During the Apollo flights to the Moon, the astronauts observed the spacecraft's orientation on a special instrument called the FDAI (Flight Director / Attitude Indicator). This instrument showed the spacecraft's attitude—its orientation—by rotating a ball. This ball was nicknamed the "8-ball" because it was black (albeit only on one side). The instrument also acted as a flight director, using three yellow needles to indicate how the astronauts should maneuver the spacecraft. Three more pointers showed how fast the spacecraft was rotating.

Since the spacecraft rotates along three axes (roll, pitch, and yaw), the ball also rotates along three axes. It's not obvious how the ball can rotate to an arbitrary orientation while remaining attached. In this article, I look inside an FDAI from Apollo that was repurposed for a Space Shuttle simulator1 and explain how it operates. (Spoiler: the ball mechanism is firmly attached at the "equator" and rotates in two axes. What you see is two hollow shells around the ball mechanism that spin around the third axis.)

The FDAI in Apollo

For the missions to the Moon, the Lunar Module had two FDAIs, as shown below: one on the left for the Commander (Neil Armstrong in Apollo 11) and one on the right for the Lunar Module Pilot (Buzz Aldrin in Apollo 11). With their size and central positions, the FDAIs dominate the instrument panel, a sign of their importance. (The Command Module for Apollo also had two FDAIs, but with a different design; I won't discuss them here.2)

Each Lunar Module FDAI could display inputs from multiple sources, selected by switches on the panel.3 The ball could display attitude from either the Inertial Measurement Unit or from the backup Abort Guidance System, selected by the "ATTITUDE MON" toggle switch next to either FDAI. The pitch attitude could also be supplied by an electromechanical unit called ORDEAL (Orbital Rate Display Earth And Lunar) that simulates a circular orbit. The error indications came from the Apollo Guidance Computer, the Abort Guidance System, the landing radar, or the rendezvous radar (controlled by the "RATE/ERROR MON" switches). The pitch, roll, and yaw rate displays were driven by the Rate Gyro Assembly (RGA). The rate indications were scaled by a switch below the FDAI, selecting 25°/sec or 5°/sec.

The FDAI mechanism

The ball inside the indicator shows rotation around three axes. I'll first explain these axes in the context of an aircraft, since the axes of a spacecraft are more arbitrary.4 The roll axis indicates the aircraft's angle if it rolls side-to-side along its axis of flight, raising one wing and lowering the other. Thus, the indicator shows the tilt of the horizon as the aircraft rolls. The pitch axis indicates the aircraft's angle if it pitches up or down, with the indicator showing the horizon moving down or up in response. Finally, the yaw axis indicates the compass direction that the aircraft is heading, changing as the aircraft turns left or right. (A typical aircraft attitude indicator omits yaw.)

I'll illustrate how the FDAI rotates the ball in three axes, using an orange as an example. Imagine pinching the horizontal axis between two fingers with your arm extended. Rotating your arm will roll the ball counter-clockwise or clockwise (red arrow). In the FDAI, this rotation is accomplished by a motor turning the frame that holds the ball. For pitch, the ball rotates forward or backward around the horizontal axis (yellow arrow). The FDAI has a motor inside the ball to produce this rotation. Yaw is a bit more difficult to envision: imagine hemisphere-shaped shells attached to the top and bottom shafts. When a motor rotates these shells (green arrow), the hemispheres will rotate, even though the ball mechanism (the orange) remains stationary.

The diagram below shows the mechanism inside the FDAI. The indicator uses three motors to move the ball. The roll motor is attached to the FDAI's frame, while the pitch and yaw motors are inside the ball. The roll motor rotates the roll gimbal through gears, causing the ball to rotate clockwise or counterclockwise. The roll gimbal is attached to the ball mechanism at two points along the "equator"; these two points define the pitch axis. Numerous wires on the roll gimbal enter the ball along the pitch axis. The roll control transformer provides position feedback, as will be explained below.

Removing the hemispherical shells reveals the mechanism inside the ball. When the roll gimbal is rotated, this mechanism rotates with it. The pitch motor causes the ball mechanism to rotate around the pitch axis. The yaw motor and control transformer are not visible in this photo; they are behind the pitch components, oriented perpendicularly. The yaw motor turns the vertical shaft, with the two hemisphere shells attached to the top and bottom of the shaft. Thus, the yaw motor rotates the ball shells around the yaw axis, while the mechanism itself remains stationary. The control transformers for pitch and yaw provide position feedback.

Why doesn't the wiring get tangled up as the ball rotates? The solution is two sets of slip rings to implement the electrical connections. The photo below shows the first slip ring assembly, which handles rotation around the roll axis. These slip rings connect the stationary part of the FDAI to the rotating roll gimbal. The vertical metal brushes are stationary; there are 23 pairs of brushes, one for each connection to the ball mechanism. Each pair of brushes contacts one metal ring on the striped shaft, maintaining contact as the shaft rotates. Inside the shaft, 23 wires connect the circular metal contacts to the roll gimbal.

A second set of slip rings inside the ball handles rotation around the pitch axis. These rings provide the electrical connection between the wiring on the roll gimbal and the ball mechanism. The yaw axis does not use slip rings since only the hemisphere shells rotate around the yaw axis; no wires are involved.

Synchros and the servo loop

In this section, I'll explain how the FDAI is controlled by synchros and servo loops. In the 1950s and 1960s, the standard technique for transmitting a rotational signal electrically was through a synchro. Synchros were used for everything from rotating an instrument indicator in avionics to rotating the gun on a navy battleship. A synchro produces an output that depends on the shaft's rotational position, and transmits this output signal on three wires. If you connect these wires to a second synchro, you can use the first synchro to control the second one: the shaft of the second synchro will rotate to the same angle as the first shaft. Thus, synchros are a convenient way to send a control signal electrically.

The photo below shows a typical synchro, with the input shaft on the top and five wires at the bottom: two for power and three for the output.

