Pneumatic analog-to-pulse frequency converter circuit has output pulse frequency proportional to the pressure level of the input signal. Converter circuit drives a pneumatic stepping motor and is part of a pneumatic control system for nuclear powered spacecraft

Dustin, M. O.

English

NASA Technical Reports Server

I I I	 Frequency  
NtI./ 
Pressure 
on qn	
2	 33	 4 
200 
160 
120 
80 
in 40 
0 
P	 40 
80 
0	 120 
0
160 0
200
August 1969	 Brief 69-10276 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information; Springfield, Virginia 22151. 
Pneumatic Analog-to-Pulse Frequency Converter 
Output selection 
- Circuit input pressure adjustment 
iressure. P -------------------------------------- -,Backward 
pulse output
Output Selector circuit pulse output 
-1----------------------
Oscillator input 
pmsscreP mtdor 
input pressure	 I I C2 JC 
LA 3 V-2 Nut	 pneirsu re
C, 
Conditioning circuit	 I	 Oscillator circuit
circuit Input Pressure. PIN. psig1 
Normal Operating Range.. 
Figure 1 
A pneumatic analog-to-pulse frequency converter 
circuit was designed in which the circuit output pulse 
frequency is proportional to the pressure level of the 
input signal. The circuit incorporates three unique 
features not used in other pneumatic circuits. 
1) A pneumatic oscillator whose output is a function 
of its input pressure and operates uniformly to zero 
pulses per second. 
2) The output frequency of the circuit is propor-
tional to the deviation of the input pressure from a null 
pressure (Figure I). The null pressure is the pressure 
at which the output frequency is zero and is set mid-
way in the normal operating range of the input pres-
sure signal. When the input pressure is increased 
above this value, the output pulse frequency increases 
and appears at one of the two output ports. As the
Prcpurtiunal amplitier 	 NOR-gate 
Bistablo amplitrer	 Center-output proportional amptitier 
Figure 2 
input pressure decreases from the null pressure, the 
output frequency also increases but appears at the 
other output port. 
3) The circuit saturates at a maximum frequency in 
either port and remains at this frequency for input 
pressures greater or less than their normal range. 
This converter circuit was developed for driving a 
pneumatic stepping motor in either a forward or back-
ward direction depending on the magnitude of an 
analog pressure signal, and was a part of a pneumatic 
control system for nuclear powered spacecraft. In loca-
tions close to a nuclear rocket engine, the nuclear 
radiation level is higher than the tolerance level of 
most electronic circuit elements. Hence systems using 
flueric devices, which appear to be inherently insensi-
tive to radiation, are being investigated. The flueric 
(Cirillriruled Irverictri) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19690000276 2020-03-16T17:47:38+00:00Z
converter may be of interest for other pneumatic con-
trol systems requiring a fluid oscillator which converts 
an analog pressure signal to pulse frequency and has a 
frequency range down to zero pulses per second. This 
converter may also be used in other situations where 
it is desirable to sense or use an analog pressure signal 
to actuate pneumatic devices. 
The oscillator circuit shown in Figure 2 is composed 
of three bistable amplifiers with R-C time lags in the 
feedback paths. Amplifier A-I outputs are connected 
through resistances and volumes to the control ports 
of two other amplifiers, A-2 and A-3. One output of 
each amplifier A-2 and A-3 is then connected to the 
control ports of amplifier A-I. The input pressure P2 
to the oscillator circuit is connected to both the remain-
ing control ports of amplifiers A-2 and A-3. 
To follow the sequence of operation of the oscillator 
circuit, assume the output of amplifier A-I is switched 
to resistance R 2 . The pressure in volume C 2 rises in 
a manner approximating a first order time constant 
system. When the pressure in volume C2 is slightly 
higher than the oscillator input pressure P 2 , amplifier 
A-2 will switch to the control port of amplifier A-I. 
This will switch amplifier A-I to resistance R 3 . As the 
pressure in volume C 2 drops below the oscillator in-
put pressure P 2 , the amplifier A-2 will switch from the 
control port of amplifier A-I. However, since amplifier 
A-I is bistable, its output will remain switched to R3. 
Similarly the pressure in volume C 3 rises until it is 
slightly higher than the oscillator input pressure P2 
at which time amplifier A-3 switches to the control port 
of amplifier A-I. Amplifier A-I then switches to R2. 
When the pressure in C 3 drops below the oscillator 
input pressure P 2 , amplifier A-3 switches from the 
control port of amplifier A- 1. This completes one cycle. 
As the input pressure P 2 increases, more time is 
required for the volume pressures to exceed P 2 . This 
decreases the pulse frequency. The frequency becomes 
zero when the input pressure P 2 equals the maximum 
obtainable volume pressure. 
The conditioning circuit is shown schematically in 
Figure 2. The circuit is required to provide an input 
to the oscillator circuit that is a maximum when P IN 
is at the null value and decreases when the input pres-
sure P IN to the converter either increases or decreases 
from the null value. The maximum conditioning cir-
cuit output pressure corresponds to that oscillator 
circuit input pressure P 2 which causes the pulse 
frequency to be zero. The minimum output pressure 
from the conditioning circuit corresponds to the oscil-
lator circuit input pressure which causes the oscillator 
circuits pulse frequency to be the desired maximum 
value. To provide flat saturation characteristics, the 
minimum conditioning circuit output pressure must 
not fall below this value.
The conditioning circuit consists of two proportional 
amplifiers. Amplifier A-4 in the diagram is a center 
output amplifier whose output is maximum when the 
control port pressures are equal. The output pressure 
decreases when the control port differential increases 
in either direction. Amplifier A-5 is a proportional 
amplifier whose differential output is proportional to 
its differential input signal. An additional flow source 
is provided through valve V-I. This source maintains 
the output pressure of amplifier A-4 at the desired 
minimum pressure. Valve V-2 is used to set the null 
pressure at the desired value. 
The output selector circuit directs the oscillator out-
put pulses to the correct output port. If, as shown in 
Figure 2, the converter input pressure PIN is greater 
than the null pressure, the pulses appear at the for-
ward pulse output port. If the input pressure PIN is 
less than the null pressure, the pulses appear at the 
backward pulse output port. 
Amplifier A-6 in Figure 2 is an NOR gate that serves 
to amplify the pulses produced by the oscillator circuit. 
The output of amplifier A-6 is applied to the supply 
port of bistable amplifier A-7. The pulses are directed 
toward either the forward pulse output port or the 
backward pulse output port depending upon which 
control port pressure of amplifier A-7 is greater. Pro-
portional amplifier A-8, which provides the control 
input pressures to amplifier A-7, serves to increase the 
switching sensitivity of the selector circuit. Valve V-3 
is set so that amplifier A-7 switches when PIN equals 
the null pressure. NOR gates A-9 and A-10 amplify the 
output pulse pressures. 
Notes: 
I. Documentation is available from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69- 10276 
2. Technical questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B69-10276 
Patent status: 
This invention is owned by NASA, and a patent ap-

plication has been filed. Royalty-free, non-exclusive 
licenses for its commercial use will be granted by

NASA. Inquiries concerning license rights should be

made to NASA, Code GP, Washington, D.C. 20546. 
Source: Miles 0. Dustin
Lewis Research Center
(LEW- 10345) 
Brief 69-10276	 Category 02


Pneumatic analog-to-pulse frequency converter

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