Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CM-76798 1~410
This invention relates to a sensing circuit and, more
particularly, to an improved sensing circuit for sensing the
status of a contact pair in a monitoring station.
BACKGROUND OF THE INVENTION
In a telemetry supervisory system which includes remote
monitoring stations, it is customary to have a number of
remote unmanned monitoring stations connected in a network
with a master station. Each of such remote monitoring
stations ususally has a number of remote points which it
monitors and whose status it reports to the master station.
These remote points are often monitored by a pair of con-
tacts, which are connected by a wire-pair of substantial
length.
A prior art system such as that described hereinabove,
suffers from a number of shortcomings and problems. Thus,
for example, the prior system is noise sensitive in that
noise induced in the long wires between the remote points
and the remote monitoring stations tend to induce errors in
the status indications which are reported. The prior art
system is also susceptible to transient surges induced on
the long wires or yenerated by the opening and the closing
of the remote contacts which tend to damage the equipment at
the reporting station. Moreover, the prior art system is
not very e~ficient in the power consumption in that in the
event of power failure battery operation is usually necessary
until the power is restored by using an auxiliary battery
provided on a standby basis.
According to the prior art, the noise problem is over-
come by having a galvanic isolation circuitry interposed
between the wires from the remote points and the reporting
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CM-76798 1~410
station equipment. To resolve the transient surge problems,
the prior art system uses filters and clippers or any equiva-
lent limiters in addition to the galvanic isolator to reduce
the magnitude of the surges.
These prior art solutions to the foregoing problems
generally require a large increase in the current drain of
the xeporting station and consequently reduces the overall
power efficiency of the station and of the system.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an
improved sensing circuitry.
It is still another object of the present invention to
provide an improved sensing circuitry in a station being
monitored that overcomes one or more of the aforementioned
shortcomings and problems.
It is yet another object of the present invention to
provide a solution to the aforementioned proble~.s with no or
minimum level of incxease in the current drain required to
operate the monitoring station.
The foregoing and other objects of the present invention
are attained by providing a pulse generator for generating
sampling pulses, and an energy storage circuitry coupled to
the contact pair and to the pulse generator through a gal-
vanic isolator and an output circuitry coupled to the galvanic
isolator for pro~iding an output signal indicative of the
status, i.e., open or closed state, of the contact pair.
According to a feature o~ the present invention, the
energy storage circuitry is used to store the sampling
pulses until the storage circuit stores enough charges to
build a DC voltage o a certain magnitude using the sampling
C~1-76798 l~V410
pulses when the contact pair is in the open state. When the
contact pair i5 closed, the storage circuit is allowed to
discharge the stored charge through the contac~ pair to the
ground and the pulse generator is allowed to apply the
sampling pulses to the ground v~a the galvanic isolator and
the storage circuitry.
According to another feature of the present invention,
the output circuitry is provided with a flip-flop circuitry
for amplifying and widening the output of the galvanic
isolator, a low pass filter for eliminating short negative
pulses and transients from the output of the flip-flop
circuitry to prevent change in the state of its output
signal while the energy storage circuitry is being charged
and remains charged and allows change in the state of its
output signal upon discharge of the energy storage circuitry
while the sample pulses are being transmitted through the
isolator to the ground, when the contact pair is closed, and
a binary gate responsive to the output of the filter for
providing the output signal signifying the change in the
state of the contact.
The foregoing and other objects and features of the
present invention will b~come clear from the following
detailed description of an illustrative embodiment of the
present invention in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l illustrates a prior art sensing circuitry in a
functional block diagram.
Figure 2 illustrates an illustrative embodiment of a
sensing circuitr~ in a functional block diagram in accordance
with the present invention.
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CM-76798 ~07~410
Figure 3 illustrates a sampling pulse generator circuit
that may be used in a present sensing circuitry.
Figure 4 illustrates an input isolation/surge protection
circuit employing the sampling technique in accordance with
the present invention.
Figure 5 shows timing diagram that illustrates the
operation of the sensing circuit.
DETAILED DESCRIPTION
Figure 1 illustrates a commonly used prior art sensing
circuit 1 which provides galvanic isolation and surge protec-
tion but which requires high current drain. The prior artsensing circuitry includes a DC to DC converter 2 for provid-
ing an isolated constant DC voltage of a suitable voltage
from a DC power supply to each of a plurality of galvanic
isolators 3 used for a plurality of monitoring stations in a
supervisory telemetry system. T~pically, the supervisory
telemetry system includes a plurality of sensing circuitry
such as that shown in Fig. 1 for monitoring a plurality of
corresponding remotely located contact pairs. For the sake
of simplicity only one sensing circuitry and one remote
contact pair is shown in Fig. 1.
