CH655778A5 - SIGNAL PROCESSING CIRCUIT FOR INTRINSICALLY SAFE PROCESSING OF SIGNALS FROM A SENSOR WORKING WITH DC VOLTAGE. - Google Patents

Description

The invention relates to a signal processing circuit for intrinsically safe processing of signals from a sensor working with DC voltage.

Such sensors are e.g. in flame monitors of oil or. Gas burners are used to monitor and indicate the presence of a flame. These flame monitors together with their sensors must be largely intrinsically safe, which means that every possible defect in a component must be indicated by the “flame extinguished” signal.

Self-monitoring can be carried out by checking the functionality of the flame monitor before starting up the system. In addition, systems with two flame monitors that work independently of one another have also been used and have been monitored for constant signaling in the same direction.

In addition, flame guards are known, in which the proper switching off of the flame guard is checked at certain intervals by means of a test cycle, with a further relay supplying the fuel valve or an additional electromagnet taking over the interruption of the sensor lighting during the test period. A flame monitor of the latter type with a signal processing circuit according to the preamble of claim 1 has already been proposed in DE-OS 3 026 787.

All of these solutions involve a relatively high outlay, the latter also having the disadvantage that the relays or the electromagnets are operated with a high switching frequency in order to achieve the shortest possible fault detection time.

The invention has for its object to find a signal conditioning circuit for intrinsically safe processing of signals of a sensor working with DC voltage, which avoids the disadvantages mentioned in continuous operation and is capable of the flame relay circuit described in the prior art or a similar circuit control without any DC drift problems.

According to the invention, this object is achieved by the features specified in the characterizing part of patent claim 1.

An embodiment of the invention is shown in the drawing and will be described in more detail below.

Show it:

1 is a block diagram of a flame monitor,

2 is a circuit diagram of a signal conditioning circuit,

Fig. 3 is a pulse diagram of a pulse generator.

The same reference numerals designate the same parts in all figures of the drawing.

Description of Fig. 1:

A sensor operated with DC voltage or emitting DC voltage, which is generally called DC voltage sensor 1 below and in the present example is a photoelectric sensor which is illuminated by the flame of an oil or gas burner, feeds a signal conditioning circuit 2 in two poles, which bipolar from a first supply device 3, the positive pole of which, for example is at ground potential, is fed with a negative first DC supply voltage —Ui. The signal processing circuit s

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2 in turn controls the input of a flame relay circuit 5 in a two-pole manner via a relay switching amplifier 4. The relay switching amplifier 4 and the flame relay circuit 5 are supplied with two poles from a common second supply device 6 with a positive second supply voltage U2. The negative pole of the supply device 6 is connected to the positive pole of the negative first supply voltage - Ui. A line AC voltage Un feeds two poles via a single-phase transformer 7, both supply devices 3 and 6.

The DC voltage sensor 1 is e.g. a cell that is sensitive to visible or UV light and whose working voltage is too low to directly control a switching transistor.

With the exception of the signal processing circuit 2, the circuit parts shown in FIG. 1 are known from the stated prior art; they are therefore not explained in more detail below. For a better understanding, however, it should be mentioned that the flame relay circuit 5 consists of two (not shown), parallel and oppositely polarized current paths, both of which have a common relay capacitor. The first current path, which corresponds to the direction of current flow of the positive second DC supply voltage U2, consists in the order given of the series connection of a first diode, a holding winding of the flame relay, a second diode and the common relay capacitor. The second current path in turn comprises the relay capacitor, a third diode, a pull-in winding of the flame relay and a fourth diode, all of which are also connected in series in the order given. The corresponding, associated graphic representation can be found in the stated prior art, DE-OS 3 026 787, Fig. 1, components 35 to 41.

Functional description of the circuit according to FIG. 1:

The low voltage of the DC voltage sensor 1 is converted in the signal conditioning circuit 2 into a series of switching pulses which periodically short-circuit the input of the relay switching amplifier 4 with the aid of an output switch. The output switch of the relay switching amplifier 4, e.g. a bipolar transistor, switches the flame relay circuit 5 briefly to the rhythm of the pulses. The two current paths of which are designed such that only the second current path can pull the flame relay in, while the first current path only supplies a holding current for the flame relay.

