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SUNLU AMS Heater Auto-Vent Controller - Technical Specification

1. System Overview

The SUNLU AMS Heater Auto-Vent Controller is an intelligent automation system designed for 3D printer filament dryers. It monitors AC current consumption to detect heater and fan operation states, automatically controlling a servo-operated vent to optimize humidity removal and temperature management.

1.1 Key Features

  • Automatic Current Monitoring: Real-time AC current sensing with offset compensation
  • Intelligent State Detection: Distinguishes between heater-on, fan-only, and idle states
  • Self-Learning Calibration: Adapts to different dryer models through automated threshold learning
  • Non-Volatile Memory: Stores learned parameters in EEPROM for persistence across power cycles
  • Visual Feedback: Multi-pattern LED status indication
  • User Interface: Three-mode button control (short/long/very-long press)
  • Cooldown Management: 3-minute delay for fan-only operation after heater shutoff

2. Hardware Specifications

2.1 Microcontroller

The controller supports two compatible microcontroller options:

Option 1: Seeed XIAO SAMD21

  • Processor: ARM Cortex-M0+ @ 48MHz
  • Operating Voltage: 3.3V
  • Flash Memory: 256KB
  • SRAM: 32KB
  • I/O Voltage: 3.3V logic levels

Option 2: Seeed XIAO RP2040

  • Processor: Dual ARM Cortex-M0+ @ 133MHz
  • Operating Voltage: 3.3V
  • Flash Memory: 2MB
  • SRAM: 264KB
  • I/O Voltage: 3.3V logic levels

Both microcontrollers use the same pinout and are pin-compatible for this application.

2.2 Current Sensor

  • Model: ACS758 LCB-050B
  • Type: Hall-effect AC/DC current sensor
  • Measurement Range: ±50A
  • Sensitivity: 40mV/A @ 5V supply
  • Output Type: Analog voltage (ratiometric)
  • Bandwidth: DC to 100kHz

2.3 Analog-to-Digital Converter (ADS1115)

  • Model: ADS1115
  • Resolution: 16-bit
  • Interface: I2C (address 0x48)
  • Input Range: ±4.096V (GAIN_ONE configuration)
  • LSB Size: 0.125mV
  • Sample Rate: Configurable, default 128 SPS
  • Channels Used:
    • A0: Current sensor input
    • A1: Servo feedback input

2.4 Servo Motor

  • Type: Feedback-enabled servo
  • Control: PWM signal (standard servo protocol)
  • Position Range: 0° to 180°
  • Feedback: Analog voltage proportional to position
  • Operating Voltage: 5V (separate power supply recommended)

2.5 User Interface

  • LED: Single status indicator

    • Type: Standard LED
    • Configuration: Active low (cathode to GPIO, anode to VCC through resistor)
    • Pin: D10

  • Button: Momentary push button

    • Type: SPST normally-open
    • Configuration: Active low with internal pullup resistor
    • Pin: D3

3. Software Architecture

3.1 Operating Modes

3.1.1 Normal Operation Mode

  • Continuous current monitoring
  • State detection (heater, fan, idle)
  • Automatic vent control
  • LED status indication (solid when active)

3.1.2 Standby Mode

  • System disabled
  • LED off
  • Vent remains in current position
  • Triggered by 2-5 second button press

3.1.3 Learning Mode

  • Three-phase automatic calibration
  • LED pattern guidance for user
  • EEPROM storage of learned thresholds
  • Triggered by 5+ second button press

3.2 Current Measurement Algorithm

3.2.1 Offset Compensation

At startup, the system measures the zero-current baseline for 5 seconds:

voltageOffset = average(ADC_readings) over 5 seconds

3.2.2 RMS Calculation

AC current is measured using Welford's online algorithm for numerical stability:

For each sample over 500ms:
    voltage = ADC_reading - voltageOffset
    Update running mean and variance
RMS_voltage = sqrt(variance)
RMS_current = RMS_voltage / sensitivity

3.2.3 Rolling Average

A 30-second rolling average smooths transient variations:

samples[] = circular buffer of 60 samples (0.5s each)
average_current = mean(samples[])

3.3 State Detection

The system uses hysteresis-based thresholds to prevent chattering:

| State | Entry Condition | Exit Condition | |-------|----------------|----------------| | Heater ON | Current > HEATER_ON_THRESHOLD | Current < HEATER_OFF_THRESHOLD | | Fan ON | Current > FAN_ON_THRESHOLD | Current < FAN_OFF_THRESHOLD | | Idle | Current < FAN_OFF_THRESHOLD | Current > FAN_ON_THRESHOLD |

Default thresholds (before learning):

  • HEATER_ON_THRESHOLD: 200mA
  • HEATER_OFF_THRESHOLD: 120mA
  • FAN_ON_THRESHOLD: 25mA
  • FAN_OFF_THRESHOLD: 20mA

3.4 Vent Control Logic

IF (heater_detected OR fan_detected):
    OPEN vent

IF (heater_off AND fan_detected):
    Keep vent OPEN
    Start cooldown timer

IF (heater_off AND fan_off AND cooldown_expired):
    CLOSE vent after 3 minutes

IF (heater_off AND fan_off AND NOT in_cooldown):
    CLOSE vent immediately

3.5 Learning Mode Algorithm

Phase 1: Baseline Measurement

  1. LED: SOLID (5 second prep time)
  2. User ensures dryer is OFF
  3. LED: FAST FLASH (10 second measurement)
  4. Records: baseline_current = avg(measurements)
  5. LED: SUCCESS BLINK

Phase 2: Heater + Fan Measurement

  1. LED: BREATHING pattern (5 second prep time)
  2. User turns dryer ON (heater + fan)
  3. LED: FAST FLASH (10 second measurement)
  4. Records: heater_current = max(measurements)
  5. LED: SUCCESS BLINK