Internally, the synchro has a rotating winding called the rotor that is driven with 400 Hz AC. Three fixed stator windings provide the three AC output signals. As the shaft rotates, the voltages of the output signals change, indicating the angle. (A synchro resembles a transformer with three variable secondary windings.) If two connected synchros have different angles, the magnetic fields create a torque that rotates the shafts into alignment.

The downside of synchros is that they don't produce a lot of torque. The solution is to use a more powerful motor, controlled by the synchro and a feedback loop called a servo loop. The servo loop drives the motor in the appropriate direction to eliminate the error between the desired position and the current position.

The diagram below shows how the servo loop is constructed from a combination of electronics and mechanical components. The goal is to rotate the output shaft to an angle that exactly matches the input angle, specified by the three synchro wires. The control transformer compares the input angle and the output shaft position, producing an error signal. The amplifier uses this error signal to drive the motor in the appropriate direction until the error signal drops to zero. To improve the dynamic response of the servo loop, the tachometer signal is used as a negative feedback voltage. The feedback slows the motor as the system gets closer to the right position, so the motor doesn't overshoot the position and oscillate. (This is sort of like a PID controller.)

A control transformer is similar to a synchro in appearance and construction, but the rotating shaft operates as an input, not the output. In a control transformer, the three stator windings receive the inputs and the rotor winding provides the error output. If the rotor angle of the synchro transmitter and control transformer are the same, the signals cancel out and there is no error voltage. But as the difference between the two shaft angles increases, the rotor winding produces an error signal. The phase of the error signal indicates the direction of the error.

In the FDAI, the motor is a special motor/tachometer, a device that was often used in avionics servo loops. This motor is more complicated than a regular electric motor. The motor is powered by 115 volts AC at 400 hertz, but this won't spin the motor on its own. The motor also has two low-voltage control windings. Energizing the control windings with the proper phase causes the motor to spin in one direction or the other. The motor/tachometer unit also contains a tachometer to measure its speed for the feedback loop. The tachometer is driven by another 115-volt AC winding and generates a low-voltage AC signal that is proportional to the motor's rotational speed.

The photo above shows a motor/tachometer with the rotor removed. The unit has many wires because of its multiple windings. The rotor has two drums. The drum on the left, with the spiral stripes, is for the motor. This drum is a "squirrel-cage rotor", which spins due to induced currents. (There are no electrical connections to the rotor; the drums interact with the windings through magnetic fields.) The drum on the right is the tachometer rotor; it induces a signal in the output winding proportional to the speed due to eddy currents. The tachometer signal is at 400 Hz like the driving signal, either in phase or 180º out of phase, depending on the direction of rotation. For more information on how a motor/tachometer works, see my teardown.

The amplifiers

The FDAI has three servo loops—one for each axis—and each servo loop has a separate control transformer, motor, and amplifier. The photo below shows one of the three amplifier boards. The construction is unusual and somewhat chaotic, with some components stacked on top of others to save space. Some of the component leads are long and protected with clear plastic sleeves.5 The cylindrical pulse transformer in the middle has five colorful wires coming out of it. At the left are the two transistors that drive the motor's control windings, with two capacitors between them. The transistors are mounted on a heat sink that is screwed down to the case of the amplifier assembly for cooling. Each amplifier is connected to the FDAI through seven wires with pins that plug into the sockets on the right of the board.6

The function of the board is to amplify the error signal so the motor rotates in the appropriate direction. The amplifier also uses the tachometer output from the motor unit to slow the motor as the error signal decreases, preventing overshoot. The inputs to the amplifier are 400 hertz AC signals, with the magnitude indicating the amount of error or speed and the phase indicating the direction. The two outputs from the amplifier drive the two control windings of the motor, determining which direction the motor rotates.

The schematic for the amplifier board is below. 7 The two transistors on the left amplify the error and tachometer signals, driving the pulse transformer. The outputs of the pulse transformer will have opposite phases, driving the output transistors for opposite halves of the 400 Hz cycle. This activates the motor control winding, causing the motor to spin in the desired direction.8

History of the FDAI

Bill Lear, born in 1902, was a prolific inventor with over 150 patents, creating everything from the 8-track tape to the Learjet, the iconic private plane of the 1960s. He created multiple companies in the 1920s as well as inventing one of the first car radios for Motorola before starting Lear Avionics, a company that specialized in aerospace instruments.9 Lear produced innovative aircraft instruments and flight control systems such as the F-5 automatic pilot, which received a trophy as the "greatest aviation achievement in America" for 1950.

Bill Lear went on to solve an indicator problem for the Air Force: the supersonic F-102 Delta Dagger interceptor (1953) could climb at steep angles, but existing attitude indicators could not handle nearly vertical flight. Lear developed a remote two-gyro platform that drove the cockpit indicator while avoiding "gimbal lock" during vertical flight. For the experimental X-15 rocket-powered aircraft (1959), Lear improved this indicator to handle three axes: roll, pitch, and yaw.

Meanwhile, the Siegler Corporation started in 1950 to manufacture space heaters for homes. A few years later, Siegler was acquired by John Brooks, an entrepreneur who was enthusiastic about acquisitions. In 1961, Lear Avionics became his latest acquisition, and the merged company was called Lear Siegler Incorporated, often known as LSI. (Older programmers may know Lear Siegler through the ADM-3A, an inexpensive video display terminal from 1976 that housed the display and keyboard in a stylish white case.)

The X-15's attitude indicator became the basis of the indicator for the F-4 fighter plane (the ARU/11-A). Then, after "a minimum of modification", the attitude-director indicator was used in the Gemini space program. In total, Lear Siegler provided 11 instruments in the Gemini instrument panel, with the attitude director the most important. Next, Gemini's indicator was modified to become the FDAI (flight director-attitude indicator) in the Lunar Module for Apollo.10 Lear Siegler provided numerous components for the Apollo program, from a directional gyro for the Lunar Rover to the electroluminescent display for the Apollo Guidance Computer's Display/Keyboard (DSKY).

