CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims the priorities of EP 16155357.3 and EP 16155358.1 (hereinafter referred to as “the priority documents”).

FIELD

The present invention relates to a method for detecting a deterioration state in a suspension member arrangement for an elevator.

BACKGROUND

Elevators typically comprise a car and, optionally, a counterweight which may be displaced for example within an elevator shaft or hoistway to different levels in order to transport persons or items for example to various floors within a building. In a common type of elevator, the car and/or the counterweight are supported by a suspension member arrangement comprising one or more suspension members. A suspension member may be a member which may carry heavy loads in a tension direction and which may be bent in a direction transverse to the tension direction. For example, a suspension member may be a rope or a belt. Typically, suspension members comprise a plurality of load carrying cords. The cords may be made for example with an electrically conductive material, particularly a metal such as steel.

During operation of the elevator, suspension members have to carry high loads and are typically repeatedly bent when running along for example a traction sheave, a pulley and/or other types of sheaves. Accordingly, substantial physical stress is applied to the suspension member arrangement during operation which may lead to deteriorations in the suspension members' physical characteristics such as e.g. their load bearing capability.

However, as elevators may typically be used by people for transportation along significant heights, safety requirements have to be fulfilled. For example, it has to be safeguarded that the suspension member arrangement can always guarantee safe support of the car and/or the counterweight. For such purposes, safety regulations rule for example that any substantial deterioration of an initial load bearing capacity of a suspension member arrangement can be detected such that for example counter-measures such as replacing a substantially deteriorated or faulty suspension member from the suspension member arrangement may be initiated.

Generally, a load bearing capacity of a suspension member may be specified when designing the suspension member and may then be physically tested upon completion of a fabrication of the suspension member. Physical tests may comprise for example tensile loading of the suspension member and measuring the suspension member's reaction to an application of high tensile forces.

However, during actual operation of the elevator, it may be difficult or even impossible to perform such physical tests. With conventional steel ropes serving as suspension members, visual checking of a rope condition has been possible. However, in modern suspension members, load bearing cords are typically enclosed in a coating or matrix and are therefore not visible from outside. Therefore, alternative approaches for determining a load bearing capacity in a suspension member arrangement or determining parameters related thereto have been developed.

For example, elevator load bearing member wear and failure detection has been described in EP 1 730 066 B1. A method and apparatus for detecting elevator rope degradation using electrical resistance is described in U.S. Pat. No. 7,123,030 B2. Electrical signal application strategies for monitoring a condition of an elevator load bearing member are described in US 2011/0284331 A1 and U.S. Pat. No. 8,424,653 B2. Electrical signal application strategies for monitoring a condition of an elevator load bearing member are described in US 2008/0223668 A1 and U.S. Pat. No. 8,011,479 B2. A simplified resistance based belt type suspension inspection is disclosed in US 2013/0207668 A1. An elevator system belt type suspension having connecting devices attached thereto is described in WO 2011/098847 A1. A method for detection of wear or failure in a load bearing member of an elevator is described in WO 2013/135285 A1. Electrical signal application strategies for monitoring a condition of an elevator load bearing member are described in EP 1 732 837 B1. “Health Monitoring for Coated Steel Belts in an Elevator System” have been described in a research article of Huaming Lei et al. in the Journal of Sensors, Volume 2012, Article ID 750261, 5 pages, doi: 10.1155/2012/750261. Most of these prior art approaches are generally based on measuring electrical resistance characteristics upon applying an electrical direct current (DC).

SUMMARY

There may be a need for an alternative method for detecting a deterioration state in a suspension member arrangement for an elevator. Particularly, there may be a need for such method which enables fulfilling high safety requirements, simple implementation and/or low cost. More specifically, there may be a need for a method and device which enable detecting both non-uniform as well as uniform deteriorations throughout a suspension member arrangement of an elevator.

