Note: Descriptions are shown in the official language in which they were submitted.
TEMPERATURE CONT~OLLED PRESSURE TRANSDUCER
Field of the Invention
This invention relates to apparatus for
measuring the pressure in a confined fluid and, in
particular, to temperature controlled apparatus for
measuring the pressure in a confined fluid. The inven-
tion further relates to apparatus for measuring weight
supported by a confined fluid and, in particular, to
apparatus for providing a temperature independent
measurement of such weight.
~escription of the Prior Art
Measurement of the pressure in a confined
fluid (the "fluid" may be a gas such as a pneumatic
fluid confined within a pneumatic circuit, or it may be
a liquid such as a hydraulic fluid confined within a
hydraulic circuit) has conventionally been effected by
means of a fluid pressure gauge coupled directly to a
line containing the pressurized fluid. However,
pressure gauges are typically only capable of approxi-
mating the actual fluid pressure. In an effort toobtain more accurate pressure measurements, load cells
which provide an electrical output signal representative
of the fluid pressure have been substituted for pressure
gauges. However, the mere substitution of a load cell
for a pressure gauge will not of itself significantly
improve the accuracy of the pressure readings obtained
because the load cell output signal will fluctuate with
the temperature of the transducer which is used to
measure pressure. Pressure transducers such as those
used in load cells are inherently temperature sensitive.
- 1- `~'
Many attem~ts have been made to compensate ~or
tem~erature-induced errors which affect pressure
readings obtained from a load cell. Although such
compensation methods enable reasonably accurate pressure
measurements -to be made over a narrow temperature range,
they do not o~er accuracies of, say, 1% over an
environmental temperature range.
The environmental temperature range of a
confined hydraulic fluid (i.e. the range of operating
temperatures to which the fluid may be exposed) may
typically vary from about -50 F to about +200 F.
Accurate measurement of the pressure in a confined fluid
is important if, for example, a reliable indication of
the weight supported by the confined fluid is to be
lS obtained by mathematical conversion of the fluid
pressure~ If an accurate measurement of the weight of a
load supported by a fluid confined within a hydraulic
circuit can be obtained, then the weight of the load
itself can be determined.
Summary of the Invention
The present invention provides an active means
for con-trolling the temperature of a pressure transducer
to maintain that temperature within a prescribed temper-
ature range. By maintaining the temperature of the
pressure transducer constant, one provides an accurate
basis for comparing individual pressure readings
obtained from the transducer. The procedure used to
correct such measurements to eliminate temperature-
induced errors may be simplified since one need only
-- 2 --
deal wi~h a single temperature rather than a range oE
temperatures~
In accordance with the present invention,
there is provided new and improved apparatus for measur-
ing the pressure in a confined fluid. The apparatuscomprises pressure sensing means for sensing the fluid
pressure and producing a Eirst output signal representa-
tive of such pressure; and, temperature control means
for maintaining the temperature of the pressure sensing
means near a selected temperature. The temperature
control means includes means for raising and lowering
the temperature of the pressure sensin~ means. The
apparatus includes temperature sensing means for sensing
the temperature of the pressure sensing means and
producing a second output signal representative of such
temperature.
The apparatus may also include signal
processing means for comparing the second output signal
to a reference signal representative of the selected
temperature, for actuating the temperature control means
to raise the temperature of the pressure sensing means
when the temperature falls below the selected tempera-
ture and for actuating the temperature control means to
lower the temperature of the pressure sensing means when
the temperature rises above the selected temperature.
Preferably, the temperature con-trol means
is a Peltier Effect device.
The confined fluid may be a hydraulic fluid or
a pneumatic fluid.
Preferably, the driving signal which actuates
the temperature control means is varied in inverse
proportion to the rate at which the temperature of the
pressure sensing means changes with respect to the
selected temperature.
The invention also provides new and improved
apparatus ~or weighing a load supported by a confined
fluid. The apparatus comprises pressure sensing means
for sensing the pressure in the confined 1uid and
producing a first output signal representative o~ such
pressure and of the weight of the load; and, temperature
control means for maintaining the temperature of the
pressure sensing means near a selected temperature, the
temperature control means including means for raising
and lowering the temperature of the pressure sensing
means .
