I'm a final year engg student of instrumentation branch. In my final year project, with lot of confusion and improper guidance, we chose the topic "greenhouse temperature control using PLC" (we had to actually include PLC S7-200 CPU 212 for giving weight to our project). The idea I developed was very simple. Sense temperature using IC LM35, check whether the temperature is in specified range or not! and then give corresponding input to PLC via relays(as this model has only inbuilt digital i/o module, as far as i know). The PLC would then operate a fan or heater depending on the condition.

But now this project has been disapproved as it doesn't meet the level of final year project. The College guide wants us to directly give the analog input to PLC from comparator (or give the temperature sensor output to PLC directly and let PLC perform comparison and following process). As far as i know it is not possible with the given digital i/o module. The guide, is not so aware of PLC. He and many others have already humiliated my work (i know i didn't do a great job by sensing temperature, comparing it with upper and lower limits and then switching on corresponding relay) but i have done it very sincerely. Its my humble request to everyone who is going to answer to my post, to not humiliate me more.

I have to finally submit and appear for final viva for project on 4th may. I haven't yet started writing thesis that has to be submitted on that day.I'd be probably failed if present status of project won't change. So plz help me.

I searched on net for what can be done that this project would be presentable. I thought to better design a good ladder logic program using many parameters rather to concentrate on single parameter- temperature. On another site i got a post on 'GREENHOUSE AUTOMATION' by user. He wrote the following in his post

"In a first phase I would like to control the ventilation system (window vents in the roof of the greenhouse) in relation to the temperature and the relative humidity in the house and the wind-velocity outside of it (When there is excessive wind the greenhouse should be closed, whatever the temperature and the humidity).

Logically I think I need 2 analog-inputs for the temperature- and the humidity-sensors, 1 high-speed counter to count the pulses of an anemometer and 2 digital inputs for 2 switches that can tell the PLC when the limits of the window vents opening and closing are reached. On the output I think there must be 2 relays, for driving an engine in 2 directions, for the opening and closing of the vents."

with the number of inputs as mentioned above,i feel that the ladder logic code could be expanded. But i cannot understand the role of each parameter in control of vent. I'd give those inputs manually.

Can anybody plz help me in understanding the whole process, i mean, when will vent be closed and opened on the basis of inputs fed to PLC.

I shall be very grateful for this help..

Thanks

 

First of all, analog input modules ARE available for the S7-200 (though not built in IIRC). That might be the easiest solution.

Or you could use the newer S7-1200, which is inexpensive and I believe some (all?) models come with built-in analog inputs.

Finally, one standard solution for getting analog data into a PLC with only digital inputs is to encode the analog into a digital signal with frequency or pulse width conveying the analog value, or binary coded if you can dedicate multiple inputs to a single analog signal.

 

Friend why u r not trying to another PLC instead of siemens, if u r interesting to another PLC then i have one Suggestion to u. I am very interesting to your Green house project.

Avinash ,
jain Irrigation Ltd.
jalgaon