In 1974, Lear Siegler obtained a contract to develop the Attitude-Director Indicator (ADI) for the Space Shuttle, producing a dozen ADI units for the Space Shuttle. However, by this time, Lear Siegler was losing enthusiasm for low-volume space avionics. The Instrument Division president said that "the business that we were in was an engineering business and engineers love a challenge." However, manufacturing refused to deal with the special procedures required for space manufacturing, so the Shuttle units were built by the engineering department. Lear Siegler didn't bid on later Space Shuttle avionics and the Shuttle ADI became its last space product. In the early 2000s, the Space Shuttle's instruments were upgraded to a "glass cockpit" with 11 flat-panel displays known as the Multi-function Electronic Display System (MEDS). The MEDS was produced by Lear Siegler's long-term competitor, Honeywell.

Getting back to Bill Lear, he wanted to manufacture aircraft, not just aircraft instruments, so he created the Learjet, the first mass-produced business jet. The first Learjet flew in 1963, with over 3000 eventually delivered. In the early 1970s, Lear designed a steam turbine automobile engine. Rather than water, the turbine used a secret fluorinated hydrocarbon called "Learium". Lear had visions of thousands of low-pollution "Learmobiles", but the engine failed to catch on. Lear had been on the verge of bankruptcy in the 1960s; one of his VPs explained that "the great creative minds can't be bothered with withholding taxes and investment credits and all this crap". But by the time of his death in 1978, Lear had a fortune estimated at $75 million.

Comparing the ARU/11-A and the FDAI

Looking inside our FDAI sheds more details on the evolution of Lear Siegler's attitude directors. The photo below compares the Apollo FDAI (top) to the earlier ARU/11-A used in the F-4 aircraft (bottom). While the basic mechanism and the electronic amplifiers are the same between the two indicators, there are also substantial changes.

The biggest difference between the ARU-11/A indicator and the FDAI is that the electronics for the ARU-11/A are in a separate module that was plugged into the back of the indicator, while the FDAI includes the electronics internally, with boards mounted on the instrument frame. Specifically, the ARU-11/A has a separate unit containing a multi-winding transformer, a power supply board, and three amplifier boards (one for each axis), while the FDAI contains these components internally. The amplifier boards in the ARU-11/A and the FDAI are identical, constructed from germanium transistors rather than silicon.11 The unusual 11-pin transformers are also the same. However, the power supply boards are different, probably because the boards also contain scaling resistors that vary between the units.12 The power supply boards are also different shapes to fit the available space.

The ball assemblies of the ARU/11-A and the FDAI are almost the same, with the same motor assemblies and slip ring mechanism. The gearing has minor changes. In particular, the FDAI has two plastic gears, while the ARU/11-A uses exclusively metal gears.

The ARU/11-A has a patented pitch trim feature that was mostly—but not entirely—removed from the Apollo FDAI. The motivation for this feature is that an aircraft in level flight will be pitched up a few degrees, the "angle of attack". It is desirable for the attitude indicator to show the aircraft as horizontal, so a pitch trim knob allows the angle of attack to be canceled out on the display. The problem is that if you fly your fighter plane vertically, you want the indicator to show precisely vertical flight, rather than applying the pitch trim adjustment. The solution in the ARU-11/A is a special 8-zone potentiometer on the pitch axis that will apply the pitch trim adjustment in level flight but not in vertical flight, while providing a smooth transition between the regions. This special potentiometer is mounted inside the ball of the ARU-11/A. However, this pitch trim adjustment is meaningless for a spacecraft, so it is not implemented in the Apollo or Space Shuttle instruments. Surprisingly, the shell of the potentiometer still exists in our FDAI, but without the potentiometer itself or the wiring. Perhaps it remained to preserve the balance of the ball. In the photo below, the cylindrical potentiometer shell is indicated by an arrow. Note the holes in the front of the shell; in the ARU-11/A, the potentiometer's wiring terminals protrude through these holes, but in the FDAI, the holes are covered with tape.

Finally, the mounting of the ball hemispheres is slightly different. The ARU/11-A uses four screws at the pole of each hemisphere. Our FDAI, however, uses a single screw at each pole; the screw is tightened with a Bristol Key, causing the shaft to expand and hold the hemisphere in place.

To summarize, the Apollo FDAI occupies a middle ground: while it isn't simply a repurposed ARU-11/A, neither is it a complete redesign. Instead, it preserves the old design where possible, while stripping out undesired features such as pitch trim. The separate amplifier and mechanical units of the ARU/11-A were combined to form the larger FDAI.

Differences from Apollo

The FDAI that we examined is a special unit: it was originally built for Apollo but was repurposed for a Space Shuttle simulator. Our FDAI is labeled Model 4068F, which is a Lunar Module part number. Moreover, the FDAI is internally stamped with the date "Apr. 22 1968", over a year before the first Moon landing.

However, a closer look shows that several key components were modified to make the Apollo FDAI work in the Shuttle Simulator.14 The Apollo FDAI (and the Shuttle ADI) used resolvers as inputs to control the ball, while our FDAI uses synchros. (Resolvers and synchros are similar, except resolvers use sine and cosine inputs, 90° apart, on two wire pairs, while synchros use three inputs, 120° apart, on three wires.) NASA must have replaced the three resolver control transformers in the FDAI with synchro control transformers for use in the simulator.

The Apollo FDAI used electroluminescent lighting for the display, while ours uses eight small incandescent bulbs. The metal case of our FDAI has a Dymo embossed tape label "INCANDESCENT LIGHTING", alerting users to the change from Apollo's illumination. Our FDAI also contains a step-down transformer to convert the 115 VAC input into 5 VAC to power the bulbs, while the Shuttle powered its ADI illumination directly from 5 volts.

