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Reverse-engineering an airspeed/Mach indicator from 1977

Reverse-engineering an airspeed/Mach indicator from 1977

2023-01-12 12:28:42

How does a classic airspeed indicator work? CuriousMarc picked one up for a mission, however it did not have any documentation, so I
reverse-engineered it.
This indicator
was used within the cockpit panel for enterprise jets such because the
Gulfstream G-III, Cessna Quotation, and Bombardier Challenger CL600.
It was in all probability manufactured in 1977 primarily based on the dates on its transistors.

You may anticipate that the symptoms on an plane management panel are easy dials.
However behind this dial is a big, 2.8-pound field
with a posh system of motors, gears, and suggestions potentiometers, managed by two boards of electronics.
However for all this complexity, the indicator does not have any smarts: the pointers simply point out voltages fed into it from an air information laptop.
It is a fast weblog submit to summarize what I discovered.

Front view of the indicator.

Entrance view of the indicator.

The dial has two rotating pointers:
the white pointer signifies airspeed in knots whereas
the striped pointer signifies the utmost airspeed (which varies relying on altitude).
The “digital” indicator on the prime exhibits Mach quantity from 0.10 to 0.99, applied with rotating digit wheels.
When the unit is working, the OFF indicator flag switches to black. The flag switches to a vivid VMO warning if
the pilot exceeds the utmost airspeed.1
On the rim of the dial, two small markers known as “bugs” will be manually moved to point essential speeds akin to takeoff pace.

In use, the indicator is linked to a Sperry air information laptop and receives voltage indicators to manage the dial positions.3
The air information laptop measures the static and dynamic air stress from pitot tubes and determines the airspeed, Mach quantity, altitude,
and different parameters. (These calculations develop into nontrival close to Mach 1 as air compresses and the fluid dynamics change.)
Since we did not have the air information laptop or its specs, I wanted to determine the connections from the pc to the show.

With the unit’s cowl eliminated, you’ll be able to see the interior mechanisms and circuitry.
Every of the three indicators is managed by a small DC motor with a potentiometer offering suggestions.
To the appropriate, two circuit boards present the electronics to drive the symptoms.4
On the higher proper, the black blob is a 26-volt 400-Hertz transformer to energy the unit.
Some energy provide elements are in entrance of it.
Under the transformer is an orangish versatile printed-circuit board, which appears superior for the timeframe.
This versatile ribbon connects the transformer, the exterior connector, and the printed-circuit board sockets, offering the backplane for the system.

A side view of the unit shows the gears to control the indicators.

A aspect view of the unit exhibits the gears to manage the symptoms.

The diagram under exhibits the precept behind the servo mechanism that controls every indicator.
The aim is to rotate the indicator to a place comparable to the enter voltage.
A suggestions loop is used to attain this.
The potentiometer offers a voltage proportional to its rotation.
The enter voltage and the suggestions voltage are inputs to an op amp, which generates an error sign primarily based on the distinction between the inputs.
The error sign rotates the DC motor within the acceptable path till the potentiometer voltage matches
the enter voltage. As a result of the indicator and the potentiometer are geared collectively, the indicator shall be within the right place.
Because the enter voltage modifications, the system will repeatedly monitor the modifications and maintain the indicator up to date.

A diagram illustrating the servo feedback loop.

A diagram illustrating the servo suggestions loop.

As a result of the DC motor spins a lot quicker than the dial strikes, discount gears sluggish the rotation.
The photograph under exhibits the gear practice within the unit. A potentiometer is on the upper-right with three wires connected.

A closeup of the gear train. A potentiometer is on the right.

A closeup of the gear practice. A potentiometer is on the appropriate.

The Mach quantity has further gearing to rotate the numbered wheels. When the low-digit wheel cycles round, it advances the high-digit wheel, just like an odometer.

The mechanism to rotate the digit wheels for the Mach number.

The mechanism to rotate the digit wheels for the Mach quantity.

Fault checking

One attention-grabbing function of the indicator unit is that it implements fault checking to alert the pilot if one thing goes fallacious.
The entrance panel has a three-position flag. By default it is within the OFF place. Powering the coil in a single path rotates the
flag to the clean aspect. Powering the coil within the different path rotates the flag to the “VMO” place which signifies that the pilot has
exceeded the utmost working pace.

I figured that powering up the unit would transfer the flag out of the OFF place, however it’s extra sophisticated than that.
First, the unit checks that the air information laptop is offering an acceptable reference voltage.
Second, the unit verifies that the motor voltages for the 2 needles are inside limits;
this ensures that the servo loop is working efficiently.
Third, the unit checks that indicators are obtained on standing pins Ok and L.
The unit solely strikes out of the OFF state if all these situations are happy.5
Thus, if the unit receives dangerous indicators or is malfunctioning, the pilot shall be alerted by the OFF indicator, quite than trusting the defective show.

