PLC Controlled greenhouse

[northpad]

WoW

Thanks everyone for all your help. Over the last week I have learned a ton, my head is spinning. Lancie1 you are the man, the drawing you posted...wow thanks
I am working "nights" for the next 2 weeks, so I won't be able to post much. But I'll still be working on some of issues Lancie1 has brought up. I still don't know if I have the right PLC, but I am going to buy some relays, and enclosure and start on JUST the relay/switch enclosers. Should I use 3 position switch for all my outputs (Auto/off/Manual) like JBJennings suggest, or is a 2 position fine? I just don't know......Anyway I'm super excited about this. THANKS again

 

[OkiePC]

I would go for three position switches. Then you can put them in AUTO (future from PLC) OFF, or HAND (Always ON).

We have a refrigeration system that has one for every device mounted on a swingout panel inside the enclosure. They are small toggle type switches, and probably MUCH less expensive than 30mm selectors. I will see if I can find you a part number...

 

[Lancie1]

Well, I agree with Okie - 3-position Auto-Off-Hand switches are nice if you can get them. I think for your application, the simple Allied Electronic toggle switches will do fine, and they actually come in 3-positon ON-OFF-ON versions at about $4/each, so no reason not to get those. If you don't need one of the positions, you don't have to wire up all the terminals on every switch.

Really in this case because you are using relays between the PLC and the devices, all the switch needs to do is bypass the relay, until the PLC is installed or whenever it is removed.

 

[northpad]

OK, 3 position switches it is. It can't be that much more work right? haha

 

[OkiePC]

We have two different types of switches. The tiny ones are ALCO MTA 306E.

The others look like they are more heavy duty, but have no part numbers on them...

 

[Lancie1]

Well not much more work, but more space required on the back of the door for wiring, and of course, more money!

At this point you may want to make a decision on the function of these proposed switches. In an industrial plant, Hand-Off-Auto switches must be wired into the control circuit, because the motor power is usually 480 volts and nobody wants that in a control room. But in your greenhouse relay panel, 120 volts-to-ground is your highest voltage, and it can be wired directly to your toggle switches (provide you buy switches rated for 120 volts), and then from the switches out to the equipment.

That is one alternative, and the one I prefer because it eliminates the relays from the picture and you will have a true "Hand" operation with the minimum required equipment. The second alternative is to wire 24 VDC control power to the toggle switches, and use the switches to energize the 24 VDC relay coils. I think that is what Paul (Okie) has in mind, and this method will be somewhat safer for you to work around. BUT it also means you would have to go ahead and buy the PLC box and at least the 24 VDC power supply, otherwise you could not run the greenhouse equipment.

Your money, your greenhouse, your choice!

PS: I remember the days when we used 480 volts for motor control circuits! How things have changed.

 

[Lancie1]

In the end, I need to have ability to monitor the environmental sensors in real time, with a PC.

Northpad, if you want this capability, you will have to buy another program of some type, such as Rockwell FactoryTalk (or something similar) to run on your PC and provide an operator graphical interface with the PLC. FactoryTalk would be an expensive solution, overkill for a greenhouse operation. There are other brands of PC-type operator programs, such as Automation Direct "Think & Do". Still this is going to require a large committment in time to learn how to program the operator interface.

A cheaper solution would be a graphic panel (Red Lion, Automation Direct, and others) that connects to and interfaces with the PLC through a comm cable. You can get a 6" Automation Direct C-More color operator panel EA7-T6CL for $699, plus $129 for the C-more programming software EA-PGMSW.

 

[BobB]

I am doing something similar on a much smaller scale. It is for growing orchids.
I have puchased a couple of quite accurate humidity/temperature sensors with Modbus RTU communications.
The data is taken back into an Omron CJ1 PLC and used to operate a fogger - 1000psi - to increase humidity. Different humidity settings for different shade houses. Orchid growers have generally found most orchids can handle the temperatures provided the humidity is up. The disanvantage is that if the humidity is too high damping off can occur.
Have had to do a bit of fancy stuff around the operation of the fogger with valves to different shadehouses and if one has called for the fogger lock the others out until the one calling for fog has run its course. One shade house runs at 75% humidity, another at 70% and the third at 65%.
I also have a watering system that is automated through the the PLC. Sensors have been inserted in certain pots in the shadehouse to call for water.
Ph is also monitored via Modbus RTU sensors so that I know when to apply lime to some species to up the ph.
Also have a wireless network set up extracting data back into Citect SCADA system for trending temperature, ph and humidity.
One of these days I may even look into the application of fertliser and fungicide via the PLC but I think fungicde will be a problem as overhead sprayers will not get all the leaves - I think it will really have to be done by hand.
Good luck and come back and let us know how you go.

