Motor Supply Sensor Circuit; Motor Servo System Block - HP 7475A Service Manual

Section VI
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1
STOP-I
(7 .5°)
7475-A-15-1
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L30msecj
(a)
CW SEQUENCE
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Model 7475A
(b) CCW SEQUENCE
Figure 6-4. Quadrature Waveforms
r
ENCA
FROM
PROCESSOR DEC
SERVO ENC B
THROUGH
IC
GATE ARRAY
X 2
X4
SIGN
SERVO
CONDITION
MOTOR
._
OPTICAL
PULSE CIRCUIT
DRIVERS
__.
ENCODER
(PART OF
GATE ARRAY)
STRETCHED
PULSE
PULSE WIDTH
=>
DIGITAL
DATA FROM
GAIN ADJUST
+18/26V
PROCESSOR
(PART OF
UNREGULATED
GATE ARRA YI
7470-A-15-1
Figure 6-5. Motor Servo System Block Diagram
6-39. Digital move commands are generated and sent
by the microprocessor through the gate array U5 to the
servo chips U3 and U4. The servo chips provide the
interface to the microprocessor, decodes the encoder sig-
nals, sums position errors, estimates velocity and sums
it, and transforms the servo error to a pulse-width-
modulated output. The servo chips output the pulse-width-
modulated signal back to the gate array where the motor
drive pulses are stretched to the proper width in the digital
gain circuit. The servo conditioning circuit, also in
the gate array, then passes the pulses on to the motor
drivers through either the XA or XC line, depending on
the direction of rotation indicated by the sign voltage
from the servo chip. As the mechanical system moves,
optical encoders mounted on the shaft of each motor
send hack digital pulses to the servo chip to close the
servo loop.
6-40. To maintain a consistant and predictable move-
ment, it is essential to control the amount of power
applied to the motor by each pulse. The pulse amplitude
6-6
depends on the actual voltage output of the motor drive
power supply. The pulse width is modified to compen-
sate for pulse amplitude so that the pulse represents the
proper amount of power.
6-41. MOTOR SUPPLY SENSOR CIRCUIT
6-42. The power supply for the drive motors is unre-
gulated. For the gate array logic to know how much to
stretch the servo chip pulse width, the microprocessor
must know the level of unregulated voltage supplied to
the motors. The voltage sensing circuit uses comparator
UlO (U9) to provide single bit analog to digital conver-
sion at intervals of approximately five seconds. The
resulting digital information is used to regulate power
through the motor drive circuits.
6-43. A divider R16 (R14) and Rl 7 (R15) places one-tenth
of the power supply voltage at the non-inverting input to
the comparator. Pulses of known value from U6 Gate
Array B are applied to an integrating capacitor C11 (C8)
at the inverting input to the comparator. The pulse