The best of both worlds?
Published: 14 October, 2016
“Increasing the safety of your machines means reducing productivity”. This is a commonly held view among staff who are accountable for the throughput of work equipment. Warren Spiers, of Spiers Engineering Safety explains that it’s actually a false dilemma.
As technology develops, it is becoming possible to improve production and safety. We might call this “optimisation”; that point where peak safety and maximum production intersect.
Part of this process involves making the link between safety and reliability. If a machine is unreliable or difficult to interact with, this increases the probability of unsafe interactions to keep it running. A maintenance worker was killed in a factory in which I worked as a student. The root cause was due to the machine being difficult to re-start once it had been isolated to gain entry safely, so the worker climbed over the perimeter fence. Once the sensor that had caused the machine to stop was realigned, it restarted automatically, causing fatal injuries. It is clear to see that improving the reliability of that machine (in particular the manner in which it restarted) would not only have caused less downtime, but simultaneously made the machine safer. A safety system which causes problems for those who interact with the machine should be redesigned, or the motivation to defeat it will grow. The analogy I use is that a safety system is like a football referee. If they’re doing a really good job you don’t notice them. Neither should you intervene unnecessarily.
Reducing the number and duration of interactions with a machine will make it safer and more productive. Bear in mind, the intended lifespan of the control system; it is difficult to get 21st century performance from automation components manufactured 30 years ago. A well designed system using some form of intelligent safety controller, connected to the machine HMI(s), should keep fault finding tasks to a minimum. Remarkably, even new machines often do not furnish basic information such as which emergency stop button is pressed, meaning that the operator has to check them all.
A requirement for specialist equipment in order to fault find will increase the likelihood of a “night shift bypass” of the safety circuit that is too common and leads to further hazards. Modern, comprehensive safety systems are also self-checking, so that a series of faults do not accumulate which lead to the loss of the safety function. Accidents are often the result of a chain of smaller incidents, such as unnoticed component failure or bypasses, rather than a single large failure.
By using a safety PLC, it is possible to integrate the process and the safety logic. Often a machine can take many minutes to reset, simply because someone opened a safety gate at the wrong time. With a linked system, access is requested, and granted when it is convenient to enter. Solenoid locking can be used to enforce access control.
Don’t use a sledgehammer to crack a nut
The simplest control measure is not always the best. A two metre (6’ 6”) fence all the way around a machine restricts access, and impedes the view of the process. It may even create additional hazards such as difficulties manoeuvring forklift trucks past the machine. With current safety technology, it is possible to not only reduce risk to an acceptable level, but improve the material flow. Using your Risk Assessments as a starting point you may be able to replace a distance guard with a light curtain or laser scanning device.
What are the options?
There is no single way to make every machine safe. You may find it useful to draw the machine without any existing safeguards, and think about how the operators currently use the machine. This may give ideas about how to make the process more efficient. Risk assess how the machine operates now, and how you might want it to work in future. Can any hazards be designed out?
An understanding of what tasks the operator performs is essential. I recently assessed a printing machine where in-running nips on driven rolls were tightly guarded; when the power was removed or the safety circuit tripped, the driven rolls were braked so that they could not move. Unfortunately, the operator frequently needs access to those rolls for cleaning or removing waste. The design of the guarding made this extremely difficult. Safe Motion within a drive allows the system to be put into a mode where the speed, direction and torque are controlled in a fail-safe manner to allow the task to be done more safely, yet more easily at the same time by moving the rollers either in increments or at a slow control speed. In some situations, the in-running nip hazard is nullified if the rollers rotate in the opposite direction. It is common to see inflexible safety systems bypassed or removed entirely because the user finds them too obstructive.
Your process may include an industrial robot or you might be considering adding one. Collaborative robots can now share floor space with your operators, meaning that the requirement to strictly segregate the robot from the workforce is no longer an absolute. It can slow down if it detects the presence of people within a “warning” distance, and stop safely if they encroach too far. Some detect contact and/or use force limiting to ensure that no injury is caused.
One client I worked with had a problem where insects would cause lengthy stoppages by triggering photoelectric sensors. This can be a problem where the hazard requires a fine sensor resolution, but the sensor can’t determine if there’s really a person there. Safe camera systems can detect human intrusion into a zone. This has the advantage of covering multiple areas and hazards with one system. A single installation could replace a dozen or more individual light curtains and sensors. This brings many benefits; simplified installation, decreased fault finding and maintenance. The camera system also removes the binary nature of ordinary sensors, by having warning zones and hazard (stop) zones, without the pitfalls of false triggering. Whereas a normal light curtain or sensor is only sensitive in two dimensions, smart camera systems also offer protection in three dimensions.
It is crucial that the people you take safety advice or systems integration solutions from are not only adequately qualified, but that their market awareness is current enough to ensure that the solutions you are installing aren’t obsolete before they are commissioned.
For further information please visit: www.spierssafety.co.uk