The present invention is directed to a safety circuit for a passenger conveyor system, in particular for the type of a installation comprising one of an elevator or escalator system, wherein the safety circuit comprises a series chain of switch contacts connected in series to monitor equipment related to the safety of the operation of the installation, an electric power supply connected to one end of the series chain, and at least one switching device connected to another end of the series chain and which generates a control signal for an installation operation control depending on the switching status of the switch contacts.

A safety circuit for a passenger conveyor system, in particular for an elevator or escalator installation, comprises a chain of switch contacts, such as door contacts of a hoistway door, connected in series, a switch contact being provided for the purpose of, for example, monitoring the position of a hoistway door. Further contacts or switches for the purpose of monitoring, for example, the position of the elevator car door, the position of the brake, equipment serving the safety of the elevator operation, can be connected into the safety circuit and the series chain, respectively. The safety circuit is usually supplied with a supply voltage from either an AC or DC voltage source connected to one end of the series chain. At least one switching device is usually connected to another end of the series chain, the switching device being for example a safety relay. If all switch contacts of the series chain are closed, the safety relay is activated. On the other hand, if one of the switch contacts is in a disconnected condition, the safety relay is deactivated. An elevator operation control monitors the status of the safety relay and if the safety relay is activated, the elevator operation control releases, for example, a pending travel command. Such safety circuit described with reference to an elevator installation may be applied in a similar manner in an escalator installation.

In most such applications the switch contacts of a safety circuit which determine the safety status of the installation are spread over the whole

system. Usually, only a few signals are transmitted out of the series chain by a plurality of sensing wires connected to the series chain at discrete terminals, wherein the sensing wires are connected to a controller for measurement and information purposes. Here, it is often not possible to provide sensing wires for each of the switch contacts which are to be monitored in order to locate individual open switch contacts. Especially, the safety chain of the hoistway door contacts in an elevator installation is difficult to monitor as the switch contacts are distributed over the whole building. On the other hand, the hoistway door contacts are often affected and turn out to be the root cause for many callbacks. In most cases the search for a contact fault has to be done manually with use of a voltage meter in the installation. Such procedure may be very time consuming and often needs to be carried out on top of the elevator car.

The object of the present invention is to provide a safety circuit for a passenger conveyor system of the above type, which safety circuit enables the detection and localization of a switch contact being in a disconnected condition by means of an electrical sensing system which has reduced complexity.

The object is solved by a safety circuit for a passenger conveyor system according to claim 1.

The safety circuit according to the present invention, which is of the constitution set forth in the introductory part of the present invention, furthermore comprises an electrical sensing system coupled to the series chain and to a measuring and data processing device, the sensing system having a sense line and adapted to comprise at least one system characteristic which varies depending on the switching status of the switch contacts. The measuring and data processing device is adapted to measure and process a sensing signal transmitted on the sense line of the electrical sensing system. The measuring and data processing device is further adapted to compute a switching status and the position of any one of the switch contacts being in a disconnected condition based on the sense signal.

The safety circuit according to the invention enables the detection and local- ization of any one of the switch contacts being in a disconnected condition by means of an electrical sensing system having reduced complexity. The electrical sensing system requires only a single sense line adapted to transmit a sense signal which is indicative of the at least one system characteristic which varies depending on the switching status of the switch contacts. Thus, the electrical sensing system does not require a complex measurement wiring connected to each of the switch contacts to be monitored, and therefore does not require any additional sense lines or stubs for measuring the status of any one of the switch contacts. Moreover, the safety circuit according to the invention enables fast and easy service of the installation, as the safety circuit can compute the exact position of a faulty switch contact. The working time spent on top of the elevator car to service the safety circuitry can be reduced or eliminated. The safety circuit according to the invention is applicable for both an elevator installation or an escalator installation comprising a series chain of switch contacts serving the safety of the operation of the installation.

According to an embodiment of the present invention, the measuring and data processing device includes a database which associates different numbers of switch contacts being in a defined switching status to a corresponding sense signal value. For example, the database associates different numbers of switch contacts being in a closed or activated condition to a corresponding sense signal value. Thus, by determining the sense signal value the measuring and data processing device may determine the corresponding number of switch contacts being in the closed condition, and may derive therefrom the position of a switch contact being in a disconnected condition. By this technique any switch contact being in open condition can be located individually within the series chain without extensive search. In order to locate a switch contact being in a disconnected condition, the measuring and data processing device may associate the sense signal provided through the sense line and which is indicative of the switching status of the switch contacts to a corresponding number of switch contacts being in a defined switching status. The position of a switch contact being in a disconnected condition is derived therefrom.

