WO2021116527A1

WO2021116527A1 - Monitoring of elevator system

Info

Publication number: WO2021116527A1
Application number: PCT/FI2019/050879
Authority: WO
Inventors: Petteri Valjus; Henri WENLIN
Original assignee: Kone Corporation
Priority date: 2019-12-10
Filing date: 2019-12-10
Publication date: 2021-06-17

Abstract

The present invention relates to a method for monitoring of an elevator system (100), the method comprises: receiving (210) measurement data from a sensor (170) arranged to measure at least one elevator rope (130) during a motion of an elevator car (110); detecting (220) from the measurement data a variation on a surface of the elevator rope (130); determining (230) a parameter representing the motion of the elevator car (110) from the variation on the surface of the elevator rope (130); comparing (240) the parameter to a reference value; and setting a detection result to express one of the following: (i) the elevator system (100) operates as expected (250), (ii) an operation of the elevator system (100) deviates from expected (260). The invention also relates to a control unit (160), an elevator system (100) and a computer program product.

Description

MONITORING OF ELEVATOR SYSTEM

TECHNICAL FIELD The invention concerns in general the technical field of elevators. More particularly, the invention concerns monitoring of an elevator.

BACKGROUND

An elevator system is a complex solution which comprise parts and devices which wear during use. A wearing may cause variation in an operation of the elevator system and, in the worst case, reduce safety of using the elevator system.

So-called roped elevator systems are based on a solution in which an elevator car is raised and lowered by elevator ropes, typically made of metal. The elevator ropes, such as hoisting ropes or overspeed governor ropes, may be attached to the elevator car at one end and to a counter weight or to the elevator car at the other end and the elevator rope is looped at least partly around a traction sheave, or any similar entity, between the mentioned ends. For example, the traction sheave is a grooved pulley into which grooves the traction rope, or ropes, is fitted to. Moreover, the traction sheave is coupled to an electrical motor, and when the motor is controlled to rotate the traction sheave, and as a result the elevator car moves along its pathway e.g. in an elevator shaft. As may directly be understood from the above a traction system as a whole is under a physical stress and comprises parts and devices wearing out in a course of time. In view of above there is need to develop novel approaches by means of which it is possible to monitor an operation of an elevator system and to derive at least some understanding on one or more entities whose operation deviates from expected operation. SUMMARY

The following presents a simplified summary in order to provide basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

An object of the invention is to present a method, a control unit, an elevator system and a computer program product for monitoring of an elevator system.

The objects of the invention are reached by a method, a control unit, an elevator system and a computer program product as defined by the respective independent claims.

According to a first aspect, a method for monitoring of an elevator system is provided, the method performed by a control unit comprises: receiving measurement data from a sensor arranged to measure at least one elevator rope during a motion of an elevator car; detecting from the measurement data a variation on a surface of the elevator rope; determining a parameter representing the motion of the elevator car from the variation on the surface of the elevator rope; comparing the parameter to a reference value; and setting, in accordance with a comparison between the parameter and the reference value, a detection result to express one of the following: (i) the elevator system operates as expected, (ii) an operation of the elevator system deviates from expected. The variation on the surface of the elevator rope may be detected based on detection of at least one measurement data value representing a peak in a projected image of the elevator rope and at least one measurement data value representing a valley in the projected image of the elevator rope. For example, the parameter may be determined on a basis of at least one of: detection of adjacent peaks from the variation on the surface of the elevator rope; detection of adjacent valleys from the variation on the surface of the elevator rope.

A monitoring of the elevator system may be based on a monitoring of a slip of the elevator rope. For example, the slip of the elevator rope may be determined by determining, as the parameter, a speed of the elevator rope on a basis of the variation of the elevator rope. Moreover, the reference value may be determined from signal representing a speed of an electrical motor rotating a traction sheave along which the elevator rope is arranged to travel at least in part.