The dial of our FDAI was repainted to match the dial of the Shuttle FDAI. The Apollo FDAI had red bands on the left and right of the dial. A close examination of our dial shows that black paint was carefully applied over the red paint, but a few specks of red paint are still visible (below). Moreover, the edges of the lines and the lozenge show slight unevenness from the repainting. Second, the Apollo FDAI had the text "ROLL RATE", "PITCH RATE", and "YAW RATE" in white next to the needle scales. In our FDAI, this text has been hidden by black paint to match the Shuttle display.13 Third, the Apollo LM FDAI had a crosshair in the center of the instrument, while our FDAI has a white U-shaped indicator, the same as the Shuttle (and the Command Module's FDAI). Finally, the ball of the Apollo FDAI has red circular regions at the poles to warn of orientations that can cause gimbal lock. Our FDAI (like the Shuttle) does not have these circles. We couldn't see any evidence that these regions were repainted, so we suspect that our FDAI has Shuttle hemispheres on the ball.

Our FDAI has also been modified electrically. Small green connectors (Micro-D MDB1) have been added between the slip rings and the motors, as well as on the gimbal arm. We think these connectors were added post-Apollo, since they are attached somewhat sloppily with glue and don't look flight-worthy. Perhaps these connectors were added to make disassembly and modification easier. Moreover, our FDAI has an elapsed time indicator, also mounted with glue.

The back of our FDAI is completely different from Apollo. First, the connector's pinout is completely different. Second, each of the six indicator needles has a mechanical adjustment as well as a trimpot (details). Finally, each of the three axes has an adjustment potentiometer.

The Shuttle's ADI (Attitude Director Indicator)

Each Space Shuttle had three ADIs (Attitude Director Indicators), which were very similar to the Apollo FDAI, despite the name change. The photo below shows the two octagonal ADIs in the forward flight deck, one on the left in front of the Commander, and one on the right in front of the Pilot. The aft flight deck station had a third ADI.15

Our FDAI appears to have been significantly modified for use in the Shuttle simulator, as described above. However, it is much closer to the Apollo FDAI than the ADI used in the Shuttle, as I'll show in this section. My hypothesis is that the simulator was built before the Shuttle's ADI was created, so the Apollo FDAI was pressed into service.

The Shuttle's ADI was much more complicated electrically than the Apollo FDAI and our FDAI, providing improved functionality.16 For instance, while the Apollo FDAI had a simple "OFF" indicator flag to show that the indicator had lost power, the Shuttle's ADI had extensive error detection. It contained voltage level monitors to check its five power supplies. (The Shuttle ADI used three DC power sources and two AC power sources, compared to the single AC supply for Apollo.) The Shuttle's ADI also monitored the ball servos to detect position errors. Finally, it received an external "Data OK" signal. If a fault was detected by any of these monitors, the "OFF" flag was deployed to indicate that the ADI could not be trusted.

The Shuttle's ADI had six needles, the same as Apollo, but the Shuttle used feedback to make the positions more accurate. Specifically, each Shuttle needle had a feedback sensor, a Linear Variable Differential Transformer (LVDT) that generates a voltage based on the needle position. The LVDT output drove a servo feedback loop to ensure that the needle was in the exact desired position. In the Apollo FDAI, on the other hand, the needle input voltage drove a galvanometer, swinging the needle proportionally, but there was no closed loop to ensure accuracy.

I assume that the Shuttle's ADI had integrated circuit electronics to implement this new functionality, considerably more modern than the germanium transistors in the Apollo FDAI. The Shuttle probably used the same mechanical structures to rotate the ball, but I can't confirm that.

Conclusions

The FDAI was a critical instrument in Apollo, indicating the orientation of the spacecraft in three axes. It wasn't obvious to me how the "8-ball" can rotate in three axes while still being securely connected to the instrument. The trick is that most of the mechanism rotates in two axes, while hollow hemispherical shells provide the third rotational axis.

The FDAI has an interesting evolutionary history, from the experimental X-15 rocket plane and the F-4 fighter to the Gemini, Apollo, and Space Shuttle flights. Our FDAI has an unusual position in this history: since it was modified from Apollo to function in a Space Shuttle simulator, it shows aspects of both Apollo and the Space Shuttle indicators. It would be interesting to compare the design of a Shuttle ADI to the Apollo FDAI, but I haven't been able to find interior photos of a Shuttle ADI (or of an unmodified Apollo FDAI).17

You can see a brief video of the FDAI in motion here. For more, follow me on Bluesky (@righto.com), Mastodon (@kenshirriff@oldbytes.space), or RSS. (I've given up on Twitter.) I worked on this project with CuriousMarc, Mike Stewart, and Eric Schlapfer, so expect a video at some point. Thanks to Richard for providing the FDAI. I wrote about the F-4 fighter plane's attitude indicator here.

Notes and references

I needed to power some cameras using Power over Ethernet but they only supported 12V input, so I ordered some PoE extractors from ali. Here they are for your information.

Black one

White waterproof one

Conclusion

They both only support 100Mbps over 2 pairs. The wateproofing was quite annoying to remove but not impossible. It seems they both can be modified to support different input/output voltage. As you can see in the pictures, they are both using H6225K step-down regulator in a very similar design.

The circuit mostly corresponds to following schematic found on the internet:

Thank you for your attention.

We recently received an attitude indicator for the F-4 fighter plane, an instrument that uses a rotating ball to show the aircraft's orientation and direction. In a normal aircraft, the artificial horizon shows the orientation in two axes (pitch and roll), but the F-4 indicator uses a rotating ball to show the orientation in three axes, adding azimuth (yaw).1 It wasn't obvious to me how the ball could rotate in three axes: how could it turn in every direction and still remain attached to the instrument?

We disassembled the indicator, reverse-engineered its 1960s-era circuitry, fixed some problems,2 and got it spinning. The video clip below shows the indicator rotating around three axes. In this blog post, I discuss the mechanical and electrical construction of this indicator. (The quick explanation is that the ball is really two hollow half-shells attached to the internal mechanism at the "poles"; the shells rotate while the "equator" remains stationary.)

