Figure 1-17. Force Balance Transducer.

9-1270-219-13&P

Figure 1-17. Force Balance Transducer.

permanent

magnet.

For

the

Pitot-Static

transducer,

pitot

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

and sealed.

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

length

any

detectable

amount

under

normal

resultant signal is detected by the control electronics

operation

conditions.

which

supplies

feedback

current

the

forcer

coil

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

card

contains

control

loops

drive

the

transducers

voltage

buffers

and

Programmable

Read

Only

Memories

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

transducer's

temperature

response

(fig.

1-19).

(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

secondary

alters

with

respect

the

primary

the

manner, but differs in that pressure is supplied to the

pick-off turns from its null position. The phase-shift is detected

1-19