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Temperature Based Control Using Plc


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Hi !!


I am planning to use a Twido (telemecanique owned brand) Compact PLC for the following application.



Control Logic


it is temperature based Fan and water valves control .


Say at 25 deg c fan should come on and 27deg c another fan should come on and at 30 deg c two water solenoid valves should come ON.


Now, i am using a PT100 as temprature sensor and a temp transmitter to convert it to 4-20 ma signal...Now how this signal will be incorporated into Ladder logic.. has anybody done this programing in Twido..?


a sample ladder logic will be appreciated.








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Hello Chaterpilar


I have no experience with the Twido so I can not really comment on how it would appear on the ladder diagram, but I would expect that normally you would transfer the data from the analog input into a register and process/scale it from there.


The big question is to be the scaling and conversion that occurs in the temp transmitter.

There will be a scale that shows the current output against temperature. Once you have this, you can determine the thresholds to use in you ladder.


Another option to explore is the use of a PLC that has the PT100 input built in.

For example, the Horner XLE OCS HE-XE105 has two analog input channels that can be configured as PT100 inputs. The analog input then shows 20 counts per degree C.


To achieve your requirements, you need to use three "greater than or equal to" comparators operating off the Analog input. In the case of the Horner, you would use the first comparator to test the input for 25 x 20 (= 500) and this would turn on output 1. The second comparator would compare the analog input with 27 x 20 (= 540) and turn on the second output and the third comparator would compare the analog input with 30 x 20 (= 600) and turn on the third output.


I expect that this is easy to do with the Twido, so the only difficulty is in determining the scaling factor.


Best regards,

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Hello Chaterpilar


I have experience with Allen Bradley SLCs. In a case where an analog signal is to measure, an Analog Input card is used which converts 0-10V or 4-20ma signal in to 0-32767. Now we easely scale this signal.

I aspect, that almost same method would be adopted in Twido.


According to my experience in such cases, there are two methods used for switching on or off the out put.

First method is that we use a Limit Tester (comparator) and Out Put Enable instruction for switching on and off the out put. In this case we test the accumulated value (signal input) with two preset values i.e. Low Limit and High Limit. The out put remains on if accumulated value in between these two limits.

Second method is to use a Delay On Timer with instructions Greater Than or Equal and Out Put Enable. The comparator enables the timer which enables his Done Bit after 100-500ms (as we set) time interval. This done bit is used to enable the out put.


This is why that the analog signal continuously changes its value because of noise problem in signal cables. In normal circumstances, this noise is negligible, but in analog input cards where 1 volt converts in a value 3276.7, a small noise changes this value in one to three figures.


This problem can be minimized by using proper signal cable (Belden or equivalent) which capacitance is minimized. If we use a simple comparator and out put enable instructions, then “flickering” problem with out put would have to be faced any time when preset and accumulated value would equal in comparator.



"Don't assume any thing, always check/ask and clear yourself".

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