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Star/Canopus magnitude ratio

Figure 120.- Transfer function of star intensity signal.

When the CT Sun shutter circuit received the required power from the ACE, an effective solar spectral distribution illumination of 54 to 129 klx (5000 to 12 000 fc), it caused a Sun shutter closure for angles of ±20° ± 4° in clock angle and ±35° ± 5° in cone angle as measured from the CT optical axis. The Sun shutter opened again within 10° if the Sun angle was increased beyond the Sun shutter closure envelope.

Interial reference unit.- The IRU was comprised of two subassenblies: an inertial sensor subassembly and an inertial electronics subassembly. The sensors in the IRU consisted of three miniature, single-degree-of-freedom, floated, rate-integrating gyroscopes and one miniature, pulse-captured, linear, single-axis, pendulous accelerometer. Position information was generated by integrating the rate signals. Precision biases of either polarity could be introduced into the roll and yaw integrator inputs in response to bilevel signals from the ACE based upon CCS commands to the ACE to perform commanded turns. The IRU provided three-axis rate signals for damping and a three-axis inertial reference during those times when the VO was not locked onto its celestial references. It also provided a signal from which the linear change in velocity of the VO was derived during PROPS engine burns. Two identical IRU's were provided. Only one IRU was used at any given time, the other provided standby redundancy. Both IRU's were powered during launch and pyrotechnic events to protect the sensors; otherwise, only one IRU was powered. Neither IRU was normally on during cruise. The orientation of the sensing elements is shown in figure 121.

Power was supplied directly to the IRU from PWRS through switches that were controlled by signals from the ACE. The 50 V rms, 2.4-kHz, square-wave power, converted and conditioned as required by the IRU, was used for all functions except powering the gyro spin motors. The 27.2 V rms, 400-Hz, three-phase, stepped square-wave power was supplied from the 400-Hz inverter in PWRS for the spin motors.

Figure 121.- Inertial sensor orientation.

The IRU had five operating modes as follows:

Rate mode: The IRU provided three-axis rate signals and accelerometer pulses.

All axes inertial mode: The IRU provided three-axis rate signals, three-axis position signals, and accelerometer pulses.

Roll inertial mode: The IRU provided three-axis rate signals, roll position signal, and accelerometer pulses.

Commanded turn mode: A turn bias was input to the rate integrator while in an inertial mode. Yaw turns were performed in the all axes inertial mode. Roll turns were performed in the roll inertial or all axes inertial mode.

Inhibit mode: The IRU was powered, but its rate and position signals and its accelerometer output were inhibited (zero).

The pitch, yaw, and roll rate signals were analog voltages proportional to the rates about the respective axes and had a transfer function as shown in figure 122 for RCA control.

Figure 122.- Transfer function of rate signal for RCA control.

The pitch and yaw rate signals (roll rate was also provided for telemetry) were analog voltages proportional to the rates about the respective axes and had a transfer function as shown in figure 123 for TVC control.

IRU rate, deg/sec

Figure 123.- Transfer function of rate signal for TVC control.

IRU rate, deg/sec

Figure 123.- Transfer function of rate signal for TVC control.

The pitch, yaw, and roll position signals were analog voltages that were the integral of the respective rate signal voltages from the time the inertial mode signal was received by the IRU. The transfer function was as shown in figure 124.

Position, deg

Figure 124.- Transfer function of position signal.

Position, deg

Figure 124.- Transfer function of position signal.

The accelerometer was mounted with its sensitive axis along the Z-axis such that it sensed accelerations in the +Z direction. The output of the measured proof-mass motion was demodulated and compared with a reference voltage in a comparator. When it exceeded the reference, the comparator output was high. As long as the comparator output was high, the slave flip-flop provided pulses at 400 PPS to the proof-mass restoring torquer and to the CCS. The pulses were calibrated so that the CCS measured velocity change by counting pulses. Each pulse represented a velocity change of 0.03 m/sec. The CCS counted pulses until enough had been generated to represent the desired VO velocity change at which time the CCS terminated the engine burn. The accelerometer had a nominal preset bias of 400 x io"6g to avoid a deadband and to permit in-flight calibration. An analog loop around the accelerometer provided proof-mass capture during launch. The analog loop provided a capture range for accelerations of up to 18g.

Attitude control electronics.- The ACE provided three basic functions: RCA control, TVC control, and mode control. The RCA electronics controlled the operation of the RCA jet valves in response to sensor signals to provide VO attitude control. The RCA electronics consisted of Sun sensor excitation circuitry, switching for position and rate signal selection, sensor signal buffer amplifiers, jet valve driver enable/disable circuitry and a sensor signal summing amplifier, threshold detector with deadband and minimum on-time, rate estimator derived rate circuit, and jet valve drivers for each axis. The TVC electronics controlled the GA's to orient the PROPS rocket engine in response to preaim commands and IRU signals. The TVC electronics consisted of a preaim circuit, gimbal servo electronics, compensator, and gain selection and enable controls. The preaim circuit received a 7-bit data word from the ACE memory and converted it to an analog signal. The analog signal biased the gimbal position so that at the start of engine burn the thrust vector passed through the predicted VO CM. The gimbal servo electronics consisted of compensation networks, GA drivers, and derived rate feedback. The gimbal servo operated as a closed-loop controller to position the gimbal to the commanded position. Derived rate feedback was utilized for damping. The compensator consisted of forward loop compensation and feedback compensation (path guidance). Two separate gain settings were utilized to accommodate changes in VO pass properties. The mode control electronics accepted, decoded, and processed the CCS commands in conjunction with control logic to provide the desired mode control switch states. The ACE received a 14-bit command input from the CCS, ACS enable signal from DEVS, and scan slew signal from ARTCS. It also generated internal ACS logic functions and had direct control over all ACS operating modes including CT operation. Two identical ACE assemblies were provided. Power was supplied to one and only one of the two ACE's at all times by command to PWRS from the CCS. An exception was that ±30 V dc was supplied to both ACE's during propulsive maneuvers to power quad redundant circuitry (GA drivers) which was shared by the two ACE's.

The ACE received 50 V rms, 2.4-kHz, square-wave power from PWRS. Power conditioning was provided by the ACE to derive the required internal supply voltages, jet valve drive power, dc excitation for the Sun sensors, and ac excitation voltage for the GA LVDT's. A power dropout detector was provided for proper initialization of the ACE logic at power turnon, after a prolonged (>5 msec) power dropout, and for maintaining the established logic states within the ACE during momentary power dropouts of up to 5 msec. A 2.4-kHz logic clock was derived from the 2.4-kHz input power. Power switching of the 2.4-kHz, square-wave voltage to the CT and CT Sun shutter circuitry was also provided. In addition, when the TVC electronics were to be used, 30 V dc was supplied to both ACE's from PWRS; otherwise, the inactive ACE was unpowered.

The CCS-ACE interface consisted of an enable, data, and strobe line. Signal timing and message structure were as illustrated in figure 125. Each message consisted of 14 bits. The first received bit was always a 0 and did not convey any information for the ACE. The second received bit was a parity bit. Odd parity was employed. Bits dl to d8 were data, and bits al to a4 were address bits defining one of nine blocks. The ACE derived a 1.2-kHz data transfer clock from the 2.4-kHz clock which was generated from the 2.4-kHz input power. Since the 1.2-kHz clock was derived from the VO 2.4-kHz power source, it was in synchronism with the CCS commands which were also timed from the 2.4-kHz power. The data transfer clock was used to enter the 14-bit word

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