According to one embodiment of the present invention, the electrical sensing system comprises a network of resistors connected to the series chain of

105 switch contacts providing a resistance characteristic which varies depending on the switching status of the switch contacts. A current measuring device may sense the current at the top of the series chain connected to the electrical power supply, wherein the sensed current varies dependent on the resistance characteristic of the network of resistors connected to the series chain. Such

110 network of resistors may be of rather low complexity using only low cost components.

According to a further embodiment of the invention, the safety circuit comprises a current sensor connected between the electric power supply and a us first switch contact of the series chain, the current sensor connected to the measuring and data processing device via the sense line providing the sensed current signal of the series chain which varies depending on the switching status of the switch contacts.

120 According to another embodiment of the present invention, the electrical sensing system comprises a capacitive coupling arrangement coupled to the series chain and to the measuring and data processing device providing a capacitive characteristic between the series chain and the sense line which varies depending on the switching status of the switch contacts. For example, a

125 capacitance meter measures the capacitance between the series chain and the sense line. A differential capacitance may be proportional to the length of the wire pair comprising the series chain and the sense line. If the total length of the wire pair is cut into segments as a result of an open switch contact, the measured capacitance between the series chain and the sense line varies,

130 depending on the location of the open switch contact. According to an embodiment, the sense line and the series chain are twisted at discrete positions for capacitive-coupling the sense line to the series chain.

According to another embodiment of the present invention, the electrical

135 sensing system comprises a high-frequency signal coupling network coupled into the series chain and to the measuring and data processing device

providing a pulse reflection characteristic of the series chain with respect to a high-frequency signal supplied to the series chain, which pulse reflection characteristic varies depending on the switching status of the switch contacts. uo Thus, the measuring and data processing device may be adapted to measure a reflected pulse signal received from the series chain in order to detect a propagation delay of the pulse signal supplied to the series chain which varies depending on the switching status of the switch contacts.

145 For this purpose the measuring and data processing device may comprise a pulse generator adapted for generating a pulse signal supplied to the series chain and a timing unit adapted for sensing the reflected pulse signal received from the series chain and for detecting the propagation delay of the pulse signal.

150

According to an embodiment of the invention, the series chain comprises a termination network which is coupled into the series chain and which is adapted to have a predetermined signal reflection characteristic. In particular, the termination network is coupled into the series chain downstream to an

155 ultimate switch contact of the series chain. In such configuration, if one of the switch contacts of the series chain is in a disconnected condition, there will be a significant signal reflection characteristic being different from the predetermined signal reflection characteristic associated with the termination network. The propagation delay of the pulse signal is proportional to the location of a

160 disconnected switch contact which reflects the pulse signal, which enables the measuring and data processing device to detect the position of the switch contact being in a disconnected condition.

Further embodiments, features, aspects and details of the present invention are 165 evident from the dependent claims.

The invention will now be described with reference to the following description of embodiments of the invention taken in conjunction with accompanying drawings, wherein

170

Fig. 1a is a circuit diagram of a first embodiment of a safety circuit according to the invention,

Fig. 1 b is a circuit diagram of an alternative circuit of the first embodiment of a 175 safety circuit according to the invention,

Fig. 2 is a circuit diagram of a second embodiment of a safety circuit according to the invention,

180 Fig. 3 is a circuit diagram of a third embodiment of a safety circuit according to the invention.

Referring to Fig. 1, reference numeral 12 denotes a safety circuit for an elevator or escalator installation which comprises a typical structure of a series chain of

185 switch contacts connected in series to monitor equipment related to the safety of the elevator or escalator system operation. The safety circuit 12 comprises a series chain 20 of switch contacts S1 - Sn, wherein a switch contact SI - Sn is provided for the purpose of, for example, monitoring the position of a hoistway door. The hoistway door actuates the associated switch contact such that the

190 contact is in a closed or connected condition when the hoistway door is in a closed condition. If all of the switch contacts S1 - Sn are closed, the elevator or escalator installation is set free for operation. An electric power supply 1 providing an AC or DC voltage of approximately 110 V in the present embodiment is connected to the upper end of the series chain 20. A switching

