Background
Inhalimed is developing a portable inhalation device. An ultrasonic piezo element turns liquid medication into a fine mist, and the user operates the device through a graphical display.
The challenge
A piezo element produces mist only when driven exactly at its mechanical resonant frequency, around 173.6 kHz. The problem is that this frequency is not fixed. It varies from part to part, and it drifts by about 160 Hz as the element heats up during treatment. A controller that locks to a fixed frequency loses resonance within minutes.
What we built
- Sweep and lock: the firmware sweeps a frequency range, measures the current at each point and identifies the resonance peak, including interpolation between measurement points for higher accuracy.
- Real-time tracking: a tracking algorithm that holds the frequency within ±25 Hz of the peak and follows it throughout the treatment as the element heats up.
- Hardware: tuning the resonant circuit and drive coils, and locating heat sources in the switching circuit.
- User interface: a graphical display based on LVGL on an ESP32-S3.
The result
A device that produces a stable mist through a full treatment and follows the frequency drift caused by heat without losing resonance. This is a classic hardware-firmware problem: the solution required both changes in the drive circuit and a tracking algorithm in the code, designed by the same engineer.
