【Introduction】With the advent of the era of vehicle electrification, the demand for pure electric vehicles is growing rapidly. Every electric vehicle, now and in the future, needs an affordable, high-performance electric traction motor (Electronic machine). And every electronic machine requires accurate, versatile and affordable rotor position sensors.
Synchronous motors require this sensor signal to drive motor torque control (Figure 1). Errors in the synchronization of these signals will degrade the overall performance of the system and may lead to safety issues.
Figure 1: Motor System
Rotor Position Sensor Type
There are generally three types of high-speed rotor position sensors: variable reluctance (VR) decoders, magnetic decoders, or inductive decoders (Figure 2).
Figure 2: Comparison of high-speed decoder technologies
Variable Reluctance (VR) Decoder:
VR decoders are currently the most commonly used position sensors in traction motors. It consists of a ferromagnetic rotor and a stator with several coils. VR decoders have long been used in motor position sensing applications and have advantages in harsh environments due to their superior robustness.
The magnetic decoder adopts Hall effect sensor, and the comprehensive cost advantage is very prominent. It not only has the characteristics of a VR decoder, but also has a compact size.
The inductive decoder has high precision, wide speed range and stray field anti-interference ability, and the automotive safety integrity level can reach ASIL D. At the same time, it also reduces the overall cost of the electric drivetrain compared to VR decoder technology.
How Inductive Decoders Work
An inductive decoder measures the position of a metal target placed in front of a set of induction coils. Figure 3 shows a typical configuration.
Figure 3: Example of an inductive decoder board
A metal target plate is mounted on the motor rotor. It consists of several target lobes matching the number of pole pairs of the motor. The coil set consists of a transmit (Tx) coil and a receive (Rx) coil, which are fixed to the motor stator and are usually placed directly in the PCB. The coils are in turn connected to the IC interface (Figure 4).
Figure 4: Basic principle of an inductive sensor
How Inductive Sensors Work
■ The Tx coil is controlled by the sensor IC, which can generate an AC magnetic field after excitation
■ VCSEL illumination (940 nm up to 110°FOV)
■ The Rx coil receives the field reflected from the target plate and generates a set of three AC signals (3-phase coil design)
The position sensor interface chip converts the input signal from the Rx coil to a differential sine/cosine (baseband) output signal. These signals are used to communicate with the electronic control unit. The only calculation the ECU has to perform is to calculate the angle from the sine/cosine signal via arctangent.
Designing an Inductive Decoder Module
There are four main aspects that must be considered when designing a module using an inductive decoder. i.e. coil system, target board, chip (signal processing) and PCB (Figure 5)
Figure 5: Four main design areas of an inductive decoder
The coil is the transducer of the system, equivalent to the Hall plate of the magnetic system. Inductive sensors offer a degree of design flexibility when it comes to coil design.
In terms of coil design and design scalability, since the sensor (coil set) is not integrated in the chip, it can be customized according to the needs of the application. The sensor period can be adjusted according to the number of pole pairs of the motor.
Figure 6: Example of an inductive sensor with different periods
Figure 6 shows some variable period coil designs. The variable period allows the sensor range to be matched to the electrical period of the motor, maximizing sensor accuracy. In theory, there is no maximum pole pair limit for inductive sensors. The limitations are mainly due to the constraints of space and PCB design rules.
Figure 7: Coil design flow for a 5-pole pair, 4 cm diameter inductive sensor. a) The original function. b) Individual Rx coil mode. c) Complete coil set and target plate. d) Rx voltage for one electrical cycle
Figure 7 briefly illustrates how to obtain the coil set. The Rx coil consists of a sine primitive functionand its complementary function definition (Fig. 7a).The Rx coil pattern is then obtained by polar transformation to fit N cycles within a complete revolution, where N is the number of pole pairs of the motor
Then, by adding two more Rx coils translated 120°/N, a circular Tx coil surrounding the Rx coil, and a target plate, the complete coil set is obtained (Figure 7c). This configuration will provide a 3-phase signal group as shown in Figure 7d.
The target plate is placed on the rotor element and can be adjusted according to the number of pole pairs of the motor. The energy from the Tx coil creates eddy currents in the target plate, which in turn create a magnetic field that is sent back to and received by the Rx coil.
