Robotic safety standards updated - after 14 years

With robots being used, in conjunction with manual operators, in continually widening range of applications, it is implied that the message about robot safety needs to be spread much wider.

With robots being used, inconjunction with manual operators, in continually widening range of applications, it is implied that the message about robot safety needs to be spread to a larger, more diverse audience.

Think back to the mid-1980s and you will realise how far the use of robots in industry has moved on.

Fair enough, at that time robots were becoming commonplace in automotive body-in-white assembly lines and paint plant, but a robot being used anywhere else was still considered to be an innovation.

In 1988 the HSE (Health and Safety Executive) published HSG43, its guidance for the safety of industrial robots but, remarkably, this has only recently been superseded by a revised edition.

And this revised guidance is very welcome, considering the widespread use of robots today.

As well as the many automotive applications, robots are now found in a variety of other industries, palletising all manner of goods, unloading injection moulding machines producing mobile phone handset casings, trimming fibre-reinforced structural mouldings, and serving machining centres - to name just a few.

With this wider use, there has been a significant increase in the number of people that need to work with robots, including the process designers, control engineers, installation and commissioning engineers, operatives, production engineers and maintenance staff.

This implies that the message about robot safety needs to be spread to a larger, more diverse audience.

Different people understand different things from the word 'robot'.

Leaving aside the humanoid robots that are little more than research-and-development or PR projects, the term can be used to describe everything from a simple two- or three-axis orthogonal device with a gripper for unloading small injection moulding machines, to large, multi-axis machines capable of manipulating heavy welding equipment or substantial automotive sub-assemblies.

One definition that is worth bearing in mind is that given in the European standard EN 775: 1992 (Manipulating industrial robots - Recommendations for safety), which refers to an 'automatically controlled, reprogrammable, multi-purpose, manipulative machine with several degrees of freedom, which may be either fixed in place or mobile for use in industrial automation applications'.

This is the definition that has been used by the HSE in preparing the revised HSG43, though the guidance is restricted to fixed, not mobile, robots, and the 'robot system' is taken to include the control equipment, plus any associated machinery and equipment.

Primarily the guidance is concerned with robots of the teach-and-playback type, with a pendant-type control for teaching the robot.

However, much of the guidance is also applicable to other types of robot, and even some relatively unsophisticated pick-and-place devices.

Nobody contemplating a new robotic installation would deny the importance of safety, but it should also be borne in mind that existing robots are also covered by PUWER 98.

Anybody who is responsible for the safety of an existing robot installation should therefore consider very carefully whether they need to take any further action in the light of the revised HSG43.

For new installations, safety should be considered as an integral part of the project.

It is estimated that between 20 and 50% of the cost of a robot installation is attributable to safety-related issues, so safety cannot be viewed as an add-on if it is to be implemented cost-effectively.

Many facets of the overall robot system need to be looked at, ranging from the design of the mechanical hardware, control system and safety system, to the installation/commissioning/testing, the procedures for normal operation and for teaching/programming, maintenance activities, future modifications, and training.

The first step must, of course, be a formal and rigorous risk assessment carried out to a recognised standard.

This allows the nature of the risks to be established as well as identifying who might be harmed and how.

The possible degree of harm can then be evaluated and, on the strength of this, it can be decided whether the existing precautions are adequate or if additional protective measures need to be taken.

Because of the size and power of most industrial robots, the severity of injuries is potentially great.

Furthermore, the way that teach-and-playback robots are often programmed via pendant controls operated from within the working cell, special precautions need to be taken to protect the programmer and to avoid anybody else from being put at risk at that time.

During normal operation it will be necessary to keep personnel away from the robot, with the simplest method being appropriately positioned physical barriers (of at least 2m in height), usually in conjunction with interlocked access gates.

However, these are not always the most practical when frequent rapid access is required, or where components or workpieces need to be moved into or out of the work cell.

Several alternatives are available, ranging from pressure-sensitive mats to light-beam barriers and laser area scanners.

All of the options, however, must be considered in relation to the findings of the risk assessment, and their appropriateness for the given circumstances.

Furthermore, any muting of light curtains must be programmed with extreme care, and working practices need to be defined precisely.

For teaching the robot there needs to be a means of switching from the operational mode to the setting mode, for which a method must exist that prevents the changeover being made while any person is within the safeguarded area.

This will usually involve some form of key switch or trapped key system or, for more complex installations, there may be a formalised procedure with several personalised padlocks.