195 device 2 is connected to the lower end of the series chain 20 and detects the status when all of the switch contacts S1 - Sn are in the closed condition. The switching device 2 may be a safety relay which actuates a relay contact 3 which closes a control signal loop connected to an installation operation control 4 applying a signal voltage to the relay contact 3. With closing of the relay

200 contact 3 a control signal for the installation operation control 4 is generated which releases, for example, a pending travel command. According to another embodiment, the switching device 2 may be part of a variable frequency control device for driving, e.g., a drive motor of an elevator hoisting system. Here, the variable frequency control device monitors the status of the switching

205 device 2 and actuates a drive command signal if the switching device 2 is activated. As a result, the switching device 2 acts as a energy-flow controlling contactor.

According to Fig. 1, the safety circuit 12 comprises an electrical sensing system 2io 5 coupled to the series chain 20 and to the measuring and data processing device 22, such as an A/D converter. The electrical sensing system 5 comprises a network of resistors R connected to the series chain 20. The network of resistors comprises a first resistor R connected to a terminal 201 between the electric power supply 1 and the first switch contact S1 and connected to a daisy 215 chained return path 27 of the series chain at terminal 211. A second resistor R is connected to the terminal 202 between the first switch contact S1 and the downstream connected second switch contact S2 and to the daisy chained return path 27 of the series chain at terminal 212. In an analogous manner, a further resistor R is connected to terminal 203 located between switches S2 220 and S3 and to terminal 213 of the return path. A fourth resistor R is connected to terminal 204 located between switches S3 and S4 and to terminal 214 of the return path. The ultimate resistor R connected to the series chain, which may comprise, in principal, any number of switch contacts, is connected to terminal 2On between switch contacts S4 and Sn and to terminal 21 n of the return path.

225

Furthermore, the safety circuit 12 comprises a current sensor 25 connected into the series chain, which is in particular, according to Fig. 1a, connected between the electric power supply 1 and the first switch contact S1 of the series chain 20. According to the embodiment of Fig. 1b, the current sensor 25a is 230 connected into the return path 27 of the series chain between a terminal for potential HL1 and switching device 2. This embodiment provides advantages due to the ground relation.

The current sensor 25, 25a is connected to the measuring and data processing 235 device 22 via a sense line 21 and provides a sensed current signal 210 of the series chain 20. The measuring and data processing device 22 is adapted to measure and process the sensed current signal 210 transmitted on the sense line 21. A second input of the measuring and data processing device 22 is

supplied with a current reference signal 240 provided by a current reference

240 device 24. The current reference device 24 provides a reference current 241 flowing through reference resistor R and current sensor 26 which provides the current reference signal 240 derived therefrom. The measuring and data processing device 22 is adapted to compare the current reference signal 240 with the sensed current signal 210. With such arrangement using a current

245 reference device 24 providing a reference current and a current reference signal, voltage fluctuations of the electric power supply may be compensated.

An output of the measuring and data processing device 22 is connected to a control unit 23 which provides information on the switching status of the switch contacts S1 - Sn. The control unit 23 may be used to serve the elevator or

250 escalator installation service in order to repair a defective switch contact.

The network of resistors R of the electrical sensing system 5 provides a resistance characteristic which varies depending on the switching status of the switch contacts S1 - Sn. With each switch contact SI - Sn being in a closed

255 condition the current flowing through the series chain increases with a predetermined step increment as with each closed switch contact more resistors R are parallel-circuited. The measuring and data processing device 22 measures and processes a corresponding sense signal transmitted on the sense line 21 which is indicative of the respective resistance characteristic depending on the

260 switching status of the switch contacts. The lowest current will be detected when switch contact S1 is in disconnected condition and the highest current will be detected when all of the switch contacts S1 - Sn are in the closed condition.

265 The measuring and data processing device 22 includes a database which associates different numbers of switch contacts S1 - Sn being in a closed condition to a corresponding sense signal value. In particular, the measuring and data processing device 22 associates the sense signal 210 to a corresponding number of switch contacts SI - Sn being in a closed condition. When

270 the respective number of closed switch contacts is known the defective or open switch contact must be the switch contact following the last switch contact of the detected number of closed switch contacts. For example, if a number of

three closed switch contacts is detected which means that switch contacts S1 - S3 are in closed condition, the defective switch contact being in open condition

275 must be the next one connected downstream to switch S3, i.e. switch contact S 4. Thus, the measuring and data processing device 22 may compute the switching status and the position of switch contact S4 based on the sense signal 210. Likewise, the measuring and data processing device 22 may compute a switching status and the position of any one of the switch contacts

280 S1 - Sn being in a disconnected condition based on the sense signal 210 transmitted on single sense line 21. Thus, the invention provides an electrical sensing system 5 which requires only one sense line for a plurality of switch contacts to be monitored, thus providing reduced complexity of the electrical sensing system. Any open switch contact can be located individually within the

285 safety circuit without extensive search.

