Method and system for detecting a stall condition in an elevator

Stall condition in an elevator is potentially dangerous situation if it causes slack to the ropes of the elevator. In such situation a counterweight or elevator car does not move even if the hoisting machine is still operating. This situation may be prevented by stopping the elevator as early as possible after detecting such stall condition. The detection of the stall condition is based on monitoring the torque generated by the hoisting machine of the elevator. When rapid change in the torque is detected a stall condition is suspected. The elevator may be stopped or an alarm may be launched when the stall condition is suspected.

This application claims priority to European Patent Application No. EP13190064 filed on Oct. 24, 2013, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to elevators and more particularly to detecting a stall condition of an elevator car or counterweight.

BACKGROUND OF THE INVENTION

Elevators comprising a traction sheave arranged to operate at least one, typically more, ropes to which an elevator car and a counterweight is attached are commonly used. The friction between ropes and traction sheave is critical in elevators. In normal operation conditions the friction between ropes and traction sheave is large enough so that when the motor operates the traction sheave the elevator moves up or down. The friction force is determined by a plurality of different factors, for example, the shape and texture of rope and traction sheave and weight of the elevator car and counterweight. Parameters of these factors are decided for various reasons, for example, energy efficiency and security requirements.

When the friction between the ropes and the traction sheave is high enough the ropes in the traction sheave do not slip even if the elevator car or the counterweight has stalled. Stalling can be caused, for example, by running failure where the elevator car or counterweight gets stuck in the elevator shaft. If the elevator car is going upwards and the downward going counterweight gets stuck there will be slack in the rope between the traction sheave and counterweight. The slack between the traction sheave and the counterweight, or the elevator car if the movement is in opposite direction, can cause inconveniences or even security risk. A further example of stalling is a situation wherein the counterweight, or an elevator car, is at the buffer but the hoisting machine is still operating.

Conventionally this problem has been solved by designing the system so that the friction is large enough to operate in normal conditions and in case of stalling it is not too high so that the rope will slip. This kind of designing sets limitation to the overall design of the elevator system and the additional mechanical movement causes inefficiency and unnecessary wear.

In an improved method the stalling situation is detected by monitoring the current required by the hoisting machine. For example, if the counterweight has stalled the gravity caused by the counterweight does not help in hoisting of the elevator car. Thus, the hoisting machine needs more electric current in order to maintain the upward movement of the elevator car. The stalling situation is detected if the current required by the hoisting machine exceeds a predetermined threshold value.

The drawback of the current threshold implementation is that the current does not exceed the threshold in all conditions. The current threshold must be above the highest allowable current and it is possible that it is not exceeded always when stalling has occurred.

SUMMARY

Stall condition in an elevator is potentially dangerous situation if it causes slack to the ropes of the elevator. In such situation a counterweight or elevator car does not move even if the hoisting machine is still operating. This situation may be prevented by stopping the elevator as early as possible after detecting such stall condition. The detection of the stall condition is based on monitoring the torque generated by the hoisting machine of the elevator. When rapid change in the torque is detected a stall condition is suspected. The elevator may be stopped or an alarm may be launched when the stall condition is suspected.

The present invention is applicable in all elevators having a traction sheave involving suspension means including common twisted cord steel ropes, high friction coated ropes, cogged belts and similar.

In an embodiment of the invention a method for detecting a stall condition in an elevator is disclosed. In the method torque used for hoisting said elevator is monitored. From the monitored torque the rate of change of said torque used is computed. A stall condition is detected when said computed rate of change exceeds a predetermined threshold value. In a further embodiment of the invention a computer program for to be executed in a controlling unit capable of instructing at least one elevator is disclosed.

In a further embodiment of the invention a system for detecting a stall condition in an elevator is detected. The system further comprises a control unit configured to measure the torque used for hoisting said elevator, wherein said control unit is configured to detect a stall condition when said torque changes rapidly. The rapid change may be detected with the method described above.

In a further embodiment of the invention a system described above is used for controlling at least one elevator. The system may be implemented to each of the elevators or at the group controlling level.

The benefit of the invention is that it is capable of detecting a stall condition in situations that are not covered by the conventional methods, such as torque or current threshold and thus solves the problems of prior art. A further benefit of the invention is that it does not typically require structural changes to the elevator. Typically the invention can be implemented using the existing equipment, however, when the elevator does not include the necessary components the needed components may be added without a need of further modifications to the existing system.

Thus, a benefit of the invention is that it improves the passenger security and is easy and cost efficient to implement. Furthermore, when the stalling condition controlling can be handled according to the present invention the designer has more freedom when choosing friction properties for ropes and traction sheave. This may allow better functionality and/or cheaper price of the whole elevator.

A further benefit of the invention is that it provides better passenger security also in configurations having suspension means that cannot slip. For example, when an elevator car operated with a cogged belt is going downwards, it is possible that the elevator car stalls. In such case when the hoisting machine continues operating the elevator slack between the traction sheave and the elevator car emerges. When the elevator car has stopped unexpectedly due to undesired defect it is possible that elevator car stops only temporarily. Thus, when the stall condition is released and there is slack in suspension means the car may fall freely. When the suspension means do not have slack any more an undesired impact may be caused. Such an impact is not only uncomfortable but may be also dangerous. The benefit of the invention is that it detects such an situation rapidly so that the elevator may be stopped without causing risks or inconveniences to passengers in the elevator car. Correspondingly a stall condition of the counterweight may cause risks or inconveniences.

