The higher the more wobbly - this should never be the case, of course. Quite the opposite: the standard of safety should not suffer from aiming high. Cranes and mobile cranes as well as lifting platforms must therefore be equipped with safety sensor systems to always guarantee stability and operating safety.
This is where the sensors from TWK now come into play. TWK is a leading company in the field of SIL safety sensor systems. This not only involves rotary encoders in single and multiturn designs. The products required on the market go far beyond this:
▪ Inclinometers for levelling processes and for registering the position of mobile crane booms
▪ Electronic switching cam encoders that also reliably ensure switching processes in addition to the rotary encoder position signal
▪ Vibration sensors to prevent damage to machines caused, for instance, by wear and defective mechanical components or unfavourable operating conditions such as gusts of wind and the resulting oscillations and vibrations.
Let us take a look at the electronic switching cam encoder NOCN/S3. This safety component is SIL2-certified and equipped with a CANopen Safety interface that ensures reliable data transfer to the master. The device can be extensively parametrised by the customer via this interface. A mandatory security procedure ensures that unauthorised parameter changes are not possible. The position and speed signal is transferred in normal and bit-inverted form via the two SRDOs.
That was the rotary encoder side of the NOCN. However, it is additionally equipped with 2 safety SIL2 switching contacts that initiate switching processes in specific positions. This function can be used, for example, when reaching end positions so that specific points are not passed, and is accurate down to the bit level. Each NOCN has two such switching contacts, which in turn consist of two relays that are connected in series. This design guarantees reliable contact opening even in the event of adverse current and voltage conditions on the line. Sticking contacts are therefore effectively ruled out.
One typical application for these functions is lifting drums on loading and construction cranes - also called tower cranes. The trolley and the load hook are moved using these lifting drums. So that the trolley and the attached load do not move too far, the maximum travel also has to be controlled using the lifting drum. Limit switches switch before reaching the end position, which must not be exceeded. This implementation is safety-relevant so that accidents are unable to occur. The components must therefore meet corresponding safety requirements.
Mechanical switching cam encoders are often connected to the lifting drum using a toothed gear (Figure 2).
The mechanical switching cam encoders with or without integrated rotary encoder have the disadvantage that they are often not up to the harsh operating conditions for long. They also have to be adjusted mechanically using adjustment screws, for instance, a process which is laborious. Direct movement to the switching positions may even be necessary. The electronic SIL2 switching cam encoders NOCN from TWK enable comfortable parametrisation from the control cabinet. Corresponding objects can be used to program the switching points directly (low and high limit) via the SIL2-certified CANopen Safety interface. Or the desired positions are moved to directly and the position value is then assigned to the switching contact. This significantly simplifies lifting drum commissioning. In addition to the switching contacts, the high-resolution position and speed signal from the NOCN is always available to the control system, of course. Thanks to their cast design and extremely robust structure, they comply with systems of protection up to IP69K and are optimally protected against all kinds of weather as well as shocks and vibrations. They are also a good alternative to the now commonly used absolute rotary encoders with additional incremental signal for trolley regulation.
The safety rotary encoders from TWK have also proved their worth in registering the angle position of a crane boom. If the boom is to be driven using a ring gear, the rotary encoder can be simply attached via a play-free gear ZRS. Optionally, special NOCN software always calculates the exact boom position - calibrated to 360° of the ring gear. This involves the special slewing ring software.
If CANopen Safety is not to be used, the SSI and analogue interfaces can also be selected for the electronic switching cam encoders. These interfaces only permit switching contacts of a SIL2 design. The devices are parametrised via teach-in lines.
Further sensors from the TWK portfolio for SIL2 safety applications include the NBN/S3 and NBT/S3 series inclinometers for undercarriage levelling and boom inclination in the case of mobile cranes (Figure 3). The safe CANopen Safety interface or PROFIsafe via PROFINET is also used in this application. Interfering accelerations caused by vibrations may now occur temporarily - due to an engine starting, for instance; these can prevent inclination measurement or perhaps even lead to an override fault in the NBN due to their intensity. Often, however, it is not necessary to have measured inclination values for this short period of time. An integrated software module is used to detect the interferences and the inclinometer outputs a value indicating to the control system that measurement is not possible at present. In this case, it does not initiate crane safety shut-off. Once the interference has passed, the NBN supplies its regular measured values again.
A version with integrated gyroscope sensor that extensively eliminates these interferences will be available shortly.
Finally, the SIL2-certified oscillation or vibration sensor NVA115/S3 deserves a mention. This device measures vibrations in a frequency range from 0.1 Hz to 60 Hz. Partial frequency bands, e.g. for slow tower movements, can be set in the factory. Two SIL2 switching contacts shut off reliably in the event that limit values are exceeded. Damage to the crane caused by excessive oscillations and vibrations can be avoided in this way. The application switches to a safe state to avoid danger. Adjustable time constants even enable NVA safety shut-off to take place in a specific crane vibration phase in order to bypass additional mechanical stresses due to shut-off 'at the wrong point in time'. The continuous vibration acceleration values are transferred via the CANopen Safety and/or analogue 4 ... 20 mA interfaces.
The TWK range of safe SIL2/SIL3 sensors is rounded off by rotary encoders with a PROFIsafe via PROFINET and fail-safe over EtherCAT interface. Corresponding PROFINET switching cam encoders will be available in the near future. A final mention goes to the redundant category 3 sensors - there is something for every crane.
