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 a developer of safe control systems for earth moving machinery (EMM). I felt like an odd bird among fellow machine safety specialists when exiting that world and entering the industry floor. I actually envied the industry as the components were there, ready to be picked together into safety functions. Instead building a system on an excavator to fit onto a gazillion different makes and models was like building a 5k puzzle with only white pieces. How hard could it be, right? 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, which is great! I try to learn from the best, and maybe comparing the two worlds might be interesting. So I intend to start doing that and see where it takes us. Stay tuned! Anyways, let’s 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. In contrast, 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 would be neatly stowed away behind fences on the industry floor. One almost 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 is 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 avoiding the hazard.
- There is already a way to kill everything, the ignition key. 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, suddenly, the operator is not present anymore. He is walking beside the machine! We would like to be able to kill everything quickly. We would probably want to have a three-way safety control device just like a safety control device in a robotic cell where 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? 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?
Even if we are talking about EMM or robots, we are abiding by the machinery directive, and most standards are the same. We have similarities and 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, they might need to be if you add functions like a remote control. It all comes down to… the risk assessment.
When I accumulate more experience in the safety industry, I will find more neat comparisons between EMM and industry safety and I intend to share them. 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!