Pro/Con using PLC for analog input?

[haygood2]

My company does not use our S7 300 series PLC for analog inputs.
Instead we use these:

http://bellsouthpwp.net/h/a/haygood33/karl.htm

]

http://www.dr-staiger-mohilo.de/e_kraft-weg_DMFPV3.html

I ask the engineers why not let the PLC do the work??
They say too much programming....

What are the benefits of using these instead of the PLC?
Thanks!!

 

[jabaldauf]

I am not familiar with the specific hardware that you are using. However, the PLC may be limited by any of: sampling speed, resolution (bits), voltage limits, and cost. The PLC might further have memory limitations, whereas these other units appear to have data logging capability.

Datalogging in particular can require a degree of programming that may have shaped your engineering decision.

 

[DAVEZIMM]

With the S7-300 Analog input card, you basically get a hexadecimal value in a PIW (Peripheral Input Word) and the rest is up to you. Looking at the typical applications of the devices you showed with wheatstone and tension scaling built in, you would avoid alot of coding and scaling work to achieve crudely the same result. Also, little problems or offsets that might develop over time would require a programmer to fix and address rather than most process technicians familiar with these units. Lastly, and most importantly in considering the 80kHz response time of the first unit, the typical PLC scan of the S7-300 could be 20-30ms thus you could miss a lot of data that might be important to your application.

 

[jonesy29847]

Assuming you are looking for the functionality of the DMF-P V3, I am not sure you can even come close using analog inputs on a PLC. I say this because some of the claims of the hardware may require greater capibilities than the PLC, Bandwidth for one, math functions are another, the list could go on. A PLC is limited to a frequency of about 20hz to avoid aliasing. Mathmaticly speaking integration may be a big hurdle, and it would appear that some decision making can be made in the DMF-P.

Does this help?

 

[bprinner]

The SIEMENS S7 300 series has interupt driven capabilities which can get you fast program control and advanced math capabilities. One downfall however is that as you increase you output buffer, once it hits 32767 it flips to ZERO and freezes. You have to add extra logic to limit the output buffer from 100 to 32500 to prevent the buffer over-runs.