(EFIS: Last updated by Benjamin on February 13, 2024)
The EFIS or Electronic Flight Instrument System combines the indications of the primary flight instruments, at least the artificial horizon, the ball, the turn indicator, the anemometer, the altimeter, the variometer, and the compass, on a single display. Before the widespread use of EFIS and digital technology on aircraft instrument panels, an individual device was dedicated to each function, generally based on analog, electromechanical, or pneumatic technology.
As an EFIS is first and foremost a computer, the list of other indications or functions that can be implemented is not exhaustive: density altitude, true air speed, angle of attack, outside temperature, G-meter, wind speed and direction, voltmeter, clock, autopilot… etc.
AvionicsDuino EFIS general presentation
The primary function of the main EFIS unit is to manage the screen (graphical display of the artificial horizon and all textual data) and make many complex calculations to convert raw sensor data into human-readable format.
For example, compute the aircraft’s speed from static and total pressure measurements. Or calculate the position of the ball based on lateral accelerations. This unit itself contains a very limited number of sensors, namely the two pressure sensors (absolute for static pressure and differential for the Pitot probe) and the inside air temperature sensor.
All other sensors are located in the above-mentioned modules (AHRS and Digital Compass): GNSS, gyrometers, accelerometers, magnetometers, outside air temperature, and relative humidity sensors. Complex computations are also performed in these modules.
EFIS general architecture
It is shown schematically in Figure 1. The main unit is represented in the yellow box, the AHRS in the pink one, and the magnetometer module, with outside air temperature and relative humidity sensors in the green one.
The main unit includes a 5-volt power supply (described elsewhere). It provides the 5 volts required by the other two modules. The modules communicate via the CAN bus or a UART serial channel between the AHRS and the central unit. This UART serial communication will eventually be removed in the near future and replaced by the CAN bus (the double-direction blue arrow will then be removed, and the dotted green arrow will be represented by a solid line).
The heart of the main unit is a PJRC Teensy 4.1 board equipped with an NXP i.MXRT1062 microcontroller (ARM Cortex-M7 @ 600 MHz). See the microcontroller boards page.
The other components are listed below:
- Differential pressure sensor: Analog Microelectronics AMS 5915 0050D to measure the differential pressure between the static port and the Pitot probe and to calculate the speed. The measuring range of this sensor is 0 to 50 hPa, which is suitable for the speed range of the MCR Sportster (0 – 320 km / h).
- Absolute pressure sensor: Analog Microelectronics AMS 5915 1500A to measure static pressure and calculate altitude. The measuring range of this sensor is from 0 to 1500 hPa.
- 1-Wire DS18B20 inside air temperature sensor (IAT).
- Grayhill 62SG optical rotary encoder for menu navigation.
- CAN Bus Transceiver MCP2562EP
- Adafruit RA8875 graphics controller
- 4.3 ″ 480 x 272 TFT LCD display, IPS, 1000 cd / m², Riverdi RVT43HLTFWN00
Printed circuit board
Figures 2 and 3 are 3D views of the printed circuit board in its current 2.1 version (views produced using KiCad). Compared to the previous version 2.0, there is little change. The connector for a GNSS has disappeared.
A GNSS was actually previously incorporated into the central unit of the EFIS. Indeed, before developing our own AHRS, we used an AHRS prototype provided by Naveol. This prototype included a GNSS chip, but its closed software did not allow for full access. Hence, there was a need to integrate a GNSS into the central unit. The GNSS included in the AvionicsDuino AHRS now provides all the data required for the EFIS.
Furthermore, as soon as the EFIS main unit and AHRS software are modified and updated for this purpose, the AHRS-UART connector will become useless because the AHRS will send its data to the CAN bus. It is, therefore, already necessary to plan the connection of the AHRS to the CAN bus.
Figure 4 below shows the electrical diagram of the EFIS main unit. KiCad files can be downloaded below on this page.
The aluminium enclosure
Figures 5 and 6 show the combined case, which contains the central units of the EFIS and EMS. No plans or instructions are provided on this site for making cases. Indeed, one of the characteristics of homebuilt aircraft is the huge diversity of projects. Instrument panels are highly customized, so a case suitable for one will not suit another. Therefore, we believe it is up to the builder to design its enclosures.
We have chosen to make our cases from aluminum to prevent electromagnetic interference (EMI) and electromagnetic compatibility (EMC) issues. 3D-printed enclosures can also be shielded internally or externally with copper or aluminum adhesive tape. Our cases are made of aluminum sheets and angles assembled by screws and rivets. Note in Figure 6, produced during installation on the panel, that there is still a connector for a GPS antenna. Since then, this connector has been dismantled, and its hole has been sealed… with copper adhesive tape.
Flight test results
For the flight tests, a flight data recorder (FDR) was previously developed to rigorously compare the data from our EFIS with those from a commercial EFIS serving as a reference. For our tests, this reference was a Dynon EFIS D10A.
The first COM port of the FDR received the serial output of our EFIS, the second that of the Dynon, and the third the NMEA stream of a u-blox NEO M9N GNSS, allowing a common time-stamping for all the data and the analysis of the GPS tracks.
The videos below were made during one of the many test flights carried out to test the EFIS. They demonstrate the excellent performance of the AvionicsDuino AHRS, which is perfectly suited to the flight mechanics of fixed-wing aircraft. The artificial horizon perfectly follows that of the reference EFIS and the natural horizon in a stable and prolonged manner. The position of the ball is identical on both EFIS. The pressure sensors are perfectly suited to this use. The indicated airspeed and altitude are identical to the Dynon’s.
Compared to these two videos, the latest version of the EFIS software (v2.5) brings some improvements to the presentation of data on the display(fig. 7), as well as support for the different units of speed (Km/h, knots, MPH), pressure (hPa or In Hg) and temperature (°C or °F). Users can choose the units they want by navigating the menus using the rotary encoder. Their choices are saved in the EEPROM, as well as many other parameters (magnetic deviation, UTC time correction, display brightness, latest QNH and QFE, G max, and G min).
Flight data recorder analysis
The graphs below were established with the data from the flight recordings; the x-axis is graduated in seconds. These graphs demonstrate the excellent correlation between the indications of the two EFIS. They validate our different hardware and software choices. You have to hover the mouse cursor over the curves to enlarge them.