Eddy Current Sensor Design Solution
Author: Guo Rongling
In automotive motor control systems, detecting the rotor position of a BLDC motor is crucial. However, traditional Hall switch detection has several drawbacks: First, it cannot continuously output rotor angle values, failing to provide data support for precise motor control. Second, Hall devices need to be installed inside the motor, where high temperatures, strong vibrations, and intense magnetic fields can easily damage them, making their reliability insufficient. As a result, "resolvers" have gained attention, but they are expensive, difficult to install, and have low output signal speeds, making them unsuitable for high-speed motor control. Eddy current sensors, on the other hand, effectively overcome these shortcomings.
Technical Features of Eddy Current Sensors
Motor interiors are subject to magnetic pulse interference and transmission interference. To address signal anti-interference issues, the shielding and wiring of the eddy current sensor are optimized. Stainless steel shielding plates are placed behind the encoding and decoding components, effectively blocking interference from the motor's internal pulsed magnetic fields. Two wiring solutions—4-pair shielded twisted-pair cables and 4-layer FPC flat cables—are employed, both delivering excellent results.
A space voltage vector model is adopted to solve the driving and signal decoding challenges of the eddy current sensor. The advantage of this model is that it eliminates the need for dedicated chips—only an automotive-grade general-purpose microcontroller is required for excitation driving. Additionally, just a Schottky diode bridge is needed to process the space vector voltage signal, ensuring low cost, high reliability, and no loss of detection accuracy.
Beyond rotor angle sensing in BLDC motors, eddy current sensors can also be applied in other scenarios, such as steering wheel deflection angle detection, throttle detection, brake detection, and wheel rotation detection.
Advantages of Eddy Current Sensors
The benefits of eddy current sensors are highlighted in the following aspects:
1. High Precision
The eddy current sensor incorporates a three-phase space voltage vector angle decoding method. This approach features a simple yet effective mathematical model, low requirements for signal processing circuits, and high-speed angle output. At 5,000 rpm, it achieves an angle resolution of 0.5–1°, meeting the demands of automotive control applications.
2. High Reliability
The eddy current sensor uses highly durable stainless steel (304 mirror-finish stainless steel) as the rotor encoding element and PCB coils as the decoding element, eliminating the need for fragile IC chip soldering. These robust components can withstand high temperatures, strong vibrations, and intense magnetic fields inside the motor.
3. Low Cost
The eddy current sensor is significantly more cost-effective than resolvers.
