Fiber Optic Sensor Rotational Speed ​​Experiment Report

A fiber optic sensor system (FOSS) can measure rotational speed non-invasively with high accuracy using intensity-modulated optical signals and Sagnac interferometry.

Objective

The experiment aims to measure the rotational speed of a DC motor or rotating shaft using a fiber optic sensor system (FOSS), providing a non-contact, high-resolution method for speed detection and comparison with conventional tachometers .

Experimental Setup

• Sensor Type: Fiber optic displacement sensor (FODS), intensity modulation-based extrinsic type .
• Motor Control: DC motor with variable speed (200–3000 RPM) using a chopper motor controller .
• Optical Configuration: Sagnac-type fiber ring interferometer for detecting angular velocity, with counter-propagating light beams in a closed optical path .
• Signal Detection: Silicon PIN photodiode with FET low-noise amplifier; phase modulation introduced via piezoceramic element to obtain analog signals proportional to angular velocity .
• Calibration: Validate FOSS output against a digital tachometer to ensure accuracy .

Procedure

1. Mount the fiber optic sensor probe at an optimized distance from the rotating shaft.
2. Set the motor to a specific speed using the motor controller.
3. Record the optical signal output from the sensor.
4. Apply phase modulation and capture the signal using lock-in amplifiers tuned to the fundamental and second harmonic frequencies .
5. Repeat measurements across the desired speed range (e.g., 200–3000 RPM).
6. Optionally, perform Allan Variance analysis to quantify random errors and bias instability .

Data Analysis

• Convert the optical signal to rotational speed using calibration curves.
• Apply signal filtration to reduce noise and improve measurement reliability .
• Compare FOSS measurements with conventional tachometer readings to evaluate accuracy.
• Plot rotational speed versus time or motor input to visualize performance.
• Calculate statistical parameters such as mean, standard deviation, and error margins.

Results

• Present measured rotational speeds in tabular and graphical form.
• Include error analysis and discussion of sensor sensitivity, dynamic range, and resolution.
• Highlight the advantages of FOSS, such as non-contact measurement, wide dynamic range (up to 170 dB), and high correlation with reference instruments .

Discussion

• Discuss factors affecting measurement accuracy, including fiber length, coil diameter, optical losses, and temperature effects on refractive index .
• Explain the Sagnac effect principle and how it enables detection of angular velocity.
• Compare FOSS performance with traditional methods like stroboscopes or digital tachometers .

Conclusion

• Summarize the effectiveness of fiber optic sensors in rotational speed measurement.
• Emphasize non-contact operation, high sensitivity, and reliability in laboratory and field conditions.
• Suggest improvements, such as temperature compensation, longer fiber coils, or advanced signal processing for enhanced accuracy .

References

• Shrikant M. Maske, "Design and Development of Fiber Optic Sensor System for Rotational Speed Measurement," IJIREEICE .
• Lefevre, "Fiber-Optic Rotation Sensor: Analysis of Effects Limiting Sensitivity," Springer .
• Perez et al., "Rotational motion investigation in seismology – remote sensing by an optical fiber system," Frontiers in Advanced Optical Technologies .