EP0052220B1 - Method and device for measuring the resistance in a signalling line - Google Patents

Abstract

1. A method of measuring the resistance in a signalling line of a hazard alarm system, the individual signalling lines (M, L) of which commence from a central control unit (Z) and are each provided with series-connected alarms (M1 to Mn) having a variable resistance value, where a voltage source (UV) is connected to the signalling line (ML) via a series resistor (RV) and a capacitor (C) is charged by a constant current in dependence upon the resistance value (RX) of the signalling line (ML), characterised in that the capacitor (C) is charged by a separate constant current source independent of the current in the signalling line and its charging time (T), which is proportional to the resistance value (RX), is measured in digital form by means of a counting device and a clock pulse generator (TG), the capacitor being discharged before measurement commences.

Description

The invention relates to a method and a device for measuring the resistance on a signaling line in a hazard alarm system according to the preamble of claim 1.

In alarm systems, for example for fire or burglar alarms, several detectors are often connected in series and connected to a control center via a line. Several similar signaling lines are connected to this control center. Each detector has a quiescent resistance, which is changed to an alarm resistance when the detector is actuated. In order to detect the actuation of a detector or a fault on the signaling line in the control center, it is therefore necessary to detect relatively small absolute changes in resistance on a line. The value of the change in resistance can be very different from the total value of the resistance of the signal line. For example, the value of the change in resistance in relation to the total resistance can be very small. This depends on the number of detectors connected in series and the size of the line resistance. This requires a measuring method that can measure and evaluate the relatively small changes in resistance value precisely.

It is known to measure the resulting resistance of a signal line using a bridge circuit. However, it is disadvantageous that the bridge output voltage is proportional to the resistance value only in the event of small changes in resistance. Therefore, the bridge must be adjusted to the existing total resistance. It is not possible to change the required line voltage. To monitor the signaling line or to make tampering more difficult, it is in many cases necessary to change the voltage of the signaling line in a predetermined manner.

It is also known to use a constant measuring current and thereby to obtain a resistance-proportional measuring voltage. However, this solution has the disadvantage that a relatively complex constant current source must be used. Which should also be safe against external external voltages. It is also known to use a series resistor for current injection, but the measuring voltage is no longer proportional to the resistance value, i.e. the measuring voltage dependent on the resistance value is not linear.

From DE-OS 27 16 506 a device for determining the alarm trigger location in alarm systems is known. There, a signaling loop is fed with a constant current source via a series resistor. When a contact of the signaling loop is opened, a voltage proportional to the parallel resistor in question reaches an analog memory via a rectifier, which can be a capacitor. The highest stored voltage value present is a charging voltage of the capacitor proportional to the resistance value of the loop. This analog voltage value is compared with an analog voltage value generated by a staircase voltage generator. In the case of equality, the open contact of the signaling loop is recognized and due to the analog voltage level, which corresponds to a specific stored binary value of a binary counter, the relevant contact of the signaling loop is displayed via a decimal decoding unit. However, such a device has various disadvantages. A constant current source is required in order to be able to measure a voltage that is exactly proportional to the resistance value in accordance with the resistance value of the loop on the charging capacitor. Every fluctuation of the voltage source or current source influences the measurement result and leads to inaccuracies. In the case of a large number of signaling contacts on a loop, an exact resistance measurement is necessary in order to be able to precisely determine the relevant signaling contact. The measurement of the analog voltage value at the capacitor does not always lead to exact results because the measurement voltage, which is dependent on the resistance value despite the impressed current, is not linear and is therefore not exactly proportional to the resistance value to be measured.

It is therefore an object of the invention to provide a measuring method and a device for carrying out the method which uses a series resistor for impressing current, the resistance value to be measured being converted into a quantity which is proportional to the resistance value and can be measured in a simple manner . The measuring method should remain unaffected by the supply voltage.

With regard to the method, this object is achieved in an alarm system described at the outset with the features of the characterizing part of claim 1. With regard to the device, this object is achieved with the features of claim 4.

The measured variable of the resistance value of the signal line to be measured is thus converted into a time variable proportional to the resistance value, the time being measured. This is done by measuring the charging time of a capacitor that is assigned to the signal line and through which a constant current flows. The time proportional to the resistance to be measured is measured digitally.

The capacitor is expediently short-circuited and thus discharged before the measurement begins. For this purpose, a switch or a transistor can be provided, which is connected in parallel to the capacitor. At the start of the measurement, the switch is opened so that the capacitor is charged with the constant current becomes. The charging time of the capacitor is measured by supplying pulses from a clock generator via an AND gate to a downstream counting device for this time. If the capacitor is charged, the AND gate is blocked and no further pulses can be fed to the counting device. The end of the charging time of the capacitor can be determined using a comparison circuit. This comparison circuit has two inputs and one output which emits a stop signal to the AND gate at the end of the charging time of the capacitor. This stop signal is emitted when the capacitor voltage and the line voltage are in a predetermined relationship to one another at the input of the comparison circuit.

