g.mccormick
Lifetime Supporting Member
A little background. I work in the engine testing testcell world. I am currently in the process of designing the testcell controls for a new cell of ours. This cell will be originally slated for doing burner testing on aftertreatment systems (customer is supplying a 3rd party hot gas generating burner we are providing the facitlities and cell).
We have a software product that does the full data acquisition and control of hte test cell, normally we use products from Beckhoff, Gantner, Opto22 for IO. The "IO" is usually just used as IO with no control aspects, all control from our PC. With that, it has been normal convention to use a lot of hardwired relay interlocks for differing levels of permissive. This is done usually since our system is flexible enough that someone can change spec files to alter sequencing, etc. The hardwired also incorporates a small PLC that's sole purpose is to receive pulses and do an orderly shutdown of the cell if the pulses stop (Watchdog).
On this cell, I am using Bechoff as the IO, but I am also going to utilize the PLC to handle all of the permissive interlocking that would normally be done in relay hardwire.
The estops, gas safety, and flame detection is handled through safety relays.
Question: Does anyone really rely on hardwired relay interlocks when using a PLC? Its hard to break old habits, and I keep wanting to put in relays where PLC logic will go. The only thing that I can see as a potential benefit of the relays is that it is easier to see what is stoping the process (open the panel, follow the schemits, see whta relay is off).
The fix for this, I believe will be better status between the PLC and my system. Allowing for more statuses (DI's from my system point of view, but not mapped to physical DI's on the Bechoff, just internal logic).
So, long boring story short, other than Estop related, does anyone really use physical relay interlocking?
We have a software product that does the full data acquisition and control of hte test cell, normally we use products from Beckhoff, Gantner, Opto22 for IO. The "IO" is usually just used as IO with no control aspects, all control from our PC. With that, it has been normal convention to use a lot of hardwired relay interlocks for differing levels of permissive. This is done usually since our system is flexible enough that someone can change spec files to alter sequencing, etc. The hardwired also incorporates a small PLC that's sole purpose is to receive pulses and do an orderly shutdown of the cell if the pulses stop (Watchdog).
On this cell, I am using Bechoff as the IO, but I am also going to utilize the PLC to handle all of the permissive interlocking that would normally be done in relay hardwire.
The estops, gas safety, and flame detection is handled through safety relays.
Question: Does anyone really rely on hardwired relay interlocks when using a PLC? Its hard to break old habits, and I keep wanting to put in relays where PLC logic will go. The only thing that I can see as a potential benefit of the relays is that it is easier to see what is stoping the process (open the panel, follow the schemits, see whta relay is off).
The fix for this, I believe will be better status between the PLC and my system. Allowing for more statuses (DI's from my system point of view, but not mapped to physical DI's on the Bechoff, just internal logic).
So, long boring story short, other than Estop related, does anyone really use physical relay interlocking?