Brief Description of the Drawings
Figure 1 is a block diagram of a system for
measuring the pressure in a confined fluid in accordance
with the invention.
Figure 2 is a cross-sectional view depicting
the mounting of a pressure transducer, temperature
sensor and temperature controller for measuring pressure
in a fluid confined within a hydraulic circuit.
Figure 3 is a schematic diagram of the
; preferred embodiment of an electrical circuit for
controlling the temperature of the pressure sensing
means.
Figure 4 is a schematic diagram oE the
preferred embodiment of a temperature sensor amplifier.
Figure 5 is a schematic diagram of the
preEerred embodiment oE a pressure sensing means
amplifier.
Description of the Preferred Embodiment
Figure 1 depicts in block diagram form a
system or measuring the pressure in a confined fluid.
The system may also be used for measuring the weight
supported by the confined fluid if the pressure readings
obtained are converted mathematically. The fluid may be
either a hydraulic or a pneumatic fluid. If the fluid
is a hydraulic fluid then it will typically be confined
within a hydraulic circuitO The hydraulic circuit may
be of the type commonly provided in heavy machinery used
Eor moving or supporting loads; for example, hydraul-
ically operated log yarding machines, fork lift trucks
or excavators, to name but a few.
As shown in Figure 1, the system includes a
"pressure interface" 1 comprising a temperature sensor
2, pressure sensing means 3 and temperature
controller 4. Pressure sensing means 3 produces a first
electrical output signal at 5 which is an analog repre-
sentation of the pressure in the confined fluid.
Temperature sensor 2 produces a second electrical output
signal at 6 which is an analog representation of the
temperature of pressure sensing means 3. Temperature
controller 4 is used to selectably raise or lower the
temperature of pressure sensing means 3 to maintain its
temperature near a selected temperature.
Signal processing means 7 are provided to
compare the second output signal to a reference signal
-- 5 --
representative of the selected temperature oE pressure
sensing means 3. When the temperature of pressure sens-
ing means 3 falls below the selected temperature,
temperature controller 4 is activated - as hereinafter
described - to raise the temperature of pressure sensing
means 3. When the temperature of pressure sensing means
3 rises above the selected temperature, temperature
controller 4 is ac-tivated to lower the temperature oE
pressure sensing means 3. Signal processing means 7
includes a master control device 8 such as a micropro-
cessor or computer, capable of analyzing the various
signals aforesaid and producing suitable signals to
actuate temperature controller 4. Signal processing
means 7 also includes a multiplexer 9 (for alternately
presenting either the first or the second output signal
to master control device 8) and an analog-digital
converter 11 for converting the first and second output
signals (typically measured in volts) to digital repre-
sentations convenient for manipulation by master control
device 8. Signal processing means 7 also includes a
suitable interface 13 for presentation of the analog-
digital converter output signal to master control device
8 and for presentation of output signals from master
control device 8 to temperature controller 4.
Master control device 8 may be programmed by
conventional known techniques to monitor the temperature
of pressure sensing means 3 and to actuate temperature
controller 4 as required to maintain the temperature of
pressure sensing means 3 at or near the selected temper-
ature. Similarly, master control device 8 may be
-- 6 --
programmed to convert the first output signal produced
by pres~ure sensing means 3 to provide a direc-t readin~
of the pressure in the confined fluid (in units such as
pounds per square inch). The master control device
program may include steps for "correcting" the pressure
readings to account Eor the temperature of pressure
sensing means 3. Pressure readings so obtained may also
be mathematically converted by master control device 8
to provide a representation of the weight supported by
the confined fluid. Visual representations of pressure
or weight may be presented either on alpha-numeric type
display 15 or on hard~copy printer 17. A keyboard 19
may be provided to permit the system operator to enter
commands to master control device 8, to select informa-
tion for presentation on display 15 or printer 17~ etc.
Pressure sensing means 3 is maintained at ornear a pre-selected temperature so -that all pressure
readings are taken at substantially the same temper-
ature. An appropriate algorithm may then be used to
mathematically "correct" all such pressure readings to
eliminate any temperature-induced error component.
