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Collaborative Robots: Designing for Productivity & Safety

Image Source: hvostik/stock.adobe.com

By Steven Keeping for Mouser Electronics

Edited April 22, 2021 (Originally Published August 28, 2015)

Industrial robots are commonplace in factories because they provide an effective alternative to manual workers for repetitive, high-volume assembly line tasks. Machines can continuously repeat high-precision tasks for many years with only the occasional interruption for routine maintenance. Boosted productivity ensures a return on the initial high capital investment.

But relatively low-cost human workers remain the best option for low-volume, high-mix, intricate assembly work because they are dexterous, flexible, and can solve problems that would grind a machine to a halt. Collaborative robots–the lightweight, compact, and relatively inexpensive cousins of full-size industrial robots–are now being introduced to combine the advantages of robots with the assets of humans. However, because collaborative robots share the workspace with humans, new engineering techniques must maximize productivity while keeping the workers safe. Let’s examine how cobots and humans can collaborate as co-workers.

Sharing the Workspace

Cobots fill a niche in a manufacturing environment where the product mix is consolidating, and volumes increase but not to the extent that justifies full automation. Cobots can do the picking of parts, lifting and fetching, and repetitive, routine actions while humans work on the intricate fabrication and intellectual challenges of the process.

Collaboration is not a natural extension for traditional industrial robots. The International Organization for Standardization (ISO) 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.” The description fits a machine purpose-designed for maximum productivity without human assistance.

It's not surprising that from the introduction of industrial robots in the 1970s, a division on the factory floor has remained a requirement for the safe automation of their high-volume applications. Today, workers are kept well away, and the machines are enclosed behind metal barriers to eliminate the hazards posed by rapidly moving and heavy mechanical parts (Figure 1). Basic external sensor technology provokes an emergency stop of the industrial robots when someone or something crosses a beam or triggers a switch by opening the barrier. When technicians do intentionally enter the robots’ operating envelopes for maintenance or reprogramming, the machines are powered down with their arms locked in a safe position.

Figure 1: Industrial robots operate behind safety barriers. (Image Source: Ridvan/stock.adobe.com)

Maximizing speed, strength, and precision remain important for cobots, but to maximize the advantages of collaborative working, humans, and robots need to work in harmony. To justify the introduction of a cobot, it must cost no more than the equivalent for human labor. A robot that moves parts into position and adds quick-drying adhesive is of little value if a human coworker still has the previous two to three work pieces to fit together. But more important than that, cobots must be constantly aware of where 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.

The key design objectives for cobots can be summarized as achieving:

• Safe interaction with human workers and delicate assembly equipment
• Reduced costs to justify the use of robotic labor applications
• Robotic operations at a rate compatible with human capabilities
• Clean and low-noise operations
• Compact and light form factors
• Simple and fast programming by non-expert workers to cope with high-mix production

Cobot System Design Guidelines

Key factors in cobot design relate to the fact that the machine and human share the same workspace (Figure 2). The designer needs to ensure that efficiency is high and that the cobot is constantly aware of the sometimes-unpredictable movements of its coworker and can react safely. The designer also needs to ensure that the cobot doesn’t apply excessive force if intentional or unintentional contact is made between itself and the human. This adds complexity. Unlike industrial robots in which safety systems are not an intrinsic part of the robot, cobots contain safety systems that are generally integrated into their structure and controlled by their systems.

Figure 2: Collaborative robots share the same space as coworkers. (Source: hbrh/stock.adobe.com)

Fortunately, guidance on these design challenges comes in the form of international safety standards for cobots, which have been developed in parallel with the rapid introduction of these robots in the workplace. For example, the ISO provides some guidelines for designing cobots in its ISO 10218 document. A technical specification (TS) created by the organization, the ISO/TS 15066, focuses on the safety of cobots. ISO/TS 15066 highlights the importance of safety-related control system integrity regarding controlling process parameters such as speed and force. (Note: ISO/TS 15066 represents a voluntary document and is not a standard. However, it is expected to form the basis of a standard in the future.)

ISO/TS 15066 also provides general information for a cobot 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 around a sharp object with its manipulator. In another example, the 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, design of a robot’s operations, and the transitions between a robot’s collaborative and non-collaborative operations. Specifically, the document provides extensive details for implementing the following collaborative-operation requirements, which creates safe, efficient solutions that fulfill the design objectives mentioned:

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 the 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 utilize 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 the operator and cobot to move simultaneously within the workplace by equipping the cobot with sensors to monitor the worker’s proximity. At large separations, the cobot continues to operate at medium speed, but upon closer approaches, the cobot reduces its speed. 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 intentional or unintentional contact occurs between a cobot (or any work piece) and a worker when both are in the collaborative workspace. Contact can either be quasi-static, such as the clamping part of a worker’s body between a cobot’s manipulator and a fixed object, or transient, such as the knocking into a part of a coworker’s body where the worker can recoil.