According to a first aspect of the invention, a method for detecting non-uniform and uniform deteriorations in a suspension member arrangement for an elevator is proposed. Therein, the suspension member arrangement comprises at least one suspension member having a first and a second group of electrically conductive cords. First ends of the first and second group of electrically conductive cords are electrically connected to an alternating voltage generator arrangement comprising a first voltage generator for applying a first alternating voltage U1to a first end of the first group of cords of the suspension member and a second voltage generator for applying a second alternating voltage U2to a first end of the second group of cords of the suspension member. Second ends of the first and second group of electrically conductive cords are electrically connected to each other via a neutral point at which electrical resistances to the first voltage generator and to the second voltage generator, respectively, are same in a non-deteriorated state of the suspension member arrangement. First and second electrical resistors are arranged between the neutral point and the second ends of the first and second group of electrically conductive cords, respectively. The method comprises the following steps, possibly but not necessarily in the indicated order:

applying the first alternating voltage U1to the first end of the first group of cords of the suspension member and applying the second alternating voltage U2to the first end of the second group of cords of the suspension member;

determining a neutral point voltage Unbetween the neutral point and an electrical reference potential;

determining a difference voltage Udbetween a first difference measurement point, located between the second end of the first group of electrically conductive cords and the first electrical resistor, and a second difference measurement point, located between the second end of the second group of electrically conductive cords and the second electrical resistor;

determining the deterioration state based on both the determined neutral point voltage Unand the determined difference voltage Ud.

According to a second aspect of the invention, a device for detecting non-uniform and uniform deterioration states in a suspension member arrangement for an elevator is proposed. Therein, the suspension member arrangement comprises at least one suspension member having a first group and a second group of electrically conductive cords. The device comprises:

an alternating voltage generator arrangement comprising a first voltage generator for applying a first alternating voltage to a first end of the first group of cords and a second voltage generator for applying a second alternating voltage to a first end of the second group of cords;

a voltage measurement arrangement comprising(i) a circuitry for electrically connecting second ends of the first and second groups of electrically conductive cords to each other via a neutral point at which electrical resistances to the first voltage generator and to the second voltage generator, respectively, are same in a non-deteriorated state of the suspension member arrangement,wherein first and second electrical resistors are arranged between the neutral point and the second ends of the first and second group of electrically conductive cords, respectively;(ii) a neutral point voltage determining unit for determining neutral point voltages Unbetween the neutral point and an electrical reference potential; and(iii) a difference voltage determining unit for determining a difference voltage Udbetween a first difference measurement point, located between the second end of the first group of electrically conductive cords and the first electrical resistor (Ra), and a second difference measurement point, located between the second end of the second group of electrically conductive cords and the second electrical resistor (Rb).

An additional aspect of the present invention already described in the priority documents relates to a method for detecting a deterioration state in a suspension member arrangement for an elevator. Therein, the suspension member arrangement comprises at least one suspension member having a first and a second group of electrically conductive cords. The method comprises at least the following steps, preferably in the indicated order:

A first alternating voltage U1is applied to a first end of the first group of cords of the suspension member and a second alternating voltage U2is applied to a first end of the second group of cords of the suspension member. Therein the first and second alternating voltages have same waveforms and a phase difference of substantially 180°.Then,a summed voltage (U3+U4) correlating to a sum of a third voltage (U3) between the second end of the first group of cords and a common electrical potential and a fourth voltage (U4) between the second end of the second group of cords and the common electrical potential and/or(ii) a differential voltage (U3−U4) correlating to a difference between the third voltage (U3) and the fourth voltage (U4)are determined.Finally, the deterioration state of the suspension member arrangement is determined based on at least one of the summed voltage and the differential voltage.

A further additional aspect of the present invention already described in the priority documents relates to a device for detecting a deterioration state in a suspension member arrangement for an elevator. Therein, the suspension member arrangement comprises at least one suspension member having a first and a second group of electrically conductive cords. The device comprises at least an alternating voltage generator arrangement, at least one voltage measurement arrangement and the determination unit. The alternating voltage generator arrangement is adapted for applying a first alternating voltage U1to a first end of the first group of cords of the suspension member and for applying a second alternating voltage U2to a first end of the second group of cords of the suspension member. Therein, the alternating voltage generator arrangement is configured to generating the first and second alternating voltages with same waveforms and a phase difference of substantially 180°. Furthermore, the device comprises a first voltage measurement arrangement and/or a second measurement arrangement. Therein, the first voltage measurement arrangement is adapted for determining a summed voltage (U3+U4) correlating to a sum of a third voltage (U3) between the second end of the first group of cords and a common electrical potential and a fourth voltage (U4) between the second end of the second group of cords and the common electrical potential. The second voltage measurement arrangement is adapted for determining a differential voltage (U3−U4) correlating to a difference between the third voltage (U3) and the fourth voltage (U4). The determination unit is adapted for determining the deterioration state of the suspension member arrangement based on at least one of the summed voltage and the differential voltage.

