JohnnyDrumstix said:
What is the part number of the "sensor" you are using? That ABB AX460 number posted is a "controller" made to drive an external device...
Have you considered...what if this "controller" is actually controlling an "external device"? Would you still say?...
JohnnyDrumstix said:
...I will almost guarantee you you wont need the AX460 anymore once you give me the actual sensor part number that is wire to the unit!
You are lucky you said "almost"!...
I have previous experience with these ABB pH Analyzers, having installed and commissioned them in Effluent Plants. Although they can be used just to retransmit process analog input data, similar to a simple transmitter/transducer, that is often just an "added bonus", or extra feature, more so than their primary reason for being selected i.e. it is quite possible it could be doing more here than just "conditioning" an analog signal to be passed on to the overriding control system. They are usually specified for their very specific abilities in handling process pH electrode probes, and often more so to control the pH level itself. There are analog and relay outputs which could be used in all manner of ways to control, say, dosing pumps or valves, etc. They are often used for closed loop PID control of such critical processes, and quite good at it too. Another feature is a built-in wash facility where the relay outputs can be configured to control a solenoid valve and/or a wash pump to clean the probes. So, in many applications, they are far from just "conditioners", or simple transmitters. Considering this, we should not "lightly" assume we can just eliminate them out of the equation and replace them with some clever programming.
That's just some food for thought.
But, even if in this application it actually is only "conditioning" the pH value for retransmission, we must still consider the benefits to using such devices before the PLC, as danw was good enough to start pointing out...
As danw mentions, the Analyzer's analog input resistance must be very high to handle the pH electrode's high glass bulb resistance. We are talking in the order of 1e+13. That is 10,000,000,000,000Ω or ten quadrillion ohms. Not something you can just easily whip out of the middle of a pH probe to PLC controller circuit, even if it is only "conditioning" the signal being retransmitted, wouldn't you agree?
They also provide built-in calibration, and later recalibration, to meet very high Standards - DIN, Merck, NIST, US Tech, etc. They have built-in zero span and drift compensation to maintain the original calibration accuracy, further reducing the need for future recalibration. This specific unit provides automatic or manual Nernstian temperature compensation (-10 - 200 °C / 14 - 392 °F) and between 40 and 105% Calibration Slope, user configurable. Features you yourself seem to be familiar with, yes? And again, I would also hope you would agree, not features easily replicated by trying to use these pH probes directly with generic PLC controller analog inputs?
And just to add - another key advantage of using these Analyzers is that they are more often mounted close to the probes so the cabling is kept quite short between the probes and the inputs, rather than cabling some distance back to a PLC enclosure, complicating the resistive path during calibration. The provided analog output is then "ready to go" for retransmission to a supervisory controller using suitable signal cabling.
These Analyzers have a very specific and bespoke reason for existing, and as such, we should really give them their due respect, I feel.
In my opinion, I don't think you are going to find too many Process Engineers who are going to be inclined to attempt, or hire a Programmer to attempt, re-engineering all the above "ready made" features using a PLC, when the device was specifically specified in the first place for having those features.
Back more to the topic at hand...
It is this Analyzer's analog output, provided for retransmission, that we must concentrate on here with regard to connecting the analog signal to the POINT I/O module. The specific connections for this output are on:-
Connection terminal block A - Analog Output 1
A13 +
A14 -
So that makes it a 2-wire 4-20mA analog current signal.
MTPLCUSER said:
I am trying to understand how I wire a device that has a 4-20ma output and provides the 24vdc power as well...
How have you come to this understanding? With nothing connected, are you measuring 24VDC across A13 and A14?
The Analyzer's analog output is listed as being an isolated output, and no more. This means the 2 wires (+/-) are isolated from the rest of the Analyzer's circuitry. This should permit you to wire this as a 2-wire loop-powered analog input.
i.e. the PLC may provide the power source. That is how I remember wiring them?...
Regards,
George