TM 5-6675-323-14
ag.
D i g i t a l t o a n a l o g c o n v e r t e r . The DAC U7 produces differential output
currents which are linear products of the digital input and the dc current reference
at pin 14. The op. amp. U3 converts the differential currents to a voltage source
f o r the analog switches U2 and U4.
If the plotter motors are stationary, the power
control U27 turns the transistor Q2 on, shunting the reference circuit, reducing the
output current from U7.
ah.
I n t e r p o l a t o r o u t p u t . D u r i n g each subcycle, U3 must output four discrete
s i g n a l s i n e a c h a x i s . T h e s e are:
cosine 3rd harmonic,
cosine fundamental plus 3d harmonic,
sine 3rd harmonic,
sine fundamental plus 3rd harmonic.
( 1 ) D e m u l t i p l e x e r s , i n the form of analog switches U2 and U4 with their
a s s o c i a t e d buffers U1 and U8, combine the fundamental and harmonic pairs and then
separate these pairs into the four analog signals for the motor drivers.
phase quarter amplitude 3rd harmonic.
(2) At the start of each subcycle, the X cosine 3rd harmonic is output by
U7. U 4 - S 1 is closed, and the value of the harmonic is stored on C34 and buffered by
U 8 A to the X and Y 3rd harmonic adjustment potentiometers R38 and R39. At this
t i m e , the X cosine fundamental plus 3rd harmonic is output from U7. The analog
s w i t c h U4-S2 and U4-S4 are closed, as is U2-S4.
A portion of the 3rd harmonic,
selected by the setting of R38, is subtracted from the fundamental in U3. The
result is stored in C8, buffered by U1C, and applied to the X phase-B motor driver.
The same sequence is followed for the X sine, Y sine, and Y cosine in order.
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