Design Safety Challenges

With some adaptations to limit cost, size, and complexity, cobot designers can employ existing industrial robot technology for some systems while still implementing the work methods previously described. For example, the 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 envelope.

Speed and separation monitoring demands new engineering techniques considering industrial robots are designed to come to a dead halt when a person breaches the work zone. In contrast, cobots will keep moving, albeit at a reduced speed, when workers share the workspace unless an approach is close enough to trigger a safety-rated monitored stop. The key to implementing such systems is integrating sensors into the cobot’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 power and force limiting. Designers can learn little from industrial robot design because its emphasis is on load capacity and speed. An annex for ISO/TS 15066 offers help by suggesting limits to quasi-static and transient forces for pain thresholds and 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 can recoil.

Although research continues on pain and injury thresholds, the present guidelines recommend lowering clamping risks by reducing a cobot’s speed to less than 250mm/s and its force to less than 150N during speed and separation monitoring operations. However, 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 traveled 27mm (which is acceptable with padding). This example illustrates that the robot designer must consider the differing dynamic forces generated by cobots with different payload and speed of movement capabilities.

Other advice in the ISO guidelines includes:

• Eliminating pinch and crush points on the robot
• Reducing robot inertia and mass
• Reducing robot velocity when it approaches a fixed surface so that it can stop quickly
• Increasing the surface area of contact points
• Organizing the workspace layout to limit clamping points and to allow recoil after transient collisions

Case Study: The Cobot Joint

A major challenge in cobot design is engineering lightweight, compact joints that can quickly react to forces acting on the manipulator—such as impact with a coworker—to eliminate the risk of injury.

Harmonic gears are finding favor for small robots because they enable designers to reduce joint size and weight compared to conventional mesh gears (Figure 3). However, because harmonic gears use a flexure to transmit motion between input and output, the joint exhibits low rotary stiffness compared with a mesh gear alternative.

Figure 3: Small robots use harmonic geared joints to reduce size and weight. (Source: Tatiana Shepeleva/stock.adobe.com)

A lack of stiffness presents a problem for cobot designers because the preferred method for detecting impact between a human and robot is through the change created in motor current, that is, because of a proportional change in motor torque caused by the force generated by the impact. But a lack of stiffness causes the force to wind up the slack in the joint before it has any effect on the motor torque. The result of this is a time lag just before the controller detects an increase in motor current and can respond to the impact by slowing, stopping, or reversing the manipulator. Such a delay could cause the coworker to be subject to a greater-than-recommended transient impact time of 500ms and a maximum impact force of 300N.

A mechanical solution is to use a larger harmonic gear to improve stiffness, but that increases the size and weight of the robot joint. An alternative is to use dual high-resolution encoders and a software algorithm. Such a solution will incur a small cost increase, but it won't increase the dimensions of the joint or raise its weight.

Encoders on the input and output sides of a joint will provide the controller with a real-time measure of any lack of stiffness-induced rotary deviation between the actual and programmed positions of the robot. The controller can rapidly compute the first-order compensation for an error, removing the slack from the system and ensuring that intentional or unintentional impact on the manipulator is immediately detected by increased motor torque.

Conclusion

Cobots make their mark in workspaces shared with humans as combining robot muscle with human dexterity, and problem-solving skills is dramatically improving productivity. Factory managers are just recently beginning to appreciate the number of assembly applications—currently performed solely by human labor—that cobots can perform. That’s why the impact of cobots is predicted to increase, with growth expectations roughly set to equal the size of today’s entire quantity of industrial robotics by 2025.

But it's still early for the technology, and engineers are now learning that only some of the design techniques used in engineering industrial robots are truly applicable for their collaborative cousins. A new design methodology is required to ensure that cobots remain safe around coworkers while still bringing speed, precision, and load-handling benefits to the job.

Designing cobots is a nascent discipline with little guidance to draw upon. But international safety standards for cobots are being developed in parallel with introducing the first wave models into the workplace. The ISO 10218 standard provides specific guidelines for cobots, while ISO/TS 15066 establishes safety parameters for collaborative operations. Suppliers are doing their part by teaming electronics and sensors up with advanced mechanical assemblies to create new critical components, such as specialized joints specifically engineered for the unique demands placed on cobots during everyday duties, operations, and interactions.

Steven Keeping gained a BEng (Hons.) degree at Brighton University, U.K., before working in the electronics divisions of Eurotherm and BOC for seven years. He then joined Electronic Production magazine and subsequently spent 13 years in senior editorial and publishing roles on electronics manufacturing, test, and design titles including What’s New in Electronics and Australian Electronics Engineering for Trinity Mirror, CMP and RBI in the U.K. and Australia. In 2006, Steven became a freelance journalist specializing in electronics. He is based in Sydney.