Measuring Rotation Speed ​​Using a Fiber Optic Sensor

Fiber optic sensors can measure and control rotation speed using non-contact methods such as Sagnac interferometers or intensity-modulated displacement sensors.

Types of Fiber Optic Rotation Sensors

1. Sagnac Interferometer-Based Sensors These sensors operate on the Sagnac effect, where two light beams travel in opposite directions along a looped fiber and recombine to produce a phase shift proportional to the rotation rate. The phase difference is directly related to the angular velocity of the rotating object, allowing precise measurement of rotation speed. This approach is highly sensitive and suitable for applications requiring high accuracy and immunity to electromagnetic interference . 2. Fiber Optic Displacement Sensors (FODS) Extrinsic intensity-modulated fiber optic displacement sensors can measure the distance between a sensor probe and a rotating shaft. By monitoring changes in light intensity reflected from the rotating surface, the sensor system can calculate rotational speed. These sensors are non-contact, robust, and can operate over a wide range of speeds, such as 200–3000 RPM for DC motors . 3. Fiber Bragg Grating (FBG) Sensors FBGs are primarily strain sensors but can be adapted for rotational measurements by correlating torsion or twist in a shaft to strain in the fiber. They offer multiplexing capabilities, allowing multiple sensors along a single fiber, and are immune to electromagnetic interference, making them suitable for harsh industrial environments .

Advantages of Fiber Optic Rotation Sensors

• Non-contact measurement reduces wear and maintenance.
• High immunity to EMI/RF interference, ideal for electrically noisy environments.
• Multiplexing capability, enabling multiple sensors on a single fiber.
• Wide operational range, from small laboratory setups to large industrial machinery.
• Compatibility with harsh environments, including high temperature, vibration, and submersion .

Practical Implementation

For controlling rotation speed, fiber optic sensors can be integrated with motor controllers. The sensor output, whether phase shift or intensity variation, is converted into an electrical signal using an opto-electronic interrogation system. This signal can then be fed into a feedback loop to adjust motor speed precisely. For example, in a DC motor setup, the distance between the FODS probe and the shaft is optimized to maximize sensitivity, and the sensor output is validated against a digital tachometer for calibration .

Summary

Fiber optic sensors provide accurate, non-contact, and EMI-immune solutions for measuring and controlling rotation speed. Depending on the application, Sagnac interferometers are ideal for high-precision angular velocity measurements, while intensity-modulated displacement sensors and FBG-based systems offer flexibility, robustness, and multiplexing capabilities for industrial and research applications. These sensors are increasingly used in motor control, robotics, aerospace, and other sectors requiring precise rotational monitoring.