Without restricting the scope of the invention in any way, ideas underlying embodiments of the invention may be understood as being based, inter alia, on the following recognitions and observations:

In conventional approaches for detecting a deterioration state of a load bearing capacity in a suspension member arrangement such as those approaches indicated in the above introductory portion, electrical characteristics of cords included in a suspension member have been taken as indicators for changes in the deterioration state. Generally, electrical resistances within the cords have been measured and it has been assumed that an increase of such electrical resistances indicates a deterioration of the load bearing capacity of the suspension member.

However, such electrical resistance measurements, or alternatively impedance measurements, may require substantial efforts in terms of, e.g., measuring devices, measurement analyzing devices, circuitry, etc. For example, electrical resistances have to be included, measured and compared within circuitry comprising cords of a suspension member in order to thereby enable quantitative measurements of the electrical resistance or impedance of the cords.

The inventors of the present invention have found that measuring electrical resistance/conductivity of cords, particularly measuring quantitatively such characteristics, is not necessary in order to obtain sufficient information about a deterioration state of a load bearing capacity in a suspension member to ensure safe operation of an elevator.

As an alternative approach to conventional methods and devices, it is therefore proposed to not necessarily measure any electrical resistance, resistivity or impedance within conductive cords of a suspension member directly but to provide for a method and a device which allow for deriving sufficient information about a deterioration state by measuring one or more electric voltages which at least relate to a correlation of electric voltages occurring at ends of two groups of cords of the suspension member when alternating voltages are applied to opposite ends of these two groups of cords.

In such alternative approach, electrical resistances, resistivities or impedances do neither have to be known quantitatively on an absolute scale nor in a relative manner. Instead, it may be sufficient to simply measure electric voltages, particularly sums of electrical voltages and/or differences of electrical voltages, without having any detailed knowledge about actual resistances, resistivities and/or impedances through the cords of the suspension member.

Briefly summarized and expressed in a simpler wording than in the claims, but without restricting the scope of the claims, an idea underlying the inventive method and device as already described in the priority documents may be briefly summarized as follows:

The cords comprised in a suspension member may be divided into two groups of cords. Preferably, both groups comprise the same number of cords. Further preferably, a first group may comprise all even numbered cords and a second group may comprise all odd numbered cords, such that each cord of one of the groups is arranged between two neighboring cords of the other group of cords (of course except for the two cords arranged at the outer borders of the suspension member).

Then, alternating voltages U1, U2are applied to a respective first end of each of the groups of cords using an alternating voltage generator arrangement. The alternating voltages U1, U2comprise an alternating voltage (AC) component in which a voltage periodically varies between a minimum value Uminand a maximum value Umax. Furthermore, the alternating voltages U1, U2may comprise a direct voltage (DC) component UDC. The alternating voltage generator arrangement may comprise two separate alternating voltage generators G1, G2which are synchronized in a specific manner with each other. Alternatively, the alternating voltage generator arrangement may comprise a single alternating voltage generator G comprising a direct output and an inverted output in order to provide the required two alternating voltages U1, U2. Therein, it may be important that the waveforms of both alternating voltages U1, U2are substantially the same, i.e. deviate from each other by less than an acceptable tolerance, such tolerance being for example less than 5% or preferably less than 2%. Furthermore, the alternating voltage generator arrangement shall generate the two alternating voltages U1, U2with a phase shift of substantially 180°, particularly with a phase shift of 180°±an acceptable tolerance of e.g. less than 5%, preferably less than 2%.