The galvanic isolator 3 may be of any suitable conven-
tional circuitry such as a relay or an optical isolator or a
transformer. When a remote contact pair 5 is closed, DC
current flows from the DC to DC converter 1, via the control
section 3A of the galvanic isolator 3, a low pass filter and
~lipper 4 and the remote contact pair 5 to the remote ground
6. This current flow causes a corresponding state in the
controlled section 3B of the galvanic isolator which is
sensed and sent out to a reporting equipment 8. The low
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CM-76798 ~(~'7(3410
pass filter and clipper 4 filters noise and clips surge
voltages originating at the remote contact pair 5 or on the
connecting line. A low pass filter 9 is also interposed
between the galvanic isolator and the reporting equipment to
provide additional noise immunity. Usually, separate ground
7 is provided between the remote ground 6 and the equipment
ground.
According to the prior art, the current level required
by the controlling section 3A of the galvanic isolator for
proper operation must be continuous when the remote contact
pair is closed. In a typical supervisory telemetry system
that monitors many pairs of the monitoring stations, this
current is multiplied by the number of contact pairs which
are closed at any given time. Hence, considerable amount of
current drain is involved in a typical supervisory telemetry
system that includes a large number of monitoring stations.
In accordance with the present invention, the afore-
mentioned shortcomings, including the large current drain,
are overcome by a system based on a sampling technique.
Thus, by providing sampling pulses to the sensing circuitry
for just a portion of monitoring time, in pulses, substantial
energy is saved. The conventional galvanic isolator can be
used also by using a suitable output circuitry that includes
means of stretching and/or smoothing the pulse output. The
sampling technique is used to provide appropriate output
signal indicative of the status of the contact pair being
monitored.
The principle of this invention, namely, the utilization
of the sampling technique is implemented by a sensing circuitry
10 illustrated in Fig. 2. Thus, referring to Fig. 2, there
is shown, in a functional block diagram form, a sensing
CM-7679g 107~'~10
circuitry which utilizes a sampling pulse train, instead of
a steady DC voltage, as used in the prior art sensing circuitry
illustrated in Fig. 1. As shown in Fig. 2, the inventive
sensing circuitry includes a sampling pulse generator 11
which may provide the same peak DC voltage as that of the
prior art DC to DC converter shown in Fig. 1. The low pass
filter and clipper 4 coupled to the remote contact pair 5
and the galvanic isolator 3 may be of the same type as those
used in the prior art sensing circuitry shown in Fig. 1.
Likewise, a low pass filter 4 of the type used in the prior
art sensing circuitry in the output may also be used in the
sensing circuit of the present invention.
Usually, all of the remote contact pairs 5 in a telemetry
supervisory system, are held open under a normal condition.
This permits the sampling pulses to pass through the control
section 3A of the galvanic isolator 3 into the energy storage
circuit 13 of a suitable design, such as DC storage circuit
of a conventional design. When a sufficient number of
pulses enter and charge the energy storage circuit 13 up to
a level equal to the peak voltage of the sampling pulse, the
sampling pulses cease to flow through the control section of
the galvanic isolator 3, and the energy storage circuit
remains charged and this is the case as long as the remote
contact pair 5 is open. A low pass filter g coupled to the
galvanic isolator output via a flip-flop 15, has a time
constant which is long enough to prevent switching of the
binary gate 10 during the charging of the energy storage
unit 13. This pre~ents the entry of an apparent change of
state signal to the reporting equipment 8 during the charging
of the energy storage circuit 13~
Once the energy storage circuit 13 has been loaded, the
circuit of Figure 2, is ready to respond to closure of the
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~M-76798
remote contact pair 5. When the remote contact pair 5 is
closed, as it would when a suitable circuitry such as a
relay (not shown) controlling the pair is actuated to signify
a change of a condition, it effectively shunts the energy
storage circuit 13. The charge stored in the storage circuit
is then discharged, in the form of a DC current, to the
ground 6 via remote relay contacts 5 and low pass filter and
clipper 4. Once the charge in the energy storage circuit 13
is discharged, the sampling pulses are allowed to flow to
the ground through the controlling section 3A of the galvanic
isolator, the energy storage circuit, and the contact pair.
The flow of sampling pulses through the controlling section
3A of the galvanic isolator 3 induces a corresponding pulse
train to appear at the output of the controlled section 3B.
