Collaborating robots

Introduction

A growth in the number of manufacturing workplaces at which people work very closely with robots is expected in the coming years ^[1]. By combining human abilities and dexterity with the precision, strength and speed of the robot, highly efficient production can be achieved while reducing the workload of the persons involved. In the safety strategies applied, occupational safety for the persons involved must be ensured by designing the workplace and particularly the robot system in conformity with the relevant standards ^[2][1][4]. Typical OSH issues are: Mechanical hazards and Ergonomic hazards.

Workplaces with collaborative industrial robots

This article focuses upon collaborative robots which operate in alongside human operators in the industrial sector. Similar applications exist in the non-industrial sector; these are subject to certain other requirements and will not be addressed here. This article describes the typical hazards associated with collaborative robots and the concepts for accident prevention and health protection. Special robot applications such as those for welding or laser processes require consideration to be given to further hazards; these will not be described here.

Collaborative application design

A collaborative robot shares a common workspace with a human operator in order to carry out, in conjunction with the operator and at the same time, a previously defined task. This collaborative task may have ergonomic benefits for the operator, such as reduced working loads, improved body posture or fewer repetitive movements. The collaboration may be of extended duration, as for example during joint production of assemblies, or be limited to short phases, as for example with the transfer of workpieces to the robot. A range of methods for collaboration exist for different tasks. Common to all is that all necessary protective measures must be active. The robots used must be specially designed for collaborative operation and must comply with the international standards set in the EN ISO 10218 series ^[2][3]. Physical contact may arise between persons and collaborative robots and this needs to be managed in order to protect the person.

Standardization

For collaborative industrial robots the European Machinery Directive 2006/42/EC ^[5] applies. The harmonized two-part EN ISO 10218 series of standards was developed for the specific hazards presented by industrial robots. EN ISO 10218-1:2011 ^[2] describes how safe robots may be designed and constructed. The integration of a particular robot system into a working environment also influences the safety of the industrial robots during their use. For this purpose, EN ISO 10218-2:2011 ^[3] provides instructions for protection of the personnel during integration of the robots, their installation, functional testing, programming, operation, maintenance and repair. In Europe, EN ISO 10218-1/2 superceded European standard EN 775 in 2008. For detailed requirements governing collaborative robots, EN ISO 10218-2 refers to the ISO/TS 15066 specification ^[4], which is currently under development.

Hazard identification and risk assessment

Robots possess particular characteristics that differentiate them from other machines and which must be addressed by risk assessments:

• They perform movements involving high energy levels and extending over a large operating space.
• It is difficult to determine in advance when a movement will be initiated and what path it will follow. The movements may vary according to operational requirements.
• The robot's operating space may overlap with the working areas of other robots and machinery.
• Persons must work within the operating space of the robot whilst its drive elements are energized.

Where a collaborative robot is used, consideration must be given to the anticipated unobstructed accessibility by persons during collaboration with the robot. This may result in contact between the human operator and the robot. This may be intentional during performance of the work task, or be a result of foreseeable misuse, owing for example to guards having been replaced by other protective measures. It is therefore particularly important that the collaborative task be described and specific hazards identified from the description. Collaboration may be characterized by:

• Frequency and duration of presence of operators or other workers in the collaborative space whilst the robot is energized
• Frequency and duration of contact between operators and the energized robot (e.g. hand guiding, handover of the tool or workpiece)
• Restarting of the robot following violation of the minimum separation distance or following contact
• Switching of the robot between collaborative and autonomous operation when the operator enters or leaves the collaborative space
• Collaboration with several operators simultaneously
• Energise robot arm involuntary movements
• Highest incidence of contention between human and robot is when both are accessing the same object

As with other machines, the hazards must be identified within the boundaries of the robot system, and the risks assessed. This is described in ISO 10218, ISO 12100 and other relevant standards. Mechanical hazards such as clamping, crushing, shear, impact and piercing typically occur (see Annex A of EN ISO 10218-1 ^[2]).

Suitable measures for risk reduction must then be taken:

1. Design measures for the elimination or reduction of hazards
2. Technical measures for the protection of persons against hazards and unacceptable mechanical effects
3. Provision of supplementary protective measures such as user information, warning instructions, training, personal protective equipment, etc.

The collaborative robot includes the robot itself, the end-effector and the objects moved by it. The end-effector is the tool adapted to the robot arm, with which the robot performs tasks. This gives rise to the possibility of collision with the tool and the workpiece and a risk of crushing against parts of the working area and other hazards as indicated in Annex A of EN ISO 10218-1 ^[2]. The risk assessment must therefore also cover the intended industrial workplace and the work task; the robot cannot be adequately assessed in isolation. In addition to the conditions typical of the machine, the risk assessment must consider the following: the characteristics of the robot, the hazards presented by the end-effector and the workpiece, the position of the operator and the path travelled by the robot in relation to the working environment, other machines, parts of building structures, etc.