Then, at least one voltage measurement is performed using at least one voltage measurement arrangement. Specifically, a voltage named herein “summed voltage” U+and/or a voltage named herein “differential voltage” U−is determined. Both, the “summed voltage” U+and the “differential voltage” U−may be measured at least with their alternating voltage components U+,AC, U−,ACbut preferably with both, their alternating voltage components U+,AC, U−,ACand their direct voltage component U+,DC, U−,DC. In the alternating voltage components U+,AC, U−,AC, both an amplitude and phase may be determined. As will be described further below, valuable information about the deterioration state of the suspension member may be derived particularly from the phase information included in the measurement of at least one of the alternating voltage components U+,AC, U−,AC.

Therein, the summed voltage U+correlates in a predetermined manner to a sum (U3+U4) of a third voltage (U3) and a fourth voltage (U4) whereas the differential voltage U−correlates in a predetermined manner to a difference (U3−U4) between the third voltage (U3) and the fourth voltage (U4). The third voltage (U3) occurs between the second end of the first group of cords and a common electrical potential such as e.g. a ground potential. The fourth voltage (U4) occurs between the second end of the second group of cords and the common electrical potential such as e.g. the ground potential.

The summed voltage U+and the differential voltage U−may be directly the sum (U3+U4) and the difference (U3−U4), respectively. Alternatively, the summed voltage U+may proportionally correlate to such sum (U3+U4), i.e. may be a multiple of such sum such as e.g. (U3+U4)/2. Analogously, the differential voltage U−may proportionally correlate to the difference (U3−U4), i.e. may be a multiple of such difference. As a further alternative, the voltage measurement arrangement may measure voltages (U1), (U2) occurring at opposite first ends of both groups of cords and may determine a sum (U1+U2) and/or difference (U1−U2) or a multiple of such sum/difference which, due to the fact that (U1), (U2) occur in the common circuitry with (U3), (U4), correlate in an unambiguous manner to the sum (U3+U4) and to the difference (U3−U4), respectively.

Information about the deterioration state of the suspension member may be derived from at least one ofa phase determination in the alternating voltage components U+,AC, U−,AC, of the summed voltage U+and/or the differential voltage U−,(ii) an amplitude determination in the alternating voltage components U+,AC, U−,AC, of the summed voltage U+and/or the differential voltage U−, and(iii) a value determination in the direct voltage components U+,DC, U−,DC, of the summed voltage U+and/or the differential voltage U−.

In a normal state in which no deteriorations occur in the cords of the suspension member, both the third and fourth voltage U3, U4should directly follow the applied alternating voltages U1, U2, i.e. with a same phase but with a reduced amplitude, and should therefore be both same in amplitude but with a phase shift of 180° such that the summed voltage U+should be a constant direct voltage (DC) (i.e. U+,AC=0) and the differential voltage U−should be an alternating voltage (AC) (i.e. U−,AC≠0) having double the amplitude than each of the third and fourth voltages U3, U4.

However, when any deterioration occurs in the cords of the suspension member, such as one or more local breakages of cords, significant corrosion of cords, defects in an electrically isolating cover enclosing and electrically separating neighboring cords (such defects potentially resulting in shorts between neighboring cords and/or electrical connections to ground of some cords), etc., the summed voltage U+and/or the differential voltage U−generally significantly change. Such changes may be detected and may then be interpreted as indicating specific types and/or degrees of deteriorations in the suspension member.

For example, an increase of an electrical resistance due to e.g. corrosion or even a breakage in one of the cords will significantly change a respective one of the third and fourth voltages U3, U4occurring at the second end of the respective group of cords including the deteriorated cord. Accordingly, due to such voltage change, for example no purely direct voltage (DC) is measured anymore for the summed voltage U+.

Other deteriorations of the suspension member and/or its cords generally result in other deviations of the summed voltage U+and/or the differential voltage U−from their initial “normal” behavior, as will be described in more detail further below.

Accordingly, upon applying phase-shifted first and second voltages of same waveforms to first ends of two groups of cords, valuable information about a current deterioration state in the suspension member of the suspension member arrangement may be derived by measuring third and fourth voltages U3, U4at or between the second ends of both groups of cords (or measuring any multiple thereof or any voltages correlating thereto) and correlating them as the sum (e.g. U3+U4) and/or the difference (e.g. U3−U4).