Still further, a monitoring of the elevator system may be based on a monitoring of an elongation of the elevator rope. The reference value may correspond to a reference distance between two predetermined points within a travel path of the elevator car. For example, a distance between the two predetermined points may be determined on a basis of the variation of the elevator rope for comparison. Moreover, the distance between the two predetermined points within a travel path of the elevator car may be defined as a number of detected adjacent peaks or detected adjacent valleys. The reference distance between the two predetermined points may e.g. correspond to a known distance between two floors within the travel path of the elevator car.

According to a second aspect, a control unit for monitoring of an elevator system is provided, the control unit comprising: at least one processor; at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the control unit to perform: receive measurement data from a sensor arranged to measure at least one elevator rope during a motion of an elevator car; detect from the measurement data a variation on a surface of the elevator rope; determine a parameter representing the motion of the elevator car from the variation on the surface of the elevator rope; compare the parameter to a reference value; and set, in accordance with a comparison between the parameter and the reference value, a detection result to express one of the following: (i) the elevator system operates as expected, (ii) an operation of the elevator system deviates from expected.

The control unit may be arranged to detect the variation on the surface of the elevator rope based on detection of at least one measurement data value representing a peak in a projected image of the elevator rope and at least one measurement data value representing a valley in the projected image of the elevator rope.

Further, the control unit may be arranged to determine the parameter on a basis of at least one of: detection of adjacent peaks from the variation on the surface of the elevator rope; detection of adjacent valleys from the variation on the surface of the elevator rope.

The control unit may also be arranged to perform a monitoring of the elevator system based on a monitoring of a slip of the elevator rope. For example, the control unit may be arranged to determine the slip of the elevator rope by determining, as the parameter, a speed of the elevator rope on a basis of the variation of the elevator rope.

The control unit may also be arranged to determine the reference value from a signal representing a speed of an electrical motor rotating a traction sheave along which the elevator rope is arranged to travel at least in part. The control unit may be arranged to perform a monitoring of the elevator system based on a monitoring of an elongation of the elevator rope. For example, the control unit may be arranged to apply a reference distance between two predetermined points within a travel path of the elevator car as the reference value. Still further, the control unit may be arranged to determine a distance between the two predetermined points on a basis of the variation of the elevator rope for comparison. The control unit may also be arranged to define the distance between the two predetermined points within a travel path of the elevator car as a number of detected adjacent peaks or detected adjacent valleys.

For example, the control unit may be arranged to apply a known distance between two floors within the travel path of the elevator car as the reference distance between the two predetermined points.

According to a third aspect, an elevator system is provided the elevator system comprising: at least one elevator rope; at least one sensor for measuring the at least one elevator rope; and a control unit according to the second aspect as described in the foregoing description.

According to a fourth aspect, a computer program product for monitoring of an elevator system is provided which computer program product, when executed by at least one processor, cause a control unit to perform the method according to the first aspect as described in the foregoing description.

The expression "a number of” refers herein to any positive integer starting from one, e.g. to one, two, or three.

The expression "a plurality of” refers herein to any positive integer starting from two, e.g. to two, three, or four.

Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in connection with the accompanying drawings.

The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality. BRIEF DESCRIPTION OF FIGURES

The embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. Figure 1 illustrates schematically an example of an elevator system according to an embodiment of the invention.

Figure 2 illustrates schematically a method according to an embodiment of the invention.

Figure 3 illustrates schematically some aspects relating to an embodiment of the invention.

Figure 4 illustrates schematically a control unit according to an embodiment of the invention.

DESCRIPTION OF THE EXEMPLIFYING EMBODIMENTS

The specific examples provided in the description given below should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given below are not exhaustive unless otherwise explicitly stated.

