Note: Descriptions are shown in the official language in which they were submitted.
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CONDITION RESPONSIVE SOLID-STATE SWITCH
Field of the Invention
The present invention relates to a solid-state switch that responds
automatically to change in an environmental condition to which the switch is
subjected, and particularly to such a switch which will em~ te operation of a
meçh~nical switch for retrofit into existing in~t~ tions or circuit designs.
Back~round of the Invention
There are known me~ nical switches thzt respond to changes in the switch
environment, in fact such condition responsive mechanical switches are in wide use.
10 An example is a pressure responsive switch, and a representative application is a
pressure responsive switch associated with an aircraft turbine engine. For maximum
fuel effficiency, the flow of coolant may be adjusted based on absolute pressure, such
as by a solenoid valve controlled by a pressure responsive mechanical switch.
Mechanical pressure switches can directly switch electrical leads using only two wires
15 by being connected in series between a power source and the load. Improvements
have been made to increase the useful life of mechanical switches, such as the use of
sealed contacts. Nevertheless, contact fretting, wear, fatigue, and arcing in harsh
environments have continued to be problems. In addition, in the case of a short
circuit in the line closed by the switch, no effective current limit is provided.
Solid-state switches have no wearout or cycle life within the rated usage.
However, for a system built or designed for use of a mechanical switch, substitution
of a solid-state switch may not be possible without extensive rede~ign In some fields
such redesign can greatly increase the expense, such as in the aircraft industry where
significant redesign of circuitry associated with an aircraft engine may require
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recertification. Such redesign could involve adding a third wire from airframe power
to provide a continuous power supply current path for the electronic circuit of the
solid-state switch. By comparison, mechanical switches typically require no extemal
power.
Alternatively, the solid-state switch could be designed to have a continuous
voltage drop across the switch sufficient to operate its own electronic circuitry.
Nevertheless, the voltage drop would subtract from the supply voltage and thereby
cause operational problems at low-voltage conditions. In addition, the several volt
drop across the switch required to operate its internal circuitry, combined with a large
load current, could result in several watts of power dissipation, which further
complicates the situation because of excessive switch heating. In an aircraft turbine
engine environment, heat already may be a problem given the extreme temperatures to
which the switch is subjected.
Summary of the Invention
The present invention provides an automatic condition responsive solid-state
switch that ~m~ tes a meçh~nical switch by requiring only two-wire electrical
connection, i.e., connection between a power source and a load, and which does not
require additional external power, and which does not significantly affect powersupply to the load. In the plerell~d embodiment, the switch in accordance with the
present invention is responsive to changes in absolute pressure and operates reliably
with predetermined hysteresis over a wide temperature range and for a wide range of
supplied voltage.
The switch in accordance with the present invention uses pulse techniques for
periodically monitoring the desired condition, such as pressure. In the off state,
power from the source drives a timer circuit which supplies a short voltage pulse to
the condition sensing circuitry, and to associated circuitry for temperature
compensation, amplification of the resulting pressure signal, and a comparator circuit
which determines whether or not the pressure indicated by the amplified pressuresignal exceeds a predetermined pressure threshold. If the sensed pressure is greater
than the predetermined pressure, a power switch (power transistor) is turned on to
connect the voltage source to the load. Turning on or closing the power switch
results in substantially elimin~ting the voltage drop between the switch terminals,
which previously was used to actuate the timer circuit. Accordingly, a separate timer
circuit is provided to periodically open the closed power switch for again evalll~ting
the sensed condition and, if appropriate, turning the power switch off.
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Associated circuitry controls the circuit hysteresis to prevent undesirable
chattering when the pressure stays close to the threshold pressure. Predetermined
limits are established such that the power switch remains in its on or off state for a
range of pressures which depend on whether the switch previously was open or
5 closed. The switch also can include circuitry for limiting the maximum currentconveyed through the switch and for providing a trickle current bypassing the switch
when it is open for a standard circuit integrity check.
Brief Description of the Drawings
The foregoing aspects and many of the flttçn~nt advantages of this invention
10 will become more readily appreciated as the same becomes better understood byrerelence to the following detailed description, when taken in conjunction with the
accompanying drawings, wherein:
FIGURE 1 is a diagrammatic illustration of a conventional meçh~nical
pressure responsive switch.
FIGURE 2 is a block diagram of a condition responsive solid state-switch in
accordance with the present invention.
Each of FIGURES 2-8 is a detailed circuit diagram of a component of the
circuit of FIGURE 2.
