In order to safeguard the manufacturing process of a supplier from the semi-conductor industry in a simple fashion, the R & D engineers for photoelectric sensors from Banner Cooperation in Minneapolis/USA have developed a product, which is aimed at implementing a fast and convenient means for non-contact monitoring of doors and flaps via optical sensing technology. The combination of existing technologies, such as fibre-optics and a redundant electronic design, which is a proven technique frequently implemented in safety light curtains, has been complemented by new LEDs with a high emitting power.

Interference-free and safe up to EX Zone 0

The developed safety system is extremely versatile, efficient and easy to install. The system consists of a central controller, optical elements and optical fibres for monitoring access doors, gates, guards or passage ways. PICO-GUARD™ does not require any electrical connection in the machine section or the dangerous environment. Thus, the system is inherently immune to electro-magnetic interference. Furthermore the system may be operated in explosion hazardous areas, Zone 0, because the energy transferred to the fibre-optic loop is marginal.

A key factor is the high flexibility of the PICO-GUARD system, owing to its four independent optical channels. Each of the four channels operates at a different frequency to provide cross-talk protection and is capable of integrating several optical elements within a fibre-optic loop. The system comes with a calculator software, which calculates the excess gain based on the number of components, fibre lengths and lens separation distances, and is thus capable of providing a measure of the system’s operational safety. A single optical path, for example, can be placed 30 m away from the controller, provided the system’s excess gain is sufficient. When using several safety switches in the same loop, the admissible maximum length decreases with the number of optical paths and the spacing between the lenses. Independent of the type of optical element used, the interruption of the light beam and the resulting signal loss caused by the movement of the monitored door or the guarded person, leads to an interruption of the safety circuit and thus to an immediate stop of the machine or process.

Safety technology for a large variety of applications

The controller is equipped with two safety switching outputs, so-called transistor OSSDs (Output Safety Switching Device - 24 VDC), for additional monitoring of the series-connected safety relay with forced-guided contacts and the two-channel control EDM (External Device Monitoring) outputs. This design does not differ from safety light screens specified for personnel safety category 4, so that a muting function can be implemented via a separate muting module for connection of according sensor pairs or signal inputs.

Another advantage is the patented universal safety stop interface (USSI). The USSI enables interfacing two or multiple PICO-GUARD™ controllers to monitor an individual machine. The interface is also suited for connecting other safety devices, for instance, personnel safety lights screens, emergency stop buttons or cable pulls to the process control system. Additional safety functions can thus be easily integrated into the control unit.

Each PICO-GUARD controller has two inputs – one for devices with latching output requiring a manual reset, and the other for devices with automatic reset. An external display, which can be connected via the RS232 interface of the controller, visualises the controller status. The same interface can be used to transfer data to the PLC. The status of each optical channel can be signalled via additional non-safety-relevant transistor switching outputs. Removable terminal blocks additionally simplify electrical installation.

Patented functions for safety category 4

PICO-GUARD fulfils safety category 4 according to ISO 13849-1 (EN954-1), or type 4 according to IEC61496-1-2. Consequently only a single switch point per monitored door is needed, whereas previously at least two redundant mechanical interlock switches were required for this purpose. This is enabled by the patented self-checking, diverse redundant sensing principle, particularly suited for plastic fibre-optics.

The hardware and firmware of the controller is extensively tested, applying the strict failure mode and effects analysis (FMEA) to ensure that a single system component cannot fail to danger. The controller is additionally approved for use in Ex Zone 2 and optically transfers signals to Zone 0, so that it is now possible to solve personnel or process safety applications under such difficult ambient conditions in an uncomplicated fashion.

Perfectly fit for extreme environments

The optical safety switches of the PICO-GUARD system are conveniently connected to the optical fibre via a simple snap-on mechanism and come in a large variety of designs. Alongside straight and angled types, there are also dual lenses as passive and active switching elements. Opposed mode sensors with ranges of more than 28 m per channel can also be implemented. The opposed mode system incorporates 12 and 30 mm lenses with a maximum opening angle of 2.5 °. In order to meet a large variety of application requirements, the system comes with a choice of standard and teflon-coated fibres (for aggressive media) or PVC and stainless steel jacketed versions for harsh environments.   

The author:
Dipl.-Phys. Ing. Markus Bregulla
is product specialist for photoelectric and ultrasonic sensors
within the product management team of
Hans Turck GmbH & Co., Mülheim a.d. Ruhr.