The safety of the series chain is maintained by the fact that resistor types are used which exclude a short circuit of the resistive element. Furthermore, the return path 27 of the series chain 20 is guided such that any interruptions of the 290 return path will lead to a drop within the switching device 2 which is thus deactivated.

Referring now to Fig. 2, there is shown a circuit diagram of a second embodiment of a safety circuit according to the invention. The safety circuit 13

295 comprises a series chain 30 of switch contacts SI - Sn similar to that disclosed in Fig. 1, an electric power supply 1 connected to one end of the series chain 30 and a switching device 2 connected to the other end of the series chain 30. The safety circuit 13 comprises an electrical sensing system 6 comprising a capacitive coupling arrangement coupled to the series chain 30 and to the

300 measuring and data processing device 32. The measuring and data processing device 32 is adapted to measure and process a sense signal 310 transmitted on sense line 31 of the electrical sensing system.

The sense line 31 is capacitive-coupled to the series chain 30 at a plurality of

305 terminals 301 to 3On, wherein terminal 301 is coupled to a first switch contact

S1 and switching device 2. The terminal 302 is positioned between switch

contacts S1 and S2, terminal 303 is positioned between switch contacts S2 and S3, terminal 304 is positioned between switch contacts S3 and S4, and terminal 3On is positioned between switch contacts S4 and Sn. In order to capacitive- couple the sense line 31 to the series chain 30, the sense line 31 and the series chain 30 are twisted at discrete positions 35. Thus, each of the sense line 31 and the series chain 30 work as a "capacitor plate" (Cp1, Cp2) at a twist position 35 thereby achieving a maximum coupling capacitance C at each of the twist positions 35. Furthermore, the capacitive coupling arrangement comprises a supply line 36 for supplying the series chain 30 with a periodical signal. For avoiding effect on remaining circuit parts of the safety circuit, the measuring and data processing device 32 for supplying the periodical signal on the supply line 36 is operated in a floating condition. The safety of the series chain 30 may be maintained by the fact that the measuring and data processing device 32 is operated in a floating condition with respect to the HL1 potential. Therefore, no signal energy can be induced into the contactors of switching device 2.

When a periodical signal is supplied via supply line 36, the sense line 31 provides a sensed signal 310 indicative of a sensed capacitance between the series chain 30 and the sense line 31 which varies depending on the switching status of the switch contacts S1 - Sn. The measuring and data processing device 32, which includes an A/D-converter, measures the capacitance between the series chain 30 and the sense line 31. The differential capacitance is proportional to the length of the wire pair. If an open switch contact S1 - Sn cuts the total length of the wire pair into two segments, the measured capacitance decreases depending on the location of the open switch contact. The capacitance may be measured by determining the phase deviation between the detected current and voltage.

The measuring and data processing device 32 includes a database which associates different numbers of switch contacts S1 - Sn being in a closed condition to the measured capacitance value. Thus, the measuring and data processing device 32 may associate the sense signal provided through the sense line 31 to a corresponding number of closed switch contacts SI - Sn and may derive therefrom the position of a disconnected switch contact. As the

number of closed switch contacts is known, the defective open switch contact must be the next one connected upstream to the ultimate switch contact of the number of closed switch contacts. For example, if a number of three closed switch contacts is detected, the defective open switch contact must be switch 345 contact S4.

The safety circuit 13 according to Fig. 2 further comprises a capacitance reference device 34 providing a reference signal 340 indicative of a reference capacitance, wherein the capacitance reference device 34 is coupled to the

350 measuring and data processing device 32. The latter is adapted to compare the reference signal 340 with the sensed signal 310 on the sense line 31. The output of the measuring and data processing device 32 is provided to control unit 33 similar to control unit 23 of Fig. 1. As the capacitance per meter of a twisted pair cannot be determined exactly and the wiring length may vary, it is

355 preferred that an initial learning run be made to set up the correct values. The capacitance reference device 34 includes a reference capacitor made by twisted wires.