As will be described further below, additional information about a specific type, degree and/or location of an occurring deterioration may be derived when measurements of both the summed voltage U+and the differential voltage U−are taken into account.

A possible advantage obtainable with the approach described herein is that, in contrast to most prior art approaches, no electrical direct current (DC) is applied to the cords of a belt but, instead, alternating currents (AC) are applied. Applying such alternating currents may significantly reduce a risk of any electro-corrosion at the cords.

Briefly summarized and expressed in a simpler wording than in the claims, but without restricting the scope of the claims, ideas underlying embodiments of the inventive method and device in accordance with the claims presented herein may be briefly summarized as follows:

Various different types of deteriorations in suspension members may be detected by measuring specific voltages such as the summed voltage U+and the differential voltage U−described above.

Specifically, it has been found that for detecting at least some deteriorations of major importance, it may be generally sufficient to measure only two specific voltages referred to herein as neutral point voltage Unand difference voltage Ud.

The neutral point voltage Unmay be a differential voltage between a specific point in a circuitry including the cords of the suspension member and an electrical reference potential such as a ground potential. Therein, the specific point in the circuitry is referred to as the neutral point and the measured voltage is referred to as neutral point voltage. The neutral point occurs in a circuitry in which two alternating voltages are applied to both groups of cords, respectively, with a phase shift of preferably 180° being present between the two alternating voltages and in which both second ends of the first and second groups of cords are electrically connected to each other. Specifically, the neutral point is the location in such circuitry at which an electrical resistance between the neutral point and the first voltage generator, on the one hand, and an electrical resistance between the neutral point and the second voltage generator, on the other hand, are substantially equal (i.e. differing from each other by less than a predetermined tolerance of e.g. less than 5% or less than 2%) in a non-deteriorated state of the suspension members (i.e. for example directly after manufacturing and/or installing the suspension members).

The difference voltage Udmay an absolute voltage difference or potential difference. The difference voltage Udmay be measured upon a first electrical resistor Rabeing arranged between the neutral point and the second end of the first group of electrically conductive cords and a second electrical resistors Rbbeing arranged between the neutral point and the second end of the second group of electrically conductive cords. In such case, the difference voltage Udmay be measured between the first difference measurement point, which is located between the second end of the first group of electrically conductive cords and the first electrical resistor, on the one side, and a second difference measurement point, which is located between the second end of the second group of electrically conductive cords and the second electrical resistor, on the other side.

As long as no deterioration occurs in the suspension members, an AC component of the neutral point voltage remains “0” as long as the AC voltages U1, U2applied to the first and second group of cords are in a symmetric condition, i.e. are of equal waveform and are shifted by 180°, and the voltage drops throughout both groups of cords are identical. In other words, as long as the applied voltages and the included electrical resistances at both groups of cords are equal, there is a symmetric or balanced situation in which 180° shifted voltages compensate each other at the neutral point.

As soon as specific deteriorations (referred to herein as non-uniform deteriorations) such as interruptions in cords, electrical shorts between cords or electrical connections to ground (due to e.g. damages in an isolating sheath separating cords from each other or from e.g. a traction sheave or a pulley) occur, such situation becomes asymmetric/unbalanced such that the AC component of the neutral point voltage will no more remain “0”. Such non-uniformly deteriorated situations may be detected easily by continuously or repeatedly monitoring the neutral point voltage upon same AC voltages U1, U2being applied to the first and second group of cords with a 180° shift.

However, it has been found that e.g. in case of a uniform or homogeneous deterioration of all cords, i.e. of cords in both the first and second group of cords, the situation within the circuitry also remains symmetric/balanced. For example, if all cords in the suspension member arrangement uniformly loose some of their load-carrying cross-section due to e.g. homogeneous corrosion, an electrical resistance through these cords will increase. While such uniform reduction in cross-section may result in a critical deterioration of the suspension member's load-bearing capacity, it may not change a symmetry/balance of voltage drops through the groups of cords. Accordingly, the AC component of the neutral point voltage remains “0” also in such cases of potentially severe but homogeneous deterioration of the suspension members' cords.

