×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Pro/Con using PLC for analog input?

Pro/Con using PLC for analog input?

RE: Pro/Con using PLC for analog input?

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.

RE: Pro/Con using PLC for analog input?

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.

RE: Pro/Con using PLC for analog input?

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?



RE: Pro/Con using PLC for analog input?

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources