Industrial robots are used in factories because they provide an effective alternative to manual workers for repetitive, high-volume assembly line tasks. Machines can repeat high-precision tasks for many years with only the occasional interruption for routine maintenance. The initial high capital investment is repaid by boosted productivity.
But relatively low-cost human workers remain the best option for low-volume, high-mix, intricate assembly work because they are dexterous, flexible and able to solve problems that bring machines to a halt. Collaborative robots, or ‘cobots’, the lightweight, compact and relatively inexpensive cousins of full-size industrial robots, are now being introduced to combine the advantages of both robots and humans. However, because they share the workspace with humans, new engineering techniques are required to keep the workers safe while maximising productivity.
Sharing the workspace
Collaborative robots fill a niche in manufacturing when the product mix is consolidating and volumes are increasing but not to the extent that justifies full automation. The robots can do the routine, repetitive actions while the humans work on the intricate tasks and intellectual challenges of the process.
Collaboration is not a natural function of traditional industrial robots. The International Organisation for Standardisation defines an industrial robot as “An automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes, which can be either fixed in place or mobile for use in industrial automation applications”. This description fits a machine designed for maximum productivity without human assistance.
Since the introduction of industrial robots in the 1970s, the safe automation of high-volume applications has required a division on the factory floor. Workers are kept well away and the machines are enclosed behind barriers to eliminate the dangers posed by heavy, rapidly moving mechanical parts. Basic external sensor technology instigates an emergency stop when someone or something crosses a beam or triggers a switch by opening the barrier. When a technician intentionally enters a robot’s operating space for maintenance or reprogramming, the machine is powered down with its arm(s) locked in a safe position.
Maximising speed, strength and precision remains important for collaborative robots, but to maximise the advantages of collaborative working, humans and robots need to work in harmony. To justify the introduction of a collaborative robot, it must cost no more than the equivalent for human labour. A robot that is moving parts into position and adding quick-drying adhesive is of little value if a human coworker still has to fit together the previous two or three workpieces. More importantly, robots must constantly sense where the humans are positioned, how they are moving and the force they’re applying when contact is made (whether intentionally or unintentionally) to ensure safe working.
Guidelines for the design of collaborative robots
Key factors in the design of collaborative robots relate to the fact that the machine and a human share the same workspace. The designer needs to ensure not only that efficiency is high but also that the robot constantly senses the sometimes unpredictable movements of its coworker and can react safely. The designer also needs to ensure that the robot doesn’t apply excessive force if there’s intentional or unintentional contact between itself and the human. This adds complexity, because, unlike industrial robots in which safety systems are not an intrinsic part of the robot, collaborative robots contain safety systems that are generally integrated into its own structure and controlled by its own systems.
Fortunately, guidance on these design challenges comes in the form of international safety standards for collaborative robots, which have been developed in parallel with the rapid introduction of these robots in the workplace.
For example, the International Organisation for Standardisation provides some guidelines for designing collaborative robots in its standard ISO 10218, and its technical specification ISO/TS 15066 (which is a voluntary document and not a standard, but is expected to form the basis of a standard in the future) focuses on their safety. ISO/TS 15066 highlights the importance of safety-related control system integrity with regards to controlling process parameters such as speed and force.
ISO/TS 15066 also provides general information for a collaborative robot designer to use, such as information explaining the need for a risk assessment of hazards in the workspace. For example, even the best robot design can’t be considered safe if it allows the robot to wave a sharp object around, or a workspace could be dangerous if it’s closed in by fixed objects that cause a worker to become trapped then crushed by robot movement.
The key sections of ISO/TS 15066 address the design of workspaces and a robot’s operations, and the transitions between its collaborative and non-collaborative operations. Specifically, it provides extensive details for implementing the following requirements for collaborative operation, which creates safe, efficient solutions that fulfill design objectives.
Safety-rated monitored stop
A safety-rated monitored stop is an assured robot stop without removing power and occurs when a human worker enters the collaborative workspace. The system ensures that a robot and human don’t move at the same time and is primarily employed when the robot is rapidly moving heavy parts through the workspace.
Before a hand-guided operation can start, a robot must perform a safety-rated monitored stop. During the operation, a worker is in direct contact with the robot arm and can utilise hand controls to move it. This operation is used for lift assists or highly variable tool applications.
Speed and separation monitoring
This collaborative work method is perhaps the most relevant, as it allows an operator and robot to move simultaneously within the workplace by equipping the robot with sensors to monitor how close the worker is. At large separations, it continues to operate at medium speed, but on closer approaches, it reduces its speed, and at very close approaches, it comes to a complete safety-rated monitored stop.
Power and force limiting
Power and force limiting are required in applications where there could be intentional or unintentional contact between a collaborative robot (or any workpiece) and a worker when both are working in the collaborative workspace. Contact can either be quasi-static, such as clamping part of a worker’s body between a robot’s manipulator and a fixed object, or transient, such as knocking into a part of a coworker’s body where the worker is able to recoil.
Design safety challenges
With some adaptations to limit cost, size and complexity, collaborative robot designers can employ existing industrial robot technology for some systems while still implementing the work methods described above. For example, a safety-rated monitored stop is an established technology for industrial robots that uses safety barriers to implement an emergency stop when a human enters the operational space.
Considering that industrial robots are designed to come to a dead halt when a human enters the work zone, speed and separation monitoring demands new engineering techniques. Collaborative robots will keep moving, albeit at a reduced speed, when workers are sharing the workspace, unless an approach is close enough to trigger a safety-rated monitored stop. Key to implementing such systems is integrating sensors into the robot’s control systems so that the closed-loop feedback enables rapid motor response when speed reduction is necessary.
But the most difficult design challenge is limiting power and force. Designers can learn little from industrial robot design because its emphasis is on load capacity and speed. An annexure to ISO/TS 15066 offers help by suggesting limits to quasi-static and transient forces for pain thresholds as well as minor, reversible and irreversible injury thresholds for humans. Transient force thresholds can be twice as high as quasi-static ones, because they occur within a shorter timeframe and the worker is able to recoil.
While research continues on pain and injury thresholds, the present guidelines recommend lowering clamping risks by reducing a robot’s speed to less than 250mm/s and its force to less than 150N during speed and separation monitoring operations, while transient forces can be twice as high but must not be applied for longer than 500ms.
Meeting these thresholds is challenging. For example, a 2kg robot arm carrying a 0.5kg load and moving at 1m/s must decelerate at 60m/s2 to limit its crushing force to below 150N if unintentional contact occurs. In that time, the arm will travel 8mm, which is acceptable for collaborative operation. An identical robot arm carrying a 3kg load would need to decelerate at 19m/s2 to limit its crushing force to less than 150N, during which time it will have travelled 27mm (which is acceptable with padding). This illustrates that the robot designer must consider the differing dynamic forces generated by collaborative robots with different payload and speed of movement capabilities.
Other advice in the ISO guidelines includes: eliminating pinch and crush points on the cobot, reducing its mass and inertia, reducing its velocity when it approaches a fixed surface, so it can stop quickly, increasing the surface area of contact points and organising the layout of the workspace to limit clamping points and allow recoil after transient collisions.
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