Figure 2 is a cross-sectional illustration of
one way in which pressure sensiny means 3 (hereinafter
referred to as a "pressure transducer"), temperature
sensor 2 and temperature controller 4 may be mounted for
measuring the pressure in a fluid con-fined within a
hydraulic circuit. Pressure transducer 3 is rigidly
mounted on metal plate lO in close thermal contact
therewith. Pressure transducer fluid inlet port 12
protrudes through plate 10 and through metal plate 14
for connection to hose 18 which couples pressurized
hydraulic :Eluid to pressure transducer 3. Metal plates
14, 15, 16 and 17 form an enclosure for pressure trans-
ducer 3. Plate 14 includes a plurality of fins 19 for
conveying heat to or from plate 14 as hereinafter
described. The enclosure is filled with a fibre type
insulating material 20 to thermally isolate pressure
transducer 3. Electrical leads for temperature sensor 2
and pressure transducer 3 are also passed through plates
10 and 14 to emerge at 21.
Temperature controller 4 includes three indiv
idual temperature control devices 24 which are rigidly
affixed, for example, by gluing, between metal plates 10
and 14 so as to maximize thermal conductivity between
fins 19, plate 14, devices 24 plate 10 and pressure
transducer 3. Temperature control devices 24 are
"Peltier Effect" devices. A Peltier Effect device is a
two port electrical component which will "pump" heat one
way when a current passes through the device in one
direction and which will "pump" heat the other way when
a current passes through the device in the opposite
direction. Peltier Effect devices 24 are electrically
connected in series so that a current applied to devices
24 in one direction will cause them to "pumpl' heat from
fins 19 and plate 14 to plate 10 and pressure transducer
3 (thus raising the temperature of pressure transducer
3) and so that a current applied in the opposite
direction will cause devices 24 to l'pump" heat from
pressure transducer 3 and plate 10 to plate 14 and fins
19 (thus lowering the temperature of pressure transducer
-- 8 --
3). Electric leads from Peltier Effect ~evices 24
protrude throu~h plate 14 a-t 21.
Temperature sensor 2 is insert~d into plate
10 near pressure transducer 3. A suitable adhesive such
as epoxy is used to seal temperature sensor 2 in plate
10 to provide good thermal conductivity between temper-
ature sensor 2 and pressure transducer 3. Preferably,
temperature sensor 2 is not positioned directly between
any of Peltier Effect devices 24 and pressure transducer
3; the object being to minimize the effect of Peltier
E~fect devices 2~ on temperature readings taken by
temperature sensor 2 so that temperature sensor 2
provides a reliable indication of the temperature of
pressure transducer 3.
Figure 3 is an electrical circuit schematic
diagram o:E a tempera-ture controller for controlling
~eltier Effect devices 24 to selectably raise or lower
the temperature of pressure transducer 3. An input
signal comprising two binary digits is applied by master
control device 8 (via interface 13) to input terminals
Ao and Al of the temperature contro].ler.
If a logic "low" signal is applied to both
input -terminals Ao and Al, then the output of each
logical '~nand" gate Nl and N2 will be "high"O These
"high" signals are applied to the base of each of trans-
istors Ql and Q2 via resistors Rl and R2 respec-
tively, causing each of transistors Ql and Q2 to
turn "on"~ The collector voltayes of transistors Ql
and Q2 will therefore be "low", causing each of
transistors Q3 and Q~ to turn "off". The collector
g _
voltages o:E transistors Q3 ancl Q~ will then be
"high" and thelr emitter voltages will be "low". Hence,
each of Darlington power transistor pairs Q5, Q6~
Q7 and Q~ will be "off", and no current will Elow in
Peltier Effect device Pl. Similarly, i a logic
"high" signal is presented at input terminal Ao and a
logic "low" signal is presented at input terminal Al
then the output of both nand gates Nl and N2 will be
"high" and again no current will flow in Peltier Effect
device Pl.
If master control device 8 determines that the
temperature of pressure transducer 3 is equal to a pre-
determined selected temperature, no action need be taken
to raise or lower the temperature of pressure transducer
3. Accordingly, no current need be passed through
Peltier Effect device Pl. This may be achieved by
holding input terminal Al of the temperature
controller "low" so that Pel-tier Effect device P
remains "off".
