Guidance for Light Curtains & Laser Scanners

The most misapplied safety devices in the industry are light curtains and laser scanners, common issues with installations include:
  1. Application not suitable for light curtain/scanner, eg; the machine ejects parts, the machine has a long stopping time, environmental influences
  2. Light curtain placed too close to the hazard – Insufficient safety distance 
  3. Scanner safety field size is too small – Insufficient safety distance 
  4. Stopping performance monitoring not provided when it should be
  5. Muting sensors not mounted correctly
In the past, it hasn’t been easy for installers/designers to find guidance on all these topics in the one reference. We have had AS 4024.2801 in Australia since 2008, but this standard only provided sufficient guidance for safety distance calculation which addressed issues 2 and 3 from the above list.
Guidance is now at hand with the new standard AS 4024.2802:2017 being introduced. This standard provides information on all aspects of designing/installing presence sensing system such as light curtains and laser scanners.

AS 4024.2802:2017 covers safety distance calculation to address issues 2 and 3 in the above list, but it does a lot more as well.

It also provides an explanation of how to ensure the application is suitable for presence sensing devices, this guidance can help address issue 1 from the above list.

Issue 5 a major problem in the industry, it is common to see muting sensors mounted incorrectly and this increases the risk of operators inadvertently muting the light curtain and being exposed to hazards. AS 4024.2802:2017 has information on all common muting configurations and provides clear instructions on how the sensors are mounted and the timing sequence of the muting operation.
Issue 4 reflects the fact that many designers/installers aren’t aware of the requirement of stopping performance monitoring. If the light curtain/scanner is used as a trip device then the safety distance is integral to ensure the risk is controlled. If the machine’s stopping time is subject to deterioration (eg: brake wear) then the stopping time of the machine should be monitored. This information can be used to schedule preventative maintenance to ensure the safety risk is controlled and reduce unexpected downtime.

If you design/install or maintain presence sensing systems, such as light curtains or laser scanners, I recommend referencing the new AS 4024.2802:2017 standard.

Published: 8 February 2018

Safety for Collaborative Robots

In recent times there has been a strong growth in the use of robots in Australian manufacturing, thus why collaborative robots is currently a hot topic. These robots are designed to operate in cooperation with humans, which presents some new safety considerations compared to traditional robots that operate behind a safety fence. There is a new Australian Standard, AS 4024.3303:2017, which provides guidance on the process involved to ensure your collaborative robot doesn't pose a threat to its human work colleagues.

A risk assessment must be carried out to determine if a collaborative robot is suitable for the application. This should also include determining the collaborative workspace of the robot and estimating the risk of the hazards so the appropriate risk reduction measures can be applied.

The collaborative workspace is the area where the robot and human co-inhabit during normal operation, see Figure 1 below.
Fig. 1 - The Collaborative Workspace
To reduce the risk associated with robots and humans working in this collaborative workspace one or more of the following methods can be utilised.

Safety-rated Monitored Stop

This method may be used to provide access for the operator to perform tasks, such as loading a part into the end effector. In this method, the robot will move to the collaborative workspace and perform a safety stop. This allows the operator to enter the collaborative workspace and perform their task. Once the operator is out of the collaborative area, the robot can resume normal operation. If the operator enters the collaborative workspace, when the robot is moving in the collaborative workspace, the robot will perform a safety stop and need to be manually reset.

The robot system must be able to detect the presence of an operator inside the collaborative workspace. The size of the collaborative workspace must be determined to take into consideration the speed of the robot, the reaction time of the robot, stopping time of the robot, speed of human movement and resolution of the system used to detect the presence of the operator.

Hand Guiding

This method works similar to the "Safety-rated monitored stop" however, once the operator is inside the collaborative workspace, they can operate the robot with a hand guiding device. This allows the operator to manually control the robot in close proximity for detailed tasks. When the robot is manually controlled, it will perform its movements at a controlled speed deemed acceptable from a risk assessment. If the operator releases the hand guiding device, the robot will stop and when the operator has left the collaborative workspace, the robot can resume normal operation.

Speed and Separation

In this method, the robot and operator can work at the same time in the collaborative workspace. The robot maintains a protective separation distance from the operator. If the distance between the operator and robot becomes less than the protective separation distance the robot will stop.

The speed of the robot must be monitored because the protective separation distance is reliant on the speed of the robot. The protective separation distance is also reliant on the on the robot’s reaction time and the accuracy/resolution of the system used to detect the distance of the operator.

The robot may change its speed depending on the position of the operator to reduce the protective separation distance or the robot may use alternative paths that ensure the protective separation distance is maintained.

Power and Force Limiting

In this method, the robot and operator can work at the same time in the collaborative workspace and contact between the operator and robot can occur. The energy and force of these collisions are limited below an established threshold limit. A risk assessment process is used in conjunction with data from Annex A of the standard, to determine the suitable energy and force thresholds for the tasks to be performed.

The robot keeps energy and force of contact below the threshold by:

  • Increasing contact surface areas; rounded edges, smoothed edges, etc.
  • Absorbing energy; using padding/cushioning, deformable components, etc.
  • Limiting forces, speed
  • Using sensors to anticipate collisions 

When considering a collaborative robot, it is essential that a risk assessment process is conducted to understand the risks associated with the application. With the use of the new standard, AS 4024.3303:2017, the appropriate collaborative methods can be selected. The standard also provides guidance, on what safety features the robot requires for each collaborative method.

Published: 25 July 2017

How do I validate my Safety System?

The most common step that is not performed or performed incorrectly when implementing a safety system is validation. This step is essential to confirm the specification and conformity of the safety system, however many people are unsure how to validate or don't even consider performing a validation.

Here are some common mistakes made with validation:

No Specification

You can’t validate an unspecified safety system, thus if there is no specification document then what are you validating?

The specification document has two purposes:
  1. It provides a framework for the system to be designed
  2. It provides a specification to validate

The specification should explain the following:
  1. The functional behaviour of the safety system - For example if the system is an E-Stop the specification should explain; how the E-Stop is initiated, what hazardous movements are inhibited by the E-Stop, what Stop Category is performed, how quickly are these movements inhibited, how is the system reset to allow machine operation to continue, etc.
  2. Operational and environmental conditions
  3. Integrity Requirements - What is the level of risk reduction required by the safety system? This can be measured by a required Safety Category (CAT), Performance Level (PL) or Safety Integrity Level (SIL)
Once the Specification exists then the system can be validated according to its functional, environmental and integrity requirements.

Only Normal Operation of Safety System is Tested

It is common for validation to be performed on a safety system with no fault simulation testing.

For example, if validating an E-Stop the machine is started under its maximum expected operational load and the E-Stop hit. The safety function is validated by confirming the hazardous movements have been ceased in the required time according to the specification and the machine can’t be restarted until the E-Stop operator is manually reset.

The above validation may prove the functional behaviour of the E-Stop but many safety systems also require fault simulation to validate their integrity requirement. If the above E-Stop had a requirement of CAT 3, then all single fault modes would need to be simulated to confirm that the system will not lose safety function due to a single fault.

No Documentation

As like any activity performed during the implementation of a safety system, validation does not exist if it is not documented. All relevant analysis, tests reports, calculations, data sheets, etc. must be recorded to prove the process undertaken.

For help with validation plans, register for the NHP Safety Reference Guide, in the 'Safety Function Document' section there are numerous examples of pre-engineered Safety Functions with validation plans at the back of each document.

For more information on the process of validation, activities to be performed and the documentation required reference AS 4024.1502-2006.

Published: 18 May 2017