Figure 1-17. Force Balance Transducer.
c a p s u l e only, with the precision bellows being evacuated
p r e s s u r e is delivered to the interior of the bellows and
static pressure to the interior of pressure capsule. If the
P i t o t pressure is greater than the Static, the bellows will
(d) Response to pressure changes is very rapid. The
bellows range of movement is restricted to less than 0.002
attempt to expand, and cause the rotary pick-off to turn.
This minute rotation is sensed by the pick-off and the
i n c h total movement, therefore the bellows will not alter
resultant signal is detected by the control electronics
r e t u r n bellows and beam to the original (null) position. A
drop in pitot pressure will have the reverse effect (less
(e) A small temperature-sensing coil is mounted
within each transducer so that transducer temperature
current will be required by the forcer coil).
may be monitored by the CPU.
(a) The current required to null the pick-off is
(2) C o n t r o l L o o p E l e c t r o n i c s . The printed circuit
proportional to the pressure difference, and this current
v a l u e is m o n i t o r e d by th e PT U con t r o l loo p ele c t r o n i c s .
(PROM), to supply the CPU with information for each
(b) Output linearity of the transducer is improved
by supplying the Linearity coil, mounted in the magnet
(a) A 5kHz oscillator drives the rotary pick-off
assembly, with a proportion of the forcer coil's current.
primary coil. The phase of the induced signal in the
(c) The static transducer functions in a similar
manner, but differs in that pressure is supplied to the
pick-off turns from its null position. The phase-shift is detected