Position sensors for motor control feedback loop

What’s the use of a position sensor for motor drivers?

This tech talk provides an overview of Melexis magnetic position sensors for motor commutation and/or positioning. Discover magnetic solutions: latch/switch, linear Hall or resolver. To learn about the technical concepts, please refer to the complete application note (chapter 5).

Position sensors for motor driver

Electrical motor drive is a complex domain with many use cases. In this area there are motor drivers that can perfectly operate without a position sensor (sensorless). So why add extra component(s)? Let’s first distinguish the two possible roles that can play a position sensor here.

Position sensors for motor driver
Figure 1: general building blocks and paths to enable motor commutation and application positioning

Motor control can have multiple feedback loops: one for the motor commutation and one for the positioning. These control loops do not necessarily use the same position sensor since different characteristics are needed to optimize the system and BOM. Position sensors can improve the control of the system in the following areas: position, torque or speed.

Position Control

  • Enables a known (safe) position at startup and throughout motion
  • No missed steps when using stepper motor

Torque Control

  • Enables low speed and low noise generation especially for larger motors like pumps

Speed Control

  • Enables reliable startup
  • Low speed drive
  • Capability to handle highly dynamic loads

Benefits of a position sensor in motor drive application
Figure 2: Benefits of a position sensor in motor drive application

Motor commutation

Position sensors can be an integral part of the motor commutation control loops for several types of electrical motors.

The motor control algorithm determines the currents through the coils and the timing of those currents. The angle of applied field has to be in quadrature to the rotor’s field direction for maximum efficiency.

Concepts are fully detailed in the application note (chapter 5), but note that the type of motor control algorithm is linked with the motor design and the sensor type. For example, brushless motors can work with trapezoid control, sinusoidal control and field oriented control:

  • Trapezoid control, the basic, can use latch/switch readout for the position of the rotor. It is sufficient for a Brushless DC motor (BLDC). This concept supports high speed but torque ripple might be present and unwanted. Imagine an accelerating electrical car where the acceleration is not smooth.
  • Sinusoidal control or Field Oriented Control will behave better however they require an accurate angle position of the rotor (an angle with much higher resolution). The more accurate, the higher the efficiency will be and in some cases even better safety. These control algorithms can also be used for a Permanent Magnet Synchronous Motor (PMSM).

Latch and linear principle
Figure 3: latch and linear principle

Resolver principle
Figure 4: resolver principle

Application positioning

Position sensors can be required for the application positioning. Take for example a critical smart valve. The position sensor ensures a correct position of the valve independently from its motor position. Another example is a robot arm where the servo motor accurately positions a joint.

Here, the sensors needed for motor positioning might be the one(s) used for motor commutation. This is especially true if the rotor shaft’s position and the motor’s output shaft are 100% correlated. This is not always the case however. In some cases, an internal gear box converts a high speed/low torque rotor to a lower speed/higher torque output shaft. It then might be necessary to put an additional – lower speed- sensor on the output sensor.

Application positioning

Melexis Products

Sensor ICs for motor commutation

This chapter gives a high level overview of 3 different hall based product categories for motor commutation: latch/switch, linear hall and resolver.

Motor Commutation (High speed) Low resolution High resolution
Multiple IC Latch/Switch Linear Hall
Single IC (or dual IC for redundancy)
Resolver

Motor commutation with Latch/Switch

Latch/Switch products are placed in the stator in a multi-IC configuration. They are interesting for trapezoid control of BLDC motors: 3 ICs are used, 1 for each phase.

Lateral or perpendicular sensing
Figure 5:
Lateral or perpendicular sensing

Melexis provides a whole range of Hall Effect latches with fixed, pre-programmed or programmable parameters. Next to the traditional sensors which are sensitive to the magnetic flux density that is applied perpendicular to the die surface, Melexis offers latches that sense a lateral magnetic flux density. This feature brings a large flexibility in the positioning of the sensor versus the magnet (rotor or sensing magnet). The sensors are available in a single die TSOT-3L or TO92-3L. Melexis also offers the first ASIL-B capable latch/switch on the market.

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Lateral / X-axis
Figure 6:
Lateral / X-axis

Perpendicular / Z-axis
Figure 7:
Perpendicular / Z-axis

Motor commutation with Linear Hall

A linear Hall Effect sensor can be used to replace the hall Latch sensors. Using multiple sensors in quadrature gives the absolute angle of the rotor with high angle resolution. Their analog output makes it possible to calculate, with a dedicated algorithm, a much more accurate rotor position. This makes them not only suitable for detecting the motor commutation point but also for an accurate position control.

Two linear hall sensors placed at a 90° magnetic phase shift can also be used as a sine cosine angle sensor. The angle α is calculated from the arctangent of SIN over COS.

