Issue 1/2006
01/02/06
Interfaces between absolute-value transducers and elevator controls
Christoph Stratmann (K.A. Schmersal GmbH, Wuppertal)
When sensing the position of the elevator car, absolute-value transducers offer significant advantages when compared with systems which register only the relative position. Not only is the design of the position-sensing system important. Another question which plays a major role is how data is transmitted from the position sensing system to the elevator controls. Only where the system offers multiple options will the prerequisites for flexible, universal utilization in elevator engineering be satisfied. The following article describes the interfaces which Schmersal uses for the non-contacting USP position sensing system.
Category: Issue 1/2006
Posted by: Editor
In all absolute value transducers current measured values are output through a data interface to the elevator’s control system. The controls then calculate the current position and forward the appropriate commands to the drive system.
The measurement system itself is thus only one part of the technical challenge for developers of positioning systems. At least as important is providing high-performance interfaces through which position data can be forwarded to the controls. This task is a challenge because there are numerous elevator control concepts in use and they all employ differing data standards and interfaces. The situation is unfortunately that – in contrast to other branches of industry – standardization of data protocols in elevator technology is still in its infancy; proprietary systems prevail and the number of data interfaces on the market is large and more than confusing.
Versatility in application
With the introduction of the USP system (Fig. 1) the Schmersal Group has developed a positioning system whose design is fundamentally different from conventional absolute value transducers. The position values are registered without any physical contact, using the propagation time for an acoustic pulse which is impressed onto a special wire. This wear-free measurement system is extremely dependable, even under harsh conditions. One great advantage is the elimination of the costly mounting of sensors and transducers inside the hoistway. The installer need only mount the special wire and the evaluation unit in the shaft, attach the transmitter unit (which impresses the pulse onto the wire) on the car and – depending on which controls are being used – program the landings using a simple teach-in procedure. This represents a major simplification in the installation of the positioning system.
As original equipment and in modernization
USP is equally suited for new elevator installations and modernization projects since no structural changes are required when installing the system. From the view of the electronics developer, however, this range of applications brings about further challenges since interfaces will have to be devised to match older controls (including electromechanical types).
In order to cover these application areas and to offer control engineers the advantages of non-contacting position registration all across the board, three differing data interfaces are available in the USP concept:
• SSI,
• UART
• CANopen.
Synchronous data transmission through an SSI interface
The SSI interface is used for synchronous data transmission. The controls apply a clock signal, with a fixed number of clock pulses, to a line. The absolute value transducer returns one data bit in response to each clock pulse (Fig. 2). At the hardware level, the SSI interface is realized using an RS 422 interface: there are two data channels with one clock signal each and a data signal; the two channels run opposite one of the other. Channel B thus transmits a signal inverse to that in Channel A (Fig. 3). This ensures great transmission reliability.
SSI: Large number of variation options in signal transmission
There are numerous possibilities for outputting the data signal to the data line. Firstly, either the highest bit (MSB mode) or the lowest bit (LSB) can be transmitted first. The data can be transmitted in either a 24 or 25 bit format and the data are available either in binary encoding or in Gray code. Moreover, there are single and double reading processes available in which all the data information is transmitted either one time only or in two packages, one directly after the other. Thus a range of SSI interfaces is available for USP and the controls made by many manufacturers can be connected here (Fig. 4: Table of control manufacturers from the Plug & Lift flyer).
UART: Two protocols to choose from
The asynchronous, bi-directional interface is a standard UART, such as that found in the serial ports in a PC. Here again the hardware layer is realized with an RS 422 interface. During data communications the controls use a query command to call the measured value present at the absolute-value transducer. The transducer responds by transmitting the current measured value in serial form.
As in other serial interfaces, there are various adjustment options here, too. The default setting for the USP interface is: 1 start bit, 9 data bits (the 9th is always “0”), no parity, 1 stop bit, with a transmission rate of 38.4 kBaud. The following protocol is also possible, as an option: 1 start bit, 8 data bits, no parity, 1 stop bit (Fig. 5). Both 19.2 and 38.4 kBaud transmission rates are possible.
New: Updated CANopen interface for USP
The CAN (Controller Area Network) serial data bus system was developed in the 1980s for data transmission in motor vehicles. The physical layer and the data security layers are described in the CAN specification. The CANopen Lift concept is based on the CAN hardware layer and is modified specially to suit the needs of elevator engineering. The CANopen application layer is described by the DS 301 specification published by CIA (CAN in Automation) and DSP 417 (Fig. 6).
Bus wiring is described in ISO 11?898 (CAN high-speed). Recommended for use here is a twisted pair with a surge impedance of from 108 to 132 ohms is recommended. Maximum line length will depend on the data transmission rate. The CAN cable has to be fitted with a 120-ohm terminating resistor at both ends.
CAN differs from other bus systems in that the modules are not addressed; instead, the messages are identified. The messages are assigned a priority value so that time-critical data can be transmitted immediately. The multi-master concept is laid out so that the bus modules send data themselves. They can, however, be requested to send data whenever necessary. Schmersal has since November 2005 offered the USP system with an updated CANopen interface.
Parallel interface and selector simulator: Virtually unlimited possibilities
Whenever older elevators are to be modernized the parallel interface is available as an additional module. The programmable switching outputs at the interface make it possible to connect all types of relay-based elevator controls. Thus the range of options for elevator and control engineers to use the USP is vast. Over and above this, a modernization project can initially leave old electromechanical controls in place, since the selector simulator module provides the required signals to the controls and thus assumes the functions of a Hartmann-type selector.
Testing and logging interfaces
A further new development at Schmersal facilitates servicing the positioning and control systems. When carrying out diagnostics work in the field it can make good sense to log the stream of data communications between the controls and the position transducer. Due to the high data rates involved here, large volumes of data can be accumulated in a short period of time. In addition, the measurement systems available to date have had problems with reading, processing and storing this data stream in realtime.
That is why Schmersal has in recent months developed a diagnostics computer which logs such data (Fig. 7); it can be made available to the customer on request. This test PC makes it possible to address and query the USP interfaces (SSI, UART, CAN). The measurement device can address the transducer directly, acting as the master unit, and query the position values at regular intervals. It is, however, also possible to log current data communications between the controls and the absolute value transducer. Moreover, two different transducer systems, including one for reference measurements, for example, can be queried simultaneously, even if the two transducers work with differing interfaces or at differing scanning rates.
The data are stored and can subsequently be evaluated. The measured values can be depicted graphically so that various events can be identified in the analysis. Triggering in response to outside events, such as activating an electrical output in case of a malfunction, can also be registered in this way. Thus diagnosis is simplified in case of a malfunction.
What does the future hold in store?
It is easy to see that the multitude of potential interfaces and their adjustment options are quite large. Implementing every possible interface parameter involves an amount of development effort which should not be underestimated – a challenge which the elevator technology division at Schmersal is happy to take on. Numerous interfaces have already been developed on the basis of customer specifications. And in spite of this it would be desirable if, in the future, the elevator industry could agree upon a common and standardized interface. Perhaps CANopen Lift represents such an opportunity for the future.
1/2006
