Issue 5/2005
09/02/05
“MOSIS” – Modular Shaft Information System
Enrico Marchesi (Schindler Switzerland)
The car races at a speed of 10 m/s from the lobby to the 47th floor, and stops exactly level with the landing. Thanks to MoSIS, the elevator control system knows the position of the car at any given moment to accuracy of within less than half a millimetre. The same system also performs a whole series of safety functions which in traditional elevators had to be distributed over various individual components and subsystems.
Category: Issue 5/2005
Posted by: Editor
The foundation was laid for this enormous step forward by a whole series of technological, process and product innovations. And the benefits of this development work are not restricted to the product MoSIS. The results and findings gleaned from the project also provide a solid basis from which other products can be economically and efficiently derived in the future.
MoSIS makes no detours
With the development of MoSIS, a system has been created which is capable of providing a direct, absolute measurement of the car’s position in the shaft without making any detour – for example via the incremental encoder at the traction sheave. A magnetized tape in the shaft carries the precise position information which is read out continuously by the sensor’s electronic circuit. This allows the position of the car to be determined at any time to an accuracy of within less than half a millimetre.
In terms of measurement technology, direct determination of the car position offers major benefits – primarily where greater travel heights and speeds are involved. This allows typical measurement errors such as that caused by rope slip to be completely eliminated. Dynamic effects caused as a result of the rope’s inherent elasticity can also be more successfully dealt with. It is only with direct position determination that it is possible at all to deploy efficient control algorithms which take account of dynamic rope effects. These control algorithms in turn permit transport performance to be improved.
The existence of absolute position information also enhances elevator availability. After a power failure, for instance, the system is immediately capable of directly determining the precise car position in the shaft, and resuming normal service. The need for synchronization and learning trips is completely eliminated in this situation.
Floor referencing is performed by MoSIS using magnetic position markers at the landing doors. A small supplementary sensor detects the position of the respective door threshold every time it passes by, and compares it to the stored shaft image. If the measured position deviates from the stored position – as can occur for instance due to building shrinkage over time – the shaft image is automatically adjusted. As a result, following its initial installation, MoSIS provides all the measured variables required by the controller at any time and with a constant degree of accuracy and reliability.
Core technology – magnetic encoding
Magnetic length encoding has is established in various fields of industrial application as a durable, efficient and low-cost technology for position measurement. It uses magnetic North and South poles on a suitable carrier in a defined sequence. In terms of signal technology, these poles can be defined as a binary code. Using hall or MR sensors, the magnetic fields are detected and the required position information is generated using a suitable signal processing system. A positive aspect of this technology, particularly at high speeds, is that scanning can take place on a totally non-contact basis.
In the most basic application, a magnetic incremental code with alternating north and south poles is used. (Fig. 2). Here, position information is typically provided as a standardized impulse signal such as with incremental encoders. In actual fact, this measurement principle is used not only for linear position measurements, but is also able to cover rotary applications as an alternative to the optical rotary encoder.
An absolute position code can be generated by varying the physical width of the individual poles. If the resulting magnetic pole-pattern does not repeat itself over the whole travelling distance it can be interpreted as an absolute position information. However, in order to process this code, a parallel incremental track is required. This incremental track is used by the reader electronic as a clock signal to synchronize the sampling points. Nowadays, this type of dual-track system is the generally accepted standard, and is offered by a variety of manufacturers.
However, the suitability of these types of systems for deployment in elevators is limited. The dual-track magnetic tapes require dual-track sensor heads. As well its greater space requirement and the higher costs of the sensor and tape, the technical measures necessary to guarantee precise guidance of the sensor heads relative to the tape also come at a price. Lateral displacement of the sensor head will quickly lead to measurement errors and system failure.
One of the most important achievements of the MoSIS project has been the development of a method of encoding the absolute position as well as a clock information into one single-track tape. By using a Linear Feedback Shift Register (LFSR) algorithm, a binary pseudo random code with a width of n bits is generated. This code is guaranteed not to repeat itself over a distance of 2n-1 bits.
The LFSR algorithm also guarantees that a bit change takes place after two identical bits at the latest. This property can be used to extract a reliable clock signal since all magnetic poles have the same physical width. The result is a code with a width of n bits (n = 17 in the case of MoSIS), which permits one-to-one assignment of the absolute position over the entire travel distance. The pole width of 7 mm as used by our systems therefore permits a maximum measurement length of 917 metres: A value which will safely cover future elevator applications.
Precision at a rate of milliseconds
MoSIS has also broken new ground in the field of signal processing. As the magnetic poles have a physical width, a new position value can not be read until after there has been a shift by one pole width. Seen from this aspect, the pole width corresponds to the measurement resolution, which is 7 mm. As a basis for elevator control, needless to say this resolution is far too coarse. Although reducing the pole width would increase the resolution, it would also give rise to a whole series of other drawbacks. First and foremost, the reliable scanning distance would be reduced to such a degree that the technology would become unsuitable for use in elevators.
These conflicting objectives have been resolved by taking an innovative new approach to signal processing. This involves processing the magnetic signals using both digital and analogue technology in parallel. The digital evaluation produces the raw and coarse resolution of 7 mm. However, the magnetic field within the poles is not constant. A series of special analog sensors is therefore used to measure the individual magnetic field strengths across several poles. By using a complex interpolation algorithm the fine position with a resolution of less than half a millimetre is calculated – and all that in real-time at operating speeds of modern high-rise elevators.
With MoSIS, CAN (Controller Area Network) takes care of reliable, efficient data communication to the elevator control system. Even before CAN became a widely used technology in elevator applications, Schindler decided to adopt this durable technology for use in MoSIS. With its high data rates and long transmission distances, CAN offers the ideal complement to the high-precision MoSIS sensory system. The result is an overall system which responds to a request from the elevator control with the current position value of the elevator car to an accuracy of just tenths of a millimetre in less than 2 milliseconds.
