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06/06/2005
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Dean Palmer reports on a new approach to controlling access to hazardous machinery that enables mechanical trapped key interlocks and electrical safety gate switch interlocks to be integrated in one unit
 Dean Palmer reports on a new approach to controlling access to hazardous machinery that enables mechanical trapped key interlocks and electrical safety gate switch interlocks to be integrated in one unit
A compact access and control system has been developed that enables a selection of modules including mechanical trapped key interlocks, electrical safety gate switch interlocks and electrical operator controls to be integrated in one unit.
The system, which was launched earlier this year, features patented mechanical and electrical connections between every module. It simply clips together and the internal network is self-configuring. With more than 4,000 billion combinations of modules, it can be customised for any access and control application.
The developer of 'eGard', Fortress Interlocks based in Wolverhampton, designed eGard after identifying the need in the market for an intelligent, modular system for machinery guarding applications. The firm, which has traditionally produced heavy duty interlocking products for power generation, automotive manufacturing, steel, pulp and paper, textiles and material processing applications, wanted to transfer the functionality of interlocking to medium and light duty industries, an area dominated by lower capability products such as simple tongue switches.
Initial development work focused on mechanical trapped key interlocks and the firm patented a mechanism for interlinking modular interlocks. The idea was to develop a control system that encompassed safe and standard I/O in one simple, modular arrangement.
The company wanted the new system to be easily fitted to any machine, with electrical connections at one end, minimising wiring. It had to be engineered for Installation EN Category 4 applications. eGard is rated to IP65, has a polymer body for corrosion resistance and has stainless wear parts for long life.
The development process produced a number of problems, including the number of interacting parts. Fortress was used to producing products made from alloys and stainless steel but eGard was the first product designed using polymer housings, so the design team had to familiarise themselves with polymers, snap fits and ultrasonic welding.
Pro/Engineer CAD software was used for the mechanical design, along with Pro/Intralink to manage the CAD data, allowing several design engineers to work on the concept at the same time. And, to ensure the final product was affordable, the basic external polymer housing was designed to be exactly the same for each module, despite the fact that there are many different types of module.
These modules include mechanical locks, electrical locks, safety switches, service modules such as indicators and buttons, internal releases, emergency stops, and a selection of actuators suitable for both hinged and sliding doors. As the project advanced, the polymer body and base went through many design variants before it being finalised. Now, the basic modules can accommodate any type of standard controls such as switches, buttons and lamps, as well as trapped key and electrical interlocks.
The mechanical links use the patented device from the first R&D project, although the links run down one side of the module rather than the spine, as had first been planned. This allows the electrical connections to run down the other side of the module. The runner bar connects without having to be screwed together. It simply clips together and is then held securely in place by the screw tube that is used to install the product on site. The fixing holes are designed for aluminium extrusions, but the system can be fitted to any other machine or gate housing.
The product's true innovation is in the separation of 'safety' and 'control' information within the PCBs. A set of hard-wired, dual-channel safety circuits are implemented on every module, to allow modules to be configured in any order. The dual safety circuits ensure the safety of the product by shutting down the system if there is a malfunction or emergency stop condition. An isolated internal data bus system had to be developed using a series of PIC microprocessors to convey control data. It is this internal network that controls the components such as the switches and lights.
The internal electrical connections were developed to ensure that the network uses a minimal number of terminals to relay the I/O information. The internal network is also self configuring, for ease of installation and reconfiguring. Each module has a PCB, some with a processor, and at the end of the modular stack is a control module. The processor in each module is programmed to tell it what type of module it is, and the processor in the control module knows where each module is and what its address is.
There are three electrical end modules: a safety and control module; a safety-only module; and an AS-interface module. And if eGard is used for purely mechanical interlocking, cap and foot are also available. With the correct selection of end module, eGard can simply connect to PLCs and mimic panels, or can be used on an AS-i network.
Developing the crucial software that handled the non-safety data took considerable time. Also, space considerations were different within each type of module and the PCBs feature different components. Fortress was eventually able to design a module PCB with eleven variants that would fit and work within each module.
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Author
Tom Shelley
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