Today’s industrial robots are fairly well-behaved creatures – doing only what they are programmed to do – as long as nobody gets in their way. Since robots move at astonishingly high speeds and carry loads up to 600 kg, humans must keep well out of their way.
Man and machine are usually separated by fences, like visitors and tigers in the zoo. However, the cost of such traditional safety equipment is high and has become a driving factor for the advancement of robot-based automation in highly industrialised countries.
ABB now offers a solution that cuts the cost of robot installations by replacing expensive mechanical safety equipment with dedicated electronics and software.
According to European and North American regulations for occupational health and safety, even a potential malfunction of a robot's controller hardware or software - however unlikely - is considered a risk and must be accounted for. This means that if the door to a robot cell is opened, for whatever reason, a contact must be tripped and the machine shut down immediately. To avoid even a theoretical failure of this safety device, dual channel switches and circuits are required. To avoid accidents caused by robot collisions, mechanical cams are used to activate position switches mounted on the robot axes, thereby limiting the robot's range of motion. These are expensive, hard automation methods that curb the efficiency of a machine that was originally intended to provide flexible and affordable automation.
Next generation robot safety
By replacing expensive mechanical safeguarding equipment with more efficient and re-configurable electronic motion safety for robots, ABB's SafeMove concept can even enhance the flexibility of ABB's robotic products.
SafeMove builds on the latest developments in redundant software, electronics-based safety technology and changes in safety regulations (ISO 10218) for reliable, fault-tolerant monitoring of robot speed and position. If a safety hazard is detected, SafeMove executes an emergency stop, halting the robot within a few milliseconds. The technology also offers new functions such as electronic position switches, programmable safe zones, safe speed limits, safe standstill positions and an automatic brake test. Combined, these allow more flexible safety setups.
SafeMove can also impose speed limits on a robot and ensure that it stays out of protective, three-dimensional zones. Alternatively, the robot can be confined within three-dimensional geometric spaces, allowing significant reductions in the size of robot installations. The result is that fences can now be moved much closer to the robot, saving valuable floor space. Axis limits can be combined logically, and work-piece positioners, linear tracks and other external axes can be further restricted without any additional effort.
In safe standstill mode robot movement is inhibited completely, yet all drives are powered and the motors are actively controlled. The purpose of this operating mode is to allow the worker to approach the robot in safety, and even to load a work piece into the gripper or carry out maintenance on the tooling without the need to shut the robot down. This not only saves cycle time when operation is resumed, it also reduces wear on the brakes and contactors needed to achieve the shut down.
In safe speed mode, the robot may even be allowed to move - completely or partially - at a speed that is slow enough to pose no threat to the worker, eliminating the need for a separating fence altogether. In combination with other supervision - such as confined space - workers and robots can now perform manufacturing tasks together - something that has not previously been allowed.
The safety of a robot ultimately relies on its ability to stop, or be stopped, when a hazardous situation arises. And stopping capabilities are determined by functioning mechanical brakes on the robot motors. SafeMove therefore contains an automatic brake test procedure, which periodically checks the mechanical brakes of the robot - something that would be very useful in a car!
Technical solution
SafeMove uses sensors that are already used for motion control to monitor the position of the motor. It then computes the robot position in a safety-rated computer that works independently from the robot controller. In addition, the sensor signals are checked for sanity. A model of the robot mechanics and extra reasoning about the nominal behaviour of the servo control loop further enhances the safety level (patent pending). Even though SafeMove is an independent computer that sits in the cabinet of ABB's fifth-generation industrial robot controller, the IRC5, from a user perspective, it is seamlessly integrated. SafeMove and IRC5 communicate over an internal network link.
Synchronisation between the safety computer and the robot controller must be checked after a power-down and at the beginning of each shift. Since this procedure can be easily combined with regular automatic tool service operations like cleaning, dressing or wire cutting, it does not normally add to the cycle time of the installation.
Process safety
Robots often handle dangerous process equipment - such as weld guns, laser heads, water jet guns, or even radioactive sources. Such equipment needs special attention in case a fault develops. It may be necessary to provide a protective enclosure around the complete robot cell that can withstand the process energy in the case of a robot malfunction. Imagine, for example, the consequences if a robot were to point an ultra-high-pressure water jet horizontally rather than downwards, and the jet is accidentally turned on. This is the kind of scenario that must be considered when planning a water jet cutting cell. Use of the SafeMove function now allows safety checks to ensure that the orientation and position of the robot tool are within a defined tolerance before the tool can be activated. During operation, the robot is monitored continuously while the tool orientation stays within the tolerance band. As soon as this tolerance is exceeded, a safe shutdown of both the robot and the process equipment is initiated. This can lead to drastic cost reduction for protective enclosures.
Access security
Most accidents with machine installations occur as a result of disabled safety equipment. Safety is often seen as an obstacle to productivity, and it lies in the human nature to make calculated risks if time can be saved. Industrial practice shows that it is difficult to keep passwords secret on the shop floor and this leaves the system open to manipulation. ABB's scientists and engineers have therefore developed and patented a mechanism that protects the safety configuration of SafeMove by a combination of an access-restricted configuration tool and a public activation code. The mechanism makes the safety configuration almost as secure as a bank account and very convenient to use.
Safe, compact, fast and flexible
By exploiting SafeMove's features, it is possible to reduce significantly the number of safety devices employed, including light curtains, safety relays, mechanical position switches, protective barriers, etc. By replacing mechanical position switches for robots and additional axes, there is no longer any need to maintain these devices, which are often exposed to severe environmental conditions and therefore have a limited lifetime. This allows robot cells to become more compact. Flexibility is increased as safety configurations can be reconfigured easily using software. Replacing broken-down robots equipped with dedicated cams and position switches used to be a lengthy procedure. Today, the time required for such repairs is significantly reduced since the safety sits in the controller and limit switches no longer exist. This can lead to more compact robots, since cam rings of the past required a significant amount of space; robots without position switches provide reductions in cost.
Planning and engineering safety
ABB offers RobotStudio, an off-line programming tool that allows the visualisation, programming and testing of a robot installation on an office PC, and the SafetyBuilder, a secure tool for parameterising and activating the SafeMove controller. The combination of these powerful tools allows the engineer to design and test the safety zones in a virtual environment during the planning phase, and later use the data for engineering and commissioning. It is possible to retrofit IRC5 with SafeMove, so that new functions can be introduced into existing IRC5 installations.
Flexible manufacturing
In the future, SafeMove will enable completely new manufacturing concepts with ABB robots. Since humans and robots are now able to work closely together, they will team up to become real colleagues. The powerful robot can present heavy work pieces to the worker, and the worker can perform tasks that are harder to automate. Or the worker can load small parts from a container box directly into the robot gripper, without the need for separating turntables, receiving fixtures or roll doors, and the robot can then do the work - perhaps even in co-operation with another robot or another worker.
For more information contact Lars Mandel, ABB South Africa, +27 (0)11 653 3100, [email protected], www.abb.co.za
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