If a logic 'llow" signal is presented at input
terminal Ao and a logic "high" signal presented at
input terminal Al, then the output of nand gate Nl
will be "low" (due to presence of logical inverter
Il). The output of nand gate N2 will remain
"high", thus power transistor stages Q6 and Q8 will
remain "oEf" as described above. However, the "low'
signal at the base of transistor Ql will turn Ql
"off", resulting in the coupling of a "high" signal to
the base of transistor Q3 causing it to turn "on".
The voltage at the collector of -transistor Q3 then
10 -
drops to enable the flow of emitter-base ~urrent in
power translstor stage Q5 thereby driving Q5 to an
"on" condltion. When Q5 is "on" the source voltage of
30 volts (less tlle emitter-collector drop across Q5)
ls applied to right hand terrninal "~" (as viewed in
Figure 3) of Peltier Effect device P1. Similarly, the
voltage at the emitter of transistor Q3 rises to
enable the flow of a base-emitter curren-t in power
transistor stage Q7 thereby drivin~ Q7 to an "on"
condi-tion. This action applies electrical ground
reference (plus the collector-emitter drop across Q7)
to the left hand terminal "-" (as viewed in Figure 3) of
Peltier Effect device Pl Accordingly, a current path
is established from the 30 volt source, through Q5 and
Peltier Effect device Pl to ground via Q7. Current
will thus flow through Peltier effect device Pl from
the "+" to the "-" terminals (as viewed in Figure 3).
In the preferred embodiment, Peltier effect device P
pumps heat away Erom pressure transducer 3 when a
current passes from the "+" to the "-" terminals oE the
Peltier effect device.
It may similarly be determined tha-t current
will Llow in the opposi-te direction through Peltier
Effect device Pl frorn the "-" to the "-~" termina~s (as
viewed in Figure 3) which in turn means that heat is
pumped towards pressure transducer 3 in the preEerred
embodiment, if a "high" signal is presented at each of
input terminals Ao and Al. The output of nand gate
Nl will then be "high" causing power transistor s-tages
Qs and Q7 to remain "off" as explained above.
However, the output of nand gate N2 will be "low"
causin~ transistor Q2 to turn "off" and
transistor Q4 to turn "on" due to the presence o~ a
"hicJh" signal at its base. The "high" voltage present
at the collector and emitter oE conducting transistor
Q~ will cause power transistor stages Q6 and Q8 to
turn "on'l, thus causing a current to flow rom ~6
through Peltier Effect device Pl to yround via Q8.
Figure 4 is an electronic circuit schematic
diagram of a differential amplifier for comparing the
output signal of temperature sensor 2 with a reference
signal representative of the selected temperature of
pressure transducer 3. The temperature sensor used in
the preferred embodiment is a National Semiconductor
LM135 precision temperature sensor which operates as a
two terminal zener diode, shown in Figure 4 as D2.
Because the vol-tage drop across zener diode D2 varies
with temperature, the voltage presented at input
terminal X2 of amplifier Al will vary with the
tempera-ture of pressure transducer 3 (because temper-
ature sensor 2 is placed in thermal conductivity with
pressure transducer 3).
Regulator Ul provides an accurate, temper-
ature independent reference voltage for amplifier A1.
Resistor Rf and capacitor Cf shift and scale the
output of amplifier Al into the 0-5 volt range which
is suitable for input to analog-digital converter 11.
To calibrate the difEerential amplifier, diode D2 is
heated to the selected -temperature of pressure trans-
ducer 3 and potentiometer R18 is then adjusted so
- 12 -
that the output voltage o:E ampli:Eier ~l is 2.5 volts
~the midpoint o:E the 0-5 volt amp:Lifier output voltage
range).
If the temperature of pressure transducer 3 is
less than the selected temperature, then the output
voltage o~ amplifer ~l will be less than 2.5 volts.
