Greenhouse Automation

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 this very site i got a post on 'GREENHOUSE AUTOMATION' by marc(username mdl). 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 marc mentioned,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
Sagar

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

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It seems that you may have selected the wrong model of S7-200. For analog control, you need the S7-222 model, or better.

You would 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.

When there is excessive wind (as measured by the anemometer and counted by the PLC High-Speed Counter Input - say greater than 10 MPH), the greenhouse [vent] should be closed, whatever the temperature and the humidity.

Some of the S7-200 PLCs (S7-222) do have a provision for adding analog input modules, as discussed in this recent similar thread on rainwater control with S7-200 PLC:

http://www.plctalk.net/qanda/showthread.php?t=69735&page=2

If you can buy a S7-222 PLC with add-on auxiliary analog input and output modules, then you can implement true proportional control of your ventilation damper and heater. Instead of using On/off controls, you could use a control scheme that controls a heater or vent a percentage amount based on amount cooling needed (difference between setpoint temperature and actual temperature.

Thanx for ur kind reply, lancie.

But the problem is that the PLC we are using belongs to the institute and we have to use only that plc. I dont have options. S7-200 cpu 212 is the only choice.

Now the analog inputs are not of that great concern. I'd try and convince my guide to better focus more on the ladder logic assuming that we are already receiving all the inputs. I'd use relays or manual switches for the demo purpose and that much would be acceptable by the guide, i suppose!

Someone also told me that i can assume these virtual analog inputs to be saved in memory. and if it is not that possible, i'm thinking to use manual switches for representing all inputs. For instance,for the temperature analog input i shall assume the analog output of temp sensor to be fed into two comparators for limit check and then actuating relays which would be inputs to pLC. I shall do this for all analog inputs. Is this a bad idea? (I dont know much)

The main problem bothering me is what is actual problem statement of this automation for which i'd be designing the ladder logic. I mean what are the conditions to be met for open and closing of vent, fan or heater etc. I need a very clear description of automation procedure.

Thanx
Sagar

My advice to you would be to select a different topic to apply a PLC to. Your instructor feels that your choice of "greenhouse temperature control using PLC" requires analog inputs to be of sufficient scope to qualify for credit. Since you don't have a PLC with analog inputs, you need to choose a model to automate that is sufficiently complex without needing analog inputs.

I suggest a washing machine.

From what you have submitted I understand
1. You have to submit a thesis (ie proposal) by May 4.
2. Your advisor does not like your topic or application.
3. If you do not work this out you are gonna FLUNK.

You have some choices - the first issue I think is politics.
You can satisfy the current advisor - what is he interested in?
You can get another advisor - I recommend this - but you have a time problem if you are going to get this done and write the proposal by May 4.

IF you learn nothing else from this experience you will gain from the experience of what happens with poor planning. You are GOING to learn this lesson the hard way.

Yes I am somewhat qualified to make these statements. I have a BS and MS which required research and thesis at U of Washington in USA. My master thesis is 225 pages long.

IF I had planned things better I would have saved myself a lot of work.

I also had supportive advisors who gave me a lot of latitude (too much actually) and support.

RECOMMEND.
1. Get a different advisor
2. Improve your greenhouse project theoretical design
3. Put together a list of equipment that will adequately support your design.

I did NOT do any of what I am recommending and let me tell you I learned some lessons. That is what research is all about.

"If we knew what we were doing it would not have been research"
is the best summary of my project.

Dan Bentler

Sagar29 said:

Thanx for ur kind reply, lancie.

But the problem is that the PLC we are using belongs to the institute and we have to use only that plc. I dont have options. S7-200 cpu 212 is the only choice.

Now the analog inputs are not of that great concern. I'd try and convince my guide to better focus more on the ladder logic assuming that we are already receiving all the inputs. I'd use relays or manual switches for the demo purpose and that much would be acceptable by the guide, i suppose!

Someone also told me that i can assume these virtual analog inputs to be saved in memory. and if it is not that possible, i'm thinking to use manual switches for representing all inputs. For instance,for the temperature analog input i shall assume the analog output of temp sensor to be fed into two comparators for limit check and then actuating relays which would be inputs to pLC. I shall do this for all analog inputs. Is this a bad idea? (I dont know much)

The main problem bothering me is what is actual problem statement of this automation for which i'd be designing the ladder logic. I mean what are the conditions to be met for open and closing of vent, fan or heater etc. I need a very clear description of automation procedure.

Thanx
Sagar

Click to expand...

Hi Sagar,

Don't worry they won't fail you unless you don't have any thing to show or at very least "explaining" them.
Ask your adviser to support you in every possible way...If you have any PLC training centre near by, you can reach out there for help...usually they have some spare PLCs & modules which you can take on rent...Also If you have SCADA you can write values in memory area of your PLC to compare and trigger on your defined condition, I think this will solve your analog module problem...& Its your project you must know what you are trying to build...

I mean what are the conditions to be met for open and closing of vent, fan or heater etc? I need a very clear description of automation procedure.

Click to expand...

Sagar,
You have said the problem precisely. You do not know enough about growing plants in greenhouses, so you cannot write a description of how to do it. If you can arrange to talk so someone who grows plants in greenhouses, that would help you a lot.

I have grown vegtables in a home greenhouse, and basically it comes down to temperature, light, water, and nutrients. You have to control all 4 variables to successfully grow plants. The following conditions must be avoided for the type of plants being grown: Too hot, too cold, too dry, too wet, not enough sunlight, too much sunlight, not enough fertilizer, too much fertilizer. Sometimes if two or more plant varieties are being grown, some of the variables will be in conflict.

A PLC program is like many other projects. You must learn enough about your subject in order to provide adequate control methods.

But the problem is that the PLC we are using belongs to the institute and we have to use only that plc. I dont have options. S7-200 cpu 212 is the only choice.

Click to expand...

You are probably wrong about this. There ARE usually other options - just not ones that you have considered. I have sometimes been told that "you have to use this PLC because it is the one we have on hand". If I thought the project would be endangered by using the wrong PLC, I would go out and buy the correct one, install it, make it work, then if the project proved successful, I would submit a bill to be reimbursed for the PLC cost. If on the other hand, I chose wrong and it was a flop, I had to pay the cost personally. It comes down to how much you want something, and how much you believe in your abilities.

Are you willing to do what it takes to get the job done? Are you willing to risk any personal assets, or do you expect to always have a free ride and that someone else will always pay the cost of your screw-ups? Lack of knowledge about the project situation is never an excuse - you should have checked it out yourself! You will find that in the real world, these things are what separates the successful project managers from the students.