Integrated safety
By determining the car position accurately and directly in real-time, MoSIS can handle all safety-relevant functions of conventional hoistway information systems. Not only does MoSIS always know where the car is, but also, for example, if the maximum speed is exceeded it interrupts the safety circuit. The following safety functions are integrated in MoSIS:
• Shaft end limit switch.
• Emergency Terminal speed limit control at the shaft end.
• Detection of door zones and door overbridging.
However, MoSIS can only handle safety-relevant functions if its measurement of the car position is absolutely reliable. The system must even continue to function if an electronic component fails. For this reason, its entire circuit board is redundantly duplicated (Fig. 3). Also, safety-related components must be separately certified. In this respect, our company is a pioneer. Until now, only safety-related systems constructed from traditional electromagnetic components such as relays and contactors were certified. MoSIS is the first stored-program electronic hoistway information system to be examined by the TÜV and approved as a safety circuit according to EN 81-1:1998. The result is a single system where the safety functions are implemented completely in software.
Benefits in the system network
Due to its high degree of integration in elevator systems, the innovative MoSIS concept offers a whole range of benefits (Fig. 4). The fact that all higher functions are implemented in software results in a degree of flexibility unattainable using conventional systems.
Traditionally, a whole series of different components had to be placed in the shaft and the machine room. Today their various functions have been completely integrated in a single compact system with a few defined interfaces. The enormous time savings made possible as a result begin at the installation stage. The magnetic tape is inserted in the neck of the rail; adhesion is taken care of by the tape’s own magnetic properties. The sensor unit is fixed on the car. A roller carriage ensures that the sensor head is correctly guided along the tape (Fig. 5). The system is commissioned fully automatically by simply pressing a button. The entire installation and commissioning process takes just a few hours.
MoSIS is virtually maintenance-free. If building shrinkage takes place over time, in the past this frequently used to entail time-consuming readjustment of switches and position flags as an established part of the routine maintenance work at the elevator. These adjustment processes are performed today fully independently by the system. MoSIS stores the entire shaft image and records travel paths, floor positions and end points. If the current measured values begin to gradually deviate from the saved shaft image, the errors are automatically corrected and transmitted to the control system. This also results in major savings in terms of maintenance.
Implementation of all the higher functions in MoSIS as software means that Schindler is able to respond quickly and efficiently to any new or changing requirements. Any amendments to currently valid codes and standards, for instance, or customer-specific and country-specific requirements can be implemented at relatively low cost. In the ideal case, all that is necessary is an adjustment of software parameters within the secure hardware architecture. Extended functions can be realized by additional software modules. Development efforts in these cases are reduced to an absolute minimum.
Innovation and business benefit
MoSIS as a product is an innovation per se, comprising major technological steps. However, this product innovation is the result of a whole series of process- and technology innovations. In considering the benefits of the investment which culminated in MoSIS, looking at this particular product in isolation would be taking too narrow a viewpoint. Using this approach, the investment occurred would not be justified. Within this narrow framework, a simple calculation of return on investment would show this type of development project to be uneconomical.
Magnetic measurement technology in itself is not an innovation. It has already been successfully applied in a number of industries for decades. However, the technology available was inadequate for application in elevators. The travel paths were too short, the encoding methods used unsuitable and costly, the scanning distances too small and the hardware too expensive for the applications we required. It is only with the development of MoSIS that a variety of innovative technological steps was achieved which allowed the different problems in all these areas to be solved, making this technology accessible for deployment in elevators. The use of this technology in MoSIS represents just a first step. In actual fact, Schindler now has at its disposal a basic technology with potential applications in every market sector.
Depending on the specific requirements involved, this technology provides a basis for the derivation of suitable products. The development risks and time periods involved are minimal, as the same technological nucleus is used again and again. Because every product line involves the use of largely identical electronic components, economies of scale will yield considerable cost benefits on the product level. Knock-on benefits will also include lower installation and maintenance costs as a result of reduced complexity. No matter which type of elevator is involved – the shaft information works according to the same principles. Consequently, the use of magnetic measurement technology has created a platform with a whole catalogue of benefits, many of which will only be brought to bear over the course of the next few years.
This applies analogously to the safety system used in MoSIS. Alongside its technological implementation as such, the innovative benefits here are largely to be found in the process sector. Type testing is just the last in a whole series of processes which permit the deployment of MoSIS as an electronic safety system in elevators. Armed with this expertise, today Schindler has at its disposal another solid basis on which to build for future projects. Here too, with the creative precedent of MoSIS, future risks have been eliminated and potential opened up for future developments.
Taking these aspects into consideration, it is clearly evident that the entire benefit of innovations in the investment goods sector cannot be seen in the light of short-term ROI in individual products, but in terms of the technological and process innovations evolved from new expertise and new discoveries in the long term as a basis for establishing a unique competitive position in the marketplace.
Lecture on the occasion of the European Lift Congress Heilbronn (ELCH), 28 and 29 June 2005.
Biographical details
Enrico Marchesi: Graduated from the Swiss Federal Institute of Technology in Zurich as a Mechanical Engineer in 1995. Subsequently studied at the University of California, Davis, graduating as Master of Science in Transportation Technology and Policy. Various research activities in the field of alternative vehicle propulsion systems and individual mobility in California and Zurich. Joined the R&D – Technology Management Department of Schindler Elevators in 1999, heading up technological studies and pre-development projects. Further qualification at the University of Basel graduating as Master of Advanced Studies in Marketing. Since 2002, responsible for the Group-wide Hoistway Systems Development Division.
5/2005
