Issue 6/2008


11/02/08

Pessral – Present Situation and Technological Trends

Alfredo Gómez, Ángel Gimeno, David Díez and Miguel Ángel García (Instituto Tecnológico de Aragón, Spain)
The Introduction of Pessral (Programmable Electronic System in Safety Related Applications for Lifts) as a normative amendment in EN 81, Amendment A1, could represent a big revolution in the lift industry, comparable to the effect caused years ago when the normative Amendment A2, related to MRL came into light.
Category: Issue 6/2008
Posted by: Editor

Nevertheless the situation is quite different. Amendment 2 appeared when quite a lot MRL solutions were available and therefore it was meant to standardize those existing solutions. In the case of Pessral, standardization came before commercial solutions were generalized, which has caused a different impact in the market. As a result very few systems are commercially available as stand alone solutions.

The strict conditions introduced by the standard and in particular by the standards IEC 61 508 (parts 1, 2, 3, 4, 5 and 7) on which Pessral are based, obviously has an economic impact in the fi nal product that could explain this fact. More important, maintenance costs could also increase for the need of specific equipment to verify the system reliability.
Trying to put some light in this subject, this paper presents a thorough study of patents published since the introduction of Amendment 1 in EN 81, November 2005, as a way to identify the key players in Pessral development and the technological trends followed by these systems. This information will be useful to anticipate and orientate future development of Pessral.
Pessral description and implications
The possibility of use of programmable electronic systems in safety related applications for lifts gives rise to the appearance of a particular concept in lift design technologies. This new concept is Pessral (Programmable Electronic System in Safety Related Applications for Lifts).
According to the use of electric safety devices, EN 81-1 takes into account the use of electronic components and presents a set of guidelines relatives to hardware configuration, but up to the publication of Amendment A1 in November 2005 there was a gap in relation to programmable electronic devices. This situation was solved with EN 81-1; 1998 / A1:2005 where all the requirements for the inclusion of Pessral were established.
The amendment is based on the standard IEC 61 508 (Functional safety of electrical/ electronic/programmable electronic safety- related systems). All Pessral related terminology, design methodologies, risks analysis and classification and even the recommendations about application of technical solutions are derived from this standard.
According to the definition included in EN 81-1 a Pessral is a command, protection or control system based in one or more programmable devices, including all system elements such us power supply, sensors and other input devices, data buses and communication means, actuators and other output devices, used in safety related applications. A programmable electronic device is based on computing technology, that can include hardware, software, and input-output units. Programmable electronic devices are microprocessors, microcontrollers, PLCs, ASICs, etc. The most important concepts to take into account regarding Pessral design are safety function and safety integrity. Safety function is basically aimed to obtain or to keep a state of safety with regard to a specific dangerous failure in the lift system. Safety integrity is the probability of a safety-related system to carry out successfully the safety functions required in all the specified situations during a certain period of time. From these two definitions another fundamental concept appears, SIL (Safety Integrity Level), which is the discrete level for the specification of the safety integrity requirements for the safety functions to be included in the Pessral.
Every safety functions of each safety electric device of a lift is associated to a certain SIL, from 1 to 3 (SIL1 represents the lowest level and SIL3 the highest level). The higher the SIL, the less the probability of safety related applications to fail in the execution of all safety functions required.
Functional safety requirements associated with Pessral systems affects directly to the design of this type of devices, at both technical level (hardware and software) and design and implementation process. Also it is worth noting that the interactions with the environment (hardware) not related to safety functions, for example if a Pessral and a system not related to safety are sharing the same hardware, the conditions for Pessral must be accomplished.
The safety requirements that must be fulfilled are very strict, so lift safety never must be put at risk due to a failure of a Pessral. Additionally not only the Pessral must perform correctly its function, but it must demonstrate error-proof reliability along its complete life.
Therefore Pessral will have to satisfy the specified requirements associated to the corresponding SIL. In addition it will have to meet other common requirements, independently of SIL, concerning hardware design (including restriction measures to access to the code program and other measures related with hardware environment outside safety functions), software design (programming) and design and implementation process. That will produce systems with increased reliability, robustness, with minimized failure risks, both accidentals and systematic, and capable of detecting and identifying failures and redirect the lift to safe mode.
The fulfillment of all specified requirements will have to be documented, including technical data (hardware, software, interaction among them) and reliability and robustness data (for example using AMFE). Also the instructions to make test and inspections must be documented.
Despite the strict technical requirements set up by the SIL classification, possibilities for new solutions with respect to safety devices for lifts with Pessral are enormous. Nevertheless the introduction of new technologies in any sector with traditional technology (in this case the programmable electronic in safety systems for lifts) is questionable and doubtful in reference to the return of the inversion, but the experience demonstrates that, after a period of uncertainty, the new technologies end up prevailing. Anyway the use of functional safety electronic devices is not something new, so the acquired experience in other near sectors, for example automotive, can be valuable.
In any case difficulties for Pessral development can be found:
  •  Necessity of great market for greater returns.
  •  Necessity of highly qualified personal.
