ESP32C3_BLUETOOTH_AUDIO_VARIO

Accurate, zero-lag audio variometer using Kalman filter fusion of accelerometer and pressure sensor data. This project uses the KF4D kalman filter algorithm from the ESP32_IMU_GPS_BARO_VARIO project.

Other features :

WiFi Vario configuration via web page.
WiFi Over-the-air (OTA) firmware updates.
Bluetooth LE transmission of $LK8EX1 sentences, to provide apps like XCTrack with accurate barometric pressure altitude and climb-rate data.
Soft-switched power on/off.
'No activity' time-out power down to conserve battery life.
2-layer PCB sized for standard Hammond 1551K enclosure.

Software Build Environment

Ubuntu 20.04 LTS AMDx64
Visual Studio Code with PlatformIO plugin using Arduino framework
The file platformio.ini specifies the framework packages, toolchain and libraries used by the project.
LittleFS partition for hosting HTML web page content.

Hardware

AI-Thinker ESP-C3-12F C3FN4 module (4MByte flash, Wi-Fi and Bluetooth LE)
CJMCU-117 module with MPU9250 9-DOF IMU and MS5611 barometric pressure sensor
TLV75533 LDO regulator, max current 500mA
KPEG006 broadband-audio piezo transducer
74HC240 used as a push-pull piezo driver for louder volume
MCP73871 Li-poly battery charger, max 500mA charging current
1800mAHr Lipoly battery
Hammond 1551K standard size enclosure (80 x 40 x 20mm).
On Rev B hardware, the USB C interface supports Li-Poly battery charging and program flash/debug. On the Rev A hardware, the USB C interface only supports battery charging. An external USB-UART adapter is required for flash/debug operation.

Current Drain

With Bluetooth LE disabled, current drain is ~30mA.

With Bluetooth LE enabled and transmitting LK8EX1 messages at 10Hz, current drain is ~85mA.

Software Build Notes

Build Steps

Select Rev A or Rev B hardware configuration options in the platformio.ini file.
The first time you flash the ESP32-C3 with this project code, select PROJECT TASKS -> esp32c3 -> Platform -> Erase Flash. This will wipe the entire flash including any previous partition tables.
Next, select Platform -> Build Filesystem Image. This will build a LittleFS flash partition with the contents of the /data directory. The /data directory contains the static HTML and CSS files for the WiFi server webpage.
Next, select Platform -> Upload Filesystem Image. This will flash the LittleFS data partition to the ESP32-C3.
Next, select General -> Clean All, then Build. This is only required once, to download the source code for the required libraries and build them.
If the libraries have already been downloaded and compiled, select General -> Clean to rebuild only the project application code.

Rev A hardware

Since the vario circuitry uses soft-switched power control, it helps to have a debug interface that includes both the serial port pins (RX,TX,GND) as well as the PWR button pins. Connect a slide switch or jumper across the PWR button pins. Ensure the vario is switched off. When you want to flash or erase the ESP32-C3, press and hold the PCCA button while turning the slide switch/jumper on. When done flashing, turn off the slide switch/jumper. Without this switch/jumper, you would have to keep the PWR button pressed for the full duration of the erase & flash process.
Select Build and then Upload and Monitor to build and flash the application firmware binary.
Ensure the serial debug monitor is visible, then reset or power-cycle the ESP32-C3 module.

Rev B hardware

Ensure the vario is switched off. Connect the USB cable from your PC to the vario. Press and hold the PCCA button. Now close the flash/debug jumper or slide switch (JP1 or SW2). Release the PCCA button. You will see the CDC port show up as a new serial port. On Ubuntu it will be /dev/ttyACMx instead of /dev/ttyUSBx. Set this new port as the upload and monitor port in platformio.ini.
Select Build and then Upload to build and flash the application firmware binary.
To see serial debug prints and prompts on boot, open the flash/debug jumper / slide switch (JP1/SW2) to turn off the vario. Select Monitor in VSC and then switch on the vario using the PWR button.
For both Rev A and Rev B hardware : After a full chip erase and firmware upload, there is no calibration data. So you will see a calibration error message. Follow the prompts to calibrate both accelerometer and gyroscope. Once complete, the calibration parameters will be saved to flash.

This is a startup serial monitor log after a full flash erase, i.e. no calibration parameters.

The gyroscope is re-calibrated each time on power-up. You should leave the vario un-disturbed when you hear the count-down beeps for gyroscope calibration. If the vario is disturbed during the gyro calibration process, it will use the last saved gyro calibration parameters.

