Position sensors enhance the accuracy and efficiency of motor operations

Position sensors for motor commutation and positioning

Motor drive is a complex field applied in numerous use cases. While some motor drives can operate perfectly without position sensors (sensorless), there are instances where adding additional position sensors becomes necessary for more precise motor control. Motor control can have multiple feedback loops, with one used for commutation and another for positioning. These control loops may not necessarily use the same position sensors, as optimizing the system and Bill of Materials (BOM) requires different characteristics. Position sensors can enhance the system's control over position, torque, or speed.

Position control in motors can enable known (safe) positions during startup and throughout the entire operation. It helps avoid missed steps, especially when using stepper motors, and facilitates torque control for low-speed and low-noise operations, particularly suitable for large motors such as pumps. Additionally, speed control in motors demands reliable and low-speed drive startup capabilities, capable of handling high dynamic loads.

For motor commutation, position sensors serve as components in the motor commutation control loop for various types of motors. Motor control algorithms determine the currents through the coils and timing of those currents to ensure that the angle at which the magnetic field is applied is quadrature to the rotor's magnetic field direction for maximizing efficiency. The type of motor control algorithm depends on the motor design and sensor type. For instance, Brushless DC motors (BLDC) can operate in trapezoidal control, sinusoidal control, and field-oriented control modes.

Trapezoidal control serves as the basic mode, determining the rotor's position based on the readout of the latch/switch. It can meet the requirements of BLDC, enabling high-speed commutation but may lead to unexpected torque ripple, potentially causing uneven acceleration, especially in applications like electric vehicles.

Sinusoidal control or field-oriented control performs better but relies on the accurate of rotor angle position (an angle with much higher resolution). The more accurate the angle position, the higher the efficiency, and in some applications, this can translate to increased safety. These control algorithms are also applicable to Permanent Magnet Synchronous Motors (PMSM).

Position sensors are also used for applications like positioning, as seen in a critical smart valve where a position sensor ensures the correct valve position, independent of the motor position. Another example is the accurately positioning of joints on a robotic arm using servo motors. In this case, the position sensor used for motor commutation can be different from the one used for motor positioning.

Rotor position sensors meeting the requirements for motor positioning

With the advent of the era of automotive electrification, the demand for pure electric vehicles is rapidly growing, and various electrical appliances with transmission needs also require a variety of motors, driving the market's rapid expansion. Whether now or in the future, many appliances, including every electric vehicle, requires an economical and high-performance motor. And every motor needs an accurate, versatile, and cost-effective rotor position sensor.

For example, synchronous motors require the use of sensor signals to drive motor torque control, and errors in these signals during synchronization will reduce the overall performance of the system and may lead to safety issues. Therefore, the use of rotor position sensors will provide higher safety for torque control of the motor.

There are typically three types of high-speed rotor position sensors, including Variable Reluctance (VR) resolver, magnetic resolver, or inductive resolver. VR resolver are the most commonly used position sensors in traction motors of electric vehicles and consist of a ferromagnetic rotor and a stator with several secondary coils. VR resolver have been widely used in motor position sensing applications for a long time and have an advantage in harsh environments due to their excellent robustness.

Magnetic resolver uses Hall effect sensors and offer a significant cost advantage. They not only have the characteristics of VR resolver but also have a compact size. Inductive resolver has high accuracy, a wide speed range, and immunity to stray magnetic fields. At the same time, compared to VR resolver technology, they reduce the overall cost of the electrical drivetrain system.

To ensure accurate and reliable operation, an excellent rotor position sensor should provide precise and reliable position sensing at speeds greater than 200,000 e-rpm (electrical revolutions per minute). It should support highly accurate position sensing to ensure maximum efficiency, optimal torque control, and low torque variation. The sensor should also have immunity to stray magnetic fields and flexible design for different motor designs and sensor placements. Additionally, it should also have the advantages of being lightweight and cost-effective.

Various solutions to meet different motor application needs

When applied to high-speed motor commutation applications, the solution can be selected based on resolution. In a multi-chip architecture, latches/switches can be used at low resolution, and linear Hall sensors can be employed at high resolution. In a single-chip architecture (or dual-chip for redundancy), an angle resolver can be used at high resolution.

In the case of using latches/switches for motor commutation, latch/switch products are placed in the stator in a multi-chip configuration. They are well-suited for trapezoidal control of brushless DC motors, for example, using 3 chips, one for each phase.

When using linear Hall sensors for motor commutation, linear Hall effect sensors can replace latch-type Hall sensors. Multiple sensors used quadrature can provide rotor absolute angle with high angular resolution, and their analog outputs can be used to calculate a more accurate rotor position through dedicated algorithms. Therefore, they are suitable not only for detecting motor commutation points but also for accurate position control.