If the pulses appearing in the controlled section 3B of
the galvanic isolator 3 are narrow, it is necessary to
amplify and/or widen them, before they are averaged. The
most economical way of achieving this is with a flip-flop
15. As shown, such a flip-flop can be interposed between
the galvanic isol~tor 3 and the low pass filter 9. Pulses
from the reset pulse signal source 12, which drives the
sampling pulse generator 11, can also be used as reset
pulses to the flip-~lop 15. The reset pulses are set to
occur just prior to the occurence of the sampling pulses.
~eset pulses drive the flip-flop 15 output high, and the
output pulses of the isolator 3 then drive the output low.
Consequently, the flip-flop output will be maintained at the
positive supply voltage of the ~lip-~lop continuously, while
the contact pair is open. When the contact pair is closed,
the sampling pulses induce output pulses in the galvanic
isolator output. The output of the isolator and the reset
CM-76798 1~410
pulses causes the flip-flop to change its output from a high
DC value into a train of pulses. The output of the flip-
flop 15 is then filtered by the low pass filter g to eliminate
the short negative pulse and other transients. The low pass
filter 9 is also designed to prevent a change-of-state
signal during the charging and discharging of the energy
storage circuit 13. This prevents the sensing circuit from
sending out a false output signal caused by noise or transients.
Following a period of time delay after the closing of the
contact pair, the output voltage of the low pass filter 9
reaches the switching threshold voltage of a binary gate 14
and causes it to switch. This generates a change of state
signal and this signifies a contact closure, which is then
processed by the reporting equipment 8.
The low pass filter and clipper 4 serves the same
function as that of the prior art as shown in Figure 1. The
isolation between the input and output of the galvanic
isolator is improved substantially by the use of separate
grounds 6 and 7 as illustrated in Figure 2. This provides
improved surge protection function.
Figures 3 and 4 illustrate, respectively, specific cir-
cuitry used for generating the sampling pulses and providing
an isolation/surge protection to the sensing circuit in
accordance with the present invention.
Referring to Figure 3, the sampling pulse generator
circuit includes three conventional complementary emitter-
follower transistor pairs, Ql and Q2, Q3 and Q4, and Q5 and
Q6 coupled in series, a transformer Tl that is interposed
between the first two emitter-transistor pairs and that
provides a galvanic isolation between the input and output
terminals, 31 and 32. Three emitter-follower transistor
pairs, transformer Tl and a passive circuit network are
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1~)7V410
CM-76798
designed to elements provide a sampling pulse at the output
35, which is slightiy time delayed with respect to the reset
pulses applied at the input thereof from reset pulse source
12. The passive circuit network includes capacitor C7 and
diodes CRl, CR2, which are operatively coupled to the trans-
former as illustrated to generate a ringing waveform when
the reset pulse is finished. By virtue of the polarity
inversion achieved by Tl, the first pulse of the ringing
waveform is positive. This pulse is the sampling pulse and
is present at the end of the reset pulse.
Figure 4 shows an illustrative detail of circuit
elements that provide isolation/surge prevention function
for the galvanic isolator 3, flip-flop 15, and low pass
filter 9 of the output circuitry. The galvanic isolator may
be in the form a a transformer Tll. The energy storage
circuit is a peak detector, consisting o~ diode CR3 and
capacitor Cll. Cll is also a part of the low pass filter
and clipper, which also includes resistor Rll, and diodes
CRll and CR12. The inductance of Tll, together with the
capacitor C12, generate a ringing waYeform, in the same way
as the sampling pulse generator does as described above.
The first pulse of the ringing drives the flip-flop
which consists of two NOR gates Gll and G12. The low pass
filter is composed of resistor RlZ and capacitor C13.
Binary gate G13 then provides the change of state signal in
the form required for the reporting equipment.
For each remote contact pair, a circuit such as the one
shown in Figure 4 is required, but only a single sampling
pulse generator is required for a complete set of remote
stations being monitored.
The overall operation of the present sensing circuit
shown in Figures Z, 3, and 4 will now be explained with
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C.~-7679~ 107V410
reference to the timing diagram waveforms shown in Figure 5.
The reset pulse source 12 provides a train of reset
pulses as shown in the waveform A in Figure 5. The sampling
puise generator responds to the reset pulses and generates a
train of sampling pulses as shown in waveform B in Figure
5. Note that the sampling pulses are essentially the same as
those of the reset pulses except for the time delay introduced
by the sampling pulse generator. While the contact pair 5
is open, the storage output of the storage circuit 13 remains
high as illustrated in waveform C in Figure 5. While the
contact pair 5 is open, there is no output in the output of
the galvanic isolator 3 as shown in waveform E in Figure 5.
The output of the flip-flop 15 and the low pass filter 9
remains high and the output of the binary gate 14 remains
low as illustrated in the waveforms F, G, and H respectively
in Figure 5 while the contact pair 5 remains open.