As long as the output switch of the signal conditioning circuit 2 is open, the output switch of the relay switching amplifier 4 is also open and the common relay capacitor is charged via the first current path. The flame relay is held as long as a charging current is flowing, if it was activated at the beginning of the charging process. The time interval between two charging processes is selected such that the charging current of the relay capacitor, when the circuit is in the correct state, is just sufficient to cause the flame relay to hold. In the present example, the time interval may not exceed 900 ms.

If the output switch of the signal conditioning circuit 2 is closed, that of the relay switching amplifier 4 is also closed, the flame relay circuit 5 is short-circuited, and the common relay capacitor discharges via the second current path and thereby excites the flame relay. Its switch contact indicates the presence of a flame when excited.

The relay capacitor is thus periodically charged and discharged by the output pulses of the signal processing circuit 2. However, this only takes place as long as all components function correctly and the DC voltage sensor 1 detects a flame.

The duration of the discharge current of the relay capacitor:. should have a predetermined value, e.g. speak approximately 50 "ms, .ent-". If it is much longer, for example due to some fault, the capacitor will be over-discharged and will. s then no longer able to supply the holding current. ' As well. .. this duration must not be significantly shorter, since otherwise the R relay capacitor will discharge too little and the charging current in the subsequent charging phase will be too weak to hold the flame relay.

10 Description of Fig. 2:. ,. . -

2, the DC voltage sensor 1 feeds the signal conditioning circuit 2 by connecting its first pole via a first resistor R1 to a first pole of the input of a Pi network 8, the output of which, in turn, 15 is the control input of a first Switch 9 controls. The input cross branch of the Pi network 8 consists of? a, first capacitor C1, its longitudinal branch from an inductor 10 and its output transverse branch from a second r. Switch 11, the control input of which was connected via a second Widér-20 R2 to the output of a pulse generator 12. The first switch 9 contains e.g. a bipolar semiconductor switch 13, the base of which works as a control input and is controlled via a third resistor R3, "_ wherein the base-emitter path is connected to a voltage limiting diode 14 in parallel. The diode cathode is connected to the base of the bipolar semiconductor switch 13, while the collector forms the single-pole output signal processing circuit 2. The bipolar semiconductor switch 13 is, for example, an NPN transistor, and the second 3 switch 11 is, for example, an N-channel field-effect transistor, the second pole of the DC voltage sensor 1, the common pole of the first capacitor C1 and the second switch 11, the anode of the voltage limiting diode 14 and the emitter of the bipolar semiconductor switch 13 are connected to one another and are connected to the positive pole of the negative first DC supply voltage -Ui, which is connected, for example, to the ground potential.

The pulse generator 12 consists of a first astable multivibrator 15, the output of which feeds the control input, of a first monostable multivibrator 16 connected downstream, the output of which in turn controls the enable input of a downstream second astable multivibrator 17 which is higher in frequency than the multivibrator 15. The output of the latter is connected to the control input of a second monostable multivibrator 18 connected downstream

switched, the output of which forms the output of the pulse generator 12.

The astable multivibrator 15 or 17 consists of a first, two-input Schmitt-Trigger-Nand-50 gate 19 or a second two-input Schmitt-Trigger-Nand gate 20, which is connected to a first RC series circuit R4-C2 or a second RC series circuit R5-C3, the respective common first pole of the resistor forming the RC series circuit; 55 stand and capacitor with a first input of the associated Schmitt trigger nand gate 19 or, 20, is connected. The RC series circuit R4-C2 consists of a fourth resistor R4 and a second capacitor C2 and the RC series circuit R5-C3 consists of a fifth resistor R5 and a third capacitor C3. The output of the nand gate 19 is connected to the second pole of the resistor R4 and at the same time forms the output of the multivibrator 15. The output of the nand gate 20 is connected to the second pole of the resistor R5,::, it and simultaneously forms the output of the Multivibrä. tors 17. The second pole of the capacitors ÇZ and Ç3 lies at the negative pole of the DC supply voltage —Ui. The 'second inputs of the two Nand gates 19 and 20 are the

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respective enable inputs, the one of the nand gate 19 being permanently connected to ground potential and thus to the logic value “1” and thus being permanently enabled.

