Industrial robots are found throughout industry wherever high productivity demands must be met. The use of robots, however, requires design, application and implementation of the appropriate safety controls in order to avoid creating hazards to production personnel, programmers, maintenance specialists and system engineers.
Why Are Industrial Robots Dangerous?
One definition of robots is “moving automatic machines that are freely programmable and are able to operate with little or no human interface”. These types of machines are currently used in a wide variety of applications throughout industry and medicine, including training. Industrial robots are being increasingly used for key functions, such as new manufacturing strategies (CIM, JIT, lean production and so on) in complex installations. Their number and breadth of applications and the complexity of the equipment and installations result in hazards such as the following:
- movements and sequences of movements that are almost impossible to follow, as the robot’s high-speed movements within its radius of action often overlap with those of other machines and equipment
- release of energy caused by flying parts or beams of energy such as those emitted by lasers or by water jets
- free programmability in terms of direction and speed
- susceptibility to influence by external errors (e.g., electromagnetic compatibility)
- human factors.
Investigations in Japan indicate that more than 50% of working accidents with robots can be attributed to faults in the electronic circuits of the control system. In the same investigations, “human error” was responsible for less than 20%. The logical conclusion of this finding is that hazards which are caused by system faults cannot be avoided by behavioural measures taken by human beings. Designers and operators therefore need to provide and implement technical safety measures (see figure 1).
Figure 1. Special operating control system for the setting up of a mobile welding robot
Accidents and Operating Modes
Fatal accidents involving industrial robots began to occur in the early 1980s. Statistics and investigations indicate that the majority of incidents and accidents do not take place in normal operation (automatic fulfilment of the assignment concerned). When working with industrial robot machines and installations, there is an emphasis on special operation modes such as commissioning, setting up, programming, test runs, checks, troubleshooting or maintenance. In these operating modes, persons are usually in a danger zone. The safety concept must protect personnel from negative events in these types of situations.
International Safety Requirements
The 1989 EEC Machinery Directive (89/392/EEC (see the article “Safety principles for CNC machine tools” in this chapter and elsewhere in this Encyclopaedia)) establishes the principal safety and health requirements for machines. A machine is considered to be the sum total of interlinked parts or devices, of which at least one part or device can move and correspondingly has a function. Where industrial robots are concerned, it must be noted that the entire system, not just one single piece of equipment on the machine, must meet the safety requirements and be fitted with the appropriate safety devices. Hazard analysis and risk assessment are suitable methods of determining whether these requirements have been satisfied (see figure 2).
Figure 2. Block diagram for a personnel security system
Requirements and Safety Measures in Normal Operation
The use of robot technology places maximum demands on hazard analysis, risk assessment and safety concepts. For this reason, the following examples and suggestions can serve only as guidelines:
1. Given the safety goal that manual or physical access to hazardous areas involving automatic movements must be prevented, suggested solutions include the following:
- Prevent manual or physical access into danger zones by means of mechanical barriers.
- Use safety devices of the sort which respond when approached (light barriers, safety mats), and take care to switch off machinery safely when accessed or entered.
- Permit manual or physical access only when the entire system is in a safe state. For example, this can be achieved by the use of interlocking devices with closure mechanisms on the access doors.
2. Given the safety goal that no person may be injured as a result of the release of energy (flying parts or beams of energy), suggested solutions include:
- Design should prevent any release of energy (e.g., correspondingly dimensioned connections, passive gripper interlocking devices for gripper change mechanisms, etc.).
- Prevent the release of energy from the danger zone, for example, by a correspondingly dimensioned safety hood.
3. The interfaces between normal operation and special operation (e.g., door interlocking devices, light barriers, safety mats) are necessary to enable the safety control system to automatically recognize the presence of personnel.
Demands and Safety Measures in Special Operation Modes
Certain special operation modes (e.g., setting up, programming) on an industrial robot require movements which must be assessed directly at the site of operation. The relevant safety goal is that no movements may endanger the persons involved. The movements should be
- only of the scheduled style and speed
- prolonged only as long as instructed
- those which may be performed only if it can be guaranteed that no parts of the human body are in the danger zone.
A suggested solution to this goal could involve the use of special operating control systems which permit only controllable and manageable movements using acknowledgeable controls. The speed of movements is thus safely reduced (energy reduction by the connection of an isolation transformer or the use of fail-safe state monitoring equipment) and the safe condition is acknowledged before the control is allowed to activate (see figure 3).
Figure 3. Six-axis industrial robot in a safety cage with material gates
Demands on Safety Control Systems
One of the features of a safety control system must be that the required safety function is guaranteed to work whenever any faults arise. Industrial robot machines should be almost instantaneously directed from a hazardous state to a safe state. Safety control measures needed to achieve this include the following safety goals:
- A fault in the safety control system may not trigger off a hazardous state.
