An analysis of the electronic simulation of barometric pressure used to self-test the counter electronics of the digital barometer is presented. The barometer is part of the Meteorological Monitor Assembly that supports navigation in deep space communication. The theory of operation of the digital barometer, the design details, and the verification procedure used with the barometric pressure simulator are presented

Duff, L. W.

Guiar, C. N.

English

NASA Technical Reports Server

14335
TDA Progress Report 42-71 July - September 1982
Electronic Simulation of a Barometric Pressure Sensor
for the Meteorological Monitor Assembly
C. N. Guiar and L. W. Duff
DSN Engineering Section
This article presents an analysis of the electronic simulation of barometric pressure
used to self-test the counter electronics of the digital barometer. The barometer is part of
the Meteorological Monitor Assembly that supports navigation in deep space communica-
tion. The theory of operation of the digital barometer, the design details, and the verifica-
tion procedure used with the barometric pressure simulator are presented.
I. Introduction
The Meteorological Monitor Assembly (MMA) is one of the
subsystems in the Deep Space Network's technical facility
which provide the meteorological information necessary for
calibrating radiometric data in support of navigational require-
ments for various deep space communication projects. The
assembly as shown in Fig. 1 includes equipment required for
the measurement, data processing, and handling of the follow-
ing major types of data:
(1) Barometric pressure.
(2) Rainfall/water precipitation rate.
(3) Dewpoint temperature of ambient air.
(4) Azimuth and elevation angles of the large, 64-m
antenna.
(5) Faraday rotation.
(6) Polarization angle.
(7) Ground level solar radiation.
(8) Dry bulb temperature of ambient air.
In supporting field operators, a need evolved for verifica-
tion of the operation of the barometric pressure instrumenta-
tion (digital barometer) using self-test electronics. The baro-
metric pressure simulator was developed for this purpose and
is presented by the block diagram in Fig. 2. This article reports
on the design details and verification procedures incorporated
in the barometric pressure simulator development.
II. Barometer Functional Description
Schematic diagrams of the Bell & Howell digital barometer
(P/N 4-461-004) are shown in Figs. 3 and 4. The digital barom-
eter basically consists of a digital pressure transducer and a
front panel light-emitting diode (LED) display. The barometric
pressure to be measured is received at the rear panel connector,
where it is applied to the input of the transducer. The trans-
ducer converts the pressure into a frequency which varies
linearly with pressure. This signal is fed into a presettable
counter where it is converted to a digital data signal. The digital
output signal is applied to the rear panel connector as a paral-
lel binary coded decimal (BCD) output, and also to the front
panel LED assembly, which produces a five-digit display.
95
https://ntrs.nasa.gov/search.jsp?R=19830006064 2020-03-21T04:53:05+00:00Z
A precision dual-capacitor pressure-sensing element gener-
ates two variable frequencies, one varying directly and the
other varying inversely with the pressure applied to the input
fitting. The mixer accepts these two frequencies as input and
provides an output signal which has a frequency that is the
difference between the two input frequencies and varies
linearly with pressure change. The purpose of processing the
signal in this manner is to provide a more stable and accurate
frequency input for the counter electronics. The frequency
signal is fed to a digital counter that is programmed to sample
the output frequency of the sensing element. The time period
of the sample count can be varied by setting the span-adjust
switches which will provide a readout in the desired units.
The digital barometer is currently equipped with a capa-
bility to self-test the display. When the test switch on the front
panel is momentarily pressed to the ON position, all segments
of the digital display are lit, producing a display of 88888.
Any LED failure in the display will thus be revealed. This was
the only self-test feature available with this type of barometer.
In the event that any counter circuit in the group U1-U5 as
shown in Fig. 4 fails (e.g., bit malfunction), there is no self-
contained means of determining this failure. The counter cir-
cuits can be tested by providing a known frequency /0 to the