The difference between the amplifier output voltage and
2.5 volts will represent the difference between the
selected temperature of pressure transducer 3 and i~s
actual temperature. If the temperature of pressure
transducer 3 is grea-ter than its selected temperature,
then the output voltage of ampliEier Al will be
greater than 2.5 volts - the magnitude of the difference
in voltages representing the difference between the
actual and selected temperatures of pressure transducer
3. The analo~ output voltage signal produced by
amplifier Al is converted into a digital represen-
tation by analog-digital converter ll or presentati.on
to master control device 8. By sensing whether the
amplifier output voltage is greater than 2.5 volts, less
than 2.5 volts, or equal to 2.5 volts, master control
device 8 may determine whether the temperature of
pressure transducer 3 is too high (greater than the
selected temperature) or too low (lower than the
selected temperature) or "correct" (equal to the
selected temperature). If the temperature of pressure
transducer 3 is too high, then -temperature controller
may be activated as described above to lower the
temperature of pressure transducer 3. Conversely, if
the temperature of pressure transducer 3 is too low,
- 13 -
then temperat~lre controller 4 rnay be activated as
described above to raise the temperature of pressure
transducer 3. If master control device 8 detects that
the temperature of the pressure transducer 3 is equal to
the selected temperature, then temperature controller 4
may be turned "off".
Preferably, the drive signal for Peltier
effect devices 24 includes two components. The Eirst,
as just described above, is directly dependent upon the
temperature of pressure transducer 3. To avoid "over-
shooting" the selected tempera-ture of pressure trans-
ducer 3 or perhaps causing the pressure transducer
temperature to "oscillate" about -the selected temper-
ature, a second drive signal component is preferably
used to drive Peltier Effect devices 24 in inverse
proportion to the rate of -temperature change which is
considered appropriate to change the actual temperature
of pressure transducer 3 to the selected temperature.
For example, if the temperature of pressure transducer 3
is 20C below its selected temperature, then it will
likely be desirable to apply a maximum drive signal to
Peltier Effect devices 24 to "pump" heat toward pressure
transducer 3 in order to raise its temperature as
quickly as possible. However, as the temperature of
pressure -transducer 3 approaches the selected temper-
ature, the driving signal for Peltier Effect devices 24
should be reduced so as to reduce the amount of heat
"pumped" toward pressure transducer 3. (If this is not
done then pressure transducer 3 may be heated well above
the selected temperature and will therefore have to be
-- 1'1 --
cooled.) The drive signal Eor Peltier EfEect devic2s 24
may there~ore be continually changed in response to the
changing temperature of pressure transducer 3. It may
even be appropriate in some cases to drive Peltier
Ef~ect devices 24 so as to "pump'; heat away from
pressure transducer 3 even thouyh the temperature of
pressure transducer 3 is below the selected temperature.
This would occur, for example, iE Peltier Effect devices
24 had been driven to "pump" heat toward pressure trans-
ducer 3~ raising its temperature near the selected temp-
erature. As the pressure transducer temperature
approaches the selected temperature, heat is pumped away
to avoid overshooting the selected temperature. Similar
"proportional control" of the driving signal for Peltier
Effect devices 24 should be used when the pressure
-transducer temperature exceeds the selected temperature.
An appropriate algorithm for driving Peltier Effect
devices 24 in response to changing temperatures of
pressure transducer 3 may be programmed into master
control device 8.
Figure 5 is an electronic circuit schematic
diagram of an amplifier for the analog voltage output
signal produced by pressure transducer 3~ The pressure
transducer is represented in Figure 5 as Px~
Potentiome-ter R20 may be adjusted to select the
amplifier gain. Potentiometer R23 may be adjusted
to provide an offset voltage to compensate Eor
variations in output voltage ranges between different
pressure transducers. For example, the pressure trans-
ducer used in the preferred embodiment has an output
- 15 -
voltage range of 0 - 5 volts (representing pressures of
0 psi through 3,000 psi respectively).
In operation, a hose containing pressurized
hydraulic fluid is coupled to pressure transducer 3.
Pressure transclucer 3 produces a first electrical output
signal which is representative of the pressure in the
confined fluid. The first output signal is amplified as
described above for presentation to master control
device 8 via multiplexer 9 and analog-digital converter
ll. Temperature sensor 2 produces a second electrical
output signal which is representative of the temperature
of pressure transducer 3. As described above, the
second output signal is compared by temperature sensor
amplifier ~l with a reference signal representative of
the selected temperature of pressure transducer 3. The
temperature sensor amplifier output signal is presented
to master control device 8 via multiplexer 9 and
analog-digital converter ll. Temperature controller 4
is activated by master control device 8 (preferably, by
means of "proportional control" as decribed above) to
pump heat toward or away from pressure transducer 3 in
order to raise or lower its temperature., depending upon
tlle actual temperature oE the pressure transducer
relative to its selected temperature, and the desired
rate of temperature change.