- A fault in the safety control system must be identified (immediately or at intervals).
Suggested solutions to providing reliable safety control systems would be:
- redundant and diverse layout of electro-mechanical control systems including test circuits
- redundant and diverse set-up of microprocessor control systems developed by different teams. This modern approach is considered to be state-of-the-art; for example, those complete with safety light barriers.
Safety Goals for the Construction and Use of Industrial Robots.
When industrial robots are built and used, both manufacturers as well as users are required to install state-of-the-art safety controls. Apart from the aspect of legal responsibility, there may also be a moral obligation to ensure that robot technology is also a safe technology.
Normal operation mode
The following safety conditions should be provided when robot machines are operating in the normal mode:
- The field of movement of the robot and the processing areas used by peripheral equipment must be secured in such a way as to prevent manual or physical access by persons to areas which are hazardous as a result of automatic movements.
- Protection should be provided so that flying workpieces or tools are not allowed to cause damage.
- No persons must be injured by parts, tools or workpieces ejected by the robot or by the release of energy, due to faulty gripper(s), gripper power failure, inadmissible speed, collision(s) or faulty workpiece(s).
- No persons may be injured by the release of energy or by parts ejected by peripheral equipment.
- Feed and removal apertures must be designed to prevent manual or physical access to areas which are hazardous as a result of automatic movements. This condition must also be fulfilled when production material is removed. If production material is fed to the robot automatically, no hazardous areas may be created by feed and removal apertures and the moving production material.
Special operation modes
The following safety conditions should be provided when robot machines are operating in special modes:
The following must be prevented during rectification of a breakdown in the production process:
- manual or physical access to areas which are hazardous due to automatic movements by the robot or by peripheral equipment
- hazards which arise from faulty behaviour on the part of the system or from inadmissible command input if persons or parts of the body are in the area exposed to hazardous movements
- hazardous movements or conditions initiated by the movement or removal of production material or waste products
- injuries caused by peripheral equipment
- movements that have to be carried out with the safety guard(s) for normal operation removed, to be carried out only within the operational scope and speed, and only as long as instructed. Additionally, no person(s) or parts of the body may be present in the area at risk.
The following safe conditions should be assured during set up:
No hazardous movements may be initiated as a result of a faulty command or incorrect command input.
- The replacement of robot machine or peripheral parts must not initiate any hazardous movements or conditions.
- If movements have to be carried out with the safety guard(s) for normal operation removed when conducting setting-up operations, such movements may be carried out only within the directed scope and speed and only as long as instructed. Additionally, no person(s) or parts of the body may be present in the area at risk.
- During setting-up operations, the peripheral equipment must not make any hazardous movements or initiate any hazardous conditions.
During programming, the following safety conditions are applicable:
- Manual or physical access to areas which are hazardous due to automatic movements must be prevented.
- If movements are carried out with the safety guard(s) for normal operation removed, the following conditions must be fulfilled:
- (a)Only the command to move may be carried out, and only for as long as it is issued.
- (b)Only controllable movements may be carried out (i.e., they must be clearly visible, low-speed movements).
- (c)Movements may be initiated only if they do not constitute a hazard to the programmer or other persons.
- Peripheral equipment must not represent a hazard to the programmer or other persons.
Safe test operations require the following precautions:
Prevent manual or physical access to areas which are hazardous due to automatic movements.
- Peripheral equipment must not be a source of danger.
When inspecting robot machines, safe procedures include the following:
- If it is necessary to enter the robot’s field of movement for inspection purposes, this is permissible only if the system is in a safe state.
- Hazards caused by faulty behaviour on the part of the system or by inadmissible command input must be prevented.
- Peripheral equipment must not be a source of danger to inspection personnel.
Troubleshooting often requires starting the robot machine while it is in a potentially hazardous condition, and special safe work procedures such as the following should be implemented:
- Access to areas which are hazardous as a result of automatic movements must be prevented.
- The starting up of a drive unit as a result of a faulty command or false command input must be prevented.
- In handling a defective part, all movements on the part of the robot must be prevented.
- Injuries caused by machine parts which are ejected or fall off must be prevented.
- If, during troubleshooting, movements have to be carried out with the safety guard(s) for normal operation removed, such movements may be carried out only within the scope and speed laid down and only as long as instructed. Additionally, no person(s) or parts of the body may be present in the area at risk.
- Injuries caused by peripheral equipment must be prevented.
Remedying a fault and maintenance work also may require start-up while the machine is in an unsafe condition, and therefore require the following precautions:
- The robot must not be able to start up.
- The handling of various machine parts, either manually or with ancillary equipment, must be possible without risk of exposure to hazards.
- It must not be possible to touch parts that are “live”.
- Injuries caused by the escape of liquid or gaseous media must be prevented.
- Injuries caused by peripheral equipment must be prevented.