counter inputs, producing a BCD output that should correspond
with /0. Therefore, if the BCD output does not correspond
properly to the input frequency, it can be assumed that the
counter electronics within the barometer have malfunctioned.
III. Barometric Pressure Simulator
The barometric pressure simulator provides known fre-
quencies to the inputs of counters U1-U5 (Fig. 4) to deter-
mine if they are counting correctly. Frequency-generating cir-
cuitry and delay, latch, and flag electronics as shown in Fig. 5
comprise the barometric pressure simulator. Details of these
electronics are given in the remainder of this section.
A. Frequency and Barometric Pressure Relationship
In determining the factor by which the circuit scales simu-
lated barometric pressure according to frequency, a frequency
generator was connected directly to the input of the barom-
eter counter electronics (U13) in parallel with a frequency
counter measuring system as shown in Fig. 6. Several baro-
metric pressure readings were taken for various frequency
values. The resulting test data given in Table 1 indicate that a
change in frequency of approximately 7.4 kHz produced a
1.0 mbar1 change in barometric pressure. The linear frequency-
pressure relationship can be assumed to take the form:
f = m p + b (1)
where / is the frequency in kHz, p is the pressure in mbars, m
is the slope in kHz/mbar, and b is the ordinate intercept in
kHz. If frequency is zero, the pressure is zero; therefore, b
is zero. The average slope, determined using the data in
Table 1, is 7.4 ± 0.5 kHz/mbar.
A resolution of 12 bits was chosen to provide a suitable
range of frequencies for use in barometric pressure simulation.
The 12-bit word, transferred through the latching network
from the HP9821A calculator, is used as the input for three
cascaded, synchronous, 4-bit counters as shown in Fig. 5. The
least significant bit output (LSB) provides the necessary fre-
quency and is determined by:
clock frequency
JLSB (2)
'One millibar (mbar) is equal to 0.0147 psi or 0.101 kPa.
where b± is the input bit value for first counter, Z>2 is the input
bit value for second counter, and i3 is the input bit value for
third counter.
The value 16, used in the denominator, corresponds to the
number of increments from binary 0000 to 1111.
A 6-MHz crystal-controlled oscillator provides a constant,
stable clock signal for control of the digital electronics. The
actual frequency used for barometric pressure simulation (/0)
is generated by subtracting the LSB frequency from the clock
frequency. Table 2 shows a few examples for various 12-bit
inputs and their corresponding simulated barometric pressure.
B. Handshake
A simplified block diagram of the HP11202A I/O interface
is shown in Fig. 7. The interface provides 8-bit data exchange
between the HP9821A calculator and the barometric pressure
simulator. Data bits DO-D5 are used for data information and
D6 and D7 for control.
Data is transferred in a "half-duplex" mode, meaning data
can be either input or output, but not both at the same time.
All data and control lines are compatible with standard TTL
levels.
When the calculator is given a WRITE command a Select
Code Signal is output. The Select Code Signal is used to address
the correct device. The status of the Service Interrupt line
(SIO) is then checked by the calculator. If a low signal is
detected on the SIO line the interface is not busy, allowing
data to be held low on the chosen calculator output lines
96
(DOO-D07). The Calculator Status Line (SOS) signal then sets
the I/O line low to indicate that an output operation has been
initiated. Next, the Control Enable Line (CEO) and SELECT
code signals are output. The CEO signal provides the correct
timing for interface operations. If the Select Code Signal
addresses the correct device, data is ready to be transferred to
the output lines. The device control line (CTL) is transmitted
to the barometric pressure simulator 5 /usec later, indicating
data has been transferred to the output lines (DO-D7). All
data will be held on these lines until the barometric pressure
simulator returns the Device Ready (FLG) signal, indicating
data have been accepted.
C. Latch and Delay
The six data bits (DO-D5) are connected to three 4-bit
bistable latches. These latches are used as temporary storage
for binary information between HP9821A calculator and fre-
quency generating electronics. Since 12 data bits are needed in
the frequency-generating circuitry, data transfer takes place in
two segments. The first segment involves sending six bits
