Guard connections made at light speed

Tom Shelley reports on a Category 4 machine safety system that is remarkably quick and easy to install



A US devised machine guarding system uses optical fibre in a way that takes most of the hassle out of installing and maintaining such systems while giving Category 4 protection and suitability to be used in hazardous zones.

The 'Pico-Guard', from Banner Engineering is available in the UK from Turck Banner and solves most of the problems normally associated with installing and maintaining such systems at a stroke.

Most noteworthy is that the cut, low cost, plastic optical fibre needs only to be pushed into a connector and secured by a quick push down on a tab. When shown to a bunch of trade journalists, everyone present had succeeded in successively making connections before one of their number, quite technically competent, was still finishing wiring up an equivalent single system reliant on screw up terminals.

When the new system was being presented, the comment was made that since mechanical micro-switches had been largely replaced by photoelectrics in the 1970s, it was surprising that mechanical interlocks were still the norm in machine guarding systems. Apart from wearing out, leading to the often encountered need to slam guards repeatedly to get them to latch, such devices cease to work if guard doors become misaligned, malfunction when vibrated and are easily defeated. Far too many maintenance engineers have a little device with a tongue that can be used to ensure contacts are kept made while the guard door is open. Many types can be held latched using adhesive tape.

The Pico-Guard System, on the other hand, is virtually impossible to defeat, because unless the coded light signal is received, there is no way it can be made to latch. A single switch point per door meets Safety Category 4 applications, without the need for additional door monitoring relays. Because the optical fibre terminations are small, they can be embedded in doors, giving monitored enclosures a cleaner look, while continuing to function if the doors become slightly misaligned. Having no mechanical parts, the interlocks are completely unaffected by vibration.

Each controller can monitor up to four loops, each of which can contain multiple switch points. Receiver units have two solid state safety outputs to control 24VDC loads. In addition, the patent pending Universal Safety Stop Interface allows two or more controllers to be connected to control a single machine, and provides an interface to other safety devices such as light curtains, emergency stop buttons and rope pulls.

The systems are certified to ISO 13849-1 (EN 954-1) Category 4 and IEC 61496-1 Type 4. They monitor themselves every 15ms. Cost of a controller and four switches is about £600, which is similar to that of a mechanical category 4 system with two monitoring relays per door. The 'Pico' in the title should not be taken to imply anything is sized to one billionth of a mm, but is used by the company as a generic name for its miniaturised products.

Banner engineering, which manufactures in the US and China, claims to launch a minimum of ten new products each year and uses an MRP system that adjusts production schedules every 15 minutes in response to order entries.

Turck Banner

Pointers

* Through use of low cost, plastic fibre optic cable, it is possible to overcome problems arising from vibration and slight misalignment.

* The system cannot be easily defeated

* Connections can be made in seconds

* Meets Category 4 safety requirements

* Can be used in hazardous areas

Author
Tom Shelley

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