Assume at time tl the contact pair closes and thereafter
remains closed. The sampling pulse generator continues to
apply the sampling pulses in response to the reset pulses as
shown in waveform B of Figure 5. However, the storage
circuit 13 begins to dischar~e at time tl and this continues
until time t2. The stored charge is discharged through the
contact pair gradually by the action of the low pass filter
and clipper as illustrated in waveform C in Figure 5. This
continues until the charge stored at the capacitor 11 is
completely discharged by the time t2. As the capacitor
discharges the transformer Tll begines to induce a train of
output pulses in the output pulses in its output windings of
the transformer Tll. The pulse amplitude of the output of
the galvanic isolator at capacitor C12 begins to increase
until it reaches the amplitude corresponding to that of the
input sampling pulses and this takes place during the transition
CM-76798 1(~7~'~10
~etween time tl and time t2 while the charge stored in the
storage 13 is drained. Thereafter the output of the galvanic
isolator provides a train of pulses in response to the train
of pulses applied to the galvanic isolator by the sampling
pulse generator. The output pulse train of the galvanic
isolator is the same as the input pulse train in the form of
the sampling pulses except for a time delay introduced by
the output windings of the transformer and the capacitor
C12. The reset pulses (Fig. 5; A) are applied to the gate
G12 of the flip-flop 15 and the output pulses (Fig. 5; E) of
the galvanic isolator are applied to the other gate Gll of
the flip-flop. These two pulse trains cause the flip-flop
15 to flip-flop its output between a low and high voltage.
Referring to ~ig. 5, waveforms A, E and F, it is evident
that until the amplitude of the output pulses of the galvanic
isolator reaches a certain level, the gate Gll does not
cause the flip-flop output to change. Only when the voltage
amplitude of the isolator is high enough does the flip-flop
change its output voltage~ Once the output amplitude is
high enough, then as noted, the reset pulse causes the flip-
flop output to go high and the pulse from the isolator
causes the output to go low. In this manner, the flip-flop
15 begins to provide a train of pulses toward the end of the
time t2 when the storage 13 is discharged. Thereafter, the
output of the flip-flop tracks the pulse train output of the
galvanic isolator and the reset pulse train.
The output of the flip-flop is applied to the low pass
filter 9 made of RC circuit R12 and C13. As the output of
the flip-flop changes from DC to AC pulse train, the capacitor
C13 begins to discharge and consequently the voltage output
at the low pass filter 9 begins to decrease as the output of
the flip-flop applied to the low pass filter begins to
CM-76798
107(~410
change from DC to AC pulse train. The pulse train of the
flip-flop continues to cause the capacitor C13 to discharge
to a certain potential where the potential causes the binary
gate G13 to change its output from a low to a high level
output at time t3. Change of the output of the binary gate
signifies the fact that the contact pair 5 is now closed.
Now, when the contact pair 5 again opens up, the
process will be reversed and the storage 13 will begin to
charge and thus its output will climb hack to a DC voltage
as shown at the beginning of the time waveform C in Figure
S. This will cause the output of the galvanic isolator in
the form of pulse train to decrease and eventually dis-
appear as the storage circuit capacitor C11 charges to a
potential enough to prevent transformer Tll from inducing
any output voltage in the galvanic isolator. In turn this
will prevent the galvanic isolator from generating pulse
train output and this will ir, turn cause the output of the
flip-flop to stop generating a pulse train and have its
output go high. In turn, this will cause the output of the
low pass filter go high and the output of the binary gate
14 go low thereby indicating the open state of the contact
pair.
The time delay introduced between the opening and
closing of the contact pair 5 and actual sensing of the change
is by design and deliberate. This is to prevent any tran-
sient pulses or noise signals that are likely present in
the sensing circuit from falsely inducing changes in
the output of the binary gate 14. Usually the transient
or noise is of a short duration. The interference by the
noise or transient signal is eliminated by the use of the
low pass filters 4 and 9 and flip-flop 15. However, the
107()410
CM-76798
filters and flip-flop cause the time delay between the
time the change in the state of the contact pair and the
sensing of the change in the output of the binary gate 14.
But, this delay is of no significance and does not adversely
affect the end result in that usually the time delay is not
critical in applications.
In summary, then, in accordance with the present
invention, there is shown an illustrative embodiment that
is based on a sampling technique for improving a sensing
circuitry that overcomes various shortcomings and diffi-
culties of the prior art sensing circuitry and that provides
better noise isolation and surge current prevention and that
improves power efficiency by reducing the energy required
by the sensing circuitry.
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