analog input or connect to switch

Beginner here, selecting new PLC system that has the capability to stop everything if the air pressure drops below some point. My dilema is whether to put an analog signal from a pressure sensor into the PLC, or have a pressure switch. One way requres adjusting the program if needed, the other way requires adjsting the pressure switch. No big deal either way, but since I'm new to PLC's I don't know the pros/cons on that end of the system.
Help!
Thank you!
D²

If the switch is detecting a safety situation then attach the air switch directly to safety equipment as needed. It would still be appropriate to bring some type of contact to the PLC so that it may perform some housekeeping (possibly informing an operator via a display).

If this is just a 'shut the machine down because it won't be able to perform it's job anymore' situation then by all means attach the switch to the PLC letting it perform an orderly shutdown, if that is possible with low air.

Whether or not the setpoint of the switch is adjusted manually or via a link with a PLC depends solely on the switch design. If it is for a safety situation the keep setting the sensing point a manual operation. Depending on how critical sensing this drop in pressure is for personnel safety you may need more than one for redundancy and a fail safe system.

Either type of sensor can handle the situation you've described.

In general, pressure switches cost less than analog pressure transmitters. On the PLC side, discrete (On/Off type) inputs cost less per point than analog inputs. Programming for the discrete sensor will probably be simpler than programming for the analog sensor. If you're starting from scratch, and have no other need for analog pressure data, use the pressure switch.

Assuming this in a non-safety system, go with the switch.
Switches are cheaper than transmitters, digital inputs are cheaper than analog.

Ken

If you go with the switch you wont need a programmer to adjust it later. It will also simplyfy your program.

anolog input

ddouglass, Unless you are using an application that consist of using various pressures and would need multiple readings, it would probably be best to just use an adjustable pressure switch and use the input from that to shut every thing down. It would be easier to program and less expensive.

I'm going to be a contrarian. I use a transmitter for my applications instead of a switch, and I set the alarms inside my logic. (Note that this is for process critical functions, but not an alarm that involves personell safety.)

The transmitter lets me monitor the pressure for diagnostics purposes, including trending if I want it. I can readily adjust the setpoint and deadband from my operator interface, avoiding messing around opening up the switch and tweaking with a screw driver etc. It also eliminates the need for an extra guage, since I have local indication plus indication on the operator interface. I have lots of flexibility, including adding high-high or low-low or warning alarms or whatever. The transmitters are a few hundred dollars more than the switch, but compared to the cost of engineering and installation and programming and deducting the guage this is a small incremental cost.

Tom Jenkins said:

The transmitter lets me monitor the pressure for diagnostics purposes, including trending if I want it. I can readily adjust the setpoint and deadband from my operator interface, avoiding messing around opening up the switch and tweaking with a screw driver etc. It also eliminates the need for an extra guage, since I have local indication plus indication on the operator interface. I have lots of flexibility, including adding high-high or low-low or warning alarms or whatever.

Click to expand...

I'm liking that idea!...

I've always use an adjustable pressure switch inline with air preparation equipment (the modular style), but I can think of many projects that would have benefited by the use of a transmitter instead. Thanks for the idea, Tom... :nodi:

beerchug

-Eric

An analog transmitter can be a force multiplier when compared to a digital switch.
Maybe your application right now is as simple as described, but who knows what the future brings ?
If you have an analog signal, you will find yourself getting new ideas on how to use the signal, maybe in combination with other signals.

Generally I recommend to get all signals into the PLC if possible.
Then create the application in PLC and HMI code rather than in hardware. To add new and improved functionality can be as simple as downloading new code to the PLC and HMI with no hardware changes at all.

A good quality adjustable pressure switch can be MORE expensive than an analog pressure transmitter. But of course you still have to add the expense of the analog input and the HMI. If you have these allready, then there is really no doubt about it.

Tom Jenkins said:

The transmitter lets me monitor the pressure for diagnostics purposes, including trending if I want it. I can readily adjust the setpoint and deadband from my operator interface, avoiding messing around opening up the switch and tweaking with a screw driver etc. It also eliminates the need for an extra guage, since I have local indication plus indication on the operator interface. I have lots of flexibility, including adding high-high or low-low or warning alarms or whatever. The transmitters are a few hundred dollars more than the switch, but compared to the cost of engineering and installation and programming and deducting the guage this is a small incremental cost. [/B]

Click to expand...

I understand use transmitter air pressure, sometime, not know build alarms and "the flexibility"->main functions