  •  Electronic developments are expensive, complex and with strict requirements.
  •  Necessity of rigorous design methodology difficult for SME.
  •  More difficult for design and certification. But also important advantages:
  •  Once developed, the production is cheap and the final product is very competitive.
  •  Easy to update, to extend, to reprogram …
  •  Practically no wear-out failures.
  •  Allow for the substitution of mechanical systems with intense and continuous maintenance.
  •  Saving of physical space.
  •  More intelligent fault management, based on type of fault and car position, (fewer trapped passengers) and more complex safety functions.
Component manufacturers and lift companies must determine their capabilities regarding the development of electronic product in safety related systems and to act consequently.
With respect to no-elevator sector companies that can be a potential threat:
Maintenance of Pessral systems is one of the controversial issues today. Diagnosis electronic tools to verify the system reliability would be needed and also a specific training and recycling of technicians would be required. Additionally there is no a well established standard regarding communications and data transference technologies. Therefore systems are not compatible making the test and verification tasks different and almost proprietary for each individual system. So far, the market impact of Pessral systems have been very limited and very few systems are commercially available as stand alone solutions. The strict conditions introduced by the standard, obviously has an economic impact in the final product that could explain this fact. More important, maintenance costs could also increase for the need of specific equipment to verify the system reliability. Therefore market situation confronts with the enormous design possibilities that Pessral offer for new lift systems with improved performance and capabilities.
One of the main objectives of the present work is to obtain a measurable indicator for the ongoing activity in the lift industry related with the design and development of Pessral systems. The number of patent applications filed during a certain period of time is one of the most reliable and convenient indicator for the activity in a specific technological field.
Additionally many other useful information can be extracted from a related set of patents such as companies involved, inventors, applications, scope and many other technical details. Besides, patent activity is strategic in the lift industry and therefore a big amount of patents are produced yearly by the main lift and lift components manufacturers.
General patent analysis
Motivation and selection methodology
For the aforementioned reasons, Pessral related patent applications have been chosen as the primary source of information for this work and as a good indicator of activity in this particular technological field.
Patents selection has been made using different searching criteria in the Spacenet and Goldfire Innovator worldwide databases. The worldwide database enables the user to search for information about published patent applications from over 80 different countries and regions.
The searches have been restricted to the B66B group of the European Classification system (ECLA). ECLA is used by the EPO (European Patent Office) for carrying out patent application searches. It is built on top of the International Patent Classification system (IPC), and is constantly being revised and updated. The B66B group is related with Elevators, Escalators and Moving Walkways and is exploited into 31 specific sub-groups. Probably some other interesting patent applications could have been found out of the B66B group but for the purpose of this work, which only tries to identify trends and general situation of Pessral development, this restriction is considered reasonable.
The other restriction used in the searching process is the date. Patents applications published before year 2005 have not been considered due to the fact that only activity related with Pessral development after the introduction of Amendment A1 is the objective of this analysis. This is a very restrictive condition due to the long period of time required for patents publication which could be minimum 18 months for an european or PCT patent.
The different searching criteria combine the specified restrictions with keywords in the advanced search function of the Espacenet worldwide database. Keywords used are related with electronic, program, control, safety, micro and others.
Additionally, the software Goldfire Innovator has been used. It includes a proprietary patent application database and search capabilities using semantic algorithms. With the combination of these tools an initial set of patent applications is retrieved. Afterwards each one is revised for classification.
It is important to note here that not all patent applications considered in the analysis fulfil the strict conditions established by EN 81-A1. In any case and for the purposes of this work which is focused in general trends and on the general situation of Pessral related patents, all patents found to agree with the definition of Pessral as specified in Chapter 3. Definitions of EN 81 have been considered even though they do may not fulfil the specific measures for failure control according to the SIL classification.
Patent activity in the elevator industry
In the following graph, general patent activity in the elevator sector is shown from the yearly number of patents published from year 2000. The graph includes the distribution by the most active companies in this area.
The importance of this activity in the lift industry is clear if we put it aside other industrial sectors or activities such as railway, aeroplanes or nanostructures which could be initially considered as more technologically advanced or as cranes and devices for cranes which could be at a comparable technological level.
To know what technological trends or work fields have been developed in depth in recent years the different classification groups have been revised. From this analysis the ECLA groups related with indicating or signalling operating conditions (B66B3/00) and checking, fault-correcting, or safety devices (B66B5/00) are the most active considering patent publication numbers. In particular, the following graph shows patent activity in the elevator sector in areas related with safety applications , in particular in classification B66B5 Application of checking, fault correcting , or safety devices. This is the group which could be closer to Pessral solutions. The total number of Pessral patents identified applying the previously defined methodology is also shown.
 
