Pilot Gauges


The left hand pilot panel of the beech baron 58 aircraft consists of the so-called six-pack and the Radio Magnetic Indicator. I extended this configuration by a Radar Altitude indicator, so we finally have at pilot side eight gauges and I call it my 'eight-pack':

In the virtual cockpit there is also an Instrument Air indicator, which shows the current vacuum pressure in the gyroscopic instuments, like the Artificial Horizon and the Horizontal Situation Indicator. However it was not considered mandatory to implement this Instrument Air indicator, because my implemented indicators are working with the standard environmental pressure.

The above mentioned gauges were built up by using step motors, servos, rotation encoders, micro-controllers, photo-interrupters, 3D printed parts, printed scales, thousands of lines of software code and of course some few hours of spare time. If I remember correctly, it took about 1.5 years to complete these instruments. This is also because some instruments had to be built twice, one for the pilot panel and one for the co-pilot panel.

Some of the gauges are straight forward, because they only have one needle and hence only one step motor. The next more complex gauges are the ones with 2 or even 3 needles, which consequently require some additional mechanical parts like gears. But the most complex gauges were the Aritificial Horizon with the Flight Director followed by the beast of Horizontal Situation Indicator, which almost drove me crazy. Nevertheless, I finished all, sometimes twice, because the co-pilot also wants to fly sometimes.

The following sections shortly describe the functional performance and the design of the eight gauges.

1. Altimeter

The Altimeter gauge is beside the Horizontal Situation Indicator one of the most complex instruments to be implemented, because it has four step motors and one bug to set the air pressure (QNH). Therefore it was decided to design and manufacture this gauge first to learn as much as possible regarding manufacturing the 3D printer parts, assembling and aligning them, calibrating the needles and the QNH plate, as well as to develop the micro controller software.

The altimeter consists of three needles showing the current indicated altitude. The three needles provide information in terms of:

In addition the air pressure setting (QNH) in InHg is shown in a small window at the right side. The actual air pressure can be set by the knob at the lower left side of the gauge in the range from 28.1 to 31.5 InHg. The right pictures show the design of the altimeter in the virtual cockpit and the built design.

The resulting functional design and interfaces of the altimeter gauge controller shows the following features:

In addition, the controller provides two interfaces for debugging and device programming purposes (ISP and RS-232 I/F). The “Main Controller for Gauges” is nearly identical with the nominal Main Controller, however upgraded for the special functional request on motor power supply and LED dimming capability. The software is fully identical with the one of the Main Controllers.

Mechanical Design

To increase the turn speed of the needles with the given step motor it was necessary to include an additional gear train with a ratio of 2:1, hence the turn speed of the needle is doubled. This is necessary, because the step motor alone is not able to turn with such a speed, when you are in a stalk descent, al least for the 100 feet needle. However, I hope to never see the needle to move so fast 😱

For this gear train a dedicated housing has been designed to offer a guidance for the gear axes and a mounting element for final placement on the motor plate. The plastic gears were bought and have a gear module of 1.0, the first gear with 20 teeth, the second one with 10 teeth. The larger gear is mounted on the axis of the motor by first increasing the centre hole to 5 mm, then rasping a notch in the hole of the gear – radially – to allow a proper fixation on the motor axis by finally putting 2K glue into this notch.

The next step is to manufacture the motor axes by 3 mm diameter aluminium, which shall be 85 mm long. The small gear is drilled with a 3 mm drill, which will then fit rigidly on the motor axis without any further fixation (glue or headless screw). This axis then fits into the 3 mm hole in the 3D gear housing, which finally can be closed by a dedicated 3D printed gear cover.

The inner and outer diameter dimensions in mm of the 4 central axes parts are shown below and are made out of rigid aluminium round material for the 100 needle axis and out of brass pipes for the other three needle axes.

The front part of the altimeter gauge consists of the following parts:

All four parts are mounted together by four 2 mm screws from the bezel front. Before assembling, the two LEDs and cables had to be mounted in the needle chamber. The gauge needle scale, designed by Power Point and printed with an inkjet printer on glossy photo paper, is glued into the needle chamber. The same was done for the QNH scale, which is glued on the QNH plate.

Electrical Design

The electrical design is straight forward and consists basically of an ATmega32 controller and two step motor driver ICs of type ULN2803. The schematics of the entire board is shown below. The upper left four connectors JP1 to JP4 serve the infrared barriers, which determine the exact position of the needles and the plate relative to the scales of the gauge. The bottom SV6 connector serves the rotation encoder for setting the required QNH, while the upper right JP8 connector is the In-System Programming interface (ISP).