In order to enable also detection of such uniform or homogeneous deterioration of the suspension members' cords, it is proposed to perform a second voltage measurement, i.e. to additionally measure the difference voltage Ud.

Therein, the difference voltage Udmay be or may correlate to the absolute potential difference between the voltages applying between the first and second difference measurement points. Specifically, due to the difference measurement points being separated from the neutral point by the first and second resistors, such difference voltage will indicate a voltage difference applying between the second ends of both groups of cords. While the voltages U1, U2applied at the first ends of both groups of cords are generally identical with respect to the waveform and shifted with respect to each other, the voltages resulting at the opposing second ends of both groups of cords strongly depend on actual electrical characteristics within these groups of cords.

Of course, if the electrical characteristics non-uniformly change in only one of the groups of cords, then the symmetry/balance between both cords is broken and the neutral point voltage will be affected. However, if electrical characteristics uniformly change in both groups of cords, such symmetry/balance will be maintained. Accordingly, the voltages reaching the neutral point through each of the first and second group of cords will still neutralize each other due to their phase shift and the differential neutral point voltage will still comprise no AC voltage component.

However, upon such uniformly changing electrical characteristics, particularly upon uniformly increased electrical resistances throughout both groups of cords, the AC voltages occurring at the ends of both groups of cords will be affected. Specifically, their amplitude will be affected. Accordingly, by measuring the difference voltage between the corresponding difference measurement points at the second ends of the groups of cords, an indication indicating whether or not the electrical characteristics have changed uniformly/homogeneously in the groups of cords may be derived.

It may be noted that, according to the present proposal, it may be sufficient to measure only the two described voltages, i.e. the neutral point voltage Unand the difference voltage Ud, in order to thereby enable detecting both non-uniform deteriorations in the cords as well as uniform deteriorations in the cords.

Particularly, both voltage measurements Unand Udmay be determined at the same end of the suspension member. Thereby, both voltage measurements units, i.e. the neutral point voltage determining unit and the difference voltage determining unit, may be arranged at the same end of the suspension member and may possibly be included in a common device. Accordingly, the voltage measurement arrangement may be simple. Particularly, it is sufficient to only measure resulting voltages Unand Udwhile no resistance measurements need to be performed.

According to an embodiment, the process of determining of the deterioration state is based on a comparison of the difference voltage Udas currently determined with a reference difference voltage value Udr.

In other words, for determining the deterioration state of the suspension members, the neutral point voltage may be measured and analyzed in order to detect specific types of non-uniform deteriorations such as cords being shorted or connected to ground. Therein, the neutral point voltage may be analyzed in various ways. In a separate step, the difference voltage may be measured and analyzed in order to detect at least one other type of deterioration, particularly in order to detect uniform/homogeneous deterioration of the cords in the suspension member. For this purpose, the difference voltage is compared to a reference difference voltage value and based on such comparison it may be determined whether the suspension members are excessively deteriorated in a homogeneous manner.

Specifically, according to an embodiment, the reference difference voltage value Udrcorrelates to the difference voltage Udas measured in a non-deteriorated state of the suspension member arrangement.

In other words, the reference difference voltage value may be the difference voltage which is measured as long as the suspension members are not substantially deteriorated, i.e. for example in a state of the suspension members directly after production thereof. While also in such non-deteriorated state there will be a non-zero AC component of the difference voltage, such initial difference voltage will typically significantly differ from any difference voltage measured after some lifetime of the suspension members, i.e. measured upon the cords in the suspension members being for example corroded to a certain degree and therefore having deteriorated and having changed their electric resistance.

Accordingly, upon repeatedly comparing a currently measured difference voltage with e.g. an initially measured difference voltage or any reference difference voltage value directly correlating thereto, information about a current deterioration state of the suspension member including information about any uniform deteriorations may be derived.

Thus, in accordance with an embodiment, any deviation from a state in which

as a first criterion, the neutral point voltage Uncomprises no alternating voltage component Un,AClarger than a predetermined first tolerance, andas a second criterion, the difference voltage Uddiffers from the reference difference voltage value Udrby less than a predetermined second tolerance,
is interpreted as indicating an excessive deterioration in the suspension member arrangement.