The monostable multivibrators 16 and 18 each consist of a third Schmitt trigger nand gate 21 or a fourth Schmitt trigger nand gate 22 and a first CR network C4-R6 or a second CR network C5-R7 , wherein the respective common first pole of the resistor and capacitor forming the relevant CR network is connected to the first input of the associated nand gate 21 or 22. The CR network C4-R6 consists of a fourth capacitor C4 and a sixth resistor R6 and the second CR network C5-R7 consists of a fifth capacitor C5 and a seventh resistor R7. The control input of the monostable multivibrator 16 is connected to the second pole of the capacitor C4, the control input of the monostable multivibrator 18 to the second pole of the capacitor C5 and the respective second pole or input of the resistors R6 and R7, the Nand gates 21 and 22 the positive pole of the DC supply voltage —Ui, which, for example is connected to ground potential.

For the four Schmitt trigger nand gates 19, 20, 21 and 22, e.g. a CMOS “Quad 2-Input Nand Schmitt Trigger” of the type MC 14093B from Motorola, Phoenix, Arizona, can be used.

Functional description of the circuit according to FIG. 2 using the diagram according to FIG. 3:

The astable multivibrator 15 continuously generates low-frequency square-wave pulses with a period of e.g. T = 500 ms, which are shown as a characteristic A in FIG. 3 as a function of time t. The subsequent monostable multivibrator 16 is triggered by the negative going edge of the output pulses of the astable multivibrator 15 and in turn provides at its output a positive pulse with a duration of e.g. for each negative going edge, i.e. with a repetition time of also T = 500 ms. tP = 50 ms. This output signal is shown in FIG. 3 as characteristic curve B as a function of time t.

The second astable multivibrator 17 oscillates at a frequency which is about a factor 1000 greater than the frequency of the output signal of the astable multivibrator 15, but only during the release time tP -50 ms given by the monostable multivibrator 16; this signal appears in FIG. 3 as characteristic curve C.

The second monostable multivibrator 18 is triggered by the negative going edges of the output pulses of the astable multivibrator 17 and delivers short positive pulses with a pulse duty factor of e.g. 1/2 to 1/3 of the period of the astable multivibrator 17, depending on the dimensioning of the capacitor C5 and the resistor R7. These output pulses correspond to the characteristic curve D in FIG. 3. All the pulses shown in FIG. 3 have the level values OV and - Ui. At most, part of the resistor R7 can be short-circuited to increase the sensitivity at the input of the switching amplifier 4 by means of a switch (not shown), since this reduces the output pulses and thus the energy consumption from the capacitor C1 is reduced.

In contrast to the previously known or proposed solutions, the low DC sensor voltage is not amplified directly as a DC voltage signal in the present solution, but chopped with the aid of the second controllable switch 11 and transformed up by means of the inductor 10. This eliminates the drift problem that occurs with the otherwise required DC voltage amplifiers.

The voltage generated by the DC voltage sensor 1 charges the capacitor C1. The energy stored in this way is then removed from this capacitor C1 within a very short time and fed to the semiconductor switch 13 at a higher voltage level. This happens periodically about 2 times per second. The output of the semiconductor switch 13 controls the subsequent relay switching amplifier 4.

In order to transmit the energy stored in the capacitor C1 with as little loss as possible to the control input of the semiconductor switch 13, the voltage of the capacitor C1 is switched on and off to the inductor 10 at high frequency in the above-mentioned half-second cycle in each case for approximately 50 ms. The choice of a high frequency has the advantage that the inductance 10 can be chosen to be very small. In addition, the pulses controlling the second switch 11 are selected asymmetrically, in such a way that the on-time is shorter than the off-time, which has a favorable effect on the efficiency of the energy transmission.

3, the control pulses are shown purely schematically as characteristic curve D.

In the time tp ^ 50 ms during which the output pulses of the pulse generator 12 make the second switch 11 conductive, the capacitor C1 is largely discharged. The O pulses occurring at the output of the semiconductor switch 13 during this time are smoothed in the subsequent relay switching amplifier 4, so that ultimately a 50 ms pulse is available for the flame relay circuit 5. In order to receive this pulse periodically every half second, the capacitor C1 must always be sufficiently recharged, which can only happen when the second switch 11 is open and the DC voltage sensor 1 generates sufficient voltage. In principle, it would also be possible to generate a single pulse from the capacitor charge and use it to trigger a monostable multivibrator that then delivers the 50 ms pulse. However, the control with a series of switching pulses offers a higher level of security against individual interference pulses, and the use of a simple integrated module is almost obvious for the type of pulse processing selected. Further circuit variants are conceivable and in no way impair the essential principle of the invention.

