Cutting risks in human-robot interaction

Sensing risks to prevent accidents

By Morand Fachot

Industrial activity requires tools or equipment that is capable of causing serious injuries or even fatalities if not used with due care. The introduction of automation and robots in manufacturing has greatly improved safety at work by transferring a number of hazardous and harmful tasks from humans to machines. However, it has also introduced its own set of risks for workers and operators. The use of a variety of sensors and other devices in the production chain has improved safety for industrial staff, yet there is still room for improvement in this field. The IEC is actively working to achieve this.

ABB robot in its safe enclosure (Photo: ABB)

Hazardous environment

The operation of any industrial power tool, stationary or portable, such as saws, grinders, presses or drills, entails risks. Even when used carefully, these tools may cause serious injury or death.

International data on occupational workplace injuries and fatalities is difficult to collect, let alone interpret, owing to significant divergences of health and safety regulations and the existence of different levels of industrialization. Yet statistics from the US Department of Labor give an indication of the seriousness of the problem in industrial societies. Approximately half of all occupational-related amputations in the country occur in the manufacturing sector, with stationary and portable machinery being the primary cause of the amputations.

Automation: reducing workplace risks?

As automation – the use of machines directed primarily by electronic control systems – spread into manufacturing, replacing workers for straightforward but unsafe, repetitive and unpleasant tasks, the incidence of machine-related injuries and accidents dropped.

However, automated machines introduced their own set of hazards, mainly at the point of operation, and also where components such as pulleys, belts, rods, or chains transmit energy to the machine and where moving feed mechanisms and auxiliary parts are found.

Appropriate training and the gradual compulsory introduction of protective gear and safety devices have cut risks markedly in the industrial environment over the years. Physical safeguarding mechanisms, as well as electromechanical sensors and switches that may halt operation automatically in case of danger, are some of the devices used to prevent accidents and reduce their severity when using powered equipment.

From industrial automation to robots

In contrast with an automated machine with its limited range of motions and tasks, a robot is defined by ISO (International Organization for Standardization) as "an automatically controlled, reprogrammable, multipurpose, manipulator programmable in three or more axes, which may be either fixed in place or mobile for use in industrial automation applications." The definition has been adopted by the International Federation of Robotics and many other organizations. 

Robots can be programmed to perform different jobs, react to changes in their environment and even make a limited number of choices. Although they share many of the more common safety issues with industrial automated machines, robots operate on more axes and with greater freedom to determine their work envelope (the volume of working or reaching space). They also present three major differences regarding the safety of workers: differences that concern the speed and predictability of movement and hazard zones that, unlike those for automated machines, are not fixed.

Operations safer than robot set-up, maintenance or repair

Studies in countries that use robots extensively, notably Japan and Sweden, have shown that many accidents involving robots do not occur during normal use when operators, if needed, run robots from a safe distance and area. Accidents tend to occur during programming, maintenance, testing or repair. 

Unlike automated machines, robots perform more advanced tasks, often apparently randomly and on a multitude of planes. Therefore, proper and accurate programming of robots is essential for safety. This may be done remotely by computer or directly through "personal" lead-through or walk-through teaching, with an operator physically present in the work envelope, directing a robot to scan and store values and coordinates for the work to be done. Other individuals who may find themselves close to a robot's operating range and in harm's way include materials' handlers and maintenance or repair staff.

Keeping a safe distance

For humans, the main hazards associated with robots include being struck by the moving part, being trapped between a moving part and a machine or surface, being hit by an object or material dropped or ejected by a robot, falling from the equipment or structure and exposure to dangerous levels of heat or electricity. Robots may also malfunction following software failure and error or electromagnetic and radio-frequency interference. It can be inferred from all this that keeping humans away from a robot's work envelope and protected will be the safety option of choice.

As a general rule, the design of robot workcells comprises a number of safeguarding systems aimed at keeping humans at a safe distance. These may include an external physical barrier (a secure fence or enclosure) around the workstation perimeter and a variety of sensors to detect the presence of humans in restricted or dangerous locations within that perimeter and to indicate unsafe or potentially unsafe operating conditions or events. Sensors may be installed to shut the robot down completely upon detection of an intruder in the perimeter.

Safety sensor systems cover all areas and at least three levels of hazards. The first level of system identifies perimeter penetration, the second detects a presence inside the workcell and the third a presence within the immediate vicinity of the robot.

Sensors, switches and other safety devices

Different types of sensors are used to ensure the safety of operators close to robots. They are linked to other devices that can activate alarms and shut down operations if necessary. The most common are:

  • Pressure sensitive sensors placed in floor mats within the workcell perimeter that react to the weight of an individual and can be used to detect intrusion in the perimeter and inside the workcell
  • Light curtains consisting of photoelectric barriers of several aligned beams between emitting and receiving columns. Interrupting a single beam will trigger the emergency stop for any machine. Different resolutions permit intrusion of a finger, hand, limb or body. These photoelectric sensors can also detect the presence of an unauthorized individual in the restricted zone. IEC International Standards for electro-sensitive protective equipment apply to these devices
  • Key-operated safety switches and emergency stops and switches
  • Other common safety measures and devices include the presence of a workcell controller who can shut down the robot and activate warning alarms, and the installation of flashing lights of different colours, repetitive beepers and continuous horns as well as emergency stop/live-man switches.

IEC standards central to automation and robotics safety

Many IEC TCs (Technical Committees) and SCs (Subcommittees) prepare International Standards for the devices mentioned and relating to the safety of automation and robotic systems. They include, among others, TC 65: Industrial-process measurement, control and automation, which developed the 61508 series on Functional safety of electrical/electronic/programmable electronic safety-related systems; TC 17: Switchgear and controlgear, working on safety and emergency stops and switches; TC 44: Safety of machinery - Electrotechnical aspects; SC 47E: Discrete semiconductor devices, which develops International Standards for pressure sensors; and TC 79: Alarm and electronic security systems.

No technical silver bullet

However safe automated and robotic systems have become over the years, following major technical improvements, the possibility of component failure or malfunction still exists. The weakest link in the industrial safety chain is likely to remain the human factor: most occupational accidents result from unsafe actions by workers. The cause may be insufficient training of operators, careless or improper actions by staff, incorrect programming or other activities.

To improve safety and minimize the risk of accidents it is crucial to understand ergonomics and be familiar with human factor issues. Training is also essential. Industrial staff who work with robotic systems must be thoroughly trained and understand that they are inherently unpredictable: if a robot is not moving, it does not mean that it is not going to do so; if the robot is repeating patterns, it should not be assumed that it will continue to do so.

Improving safety in industrial automation and robotics is, and will remain, a combination of technical solutions and human factors.

abb_robotized_assembly_cell ABB robot in its safe enclosure (Photo: ABB)
guillotine_01 Cutting machine with physical barrier, emergency stop and two-hand switch
safety_light_curtain_lrg Safety light curtain