FIGURE 9 is graph illustrating operation of a component of the circuit in
20 accordance with the present invention, namely, temperature compensation.
Each of FIGURES 10-14 is a detailed circuit diagram of a component of the
circuit of FIGURE 2.
Detailed Description of the Pl e~led Embodiment
FIGURE 1 illustrates diagl~ llalically a conventional me~h~nical switch
25 automatically responsive to an environm~nt~l condition to which the switch issubjected, namely, pressure. The conventional switch 1 can include a housing2
having an orifice 3 such that the interior of the housing is exposed to surrounding
atmosphere. Housing 2 supports a movable diaphragm 4 having a central electrically
conductive contact 5. For pressure exceeding a predetermined pressure, the
30 diaphragm moves to engage its contact 5 against conductive terminals 6 in the line 7
between a voltage source (V+) and a load 8. In addition, known mechanical pressure
switches may be provided with a resistor Rt connected across the contacts 6 so that a
conventional line integrity check can be made even when the switch is open. For
example, such an integrity check may be desirable to confirm that the load has not
35 been short-circuited, which could not be determined upstream from the switch with
the switch open unless there is some parallel current path across the contacts 6.
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With reference to FIGURE 2, the switch circuit 10 in accordance with the
present invention provides the advantages of solid-state operation, which include
advanced temperature compensation, hysteresis, current limit, reliability and long life,
without requiring an independent power source or line, and without significantlyaffecting voltage supplied to the load. Rather, switch circuit 10 is simply connected
between line terminals 11 and 12 between the voltage source (V+) and the load 8.Consequently, switch 10 can be substituted for known mechanical switches in existing
decign.c, and even be rellor~l~ed into existing in~t~ tions. The particular application
for the plerelled embodiment in accordance with the present invention is in an aircraft
turbine engine environment. To illustrate the complexity of the specifications that
must be met, the pleÇ~ d embodiment described below meets the following design
criteria:
Trigger point: at falling pressure, switch to close at 70 PSIA +
3 PSIA;
Hysteresis: switch to open at 7.5 PSIA above the actual switch
point, ~ 2.5 PSIA;
Temperature: switch to operate reliably through a temperature
range of-65~F to positive 300~F;
Power requirement: switch to operate for standard airframe
power, varying from 16volts DC to 32volts DC (as compared to
nominal air frame power of 28 volts DC);
Load: switch effective to drive a load (solenoid valve) of about
240 millih~nries and 40 ohms, maximum current 1 amp.
In general, the representative and plerelled embodiment circuit of the switch inaccordance with the present invention is shown in block diagram form in FIGI~RE 2,
and the detailed circuit diagrams are shown in FIGURES 3-8 and 10-14. Reference
letters A through O indicate the points of interconnection of the di~elell~ circuit
components. Beginning at the line terminals 11 and 12 between the power source
(V+, e.g., standard airframe power) and load 8, the initial component is a filter and
transient suppression circuit 13, shown in detail in FIGURE 3, which filters theairframe 28 volt DC power (variable from 16 volts to 32 volts) through a network of
inductors, capacitors, and ferrite beads. These filter components help assure
compliance with electromagnetic interference specifications. Input voltage transients
are suppressed though a fast-action zener diode CR1 connected across the input.
Returning to FIGURE 2, the heart of the circuit in accordance with the
present invention is the main power switch 14, for which the detailed circuit is shown
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in FIGURE 4. Such switch consists of a power MOSFET Q1 which, based on a
charging or discharging signal supplied at connectionC, is m~int~ined in its on
(closed) state or its off (open) state. Capacitor C13 dampens the flyback or ringing
from an inductive load whenever Q1 opens. R35 helps limit peak current through Q1
by providing a voltage drop that subtracts from the gate drive voltage. With Q1 "off"
(main power switch opened), there would be an essPnti~lly open circuit between
switch terminals 11 and 12 shown in FIGURE2, except for resistor Rt which
functions to provide a trickle current for standard line integrity checks. In addition, in
this condition there is substantial voltage potential between points A and B usable to
power the rem~inder of the circuit.
Contin~ling with the main power switch 14 in its open or off condition, the
next circuit component is a voltage regulator 15 for which the detailed circuit diagram
is shown in FIGURE 5. Unre~ tecl air frame provided DC power is re~ ted by a
zener diode CR9 and buffered by a source follower to provide an output of about
10voltsDC at outputD. The regl-l~ted DC voltage output at pointE is about
12 volts.