Figure 8: Eddy currents in the target plate can be viewed as a single current or can be decomposed into independent current loops
Turbine is a current loop generated in a conductor according to Faraday’s law of induction, the change of the magnetic field in the conductor. In a plane perpendicular to the magnetic field, eddy currents flow in closed loops within the conductor (Figure 8).
Inductive and magnetic decoders actually have similarities (Figure 9). For example, a three-lobed target corresponds to a hexapole magnet.
Figure 9: A three-lobed inductive target can be viewed as a hexapole magnet
The receiver coil connected to the inductive target plate is similar to the Hall plate of the magnetic system (Figure 10).
Figure 10: Rx of an inductive decoder clockwise/counterclockwise with respect to Hall plate positive/negative bias of a magnetic decoder
IC (Signal Processing)
With an inductive decoder, the signal processing is integrated directly inside the module (Figure 11). The Rx signal is processed by the integrated interface chip and provided to the ECU in the form of sine/cosine. Signal processing includes common mode removal, offset compensation, digital angle calculation, linearization, and propagation delay compensation.
Figure 11: Analog conditioning and digital signal processing (DSP) done by the IC
Coil design requires neither special PCB design tools nor special PCB technology.
A good rotor position sensor has the following characteristics:
● Provides accurate and reliable position sensing at speeds greater than 200,000 e-rpm (rpm: revolutions per minute).
● Highly accurate position sensing ensures maximum efficiency, optimum torque control and low torque variation.
● Anti-interference ability of stray field.
● Scalable and flexible design for different motor designs and sensor locations.
● Light weight and economical.
● Automotive Safety Integrity Level up to ASIL D.
Melexis’ MLX90510 Inductive Decoder is an inductive position sensor chip for absolute rotary motion/position sensing in safety-critical automotive applications.Its main specifications bring many specific benefits for electronic machine type applications
3-phase coil design
Melexis uses a 3-phase (three-phase) coil design, while a quadrature-phase coil design is more intuitive for magnetic systems (Figure 12).
The three-phase approach has no additional immunity to the familiar error sources (offset, mismatch, quadrature…). However, it has an advantage because the induction signal is not sinusoidal like the Hall signal. The 3-phase method simplifies linear optimization by automatically filtering out some non-idealities, such as removing even harmonics. For in-depth information on 3-phase coil design, including full theory and verification, read our white paper.
Figure 12: Magnetic Quadrature Input Signal and Inductive Three-Phase Input Signal
The MLX90510 employs an innovative digital infrastructure that enables high accuracy and high automotive safety integrity levels.
● The system propagation delay is <+/- 120 ns over the entire operating range.
● High accuracy < +/- 0.36° el up to 240,000 electronic rpm over the entire working range.
Mechanical installation position flexibility
The coil system can be printed on PCB and can be easily adapted to different types of electronic machines. In addition, it can be flexibly fixed on the shaft (Fig. 13):
● Shaft end: small size, low cost, excellent mechanical stability
● Thru axle: medium size, medium cost, excellent mechanical stability
● Shaft side: large diameter, small size, low cost, sensitive to mechanical tolerances
Figure 13: Three axis fixation positions of the coil system.
Improve cost structure
This inductive position sensor operates within a narrow-band high frequency, well beyond the switching frequency of high-power inverters. This means that no expensive shielding is required.
Using a 3-phase coil set instead of a typical sine/cosine pair can filter some unwanted harmonics in the Rx signal, thereby improving linearity. Additionally, the MLX90510 offers 16-point linearization, which mitigates residual errors from mechanical tolerances and other sources.
The MLX90510IC is ISO 26262 ASIL C (D) compliant, enabling system integration up to ASIL D levels.
● The analog output of differential sine and cosine is independent of analog input voltage amplitude changes caused by air gap changes.
● Input and output are decoupled from each other, enabling unprecedented EMC performance while maintaining stable output amplitude (independent of air gap-related input signal strength).
● Wide operating supply voltage range with overvoltage and reverse polarity protection: -24 V to +24 V
● Immunity to DC and AC stray magnetic fields (ISO 11452-8).
● Meet the requirements of AEC-Q100 vehicle regulations
● Operating ambient temperature range: -40°C to 160°C.
The MLX90510 is suitable for a variety of EV applications (Figure 14)
Figure 14: In addition to motors, application directions include power steering, electric brake boosters, and electric axles