The signal at the first input of the Schmitt trigger nand gate 19 or 20 of the astable multivibrator 15 or 17 is inverted in the nand gate when its second input is enabled, in the downstream RC series circuit R4-C2 or R5-C3 with a time delay and then returned to the first input of the nand gate 19 or 20, so that its input signal and with it the output signal of the astable multivibrator 15 or 17 continuously change its digital value with a time delay and thus the circuit in question as a generator rectangular signals works.

When a negative going edge appears at the control input of the monostable multivibrator 16 or 18, a pulse-shaped charging current flows through the capacitor C4 or. the capacitor C5 and generates a negative voltage pulse in the downstream resistor R6 or R7, the duration of which is determined by the values of the relevant capacitor and resistor and which is inverted in the subsequent Nand gate 21 or 22 into a positive voltage pulse. Nothing happens on positive going edges of the control input signal, since both poles of the relevant capacitor C4 or C5 are then grounded.

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2 sheets of drawings

Claims (8)

655778 1. signal processing circuit for intrinsically safe processing of signals of a sensor working with DC voltage, characterized in that the sensor (1) is connected via a first resistor (R1) to a Pi network (8), the input cross branch of which consists of a first capacitor ( C1), the longitudinal branch of which consists of an inductance (10) and the transverse output branch of which consists of a second switch (11) controlled by a pulse generator (12), and that the output of the pi network (8) in turn is connected to the control input of a first switch (9 ) is switched. 2. Signal conditioning circuit according to claim 1, characterized in that the two switches (9; 11) consist of semiconductor switches. 2nd PATENT CLAIMS 3. Signal conditioning circuit according to claim 2, characterized in that the second switch (11) is an N-channel field effect transistor and its control input is controlled via a second resistor (R2). 4. Signal conditioning circuit according to claim 2, characterized in that the first switch (9) has a bipolar NPN transistor, the base of which is driven by a third resistor (R3) and the base-emitter path of a diode (14) is connected in parallel , wherein the diode cathode is connected to the base of the bipolar NPN transistor (13). 5. signal processing circuit according to one of claims 1 to 4, characterized in that the pulse generator (12) consists of a first astable multivibrator (15), the output of which is connected to the input of a downstream, first monostable multivibrator (16), the The output in turn is connected to the release input of a downstream second astable multivibrator (17) which is higher in frequency than the first astable multivibrator (15), the output of which in turn is connected to the input of a second monostable multivibrator (18) connected downstream, the duration of the output pulses of both monostable Multivibrators are smaller than the period of their input signals. 6. signal processing circuit according to claim 5, characterized in that each of the two astable multivibrators (15; 17) consists of a two-input Schmitt trigger nand gate (19; 20), the output of which with an RC series circuit ( R4-C2; R5-C3) is loaded, the common pole of the resistor (R4; R5) and the capacitor (C2; C3) of the RC series circuit (R4-C2; R5-C3) with a first input of the two inputs having Schmitt trigger nand gate (19; 20) is connected, while the second pole of the capacitor (C2; C3) in question is connected to the negative pole of a negative first DC supply voltage (- Ui) and the second input of this Schmitt trigger nand -Gatters (19; 20) is a release input, that of the first astable multivibrator (15) being continuously released due to its connection to the positive pole of the negative first DC supply voltage (-Ui). 7. signal processing circuit according to claim 5, characterized in that each of the two monostable multivibrators (16; 18) consists of a two-input Schmitt trigger nand gate (21; 22), the first input via a CR network (C4-R6; C5-R7) is driven, the common pole of the capacitor (C4; C5) and the resistor (R6; R7) of the CR network (C4-R6; C5-R7) with the first input of the Schmitt Trigger nand gate (21; 22) is connected and the second pole of the resistor (R6; R7) of the CR network (C4-R6; C5-R7) and the wide input of the Schmitt trigger nand gate ( 21; 22) is connected to the positive pole of a negative first DC supply voltage (—Ui). 8. Flame monitoring device for oil or gas burners with a signal conditioning circuit (2) according to one of claims 1 to 7.