motor control
To achieve precise control and efficient commutation of motors, high-resolution current and rotational position information are crucial. While typical rotator system designs can achieve high resolution and accuracy, they still require consideration of large physical footprint and high cost. Sensorless solutions can be used to detect back EMF current and can reduce sensor weight and cost, but motor starting performance may be affected because effective position data cannot be obtained to generate back EMF. Other solutions, such as using three Hall effect sensors to detect the position of the motor magnets, are typically used in cost-sensitive applications. In this case, while resolution is achieved, three sets of signals must be monitored simultaneously. Sensor installation also presents a technical challenge in space-constrained environments, representing another potential problem.
Another approach is to use an angle sensor based on anisotropic magnetoresistive (AMR) technology. AMR technology is a type of magnetic sensing application that achieves high angular accuracy and integrates a single sensing element with electronic circuitry in the same package. This enables miniaturization of the sensor system and makes it possible to position the sensor within the motor assembly.
Comparison of AMR and optical sensing technologies
Compared to traditional optical sensing technologies, AMR technology offers a smaller footprint, lower cost, and is better suited for harsh environments, such as dirty and temperature-fluctuating conditions. Table 1 compares the two, clearly demonstrating the advantages of using AMR in brushless DC motors.
Table 1. Comparison of photoelectric and AMR sensing technologies
AMR sensor technology principle
The resistivity of a sensor based on the AMR principle depends on the magnetization direction relative to the current direction. AMR sensors operate in a magnetically saturated state, therefore the applied magnetic field dominates the resistance change. The resistance is maximum when the external magnetic field is parallel to the current direction, and minimum when the applied electric field is perpendicular to the plane of the current-carrying magnetic induction alloy. Figure 1 shows a simplified diagram of the AMR sensor's operating principle.
Figure 1. Working principle of AMR sensor
Selection of AMR Sensors
To ensure high precision in motor rotation, the following suggestions are made when selecting an angle sensor:
- It can sense a 360° rotation angle and accurately measure the absolute angular position;
- Low angle error rate;
- It has a linear and stable error rate, which makes it easy for the main microcontroller to perform offset correction calculations.
Taking Analog Devices' ADA4571 series as an example, Figure 2 shows the typical high output level and angle error of the ADA4571 when a rotating magnetic field is applied to a 360° mechanical rotation. After offset correction by the microcontroller, the typical error is less than 0.1°.
Figure 2 shows the ADA4571 error (gray) and output waveform (orange/blue) under 360° mechanical rotation.
Installation of angle sensor
For most BLDC control systems, there are many ways to configure and install sensors, depending on the available space and the accessibility of the motor shaft. Figure 3 shows two common installation configurations for the ADA4571.
Figure 3, (a) Motor shaft end system configuration (b) Motor shaft side system configuration
Figure 3a shows a typical motor shaft-end system, where a diameter magnetized disk magnet is mounted on the rotating shaft, providing a magnetic field that passes through the sensor plane. In this configuration, the rotor angle can be read directly without contact between mechanical and electrical components. Because AMR technology is independent of field strength, it has a high tolerance for air-gap variation, giving engineers greater flexibility in material selection and the mechanical tolerance requirements of the magnet material.
Figure 3b shows a shaft-side system, a configuration suitable for applications where the shaft to be detected cannot have a magnet mounted at its end. In this configuration, the magnet provides the magnetic field, and the sensor and magnetizing disk can be mounted anywhere on the shaft, providing another option for space-constrained applications.
Summarize
For designers of industrial and automotive BLDC motor control systems, magnetic angle sensors offer a compact, robust, and easy-to-assemble position sensing solution. When selecting an angle sensor, the following three key points should be considered:
- It can sense a 360° rotation angle and accurately measure the absolute angular position;
- Low angle error rate;
- It has a linear and stable error rate so that the main microcontroller can perform offset correction calculations.
In terms of installation, the angle sensor also provides engineers with more flexible installation options, including "shaft-end system" configuration and "shaft-side system" configuration, to cope with application scenarios with limited space.
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