Figure 1 illustrates schematically an example of an elevator system 100 into which the present invention may be implemented. The elevator system 100 of Figure 1 comprises an elevator car 110 and a counterweight 120 connected to each other with at least one elevator rope 130. The term “elevator rope” in the context of the present invention shall at least be understood to cover such as hoisting ropes or overspeed governor ropes. The elevator rope 130, especially in a case of hoisting rope, is arranged to travel though a traction system, and especially at least in part over a traction sheave 140. In other words, the elevator rope 130 is looped over the traction sheave 140 which, through rotational motion, causes the elevator car 110 and the counterweight 120 to move vertically in an elevator shaft. The energy for the rotational motion of the traction sheave is generated with an electrical motor 150 controllable by a control unit 160 with control signals. In addition to the above mentioned entities at least one sensor 170 is arranged to monitor the at least one elevator rope 130, wherein the sensor generates measurement data representing at least one characteristic of the at least one elevator rope 130. The measurement data is obtained by the control unit 160, which is arranged to analyse the measurement data in a manner as is described in a forthcoming description in order to generate a detection result of a monitoring of the elevator system. A non-limiting example of an applicable sensor 170 is an inductive sensor generating a magnetic field interacting with metallic objects, such as the at least one elevator rope, and in that manner generating an output signal from which is possible to analyse aspects relating to the at least one elevator rope 130, and, hence, aspects with respect to the elevator system 100. Another applicable sensor 170 type may be one based on an electromagnetic radiation. There a transmitter of the electromagnetic radiation, such as a visible light, is arranged on one side of the at least one elevator rope 130 and a detector is arranged on the other side of the at least one elevator rope 130. Hence, by transmitting electromagnetic radiation through the at least one elevator rope 130 it is possible to receive an output signal from the detector from which it is possible to derive characteristics of the elevator rope 130 for further use as will be described. In an implementation of Figure 1 there is illustrated only one control unit 160 controlling both the electrical motor 150 and the sensor 170, and, thus, the whole elevator system 100. However, each of the entities may comprise a control unit 170 on their own, wherein one control unit 160 may be arranged to operate as a master unit and the other control units as slave units at least from the perspective of the present invention wherein the master unit may perform the method as is described in the forthcoming description. Moreover, in Figure 1 the sensor 170 is arranged to measure the at least one elevator rope 130 at the traction sheave 140, but the measurement may be performed at some other entities, such as at diverter pulleys along which the elevator rope 130 may be arranged to travel. The term “diverter pulley” shall also be understood to cover an overspeed governor pulley. As said, the measurement shall be performed at such location where the at least one elevator rope 130 under monitoring is arranged to travel during an operation of the elevator.

In accordance with at least one inventive idea of the present invention an aim is to obtain measurement data from the elevator system 100 from which it is possible to derive information on a condition of the elevator system 100, such as on a condition of a traction system, for example. The present invention provides a mechanism to monitor e.g. a slip of the elevator rope 130 with respect to an entity it travels over, such as the traction sheave, as well as an elongation of the elevator rope 130 both having relevance in an evaluation of the condition of the elevator system.

Figure 2 illustrates schematically a method according to an embodiment of the invention. First, a control unit 160 may be communicatively coupled to at least one sensor 170 arranged to generate measurement data with respect to at least one elevator rope 130. In other words, the at least one sensor 170 may be arranged to measure at least one elevator rope 130. Now, since the measurement is performed during an operation of the elevator system, i.e. when the elevator car 110 travels, the at least one sensor 170 generates a plurality of measurement data values representing consecutive slices of the elevator rope 130 into a direction the elevator rope 130 has traveled within a detection area of the sensor 170. Flence, the measurement data values may e.g. be an output signal values of an inductive sensor or of an electromagnetic radiation detector receiving the electromagnetic radiation from the transmitter, wherein the elevator rope 130 under monitoring is arranged to travel between the transmitter and the detector. Flence, the control unit 160 may receive 210 the measurement data from the at least one sensor 170 by reading the measurement data from an output of the respective sensor 170.