In other words, if either the neutral point voltage comprises a significant non-zero AC voltage component or the difference voltage significantly differs from the predetermined reference difference voltage value, or both, it may be assumed that an excessive deterioration has occurred within the suspension members.

Therein, the first and/or second tolerances may be determined based for example upon specific experimentations and/or regulations to be fulfilled. For example, a difference voltage which differs from the reference difference voltage value by more than a tolerance of e.g. 2%, 5%, 10%, 20%, 30%, 40% or 50% or, alternatively, by more than a specified absolute voltage difference, may be interpreted as indicating an excessive deterioration in the suspension member arrangement.

Therein, the acceptable tolerances may be determined each for example using experiments in which suspension members are specifically deteriorated in a purposeful manner up to a degree at which their load bearing capacity decreases beyond an acceptable limit and determining the difference voltage under such specifically deteriorated conditions.

According to an embodiment, in the above second criterion, an amplitude of an alternating voltage component Ud,ACof the difference voltage Udshall differ from an amplitude of the reference difference voltage value Udrby less than a predetermined second tolerance.

In other words, whether or not the suspension members have uniformly deteriorated may be determined based on differences of amplitudes of AC components of the difference voltage as currently measured and of the reference difference voltage value corresponding e.g. to the difference voltage as measured at an earlier point in time. For example, amplitudes of both difference voltages should preferably differ by at least 2%, 5% or 10% in order to assume any excessive uniform deteriorations. Generally, the more the currently measured difference voltage differs from the reference value with respect to their amplitudes, the more any deteriorations within the cords of the suspension members will influence the difference voltage such that such deteriorations may be detected more easily.

According to an embodiment, an electrical resistance of the sum of the first and second resistors Ra, Rbis larger than an electrical resistance through each one of the first and second groups of cords in a non-deteriorated state of the suspension member arrangement.

In other words, the first and second resistors Ra, Rbshould have a substantial electrical resistance compared to the electrical resistances through each of the groups of cords. Particularly, each of the first and second resistors Ra, Rbshould preferably have an electrical resistance which is at least half the electrical resistance through a group of cords as long as no resistance increases occurred in the cords due to deteriorations. For example, the resistors may have electrical resistances of more than 10 Ohms, preferably in a magnitude of approximately 100 Ohms. Preferably, both resistors have same electrical resistances, particularly when both groups of cords comprise same configurations and therefore generate same electrical resistances.

Accordingly, the neutral point is separated from the first voltage generator by the intermediate first group of cords and the intermediate first resistor whereas it is separated from the second voltage generator by the intermediate second group of cords and the intermediate second resistor. Accordingly, with resistances through the groups of cords being identical and the first and second resistors having identical resistances, the differential voltage at the neutral point will be compensated upon same voltages U1, U2with 180° phase shift being applied to the groups of cords. However, as the first and second difference measurement points are separated from the neutral point by the intermediate first and second resistors, an absolute voltage difference between these difference measurement points will be substantially influenced by the actual electrical resistances throughout the groups of cords.

Accordingly, while only a voltage, i.e. the difference voltage Ud, is measured but no resistance or current is measured, the measured difference voltage Udcomprises information about actual resistances throughout the cords, thereby providing information about any uniform deteriorations throughout the cords.

It shall be noted that the applicant of the present application has filed a US provisional application U.S. 62/199,375 and a US non-provisional application U.S. Ser. No. 14/814,558 which relate to a more generalized approach for determining deteriorations in a suspension member arrangement for an elevator. Many features described therein may be applied directly or in a modified manner to embodiments of the present invention. Accordingly, the content of this application shall be incorporated herein by reference.

It shall be noted that possible features and advantages of embodiments of the invention are described herein partly with respect to a method for detecting a deterioration state in a suspension member arrangement and partly with respect to a device which is adapted for performing or controlling such method in an elevator. One skilled in the art will recognize that the features may be suitably transferred from one embodiment to another, i.e. from the method to the device or vice versa, and features may be modified, adapted, combined and/or replaced, etc. in order to come to further embodiments of the invention.

In the following, advantageous embodiments of the invention will be described with reference to the enclosed drawings. However, neither the drawings nor the description shall be interpreted as limiting the invention.