Requirements for collaborative robot operation

Robots for collaborative operation must satisfy the requirements of EN ISO 10218-1 including functional safety. Functional Safety is achieved when every control based protective measure – the so-called safety function - is carried out and the level of performance required is met. Safety-related parts of the robots control system must satisfy

• EN ISO 13849-1 ^[1] at Performance Level "d" with Category 3 or
• EN 62061 ^[7] Safety Integrity Level 2 with a hardware fault tolerance of 1.

This particularly applies to the safety functions described below for the various collaboration methods.

Hazards arising during collaboration can be avoided by reduction of interaction and of the collaborative space, to the extent reasonable in consideration of the task. Any unnecessary approaching of the robot must be avoided. Persons not involved in the task should be prevented by guards (e.g. fixed or interlocking guards, trip devices, fences) from accessing the collaborative space. Entry by a person into the guarded area around the robot beyond the collaborative space must cause immediate stoppage of the robot.

The collaborative space must be clearly defined and demarcated. In order to provide sufficient space for the robot to move, adequate clearance must be ensured and suitable protective equipment provided in order to prevent crushing against parts of building structures or machinery. The alternation between autonomous and collaborative operation is particularly critical for operating personnel, because the robots movements may change without an intermediate stop.

Safeguarding

During collaborative operation, operating personnel are protected by a safety function or a combination of several safety functions. The operator must be able to stop the robot movement or to end collaborative operation by a single action. Protective measures must be available to all operating personnel within the collaborative space. The following safety functions are typical:

• Stop functions: each collaborative workplace must be equipped with an emergency-stop and a protective-stop function for the robot.
• Enabling device: a robot movement occurs only following an enabling action by the operator (pushing a button for example during hand guiding).
• Limited collaborative speed: a maximum permissible robot speed is determined by the risk assessment. This limit value is stored in the control system, either as a fixed value or as a variable dependent upon the distance from the operator, and is monitored continually.
• Minimum separation distance: a sensor system continually detects the position of the operator and his or her distance to the robot. The required safe distance is typically determined dynamically from the relative speeds of the operator and the robot, the response times of the brake and control system, and the accuracy of measurement of the sensor system or the robot. If this safe distance is violated, a protective stop occurs.
• Limited force/pressure: a sensor system on or in the robot detects a contact between the operator and the robot and stops in order avoid exceeding the allowable force or pressure. A protective stop is then initiated and possibly reversal of the robot movement to enable the operator to free him or herself.

Methods of collaborative working

For safe collaborative operation, one or more of the following methods and safety functions must be selected as appropriate. Whenever a failure of the safety functions is detected, a safe stop must be initiated. Autonomous operation of the robot following a stop may be resumed only following a deliberate restart executed by the operator from outside the collaborative space.

Safety-rated monitored stop

With this method, simultaneous movement of the operator and the robot is not permissible. The robot functions autonomously until a person enters the collaborative space. The robot must then perform a stop. The stop is monitored by the control system in order to detect control failures. The operator can now work directly at the robot, and for example chuck a workpiece. As soon as the operator leaves the space, the robot can continue its autonomous movement. When the robot is working outside the collaborative space whilst the operator is within it, the robot must not penetrate this space. Movement into this space must again cause the robot to be halted with a stop at the boundary of the collaborative space.

Hand guiding

With this method, the operator and the robot can move simultaneously within the collaborative space, and also work closely together. This may be the method selected when performing some maintenance tasks. The robot however must not move autonomously, and must instead be guided manually by the operator. Hand-guided operation of the robot may be started when the robot is in the safety-rated monitored stop status at a transfer position. In order to position the robot manually, the operator must have a guidance facility (push buttons, joystick) with an emergency-stop feature. The operator must have a clear view of the collaborative space and of the robot's movements. Safe limits are imposed upon the robot's speed and position. The position and posture of the operator and the guidance facility itself must not give rise to additional hazards. As soon as the guidance facility is released by the operator, the robot must stop again. As soon as the operator leaves the space, the robot can continue its autonomous movement.

Speed and separation monitoring

With this method, the operator and the robot can move simultaneously and autonomously within the collaborative space but cannot work closely together. The risk is reduced by a permanent safe distance between the operator and the robot. This safe distance can be determined dynamically from the instantaneous speed of the robot. At low speeds, the safe distance may be reduced. The maximum robot speed, the minimum distance and other parameters must be determined by the risk assessment. If the safe distance is violated (too low), the robot comes to a halt. As soon as the operator moves away from the robot, the robot can once again move autonomously, with assurance of the minimum separation distance. The safe distance can also be assured by adoption by the robot of an alternative path by which it avoids the operator in the collaborative space. This safety function is based upon the position of all persons within the collaborative space being reliably detected.

