Condition monitoring in lifts
Maintenance in modern lifts is normally carried out on the principle of preventive maintenance. This means that lifts are regularly maintained at set intervals. This maintenance strategy ensures high availability of the lifts, but this availability is bought at a relatively price.
High availability can only be guaranteed with short maintenance intervals. If necessary, the repairs needed and the replacement of components are carried out as a precaution to guarantee trouble-free operation. In many cases this preventive replacement of components is often unnecessary, since the components have usually not yet even reached their wear margin (wear limit).
So-called condition-oriented maintenance represents an alternative to preventive maintenance, i.e. the maintenance measures are performed according to the particular condition of the system. Here components are permanently and continuously monitored using suitable measuring equipment. Maintenance is only carried out if a corresponding key value reaches a predetermined value.
In this way better use is made of the wear margin of individual components, since these are only replaced when the wear limit has been reached. Better predictions regarding the next maintenance can be made and spare parts can be stocked more efficiently. The down times of the system are minimised by having maintenance take place at times when the availability of the lift is of less importance.
Condition monitoring systems, which continuously record wear or wear-relevant data, are still an exception in the lift industry.
The partially used read-out of fault memories of controls and frequency converters via remote maintenance does not really constitute a condition monitoring system for wear monitoring, because for this three partial steps are in principle necessary:
1. The condition recording reflects the current condition of the machine.
2. The condition comparison, representing a comparison of the actual condition with a prescribed reference value.
3. The diagnosis, whose task is to localise any faults as early as possible on the basis of the condition comparison and determine their cause.
Henning GmbH & Co KG has developed a condition monitoring system that meets these criteria. The WEARwatcher is an autonomously operating measurement and evaluation system, which permanently and continuously records and evaluates vibrations and accelerations via acceleration sensors.
In this way over 50 parameters are determined and stored per lift trip. Given several hundred trips a day an enormous quantity of data is accumulated over the course of time. The data quantity is reduced using statistical methods. These methods were selected to ensure that not only trends over a lengthy period can be recognised, but also sporadic events.
In the first step the individual trips are split up into journey sections in order to be able to evaluate these separately from each other later. This evaluation occurs online in real time. The parameters determined are then stored and for example assigned to the number of stops (floors). This apportionment is then illustrated taking the journey from the first to the sixth floor as an example. Depending on the journey section, the parameters are inserted in the corresponding statistical tables. The parameters "Close door” and "Acceleration” are assigned to the starting floor using this method. The parameters "Brake”, "Creep speed” and "Open door” are allotted to the destination stop. The stops in between are only assigned the parameter "Constant journey.”
In order to be able to assign the results specifically to individual modules or components, the results must be logically connected to each other. For example, a malfunction that occurs at every stop during the opening and closing of the door indicates a problem with the lift car door. If the same malfunction only occurs at several stops, this may indicate a problem with the landing doors.
Malfunctions and unusual features are automatically forwarded by the system in the form of warning messages. These can for example be sent to the lift control and from there to the building technology centre. Transmission of the reports to a cloud is likewise possible. In this way the data is available via the Internet almost from any location, even on mobile terminals like smartphones or tablets.
A condition monitoring system certainly cannot be used in every lift. But lifts which form part of an industrial production process or are operated in public areas for the transport of people and in parking garages have to provide a high degree of availability.
Here enormous potential lies dormant which will make a technological contribution to improving the effectiveness and efficiency of lifts. In particular, the sometimes enormous cost-saving trends observable in the lift component sector can only be countered by suitable measures in the form of safety and function monitoring of critical components.
Automatic condition monitoring systems offer appropriate and efficient solutions and guarantee optimal resource efficiency combined with a high degree of lift availability.
Martin Hauk, Henning GmbH