As seen in FIGURE 2, the 10-volt DC output (D) of the voltage regulator is
supplied to a timer circuit 16, rerelled to as the "offtimer" because it operates when
the main power switch 14 is offor opened. As seen in FIGURE 6, the offtimer 16 is
an astable multivibrator providing output voltage pulses that power several circuit
components, described in more detail below. In the pr~rell~d embodiment, the output
pulses supplied at point F have a duration of about 1 millisecond, with a period of
about 32 milli~econds between consecutive pulses, and a pulse height of about
10 volts. Consequently, the offtimer subst~nti~lly reduces average current drain for
the condition when the main power switch is off.
Output pulses from the off timer are supplied to a combined temperature
sensing and pressure sensing circuit 17 shown in FIGURE 7. For pressure sensing,silicon piezoresistive strain sensing resistors RA, RB, RC and RD are arranged in a
bridge configuration on a pressure sensing diaphragm. The bridge di~elelllial output
voltage across points O and H varies in proportion to pressure when excited with a
constant voltage source, such as a 1 millisecond 10-volt pulse from the off timer
(point F). The temperature sensing resistor RE varies proportional to temperature.
The plerelled pressure sensing and temperature sensing circuit is a silicon on sapphire
bridge pressure sensor with an integral temperature sensing resistor.
The pulse output of the offtimer also is supplied to an offset and temperature
compensation circuit 18, shown in detail in FIGURE 8. Solid-state pressure sensors
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provide a consistent voltage output, but are not perfect in providing outputs identical
for di~ele"L sensors. Each individual pressure sensor voltage output varies as afunction of temperature. In addition, zero pressure does not necessarily result in zero
output volts. In the tel-~p~ re compensation and offset circuit, a pair of voltage
dividers is set up such that the difference between them compensates for the pressure
sensor offset. Further, the divider resi~t~nces are calculated such that, in combination
with the integral temperature sensor çh~nging resistance, the compensation offset
voltage varies applopliatély with temperature to compensate for the pressure sensor
voltage output that changes as a function of temperature. Values for resistors R31
and R32 must be selected judiciously after testing the pressure-temperature sensor to
determine its individual characteristics. In a representative embodiment, R31 may be
6.8 K ohms and R32 may be 2.2 K ohms. The desired relationship of varying
pressure voltage output and the output of the temperature compensation circuit is
illustrated in FIGURE 9. Line 19 represents pressure signal variation as a function of
temperature, whereas line 20 represents the voltage output (point K) of the offset and
temperature compensation circuit 18. Preferably, the two lines have slopes of equal
but opposite m~gnitlldes such that, downstream, variation of the voltage of the
pressure signal as a function of te",pe~ ~t~lre is compensated for by the signal from the
offset and temperature compensation circuit 18.
The voltage pulses from the off timer (F) also are used to power the
di~erenlial amplifier circuit 19 shown in detail in FIGURE 10. The voltage gain is
adjusted by selection of resistor R19, based on the characteristics of the individual
sensor used. In a representative embodiment, the value of R19 can be 6.9 K ohms.The amplifier circuit incl~ldes a low pass filter, R21 and C6, to reduce transients
caused by the pulsed power application.
Returning to FIGURE 2, the output from the amplifier and filter circuit 19
(point L) is proportional to pressure and reliable for the desired temperature range.
Such output serves as one input to a co~"pal~lor and hysteresis control circuit 20.
The other input, point M, is a reference voltage derived by applying the off timer
pulse output (F) across a voltage divider consisting of resistor R22 and resistor R23.
The detailed circuit of the comparator20 is shown in FIGURE 11. The
comparator supplies an output (pointG) if the pressure signal (L) exceeds the
reference voltage (M), with the desired hysteresis by virtue of the feedback loop
consisting of R27 and CR13. However, the switch point is adjusted based on whether
the main power switch was turned on or off during the last cycle, i.e., during the
evaluation or monitoring period of the previous pulse supplied by the off timer. This
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adjustment is accomplished by the hysteresis memory circuit 21 shown in
FIGURE 12. Power to the hysteresis memory circuit comes directly from the voltage
regulator's zener diode (point E) to minimi7e interaction with other circuit
components. Depending on whether the switch previously was on, the effect of the5 hysteresis memory circuit is to adjust the reference voltage at point M. Recall, for
example, that the design criteria requires initial switchover at decreasing pressure of
70 PSIA (~ 3 PSIA) with hysteresis of 7.5 PSIA from the actual switch point
(~ 3 PSIA). If the power switch is open, reg~ ted power at pointE charges
c~pacitor C5 sufficiently to turn on transistor Q11 which adjusts the reference voltage
10 at point M. If the power switch is closed, there is insufflcient voltage to operate the
voltage regulator and transistor Q11 remains off. As described in more detail below,
the closed power switch is opened for short periods during circuit operation, but the
duration is not long enough to charge C5 sufficiently to actuate transistorQ11.