2. Vertical Speed, Indicated Airspeed and Radar Altimeter

These three gauges are the most simple ones, because they only have one needle and are providing the following information:

None of the indicators have any knobs for setting specific information, also not the radar altitude indicator, where a dedicated knob for setting the usual decision height was omitted. Also the indicated air speed indicator has no knob for setting the outside temperature, which typically enables the display for the true air speed. The reason for this is that FSX Simconnect does not provide this specific information. All three indicators are shown below, in the top row the ones within the virtual FSX cockpit and in the bottom row the ones I built.

Because we are only using 3 step motors for the three indicators, it was decided to drive the three gauges by one common board, called VS-AS-RA controller. The resulting functional design and interfaces of the gauge controller is shown below, having the following features:

In addition, the controller provides two interfaces for debugging and programming purposes (ISP and RS-232 I/F). The “Main Controller for Gauges” is nearly identical with the nominal Main Controller, however upgraded for the special functional request on motor power supply and LED dimming capability. The software is fully identical with the one of the Main Controller. It shall be noted that up to 6 sub-controllers can be connected to the “Main Controller for Gauges”.

Mechanical Design

As for all other gauges already described, most of the parts are manufactured by a 3D printer. The scales of the indicators are designed with Power Point. Each of the three gauges consists of the following major parts, as shown in the right and lower picture for the vertical speed indicator:

Electrical Design

The VS-AS-RA controller is built around an Atmel ATmega32 micro-processor, running with a clock frequency of 10 MHz, because only such a device offers enough output ports to control the three step motors. The step motor output lines are buffered with current drivers of type ULN2803, featuring eight individual drivers - two of these driver ICs are necessary for three step motors. In the software design it has to be considered, that these drivers are inverters. The step motor connectors are SV2, SV3 and SV4. These 6-wire connector ribbon cables are soldered to the coloured wires of the step motor.

The board layout is shown in the right picture, has a size of approximately 84 x 84 mm and is equipped with 12 connectors. It has a red bottom and a blue top layer. I have not managed so far to manufacture double sided boards, because both layers must be aligned with an accuracy of better than 0.1 mm. I therefore did the top layer by hand wiring, because there are only a few routings to implement. Of course, such a makeshift can only be done, if it is a single-unit production.

Needle Calibration

The connectors JP1, JP2 and JP3 are the interfaces to the Infrared (IR) barriers, which are necessary to calibrate the needles of the three indicators. The external wiring for these IR barriers is shown below. The IR emitting diode is pulling a current of roughly 16 mA by the board internal 270 Ω resistor, while the IR photo transistor collector board internal resistor has a high value of 220 KΩ.

The needle calibration procedure is performed in the following way:

This procedure works very precise, because the controller can determine the 'zero' point from one to the other step of the motor, having a pointing resolution of better than 0.1 degree. 

Gauge Illumination

All three gauges have a built-in pair of warm white 3 mm LEDs (2950 K) to allow for dawn-dusk and night flights. The brightness of the LEDs is automatically set by FSX, depending at which time we are flying. In addition, the brightness can be adjusted by the pilot. This is accomplished by an external dimming control voltage, to be applied to connector JP4, as shown in the most right picture. Pin 3 of this connector is board internally routed to the ADC port PA4, which measures the applied external voltage. It basically pulls no current. The external voltage is supplied by a linear potentiometer located within the pedestal panel.  All shown 4-pin connectors are encoded (black dot) to avoid a wrong plug-in of the female connector, i.e. pin 4 is disabled by filling it with glue.

Because of using a linear potentiometer, the brightness of the LEDs will be corrected by software within the sub-controller by the Weber-Fechner law, which is basically an exponential function. This means that the user recognised brightness is proportional to the turn angle of the potentiometer. This feature is implemented by a pulse width modulation (PWM) signal applied to the LEDs. The PWM coefficients are calculated along the formula, as shown right. The resulting 256 PWM coefficients are stored in the EEMEM area of the micro-processor, which avoids to always perform the time consuming computations.


3. Horizontal Situation Indicator

The Beast is ready and it really Works !

This is the most complex gauge I ever built, because it has so many functional elements, that sometimes I thought I was losing track. All functional elements of the Horizontal Situation Indicator (HSI) are basically comparable with the ones of the original King / Bendix gauge. The only exception is the Glide Slope Indicator, which was not realised by a moving yellow indicator at the left and right side of the gauge, but with two yellow LED bar graphs, similar to those found in audio equipment, because the associated mechanisms would have been far too complicated and prone. Although this complicates the electronics, but it will definitively not fail within the next 50 years.

4. Artificial Horizon

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5. Turn Coordinator

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6. Radio Magnetic Indicator

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