With this measuring method, the resistance value of the respective line can be measured in an advantageous manner by cyclically querying the individual signal line. The measuring device can be designed so that each signal line is assigned its own series resistor. It is also possible to design the measuring device in such a way that a single series resistor is provided, which is then also connected to the individual signal lines during the cyclical interrogation.

A device for performing the method is to be explained in more detail below with the aid of circuit examples.

Show it

• 1 shows a basic detector arrangement in series connection, which are connected via signal lines to the control center,
• 2 shows a circuit arrangement of a measuring device for measuring the resistance-proportional charging time of the capacitor,
• 3 shows a part of the circuit according to FIG. 2, in which the start and stop signals are sent to the measuring device via optocouplers,
• Fig. 4 shows a measuring point switch for a cyclical query of the respective message line.

1 shows a control center Z with a plurality of signaling lines ML1 to MLn. Each signal line ML has a resistor RXI to RXn. In the message line ML1 individual detectors M1 to Mn are shown, which are connected in series. If there is a detector, e.g. MI, in the idle state, the detector M1 has an idle resistor RR1. If the detector M1 responds, the detector M1 is switched to the alarm resistor RA1 via the contact K1. When querying, a relatively small absolute change in resistance, for example RR1-RA1, is to be measured and evaluated in the center Z.

2 shows its device for carrying out this resistance measuring method. The circuit arrangement shows the resistance RX of the signal line ML to be measured. This resistor RX is connected to the supply voltage UV via the series resistor RV. The bonding sensor C is connected to the positive pole (+) of the direct voltage source UV with one electrode and to the collector of the transistor TR with the other electrode and to the first input B3 of the comparator D3. The emitter of the transistor TR is connected via a first resistor RI to the negative pole (-) of the direct voltage source UV. The second input A3 of the comparator D3 is connected to the output of a first amplifier D1. Its first input A1 is connected to the common connection point X of the measuring resistor RX of the signal line ML and the series resistor RV. The second input B1 of the first amplifier D1 is connected to the output of the amplifier D1, which is connected to the negative pole (-) of the direct voltage source UV via the series connection of the two resistors R1 and R2. The common connection point Y of the series connection of the two resistors R1 and R2 leads to the first input A2 of a second amplifier D2. The second input B2 of the second amplifier D2 leads to the emitter of the transistor TR. The output of the second amplifier D2 is connected to the base of the transistor TR. Furthermore, a switch S is connected in parallel with the chargeable capacitor C. The voltage UL on the signal line is in a certain relationship to the voltage UC on the capacitor C and the supply voltage UV. The voltage that is present between the negative pole (-) of the DC voltage source UV and the second input B3 of the comparator D3 is designated UB, the voltage that is present at the first input A3 of the comparator D3 is designated UA. Furthermore, the output of the comparator D3 is routed to the first input of an AND gate G. A clock generator TG is connected to the second input of the AND gate G. The output of the AND gate G leads to a counting device ZV.

If the switch S is now closed before the start of the measurement, the capacitor C is discharged. The measurement of the resistance value RX of the line ML is started when the switch S is opened. The capacitor C is charged with the constant current I. At the time when the voltage UA at the first comparator input A3 is equal to the voltage UB at the second input B3 of the comparator D3, the capacitor C is charged and the comparator D3 outputs a stop signal STO to the AND gate G. This means that the AND gate G is no longer acted on by a signal, so that the pulses of the clock generator TG can no longer reach the counting device ZV.

The following relationships apply to a simple representation and provided ideal components:

• LT = O.UC
• Damit ergibt die Aufladezeit T des Kondensators C
  

If the charging time of the capacitor C is designated T, the following applies:

• LT = O.UC
• This results in the charging time T of the capacitor C.
  

That is, the charging time T of the capacitor C is directly proportional to the measured value of the resistor RX and is independent of the supply voltage UV. Since the supply voltage UV is in a certain relationship to the line voltage UL and thus to the capacitor voltage UC, the charging time T of the capacitor C can be measured relatively easily with this circuit arrangement. At the inputs A3 and B3 of the comparator C3, only the condition of voltage equality has to be fulfilled, i.e. UA = UB, as can be easily illustrated from the relationships shown above.

