Fiat’s operatic TV ad for its all-new Strada model of the late-70s alerted a wide-eyed public to the future of robot-intensive manufacturing. In the ensuing four decades, robots have become commonplace in a wide variety of industrial scenarios. Always separated from humans, for safety’s sake, they have worked as distinct entities: not to be approached except under special circumstances.
The robots are now coming out of their cells. Driven by the imperative to compete economically, manufacturing businesses are deploying a new type of robot on today’s factory floor. Also, workers familiar with smart technologies in the gadgets they use every day are ready to interact with the emerging species.
The collaborative robot, or “cobot” is emerging as a powerful new industrial tool. The term encapsulates the notion that robots and humans working together can deliver the best of both worlds: the speed and repeatability of robotic assembly with the adaptability of trained humans.
As OEMs come under increasing pressure to increase automation in order to satisfy extremely high-quality expectations at the right price, collaboration between robots and humans working side by side on the production line can eliminate unwanted variabilities without losing the human touch needed to complete complex assembly tasks, individualize products where needed, or deal with exceptional events. Collaborative robots can also assist operators in space-constrained workplaces such as laboratories, which can benefit from automating tasks such as dispensing or loading/unloading but are unable to accommodate the guarding needed for a conventional robot.
Traditional industrial robots are typically seen as being expensive, custom-designed systems that are affordable only for a relatively few large businesses with high-volume production needs. In contrast, collaborative robots can be affordable, generic, programmable devices that can be “taught” to perform simple processes such as light assembly. Humans working alongside can be relied upon to carry out related tasks that would otherwise be too expensive to automate.
Figure 1: Six-joint collaborative robot ready to be fitted with a tool such as a gripper. (Picture F&P Personal Robotics)
Collaborative robots in the market today tend to be arm-type robots that are articulated at several points to provide a wide range of motion, such as the six-jointed robot shown in Figure 1. Typical capabilities give such robots a maximum reach from about 300 mm to 1.5-1.8 meters for larger systems, with a maximum payload from 2-3 kg to about 10-15 kg. On the other hand, ABB’s dual-arm YuMi® (Figure 2) has a more human-like appearance.
Figure 2: YuMi is a lightweight, desktop collaborative robot with padded arms for safety.
As collaborative robots become more widely deployed in the industry, use cases continue to evolve. These may range from straightforward execution of a single, simple process, to interacting extensively with a human to complete a complex set of tasks by working together.
Safety imperative and standards
Long-term successful integration of collaborative robots in the workplace is critically dependent upon safety. Any type of physical discomfort caused to a human by a robot classed as safe for unguarded operation may include financial liability on the part of the robot owner or supplier, and could damage confidence in the concept of collaborative robotics.
The major international safety standards applicable to collaborative robots are ISO 10218-2011 parts I and II, which cover Robots and Robot Systems & Integration respectively, and the technical specification ISO/TS 15066. ISO/TS 15066 provides guidance on how to apply the ISO 10218 standards, such as assessing risks and determining suitable force and speed limits.
ISO 10218-1 defines collaborative operation as the “state in which purposely designed robots work in direct cooperation with a human within a defined workspace”. The standards cover various means of collaborative working, such as exchanging items through a window, or interacting intensively within a common workspace. According to ISO 10218-1, a collaborative robot must satisfy at least one of four criteria to ensure human safety, as shown in Table 1.
|ISO 10218-1 clause||Safety feature||Typical application||Separation distance||Main risk reduction|
|5.10.2||Safety-rated monitored stop||Loading/unloading||Small/zero||No motion in presence of human|
|5.10.3||Hand guiding||Assembling, filling||Small/zero||Motion only by direct operator input|
|5.10.4||Speed and separation monitoring||Handling, inspection||Safety-rated monitored speed/distance||Prevention of contact|
|5.10.5||Power and force limiting||Collaborative assembly, loading||Small/zero||Inherent design, or control, to prevent excessive force|
Table 1. Collaborative robots must satisfy at least one of the ISO 10218-1 safety clauses.
Sensing for safety
A variety of sensors are needed to implement features such as safety-rated stop, speed and separation monitoring and force limiting.
There are various means of detecting the presence of a large object, such as the human body, in proximity to the robot. A capacitive sensor such as the Omron E2KQ has a sensing range of a few millimeters, and can be inserted in a 13 mm-diameter aperture. Alternatively, an optical sensor such as the Panasonic RX-LS200 has a longer detection range of up to 200 mm, and has response time of less than 1 ms. It is housed in a 45 mm x 35 mm x 14 mm die-cast zinc-alloy package.
If monitoring of the separation distance is to be employed, to adjust robot speed, a time of flight sensor such as the STMicroelectronics VL6180X may be suitable. Distance to an object is calculated at high speed by measuring the time taken for the emitted IR signal to be reflected and received by the built-in sensor. The VL6180X has a range up to a few centimeters, and accuracy is unaffected by the reflectivity of the detected object. The sensor can be connected to a microcontroller via I2C, and comes with an Application Program Interface (API) consisting of a set of C functions that facilitate control by the host application.
Power and force limiting are intended to prevent the robot from exerting enough force to harm a human within the robot’s range of motion. Risk assessment is needed in order to determine suitable limits: if the robot is carrying a sharp object, a very low force limit may be needed to prevent injury resulting from contact.
Inherent safety features of collaborative robots are likely to include measures such as rounded contours that effectively spread the force of any impact, and soft, conformable shells. The robot shown in Figure 1 is fitted with a soft cover that features a synthetic leather skin. This not only protects human workers, but can also prevent damage to critical robot mechanisms and control modules.
Although a soft shell may mitigate any injury, force sensing is needed to activate emergency stop and will be a critical aspect of the collaborative robot’s safety control systems. This may be implemented in various ways, such as detecting excessive motor torque by sensing rotor position and current. If the motor encounters an obstruction, the current will increase above a predetermined threshold causing the system to turn the motor off. This approach requires minimal additional components, since rotor-position sensors and current-sensing resistors are usually a part of the motor-control system. On the other hand, software or logic may be needed to enable the system to distinguish between normal changes in motor torque when handling normal loads and an increase in torque due to obstruction by a human co-worker.
Alternatively, force could be measured directly using one or more sensors embedded in various parts of the robot such as gripper fingers. Honeywell’s FSS series surface-mount force sensors, including the FSS1500NGT with sensing range of 0-1.5 kgf, feature a piezoresistive micromachined silicon sensing element. The force to be sensed is applied to this element, causing a change in the resistance of the sensing element. These sensors benefit from high mechanical stability, as well as low sensitivity to mounting stresses. The electrically ratiometric output generates a voltage of up to 360 mV depending on the force applied. Sensors such as the FSS1500NGT are already widely used for controlling robot end effectors.
Collaborative robots are becoming established throughout the industry; particularly where the opportunity to reduce operating costs and drive up quality and productivity make economic sense for manufacturers. Safety standards are already in place, and guidelines are emerging to aid risk assessment and help determine practicable limits such as speed and force. Ensuring human safety will be critical to establishing industry-wide confidence in the concept of the collaborative robot.