Uppdaterad: 30 mar 2020
When a machine safety specialist sits in an excavator for the first time one of his first questions might be: "Where is the e-stop? This can't be right? Every machine needs an e-stop!" So where is the e-stop in the excavator?
I started my journey into functional safety as developer of safe control systems for earth moving machinery (EMM) and when exiting that world and entering the industry floor I felt like an odd bird among fellow machine safety specialists. I actually envied the industry as the components were there ready to be picked together into safety functions (how hard could it be, right?) where instead building a system on an excavator to fit onto a gazillion different makes and models were like building a 5k puzzle with only white pieces. To be honest there was a bit of the classic dunning kruger effect over that envy and I was right up there at mount stupid looking down. I now know I have a ton to learn and that is great! I try to learn from the best and maybe compare the two worlds might be interesting. So I intend to start doing that and see where it takes us. Stay tuned! Anyways, lets get back to it.
So, about that e-stop
In a mobile machine the operator is always present (up until it is not, but that is a different story) and may actually be able to avert a hazardous situation whereas a dangerous machine on the industry floor is considered to sooner or later cause a harmful situation. This is why you actually allow lower safety levels (PL) on functions/movements on the excavator that on the industry floor would be properly stowed away behind fences. One always needs to have an e-stop on the factory floor and you never have it in an excavator. Why? Well I would say that it two things:
In EN 13850 there is only one way to avoid using e-stop and that is if "an emergency stop would not reduce the risk." The e-stop in itself won't stop (well it will, but not in short enough time) a machine where the slewing has stuck or the machine has a stuck gas pedal. Instead, searching for the e-stop will take more time than simply shoving the bucket into the ground or by other means avoiding the hazard.
There is already a way to kill everything, the key switch. While not as neat as the e-stop it is there, the operator knows about it (because he started the machine after all) and can easily reach out and kill everything (engine, power and hydraulics).
Would we never consider having e-stops then on EMM?
Well, actually we would. Let us think about a remotely controlled wheel loader where the operator walks beside the machine. On the remote "control" of the wheel loader you would most certainly have an e-stop because now, all of the sudden, the operator is not present anymore, he is walking beside the machine! We would like to be able to kill everything quickly and would probably like to have a three way safety control device, like a safety control device in a robotic cell, so you would need to hold the switch in the middle position to allow movement of the machine. A safety control device? Can we compare the machine to a robot? Well, yes in a way.
Safety control devices in robotic cells
The idea with the safety control device is to allow operators to enter very dangerous areas (robotic cells) while holding the device. Consider for a moment if we would have an autonomous excavator? Well, the use of a safety control device might be just the trick to allow operators to enter otherwise very dangerous areas, the autonomous site.
What to make of this?
Well, for one thing we are, even if we are talking about EMM or robots, abiding to the machinery directive and most standards are the same. We have similarities but also differences explaining why safety is considered differently in the two applications. We conclude that while e-stops should not be a part of the EMM but it might very well need be if you add functions like remote control. It all comes down to... the risk assessment.
I think that when I accumulate more experience from the industry safety I will find more neat comparisions between EMM and industry safety and I intend to share it when I do. Maybe you have your own reasoning to add to this? Please feel free to comment or reach out. If you like the post, share it or at least subscribe!
Consultant and founder, Zatisfy AB in Umeå. Roberth is passionate about helping companies make their CE marking a natural part of their working day. As an expert in EN 13849 he is working to develop the standard in the swedish standardization association SIS.
Find out more about Roberth here