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Setup with 3 sensors
Figure 8: setup with 3 sensors

Setup with 2 sensors
Figure 9: setup with 2 sensors

Motor commutation with Magnetic Resolver

Magnetic Resolvers, also known as resolvers or motor resolvers, are fast IC solutions which provide ratiometric analog sine-cosine outputs. These outputs are representative of the rotor magnetic flux and thus can be used to detect the motor position. The latest generation can be placed either on-axis (End of Shaft) or off-axis (Though Shaft).

End of shaft
Figure 10: End of Shaft

Trough shaft
Figure 11: Through Shaft

Triaxis® Resolvers have an X-Y-Z magnetic axis configuration. This feature brings a large flexibility in the positioning of the sensor versus the magnet (rotor or sensing magnet).

X/Y Magnetic Axis
X/Y Magnetic Axis

X/Z Magnetic Axis
X/Z Magnetic Axis

Y/Z Magnetic Axis
Y/Z Magnetic Axis

OUT1 and OUT2 (Sine and Cosine signal) of the sensor can be configured in X/Y magnetic axis, X/Z magnetic axis or Z/Y magnetic axis for End Of Shaft or Through Shaft sensing.

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Sensor ICs for positioning

Some motor applications have an extra control loop for positioning. Think for example a valve for which the actuator runs at lower speed. Or when the application uses an internal gearbox to convert a high speed/low torque rotation to a low speed/high torque rotation. Due to wear in the gearbox, the 1-1 relation between rotor position and motor output shaft is lost. As such, some designs put an extra position sensor on the output shaft.

Application Positioning (Lower speed) Raw information (Embedded) Computed angle (Standalone)
Combined with the motor commutation Latch/Switch
Resolver
Linear Hall
Specific performances Triaxis Value Optimized (Magnetometer) Triaxis Mainstream (MLX9042x)
High performances
Triaxis Performance (MLX9037x)

The products mentioned for the motor commutation are very focused on high speed. Their update frequency is expressed in few µs, and they generally output raw information. This information is then treated by the related MCU. For the application positioning, the fast response is no longer the main focus. Therefore, we can use “smarter” sensors that output already computed angle information. Such a sensor benefits from a DSP and therefore offer more functionalities (signal compensation, programmable diagnostics…) which can further simplify the system design. Melexis offers a wide range of angular position sensors at lower speeds. Here, low speed means an update frequency from 200µs.

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Driver ICs for actuator

Melexis’ all-in-one LIN motor driver and pre-driver reduces BoM and simplifies design of automotive mechatronic applications: motor-controlled flaps, valves, fans and pumps. The devices are also used in robotic systems and e-bikes/e-scooters.

They support 2-wire DC motor, 3-wire BLDC motor or 4-wire bipolar stepper motor, using either sensored or sensorless field-oriented control (FOC) algorithms.

For sensored applications, Melexis is your one-stop-shop with Sense & Drive solutions. Absolute angle position sensors can be used with Melexis' driver ICs, to drive and sense the position of a smart valve for efficient cooling of the battery or engine. It is also compatible with Melexis' pre-driver ICs to track the rotor position in smart pumps, to control the pump efficiently and dynamically with the right torque.

Our complete third generation of embedded drivers includes:

  • the LIN drivers MLX81330 and MLX81332 for 1..10 W applications
  • the LIN pre-drivers MLX81340, MLX81344 and MLX81346 for 10..2000 W applications

Note that with the MLX81346, Melexis is the first company to bring a fully integrated 48 V pre-driver System-on-Chip (SoC) to the market.

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Driver ICs for actuator

General concepts summary

For the details, please refer to the application note (chapter 5).

Here is a summary of the concept:

System Latch/Switch Linear Hall Resolver

Latch & Switch Linear Hall Resolver
Concept Latch & Switch: Sensor output Magnetic field detected by sensor Magnetic field detected by sensor

3 latches for a 3 phase system (placed at 60°)
Use the duty cycle (DC) to operate
2 linear sensors (placed at 90°)
Generate Sine and Cosine to operate
1 resolver
Provide Sine and Cosine to operate
Pros - Cost - High resolution
- Detect motor commutation point
- High resolution
- Allows absolute position control (e.g. 2 pole magnets
is sufficient)
- End of shaft position is possible
Cons - Low resolution
- Multiple sensors (x3) implying:
* precise positioning
* part to part variation to compensate
- Multiple sensors (x2) implying:
* precise positioning
* part to part variation to compensate
- Cost

Conclusion

Motor drivers can be supported by position sensors to meet specific application requirements related to the position, torque and speed control. Melexis Sense & Drive offers the right solutions to optimize your mechatronic applications.


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