Minimising the risks associated with the moving parts of machines can only be guaranteed by using the appropriate sensor technology. Alex Bryce of Pilz Automation Technology, looks at the key selection criteria that designers should consider when specifying sensor systems for safety critical applications

Selecting the most appropriate safe sensor or actuator/sensor combination for plant and machinery is not an easy task due to the myriad of options available to designers. Sensor systems operate differently, so it’s important that designers fully understand the implications of using each type.

Ultimately the goal is to harmonise productivity with machine safety and user friendliness. If a safeguard is not user friendly, there may be a temptation to manipulate a safety guard, putting the machine operator at risk. Designers need to identify potential cases in which safety devices might be manipulated and then eliminate them.

Certain considerations will help the designer to decide which components should be used for the safe monitoring of movable guards in accordance with EN 60947-5-3. Typical questions that should be asked include: is the cover frequently opened and closed? Does the process need protecting as well as the operator? Are components exposed to high vibration or extreme temperatures, or are they used in potentially explosive areas?

Mechanical safety switches

For simple safety gate applications where there is no increased risk of manipulation, mechanical safety gate switches are widely used. These are used for simple safety gate monitoring or with a mechanical guard locking device, which uses increased extraction force on the actuator to prevent the gate or cover from being opened unintentionally. Many applications require two mechanical switches to guarantee the required level of safety. This significantly increases engineering costs.

Another consideration here is that the sensor’s function can be quickly restricted by the deposit of swarf, and if safety gates drop over the course of time, the lifetime of a mechanical switch is reduced considerably.

Mechanical safety bolts, comprising a safety switch, handle and bolt, provide protection where safety gates are difficult to adjust, as well as on safety gates that are frequently opened and closed. A mechanical guide on the actuator ensures that the actuator engages with the safety switch correctly when the safeguard is closed.

Mechanical hinge switches provide a solution for rotating or hinged gates and flaps. These provide better protection against manipulation compared to other mechanical solutions because they are concealed within the hinge, which also saves on installation space.

Non-contact safety switches

Gates, covers and flaps often need to be opened repeatedly for operational or assembly purposes, to enable operators to access or reach into the protected area. Here, there is a risk of a gate being mechanically misaligned.

The disadvantage of mechanical guard locking devices is that they only have limited tolerance for misaligned gates. For this reason, non-contact safety switches provide an alternative, particularly if there are high vibrations. These switches have a higher response tolerance, allowing greater flexibility in the way they are attached to the machine.

Rugged non-contact switches are also suitable for applications that require protection types higher than IP67. These switches are able to meet safety requirements up to PLe of EN ISO 13849-1, and SIL 3 of EN/IEC 62061, or Category 4 of EN 954-1.

When it comes to deciding on the acting principle of the sensor, important distinctions need to be made. Whilst magnetic safety switches provide only limited protection against manipulation due to the technology used, coded safety switches achieve up to 100% protection against manipulation. As a result, magnetic safety switches should be concealed or installed in such a way that they are invisible to operators.

If a safety switch cannot be concealed or if 100% protection is required, RFID coded safety switches are a solution. RFID technology enables a clear assignment between sensor and actuator. RFID coded safety switches operate in accordance with the ‘key lock principle’ whereby only one key fits a lock. Given that the actuator is installed using one way screws, which can only be loosened via a special tool, a high level of protection against manipulation is achieved.

In addition, magnetic safety switches have limited suitability if metal swarf is present, which can be attracted to the magnet and deposited on the switch, leading to operational problems or even failures, meaning plant down-time and reduced productivity. Here, coded switches are ideal as RFID is unaffected by metal swarf.

Gate closed in case of danger

Due to moving parts, risks remain even after a machine has been shut down. In these ‘hazardous overrun’ cases, the use of a safe guard locking device is mandatory. This means the gate cannot be re-opened until standstill has been safely detected or a time delay has elapsed. Here, mechanical guard locking devices with spring interlocks are typically used. These guarantee that the gate can only be opened when it is safe to do so.

Integrated systems

More can be achieved by using integrated safety gate systems that combine safe monitoring, safe guard locking and control elements. All components, whether mechanical or electronic, are integrated. Complete systems such as these help to reduce the time and costs associated with project configuration, design, documentation, purchasing and installation. Also, if sensors and evaluation devices are sourced from a single supplier, the interfaces between the various components are already compatible.