Coming to an emergency stop

What happens when you hit an emergency stop button? Hopefully the emergency will stop being an emergency, but how that comes about is not necessarily so simple. And later, when the equipment restarts, you don't want it to damage itself as a result of an unexpected situation arising during the shutdown.

The design of the shutdown procedure has to be carried out very carefully to ensure that the equipment shuts down safely and in a controlled manner; you want humans to be protected and, in addition, you don't want an expensive pick-and-place robot to become a costly drop-and-damage liability. Oh yes, and don't forget that the equipment must still meet the essential health and safety requirements as laid down in the Machinery Directive, as well as conforming to any other relevant legislation.

If a fieldbus network is being used, it may be possible to monitor the equipment status after the emergency stop has been initiated, but it will probably be necessary to disconnect the power supply from actuators and equipment. However, for some fieldbus networks where power and signals are conveyed on the same wiring, this is likely to be difficult if not impossible.

At a recent press launch hosted by Asco Joucomatic, journalists were told about the VCS (valve control system) that is effectively a control loop for valves. The beauty of this approach is that a maximum of 127 bistable solenoid valves and 254 sensor inputs can be installed, yet only one fieldbus output is required for communications with the VCS controller unit. The loop uses a green two-wire cable that, coincidentally, has the same cross-section as the familiar yellow AS-Interface cable, and this green cable takes power and signals to the valves and sensors. Current is limited to 100mA per valve coil so, for larger valves requiring more power, another cable has to be routed and this is the same profile as the green cable, but coloured black. For valve solenoids fed by the black cable, the maximum current rating is 1A.

This arrangement may be particularly useful in emergency shutdown situations where valve positions need to be known, but the main power supply needs to be isolated. In other words, the green cable would remain live, but the black cable could be isolated.

Returning to the question of what happens when the emergency stop switch is operated, with the VCS arrangement the operator may be able to see on a display the status of all of the actuators, safe in the knowledge that the equipment is unpowered.

There are almost always multiple solutions to any problem, and one of the design engineer's tasks is to select the best solution for a given application. For instance, simply cutting the power to a servomotor may result in continuing motion due to momentum. Safety relays can be incorporated in the safety circuit so that the guards remain locked until a motion sensor has detected that all motion has stopped, but this may not prevent the equipment from damaging itself or the work in progress. Friction - or an end stop - may cause motion to cease, but it may be preferable to fit a spring-applied emergency brake that is actuated in the event of the power being isolated.

In more complex situations, it may be necessary to have a programmed shutdown procedure. For this it might be that a programmable safety system is required, though these tend to be costly to buy and install, needing specialist knowledge and hardware. But the important thing to remember is that when an operator hits an emergency stop button, the situation must be made as safe as practicable.

There are almost always multiple solutions to any problem, and one of the design engineer's tasks is to select the best solution for a given application