This video presents the second part of a detailed reverse engineering analysis and teardown of a BDHI (Bearing Distance Heading Indicator) aviation instrument, focusing specifically on the electromechanical servo control systems. The indicator features three main moving components: the compass card, the single VOR pointer, and the dual ADF pointer, each driven by a classic AC servo loop consisting of a control transformer (CT), a servo amplifier, and an AC motor.
The most significant discovery is the miniaturization and integration of the servo amplifiers—typically separate modules—directly onto small PCBs located on the back of each AC motor assembly, simplifying wiring and reducing the unit's overall size. The video also details the complex digital input architecture for the dual pointer, which utilizes an ARINC 429 interface. This digital data is processed through a Digital-to-Resolver and subsequent Resolver-to-Synchro conversion chain, enabling two distinct modes: an "Absolute Mode" based on external digital input, and a "Relative Mode" where the dual pointer's angle automatically tracks changes in the compass card's heading. Successful functional tests of all three servo loops are demonstrated using external synchro inputs and a custom ARINC 429 generator.
BDHI Indicator Electromechanical Analysis and Digital Control
0:04 Classic Servo System Architecture: The BDHI indicator's electromechanical assembly uses a classic servo control system for positioning, utilizing Control Transformers (CTs), servo control amplifiers, and AC motors.
0:43 Three Controlled Movements: The instrument requires three separate servo controls to move the: 1) Compass card, 2) Dual pointer (typically for ADF), and 3) Single pointer (typically for VOR).
1:01 Control Loop Operation: The rotor of the Control Transformer provides an error voltage, which, when amplified, drives the AC motor. The system stops when the error voltage is null. The physical adjustment of the CT sets the final angular position of the corresponding needle.
2:40 Key Discovery: Integrated Servo Amplifiers: Unlike typical systems, the servo amplifiers are miniaturized and located directly on small, round PCBs attached to the back of each AC motor (2:56). This integration reduces the unit's size and complexity of wiring.
4:28 Flag Activation Conditions: The indicator's "flag" (error state) is deactivated only when three conditions are met: presence of AC reference voltage for the motor, low error voltage from the CT, and 28V DC presence confirming the horizontal gyro is operational.
6:50 Digital Drive for Dual Pointer: The dual pointer features a digital input using the ARINC 429 interface (two lines, A & B). This input is decoded to extract 12 bits of angular data.
7:22 Digital Conversion Chain: The digital data is converted into two AC resolver signals (sine/cosine of the angle) via a Digital-to-Resolver (D/R) converter. These are then converted to synchro signals via a Resolver-to-Synchro (R/S) converter to feed the servo control loop.
8:08 Dual Pointer Modes: The dual pointer can operate in two modes:
Absolute Mode (8:35): The pointer is set directly according to the digital input data (or external synchro).
Relative Mode (9:58): The synchro input is offset by the compass card angle, causing the dual pointer to track any change in the compass card's heading.
9:11 Functional Testing: The video demonstrates successful testing of the dual pointer using an ARINC 429 generator to change the position in 15-degree steps in both Absolute Mode (9:12) and Relative Mode (9:58), verifying that the pointer follows the compass card in the latter mode.
10:44 Key Takeaway: The complete reverse engineering of the electromechanical and digital systems took approximately one full week.