Phase Sequence Of Stepper Motor - Control Data Corporation BR8 SERIES Hardware Maintenance Manual

h. At initial voltage application, the two flip flops of U25 are asynchronously reset through
C
the action of comparater U30. At the instant the 5Vappears, U30 pin 1 will go high, ./
4-4
forcing pins 1 and 13 of U25 low, and forcing output pins 6 and 8 of U25 high. As
capacitor C43 is charged, U30 pin 1 will switch low, assisted by hysteresis. This
will cause the reset signal to go high and remain high as long as the +5 volts remains
present.
Each step co'mmand received sequentially energizes one of the three phases of the
stepper motor. Either the logic level of the Direction signal or the use of Step
In
or Step Out signals determine whether the phase sequence is 0 A, 0B, 0C,
0A . ..
or
0A,
0C, 0B, and
0A.
Movement of the Read/Write head is initiated by the step commands from the controller.
the head is stepped one track, either toward the spindle
(In)
or away from the spindle (Out),
with each step command. The direction is regulated by the status of the Direction line
(a low level of the Direction line causes the Read/Write head to step toward the spindle,
and a high level causes the Read/Write head to step away from the spindle) or by com-
manding either the Step
In
or Step Out Line.
Drive to the specific stepper motor phases, A, B, and C, is provided by logic-driven
darlington transistors Q3,4, and 5. The logic drivers also decode the activated step-
per phase information from the outputs of U25 's flip flops. The leading edge of a step
command triggers the three microsecond one-shot U16, whose trailing edge clocks the
flip flops.
The phase sequence through which the stepper motor is driven (see Table 4-1) is
controlled by the steering logic: gates U27 pin 8; U27 pin 11; U28 pin 3; and U28
pin 6. These gates receive direction information from U22 pin 4. The output of
this gate is high during the rising edge of the flip-flop clock for the step in sequence.
It is low during the step out sequence. Encoded information, indicating the active
stepper motor phase at the time a step command has been issued, is fed to the"steer-
ing logic from flip flop U25 output pins 52 6, 8, and 9.
Tabl e 4-1. Phase Sequence of Stepper Motor
Change the
To Step
Active Phase
0
From
To
A
B
B C
IN
C
A
A
C
B A
OUT
C B
The power reduction option, which reduces the stepper motor power 15 milliseconds
after the last step command, is accomplished through the retriggerable one shot
multivibrator U26 at pin 7. The time out of this one shot is approximately 15 milli-
seconds. Step commands which occur at a repetition rate of 10 milliseconds will
hold U26 low. This has the effect of holding transistor Q2 on, and clamping the center
(~
,--,
tap of the stepper motor at
+
24 volts. When the last step command is received, the ( '
one shot will time out and go high. This turns Q2 off, leaving the center tap of the .
stepper floating. The inductive reaction of the stepper motor winding in use at t h a t -
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