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I have a few questions with regards to using safety relays in safety interlock circuits. The application area is small fully automated assembly machines (pick and place, small resistance welders, etc.). Actuators are
typically pneumatic cylinders with occasional small servo or stepper drives for simple positioning purposes. These questions do not refer to robot or stamping press applications.
E-stop removes all energy from the system including dumping the air. Access to the machines is provided by hinged doors which allow you to reach into but not fully enter the machines (i.e. the machines are too small for
you to walk inside).
The first question is how do you commonly see these devices actually being used in machines built within the past five years? Note that I am not asking you to give recommendations or application advice, I am just asking you to describe how you most commonly see people apply them whether you agree with that method or not.
I am particularly concerned with how access to the machine by qualified personnel for occasional set-up and troubleshooting is affected.
For example, I have seen a number of machines (implemented by people other than myself) in which a safety relay is simply used to replace a conventional contactor as the MCR, and the door switches and e-stop buttons (positive break) are wired with redundant contacts to this device.
I have seen set-up and trouble shooting access provided as either no access at all (which of course simply requires people to find various ways around the guards), or access is provided by using a key switch to over-ride the guards. The latter case relies upon the PLC program to ensure that no unexpected machine motions occur in this mode.
I would like to know if either of the above (no 'official' access, or access with an over-ride key) are common, or if some other means is typically used.
There has been some speculation that a proper implementation requires two safety relays, each performing a separate function. One relay
acts as an emergency stop relay and is tripped by an e-stop button (or pull cord, etc.) only. All energy is removed from the system including
disconnecting electric control power and dumping the air. This also provides the low voltage drop-out function. A 'start' button must be pressed to
re-enable this relay once it has tripped.
The second relay is connected to the guard switches. If a guard door is opened, electrical power is removed from the interlocked devices (i.e. power is disconnected from the PLC output cards). No machine motion can occur with the guards open regardless of what the PLC may be attempting to do. Since the air is not dumped from the machine, this still provides access
to the machine for set-up and trouble shooting. The relay is reset (re-enabled) upon closing the door.
I would like to know if the above method is commonly implemented. It is somewhat more expensive (two relays rather than one), but it would seem to offer greater security than the previously mentioned methods. I have not thus far seen this method used in a machine.
The final question is to ask for your opinions on the relative merits of the various makes of safety relays, particularly Jokab versus Pilz (although I would be interested in any other brands you would like to recommend as well). We currently have several brands installed and we would like to standardise on just one for spare parts.
The Ministry of Labour here has issued new regulations (with additional guidelines forthcoming) on machine guarding. I would like to get a better idea of what the general practice is with safety relays in places where they have been longer established.
**********************
Michael Griffin
London, Ont. Canada
[email protected]
**********************
typically pneumatic cylinders with occasional small servo or stepper drives for simple positioning purposes. These questions do not refer to robot or stamping press applications.
E-stop removes all energy from the system including dumping the air. Access to the machines is provided by hinged doors which allow you to reach into but not fully enter the machines (i.e. the machines are too small for
you to walk inside).
The first question is how do you commonly see these devices actually being used in machines built within the past five years? Note that I am not asking you to give recommendations or application advice, I am just asking you to describe how you most commonly see people apply them whether you agree with that method or not.
I am particularly concerned with how access to the machine by qualified personnel for occasional set-up and troubleshooting is affected.
For example, I have seen a number of machines (implemented by people other than myself) in which a safety relay is simply used to replace a conventional contactor as the MCR, and the door switches and e-stop buttons (positive break) are wired with redundant contacts to this device.
I have seen set-up and trouble shooting access provided as either no access at all (which of course simply requires people to find various ways around the guards), or access is provided by using a key switch to over-ride the guards. The latter case relies upon the PLC program to ensure that no unexpected machine motions occur in this mode.
I would like to know if either of the above (no 'official' access, or access with an over-ride key) are common, or if some other means is typically used.
There has been some speculation that a proper implementation requires two safety relays, each performing a separate function. One relay
acts as an emergency stop relay and is tripped by an e-stop button (or pull cord, etc.) only. All energy is removed from the system including
disconnecting electric control power and dumping the air. This also provides the low voltage drop-out function. A 'start' button must be pressed to
re-enable this relay once it has tripped.
The second relay is connected to the guard switches. If a guard door is opened, electrical power is removed from the interlocked devices (i.e. power is disconnected from the PLC output cards). No machine motion can occur with the guards open regardless of what the PLC may be attempting to do. Since the air is not dumped from the machine, this still provides access
to the machine for set-up and trouble shooting. The relay is reset (re-enabled) upon closing the door.
I would like to know if the above method is commonly implemented. It is somewhat more expensive (two relays rather than one), but it would seem to offer greater security than the previously mentioned methods. I have not thus far seen this method used in a machine.
The final question is to ask for your opinions on the relative merits of the various makes of safety relays, particularly Jokab versus Pilz (although I would be interested in any other brands you would like to recommend as well). We currently have several brands installed and we would like to standardise on just one for spare parts.
The Ministry of Labour here has issued new regulations (with additional guidelines forthcoming) on machine guarding. I would like to get a better idea of what the general practice is with safety relays in places where they have been longer established.
**********************
Michael Griffin
London, Ont. Canada
[email protected]
**********************