EVEN \
I
ODDI
CYLINDER CENTER
I DATA HEAD
SERVO HEAD
I
EVEN ODDFIGURE 3-25. DEFINITION OF CYLINDER CENTER
3-31
Track following the cylinder center is accomplished by the servo head. It predetermined number of microinches (proportional to position signal amplitude) off either side of the cylinder center (offset). Performing this operation is useful during error recovery and head alignment verification procedures. For example, if data errors are experienced, the heads can be offset to both sides in an attempt to recover the data.
To initiate an offset operation, the DCl sends amount-of-offset information and an OFFSET START command to the drive. The drive induces into the servo circuits an "error" voltage which is proportional to the required amount of offset. The data heads track follow off cylinder center by an amount and direction proportional to the amplitude and polarity of the error voltage.
3.4.2.3 Emergency Retract Mode
Only one positioning operation can occur in this mode, this being an emergency-retract operation.
In the emergency-retract operation/mode, the heads are unloaded and retracted from the pack whenever a serious malfunction warrants such action. Examples are logic voltage failure, retract failure, overcurrent failure, and supply voltage failure. During this operation, heads are cammed away from the disc surfaces as the heads move in reverse out of the pack.
Unlike the "normal" retract operation in the Velocity Control mode, the emergency-retract operation cannot be initiated by either the host computer or DCL.
677-01/51.20-00-12/75
3.4.3 Access Mechanism
The access mechanism, consisting of the VCM (Voice Coil Motor) and carriage assembly, is used to position the read/write heads over a preselected cylinder. The VCM and carriage assembly are part of the drive's deck plate, shown in Figure 3-26. Referring to this figure, the heads are
The read/write heads and the servo head are mounted on precision stainless steel arms. As discussed in paragraph 3.4.2.1, heads are cammed away from the disc surfaces as they move in reverse out of the pack, or cammed down to flying position as they move forward into the disc pack. All heads are similarly constructed. See paragraph 3.5 (Read/Write System) for a mechanical description of the heads.
The tachometer assembly consists of a magnetized rod, attached to the back of the T-block, and a tachometer coil. As the carriage moves back and forth, the signal generated within the coil corresponds to the speed at which the rod is traveling. This signal is fed to the servo electronics to control the speed during seek operations.
A continuous seeking operation causes the coil to rise in temperature. For this reason a continuous stream of cool air is circulated through and around the coil. This air stream is generated by pack rotation and is guided around the entire motor area. For details of the cooling provisions built into the drive, see paragraph 3.8 (Airflow and Filtration System).
The carriage, T-block, carriage way, tachometer rod, tachometer coil, and motor assembly are precision aligned at the factory. Clearances are maintained between the stationary section and the moving or suspended sections. Field adjustments are possible using precision tools which are available. Adjustment procedures are included in Section 4.0 (Maintenance)).
See the 677 OEM Illustrated Parts Catalog for a breakdown of parts in the access mechanism, tabulated in the recommended order of disassembly down to available field replaceable units, and information on the required maintenance tools.
3-32
PACK COVER SENSOR ASM
READ/WRITE HEADS (SERVO HEAD 5th FROM TOP.
LEFT SIDE)
SHROUD ,.-- ASSEMBLY
!I&~-",,.I----BAFFLE
TACH HOUSING (END VIEW ONLY)
AIR EXHAUST DEFLECTOR ASSEMBLY (WIND TUNNEL)
FIGURE 3-24). MAJOR COMPONENTS OF DECK PLATE 677-01/51.20-00-12/75
3.4.4 Servo Surface
Using a sketch of segments of the servo surface and the recording surface located directly underneath, Figure 3-27 presents salient points of the servo disc format. This figure is self-explanatory and should be understood before proceeding.
3.4.5 Servo Head
The paragraphs below describe how signals from the servo head enable the system to accomplish proper positioning of the read/write heads. The description focuses on the basic concept of Even Servo Data bordering on Odd Servo Data. As shown in Figure 3-27, this border is the common centerline of data tracks in the cylinder located directly above and below in the disc pack.
3.4.5.1 Servo Carrier
Signals from the two adjoining bands of Even Servo Data and Odd Servo Data are termed E and 0, respectively. The servo head contains a read element spanning across one track width. Therefore, if the head is centerE~d
over Even Servo Data, the peaks (positive or negative) in the E wavetrain are equal to 100% of maximum E. At the same time, the peaks (negative or positive) in the
a
wavetrain are equal to approximately 100% of maximum O.The combination of E and
a
signals from the adjoining servo bands constitutes a servo carrier containing information on the magnitude and direction of off-track. This means that for a given carrier, the head is either centered, off-track on one side of the border, or off-track on the other sid,e.3-33
DISC PACK FLUX REVERSALS PRERECORDED.
(SOUTH-SOUTH, NORTH-NORTH) FLUX REVERSALS PRERECORDED.
(NORTH-NORTH, SOUTH-SOUTH)
CYLINDER-SERVO TRACK RELATIONSHIP
• SERVO CIRCUITS DETERMINE DATA CYLINDER POSITIONS BY ANAL YlING SIGNALS RECEIVED FROM EVEN AND ODD SERVO