The motor position sensor is a device that detects the position of the rotor (rotating part) in the motor relative to the stator (fixed part). It converts the mechanical position into an electrical signal for use by the motor controller to decide when to switch the motor's current direction and strength, thereby controlling the motor's rotational speed and torque.
In new energy vehicles, the precise control of the motor is directly related to the driving safety and stability of the vehicle, and the accurate work of the position sensor resolver can ensure the correct response of the motor at critical moments such as emergency braking, acceleration or steering. This is particularly important for permanent magnet synchronous motors (PMSM), which do not have physical contact commutators and therefore rely on the position information provided by the sensor to decide when to switch the direction of current and ensure smooth operation of the motor.
At present, there are two kinds of motor position sensors commonly used in new energy vehicles, eddy current sensors and rotary transformers (rotary sensors).
01.
The difference between whirling and eddy currents stems from their basic principle
Although eddy current sensors and rotary transformers can well meet the requirements of motor position detection, due to their different signal generation machines and signal processing methods, there will be differences in specific product applications according to different requirements.
The choice of the type of motor position sensor also needs to consider other factors, such as cost, accuracy requirements, environmental adaptability, reliability, and system integration complexity, which are closely related to the basic signal generation and processing mechanism.
Take the most commonly used rotary sensor as an example, its working principle is based on the principle of electromagnetic induction. The principle of signal generation is that the motor controller provides A constant frequency AC excitation signal to the excitation coil (coil A), and this excitation signal generates an alternating magnetic field inside the rotary sensor. As the rotor rotates, the magnetic field generated by the excitation coil is cut, resulting in the induction of AC voltage in the sinusoidal coil B and the cosine coil C. By measuring the phase difference and amplitude of these two signals, the absolute position and rotation direction of the motor rotor can be accurately calculated.
◎ In signal processing, the motor controller receives and analyzes the sine and cosine signals of the rotary sensor, and calculates the precise Angle information through a software algorithm (usually the rotary encoder analysis algorithm). In order to achieve better signal processing, it is usually necessary to apply a special decoding chip, which is installed in the motor controller, and of course, it can also be achieved by software decoding.
Therefore, in the specific shape of the rotation sensor, it is usually composed of an exciting coil (primary coil, coil A), two output coils (sine coil B and cosine coil C) and an irregularly shaped metal rotor. The rotor is coaxial with the rotor of the motor and rotates with the rotation of the motor.
The eddy current sensor uses the electromagnetic induction principle to transmit and receive the induced AC signal with the corresponding coil at the transmitting end and the receiving end, so as to calculate the position of the target wheel. The target wheel is fixed on the rotating shaft and rotates together with the rotor. The relative position of the motor rotor and stator can be measured by detecting the position of the target wheel.
◎ In terms of signal processing, when the eddy current sensor is powered on, the sensor transmitting coil generates an exciting magnetic field, and the target plate follows the motor to rotate and cut the exciting magnetic field, so that the receiving coil generates coil voltage, and the sensor module demodulated and processed coil voltage to obtain the voltage signal of the corresponding position. Different from the rotary sensor, the signal processing chip of the eddy current sensor is integrated with the sensor, and the digital signal can be output directly.
Therefore, the eddy current sensor usually consists of a number of target lobes matching the number of pole pairs of the motor. The coil group consists of a transmission coil and a receiving coil, which are fixed on the motor stator, and the eddy current sensor is usually arranged directly in the PCB, and the signal processing chip is integrated.
02.
Different principles lead to different technical focus
It can be seen that the main differences between the rotation sensor and the eddy current sensor in principle lie in the excitation mode, the signal generation mechanism and the complexity of signal processing. The advantages of the rotary sensor are mainly in the stability of the excitation signal and the tolerance of the working environment, but the disadvantages are that the influence of the change of the motor scheme is greater, and the platform compatibility is poor. The advantage of eddy current sensor is its high degree of electronization, easy to meet the needs of the platform, and strong anti-EMC ability. The disadvantage is that it is slightly weaker than the rotary sensor in terms of environmental tolerance, and the cost is higher than the rotary sensor in some scenes.