If teaching requires the programmer to be in close proximity to the robot, the speed of the robot must be limited.

This is to minimise the risk of trapping and to reduce the kinetic energy to a level that will not cause a significant injury.

No two cases will ever be the same, but it is unlikely that a speed in excess of 0.25m/s would be considered acceptable.

Whatever safeguarding methods are used, they will probably be interfaced with the robot controller, though the overall safety system must be capable of safeguarding against any failure of the controller that might lead to dangerous movements of the robot.

Two important aspects of the robot control system that need to be considered are the behaviour in the event of removal of the power supply, and how the robot behaves when power is restored.

If the robot can be brought to rest in a controlled manner, the danger from robot recovery will be greatly reduced.

Whereas electrical safeguarding systems have traditionally been hard-wired, today it is permissible for the systems to depend on programmable or complex electronic systems.

Both types of system have their benefits, but the relative merits should be considered for each application.

A further option is to use a programmable system with a hard-wired backup.

This would, for example, allow a controlled shutdown of the robot, but use a trapped key interlock to subsequently cut the power to the robot.

However, this method is not recommended for situations where whole-body access to the robot cell is required.

In this case, it may be better to use a gate-locking key-exchange system.

Again, the robot benefits from a controlled shutdown and the power is also removed before access can be gained.

Additional safety stems from the operative retaining a key as he enters the cell, thereby preventing the robot from being started by anybody else.

All of the methods described so far require the power supply to be isolated from the drive motors.

Although software-reliant systems can be designed so that there is a controlled shutdown and the robot is held stationary by the software in the robot controller, a high integrity in the performance of the safety functions would have to be demonstrated for this to be acceptable.

For older machinery, perhaps hydraulically-powered, or for robots equipped with high-inertia tooling such as grinding wheels, it may be appropriate to incorporate a timer and/or a standstill monitor into the system.

This would be set to give a delay that is long enough for the robot to complete a controlled shutdown to a known safe point in the cycle, or for the grinding wheel or other tooling to run down to a safe speed.

Only then would a key be released that would allow access to the robot.

Another technique is to use an external safety system to monitor the critical safety functions and put the robot into a safe state.

Such systems are particularly appropriate when access is required for teaching or maintenance.

In a typical application, the operation of the actuation device would cause the controller to put the drive motors into a servo-hold; the monitoring system would check that the robot is maintained in this mode.

If any deviation is detected, the monitoring circuit would cause the power supply to be isolated from the drive motors.

When the robot is in 'teach' mode, the monitoring system can monitor critical functions - such as speed - to ensure that a preset maximum is not exceeded.

As an alternative, a robot controller with a programmable electronic backup may be used.

Essentially the robot controller carries out the safeguarding functions but with a separate backup provided by programmable electronic equipment.

For both of these last two cases, it is necessary to demonstrate a high integrity in the performance of the safety functions.

Robot safety is a complicated subject, and one which it is essential to get right.

Although there is guidance available from the HSE in the form of HSG43, this can still be a daunting document.

And understanding the guidance is still only half the battle; there are several regulations and standards that also need to be read, understood and correctly acted upon.

Further advice and assistance is available, however, from LC Automation.

LC Automation has offices in Preston, Sunderland and Chippenham and has first-hand knowledge of robot installations and safety equipment.

As well as being an authorised distributor for many of the leading automation, control and safety equipment brands, LC Automation also provides total integrated safety solutions, training, advice, design assistance, installation support and after-sales service.

* About HSG43, Industrial Robot Safety - The HSE's publication HSG43, Industrial Robot Safety, has recently been completely revised to reflect the changes in technology, health and safety legislation, and the availability and voluntary use of international and harmonised European standards that have arisen since the original version was published in 1988.

The main emphasis of the guidance is on fixed teach-and-playback industrial robots, though many of the principles will also be applicable to other types of robot and pick-and-place device.

A substantial document running to over 50 pages, HSG43 covers all aspects of robot safety and includes useful advice on hazard and risk assessments.

Various safeguarding methods are discussed, as well as alternative means for interfacing with the robot controller.

In addition, there is an appendix that summarises the relevant health and safety law, and another that includes no less than seven case studies.

Copies of the guidance, priced at GBP 13.50, can be obtained directly from HSE Books (tel +44 (0)1787 881165) or can be downloaded from the web site http://www.hsebooks.co.uk.

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