Off the wall question about pulling an RPM gauge input into a PLC

Okay, here goes nothing....

I need to pull a speed measurement into an existing PLC. This is on a diesel generator and the existing Tachometer is a Dynalco SPD-100 which measures in frequency. Is there a method of splicing onto this line and running a parallel leg into an analog input card on a SLC 5/03?

I'm making this stuff up as I go, if it is simply impossible, let me know. The goal is to be able to provide a speed measurement into the PLC that can be used in series with the oil pressure sensor.

Right now, when we shut the generator down, the low oil pressure kicks off the safety shutdown relay, which must then be reset manually. A lot of personnel who have access to the controls don't bother to notice that the relay has opened. This wouldn't be a serious problem except that it is our standby generator and if no one notices the relay is out, then the generator won't kick on automatically in case of a failure of our main. My idea is to place the low pressure contact in series with a generator speed contact. If the generator is at 90% speed and oil pressure falls below a certain figure, both conditions are true and there is a fault. If the oil pressure falls but the engine speed is at 30%, no fault condition exists.

Except I don't have a speed input to the PLC. Hence the crazy question. Thank you for your time, I'm curious to see what you guys come up with.

No, that doesn't seem too crazy conceptually. Although I wonder if it is an analog input you really want to go into. From what I can tell this device converts a digital pulse input into a speed. If you plan to parallel off this pulse you would need a high speed counter. And you would need to be a little careful how you parallel it.

Keith

Phoenix Contact, Dataforth, Redlion, Eurotherm, and others make frequency to analog converters.

If the generator can be started automatically I’m guessing it can be shut down in the same way in which case there must be some sort of electronic control. If so, I’d guess that there is an “on/off” signal you could tap into (probably in the fuel control system). If all of this is true, couldn’t you simply monitor the status of the generator that way? If the switch is on and the oil pressure drops then the PLC can react and shut down the generator. If the switch is off and the oil pressure drops then there is nothing to do.

I think the challenge here is that the existing instrument is actually powered by the pulses from the magnetic pickup. That's why it uses an LCD and is intrinsically safe (so it can be used around flammable vapors).

You really might need a purpose-built speed meter rather than a conventional frequency input module or signal conditioner.