Phase 3: Fan-Only Measurement

  1. LED: DOUBLE BLINK pattern (5 second prep time)
  2. User turns heater OFF (fan continues)
  3. LED: FAST FLASH (10 second measurement)
  4. Records: fan_current = avg(measurements)
  5. LED: COMPLETE BLINK (3 long flashes)

Threshold Calculation

FAN_ON_THRESHOLD = (baseline + fan_current) / 2 + 2mA margin
FAN_OFF_THRESHOLD = FAN_ON_THRESHOLD - 5mA hysteresis

HEATER_ON_THRESHOLD = (fan_current + heater_current) / 2
HEATER_OFF_THRESHOLD = (baseline + fan_current) / 2 + 5mA margin

Thresholds are saved to EEPROM with magic number validation.

3.6 EEPROM Storage

struct ThresholdData {
    uint32_t magic;  // 0xABCD1234 for validation
    float fanOnThreshold;
    float fanOffThreshold;
    float heaterOnThreshold;
    float heaterOffThreshold;
};

SAMD21: Uses FlashStorage library for EEPROM emulation in flash memory. RP2040: Uses LittleFS or EEPROM emulation library for persistent storage in flash memory.

3.7 LED Patterns

| Pattern | Timing | Meaning | |---------|--------|---------| | SOLID | Continuous on | Normal operation / Phase 1 prep | | OFF | Continuous off | Standby mode | | FAST FLASH | 100ms on/off | Measuring (learning mode) | | SLOW FLASH | 250ms on/off | Calibrating servo | | BREATHING | 2s cycle fade | Phase 2 prep (turn dryer ON) | | DOUBLE BLINK | 2 blinks, pause, repeat | Phase 3 prep (turn heater OFF) | | SUCCESS BLINK | 2 quick flashes | Phase complete | | COMPLETE BLINK | 3 long flashes | Learning complete |

3.8 Button Interface

| Press Duration | Action | Result | |---------------|--------|--------| | < 2 seconds | Short press | Recalibrate servo | | 2-5 seconds | Long press | Toggle standby mode | | > 5 seconds | Very long press | Enter learning mode |

4. Timing Specifications

| Parameter | Value | Notes | |-----------|-------|-------| | ADC Sample Period | 500ms | ~30 AC cycles @ 60Hz | | Rolling Average Window | 30 seconds | 60 samples | | Offset Measurement Time | 5 seconds | At startup | | Learning Phase Duration | 10 seconds | Per phase | | Learning Prep Time | 5 seconds | Between phases | | Fan Cooldown Delay | 3 minutes | After heater turns off | | Servo Settle Time | 1 second | After position change | | Update Display Interval | 5 seconds | Serial output | | Button Debounce | 50ms | Implicit in timing |

5. Memory Usage

SAMD21:

  • Flash: ~57KB / 256KB (22%)
  • SRAM: Estimated ~4KB / 32KB (12%)
  • EEPROM: 20 bytes (threshold storage)

RP2040:

  • Flash: ~57KB / 2MB (<3%)
  • SRAM: Estimated ~4KB / 264KB (<2%)
  • EEPROM: 20 bytes (threshold storage in flash)

6. Communication Interfaces

6.1 I2C Bus

  • Speed: 100kHz (standard mode)
  • Device: ADS1115 @ 0x48

6.2 Serial Console (Debug)

  • Baud Rate: 115200
  • Format: 8N1
  • Purpose: Debug output and manual control commands

6.3 Serial Commands (Debug Mode)

| Command | Action | |---------|--------| | O or o | Open vent | | C or c | Close vent | | R or r | Recalibrate servo | | F or f | Read servo feedback | | S or s | Toggle standby mode | | L or l | Enter learning mode | | 0-180 | Move servo to specific angle |

7. Power Requirements

  • Microcontroller: 3.3V @ ~50mA (active), ~15mA (idle)
  • ADS1115: 3.3V @ ~1mA
  • Servo: 5V @ 100-500mA (load dependent)
  • Total System: 5V @ 200-600mA typical

Recommended: Use separate 5V power supply for servo, common ground with microcontroller.

8. Environmental Specifications

  • Operating Temperature: 0°C to 50°C
  • Storage Temperature: -20°C to 70°C
  • Humidity: 10% to 90% non-condensing
  • Altitude: Up to 2000m

9. Safety Features

  • Offset Compensation: Eliminates DC bias errors
  • Rolling Average: Filters transient spikes
  • Hysteresis: Prevents rapid state changes
  • Cooldown Delay: Protects dryer fan bearing
  • Watchdog: None (consider adding for production)
  • Overcurrent Protection: Relies on ACS758 internal protection

10. Calibration Procedure

10.1 Servo Calibration

Performed automatically at startup:

  1. Move to closed position (180°)
  2. Record feedback voltage
  3. Move to open position (0°)
  4. Record feedback voltage
  5. Create linear mapping between feedback and position

10.2 Current Threshold Learning

See Section 3.5 for detailed algorithm.

11. Error Handling

  • ADS1115 Not Found: System halts with error message
  • Servo Feedback Out of Range: Warning message, continues operation
  • EEPROM Read Invalid: Uses default thresholds

12. Future Enhancements

Potential improvements for future versions:

  1. Watchdog Timer: Auto-reset on system hang
  2. Temperature Sensing: Direct dryer temperature monitoring
  3. WiFi Connectivity: Remote monitoring and control
  4. Data Logging: Historical current and state data
  5. Adjustable Cooldown: User-configurable delay time
  6. Multiple Profiles: Store settings for different dryers
  7. OLED Display: On-device status display
  8. Emergency Stop: Hardware override for safety