As can be seen Pessral patents are very few (less than 2 % on average) even though they have an increasing evolution. Main reason for that is the patent process time schedule which requires a minimum of 18 months between patent application and publication. Considering that the analysis is restricted to patents published from 2005, not too much time has passed ever since. Another reason is that some of the already existing patents are general and strong enough to limit in some way this activity.
Pessral related patent analysis
After the general analysis, the initial set of patents after the selection process is reviewed eliminating all non Pessral related patents applications and finally all the remaining relevant ones are classified according to the defined criteria used in this analysis. This criteria with the final categories included is shown in the following graph.
Patents application in each sub-group of the referred classification is then analysed considering the relative importance of all the categories considered in each sub-group (bubble charts) and including the distribution by priority country (bar charts). Conclusions are drawn afterwards from these results.
Safety device and SIL level
With the introduction of Pessral, the electric safety devices listed in EN 81 annex A can be executed by means of programmable systems adding new functionalities and capabilities to these devices. According to the degree of ‘responsibility’ assigned to the safety device, a SIL level is associated with each of them imposing technical conditions for its development. For the higher SIL levels, these conditions are highly restrictive making the development of Pessral systems a complex task in these cases.
In the following graph the different Pessral solutions found in the patent analysis are grouped according to the safety device they are referenced to. The SIL level is also shown for each case with different colours.
Overspeed detection is the safety device approached by most Pessral solutions. It is classified as SIL 1 and therefore it is one of the simplest solutions regarding the implementation of specific measures for failure control. Speed reduction control for reduced buffers is also a numerous solution even though it is assigned with SIL 2. Other safety devices with existing Pessral solutions are classified as SIL 3 despite the high restrictive conditions it represents, which finally will result in cost increase.
From these results obviously the simplest solutions are the most widespread but it seems that SIL level is not a definitive barrier for the existence of systems solve by the use of programmable electronics. Anyway it is not possible in many cases to know if the proposed systems are developed according to the SIL requirements from the patent information available.
Pessral application
Pessral systems are implemented with two basic objectives:
  • To solve technical problems in new designs for out-of-standards solutions
  • To improve functionality or cost as substitutes of standard systems
As can be seen in the following graph, most systems are oriented to this first application, the solution of new technical challenges associated to new designs and concepts.
The overspeed governor elimination is one of the applications more frequently implemented. They try basically to overcome some of the drawbacks of mechanical overspeed governors such as installation time, reliability, quality, manufacturing costs, and operational characteristics, particularly the response time. Additionally standard overspeed governors require numerous mechanical components and significant maintenance.
Other important application is related with the safety conditions of elevators with reduced pit and head. This is an important subject these days solved with systems capable of controlling the speed of the car according to the actual position of it in the shaft. The use of multiple cars in one shaft as a solution proposed for different companies implies complex technical problems with clear safety implications, for example, to assure a certain minimum distance between adjacent cars, which can be solved with Pessral systems.