An advantageous development of the circuit arrangement is shown in FIG. 3, which shows only part of the original circuit according to FIG. 2. Here, the switch S according to FIG. 2, which is connected in parallel to the capacitor C, is replaced by a transistor STR. The transistor STR is connected in parallel with the capacitor C with its collector-emitter circuit. The base of the transistor STR is connected via a further resistor R3 to the negative pole (-) of the supply voltage UM and to an optocoupler OK2. The optocoupler OK2 is used for the electrical isolation of the actual measuring device or measuring circuit from the other evaluation device of the control center. For example, a start signal STA for the measurement can take place via the optocoupler OK2, so that the transistor STR which has short-circuited the capacitor C is then opened. The output signal of the comparator D3 is also passed via a further resistor R4 to a further optocoupler OK1, so that the stop signal STO reaches the downstream AND gate G here too, galvanically isolated. This is followed by the counting device ZV.

A particular advantage of such a resistance measuring arrangement results if this measuring arrangement is provided only once in a control center and measures the resistance values of the respective signal line in rapid succession. For this purpose, a measuring point changeover switch MU, as shown in FIG. 4, can be provided. The measuring point switch MU is connected in turn to the respective signal line ML1 to MLn, which are symbolized here with the resistors RX1 to RXn. A series resistor RV1 to RVn is assigned to the respective signal lines ML1 to MLn. It is expedient to carry out a cyclical query with a multiplex switching device which is controlled by a microprocessor. Likewise, the start and stop signals STA, STO are sent from a microprocessor, for example via optocouplers OK1, 2 to the measuring circuit or from the measuring circuit to the microcomputer.

Claims (6)

1. A method of measuring the resistance in a signalling line of a hazard alarm system, the individual signalling lines (M, L) of which commence from a central control unit (Z) and are each provided with series-connected alarms (M1 to Mn) having a variable resistance value, where a voltage source (UV) is connected to the signalling line (ML) via a series resistor (RV) and a capacitor (C) is charged by a constant current in dependence upon the resistance value (RX) of the signalling line (ML), characterised in that the capacitor (C) is charged by a separate constant current source independent of the current in the signalling line. and its charging time (T), which is proportional to the resistance value (RX), is measured in digital form by means of a counting device and a clock pulse generator (TG), the capacitor being discharged before measurement commences.

2. A method as claimed in Claim 1, characterised in that the capacitor (C) is connected to the supply voltage (UV) via a transistor (TR). and is short-circuited before a measurement commences by a parallel-connected switching element (S; STR) that is opened when the measurement commences and that during the charging time (T) Pulses of the clock-pulse generator (TG) are fed via an AND-gate (G) to the subsequently-connected counting device, and that the end of the charging time (T) of the capacitor (C) is established by a comparator circuit (T3) which emits an output signal (STO) to the AND-gate (G) at a given ratio of the capacitor voltage (UC) to the voltage (UL) of the signalling line (ML).

3. A method as claimed in Claim 1 or 2, characterised in that ihe resisiance value (RX) of ihe signalling line (ML) in question is measured by a cyclic interrogation of the individual signalling lines (ML1 io MLn), where each signalling line (ML) is assigned iis own series resisior (RV).

4. A device for the implementation of the method claimed in Claim 1 or 2, characterised in that the resistance (RX) of the signalling line (ML) is connected to the d.c. voliage supply source (UV) in series w ith a series resisior (RV), that the chargeable capacitor (C) has one electrode connected to the positive pole (+) of the d.c. voltage source (UV) and the other electrode to the collector of a transistor (TR) and a first input (B3) of a comparator (D3), that the emitter of the transisior (TR) is connected via a first resistor (R1) to the negative pole (-) of the d.c. voltage source (UV), that the second input (A3) of the comparator (D3) is connected to the output of a first amplifier (D1) whose input (A1) is connected to the common connection point (X) of the resistance (RX) to be measured, the signalling line (ML) and the series resistor (RV), and whose output is connected via the series arrangement of two resistors (R1 and R2) to the negative pole (-) of the d.c. voltage source (UV), and that a second amplifier (D2) has its input (A2) connected two resistors (R1 and R2) and is connected by its output to the base of the transistor (TR), that the capacitor (C) is connected in parallel with a switching element (S), that the output of the comparator (D3) is connected to the first input of a AND-gate (G) and a clock-pulse generator (TG) is connected to the second input thereof, and that the second output of the AND-gate (G) is connected to a counting device (ZV).

5. A device as claimed in Claim 4, characterised in that the switching element (S) is a switching transistor (STR) which can be acted upon by a start signal (STA).

6. A device as claimed in Claim 4 or 5, characterised in that the output of the comparator (D3) is connected via a first optocoupler (OK1) to the AND-gate (G). and that the switching transistor (STR) can be acted upon by a start signal (STA) via a second opto-coupler (OK2).

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