SERVO AMPLIFIER BOARD (SVA)

4.5.5.1 Board Functions. The main functions performed by the logic circuits on this circuit board include:

A. Provide drive current to positioner motor B. Generate BOT and Detent pulses

C. Energize Servo motor relay

A block diagram of the SVA circuit board, showing the principal signal paths and functional blocks, is shown in Figure 4-23.

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1132-001

-POSITION CHANNEL AMPLIFIER AMPLIFIER DETENT EVEN (-) __

(INVERTING) H

..-

-Figure 4-23. Block Diagram of Servo Amplifier Board (SVA)

4.5.5.2 D DAC Input. The SVA receives the output of the Digital-To-Analog Converter from the SLB. In the Velocity Ramp Generator it will appear as shown in Figure 4-24. The illustration represents the output of the D-to-A Converter as it converts the difference count from the sub-tractor. Note that when the difference count is maximum, the D DAC signal is maximum ampli-tude; and as the difference count reduces (value in the CAR approaches the value stored in the NAR), it will correspondingly decrease in amplitude. The output of the Velocity Ramp Generator and its inverted signal are applied simultaneously to two FET's.

During a LOAD or UN LOAD operation, the corresponding FET (LOAD or UN LOAD) will be activated by the state of the LOAD/UN LOAD signal and either the high Q or low Q output of the

FWD f-f from the SLB to produce the Servo Velocity signal.

During a Programmed Seek operation, the corresponding FET (FWD or REV) will receive the Velocity Ramp Generator signal. The selection of one of these FET's is accomplished by the high or low Q output of the FWD f-f which is enabled by the FWD/REV EN signal from the SLB. The Servo Velocity signal would then be applied to the Summing Amplifier and the FWD or REV Driver to develop the Servo Amplifier Output signal for carriage positioning.

4.5.5.3 Electronic Tachometer Network. This network monitors the Position Transducers POS CHANN E L signal. The network senses the carriage velocity and generates a voltage that is applied to the same two FET's as the Velocity Ramp Generator signal. A correction voltage will appear at the output of the Summing Amplifier to either speed up or slow down the carriage to its intended speed. The velocity signal is also applied to the LOAD/UN LOAD FET's and controls those velocities in the same manner.

4.5.5.4 Positioner Transducer Signals. These signals are routed to the SVA from the Positioner Transducer through the Detent Amplifier PWB. They are used on the SVA to produce the Elec-tronic Tach signals, Detent ODD and EVEN pulses, Detent Fault Detection signal and for final DETENT operation. The signals are called POS CHANNEL 1 and 2. The DETENT signal is a com-posite signal derived from the outputs of the photo diodes in the Positioner Transducer POS CHAN

1 and POS CHAN 2. (Generation of electronic tach signals is explained in the preceding paragraph.)

Detent ODD and EVEN pulses are generated by shaping the DETENT signal into digital pulses which are used on the SLB.

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SUPER SERIES

Figure 4-24. New Address Seek Operation Waveforms

The Servo Velocity signal is active until the ReadlWrite heads are within one-half track of the address stored in the NAR. At this time, the Detent Enable (DETENT EN) signal is generated, the Coarse Servo Loop is disabled, and the Fine Servo Loop is enabled. The servo detents (locks) the heads in place over the selected track. If the last track is even, detenting takes place on the negative slope of the DETENT signal. If the last address bit is odd, detenting takes place on the positive slope of the DETENT signal.

4.5.5.5 Track Offset. The Super Series Disk I)rives have, as a customer option, the ability to retrieve marginal data. Two inputs from the controller allow the positioner to be moved a pre-determined amount, approx. % mil, (0.0127 mm) in the desired direction (FWD or REV). These signals (TRK OFFSET FWD or TRK OFFSET REV) are used to recover marginal data during Read operations and must not be used when writing. Offsetting is accomplished by injecting a small current into the Detent circuitry.

4.5.5.6 Beginning Of Travel. The BOT pulse from the Positioner Transducer is amplified and shaped on th is board before being routed to the S LB.

4.5.5.7 Positioning Servo System. Head positioning to a desired track is accomplished by means of the Positioner servo system. This system, a block diagram of which is shown in Figure 4-25, consists of a positioner actuator, a velocity servo loop, a position servo loop, a summing amplifier, and a servo amplifier. When the spindle motor comes up to speed, the I DX pulses set the RDY 1 f-f and cause LD HD signal to be generated and the LOAD/UN LOAD line goes high. The FWD signal is generated on the Servo Logic Board (SLB) when the LOAD f-f is set. The LOAD (L) signal goes low causing the FWD f-f to be reset. Then FWD (L) goes low and LOAD/UN LOAD goes high, and the Servo Amplifier is activated in the forward direction.

4.5.5.8 Ready I nhibit. This signal is generated only for 200 TPI disk drives. It is the output of the AMBIENT TEMP DETECTOR which is derived by comparison of the two temperature sensors (baseplate and air). When the two temperatures are equal, the RDY INHIBIT signal will be high allowing the READY signal to be sent to the Controller~ This circuitry effectively inhibits controller READ or WRITE commands to the disk drive until proper temperature balance is a-chieved.

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