Recently released INAV 1.7.2 introduced new hardware support: analog airspeed sensors. I've written new but it is new only to INAV. Analog airspeed sensors aka APM Pitot Tubes aka Ardupilot Airspeed Meter and one the market for years. And they are cheap. By cheap, I mean below $30. And until recently they were usable only in APM world.

Pitot Tube for INAV

MPXV7002 differential pressure sensor

APM Pitot Airspeed Sensor for INAV

Hardware

The hardware we are interested in, are Pitot tubes based on MPXV7002 differential pressure sensor.

Those are 5V devices. All INAV flight controllers are 3.3V devices and that creates a small problem. 3.3V analog to digital converters cannot be used to measure 5V signal.

MPXV7002 Pins

To overcome this problem, a 1:2 voltage divider will be required. You will need two resistors with the same resistance. Anything from 1kOhm to 1MOhm will do. It is really up to you. I used 100kOhm resistors and it worked fine.

APM Pitot voltage divider

INAV already expects 1:2 voltage divider is used, so you do not need to set anything else.

MPXV7002 APM pitot tube voltage divider

Software

Before connecting the airspeed sensor, you also have to find a free ADC converter. Most boards have 3 analog-to-digital converters: Vbat, Current and RSSI.

  • Vbat is not a good choice. Even if it is not used (rare case), it has its own voltage divider that would break measurements
  • Current is our first candidate
  • RSSI is the second one

Since most of my airplanes have current meters, I use a RSSI channel. To prepare APM Airspeed Sensor to use RSSI ADC channel, goto CLI and:

  1. get adc_channel and note channel assigned to RSSI pin. In my case it was channel 3
  2. set rssi_adc_channel=0 - disable RSSI ADC
  3. set airspeed_adc_channel=3 - map ADC channel 3 to airspeed sensor
  4. set pitot_hardware=ADC - enable analog pitot tube
  5. save

Steps 4 and 5 can also be done via Configuration tab in Configurator, but other have to be performed in CLI.

Usefulness

In the latest official release of INAV: 1.7.2 Pitot tube usefulness is somehow limited: airspeed can only be saved in Blackbox logs. Also, a list of boards supporting to function is short: Airbot F4, AirHeroF3, KFC32F3, Omnibus F4 and OpenPilot Revolution.

But, next release (INAV 1.8 or 1.7.3) will:

  1. Enable analog airspeed sensor on more boards
  2. Display current airspeed in OSD (only boards with build in OSD)
  3. Perhaps even closed loop speed control will be added, but this is absolutely not confirmed yet

APM airspeed sensor installed on flying wing