greenhouse dream

Hi,
I want to build greenhouse controlled by plc ,I want to sense
Soil Moisture
Temperature
Humidity
Co2
I have ls glova plc (GM7U) which have digital I/O (12 +input , 8 output) , I think the analuge sensors it’s the right choose because most digital sensors I found need protocol to communicate with plc , Am I right?
Can I find sensors it’s output is4-20ma or 0-10-20 v so I can directly connect sensors to A/D(Analogue to digital converter) without need to transmitter
LS Analogue to digital converter(G7F-AT2A

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and G7F-ADHA) have only four channel !!!!! can I use A/D from anther company have more channel (Specially LS A/D connect to PLC through expansion cable and have special block in plc program (gmwin))
Where must soil moisture insert and how much I need ? which sensor should I choose what's your recommendations , cost is first priority .
I shall be very grateful for this help..
thanks alot
Hassan

Read this old thread on a PLC Controlled Greenhouse:

http://www.plctalk.net/qanda/search.php?searchid=3189636

Cheap is not in the equation - the etmperatuire and humidity sensor cost me $600 AU each!
Do not have soil moisture or CO2 - CO2 is of no interest to me.
I have EC and PH but they are not available and are experimental from a friend of mine.
You will have to spend big dollars for this sort of stuff - it is not cheap. If you want cheap do not even try - cheap is not good and will not work properly.

if you have standard signals you can switch over with relais instead of expensive extra inputs.

Thanks

BobB said:

Cheap is not in the equation - the etmperatuire and humidity sensor cost me $600 AU each!
Do not have soil moisture or CO2 - CO2 is of no interest to me.
I have EC and PH but they are not available and are experimental from a friend of mine.
You will have to spend big dollars for this sort of stuff - it is not cheap. If you want cheap do not even try - cheap is not good and will not work properly.

Click to expand...

600$ for each ! the sensor I saw in my searching in internet it's prices not exceed 10$! may be sensors you used are special case ?

You get what you pay for - the cheapest, accurate temperature sensor in use is an RTD and air RTDs cost me $200 but they are accurate. People want to use useless thermistors - geez - even the alternator manufacturers have thrown them out the door and use RTDs these days. And also they have thankfully thrown away J type thermocouples as well - one of the legs is iron and it rusts away very quickly!
Humidity sensors are a dime a dozen but not accurate and cannot be calibrated. You have to look at long term - 5 minutes accurate life is no good. The combined sensors I bought can be calibrated for temperature and humidity - I re-calibrate the humidity each month - humidity sensors are the most unreliable and inaccurate sensors in any system - they must be able to be calibrated regularly.
Yes, you get what you pay for - I wanted accuracy and adjustability - and I did not want **** from China or India either!

Thanks

shooter said:

if you have standard signals you can switch over with relais instead of expensive extra inputs.

Click to expand...

the lowest standard relay's coil voltage is 5v or there is lower than , may be 1v. which sensor should I choose :
1) digital output I mean 0V- xV
2) analogue sensor output for example humidity sensor (0-10v) can I connect it to relay coil so when the humidity high sensor output is neer 10v so the relay contact close and 24v apply to plc input , ARE this right manner ? reliable?

I'm still so confused selection sensor.

As I understand, shooter tells you to switch a couple of similar signals (4-20mA or 0-10V) to one analog input, as the analog modules are expensive. For non critical applications where time and precision is not issue - may be. But for a real application I think that is a bad idea

The lowest standard relay's coil voltage is 5v or there is lower than , may be 1v. Which sensor should I choose :
1) digital output I mean 0V- xV
2) analogue sensor output for example humidity sensor (0-10v) can I connect it to relay coil so when the humidity high sensor output is neer 10v so the relay contact close and 24v apply to plc input , ARE this right manner ? reliable?

Click to expand...

For your (2), you seem to be proposing to use an analog input sensor to operate a relay to produce a digital input to your PLC (converting an anlog signal back to digital so you can avoid buying the analog-to-digital modules). No that is not the right manner, and has no advantage over simply buying sensors that read the parameter and convert it to an ON or OFF output.

If you want the cheapest method, then go with digital sensors with ON/OFF outputs. That will mean that you do not have to buy the expensive analog-to-digital converter modules. It is unlikely that a module from a different brand will work on your Glova GM7U PLC. You do NOT need any special sensors to connect with your built-in digital Inputs, only sensors rated for the input voltage (24 volts DC, I assume) of your PLC.

Your digital-output sensors will be more expensive because they must have a way to set the moisture, temperature, humidity, and CO2 levels to produce a digital ON/OFf output (where you want your PLC to take some action).

I think the analog sensors are the right choose because most digital sensors I found need protocol to communicate with plc. Am I right?

Click to expand...

No, it is the exact opposite. It is the analog sensors that must have a certain protocol to communicate with the PLC. They must match in voltage or current levels.

It is the analog sensors that must have an output that matches any PLC analog input modules that you choose to add on to your PLC. Unless you are very serious and willing to invest lots of time and money into this project, stay away from anlaog inputs and analog modules.

Your digital-output sensors will be more expensive because they must have a way to set the moisture, temperature, humidity, and CO2 levels to produce a digital ON/OFf output (where you want your PLC to take some action).

Click to expand...

If it will be really a green house, perhaps should be a couple of actions, that the PLC should do at the different levels. So it comes again to expensive analog signals, or expensive sensors with several preset levels and several outputs. Perhaps modbus or ASCII based sensors used in home automation will come at lower price.

humidity can be done with a Pt100 sensor as extra to the dry one you are using, just put a small sok on it and dip it with water, you will get a very accurate never calibration needed humidity sensor.
And with a small relay you only need one Pt100 converter this way the humidity is only depends on the accuracy of the Pt100s. And you can calibrate them very easy by putting them in same bucket of ice and drinking water.
moisture is more difficult one way is to measure the weight of a piece of the soil, and these electronic ones (however they are not accurate as salt, sugar is playing a role here.
if needed precision on this one do it like the labs do with some soil and a weighing scale with a big lamp above, and wait for 1 hour until the scale does not change anymore.