Additionally, magnetic or inductive resolver can be used for motor commutation control. Resolver or motor resolver, are fast-chip solutions that provide ratiometric analog sine/cosine outputs to the rotor magnetic flux. These outputs represent rotor position and can be used to detect the motor position. The latest generation of products can be placed either on-axis (End of Shaft) or off-axis (Though Shaft).

Some motor applications may have an extra control loop for positioning. For example, actuators running at low speed, such as valves, or when an internal gearbox in the application converts high-speed/low-torque rotation into low-speed/high-torque rotation. Due to the continuous wear of the gearbox, the one-to-one relation between the rotor position and the motor output shaft will gradually disappear. Therefore, in certain designs, an extra position sensor is installed on the output shaft.

A comprehensive motor commutation and positioning solution

Melexis has introduced a variety of magnetic position sensor chips for motor commutation and/or positioning, including latch/switch, linear Hall sensors, or angle resolver.

LATCH & SWITCH

As many traditional Hall-effect sensors are only sensitive to magnetic flux perpendicular to the IC, the development of complex (thus large and expensive) custom magnetic structures may be required to achieve the desired measurements. Melexis offers a range of latch and switch sensors with pre-programmed or programmable fixed parameters that utilize innovative magnetic Triaxis® technology, capable of measuring lateral magnetic flux components. In addition to sensors that sense the magnetic induction intensity applied perpendicular to the die surface, Melexis has introduced latch sensors that can sense lateral magnetic flux density, significantly enhancing the flexibility of sensor positioning relative to the magnet (rotor or sensing magnet). These sensors are available in single-die TSOT-3L or TO92-3L packages. Furthermore, Melexis has launched the first latch/switch chip that supports ASIL B.

LINEAR HALL

The MLX90290 is an SMD factory programmed very fast linear Hall sensor chip with an analog ratiometric output. It is suitable for applications such as small -stroke rotary, linear position sensing, AC and DC current sense applications, and commutation applications for brushless DC motors. The linear Hall effect sensor chip is compatible with circuits with operating bias voltages of 3.3VDC and 5VDC. This chip can be temperature-compensated for position detection using most permanent magnets (such as AlNiCo, NeFeB, and ferrites). For current sensing applications, a version with 0ppm/°C is available.

RESOLVER

Magnetic

Melexis' MLX90380 is a fast pre-programmed magnetic resolver chip suitable for brushless motors, particularly in automotive applications or other applications that require rapid position readouts. Another chip, the MLX90381, is a 3D magnetic angle resolver chip that can measure rotational speeds of up to 50,000 e-rpm. This chip is optimized for reliable motor designs, featuring a smallest footprint, module-level programming capability, cost-effectiveness, and compliance with ASIL standards.

Inductive

Melexis offers state of the art inductive resolver solution: the MLX90510 and MLX90517 support very high speed at high accuracy (maximum ±0.36° el up to 240000 e-rpm).

MAGNETOMETER

Melexis also offers various low-speed angular position sensor chips, with low speed referring to an update frequency of no less than 200 µs. Among them, the MLX90392 / MLX90393 / MLX90395 / MLX90397 are value-optimized magnetometers requiring post-processing, suitable for consumer or automotive applications. 

POSITION SENSOR

Magnetic

The MLX9042x series is a mainstream series of magnetic position sensor chips designed for cost-sensitive applications with demanding requirements. The MLX9037x series is a performance-oriented series, offering high performance, high safety, and comprehensive feature set in magnetic position sensor chips.

Inductive

The MLX90513 is a top notch inductive position sensor. Inductive technology is now broadly available with Melexis. It offers full stray field immunity, high safety standard and also higher accuracy in demanding applications.

LIN MOTOR DRIVER

Melexis' fully integrated LIN motor drive chips and pre-drive chips reduce BoM costs, simplifying the design of motor control flaps, valves, fans, and pumps in automotive mechatronic applications. These devices can also be used in robotic systems and electric bicycles/electric scooters. LIN motor drive chips support two-wire DC motors, three-wire brushless DC motors, or four-wire bipolar stepper motors, and can use sensored or sensorless field-oriented control (FOC) algorithms.

Melexis' third-generation embedded drive chip series includes the MLX81330, MLX81332 and MLX81334 LIN drive chips for 1-10 W applications, as well as the MLX81340, MLX81344, and MLX81346 LIN pre-driver chips for 10-2000 W applications. Notably, Melexis is a pioneer in bringing fully integrated systems on-chip (SoC) 48 V pre-driver chips to the market with the MLX81346.

Conclusion

With the support of position sensors, motor drives can meet specific application requirements related to position, torque, and speed control, contributing to improved accuracy and efficiency of motor operation. Melexis offers a variety of sensing and driving solutions for motors, making it an ideal choice for optimizing your mechatronic applications.