WiFi Configuration

To put the vario into WiFi AP server mode, switch on the vario and immediately press and hold the PCCA button. When you hear a confirmation tone, release the button.

Connect to the WiFi Access Point Vario-AP, no password needed.

Open the url http://192.168.4.1 in a browser. You can use http://vario.local with any OS that has mDNS support. MacOS has built-in support. For Ubuntu, install Avahi. For Windows, install Bonjour.

Configuration Options

WiFi Credentials

Enter your home WiFi Access Point SSID and password so that the vario can connect to the access point as a station. Then you can just enter the webpage url http://vario.local to access the vario configuration web page.

If the external Access Point is not configured or the configured access point is not available, e.g. on the launch site, the vario will configure itself as a standalone Wifi Access Point with SSID Vario-AP, no password. You can use your phone or tablet to connect first to this access point. Then access the webpage url http://vario.local as before.

Audio Tone configuration

The climb tone threshold should be configured for what you consider the minimum climbrate that you can turn in without losing height. The range is +20cm/s to +100cm/s.

Since a paraglider has a 'hands-up' sink rate of approximately -1m/s in still air, anything more than this indicates rising air. The zero tone is used to indicate air that is rising, but not strong enough for aggressive turns. The range is -20cm/s to +20cm/s.

The sink tone is used to indicate sinking air, not a sinking glider. The range is -400cm/s to -100cm/s.

Note that these thresholds are purely personal choices, depending on how 'talkative' you want the vario to be.

The crossover climbrate is used to adjust the frequency discrimination. The vario allocates 3 octaves of audio bandwidth to climbrates below this threshold, and 1 octave above the threshold. So if the expected average thermal strength is +5m/s, set the threshold to 500.

Kalman Filter configuration

Set the variance parameter lower for conditions with soft, wide and smooth-edged thermals. Set the parameter higher for conditions with strong, narrow and hard-edged thermals.

The adapt parameter will probably be removed after flight trials. Higher values add more uncertainty into the Kalman filter so it can respond to high acceleration transitions quickly, while maintaining a high degree of damping in zero acceleration conditions.

Inactivity Time-out

The vario will power off automatically to save battery life if it does not detect climb/sink rates beyond a minimum threshold within a specified interval (specified in minutes). If you often stand on the launch site for several minutes, hooked-up to the glider and waiting for launch conditions to improve, use a larger time-out.

Bluetooth LK8EX1

If you enable this option, the vario will transmit $LK8EX1 sentences using the Bluetooth LE protocol at 10Hz. The Bluetooth device name is BLE-Vario.

XCDisplay app on Apple iPhone connected to the vario, displaying pressure altitude and climbrate.

Usage

Power On

To power on, press the PWR button and hold (~1 second) until you see the power/bluetooth LED turn on. Release.

The vario will first indicate the battery charge status with a series of long beeps. 5 beeps for fully charged, 1 beep for discharged.

After a pause, the vario will indicate the countdown to gyro calibration with a series of 10 shorter beeps. You should ensure the vario is left undisturbed, in any orientation (do not hold it in your hand). At the end of the countdown and a brief pause, you may hear an initial squawk as the filter initializes, but it should settle down in a few seconds. Now the vario is operational and providing audio feedback.

If you want to force accelerometer re-calibration, press the PCCA button during the gyro calibration countdown. Now the vario will start a 50 short beep countdown at a fast rate. At this point you should place the vario flat on a horizontal surface (confirm with a bubble level if possible) without any disturbance, vibrations etc. Do not hold or press down on the vario case. At the end of the 50 beeps, the accelerometer will be calibrated, and then the countdown to gyro calibration will start.

Power/Bluetooth LED

If Bluetooth transmission is disabled, the power/bluetooth LED will stay on.

If Bluetooth transmission is enabled, the power/bluetooth LED will start blinking once every 2 seconds after transmission starts.

Audio Mute

When the vario has finished initialization and is providing vario audio feedback, a brief press of the PCCA button will toggle the audio on / off.

This is convenient if you have set the zero-tone threshold to a negative value or close to zero and don't want the distraction of a beeping vario while you are hooked in to the glider and waiting to launch.

Power Off

To power off, press the PWR button and hold (~2 seconds) until you hear a confirmation audio tone. If the power/bluetooth LED was on, it will turn off as well. Release.

Credits

Prototype design, component sourcing, PCB assembly and testing in collaboration with Benoit Arnal.

har-in-air/ESP32C3_BLUETOOTH_AUDIO_VARIO