(7) A n a l o g I n p u t s . The Analog Channel Select sets a
c o d e corresponding to one of the analog input channels
(fig. 1-25). The input selected is routed to a comparator.
T h e microprocessor outputs an initial guess of the digital
value of the analog voltage; this is converted to an analog
value by the DAC, and presented to the comparator
alongside the input value. The comparator's output is used
Where θ represents a pitch or combined pitch-and-yaw
processor then makes a new approximation, based on the
angle of the probe. Sampled resolver secondary peak
comparator's output. After 12 such approximations the
voltages are fed into the CPU via the A-D converter
digital value is sufficiently close to the analog input.
circuit. The sine and cosine of each relevant angle is
o n l y one DAC and four analog channel select lines are
able to perform all the A-D/D-A conversions. Note also
(a) An AADS interface test is included to allow
t h a t the analog outputs are fed back through the analog
the CPU to test the operation of the sample and hold
multiplexer to enable further checking of errors. Fixed
circuitry. During this test analog switches connect the
reference voltages are used to perform calibration checks
sample and holds directly to the oscillator, bypassing the
on the DAC within the EPU.
(9) O u t p u t P r o t e c t i o n . Each ADS output signal is
(2) A i r T e m p e r a t u r e S e n s o r C i r c u i t . T h e A A D S
Air Temperature Sensor (ATS) is supplied with constant
t o l e r a t e d indefinitely without damage to the signal output
current from a current source circuit, and the resultant
voltage is sensed by a differential amplifier on the analog
interface unit. Voltage ouput from the differential
amplifier is fed to the A-D conversion circuit and the
digital result is then stored in the CPU memory. The ATS
t h a t potential damage caused by helicopter primary power
constant current source is switched on for a period of
is confined to that signal output only.
30mS once per second while the A-D conversion is
perfomed. This action is controlled by the processor to
reduce effects of self heating.
c. Analog Interface Unit (AIU). The analog Interface
U n i t (AIU) is the analog input and output interface for
(3) Radar Altimeter Input. The radar altimeter
the ADS LRUs. The signals which are processed include
provides a dc analog output (-7mV/ft) to the EPU to
AADS angle resolver signals, Air Temperature Sensor
(ATS), raw dc analog signals related to PTU pressure
i n d u c e d errors. The input voltage is applied to a buffer
i n p u t s , and outputs to the LAl and to doppler navigation
a n d scaling amplifier, the output of which is fed to the
system. A precision 10 Vdc reference is included on this
A-D conversion circuit. The digital result is stored in the
card. Peak sampling circuitry is used to convert AADS
accepted by the A-D converter.
d. P o w e r S u p p l y U n i t . The helicopter 28 Vdc supply
is filtered in a self contained EMC filter module (fig.
(1) AADS Interface. The oscillator provides a 400Hz
s i n u s o i d a l reference signal to the AADS angle resolvers
voltages,-5 Vdc, -6.2 Vdc and -10 Vdc are generated to
(fig. 1-26). The peak of this reference is detected, and
supply the boost regulator which increases the voltage up
used as a reference against which a peak sampling pulse is
to +80 Vdc by the action of a diode booster type circuit.
generated for the sample and hold circuits. Peak voltages
The 80 Vdc voltage is controlled by networks which sense
of the resolver outputs are sampled by the sample and