The F-4 aircraft

The indicator was used in the F-4 Phantom II3 so the pilot could keep track of the aircraft's orientation during high-speed maneuvers. The F-4 was a supersonic fighter manufactured from 1958 to 1981. Over 5000 were produced, making it the most-produced American supersonic aircraft ever. It was the main US fighter jet in the Vietnam War, operating from aircraft carriers. The F-4 was still used in the 1990s during the Gulf War, suppressing air defenses in the "Wild Weasel" role. The F-4 was capable of carrying nuclear bombs.4

The F-4 was a two-seat aircraft, with the radar intercept officer controlling radar and weapons from a seat behind the pilot. Both cockpits had a panel crammed with instruments, with additional instruments and controls on the sides. As shown below, the pilot's panel had the three-axis attitude indicator in the central position, just below the reddish radar scope, reflecting its importance.5 (The rear cockpit had a simpler two-axis attitude indicator.)

The attitude indicator mechanism

The ball inside the indicator shows the aircraft's position in three axes. The roll axis indicates the aircraft's angle if it rolls side-to-side along its axis of flight. The pitch axis indicates the aircraft's angle if it pitches up or down. Finally, the azimuth axis indicates the compass direction that the aircraft is heading, changed by the aircraft's turning left or right (yaw). The indicator also has moving needles and status flags, but in this post I'm focusing on the rotating ball.6

The indicator uses three motors to move the ball. The roll motor (below) is attached to the frame of the indicator, while the pitch and azimuth motors are inside the ball. The ball is held in place by the roll gimbal, which is attached to the ball mechanism at the top and bottom pivot points. The roll motor turns the roll gimbal and thus the ball, providing a clockwise/counterclockwise movement. The roll control transformer provides position feedback. Note the numerous wires on the roll gimbal, connected to the mechanism inside the ball.

The diagram below shows the mechanism inside the ball, after removing the hemispherical shells of the ball. When the roll gimbal is rotated, this mechanism rotates with it. The pitch motor causes the entire mechanism to rotate around the pitch axis (horizontal here), which is attached along the "equator". The azimuth motor and control transformer are behind the pitch components, not visible in this photo. The azimuth motor turns the vertical shaft. The two hollow hemispheres of the ball attach to the top and bottom of the shaft. Thus, the azimuth motor rotates the ball shells around the azimuth axis, while the mechanism itself remains stationary.

Why doesn't the wiring get tangled up as the ball rotates? The solution is two sets of slip rings to implement the electrical connections. The photo below shows the first slip ring assembly, which handles rotation around the roll axis. These slip rings connect the stationary part of the instrument to the rotating roll gimbal. The black base and the vertical wires are attached to the instrument, while the striped shaft in the middle rotates with the ball assembly housing. Inside the shaft, wires go from the circular metal contacts to the roll gimbal.

Inside the ball, a second set of slip rings provides the electrical connection between the wiring on the roll gimbal and the ball mechanism. The photo below shows the connections to these slip rings, handling rotation around the pitch axis (horizontal in this photo). (The slip rings themselves are inside and are not visible.) The shaft sticking out of the assembly rotates around the azimuth (yaw) axis. The ball hemisphere is attached to the metal disk. The azimuth axis does not require slip rings since only the ball shells rotates; the electronics remain stationary.

The servo loop

In this section, I'll explain how the motors are controlled by servo loops. The attitude indicator is driven by an external gyroscope, receiving electrical signals indicating the roll, pitch, and azimuth positions. As was common in 1960s avionics, the signals are transmitted from synchros, which use three wires to indicate an angle. The motors inside the attitude indicator rotate until the indicator's angles for the three axes match the input angles.

Each motor is controlled by a servo loop, shown below. The goal is to rotate the output shaft to an angle that exactly matches the input angle, specified by the three synchro wires. The key is a device called a control transformer, which takes the three-wire input angle and a physical shaft rotation, and generates an error signal indicating the difference between the desired angle and the physical angle. The amplifier drives the motor in the appropriate direction until the error signal drops to zero. To improve the dynamic response of the servo loop, the tachometer signal is used as a negative feedback voltage. This ensures that the motor slows as the system gets closer to the right position, so the motor doesn't overshoot the position and oscillate. (This is sort of like a PID controller.)

In more detail, the external gyroscope unit contains synchro transmitters, small devices that convert the angular position of a shaft into AC signals on three wires. The photo below shows a typical synchro, with the input shaft on the top and five wires at the bottom: two for power and three for the output.

Internally, the synchro has a rotating winding called the rotor that is driven with 400 Hz AC. Three fixed stator windings provide the three AC output signals. As the shaft rotates, the phase and voltage of the output signals changes, indicating the angle. (Synchros may seem bizarre, but they were extensively used in the 1950s and 1960s to transmit angular information in ships and aircraft.)

The attitude indicator uses control transformers to process these input signals. A control transformer is similar to a synchro in appearance and construction, but it is wired differently. The three stator windings receive the inputs and the rotor winding provides the error output. If the rotor angle of the synchro transmitter and control transformer are the same, the signals cancel out and there is no error output. But as the difference between the two shaft angles increases, the rotor winding produces an error signal. The phase of the error signal indicates the direction of error.

The next component is the motor/tachometer, a special motor that was often used in avionics servo loops. This motor is more complicated than a regular electric motor. The motor is powered by 115 volts AC, 400-Hertz, but this isn't sufficient to get the motor spinning. The motor also has two low-voltage AC control windings. Energizing a control winding will cause the motor to spin in one direction or the other.

The motor/tachometer unit also contains a tachometer to measure its rotational speed, for use in a feedback loop. The tachometer is driven by another 115-volt AC winding and generates a low-voltage AC signal proportional to the rotational speed of the motor.

The photo above shows a motor/tachometer with the rotor removed. The unit has many wires because of its multiple windings. The rotor has two drums. The drum on the left, with the spiral stripes, is for the motor. This drum is a "squirrel-cage rotor", which spins due to induced currents. (There are no electrical connections to the rotor; the drums interact with the windings through magnetic fields.) The drum on the right is the tachometer rotor; it induces a signal in the output winding proportional to the speed due to eddy currents. The tachometer signal is at 400 Hz like the driving signal, either in phase or 180º out of phase, depending on the direction of rotation. For more information on how a motor/generator works, see my teardown.