Next, the control unit 160 may be arranged to detect a variation on a surface of the elevator rope 130 along a length the elevator rope 130 travelled. In other words, the detection of the variation may be based on a representation of the elevator rope 130 generated from the received measurement data values. For example, the representation may be so-called peak-valley representation of the elevator rope 130, or at least one edge of the elevator rope 130, as schematically illustrated in Figure 3. The representation may be considered as a projection of an image on the elevator rope 130. The peaks and valleys may be used as a source of variation due to a structure of the elevator rope 130. Typically, the structure of the elevator rope 130 is achieved by composing a plurality of strands consisting of wires together. The number of the strands may e.g. be six. The wires in the strand may be arranged in spiral or straight. Hence, by composing the strands spirally together, the peak-valley representation may be established. Now, in response to running the elevator rope 130 over a measurement point of the sensor 170 measurement data values may be generated. In Figure 3 it is shown, as an example, positions of the rope at different instants of time t1 ...t8 in the measurement point of the sensor 170. In the non-limiting example of Figure 3 it may be derived that a sampling rate of the sensor 170 is adjusted with respect to the motion of the elevator rope 130 so that the measurement data value in question either corresponds to a value of a peak or a valley. For sake of clarity it is worthwhile to mention that the sampling rate may be set much higher in order to gain more measurement data values enabling to generate a more accurate representation of the elevator rope 130 by the control unit 160. Moreover, depending on the applied sensor type the control unit 160 may be arranged to determine from an output signal of the sensor 170 (i.e. from the respective measurement data value) which position of the elevator rope 130 the measurement data value represents to. For example, in case of inductive sensor on a basis of a voltage value of the output signal it may be determined if the measurement data value represents a peak or a valley (e.g. through comparing to a reference value). Correspondingly, if the sensor 170 is based on the electromagnetic radiation, it is possible to determine a shape of the elevator rope 130 and one or more parameters therefrom, such as a diameter of the elevator rope 130 at each measurement instance. All in all, the control unit 160 may be arranged to detect 220 a variation of a surface of the elevator rope 130 e.g. in the manner as described in the foregoing description i.e. based on a detection of at least one measurement data value representing a peak in a projected image of the elevator rope 130 and at least one measurement data value representing a valley in the projected image of the elevator rope 130. By performing the detection to all measurement data received over the length of travel of the elevator rope 130 it is possible to detect consecutive peaks and/or consecutive valleys from the measurement data.

By reverting back to Figure 2 the method may be continued, in response to the detection of the variation 220, by determining 230 at least one parameter representing a motion of the elevator car 110 from the measurement data either directly or indirectly. The at least one parameter to be determined may be dependent on an entity, or an aspect, under monitoring, but may e.g. be a speed the elevator rope 130 travels during the measurement or a distance the elevator car 110 travels during the measurement.

In response to the determination 230 of the at least one parameter expressing a characteristic based on which conclusions with respect to an operation of the elevator system may be made a comparison 240 of the determined parameter to a reference value, or values in case of a plurality of parameters, is performed. In the comparison an aim is to determine if the elevator system, or the monitored entity, operates as expected, i.e. that the operation is proper, or if the elevator system, or the monitored entity, does not operate as expected, i.e. that the operation is improper. In other words, as an output of the comparison 240 the control unit 160 may be arranged to set a detection result to either that (i) the elevator system operates as expected, or that (ii) an operation of the elevator system deviates from expected. The setting of the detection result accordingly may generate an indication in a further entity, such as in a data center arranged to monitor an operation of one or more elevator in a centralized manner. Hence, if the detection result is that the operation of the elevator system deviates from the expected, a service to the elevator system may be arranged to. Still further, in some example embodiments the detection result may comprise data indicating a reason for the deviation, e.g. based on the aspect under monitoring.

Next, further aspects of the present invention are given by describing aspects of the invention in embodiments implemented in various application areas of the present invention. First, the invention is applied to a monitoring of a slip of an elevator rope 130 in a traction sheave 140. The slip refers to a situation in which a friction between the traction sheave 140 and the elevator rope 130 under monitoring is not good enough and at least part of a power provided by the traction sheave 140 does not transfer to the rope 130 due to slipping. This kind of situation may e.g. refer to a worn-out of a groove of the traction sheave 140 into which the elevator rope 130 in question is positioned. The slip of the elevator rope 130 may be detected by determining, from at least two sources one of which is the sensor 170, a parameter representing a motion of the elevator car 110. For example, the parameter representing the motion may be a speed. The speed of the elevator rope 130, and, hence, the elevator car 110, may be determined from the measurement data received, or obtained, from the sensor 170 on a basis of the detected variation on the surface of the elevator rope 130. Namely, as the variation, i.e. the peak-valley variation, may be detected it is possible to determine the speed of the elevator rope 130 during the motion. This is possible because it is possible to define a distance between adjacent peaks or valleys in the elevator rope 130 basically defined at least in part by a diameter of a strand and the spiraling of the elevator rope 130. The distance may e.g. be defined as a technical character of the elevator rope 130 in question. Since the distance is known, it is possible to calculate a total length the elevator rope 130 travels as a whole, or only a portion of the motion, within a respective time frame t. The total length may e.g. be L = N x d_PP/w, wherein N corresponds to the number of detected peak-to-peak or valley-to-valley distances during the monitoring period and the d_PP/w corresponds to the distance between adjacent peaks or adjacent valleys (depends on the item under detection). Now, since the total length may be derived in the described manner and the time spent to the motion is known, the speed may be determined by L/t. Hence, the parameter representing the motion of the elevator rope 130 is speed.

