Small solution that packs a large punch for AGV safetySeb Strutt of Sick UK explains how the safety laser scanner has created unprecedented new opportunities to protect personnel at affordable costs and how, in particular, its versatility has offered increased collision protection from driverless guided vehicles.
There are few developments in machinery safety that can claim such a transforming impact on productivity and machinery development as the safety laser scanner. Since its introduction in the mid 1990s, it has proved to be a true enabling technology in production environments, packing a large punch in ever smaller designs. Recent technological advances, that have made the safety laser scanner even more compact, have delivered major advances in the use of Automated Guided Vehicles (AGV). They have achieved safe non-contact detection of personnel in the path of a moving vehicles travelling along complex paths and production layouts.
Before the advent of laser techniques, contact bumpers physically detected obstacles by deformation and activated an associated mechanical switch. The approach speed of an AGV 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.
Safety laser scanners scan their surroundings in a fan shape and measure distances using the time-of-flight measurement principle. 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 form the field set. 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. The need to stop the vehicle and impact production efficiency is therefore minimised.
Developments in scanner technology have continued since the early days and scanners now have increased 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. Such advances have enabled more controlled deceleration of vehicles which has reduced wear and tear on vehicles caused by harsh braking.
Reduced size and power requirements
As technology has developed, the size and power requirements have reduced allowing application of the technology onto much smaller guided vehicles. One such example is the range of compact machines developed by the UK's largest AGV manufacturer, JBT. The company's guided vehicles are used in hospitals, manufacturing plants and warehouses and minimising vehicle size can be very important for customers who want to minimize the aisle widths.
As technology has developed, so has the functionality of the scanners. Even on a small AGV there may be between two and five laser scanners to provide the required protection. This means the complexity of the AGV control system has also increased.
A significant degree of expertise is needed to integrate all the necessary control signals as well as the laser scanner itself, in order to reach the Safety Integrity Level required to comply with the essential Health and Safety requirements of such machinery. For a safety slow down and stop function in the direction of travel, the performance level of the stop function should be category 3, PLd. The complexity of cabling encoders, field switching signals, and safety signals to and from each device could incur significant cost, so the latest developments to technology have focused on integrating complex sensors and safety-related controls. For example, in order to reduce wiring in applications where several scanners are used, Sick has developed a package that utilises the power of a safety controller (Flexi Soft) connected to four laser scanners using a 2-wire bus called EFi (Enhanced Function interface).
The concept reduces the requirement for 10 or more core cables to be connected to each scanner. Instead, a two wire network cable is used to control all aspects of the scanner via safe protocols that ensure PLd conformance. Field switching, control and status signals can be connected centrally to the local Flexi Soft controller. The safety-enable function from the Flexi controller is then easily integrated with the speed and direction control of the AGV to allow intervention during hazardous events. The high-performance of the central controller also enables diagnostic functions to be set up to monitor the system and review any incidents.
This concept extends beyond AGVs and can be applied to transfer cars and shuttles as well as complex access control applications where field switching is required. And the technology can be applied to new machines as well as for retrofit to existing machines.
Laser scanning technology is likely to see even more developments in compactness and systems integration, which will make AGVs an ever more common sight, not only in factory environments and warehouses, but in environments such as hospitals, airports or large commercial buildings.
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