Consequently, Q11 is turned on only when the power switch is off, which has the
15 effect of adjusting the reference voltage at point M.
Ultimately, a signal from the colllpal~lor circuit20(G) is conveyed to a
charge storage and transfer circuit 22 for triggering the main power switch to close.
Circuit 22 is shown in detail in FIGURE 13. Power to such circuit also is supplied by
the output pulse (F) of the offtimer. At startup, a capacitor, C8, is charged. When
20 the sensed pressure is above the established threshold level (appropliate hysteresis
being considered), the signal from comparator (G) will, at the falling edge of the
power pulse (F), result in transferring the charge from capacitor C8 to the output
line N. The charge storage and transfer circuit is tuned such that the amount ofcharge is sufficient to turn the main power switch on for expected load currents, but
25 will desaturate readily for overload conditions. This is accomplished by selection of
the values for R26 and C8. More specifically, a high voltage signal at G results in
turning transistor Q10 on which results in essen~i~lly a short circuit between N and B.
In this condition, the charge stored in capacitor C8 is not transferred. A low voltage
signal at G results in Q10 ~ ;lling off. In this condition, the gate of transistor Q9 is
30 driven at the falling edge of a power pulse (F) from the off timer, such that charge
stored in capacitor C8 is transferred to point N through Q9 and R41. In general, R37,
Q13 and CR12 are in the precharge path for capacitor C8, whereas CR6, R34 and
C11 cooperate to drive Q9 at the falling edge of a power pulse.
Charge transfer is conveyed through the on timer and current limit circuit 23,
35 shown in detail in FIGURE 14. The path from line N is through diode CR2 and
resistor R1 to the trigger input C of the main power switch. The result is that the
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main power MOSFET Q1 (FIGURE 4) is turned on and will remain in the on state
until turned off.
It should be kept in mind that the entire power-up, pressure sensing, and
switch controlling process occurs during a single pulse from the off timer, i.e., in
S approximately 1 milli~econd in the pl~relled embodiment. Also, during this time
power to the off timer, required for the r~mAin~er of the circuit operation, wasobtained by virtue of the fact that the main power switch was off such that the voltage
from the power source (V+) was applied across the switch. Once the switch has been
turned on, power no longer is available for circuit operation. Consequently, the on
10 timer 23 periodically drains the charge from the trigger input (C) of the main power
switch, such as once every 32 milliseconds. In the off state of the main power switch,
the timing capacitQr C3 is charged. In the off state of the main power switch,
cApac.itor C3 discharges "gradually," i.e., in about 32 milli.ceconds, which results in
turning on transistors Q4 and Q5. Charge at C is imme~iAt~ly drained through CR315 and Q4, which momentarily opens the power switch to power up the voltage
regulator, offtimer, etc., for the periodic monitoring ofthe sensed condition. The off
timer substantially immediately supplies the 1 milli~econd 10-volt pulse for powering
up the circuit components to evaluate the pressure and, if appropriate, substAnti~lly
imme~iAtely ll~nsrer charge back to the trigger input of the main power switch to
20 Il~A;lll~;ll it in its on or closed state.
The circuit of FIGURE 14 also has the effect of limiting maximum current
applied across the main power switch. Desaturation of the main power switch Q1, in
combination with increasing voltage across its series resistorR35, both caused by
excess current, will have the effect of turning on transistor Q2 which quickly drains
25 the charge from the gate (C) of the power switch, thereby turning off the power
switch for excess current conditions.
Thus, the solid-state switch in accordance with the present invention
periodically monitors the sensed condition and controls the flow of current from a
source to a load, without requiring a separate power source for circuit operation and
30 with minim~l effect on the voltage available for the load. Consequently, the solid-
state switch can replace a mechanical switch without extensive redesign of the system
or circuit in which the switch is used.
While the prt;relled embodiment of the invention has been illustrated and
described, it will be appreciated that various changes can be made therein without
35 departing from the spirit and scope of the invention.
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