As discussed in the foregoing description the value representing the determined parameter is compared 240 to a reference value. In the context of monitoring of the slipping of the elevator rope 130 the reference value may be determined from another source of the elevator system. In accordance with an embodiment of the invention the other source may be an electrical motor 150 of the elevator, and, especially, a motor encoder arranged to generate a signal, e.g. by pulses, expressing at least speed on which the electrical motor 150 rotates to. In other words, it is possible to receive an exact value for the speed of the electrical motor 150 from the encoder, which, in optimal situation, shall correspond to the speed value determined from the measurement data values received from the sensor 170. In such a situation no slipping between the traction sheave and the elevator rope 130 occurs. As said the speed values determined from the different sources may be compared 240 and in accordance with the comparison, it may be determined in the elevator system operates as expected 250 or not 260. In some embodiments there may be set a tolerance which defines an acceptable slipping between the mentioned entities.

In some advantageous embodiments the monitoring of the elevator system, and especially the slipping, may be arranged over long period of time in order to detect changes. For example, the control unit 160 may be arranged to perform the monitoring cycle at least one in a predefined time window, and transmit information, such as the speed values from the two sources, to a data center. For example, if it is detected e.g. in the data center that the slipping continuous increase, a service request may be set to the elevator system in question.

Another application area of the present invention as discussed in the foregoing description especially relating to Figure 2 is a monitoring of an elevator rope 130 elongation. The elongation of the elevator rope 130 may e.g. refer to a constructional stretch of the elevator rope 130 due to elevator rope’s structure and heavy forces the elevator rope 130 needs to tolerate during use. The monitoring of the elongation of the elevator rope 130 may be based on a comparison of a distance the elevator car 110 travels to a reference value. The distance the elevator car 110 travels may be determined on a basis of a variation on a surface of the elevator rope 130 under monitoring. More specifically, in accordance with the present invention the variation may be detected during the motion of the elevator car 110 by detecting either peaks or valleys of the elevator rope 110 from the measurement data from the at least one sensor 170. By detecting either peaks or valleys in the elevator rope 130 it is possible to determine a number of the peak-to-peak or valley-to-valley detections during over the motion of the elevator rope 130. Again, since the distance between adjacent peaks or valleys in the elevator rope 130 may be known (e.g. defined as a technical character of the elevator rope 130 or by measuring) a total length the elevator rope 130 travels as a whole during the motion may be L = N x d_PP/w, wherein N corresponds to the number of detected peak-to-peak or valley-to- valley distances during the monitoring period and the d_PP/w corresponds to the distance between adjacent peaks or adjacent valleys (depends on the item under detection). Hence, the distance the elevator car 110 travels corresponds of the determined total length L the elevator rope 130 travels wherein the distance is used as the parameter in the comparison 240. A reference value for the comparison 240 may be determined from other sources. In accordance with the present invention, the reference value may be a known distance between one or more floors of a building the elevator system resides. Hence, the distance the elevator car 110 is arranged to travel 240 for the monitoring shall correspond to the reference distance, such as the distance between topmost floor and the bottom floor, and in the comparison step the distance determined from the measurement data received from the at least one sensor 170 is compared to the reference value. If the two deviates, e.g. more than an allowed tolerance, a comparison result may be set that an operation of the elevator system is not as expected 260 which corresponds to the situation that the elevator rope 130 has elongated more than allowed. Alternatively, if the deviation is within an acceptable tolerance, the detection result may be set so that it indicates that the elevator system operates as expected 250 i.e. the elevator rope 130 meets requirements.