The circuitry

The unit is powered by 26 volts, 400 Hz, a regular voltage for aviation.
A small transformer offers a number of outputs for the varied inner voltages.
The unit has 4 energy provides: three on the primary board and one on the again wall of the unit.
One energy provide is for the standing indicator, one is for the op amps, one powers the 41.7V motors, and
the fourth offers different energy.

One subtlety is how the suggestions potentiometers are powered.
The servo loop compares the potentiometer voltage with the enter voltage.
However this solely works if the potentiometer and the enter voltage are utilizing the identical reference.
One answer can be for the indicator unit and the air information laptop to comprise matching precision voltage regulators.[reference] As a substitute, the system makes use of an easier, extra dependable method: the air information laptop offers a reference voltage that the
indicator unit makes use of to energy the potentiometers.6
With this method, the air information laptop’s voltage reference can fluctuate and the indicator will nonetheless attain the appropriate place.
(In different phrases, a 5V enter with a 10V reference and a 6V enter with a 12V reference are each 50%.)

The diagram under exhibits the board with the servo circuitry.
The board makes use of twin op-amp built-in circuits,
packaged in 10-pin steel cans that protected in opposition to interference.7
The ICs and a few of the different elements have obscure army half numbers; I do not know if this unit was constructed for army use or if
military-grade elements have been used for reliability.

The servo board is full of transistors, resistors, capacitors, diodes, and op-amp integrated circuits.

The servo board is filled with transistors, resistors, capacitors, diodes, and op-amp built-in circuits.

The circuitry within the lower-left nook handles the reference voltage from the air information laptop.
The board buffers this voltage with an op amp to energy the three
suggestions potentiometers.
The op amp additionally ensures that the reference voltage is at the very least 10 volts. If not, the indicator unit exhibits the “OFF” flag to alert the pilot.

The schematic under exhibits one of many servo circuits; the three circuits are roughly the identical.
The center of the circuit is the error op amp within the middle. It compares the voltage from the potentiometer with the enter voltage and
generates an error output that strikes the motor appropriately.
A optimistic error output will activate the higher transistor, driving the motor with a optimistic voltage.
Conversely, a detrimental error output will activate the decrease transistor, driving the motor with a detrimental voltage.
The motor drive circuit has clamp diodes to restrict the transistor base voltages.

Schematic of one of the servo circuits.

See Also

Schematic of one of many servo circuits.

The op amp additionally receives a suggestions sign from the motor output. I do not solely perceive this sign, which works by means of a filter
circuit with resistors, diodes, and a capacitor. I feel it dampens the motor sign so the motor does not overshoot the specified place.
I feel it additionally retains the transistor drive sign biased relative to the emitter voltage (i.e. the motor output).

On the enter aspect, the potentiometer voltage goes by means of an op amp follower buffer, which merely outputs its enter voltage.
This may occasionally appear pointless, however the op amp offers a high-impedance enter so the
potentiometer’s voltage does not get distorted.

The exterior enter voltage goes by means of a resistor/capacitor circuit to scale it and filter out noise.
Curiously, the circuit board was modified by reducing a hint and including a resistor and capacitor to vary the enter circuit for one of many inputs.
Within the photograph under, you’ll be able to see the added resistor and capacitor; the lower hint is simply to the appropriate of the capacitor.
I do not know if this modification modified the dimensions issue or if it filtered out noise.
A label on the field says that Honeywell carried out a modification on November 8, 1991, which presumably was this circuit.

A closeup of the circuit board showing the modification.

A closeup of the circuit board exhibiting the modification.

The second board implements three energy provides in addition to the circuitry for the OFF/VMO flag.
The facility provides are easy and unregulated, simply diode bridges to transform AC to DC, together with filter capacitors.
A lot of the circuitry on the board controls the standing flag.
Two twin op amps test the motor voltages in opposition to higher and decrease limits to make sure that the motors are monitoring the inputs.
These outputs, together with different logic standing indicators, are mixed with diode-transistor logic to find out the flag standing.
Driver transistors present +18 or -18 volts to the flag’s coil to drive it to the specified place.

This board has power supply circuitry and the control circuitry for the indicator flag.

This board has energy provide circuitry and the management circuitry for the indicator flag.


After reverse-engineering the pinout, I linked the airspeed indicator to a stack of energy provides and succeeded in getting the symptoms to function (video).
This unit is far more advanced than I anticipated for a easy show, with servoed motors managed by two boards of electronics.
Air security rules in all probability account for a lot of the complexity, guaranteeing that the show offers the pilot with correct data.
For all that complexity, the unit is basically a voltmeter, indicating three voltages on its show.
This airspeed indicator is a bit completely different from many of the {hardware} I study, however hopefully you discovered this have a look at its inner circuitry attention-grabbing.

With the case removed, the internal circuitry is visible.

With the case eliminated, the interior circuitry is seen.

You may comply with me on Twitter @kenshirriff or rss.
I’ve additionally began experimenting with mastodon lately as @[email protected].

Notes and references

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