through one-half of the latch electronics, with the second
transfer sending six more data bits through the remaining half.
Data bits D6 and D7 are used for the interchange of data
and control of the transfer of data through the latching elec-
tronics and into the barometer. Timing between control com-
mands and data inputs is very important if latching is to take
place in the correct sequence. Once a FLG signal is returned
to the calculator at the end of each WRITE command, all data
on the calculator output lines is cleared. Delay circuitry is pro-
vided which allows the control commands to reach the latch-
ing network only when data information (DO-D5) is present.
D. Verification of Simulator Operation
The final function of the barametric pressure simulator pro-
vides a means for data verification. Since a known frequency is
provided, the BCD output of the barometer can be checked by
transferring this data back to the HP9821A calculator for com-
parison. If the calculated frequency does not agree with the
BCD output frequency by approximately ±0.5 mbar, it can be
assumed that the counter electronics of the barometer have
malfunctioned and correction measures should be applied.
IV. Summary
The barometric pressure simulator provides a method for
self-testing the counter electronics of the Bell & Howell digital
barometer using electronic simulation of barometric pressure.
A prototype model of the simulator has been built and tested
and will soon be integrated into the digital barometer for
placement in the MMA, thus supporting field operators in the
area of barometric data verification. Operating instructions,
theory of operation, and associated software for the baro-
metric pressure simulator will be published in an operation and
maintenance manual.
97
ORIGINAL PAGE li
OF POOR QUALITY
Table 1. Test data for determination of scale factor for pressure
vs frequency
Barometer
reading,
mbars
606.8
596.8
506.0
406.7
306.7
206.9
Frequency
reading,
MHz
4.509
4.435
3.763
3.031
2.292
1.556
A Pressure,
A Frequency
kHz/mbar
7.4
7.44
7.38
7.38
7.35
Average = 7.4 kHz/mbar
Table 2. Sample 12-bit inputs and their corresponding simulated
barometric pressure
12-bit input
0111
0111
0111
0111
0100
0101
0111
1001
1010
1000
0000
1001
1111
1111
1111
1111
1111
1111
1111
1111
1111
(1)
95.24
111.1
83.3
41.7
71.4
545.4
666.7
(2)
797.9
795.8
799.5
805.2
801.2
737.1
720.7
(3)
795.4
793.3
797.5
802.7
798.7
734.6
718.2
(1) Frequency at LSB = • 6 MHz
(2) Simulated barometric pressure = P = _ .7.4 kHz/mbar
(3) Corrected barometric pressure = (f - 2.5) mbars
98
TOWER-MOUNTED SENSORS
ORIGINAL PAGE IS
OF POOR QUALITY
CONTROL ROOM EQUIPMENT
1
TEMPERATURE
SENSOR
DEWPOINT
SENSOR
STATIC
HEAD
BOOM ASSEMBLY
L_
GROUND LEVEL SENSORS
r
RAINGAGE
ASSEMBLY " 1
SOLAR
RADIATION
SENSOR
ASSEMBLY
IONOSPHERIC
RECEIVER
ANTENNA
TEMPERATURE SENSING SIGNAL
DEWPOINT SENSING SIGNAL
BAROMETRIC SENSING SIGNAL
S- AND X-BAND
NOISE TEMPERATURE
ANTENNA POINTING
ANGLES
POLARIZATION ANGLE
AND MODE STATUS
PRECIPITATION SENSING SIGNAL
SOLAR RADIATION SENSING SIGNAL
DAY AND TIME
SATELLITE POLARIZATION SIGNAL
r __ _. _ _,_™- —
BAROMETER
VALIDATION
IONOSPHERIC
A/D
AND SCANNER
MAGNETIC
RECORDER
TIME CODE
GENERATOR
POWER
CONTROL
MMA
CABINET
~1
|
1
|
I
11
1
|
1
~t
1
1
1
11
1
1
11
|
1
1
I I I
Fig. 1. Overall block diagram of MMA
OUTPUT TO
TECHNICAL
FACILITIES
CONTROLLER
99
£PART OF
METEOROLOGICAL
MONITOR ASSEMBLY
SIMULATED BAROMETRIC PRESSURE
2 CONTROL BITS (D6 & 07)
6 DATA BITS (DO-D5)
I y-At-uui/zasy)
Fig. 2. Barometric self-test electronics block diagram
SPAN ADJUST SWITCHES
CLOCK COUNTER
ZERO ADJUST SWITCHES
SENSOR FREQUENCY COUNTER
LSD MSD
DISPLAY REGISTER
I I I I I
TO LED DISPLAY
•••EXTERNAL
INPUT (REAR
CONNECTOR)
(BLANKING PULSE)
Fig. 3. Functional block diagram for Bell & Howell digital barometer (P/N 4-461-004)
100
O
RIG
INAL
 PAG
E
 IS
O
F
 POO
R
 Q
UALITY
tti28 1i35O)iZ
101
ORIGINAL PAGE «
OF POOR QUALFTY
HP9821A CALCULATOR/HP 11202A I/O INTERFACE
FREQUENCY
GENERATOR
INTERSTATE
HIGH-VOLTAGE
LOG-LIN SWEEP
GENERATOR
F-47
FREQUENCY
COUNTER
HP5300A
MEASURING
SYSTEM
BELL & HOWELL
DIGITAL BAROMETER
4-461-004
Fig. 6. Test setup for frequency vs simulated barometric pressure
scale factor determination
Fig. 5. Block diagram for barometric pressure simulator
102
CALCULATOR I/O
DOO- I
OO7
PERIPHERAL I/O
BKJ-DIO- I
DI7 ~
DATA IN
10-17
'2 'l
STORAGE REGISTER
OUT
STP
Fig. 7. Simplified block diagram of the HP11202A I/O Interface
103


Electronic simulation of a barometric pressure sensor for the meteorological monitor assembly

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server