"Safety last!" - this is the motto used by Harold Lloyd in the 1920s to climb the façade of a high-rise building in his eponymous film. He also made it to the top - and how! Things shouldn't be this daring, barely scraping by catastrophe by a hair's breadth. So safety first is preferable, after all: this is how machines, systems and operating equipment should be designed so that nobody falls from a great height or is harmed in any other way. Harold Lloyd would have been delighted back then.
Nowadays, high demands are made on machines and systems so that they operate and do not injure anyone; neither the environment nor people come to harm. Thankfully, the times in which a high number of people were injured or even lost their lives when handling machines and during construction and industrial projects have been consigned to the past. The basic concept of planning and implementing each action wherever possible so that 'safety' is an essential aspect is being increasingly taken into consideration. This is occurring on the basis of legal specifications and - what is to be regarded as even more important - due to an understanding of the fact that work must and can be carried out without collateral damage. By no means least, this results in a clear competitive advantage for machine manufacturers that are able to offer systems meeting this standard, whereby the topic of safety can be broken down to each component that is used - right down to bolts of suitable quality.
Now let us turn our attention to the sensors, which measure angles and distances amongst other aspects. TWK offers a wide range which meets these high standards of safety. Because TWK backed the 'safety horse' early on and therefore has access to a wealth of experience and can offer an accordingly high number of products that have already proved satisfactory. The already familiar robustness is now joined by the particular aspect of safety.
Rotary encoders with safe interface
The SIL2 sensors that were first developed and have already been in use at our customers for a number of years are rotary encoders. They are particularly used in mobile machines. The position of crane boom arms has to be registered, firstly to move precisely and quickly to specific positions and secondly to ensure that the defined maximum positions are not exceeded. Otherwise, the risk of the crane's becoming unstable and tipping over would be run. For instance, the angle is measured at the transition point from the undercarriage to the boom, and partly also at the articulated arms further up in the boom. In the first case, a toothed gear can be used to connect the rotary encoder to the ring gear that is usually available and thus register the boom position. For this, TWK offers safety rotary encoders which always supply the exact boom position angle in degrees thanks to the special, optional slewing ring algorithm, regardless of how often it is turned in one and the same direction. If the encoder is connected to the play-free toothed gear ZRS using a flange, the bit-precise position is always available even in the case of minute forwards and backwards movements. Other special features such as special flange designs, shaft connections and additional safe relay switching outputs are also freely available.
What does safety mean?
Without going into too much detail, safety means that certain criteria which are formulated in standards and guidelines have to be adhered to. The IEC 61508 standard and the Machinery Directive EN ISO 13849 are of relevance, for instance. Many criteria requiring compliance are contained in them: the probability of a hazardous fault that must not be exceeded, the degree of diagnostic coverage, error routines and much more besides, whereby the description in the standards differs.
Our sensors are equipped with the corresponding electronics - in redundant form in some cases - to meet these criteria. However, the software algorithms are an essential aspect; these undertake constant self-monitoring of hardware and software in the sensor, thus detect faults and transfer them to the control system via the interface. The sensor can also assume a safe status - see further below under switching outputs and analogue interfaces.
If a safety sensor is to be parametrised, this is only possible subject to adherence to the safety protocol: in addition to the parameters, checksums have to be transmitted to the sensor and valid flags have to be set for subsequent activation. Naturally, transfer of the measured values to the master is safeguarded in various ways - through additional bit-inverted transfer, for instance.
Some digital interfaces have now made their way into the SIL sensor family: CANopen Safety, PROFIsafe, fail-safe over EtherCAT.
Another option for increasing the safety aspect of sensors is to design them in fully redundant form: 2 x position registration, 2 x processing in the sensor, 2 x output through 2 interfaces. Of course, this procedure must slot into the user's safety concept.
Further SIL sensors
Rotary encoders are not always appropriate. If inclination angles have to be registered in the earth's gravitational field or oscillations and vibrations have to be measured as part of condition monitoring, our MEMS-based sensors are used.
If we take another look at mobile machines, cranes and concrete pumps, etc., we discover that they first have to be aligned or levelled when used on site. An inclinometer NBN/NBT on the undercarriage, which is able to measure in two axes (x, y), determines the correct horizontal orientation of the crane. The boom can then be operated: via a 1-axis inclinometer, the vertical inclination can now be used, along with the rotational angle, to determine the position of a cage. If the boom is telescopic, a very compact draw wire measuring system with integrated SIL rotary encoder (NOCN125-D type) provides assistance in longitudinal measurement.
There is a reason for the diverse range of sensors available from TWK: for some time now, TWK has been focussing on typical applications and accordingly designed, optimised sensor systems. As a result, many functions and measurement tasks can be covered with proven TWK sensors.
Let us finally turn to the vibration sensor NVA115. This SIL2 sensor is able to measure accelerations of up to several g in a range from 0.1 to 60 Hz - optionally selected in different bands - transfer them to the control system and also evaluate them. In this case, evaluation means that optional processing of the g values in the NVA (RMS or integral) can be used to interrupt a safety chain with the SIL2 switching outputs of the NVA on exceeding limit values in order to shut down the system, e.g. a wind turbine. The two relay switching outputs are each equipped with two relays to ensure safe shut-off. If the NVA detects a fault by means of self-diagnosis, the relays interrupt the safety chain (safe status of the NVA as an actuator).
Besides the safety CAN interface, a further two 4...20 mA interfaces are available for transferring measured values. These can also be rendered SIL2-capable using ingenious measures: feedback of the current value for monitoring and reporting faults to the master due to the fact that the value of 2 mA, which is not normally permitted, is reliably accepted.
These measures ensure that the likelihood of the sensor's endangering man and the environment is minimised as far as possible.
