DESCRIPTION

Field of the invention

The invention relates to a method for detection of wear or failure in a load bearing member of an elevator.

Prior art

A known load bearing member for elevators comprises a plurality of tensile elements encapsulated in a case. For example a known load bearing member is basically an internally reinforced belt with a flat case. The tensile elements for example are represented by metal cords and each metal cord comprises a plurality of strands wound in an appropriate manner.

A traction member for elevators having a number of steel cords encapsulated in a plastic medium is disclosed in GB-A-1362514.

The traction member can have flat or toothed surfaces. A toothed traction member is provided with teeth projecting from at least one face, which engage recesses on traction sheaves to prevent slipping.

Said load-bearing members are nowadays preferred to conventional ropes. However, the visual inspection of the encapsulated tensile elements could be not possible. A periodic check of the tensile elements is necessary for obvious reasons of safety.

It has been proposed to adopt non-visual inspection based on application of electric signals to the tension members. For example, a resistance based inspection is known, which is based on the fact that a damaged or broken wire has a higher resistance than an undamaged wire. WO 2005/095252 discloses a method of monitoring a condition of a load bearing member in an elevator system, comprising the steps of: applying a first signal having a first characteristic to at least one tension member in the load bearing member; applying a second signal having a second, different characteristic to at least one tension member in the load bearing member; determining a wear condition of the load bearing member based upon a response to the first signal; and determining a failure condition of the load bearing member based upon a response to the second signal. A resistance value of a tension member provides an indication of the mechanical integrity of the tension members.

A resistance-based inspection requires the provision of an electrical connection with the encapsulated tensile elements. To this purpose, at least one end of the load-bearing member can be fitted with a connecting device which provides electrical connection with the internal tensile elements. Such connecting devices are known in the art (WO 201 1/098847).

These methods provide an absolute measure of a certain physical entity, e.g. the electric resistance of the tensile elements. However, it may be difficult to ascertain whether a tensile element is damaged, or not, solely on the basis of such an absolute measure. The measure for example can be affected by a number of boundary conditions and/or a change of the characteristics of the tensile element over time.

Summary of the invention

The aim of the present invention is to provide a novel and improved method for checking the encapsulated tensile elements of a load bearing member of an elevator.

The idea of the invention is to perform a differential measure which compares a response to a test signal of a first tensile element and the response of a second tensile element or, in some embodiments, response of a first plurality of tensile elements is compared to response of a second plurality of tensile elements. The response of first tensile element(s) to a suitable test electric signal is compared with the response of second tensile element(s). According to some embodiments, the load-bearing member comprises bridged tensile elements; the response of first bridged tensile elements is compared with the response of second bridged tensile elements.

Hence the invention proposes a method for detection of wear or failure in a load bearing member of an elevator, said load bearing member comprising electrically conductive tensile elements encapsulated in a case, the method being directed to detection of a wear or failure of said tensile elements, and comprising the steps of:

- applying a test electric signal to at least one first tensile element of said load bearing member,

- applying said test electric signal to at least one second tensile element, other than said first tensile element, - detecting a response of said first tensile element and a response of said second tensile element to said test signal, and

- determining a condition of a damage in the first tensile element or in the second tensile element, based upon a difference between the response of the first tensile element and the response of the second tensile element to said test signal.

The method of the invention involves application of a test signal to one or more first tensile elements and correspondingly to one or more second tensile elements. The second tensile element(s) is(are) other than the first tensile element(s). Preferably said test signal is an electric tension (voltage). For example, in a preferred embodiment, a test voltage is applied to the first tensile element and the same voltage is then applied to the second tensile element, then the voltage drop in the first tensile element is compared to the voltage drop in the second tensile element.

According to a preferred embodiment of the invention, the response of a first plurality of bridged tensile elements is compared with the response of a second plurality of bridged tensile elements. For example the method of the invention compares the response of the first two tensile elements with the response of another two tensile elements, and so on.

Hence a preferred embodiment of the method comprises:

- applying said test signal to a first plurality of tensile elements which are electrically bridged together,

- applying said test signal to a second plurality of tensile elements which are also electrically bridged together,

- determining a wear or failure of the load-bearing member based upon a difference between the response of said first bridged tensile elements and response of said second bridged tensile elements to said test signal.

Preferably, the test signal is applied to first ends of the tensile elements, which are located at one inspection end of the load-bearing member. The term of inspection end shall be intended to mean one end of the load-bearing member provided with a suitable connector, allowing establishment of an electrical connection with the encased tensile elements. A test device adapted to provide the signal and to measure the response is connected to said connector, in order to perform the inspection.