The amplifier

The motors are powered by an amplifier assembly that contains three separate error amplifiers, one for each axis. I had to reverse engineer the amplifier assembly in order to get the indicator working. The assembly mounts on the back of the attitude indicator and connects to one of the indicator's round connectors. Note the cutout in the lower left of the amplifier assembly to provide access to the second connector on the back of the indicator. The aircraft connects to the indicator through the second connector and the indicator passes the input signals to the amplifier through the connector shown above.

The amplifier assembly contains three amplifier boards (for roll, pitch, and azimuth), a DC power supply board, an AC transformer, and a trim potentiometer.7 The photo below shows the amplifier assembly mounted on the back of the instrument. At the left, the AC transformer produces the motor control voltage and powers the power supply board, mounted vertically on the right. The assembly has three identical amplifier boards; the middle board has been unmounted to show the components. The amplifier connects to the instrument through a round connector below the transformer. The round connector at the upper left is on the instrument case (not the amplifier) and provides the connection between the aircraft and the instrument.8

The photo below shows one of the three amplifier boards. The construction is unusual, with some components stacked on top of other components to save space. Some of the component leads are long and protected with clear plastic sleeves. The board is connected to the rest of the amplifier assembly through a bundle of point-to-point wires, visible on the left. The round pulse transformer in the middle has five colorful wires coming out of it. At the right are the two transistors that drive the motor's control windings, with two capacitors between them. The transistors are mounted on a heat sink that is screwed down to the case of the amplifier assembly for cooling. The board is covered with a conformal coating to protect it from moisture or contaminants.

The function of each amplifier board is to generate the two control signals so the motor rotates in the appropriate direction based on the error signal fed into the amplifier. The amplifier also uses the tachometer output from the motor unit to slow the motor as the error signal decreases, preventing overshoot. The inputs to the amplifier are 400 hertz AC signals, with the phase indicating positive or negative error. The outputs drive the two control windings of the motor, determining which direction the motor rotates.

The schematic for the amplifier board is below. The two transistors on the left amplify the error and tachometer signals, driving the pulse transformer. The outputs of the pulse transformer will have opposite phase, driving the output transistors for opposite halves of the 400 Hz cycle. One of the transistors will be in the right phase to turn on and pull the motor control AC to ground, while the other transistor will be in the wrong phase. Thus, the appropriate control winding will be activated (for half the cycle), causing the motor to spin in the desired direction.

It turns out that there are two versions of the attitude indicator that use incompatible amplifiers. I think that the motors for the newer indicators have a single control winding rather than two. Fortunately, the connectors are keyed differently so you can't attach the wrong amplifier. The second amplifier (below) looks slightly more modern (1980s) with a double-sided circuit board and more components in place of the pulse transformer.

The pitch trim circuit

The attitude indicator has a pitch trim knob in the lower right, although the knob was missing from ours. The pitch trim adjustment turns out to be rather complicated. In level flight, an aircraft may have its nose angled up or down slightly to achieve the desired angle of attack. The pilot wants the attitude indicator to show level flight, even though the aircraft is slightly angled, so the indicator can be adjusted with the pitch trim knob. However, the problem is that a fighter plane may, for instance, do a vertical 90º climb. In this case, the attitude indicator should show the actual attitude and ignore the pitch trim adjustment.

I found a 1957 patent that explained how this is implemented. The solution is to "fade out" the trim adjustment when the aircraft moves away from horizontal flight. This is implemented with a special multi-zone potentiometer that is controlled by the pitch angle.

The schematic below shows how the pitch trim signal is generated from the special pitch angle potentiometer and the pilot's pitch trim adjustment. Like most signals in the attitude indicator, the pitch trim is a 400 Hz AC signal, with the phase indicating positive or negative. Ignoring the pitch angle for a moment, the drive signal into the transformer will be AC. The split windings of the transformer will generate a positive phase and a negative phase signal. Adjusting the pitch trim potentiometer lets the pilot vary the trim signal from positive to zero to negative, applying the desired correction to the indicator.

Now, look at the complex pitch angle potentiometer. It has alternating resistive and conducting segments, with AC fed into opposite sides. (Note that +AC and -AC refer to the phase, not the voltage.) Because the resistances are equal, the AC signals will cancel out at the top and the bottom, yielding 0 volts on those segments. If the aircraft is roughly horizontal, the potentiometer wiper will pick up the positive-phase AC and feed it into the transformer, providing the desired trim adjustment as described previously. However, if the aircraft is climbing nearly vertically, the wiper will pick up the 0-volt signal, so there will be no pitch trim adjustment. For an angle range in between, the resistance of the potentiometer will cause the pitch trim signal to smoothly fade out. Likewise, if the aircraft is steeply diving, the wiper will pick up the 0 signal at the bottom, removing the pitch trim. And if the aircraft is inverted, the wiper will pick up the negative AC phase, causing the pitch trim adjustment to be applied in the opposite direction.

Conclusions

The attitude indicator is a key instrument in any aircraft, especially important when flying in low visibility. The F-4's attitude indicator goes beyond the artificial horizon indicator in a typical aircraft, adding a third axis to show the aircraft's heading. Supporting a third axis makes the instrument much more complicated, though. Looking inside the indicator reveals how the ball rotates in three axes while still remaining firmly attached.

Modern fighter planes avoid complex electromechanical instruments. Instead, they provide a "glass cockpit" with most data provided digitally on screens. For instance, the F-35's console replaces all the instruments with a wide panoramic touchscreen displaying the desired information in color. Nonetheless, mechanical instruments have a special charm, despite their impracticality.

For more, follow me on Mastodon as @kenshirriff@oldbytes.space or RSS. (I've given up on Twitter.) I worked on this project with CuriousMarc and Eric Schlapfer, so expect a video at some point. Thanks to John Pumpkinhead and another collector for supplying the indicators and amplifiers.

