What part of this idea does not make sense?

Wouldn’t it be great if AD analog cards came with an integrated transmitter?



Then we would just buy the PH or Temperature sensor, wire it to the card and have an analog to digital converted signal to work with in the PLC logic?



Is there any other PLC that has this capability or is this impossible? I would like to hear your comments.

There are some wireless licensed radios out there that have a few I/O points that wire directly to the radio. I've used these radios but always needed an intellligent remote station so I've never used this feature on them. I don't know if this comes in any unlicensed modems. A small brick PLC with and analog input and comm port to talk to the radio would do the trick.

It sounds good, but I don't see how it could be done. Each analog field device has it's own range, so you would still have to set a range somewhere.


Otherwise you would be talking digital communications, like Foundation Fieldbus or HART or others, which put the A-D convertor in the transmitter. These are both great, and take a great deal of work out of a start-up when you are trying to verify range settings in the field compared to those in the PLC. I've used Emerson's DeltaV in the past, and it was nice to be able to make a change in logic that was actually residing in the transmitter.

I'm not 100% sure I understand the question...

Then we would just buy the PH or Temperature sensor, wire it to the card and have an analog to digital converted signal to work with in the PLC logic?

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Go get technical, all analog cards do this, you wire a sensor to the card and you get an A to D conversion (in binary) to work with in your program. Its just not always in a convenient format.

Are you actually asking if the card could provide the process variable as an already scaled number? As in if you attached a PH meter the card would deliver a real number such as 8.85 without the need to manipulate and scale a raw binary value (like 20713 to get 8.85) in the program?

If so there are a large number of PLCs available which provide this capability.

Most PLCs have cards that will accept RTD and/or thermocouple inputs.

Basically, all else is 4-20ma, 1-5volt, -10/+10volt, 0-20ma etc.

Special purpose sensors, things like pH, Chlorine, vibration, load-cells, and other similar devices need much more than just a power-supply and an analog input.
There are all kinds of signal conditioning that need to be interposed between the sensor and the analog output.

It would be highly impractical for any PLC manufacturer to make modules that could control and deal with every kind of device out there. Also, that would require calibration people to calibrate the PLC's. It's much easier to calibrate a device/transmitter than jump into PLC code.

piscis said:

Then we would just buy the PH or Temperature sensor, wire it to the card and have an analog to digital converted signal to work with in the PLC logic?

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Are you referring to something like THIS?

Most analog devices provide a low level signal, usually non-linear, that is converted to a linear high level (4-20 mA or 0-10 VDC for exampe) signal.

There is no technical reason why this couldn't be done. The challenge is that every primary transducer has a different output. For example, a strain gauge based pressure transmitter output differs from a capacitance based pressure transmitter. Some pH probes use a lignuim vitae reference electrode, some use polymers.

A non-trivial technical issue is that low level signals, like mV or micro-amps, are difficult to transmitt any distance without a lot of EMI/RFI problems and loss of signal integrity. This might be solvable.

The biggest hurdle is economic. Because there are many different types of transducers with many different types of output, it would take many different algoritms to linearize the signal in the PLC and convert it to engineering units. It wouldn't be economical, particularly since 4-20 mA is so widely used.

Thermocouples and RTDs are standardized and used in many many applications. It is economically feasible for PLC manufacturers to develop specialty cards for these devices because the market for a standard product is significant.

Since we're all guessing as to the meaning of the question...

It's my guess, and my "answer", that piscis doesn't realize that, in general, the sensor IS the transmitter! (There is a caveat at the end.)

The sensor, whatever type it might be, determines the magnitude of the particular condition being detected and then affects the value of the current going through, or the voltage going across, the sensor according to the condition.

That effect, in turn, "transmits" the affect of the condition to the A-to-D converter.

The A-to-D converter then converts that "transmitted" effect into a number.

Then, if an engineering type value is required, that number is run through a scaler which produces that engineering value.

Caveat:
Now, it might be the case that he is talking about a sensor that is connected to a local module that replicates the same effect which then affects the signal going back to the analog input module on the PLC. In which case, he might be talking about a radio link between that module and the PLC analog input card.

Until piscis comes back and clarifies his question... my answer is every bit as valid as any other.

By the way, I'm Sagittarius. To some, that might explain a lot.

By the way, I'm Sagittarius. To some, that might explain a lot.

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Foot in mouth disease Terry? I am a late Scorpio and have some Saggi traits - that is one of them.

The devices he is talking about typically generate millivolt signals instead of the tradtional 0-5, 0-10 VDC or 4-20 mA signals. In many cases, a signal conditioner / converter is required.

Various PLCs offer specific modules to handle these signals. However, it isn't always practical when you need a module for every unique signal...one for thermocouple, one for RTD, one for millivolt, one for current, one for voltage. That can get quite costly...and very wasteful.

We have done two applications, interfacing directly to an ORP sensor.

The challenge for the traditional PLC manufacturers is offering a competitive off the shelf product that can provide the analog flexibility many applications require.

To say that it can't be done would be in error...because it can be done...why? Because we have been doing specialty signals as well as multiple combinations for years.

The issue is both the willingness and the feasibility for a company to do so. For the larger companies who's production is completely automated, makes the ability to offer anything custom, near impossible, unless your volume is such that they would do a specific production run.

piscis, in answer to your question...yes it can be done.

Hope this helps.

God Bless,

BobB...

My foot grows from my mouth... what's your excuse?

Stephen...

How do you KNOW what he is talking about? You are guessing, aren't you?

Terry,

Even though he didn't express it plainly...he wants to be able to interface certain types of sensors directly to the analog modules. The sensors he had mentioned do not transmit a standard signal, such as 0-5 VDC or 4-20 mA, therefore additional hardware is required.

In the instance of an ORP sensor, our customer had to use a $500 interface board. In doing some research on the sensor, we determined what signal the sensor was transmitting and configured our analog input for that range.

The analog sensors are special in what they actually read, however, they are not special in the sense of what they output...they just use a non standard voltage range.

I wouldn't call it guessing...I would say that I am familiar with the sensors he is talking about and know what is required...and understand his plight, in that brand PLCs are designed to accomodate the standard, not the special.