SECTOR CONVERSION.

transmissions. In other words, both speeds are converted using the same reference signal. Synchro-to-Digital (S/D) Conversion Two methods are currently in use to convert synchro data to digital words (BAMs): the sector method and the octant method. Both methods of conversion require a reference voltage input for conversion to take place. SECTOR CONVERSION. —The sector conversion method uses the reference voltage to determine the time to sample the stator voltages for conversion to take place. The ideal time to sample the stator voltages is when the reference voltage is at or near the positive or the negative peak of its cycle. Sixty-Degree Sector Determination. —Once the negative or the positive peak of the reference is detected, the sector in which the rotor is positioned may be determined. There are six 60-degree sectors within the 360-degree rotation of the rotor. The relationship of the stator voltages to the reference defines the sector. Table 13-2 shows the sector limits and the phase relationship of the stator voltages to the reference in each sector. Stator Voltage Selection. —When the sector angle is determined, two of the three stator voltages are used to identify the ratio angle within the sector. The ratio angle is determined by a ratio between the two voltage samples. The two stator voltages selected depend on the sector. The appropriate voltages are gated to the conversion circuitry and converted to binary data. The sector angle and the ratio angle of the two stator voltages are summed to determine the binary angle of the rotor position in BAMs. Table 13-2.—Phase Relationship of Stator Voltages to Reference OCTANT CONVERSION. —The octant conversion method divides the 360 degrees of angular measurement into eight 45-degree octants. The conversion process first defines the octant and then the binary representation of the trigonometric angle within the octant. Octant Determination. —The 5-wire synchro signal (R1, R2, S1, S2, and S3) is first converted into two dc voltages representing the sine and cosine of the synchro angle. The polarity of the sine and cosine voltages and their respective amplitude to each other are used to select the octant that defines the three most significant bits of the BAM word (figure 13-3). Successive Approximations. —The remaining bits of the BAM word are determined through a process of successive approximations. The sine and cosine voltages are combined into a ratio voltage that is used to determine the condition of each of the remaining bit positions in the BAM word, starting at the MSB of the remaining bits. A trial and error method is used. A trial binary angle is generated and tested against the ratio angle until the trial angle equals the ratio angle, completing the conversion process. Single-Speed/Dual-Speed Synchro Conversions Synchro-to-digital conversions do not occur on a continuous basis. The synchro data is sampled as required by the controlling computer, usually on a periodic basis. A single BAM word is generated by the S/D conversion for both single- and dual-speed synchros. When dual-speed synchro data is being converted, two S/D conversions are required to generate one BAM word. The coarse synchro signal is converted immediately before the fine synchro signal. The summation of the two conversions is represented by a single binary word, indicating one angular value. Conversions for single-speed synchros are considered coarse conversions only. NOTE. —For more detailed information on synchros and synchro systems, refer to NAVEDTRA 172-15-00-80, NEETS, Module 15, Principles of Synchros, Servos, and Gyros. DIGITAL-TO-ANALOG CONVERSION Digital-to-analog (D/A) conversion is required when digital devices must communicate with an analog system or equipment. Three types of D/A conversion are commonly encountered on shipboard systems: digital-to-linear, digital-to-scalar, and digital-to- synchro (D/S). Linear signals are ac or dc voltages that 13-7