Notes and references

Specifications9

The Bendix Central Air Data Computer (CADC) is an electromechanical analog computer that uses gears and cams for its mathematics. It was a key part of military planes such as the F-101 and the F-111 fighters, computing airspeed, Mach number, and other "air data". The rotating gears are powered by six small servomotors, so these motors are in a sense the fundamental component of the CADC. In the photo below, you can see one of the cylindrical motors near the center, about 1/3 of the way down.

The servomotors in the CADC are unlike standard motors. Their name—"Motor-Tachometer Generator" or "Motor and Rate Generator"1—indicates that each unit contains both a motor and a speed sensor. Because the motor and generator use two-phase signals, there are a total of eight colorful wires coming out, many more than a typical motor. Moreover, the direction of the motor can be controlled, unlike typical AC motors. I couldn't find a satisfactory explanation of how these units worked, so I bought one and disassembled it. This article (part 5 of my series on the CADC2) provides a complete teardown of the motor/generator and explain how it works.

The image below shows a closeup of two motors powering one of the pressure signal outputs. Note the bundles of colorful wires to each motor, entering in two locations. At the top, the motors drive complex gear trains. The high-speed motors are geared down by the gear trains to provide much slower rotations with sufficient torque to power the rest of the CADC's mechanisms.

The motor/tachometer that we disassembled is shorter than the ones in the CADC (despite having the same part number), but the principles are the same. We started by removing a small C-clip on the end of the motor and and unscrewing the end plate. The unit is pretty simple mechanically. It has bearings at each end for the rotor shaft. There are four wires for the motor and four wires for the tachometer.3

The rotor (below) has two parts on the shaft. the left part is for the motor and the right drum is for the tachometer. The left part is a squirrel-cage rotor4 for the motor. It consists of conducting bars (light-colored) on an iron core. The conductors are all connected at both ends by the conductive rings at either end. The metal drum on the right is used by the tachometer. Note that there are no electrical connections between the rotor components and the rest of the motor: there are no brushes or slip rings. The interaction between the rotor and the windings in the body of the motor is purely magnetic, as will be explained.

The motor/tachometer contains two cylindrical stators that create the magnetic fields, one for the motor and one for the tachometer. The photo below shows the motor stator inside the unit after removing the tachometer stator. The stators are encased in hard green plastic and tightly pressed inside the unit. In the center, eight metal poles are visible. They direct the magnetic field onto the rotor.

The photo below shows the stator for the tachometer, similar to the stator for the motor. Note the shallow notches that look like black lines in the body on the lower left. These are probably adjustments to the tachometer during manufacturing to compensate for imperfections. The adjustments ensure that the magnetic fields are nulled out so the tachometer returns zero voltage when stationary. The metal plate on top shields the tachometer from the motor's magnetic fields.

The poles and the metal case of the stator look solid, but they are not. Instead, they are formed from a stack of thin laminations. The reason to use laminations instead of solid metal is to reduce eddy currents in the metal. Each lamination is varnished, so it is insulated from its neighbors, preventing the flow of eddy currents.

In the photo below, I removed some of the plastic to show the wire windings underneath. The wires look like bare copper, but they have a very thin layer of varnish to insulate them. There are two sets of windings (orange and blue, or red and black) around alternating metal poles. Note that the wires run along the pole, parallel to the rotor, and then wrap around the pole at the top and bottom, forming oblong coils around each pole.5 This generates a magnetic field through each pole.

The motor

The motor part of the unit is a two-phase induction motor with a squirrel-cage rotor.6 There are no brushes or electrical connections to the rotor, and there are no magnets, so it isn't obvious what makes the rotor rotate. The trick is the "squirrel-cage" rotor, shown below. It consists of metal bars that are connected at the top and bottom by rings. Assume (for now) that the fixed part of the motor, the stator, creates a rotating magnetic field. The important principle is that a changing magnetic field will produce a current in a wire loop.7 As a result, each loop in the squirrel-cage rotor will have an induced current: current will flow up9 the bars facing the north magnetic field and down the south-facing bars, with the rings on the end closing the circuits.

But how does the stator produce a rotating magnetic field? And how do you control the direction of rotation? The next important principle is that current flowing through a wire produces a magnetic field.8 As a result, the currents in the squirrel cage rotor produce a magnetic field perpendicular to the cage. This magnetic field causes the rotor to turn in the same direction as the stator's magnetic field, driving the motor. Because the rotor is powered by the induced currents, the motor is called an induction motor.

The diagram below shows how the motor is wired, with a control winding and a reference winding. Both windings are powered with AC, but the control voltage either lags the reference winding by 90° or leads the reference winding by 90°, due to the capacitor. Suppose the current through the control winding lags by 90°. First, the reference voltage's sine wave will have a peak, producing the magnetic field's north pole at A. Next (90° later), the control voltage will peak, producing the north pole at B. The reference voltage will go negative, producing a south pole at A and thus a north pole at C. The control voltage will go negative, producing a south pole at B and a north pole at D. This cycle will repeat, with the magnetic field rotating counter-clockwise from A to D. Conversely, if the control voltage leads the reference voltage, the magnetic field will rotate clockwise. This causes the motor to spin in one direction or the other, with the direction controlled by the control voltage. (The motor has four poles for each winding, rather than the one shown below; this increases the torque and reduces the speed.)

The purpose of the capacitor is to provide the 90° phase shift so the reference voltage and the control voltage can be driven from the same single-phase AC supply (in this case, 26 volts, 400 hertz). Switching the polarity of the control voltage reverses the direction of the motor.

There are a few interesting things about induction motors. You might expect that the motor would spin at the same rate as the rotating magnetic field. However, this is not the case. Remember that a changing magnetic field induces the current in the squirrel-cage rotor. If the rotor is spinning at the same rate as the magnetic field, the rotor will encounter an unchanging magnetic field and there will be no current in the bars of the rotor. As a result, the rotor will not generate a magnetic field and there will be no torque to rotate it. The consequence is that the rotor must spin somewhat slower than the magnetic field. This is called "slippage" and is typically a few percent of the full speed, with more slippage as more torque is required.

