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Signal Flow and Data Pipeline

ADC acquisition cycle (per 0.1 s tick)

  1. ADC.acquisition_loop() runs in its own QThread.
  2. time.sleep(self.sampling_time) (default 0.1 s).
  3. collect_data() reads N channels via aio.analog_read_volt(channel, datarate, gain). The PCA9554 mux is reconfigured only when the channel range demands it.
  4. Raw row appended to adc_values DataFrame; converted row appended to converted_values.
  5. If a plasma-current setpoint is non-zero: plasma_current_control() runs simple_pid.PID(0.3, 0.1, 0) against Ip and emits send_control_voltage → MCP4725.
  6. Every STEP ticks: send_processed_data_to_main_thread() emits data_ready([dataframe, device_name]). MainApp.on_worker_step routes to _adc_step, appends to self.datadict["ADC"], calls save_data (CSV append), triggers plot update.

STEP batching

# controlunit/devices/adc.py
if step % (self.STEP - 1) == 0 and step != 0:
    self.send_processed_data_to_main_thread()
    step = 0
else:
    step += 1

STEP serves two purposes simultaneously: averages noisy ADC samples and amortises Qt signal-emission overhead. One parameter, two jobs.

STEP is set dynamically by sampling rate:

# controlunit/devices/device.py
def set_sampling_time(self, sampling_time):
    if sampling_time >= 0.9:  self.STEP = 1
    if sampling_time < 0.9:   self.STEP = 3
    if sampling_time < 0.1:   self.STEP = 5

Plasma current PID

Live. Uses simple_pid.

  • p=0.3, i=0.1, d=0, output_limits=(0, 4500) mV.
  • Setpoint from GUI; feedback from Hall-effect sensor on channel 0.
  • Actuator: MCP4725 DAC behind galvanic I²C isolator (Apr 2026).
  • baseline = 1000 mV — empirical minimum for plasma ignition (Kawabata-kun).

Membrane heater PID

Dormant. Code exists in MAX6675.temperature_control(). Hand-rolled: integral-clamped (if integral < -0.5: integral = 0), asymmetric (only positive e drives output). Gains Kp=3.5, Ki=0.06, Kd=0. Output is on-time fraction of a 10 ms cycle, software-PWMed against an SSR.

The measurement and PID have migrated to a Windows machine with NI hardware. Not returning to the Pi; if it returns at all, it will be a dedicated ESP32 unit reporting to an orchestrator.

GPIO sync signal

GPIO 26 emits a digital sync edge consumed by various external data loggers (QMS and others). The front-panel LED is a visual indicator only — not an isolation strategy, not opto-isolated triggering. The shared GPIO was always a wiring convenience.

QMS_signal in the CSV is a boolean column logging whether the sync trigger is currently active during each ADC row.

Safety stop semantics

# controlunit/main.py
def turn_off_voltages(self):
    self.workers["ADC"]["worker"].set_plasma_current.emit(0)
    self.workers["PlasmaCurrent"]["worker"].output_voltage_signal.emit(0)
    self._mfc_presets = {1: 0, 2: 0}
    self.update_current_values()
    self.workers["MFCs"]["worker"].output_voltage_signal.emit(1, 0)
    self.workers["MFCs"]["worker"].output_voltage_signal.emit(2, 0)

abort_all_threads → turn_off_voltages first, thread termination second. Plasma setpoint goes to zero before any thread dies.

Logging

Two parallel append-only paths:

  1. CSV at ~/work/cudata/cu_<YYYYMMDD_HHMMSS>.csv with a self-describing comment header. Header embeds enough channel metadata that an old CSV can be replayed without settings.yml.
  2. Event log at ~/work/cudata/controlunit.log.

Every ADC row carries commanded presets alongside measured signals: PresetV_mfc1, PresetV_mfc2, PresetV_cathode, IGmode, IGscale, QMS_signal.