Phase Selection Timing - HP 7925D Service Manual

Appendix A 7925
A8"ENCA"
A8 "ENCB"
ONE REVOLUTION
.... 1 • . - - - - - - -
(540 Hz at 2700 RPM)
-------~~"'il
I
I
IIII
I~i
I
I
l I I I I
I
U
A8 "ENCP"
-
"'"-- ' - -
-
-
' - - ' - - - I....- I.....-..
l....-
I
I
n n n
I
I
A8 "PH1+"
I
(A· B)
I
I
I
A8 "PH1 "
(1\. Bl
A8 "PH2+"
(A·
B)
I
I
I
A8 "PH2-"
n n n
(A
·S)
7311-40
Figure A-7. Phase Selection Timing
The left- and right-most bits of the shift register are moni-
tored by the speed up detector. When these bits remain
unchanged for approximately one-half a second, the motor
is declared to be at speed. The green SPD LED at the
output of the speed up detector will remain off until the
spindle is declared to be at speed.
If the spindle begins to loose speed slightly, the encoder
pulse that was supposed to shift the "I" out of the center
bit will be late. This will cause an increase in the center
bit duty cycle, an increase in the spindle current com-
mand, and more current to be delivered to the motor until
it returns to speed.
The output from the center bit of the shift register is
buffered and filtered to produce a smooth dc voltage which
represents the spindle current command.
The current command limiter reduces the spindle current
command during the braking operation. The spindle cur-
rent command is applied to the input of the current regu-
lation circuit. It can be observed at the test point labeled
"SCC".
A-38. MOTOR CURRENT REGULATION. The
motor current regulation circuitry compares the smooth dc
A-14