clavius
Member
Hi All,
In the near future I will be retrofitting an quite old piece of equipment with a new control cabinet. This is a machine that was decommissioned and the controls cannibalized for other systems. The original system used a now obsolete PLC and so I'll be building an entire new cabinet.
This is a largish piece of gear used in a pretty harsh environment, think like a mine or a quarry.
I have the schematics for the original cabinet, and can largely duplicate what was there using a modern PLC, and can get through the programming.
I have a question on the design of the original system. This unit had the typical assortment of limit switch inputs, panel switches and buttons, and the like. An interesting thing about this was that every external PLC input is isolated with a relay.
For example, all of the external limit switch inputs are 120Vac powered, but rather than those inputs directly connecting to a PLC input (which are 120V I/O modules), they power a small relay coil, and the relay is what supplies the 120V input to the PLC.
I have not had a lot of exposure to systems built for these sort of environments. Is this typical practice? Is this just to protect the PLC from outside world?
I plan to be using 24V for all of the external switching, etc., but would it be smart to replicate this in the new system? Appreciate any thoughts.
In the near future I will be retrofitting an quite old piece of equipment with a new control cabinet. This is a machine that was decommissioned and the controls cannibalized for other systems. The original system used a now obsolete PLC and so I'll be building an entire new cabinet.
This is a largish piece of gear used in a pretty harsh environment, think like a mine or a quarry.
I have the schematics for the original cabinet, and can largely duplicate what was there using a modern PLC, and can get through the programming.
I have a question on the design of the original system. This unit had the typical assortment of limit switch inputs, panel switches and buttons, and the like. An interesting thing about this was that every external PLC input is isolated with a relay.
For example, all of the external limit switch inputs are 120Vac powered, but rather than those inputs directly connecting to a PLC input (which are 120V I/O modules), they power a small relay coil, and the relay is what supplies the 120V input to the PLC.
I have not had a lot of exposure to systems built for these sort of environments. Is this typical practice? Is this just to protect the PLC from outside world?
I plan to be using 24V for all of the external switching, etc., but would it be smart to replicate this in the new system? Appreciate any thoughts.