Many household appliances use induction motors, but how do they generate a rotating magnetic field from a single-phase AC winding? The problem is that the magnetic field in a single AC winding will just flip back and forth, so the motor will not turn in either direction. One solution is a shaded-pole motor, which puts a copper bar around part of each pole to break the symmetry and produce a weakly rotating magnetic field. More powerful induction motors use a startup winding with a capacitor (analogous to the control winding). This winding can either be switched out of the circuit once the motor starts spinning,10 or used continuously, called a permanent-split capacitor (PSC) motor. The best solution is three-phase power (if available); a three-phase winding automatically produces a rotating magnetic field.

Tachometer/generator

The second part of the unit is the tachometer generator, sometimes called the rate unit.11 The purpose of the generator is to produce a voltage proportional to the speed of the shaft. The unusual thing about this generator is that it produces a 400-hertz output that is either in phase with the input or 180° out of phase. This is important because the phase indicates which direction the shaft is turning. Note that a "normal" generator is different: the output frequency is proportional to the speed.

The diagram below shows the principle behind the generator. It has two stator windings: the reference coil that is powered at 400 Hz, and the output coil that produces the output signal. When the rotor is stationary (A), the magnetic flux is perpendicular to the output coil, so no output voltage is produced. But when the rotor turns (B), eddy currents in the rotor distort the magnetic field. It now couples with the output coil, producing a voltage. As the rotor turns faster, the magnetic field is distorted more, increasing the coupling and thus the output voltage. If the rotor turns in the opposite direction (C), the magnetic field couples with the output coil in the opposite direction, inverting the output phase. (This diagram is more conceptual than realistic, with the coils and flux 90° from their real orientation, so don't take it too seriously. As shown earlier, the coils are perpendicular to the rotor so the real flux lines are completely different.)

But why does the rotating drum change the magnetic field? It's easier to understand by considering a tachometer that uses a squirrel-cage rotor instead of a drum. When the rotor rotates, currents will be induced in the squirrel cage, as described earlier with the motor. These currents, in turn, generate a perpendicular magnetic field, as before. This magnetic field, perpendicular to the orginal field, will be aligned with the output coil and will be picked up. The strength of the induced field (and thus the output voltage) is proportional to the speed, while the direction of the field depends on the direction of rotation. Because the primary coil is excited at 400 hertz, the currents in the squirrel cage and the resulting magnetic field also oscillate at 400 hertz. Thus, the output is at 400 hertz, regardless of the input speed.

Using a drum instead of a squirrel cage provides higher accuracy because there are no fluctuations due to the discrete bars. The operation is essentially the same, except that the currents pass through the metal of the drum continuously instead of through individual bars. The result is eddy currents in the drum, producing the second magnetic field. The diagram below shows the eddy currents (red lines) from a metal plate moving through a magnetic field (green), producing a second magnetic field (blue arrows). For the rotating drum, the situation is similar except the metal surface is curved, so both field arrows will have a component pointing to the left. This creates the directed magnetic field that produces the output.

The servo loop

The motor/generator is called a servomotor because it is used in a servo loop, a control system that uses feedback to obtain precise positioning. In particular, the CADC uses the rotational position of shafts to represent various values. The servo loops convert the CADC's inputs (static pressure, dynamic pressure, temperature, and pressure correction) into shaft positions. The rotations of these shafts power the gears, cams, and differentials that perform the computations.

The diagram below shows a typical servo loop in the CADC. The goal is to rotate the output shaft to a position that exactly matches the input voltage. To accomplish this, the output position is converted into a feedback voltage by a potentiometer that rotates as the output shaft rotates.12 The error amplifier compares the input voltage to the feedback voltage and generates an error signal, rotating the servomotor in the appropriate direction. Once the output shaft is in the proper position, the error signal drops to zero and the motor stops. To improve the dynamic response of the servo loop, the tachometer signal is used as a negative feedback voltage. This ensures that the motor slows as the system gets closer to the right position, so the motor doesn't overshoot the position and oscillate. (This is sort of like a PID controller.)

The error amplifier and motor drive circuit for a pressure transducer are shown below. Because of the state of electronics at the time, it took three circuit boards to implement a single servo loop. The amplifier was implemented with germanium transistors (since silicon transistors were later). The transistors weren't powerful enough to drive the motors directly. Instead, magnetic amplifiers (the yellow transformer-like modules at the front) powered the servomotors. The large rectangular capacitors on the right provided the phase shift required for the control voltage.

Conclusions

The Bendix CADC used a variety of electromechanical devices including synchros, control transformers, servo motors, and tachometer generators. These were expensive military-grade components driven by complex electronics. Nowadays, you can get a PWM servo motor for a few dollars with the gearing, feedback, and control circuitry inside the motor housing. These motors are widely used for hobbyist robotics, drones, and other applications. It's amazing that servo motors have gone from specialized avionics hardware to an easy-to-use, inexpensive commodity.

Follow me on Twitter @kenshirriff or RSS for updates. I'm also on Mastodon as @oldbytes.space@kenshirriff. Thanks to Joe for providing the CADC. Thanks to Marc Verdiell for disassembling the motor.

Notes and references

It is a very simple device for about $15. The main SoC is GX6605S. There is some limited info about this chip on C-SKY page and some technical docs at their tools repo. It communicates with a Bluetooth module TLSR8266 using UART. There is also 4MB SPI flash holding the whole bootloader called GxLoader, Linux kernel and filesystem with the main app. The last important chip is RDA5815M which handles the RF to I/Q conversion. In the future I plan to add an USB-to-Ethernet adapter and repurpose it as an cheap opensource SAT>IP server.

It is quite simple device. The main chip is Vango V9821S – a single-phase energy metering SoC chip, then there is one EEPROM 24C04, optical isolation for S0, display and power supply circuitry and a display. There is no serial interface so no way how to get more data out of the device, only JTAG for programming.