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Ventway

Bare metal STM32F407 ventilator breathing cycle simulator, inspired by the Inovytec Ventway Sparrow.

No HAL — pure register-level C.

How it maps to a real ventilator

A mechanical ventilator delivers air in a repeating Inhale → Hold → Exhale cycle. The Ventway Sparrow uses Pressure Control Ventilation (PCV) — the turbine targets a constant airway pressure, and a PID controller adjusts motor speed in real time.

Phase Duration Pressure target What happens
INHALE 1.0 s 20 cmH₂O PID drives turbine to reach inspiratory pressure. Flow is highest at start (empty lungs, large pressure gap) and decelerates as lungs fill — the classic PCV waveform.
HOLD 0.5 s 20 cmH₂O PID holds plateau pressure for gas exchange. Turbine effort is minimal — just compensating for leaks.
EXHALE 1.5 s 5 cmH₂O (PEEP) Passive exhalation driven by lung elastic recoil. PEEP valve maintains positive end-expiratory pressure to prevent alveolar collapse.

This gives a 3-second cycle = 20 breaths per minute, a typical adult setting.

Closed-loop control

Real ventilators don't use precalculated duty curves — they use closed-loop feedback. This simulator models that with:

  1. Simulated lung — a first-order single-compartment model with tunable compliance (mL/cmH₂O) and airway resistance (cmH₂O/(L/s)). Turbine duty maps to airflow, airflow integrates into volume, and volume/compliance gives elastic pressure.

  2. PID controller — reads simulated airway pressure, compares to the per-state target, and outputs a duty percentage to the PWM. Includes anti-windup (back-calculation) and derivative-on-measurement to avoid setpoint kicks.

  3. PEEP valve model — clamps lung volume so pressure never drops below the exhale target, matching a real mechanical PEEP valve.

The decelerating inspiratory flow pattern, plateau hold, and passive PEEP-limited exhale all emerge naturally from the controller chasing pressure targets into the lung model — no hand-tuned waveforms.

Physiological defaults

Parameter Default Typical clinical range Unit
Lung compliance 50 30–80 mL/cmH₂O
Airway resistance 5 5–20 cmH₂O/(L/s)
Inspiratory pressure 20 10–35 cmH₂O
PEEP 5 3–10 cmH₂O
Turbine flow gain 10 — (mL/s) / %

Integer math

All arithmetic uses Q16.16 fixed-point (no FPU). The Cortex-M4 SMULL instruction handles the 32×32→64 multiply; division happens once per tick at 100 Hz.

The PWM output on TIM3 CH1 (PA6) represents the turbine motor drive signal. In a real device this would feed a BLDC motor controller.

Peripherals used

Peripheral Purpose
TIM2 10 ms tick interrupt — drives state machine
TIM3 CH1 1 kHz PWM output — simulated turbine motor
USART2 115200 baud TX — state/cycle logging
GPIOA Alternate function pins (PA2, PA6)

Building

make

Requires arm-none-eabi-gcc. Produces ventway.elf and ventway.bin.

Running in Renode

make renode

Or manually:

renode ventway.resc

UART2 output appears in the Renode analyzer window:

Ventway starting
[cycle 1] INHALE — target 20 cmH2O, P=0.0, duty 100%
[cycle 1] HOLD — target 20 cmH2O, P=19.8, duty 12%
[cycle 1] EXHALE — target 5 cmH2O, P=20.1, duty 0%
[cycle 2] INHALE — target 20 cmH2O, P=5.2, duty 85%
...

UART commands

All parameters are tunable at runtime over UART (115200 baud, send with \r or \n):

Command Example Effect
status status Print all current parameters
<state> <ms> inhale 1200 Set state duration (must be multiple of 10ms)
target <state> <cmH2O> target inhale 25 Set pressure target
compliance <value> compliance 30 Set lung compliance (mL/cmH₂O)
resistance <value> resistance 8 Set airway resistance (cmH₂O/(L/s))
kp <tenths> kp 30 Set Kp = 3.0 (value ÷ 10)
ki <tenths> ki 10 Set Ki = 1.0 (value ÷ 10)
kd <tenths> kd 1 Set Kd = 0.1 (value ÷ 10)

Project structure

ventway.h    — types, context struct, fixed-point macros, API declarations
ventway.c    — state machine, lung model, PID controller, ring buffers, commands
main.c       — peripheral init, ISRs, main loop
startup.c    — vector table, Reset_Handler, .data/.bss init
linker.ld    — STM32F407VG memory layout (1MB flash, 128KB SRAM)
ventway.resc — Renode emulation script
Makefile     — build with arm-none-eabi-gcc