With reference now to Fig. 3, there is shown a circuit diagram of a third 360 embodiment of a safety circuit according to the invention. The safety circuit 14 includes a series chain 40 of switch contacts S1 - Sn with an electric power supply 1 connected to one end of the series chain 40 and a switching device 2 connected to another end of the series chain 40. The constitution of these basic components is similar to that of the safety circuits according to Figs. 1 and 2. 365 An electrical sensing system 7 is coupled to the series chain 40 and comprises a high-frequency signal coupling network 45 coupled into the series chain 40 and to the measuring and data processing device 42 via the sense line 41. By means of the high-frequency signal coupling network 45, a so-called HF- coupling network, the sense line 41 is coupled to the series chain 40 at terminal 370 401 connected between the first switch contact S1 and the electric power supply 1. The HF-coupling network 45, for example, includes a capacitor or transformer for coupling a high frequency signal into series chain 40. The measuring and data processing device 42 comprises a pulse generator adapted for generating a pulse signal 411 supplied to the series chain 40 and a timing

375 unit adapted for sensing a reflected pulse signal 412 received from the series chain 40.

The series chain 40 comprises a termination network 44 which is coupled into the series chain 40 and which is adapted to have a predetermined signal

380 reflection characteristic. For example, the termination network 44 comprises the characteristic wave impedance of the signal line of the series chain. Thus, when a high-frequency signal arrives at termination network 44, the signal reflection of the high-frequency signal is rather low or approximately zero. The termination network 44 is coupled into the series chain 40 downstream to the

385 ultimate switch contact Sn of the series chain.

The pulse generator in the measuring and data processing device 42 generates a pulse signal 411 with a rise time of a few nanoseconds to be supplied to the series chain 40. If all of the switch contacts SI - Sn and the sense switch S_s

390 are in closed condition there will be no or rather low signal reflection of the pulse signal due to the termination network 44. However, if one of the switch contacts S1 - Sn or the sense contact S_s are in disconnected condition, there will be high signal reflection of the pulse signal 41 1 resulting in a reflected pulse signal 412, as a result of the mismatch between the line impedance and

395 the impedance of the opened switch contact. The time difference between the generated pulse signal 41 1 and the reflected pulse signal 412 is proportional to the location of a opened switch contact, the time difference being about 3 ns to 5 ns per meter. Thus, the electrical sensing system 7 provides a pulse reflection characteristic of the series chain with respect to a high-frequency signal 411

400 supplied to the series chain, which pulse reflection characteristic varies depending on the switching status of the switch contacts S1 - Sn.

The measuring and data processing device 42 includes a database which associates different numbers of switch contacts S1 - Sn being in a closed 405 condition with a corresponding propagation delay value. As the delay constant (in particular, characteristic wave impedance and geometry of the signal line) and the wiring length may vary, there will be an initial learning run to set up the correct values and to make further plausibility checks for a calibration. As the

number of closed switch contacts is known, the defective open switch contact 4io must be the next one connected downstream to the ultimate switch contact of the number of closed switch contacts. In particular, if a number of three closed switch contacts is detected, the defective open switch contact must be Sn.

According to the embodiment of Fig. 3, a sense contact device S s is coupled

415 into the series chain 40 downstream to the ultimate switch contact Sn of the series chain. The sense contact device S_s is controlled by the measuring and data processing device 42 for a plausibility check of the electrical sensing system 7. Thus, the sense contact device S s is located at the far end of the series chain and will be used for a teach-in and plausibility check. An output of

420 the measuring and data processing device 42 is coupled to control unit 43 which acts similar to control units 23 and 33 of Figs. 1 and 2. Via the control line

46 the HF-coupling network 45 may be disconnected from power supply 1.

All of the exemplary embodiments described with reference to Figs. 1 to 3 may

425 detect an open switch contact of the series chain by means of a single sense line, wherein no additional sense lines or stubs are required to measure the status of anyone of the switch contacts. This results in an electrical sensing system having reduced complexity. Therefore, most of discrete input level converters used in the state of the art on a control printed circuit board (PCB),

430 which transform the supply voltage of 110 V to logic level voltage for detecting a switch contact status, can be replaced. Such printed circuit boards were especially used in escalator installation systems.

435