Finally general functional improvement and the reduction of costs, mainly associated to installation are considered.
Anyway some of the existing systems can solve simultaneously more than one of these applications. In these cases the application refers to the objective highlighted in the patent description.
Main input data
These systems detect an unsafe condition from the analysis of different physical and logical variables measured with different sensors in the elevator or produced from logical circuits. The analysis may include calculations, digital signal processing or, comparison with predefined or calculated levels or situations. Therefore they require obviously some data to come into the system as input data.
The following graph shows the most usual input data required by the analyzed systems in order to perform their function. Usually the required multiple inputs but here only the main variable on which the analysis is mostly based is considered.
As can be seen most systems base their functionality on overspeed detection and car position monitoring. In some particular cases both variables are combined for the identification of different safety levels depending on the speed of the car in each particular position along the shaft. Standard safety switches and contacts are input as well in electronic circuits with microcontrollers to identify unsafe conditions based on a particular combination of switches conditions. Door position is also used as an indicator of safe condition.
Main output commands
Pessral systems produce output signals and commands to effectively execute its safety protection function. These output commands basically are aimed at the elevator speed controller, the safety gears and the door control as can be seen in the following graph.
The precise control of the elevator speed profile according to a real time safety evaluation can extend the usability of the elevator well beyond the present limits making new applications possible. The use of reduced pits and heads are related with this possibility. Electric activation of safety gears is also an interesting possibility making it possible to eliminate the overspeed governor as explained previ- ously. Finally the door control must be considered when the safety device is associated with risks involving door operation despite the restrictive SIL 3 classification.
Conclusions
  •  There is patent related activity in the area of Pessral despite the relative short time available from the approval of EN 81-A1, November 2005.
  •  Pessral solutions already exist for different safety devices. Overspeed detection (9.9.11 in EN 81-A1) is the most common one.
  •  Low SIL solutions are the most numerous but not exclusive.
  •  Most existing Pessral systems are intended to solve technical problems in new designs for out-of-standard solutions.
  •  Overspeed governor removal is one of the applications more frequently implemented.
  •  Most patents are fi led under PCT and EP procedures with priorities as well in different countries, USA and China primarily.
  •  In any case total patented solutions found with the restrictions proposed account for only a 2 % of total patents published in related ECLA groups.
  •  Short time available and aggressive patent strategies could be reasons for that low indicator. Other commercial and company strategic reasons could also be considered.
References
Fundación Cotec para la Innovación Tecnológica. (2007). Los informes tecnológicos de patentes. Cotec. Madrid. 87 pp.
Fabry B., Ernst H., Langholz J., Koster M. (2006). Patent portfolio analysis as a useful tool for identifying R&D. Elsevier. e-document.
AENOR. (2006). EN 81-1:2001/A1:2006. Reglas de seguridad para la construcción e instalación de ascensores. Parte 1: Ascensores eléctricos. AENOR. 26 pp.
AENOR. (2001). EN 81-1:2001. Reglas de seguridad para la construcción e instalación de ascensores. Parte 1: Ascensores eléctricos. AENOR. 178 pp.
Bosworth D. (1980). Patent activity: A comment. Loughborough University. 9 pp.

 

6/2008

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