Platform compatibility is first reflected in the speed level, the "energy saving and new energy vehicle technology Roadmap 2.0" prepared by the China Society of Automotive Engineering pointed out that by 2025, the maximum working speed of the position sensor is 20,000r/min, and the decoder bandwidth is >2.5kHz. By 2030, the maximum working speed of the position sensor is 25,000r/min, and the decoder bandwidth is >3.0kHz. It can be seen that there are certain challenges in the rotary sensor at high speed.
This is because the excitation frequency of the rotary sensor is closely related to the speed state considered when it is designed, and usually matches the current speed state. As the speed increases, a higher frequency of excitation is required for accurate measurement, which requires a change in the design of the rotary sensor.
Eddy current sensors do not have this problem. Effie Automotive told NE Time that the design of the eddy current sensor can better adapt to the development trend of this high speed. Its wide range of support, fast response, and better performance in high-frequency signal processing mean that eddy current sensors can be "upwardly compatible" for future applications at higher speeds. Therefore, the platform solution can be better realized in the motor products with different speeds. In fact, this is one of the factors that current motor customers choose eddy current solutions,
In addition, due to the variety of eddy current sensors, such as the shaft type, the shaft end is similar, and the shaft can be divided into O-type and C-type (some are also called full circle and semi-circle). Therefore, it is relatively more flexible in adapting customer motor design schemes.
03.
Different principles lead to different cost reduction challenges
The cost of rotary sensors mainly comes from materials and hardware, including magnetic materials (such as silicon steel sheets), coils, and so on. Therefore, the overall cost is determined according to its size, usually the larger the size, the higher the cost.
The core cost of eddy current sensor mainly lies in its electronic components, processing chips, etc., the cost of electronic parts is relatively fixed, so the core cost of eddy current sensor does not increase linearly with the size.
Therefore, the cost of eddy current sensors is lower than that of rotary sensors for large scale applications. However, in small-size motor schemes, rotary sensors have certain cost advantages. Of course, when it comes to the specific application scheme, because the signal processing chip of the rotary sensor is often not included in the cost calculation, the specific cost comparison also has some differences.
In addition to the current cost comparison, it is also necessary to pay attention to the future cost reduction space. At present, because most of the eddy current sensor chips come from foreign enterprises, the cost can be further reduced with the expansion of the scale and the maturity of domestic chip enterprises in the later stage. However, the descending space of rotary sensor is relatively limited.
Therefore, when facing future cost requirements, eddy current sensors are obviously more advantageous. In recent years, the market share of eddy current sensors has risen significantly, and in the domestic market, vehicle companies, including Geely and a number of new forces, have chosen the eddy current sensor scheme.
04.
The eddy current sensor industry still needs to grow
Although the popularity of eddy current sensor applications is increasing, the most common sensors are still rotary sensors, including sales leaders BYD and Tesla. The reason for this is that, on the one hand, eddy current sensors are applied late in the automotive field, and on the other hand, there are not many suppliers that can provide eddy current sensors, and a few companies such as Effie and Sensata can supply them in the industry.
For eddy current sensors, there are three main challenges:
In fact, eddy current sensors have been applied in the industrial field, but in the automotive field, the first thing that needs to be met is the requirements of the vehicle gauge level, especially the requirements of functional safety. Take Effie Automobile as an example, in order to ensure the stable application of the eddy current sensor, the development process is strictly in accordance with the ISO26262 process to ensure the requirements of functional safety level.
◎ The challenge of the chip, the chip must not only meet the functional requirements, but also meet the car gauge level. As an eddy-current sensor enterprise, it is necessary to establish a chip verification standard to evaluate the availability of the chip, which is also crucial for the subsequent application of domestic chips. Through years of cooperation with global chip manufacturers to establish a complete verification process, Effie Automotive revealed that the introduction of domestic chips has been planned, of course, the premise is to meet the standards.
Reliability challenges, eddy current sensor due to the installation position, the working process is prone to thermal shock in the motor, cooling oil sputtering and other challenges, which is especially greater for the chip. Effie Automotive's solution is to apply adhesive treatment to the chip location, while increasing the temperature requirements of the chip itself. To improve adaptability to the environment and improve reliability.
In the future, whether the eddy current can completely replace the rotary sensor is still unknown. Rotary sensors also have their own product upgrade path to cope with the new needs of the motor. However, the growth momentum of eddy current sensors is faster than that of rotary sensors, and of course, the base of eddy current sensors is low.