Preferably the method is performed by applying the test signal to pairs of tensile elements. Accordingly, the load bearing members comprises tensile elements which are bridged in pairs. Hence the method is performed by firstly checking a pair of tensile elements, then checking another pair of tensile elements, and so on. All or some of the tensile elements can be bridged in pairs, according to various embodiments. The test signal is preferably a low voltage in order to avoid need of special equipments for safety, and also to have a low current in the tensile elements to avoid any risk of overheating and damage. The voltage may depend of course on the length of the tensile elements. Preferably the test signal is a voltage not greater than 50 volts.

Preferably the method is based on detection of a voltage drop, responsive to said test signal. The invention can be however carried out with alternate electric measurements, for example a measurement of a difference of capacitive or inductive response. Preferably a measure of difference of capacitive or inductive response is made with a high frequency excitation, for example in such a case the test signal has preferably a high frequency which is around 1 GHz or higher.

The tensile elements are made of a suitable electrically conductive material, e.g. a metal or a conductive non-metal material. In the most common cases the tensile elements are metal cords, such as steel cords, made of several strands. Each cord may comprise for example 20 strands or more. The method of the invention allows detection of a cord with a few strands or even just one strand broken. If, for example, the response to the test signal of a certain pair of cords differs from response of other pairs of cords, this means that said pair of cord is damaged. One might note that the differential measure cannot reveal the exact location of a damage; this is however of a minor importance since a damaged load-bearing member will be replaced. The purpose of the method is to determine if the load bearing member is damaged and, in the affirmative case, if it can still operate or need replacement.

The load-bearing member is preferably a belt. The load-bearing member can also be a toothed member.

The load-bearing element can be designed to permit the test method of the invention according to embodiments which provides the comparison of response of bridged elements. Accordingly, there is provided a load-bearing element for an elevator, comprising a plurality of tensile elements encapsulated in a case, the load bearing member comprising an inspection end where first ends of the tensile elements are electrically accessible, and an opposite end where second and opposite ends of the tensile elements are bridged.

Hence the tensile elements they can be electrically connected to an external source of a signal and to a suitable test device. Preferably, at least some of the tensile elements are bridged in pairs. The main advantage of the invention is that the detection of a damaged tensile element is not affected by the boundary conditions and environmental variables such as temperature, humidity, age of the elevator, etc, being based on a differential measure. The effect of boundary or environmental aspects is elided thanks to the differential approach of the inventive method. Another advantage is that much time is saved, because more than one tensile elements can be checked at one time. Another advantage is that the method can be performed by accessing only one end of the load-bearing member.

A preferred field for application of the present invention is the field of small-size elevators without counterweight, where a load-bearing belt has several advantages compared to conventional ropes.

The advantages of the invention will be now elucidated with reference to the enclosed Figs. 1 and 2 of preferred embodiments.

Detailed description of preferred embodiments

Fig. 1 shows a load-bearing belt 1 with tensile cords T1 , T2, T3, T4, ... Tn which are encapsulated in a case 5. At an end 7 of the belt, the cords are bridged in pairs by means of bridge connections 6. At an opposite end 8 of the belt, which is denoted as inspection end, the belt has a connecting device (not shown) which allows to provide an electrical connection with the tensile cords. The tensile cords are made of a conductive material, preferably a metal and more preferably steel.

In the example, the tensile cords T2, T4, ... are connected to a source S of a test electric signal, which is preferably a voltage source. The test signal is delivered via a signal connection 9. The test signal is preferably a DC voltage which has the advantage of requiring no definition of a "zero" reference.

The electrical connections 10 allow measuring the response of each pair of cords, such as the first pair T1 -T2 or the second pair T3-T4, to the test signal in the line 9. Hence, the method of the invention can be performed. The test signal is applied to first tensile elements T1 and T2, said first tensile elements being electrically bridged together by the bridge connection 6 at the other end 7 of the belt; then the test signal from line 9 is applied to second tensile elements T3 and T4 which are also bridged together at the belt end 7. The response is detected by a measure of the voltage drop in the pairs of cords, by means of the connections 10. For example the voltage drop (AV) between point 1 1 (that is line 9) and point 12 in Fig. 1 will give the voltage drop in the first pair of cords including cords T1 and T2. A damage in the first pair of cords T1 -T2 or in the second pair T3-T4 can be determined, based upon a difference between their response. If, for example, the voltage drop in cords T3-T4 is greater than the voltage drop in cords T1 -T2, at least one of the cords T3, T4 is likely to be damaged. The entity of the damage can be determined on the basis of the entity of said difference. For example the number of broken strands can be estimated with a good precision. Hence, one can determine if the belt needs replacement or not.

The collected signal can be amplified if appropriate, for example with an operational amplifier. Fig. 2 shows a simplified embodiment where the tensile elements are not bridged. Hence, the measure is taken at opposite ends 1 1 , 12 of a single tensile member. In this embodiment, both ends of the belt must be electrically accessible.