DETAILED DESCRIPTION OF THE INVENTION

InFIG. 1a block diagram of an example embodiment according to the present invention is disclosed. In the embodiment a hoisting machine10is configured to operate an elevator comprising an elevator car11and a counterweight12. The elevator car11and the counterweight12are coupled with a rope13that is arranged to be operated by the hoisting machine10. The ropes are coupled to the elevator car11and the counterweight12by diverting pulleys16,17. The ends of the rope13are coupled to the elevator shaft so that the elevator can be operated. For example, the ropes may be coupled to support structures of the hoisting machine so that the traction sheave15can be operated by a hoisting machine. A person skilled in the art understands that there are also other configurations for the elevator ropings.

InFIG. 1the hoisting machine10is coupled to the power source14so that the electric motor18is able to transform the electricity into torque for moving the elevator car and the counterweight through the traction sheave15. The hoisting machine further comprises a control unit19that is connected to the power source such that it can measure the power consumption of the electric motor18.

In the example ofFIG. 1the elevator car11is going upwards and the counterweight12downwards. For some reason the counterweight12get jammed to the shaft so that it stops moving downwards. Immediately there will be slack in the rope13on the side of the counterweight12. Because the counterweight12does not help the upward movement of the elevator car11the electric motor15must do all the work. Thus, it needs more power from the power source14.

The control unit19measures the electric current continuously. When it detects a rapid change in the electric current from power source14to electric motor14it determines that the counterweight is not moving anymore. After the determination the control unit19makes the emergency stop of the elevator in order to prevent further slack. Further actions, such as automatic emergency call, may be applied.

In the embodiment explained with referral toFIG. 1the rapid change of torque was disclosed. The torque was derived from the electric current. Thus, detecting rapid change in the electric current is an example of measuring the change of the torque applied at that moment of time. Also other means of detecting the rapid change may be applied.

InFIG. 2a method according of an example embodiment is disclosed. In the method the use of the electric current is monitored, step20. Modern elevators may be configured to monitor a plurality of different functions for various purposes. For the present invention it is interesting to measure the torque used for hoisting and it can be derived from the electric current used for hoisting. The actual measurement process is known to a person skilled in the art of signal processing and depending on the application it may be necessary process the measured signal in order to make correct conclusion from the measurement result. For example, the processing may include filtering undesired, typically high frequency components from the torque indicating signal. During monitoring the rate of change of used torque is continuously computed, step21. For example, the change may be measured during a predetermined time interval. Such interval may be chosen on an application basis, however, a person skilled in the art understands that the time interval must be relatively small period of time as elevators travel fast and the slack should be minimized. The computed value is continuously compared with a predetermined threshold, step22. The threshold is also chosen on an application basis so that the ordinary movement of the elevator does not trigger the condition. If the threshold is exceeded the elevator may be stopped, step23. Instead of stopping it is possible to launch an alarm, however, when slack in the ropes is suspected it is typical to stop the elevator for the sake of the passenger security. Typically the alarm is launched always after an emergency stop.

If the elevator does not comprise a measurement device it is possible to install an additional measurement device. The measurement device may comprise all functionality discussed. Thus, when the additional device detects rapid change it is capable of stopping the elevator and launching an alarm.

InFIG. 3a diagram disclosing a rapid change is illustrated. In figure at the moment of time x pointed by the arrow30a rapid change has occurred as the value of y changes rapidly as a function of x. Thus the derivate, f′ (x) has a high value and hoisting must be stopped. For the sake of the clarity inFIG. 3a threshold current31according to prior art is disclosed. The threshold current mechanism may be used together with the present invention, however, in the example ofFIG. 3the threshold current is not exceeded. Thus, the conventional method would not detect the possible stall condition of the example. This can be caused, for example, when an empty elevator car is hoisted. Thus, the current needed is smaller compared to the situation where full elevator car is hoisted and thus, the threshold current is not exceeded.

The above mentioned method may be implemented as computer software which is executed in a computing device able to communicate with a measurement device for detecting rapid change in torque used for hoisting and to stop the elevator when such change is detected. When the software is executed in a computing device it is configured to perform the above described inventive method. The software is embodied on a computer readable medium so that it can be provided to the computing device.

As stated above, the components of the exemplary embodiments can include computer readable medium or memories for holding instructions programmed according to the teachings of the present inventions and for holding data structures, tables, records, and/or other data described herein. Computer readable medium can include any suitable medium that participates in providing instructions to a processor for execution. Common forms of computer-readable media can include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other suitable magnetic medium, a CD-ROM, CD±R, CD±RW, DVD, DVD-RAM, DVD±RW, DVD±R, HD DVD, HD DVD-R, HD DVD-RW, HD DVD-RAM, Blu-ray Disc, any other suitable optical medium, a RAM, a PROM, an EPROM, a FLASH-EPROM, any other suitable memory chip or cartridge, a carrier wave or any other suitable medium from which a computer can read.