Sensing technology ensures safety

Step too close to one, however, and you will immediately see how it is that these vehicles can work at such speeds in close proximity to vulnerable human bodies. They will stop long before there is any likelihood of them hitting a person. The reason for this is the increasingly sophisticated array of anti-collision sensors with which they are equipped, having achieved safe, non-contact detection of personnel in the path of moving vehicles travelling along complex paths and production layouts. Prior to laser techniques, contact bumpers physically detected obstacles by deformation and activated an associated mechanical switch. The approach speed of an AGV therefore had to be limited to the impact force required to activate the collision detection system before it could cause harm. The introduction of the laser scanner immediately allowed the AGV to travel faster as it was a collision prevention system which could detect oncoming obstacles way before impact. Thus the AGV could travel faster, increasing its hourly load capacity and ensuring safety. Seb Strutt, senior product manager at Sick UK, explains: "Safety laser scanners scan their surroundings in a fan shape and measure distances using the 'time-of-flight' measurement principle[whereby the sensor assesses the distance from an object based on the time taken for a light pulse to be returned]. As soon as a safety laser scanner detects an object in the pre-programmed hazardous area, it switches off the OSSD (Output Signal Switching Device) causing the vehicle to stop. Warning fields can be also defined so that objects are detected before reaching the hazardous area." Laser scanners on mobile vehicles usually have three zones configured to. If a pedestrian enters the outer zone, a warning alarm may be sounded, in the middle zone the vehicle may decelerate and in the inner zone only would an emergency stop be triggered. Developments in scanner technology have continued and scanners now have detection ranges allowing safety fields up to 7m. Multiple fields up to 64 zones can be configured to provide high-performance and flexibility for complex vehicle paths as the AGVs turn and change direction to follow their routes. The ability to integrate laser scanners with other sensing technologies can also be a major advantage. In particular, laser scanners have the ability to safely measure the speed of a vehicle via inputs from encoders fitted to the vehicle. Based on the measurement, the detection field is automatically adjusted. As technology has developed, the size and power requirements have reduced, allowing application of the technology on to much smaller guided vehicles. One such example is the compact machines developed by the UK's largest AGV manufacturer, JBT. JBT's guided vehicles are used in hospitals, manufacturing plants and warehouses and minimising vehicle size can be very important for customers who want to minimise the aisle widths. Inevitably, this concept now extends beyond AGVs and can be applied to transfer cars and shuttles as well as complex access control applications where field switching is required. Equally, the technology can be applied to new machines as well as being retrofitted to existing ones.