Like many plant manufacturers, operators of mountain cable railways are also compelled to meet the requirements of the European Machinery Directive in the event of conversions or new purchases. Certified electrical components with a known performance level help to implement safety functions.
Bayerische Zugspitzbahn Bergbahn AG, which was formed in 1998 from Bayerische Zugspitzbahn AG and Wank-Bahn AG, operates several mountain railways and ski lifts around Garmisch-Partenkirchen and the Zugspitze. Besides the rack railway, the Alpspitz cable car and the Zugspitze cable railway (new from December 2017), this also includes the Wank cable car. The Wank cable car is a single-cable gondola lift. It runs north-east from Garmisch-Partenkirchen via the central station to the 1780 m high Wank, a mountain on the edge of the Ester mountains.
Single-cable gondola lifts have only one circulating, endless cable that simultaneously serves as support and traction cable. The cars are uncoupled from the circulating cable at the stations for passengers to embark and disembark, guided slowly over a suspended rail and then accelerated again in the other direction and coupled onto the cable. To do this, the car must be accelerated to precisely the speed of the circulating cable prior to coupling. This car speed is monitored by a TWK PROFIsafe rotary encoder. In addition to a safe position, this also supplies a safe speed signal to a safe master via the standard Profinet line.
Another safety-relevant function is maintaining the correct distance from the preceding car. In achieving this, the application benefits from the multiturn rotary encoder's high resolution of 8192 steps/revolution and its measuring range of 4096 revolutions. As a result of this, the 4 km long circulating cable can still be broken down to approx. 1 mm despite a mechanical transmission ratio of 10:1.
The SIL2/PLd-certified TRT/S3 multi-turn rotary encoders with PROFIsafe via PROFINET interface are used for both safety functions. The possibility of setting the resolution, measuring range and direction of rotation as well as setting an initial starting value (preset value) makes it easy to adapt to the existing application. Thanks to its extensive working temperature range down to -40°C, it is particularly suitable for use at high altitudes. Its robust design is also beneficial in this case.
The actual position and speed values packed in the PROFIsafe telegram are sent via the standard PROFINET line and unpacked again at the PROFIsafe master without the user having to worry about the details of PROFIsafe communication. In addition to the PROFINET communication settings, only the Profisafe address has to be assigned in this case, and the watchdog time may possibly have to be adapted. The position or speed value of the rotary encoder can then be accessed in the F program of the control system as is familiar from the standard PLC program. Which commands and data types are available in the F program varies from manufacturer to manufacturer, however.
Double word data types are now also available when using S7 control systems with Safety Advance under the TIA portal. These enable the 32-bit wide position value to be accessed and used as a single word, which considerably simplifies the use of the PROFIsafe rotary encoder for large measuring ranges with more than 65,536 steps, as is the case with these cable railways. TWK offers the TRT/S3 model with code type D as for double word for this purpose.