COVER STORY SUPPLEMENT FEATURE
SAFETY FUNCTION SOLUTIONS There are a number of different solutions that would give a field switching function which achieves PLd. One particular example using a SICK safety-rated rotary encoder and a SICK ‘Flexi Soft’ safety controller with FX3-MOC drive monitor module can be seen in figure 5.
with safe encoder inputs and PLe rated outputs. If PLe was required then two standard rotary encoders could be used.
SAFETY SIMPLE TO ACHIEVE We must apply the logic and reasoning of EN ISO 13849 to the selection and operation of the safety functions. By
important to apply the logic and reasoning of EN ISO 13849 to the selection and operation of all the safety functions. A risk assessment defines each safety function and gives it a performance level requirement (PLr). The circuit can then be assessed using a combination of manufacturer’s specifications and/or software tools such as SISTEMA to see if it achieves the assigned PLr. Performance level is measured in probability of failure per hour (1/h), ranging from PLa (> 10- 5) to Ple (< 107).
UNDERSTANDING THE SAFETY FUNCTION The safety function defines how the risk is to be reduced by protective measures. An exact definition of the safety function is necessary to obtain the required safety. The type and number of components needed for the function are derived from sensors, logic units and power control elements. So, for the winding machine example,
the safety function for stopping the rotation of the reel if the protection field is disturbed could be described as follows and shown in figure 4. Here, we have a scanner (Input), safety
controller (Logic) and the contactors (Output). The total PL can only be as high as the lowest performing element. In this example, the scanner is PLd, the safety controller is PLe and the contactors achieve PLe by calculation using EN ISO 13849 and SISTEMA. Thus, the safety function can at a maximum achieve PLd. However, the switching of the protective field must be of the same Performance Level as the safety function for operating a stop when the protection is infringed, otherwise we have a high- integrity protective measure with a low- integrity selection function. What would happen if the safety circuit contained a standard plc which has no PL (or SIL for EN 62061) rating? It cannot be assumed that the safety circuit meets the requirements of PLd and therefore, it does not comply with the Machinery Directive.
Figure 3: Slow speed
Figure 4: Typical safety function
Figure 5: PLe field switching safety function
This safety function is used to determine speed/direction of the reel. It achieves PLd since the encoder has a Sin/Cos interface and is certified to PLd/SIL2 and a PLe rated safety controller
using appropriately-rated controllers and software to support the use of rated switching devices such as laser scanners, it is easy to achieve the appropriate rating for the whole safety circuit. Corners must not be cut. It’s more or
less the same as indelibly and clearly marking one bungee rope 80 metre jump only and the other 150 metre jump only!
SICK UK T: 01727 831121
www.sick.com/gb/en
SICK launches next-generation microScan3 safety laser scanner
T
he SICK microScan3 is the first safety laser scanner to use SICK’s patented scanning
technology, safeHDDMTM (High Definition Distance Measurement), based on advanced time of flight measurement. The uniquely-developed and patented
algorithms of safeHDDM enable multi-signal evaluation that achieves a level of measurement reliability never before seen in safety scanners. Even a very dark object with just 1.8 percent remission, e.g. black clothing, is reliably detected. With excellent resolution and detection of
difficult-to-see materials with a wide range of 5.5 metres and 275 degrees, the microScan3 operates reliably in difficult ambient light, dirty or dusty conditions where other technologies can fail. “SICK has led the way in reliable safety laser scanners for more than 20 years,” says Dr Martin Kidman,
SICK UK’s safety specialist. “Developing the microScan3 was all about making it easy for users to optimise workflow efficiency and productivity without compromising the safety of personnel. “SICK has revised and optimised every detail of its laser scanner technology to set a new benchmark for
machinery safety. The microScan3 is a rugged, high-performing and unobtrusive laser scanner, ideal for multi-sided machinery and production line protection at access and loading/unloading points, hazard points, hazardous areas and other critical safety duties.” The microScan3 achieves up to 30mm object resolution, a protective field range of up to 5.5 metres and
a warning field range of up to 40 metres. Up to eight fields can be programmed with two monitoring cases and there are three universal I/O connections which can be assigned various signals.
/AUTOMATION
MACHINE SAFETY SUPPLEMENT | MAY 2016 S5
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