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The fine art of the emergency stop

March 22, 2024
Why the e-stop button needs to be reliable and swift

Have you ever opened a door or stepped in front of a light curtain, and the whole production process shuts down with a loud groan? You know that feeling you get in your stomach that you probably did something wrong?

Well, you actually didn’t. The safety system that has been put in place by the automation and control engineers is there to protect you against yourself.

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Safety devices come in various shapes and flavors in order to protect the process from people mistakes. Of course, it is there to protect the people from the process whose inert personality cares not that you chose to stand in front of a moving wrapper gantry and get clobbered.

Safety sensors stand between the operator and a bad day. They have been strategically placed into the process to protect and provide safety on all fronts.

A good safety implementation provides a balance between protection of both the process and the people surrounding it and production requirements.

Safety devices have to work—full stop. If you hit an e-stop button, it’s because it’s an emergency. A mushroom head red pushbutton was used for the longest time. However, it was a standard pushbutton with a standard contact block wired like a normal pushbutton. When the e-stop was hit, the contacts are supposed to open, cutting power to a master control relay (MCR) that controlled power given to the control circuit of the process.

Many points of failure exist in this line of protection. Contact blocks that have fallen off, loose wires, the MCR fails closed—these are just a few of the potential obstacles to saving someone’s life when the e-stop needs to halt the process in its tracks.

In the late 1980s, a new form of safety devices started to emerge into the controls field. There was a recognized lack of fault tolerance in safety systems with the MCR mentality. It may have had something to do with the level of injuries created by the failures of the MCR systems.

The e-stop relay was born, which introduced a new way of protecting people and machines alike, and we have never looked back.

Safety systems are implemented in all walks of process but primarily in machine control. Continuous process being stopped by someone walking through a door would not be a good idea. That door would be locked shut while the process was going on, so as to not cause a disturbance to that process. Machine control is different though.

Press control, palletizing, wrapping, logistics are processes with moving parts that people need to be protected from. You should not be able to enter the product wrapper while it is in motion. By default, the process is protected by not damaging the product or the machinery.

However should the need arise to stop the wrapper, it needs to stop now.

A safety e-stop is not your standard pushbutton. It has two sets of redundant normally closed contacts. It is wired into a safety device such as a safety relay or safety programmable logic controller (PLC), which also sends a low-voltage signal through the wiring to detect a dropped wire or in fact a contact block that has fallen off the back of the pushbutton.

The purpose of a safety sensor is to operate as designed when it needs to. If it is unable to perform that function, such as a contact block falling off the e-stop switch, the control will stop the process in an uncontrolled fashion.

Remember that an e-stop is a safety stop, not a process stop function. There is no controlled stop function with an e-stop—shut it down now.

There are many options available for “safetyfying” a process and/or machine control. Guarding has become an art.

Robotic cells have fencing/guarding to protect from unintended entry while the robot is moving. Door safety gate switches prevent entry while the robot is moving. When the robot is stopped, the gate is available for entry, and the normal operation of the robot is no longer allowed until the gate is closed.

Can the robot operate while an operator is in the cell and the gate switch closed? Yes, it can, unless there are additional operator protection safety devices employed to detect the presence of a human.

Safety is a necessity. Safety systems are smart, programmable and functional. They report on which device was tripped since there is no longer a large string of serial safety devices controlling control power.

Did I mention that the control power relays are redundant, too? We are protected in every way.

About the Author

Jeremy Pollard | CET

Jeremy Pollard, CET, has been writing about technology and software issues for many years. Pollard has been involved in control system programming and training for more than 25 years.