Power and force limiting

With this method, the operator and the robot can move simultaneously and autonomously within the collaborative space and work so closely together that they may come into contact with each other. This risk is reduced by limitation of the force and power of the robot system in the event of contact occurring. The limit values for allowable force and pressure are determined and verified by a risk assessment performed for the specific workplace in question.

This method is limited to robots of special design employing safe materials, soft surfaces, cushioning and the absence of sharp edges. The reason is that it mostly needs a contact to trigger the protective-stop function. Therefore, all robot movements should be logical and predictable for the operator in order to reduce the probability of unintentional contact.

Biomechanical requirements

The risk assessment determines the body regions (head, trunk, extremities) described in ISO/TS 15066 ^[4] which may come into contact with the robot during intended collaboration. Such contact may result only in strain upon the skin and the underlying connective and muscle tissue which does not lead to deeper penetration of the skin and the tissue with bleeding trauma or to fractures or other permanent damage to the skeletal system. In addition, no injuries may occur exceeding injury severity Category 1 of the Abbreviated Injury Scale ^[8] (AIS1) and the injury severities with the codes for superficial injuries of ICD-10-GM 2006] ^[9]. Should a risk of injury to the eyes, ears, nose and mouth (sensory organs) exist during use, special protective measures (such as goggles) must be taken to reduce it.

The injury severity is adequately addressed with respect to all individual regions of the body by the following criteria for a range of mechanical hazards (impact, clamping, etc.):

• Clamping/Squeezing force (unit: N)
• Impact force (unit: N)
• Pressure per unit area (unit: N/cm²)

The tolerable range of injury severity for a given individual region of the body is not exceeded when the limit values for the injury criteria are observed. A single, uniform limit value does not exist; instead, ISO/TS 15066 contains a table of limit values for a total of 15 individual body regions, with differing limit values for pressure and force during trapping/crushing for the contact area between the robot and the person.

Within standardization, the use of a two-stage risk concept employing two limit values is also under discussion. The limit for the tolerable injury severity described above relates to undesirable, very infrequent collisions which must not occur during collaboration as intended. In the event of foreseeable misuse of the robot systems, such collisions are quite possible, owing to the absence of guards.

For the contact which may occur more frequently during the intended collaboration, the limit for the acceptable mechanical impact must be set substantially lower: to the so-called pain level. The pain level in this context is defined as the transition from the perception of pressure to the perception of pain during contact. This limit must also be reviewed at each individual workplace by means of a suitable force and pressure measurement system that resembles the human body in its characteristics.

Organizational requirements, issues of occupational medicine

The suitability, in terms of their health, of persons who work with a collaborative robot and are exposed to a risk of collision should be determined at suitable regular intervals. These persons must be provided with regular instruction on the risks and emergencies and the safety measures which must be taken. This applies in particular to installation, assembly and test work, to set-up mode and during commissioning. The particular underlying conditions for the organization of workplaces involving collaborative robots (such as working hours, breaks, first-aid kits, log books, etc.) must be examined and defined. Following an unintended collision, the fitness for work of the affected individual and the correct setup of the workplace must be examined.

Ergonomics requirements, human factors

The robot system including the human machine interface and its control and display elements must be designed in accordance with ergonomics principles ^[10], in order to ensure that its operation and maintenance are straightforward. The working space in which a collision between a person and a collaborative robot may occur must be designed such that the physical mobility of the person is not restricted. The individual's perception, attention and thought processes must not be constrained or disturbed by the working environment and the collaborative robot . The beginning of the robot's movement and its course should be visible, predictable and logical. These characteristics assist in preventing contact between the operator and the robot and possible stress upon or loss of control by the operator.

Marking and instructions

Robots for collaborative operation and points of access to collaborative areas must be marked by a suitable warning sign and symbol. The collaborative space in which the operator works directly with the robot must be clearly set out and demarcated (for example by markings on the ground, signs, etc.). The instruction handbooks for machinery must also describe the collaborative aspect:

• Design and use procedures: type of collaborative operation, detailed instructions, maintenance, abnormal situations, system limitations, cautions/warnings, PPE
• Criteria for operator capabilities: skills, training, physical limitations
• Operators must be informed regularly of the risks, procedures in emergencies and required safety measures associated with collaborative operation

Verification and validation

For the "speed and separation monitoring" method, the safe distance, braking distance of the robot and response time of the control system must be verified.

For the "power and force limiting" method, the static and dynamic injury criteria must be validated. For this purpose, a suitable force and pressure measurement system modelling the human body must be employed. Further details are formulated in the annexes of the ISO/TS 15066 specification in draft [3].