Thus, the temperature of pressure transducer 3
may be actively controlled to hold it at or near a
selected temperature at which the digital representa~ion
of the :Eirst output signal representative of the fluid
pressure may be reliably converted by master control
- 16 -
w
devlce 8 to yield a measure of the Eluid pressure, a
measure oE the wei(3ht supported by the con~ined fluid,
etc.
The following table includes specifications
for components which have been used in the preEerred
embodiment:
pressure transducer Gulton Industries Inc.
GSH series high
pressure transducer
temperature sensor (D2) National semiconductor
LM135 precision
temperature sensor
Peltier Effect devices Melcor Materials
Electronic Products
Corporation
Model No. CP 1~4~71-lOL
R1 - R2r ~16 ~ R17 lOK ohm
R3 - R4, R7 - Rlo lK ohm
Rs - R6, Rll - R14, R17~ R19 4.7K ohm
R18 ~ R20 J R23 lOK ohm po
R21 - R22 lOOK ohm
Ql ~ Q4 2N3904
Q5 ~ Q6 TIP 125
Q7 ~ Q8 TIP 120
Ul, Al National Semiconductor
LM10
A2 National Semiconductor
LMll
l 7400
N 1 - N 2
Mul-tiplexer Motorola MC 14051B
Analog to Digital Converter Philbrick 8703
It will be readily apparent to those skilled
in the art that master control device ~ may be
programmed to utilize the accurate pressure represen-
ta-tions obtainable through use of the invention in a
number oE ways. As one example, a temperature
controlled pressure transducer may be inserted into a
hydraulic line used to drive the hydraulic cylinders
which raise or lower the Eorks of a ~arding machine used
for loacling logs onto logging trucks. By means of a
suitably programmed algorithm, master control device 8
may be used to convert pressure signals derived from the
temperature controlled pressure transducer into a
representation of the weight of a log or logs suspended
in the forks of -the yarding machine. Master control
device 8 may be interfaced in known fashion as shown in
Figure 1 to a display and/or printer which may be used
to present the operator of the yarding machine with a
display and/or print-out representative of the weight of
the load supported in the forks of the machine. The
operator will then be able to determine the weight of
the load he is placin~ upon a logging truck or other
transportation or storage means. Master control device
8 may also accumulate the weight of all loads suspended
in the forks of the machine since a given reference
time. For example, the operator may "initialize" master
control device 8 (typically, by striking a key on a
keyboard console) to indicate that he is about to begin
loading logs onto a truck. As each load is placed upon
the truck, the operator may be presented by master
control device 8 with a display or print-out to indicate
the weight of that load and the weight of all loads
previously placed upon the particular truck. Hence the
operator will be able to "adjust" the total load placed
upon the truck 50 that the truck carries a required
payload without exceeding any applicable weight
restrictions~
It will be readily apparent to those skilled
in the art that the temperature controlled pressure
transducer hereinbefore described may be included in
- 18 -
~-irtually any hydraulic or pneumatic circuit to provide
a reliable indication of the pressure o~ con~ined fluid
in the system or, iE desired, an accurate representation
of the weiyh~ suppor~ed by the conEined fluid.
Modifications falling within the scope o~ the
in~ention will also occur to those s~illed in the art.
As one example, it might in some cases be more expedient
to control the temperature of the fluid surrounding the
pressure transducer than to control the temperature of
the transducer itself. This could be accomplished by
applying Peltier Effect devices to the housing oE a load
cell which contains both pressurized fluid and a pres-
sure transducer for sensing the fluid pressure. The
Peltier Efect devices could be controlled as herein-
beEore described to maintain the temperature of fluid
within the load cell housing near a selected tempera-
ture; thus ensuring that pressure readings obtained from
- the transducer are taken at a substantially uniform
temperature and independen-tly of ambient temperature
fluctuations.
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