Safety scanners: Compliance matters

 

A piece of industrial equipment may be considered safe in itself under normal circumstances but, if it is used in inappropriate circumstances, it becomes inherently unsafe. 

All too often, machinery is designated as safe to the appropriate levels and yet the safety function of the wider system of which it forms a part has not been properly assessed. In virtually any potentially dangerous working situation, there are two parts to the safety equation: Firstly, is the hardware itself inherently safe? Secondly, is the way it is used safe? 

A good example of this is safety laser scanners, and switching between protection fields: Care must be taken to ensure associated controllers and systems are also safety-rated to the appropriate levels to meet machinery safety standards.

Field Switching

A laser scanner is a safety device that uses the 'time-of-flight' principle to form a detection field, as in figure 1. Fields can then be created using software in order to safely detect people when they enter an area. 

 

Figure 1: Safety laser scanner

A particularly useful feature on modern scanners is the ability to change assigned safety fields whilst in normal operation. This allows safe operation of a machine when different human interactions with it are required, but the key question is what criteria are being used to make the decision to change the fields?

The law requires you to do everything 'reasonably practicable' to protect people from harm; therefore each selection of a field needs to be assigned a safety integrity or performance level (PL).

Functional safety and EN ISO 13849

Where the effect of a protective measure is dependent on the correct function of a control system, the term 'functional safety' is used. To implement functional safety, the Type B harmonised standard EN ISO 13849 can be applied. (We’re choosing this standard for our example, rather than IEC 62061, which is not discussed here.)

EN ISO 13849 sets out the general principles, design and validation of Safety-Related Parts of Control Systems (SRP/CS). To ensure a safe system, it is 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). 

In this example, the system safety function is made up of sensors, logic units and power control elements – see Figure 2.

 

Figure 2: Typical safety function

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 function 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.

Simple to achieve

By using appropriately-rated controllers and software to support the use of rated switching devices such as laser scanners, the appropriate rating for the whole safety circuit must be achieved to comply with standards.

Using a flexible, modular safety controller, such as SICK’s Flexi Soft, provides a scalable and simple to operate solution that’s easily integrated with all safety control components without the need for complex software.