Instrumentation: Flexible Temperature Sensor for Voice Coils

Publication: Journal of the Audio Engineering Society (JAES), September 2024.
Partners: LAUM (Université du Mans), CNRS.

The Challenge: Measuring in Extreme Environments

Operating voice coils present a hostile environment for instrumentation:

• Space Constraints: The magnetic gap is narrow, leaving no room for bulky sensors.
• Magnetic Fields: High DC flux can degrade unshielded measurement signals.
• High Displacement: The assembly must withstand excursions up to $\pm 30~mm$.
• Mass Distribution: Added mass must be balanced to prevent rocking modes.

The Solution: Direct RTD Integration

We developed an ultra-thin RTD sensor using $18 \mu m$ rolled annealed copper on a Kapton former. This setup simplifies the acquisition chain: a constant $4~mA$ current source and a single ADC channel replace the complex dual-ADC and Hilbert transform requirements of indirect methods.

1. Fabrication

18µm RTD circuit on flat Kapton

2. Integration

Insertion into the voice coil

3. Hardware

4mA source & 1-ADC acquisition

4. Metrology

Direct vs. Indirect sensing validation

Experimental Validation: Air, Helium, and Vacuum

To validate the sensor’s precision and transient response, we altered the thermal conductivity of the medium without changing the motor’s mechanical properties:

• Helium: Since Helium’s thermal conductivity is significantly higher than air ($0.153~W/mK$ vs $0.024~W/mK$), the peak temperature was halved and cooling transients were much faster.
• Vacuum (100 mBar): Tests in a partial vacuum resulted in higher peak temperatures and slower cooling, confirming the sensor’s sensitivity to subtle changes in the thermal environment.

Thermal Analysis

Comparative response: Air vs. Helium vs. Vacuum

Engineering Expertise

• Metrology in constrained environments (high flux, tight clearances).
• Thermal Transfer Physics (characterization through medium variation).
• Hardware Simplification (Reduced ADC count and DSP overhead).

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