logo Welcome, Guest. Please Login or Register.
September 22, 2018, 10:23:57 PM
Home Help Search Login Register
 News: You can search the forum for answers to your question in a previous discussion. Please click on the "Search" button above and enter the keyword to search. To get to the full discussion of a listed message, please click on the TITLE of the message listed in the search listing.

  Internet PLC Forum
  General
  Technical support
(Moderator: support)
  Analogue Refresh Rates
? previous next ?
Pages: [1] Go Down Reply Notify of replies Send the topic Print
   Author  Topic: Analogue Refresh Rates  (Read 3085 times)
Ben_Whitworth
Guest

E-Mail
Analogue Refresh Rates
« on: July 21, 2003, 09:17:29 AM »
Reply with quote

We have just bought a T100MD888+ and need it to read an analogue input from a 0-5v variable proximity sensor and switch on a relay when the value of the analogue reaches a predetermined value.  The sensor is reading distances between itself and an eccentric cam on a spindle and hence will receive the value at which to stop many times and often.  At the moment it switches the relay (that stops the spindle) only occasionally.  It often will pass straight through the value and may do three or four times in a row.  The cam is presently only turning slowly and the sensors refresh rate is high enough to record the required value.  It seems as if the PLC is not reading the analogue input quick enough.

Is it possible to increase the scan rate of the program via any tricks or to increase the refresh rate of the variable given by the ADC??

Cheers
Ben
Report to moderator   Logged
support
Global Moderator
*****

Posts: 3070





View Profile WWW E-Mail
Re: Analogue Refresh Rates
« Reply #1 on: July 21, 2003, 11:28:32 AM »
Reply with quote

If you need to read the analog data at high speed then you need to execute a non-differentiated custom function {CusFn} at every scan of the ladder program. Within the custom function you are reading the ADC input value to make a decision. Note that any SETIO or SETBIT command executed within a custom function will not update the physical output until the end of the ladder logic scan. Hence, if you need to turn on an output within this custom function, then you should allow this custom function to end so that the ladder logic can scan to the end of the whole ladder program and the physical output can be updated.  

If you are already doing the above and feel that the input ADC is not updating quick enough, then you may have to remove the 0.1uF capacitor at the corresponding ADC input. The 0.1uF acts as a low pass filter and is meant to filter out the high frequency switching noise but it also means that the frequency response is altered.

Note that ADC input #7 and #8 do not have 0.1uF capacitor. So a quick way to test if the 0.1uF cap is affecting your need for high refresh rate is to move your sensor output to the ADC #7 or #8 and change your ADC(n) command accordingly.


Report to moderator   Logged

Email: support@triplc.com
Tel: 1-877-TRI-PLCS
Ben_Whitworth
Guest

E-Mail
Re: Analogue Refresh Rates
« Reply #2 on: July 24, 2003, 10:04:57 AM »
Reply with quote

Thanks support - it is better on output 8

However, on a slightly different point, how accurate are the ADC??  The value in the PLC is wavering by about 24 units i.e 24 / 4092 , but the voltage giong into the PLC is steady, at least to the limits we can measure it (2 decimal places).

Cheers

Ben
Report to moderator   Logged
support
Global Moderator
*****

Posts: 3070





View Profile WWW E-Mail
Re: Analogue Refresh Rates
« Reply #3 on: July 24, 2003, 11:00:06 AM »
Reply with quote

The built-in ADC has repeatability of about +/- 3LSB, which is about +/- 12 counts since the reading is amplified by 4 due to internal scaling.  Input with 0.1uF capacitors should be more stable due to the filtering of switching noise imposed on the analog by the digital side of the PLC.
Report to moderator   Logged

Email: support@triplc.com
Tel: 1-877-TRI-PLCS
Pages: [1] Go Up Reply Notify of replies Send the topic Print 
? previous next ?
Jump to: 


Login with username, password and session length

Powered by MySQL Powered by PHP Internet PLC Forum | Powered by YaBB SE
© 2001-2004, YaBB SE Dev Team. All Rights Reserved.
Valid XHTML 1.0! Valid CSS!