In the foregoing description a monitoring of the elongation of the elevator rope 130 is based on a determination a distance through the detected variation 220 of the elevator rope 130. Correspondingly, the monitoring of the elongation of the elevator rope 130 may be established so that the number of peak-to-peak or valley-to-valley detections over a predetermined distance, such as between one or more floors, are determined when the elevator rope 130 is new and it can be considered as a reference. The detected number of the peak-to-peak or the valley-to-valley detections over the predetermined distance is stored in a memory accessible by the control unit 160 as the reference value. Now, when the monitoring is performed and the number of the peak-to-peak or the valley- to-valley detections are determined over the corresponding distance to the one from which the reference value is defined it measurement result may be compared to the reference value, and conclusions may be done if there has occurred elongation of the elevator rope 130. Hence, the term “distance” used for the evaluation shall be understood to cover any value representing the distance either directly or indirectly.

Figure 4 schematically illustrates a control unit 160 according to an example embodiment of the invention. The control unit 160 may comprise a processing unit 410, a memory 420 and a communication interface 430 among other entities. The processing unit 410, in turn, may comprise one or more processors arranged to implement one or more tasks for implementing at least part of the method steps as described. For example, the processing unit 410 may be arranged to control an operation of at least one sensor 170, and also an operation of the elevator system and/or any entities therein, as well as any other entities relating to the present invention. The memory 420 may be arranged to store computer program code as a non-transitory computer readable medium which, when executed by the processing unit 410, cause the control unit 160 to operate as described. Moreover, the memory 420 may be arranged to store, as described, the reference data, and any other data, such as a plurality of definitions for different kinds of degradations to be applied in the manner as described in the foregoing description. The communication interface 430 may be arranged to implement, e.g. under control of the processing unit 410, one or more communication protocols enabling the communication with the entities as described. The communication interface may comprise necessary hardware and software components for enabling e.g. wireless communication and/or communication in a wired manner.

Still further, some aspects of the present invention may relate to an elevator system arranged to implement the monitoring in accordance with the method as described. The specific examples provided in the description given above should not be construed as limiting the applicability and/or the interpretation of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.

Claims

WHAT IS CLAIMED IS:

1. A method for monitoring of an elevator system (100), the method performed by a control unit (160) comprises: receiving (210) measurement data from a sensor (170) arranged to measure at least one elevator rope (130) during a motion of an elevator car (110), detecting (220) from the measurement data a variation on a surface of the elevator rope (130), determining (230) a parameter representing the motion of the elevator car (110) from the variation on the surface of the elevator rope (130), comparing (240) the parameter to a reference value, and setting, in accordance with a comparison between the parameter and the reference value, a detection result to express one of the following: (i) the elevator system (100) operates as expected (250), (ii) an operation of the elevator system (100) deviates from expected (260).

2. The method of claim 1 , wherein the variation on the surface of the elevator rope (130) is detected (220) based on detection of at least one measurement data value representing a peak in a projected image of the elevator rope (130) and at least one measurement data value representing a valley in the projected image of the elevator rope (130).

3. The method of any of the preceding claims, wherein the parameter is determined on a basis of at least one of: detection of adjacent peaks from the variation on the surface of the elevator rope (130); detection of adjacent valleys from the variation on the surface of the elevator rope (130).

4. The method of any of the preceding claims, wherein a monitoring of the elevator system (100) is based on a monitoring of a slip of the elevator rope (130).

5. The method of claim 4, wherein the slip of the elevator rope (130) is determined by determining, as the parameter, a speed of the elevator rope (130) on a basis of the variation of the elevator rope (130).

6. The method of claim 4 or 5, wherein the reference value is determined from a signal representing a speed of an electrical motor (150) rotating a traction sheave (140) along which the elevator rope (130) is arranged to travel at least in part.

7. The method of any of the preceding claims 1 - 3, wherein a monitoring of the elevator system (100) is based on a monitoring of an elongation of the elevator rope (130).

8. The method of claim 7, wherein the reference value corresponds to a reference distance between two predetermined points within a travel path of the elevator car (110).

9. The method of claim 8, wherein a distance between the two predetermined points is determined on a basis of the variation of the elevator rope (130) for comparison (240). 10. The method of claim 9, wherein the distance between the two predetermined points within a travel path of the elevator car (110) is defined as a number of detected adjacent peaks or detected adjacent valleys.

11. The method of any of claims 8 - 10, wherein the reference distance between the two predetermined points corresponds to a known distance between two floors within the travel path of the elevator car (110).

12. A control unit (160) for monitoring of an elevator system (100), the control unit (160) comprising: at least one processor (410); at least one memory (420) including computer program code (425); the at least one memory (420) and the computer program code (425) configured to, with the at least one processor (410), cause the control unit (160) to perform: receive (210) measurement data from a sensor (170) arranged to measure at least one elevator rope (130) during a motion of an elevator car (110), detect (220) from the measurement data a variation on a surface of the elevator rope (130), determine (230) a parameter representing the motion of the elevator car (110) from the variation on the surface of the elevator rope (130), compare (240) the parameter to a reference value, and set, in accordance with a comparison between the parameter and the reference value, a detection result to express one of the following: (i) the elevator system (100) operates as expected (250), (ii) an operation of the elevator system (100) deviates from expected (260).

13. The control unit (160) of claim 12, wherein the control unit (160) is arranged to detect (220) the variation on the surface of the elevator rope (130) based on detection of at least one measurement data value representing a peak in a projected image of the elevator rope (130) and at least one measurement data value representing a valley in the projected image of the elevator rope (130).

14. The control unit (160) of claim 12 or claim 13, wherein the control unit (160) is arranged to determine the parameter on a basis of at least one of: detection of adjacent peaks from the variation on the surface of the elevator rope (130); detection of adjacent valleys from the variation on the surface of the elevator rope (130).

15. The control unit (160) of any of claims 12 - 14, wherein the control unit (160) is arranged to perform a monitoring of the elevator system (100) based on a monitoring of a slip of the elevator rope (130).

16. The control unit (160) of claim 15, wherein the control unit (160) is arranged to determine the slip of the elevator rope (130) by determining, as the parameter, a speed of the elevator rope (130) on a basis of the variation of the elevator rope (130).

17. The control unit (160) of claim 15 or claim 16, the control unit (160) is arranged to determine the reference value from a signal representing a speed of an electrical motor (150) rotating a traction sheave (140) along which the elevator rope (130) is arranged to travel at least in part.

18. The control unit (160) of any of claims 12 - 14, wherein the control unit (160) is arranged to perform a monitoring of the elevator system (100) based on a monitoring of an elongation of the elevator rope (130). 19. The control unit (160) of claim 18, wherein the control unit (160) is arranged to apply a reference distance between two predetermined points within a travel path of the elevator car (110) as the reference value.

20. The control unit (160) of claim 19, wherein the control unit (160) is arranged to determine a distance between the two predetermined points on a basis of the variation of the elevator rope (130) for comparison (240).

21 . The control unit (160) of claim 20, wherein the control unit (160) is arranged to define the distance between the two predetermined points within a travel path of the elevator car (110) as a number of detected adjacent peaks or detected adjacent valleys. 22. The control unit (160) of any of claims 19 - 21 , wherein the control unit

(160) is arranged to apply a known distance between two floors within the travel path of the elevator car (110) as the reference distance between the two predetermined points. 23. An elevator system (100) comprising: at least one elevator rope (130), at least one sensor (170) for measuring the at least one elevator rope (130), and a control unit (160) according to any of claims 12 - 22. 24. A computer program product for monitoring of an elevator system (100) which computer program product, when executed by at least one processor, cause a control unit (160) to perform the method according to any of claims 1 - 11