 

[Lancie1]

There you go, Northpad, a good source of sensor brands and catalog numbers from a working model!

Bob, would you mind sharing the info for your water, humidity, and Ph sensors?

 

[milldrone]

One shade house runs at 75% humidity, another at 70% and the third at 65%.

I'm presuming by this comment you keep these numbers around the clock. I always thought "nature" had a different scheme. Higher humidity at night when it cools down and a little drier as the air warms up during the day, (unless the location depended on moisture laden breezes from the ocean). Just curious.

 

[BobB]

Firsty off when orchids are grown in shade houses it is desirable to try to simulate thier wild growing conditions but not always possible of course.
For example, I grow my slipper orchids in a 75% constant humidity and also on heating matts set at 15-18 degrees c.
Other orchids grow in swamps/bogs for example and so they grow standing in some water.
Even ordinary cymbidiums have been found to grow really well standing in 1" of water in the warmer months - root growth is quite phenominal when they are grown this way.
Growing media is also very important but that is another story - generally no 2 growers will ever agree with each other - we all have generally different mixes. Comnmercial growers often have their own blends and usually will never reveal the exact make up.
The temperature/humidity sensors I use are Comet model T3110. can be found at http://www.cometsystem.cz/ and they have distirbutors world wide. Sometimes they nned to be calibrated on the humidity side but the facilities for this are provided.
Keeping the humidity higher actually reduces the amount of watering required - particularly in the warmer months. We all have different ways of doing things.
The ph and water sensors I have were given to me by a fdriend to try out. He made them and they work really well. He is considering manufacturing them for sale but is having a lot of trouble finding funding.
I am also considering searching the web for some ph and water sensors to try them against the ones he supplied me - may help him and may help me.

 

[Lancie1]

Thanks, Bob.

It appears that the Comet T3110 would work for Northpads proposed PLC arrangement, as he plans to have 4-20 miliamp analog inputs and a 24 VDC power supply. The wiring diagram for the T3110 shows that it is a loop-powered instrument and requires two analog circuits for humidity and temperature signals.

This past summer I visited an orchid farm in Hawaii. The orchid sales area had many different varieties in pots, and the humidity in the room was very high.

 

[northpad]

Gee can't wait till my days off, so I can look in to all this new information. I did get a 12x12x8 water tight junction box. I was going to use this in the grow room that is going to have ~5 relays. One question I have is, sense it does not have hinges, can I install the switches on the side/top/bottom? Or maybe there is a safe way to install them on a DIN rail? Thanks again for all the help.

 

[Lancie1]

Yes, your switches should be on outside of boxes. You don't want to have to open the box just to flip a switch, that defeats purpose of having the switch, because if you open box, you could just flip the circuit breaker or fuse inside the box. If you split your relays up, you will still need to run cables from the PLC cabinet to each relay cabinet.

12 x 12 x 8 sounds small for wiring. Hope your hands are small and your fingers nimble.

Only 5 relays in the Grow room? Do you still have the three 5-Amp 240 volt lighting circuits? What type of lights are these - incandescent? To run those successfully from one relay would take a hefty 2-pole power relay with 20-Amp contacts. I don't see any of those on the equipment list that you posted.

Don't forget that if you run 15-Amp lights through 15 Amp-rated relay contacts, the relay most likely will be overloaded, because the total current through the contacts will also include the current due to the resistance of all the wires all the way back to your power supply (your sub-panel?), and that can add another 1 or 2 Amps to the total load. A relay will survive a small overload for short periods, but over long periods the contacts will burn out.

A good rule-of-thumb is to only load circuits to 80% of the current rating of the lowest-rated equipment. Even most circuit breakers should only be loaded to 80%, so to get 16 Amps through a breaker requires 16/0.80 = 20 Amp trip breaker. So for three 5-Amp lighting circuits, relay minimum rating = (3 x 5)/0.80 = 18.75 Amps. The next standard size would be 20 amps.

It is possible also that your light circuits are not really 5 Amps at 240 volts. That would be 5 x 240 = 1200 watt fixtures, or six 200 watt fixtures on each circuit. In which case you need to state the actual watt load for each lighting circuit.

If you put all your lights on one relay, then you will not be able to set different lighting levels by turning on 1/3, 2/3, or all lights. They will be either all on, or all off. It might be useful to stagger your 3 lighting circuits, putting every third light on one of the 3 circuits. That way, you can have 3 levels of even lighting in the Grow Room, also each on a programmed timer in the PLC.

 

[Clay B.]

Devil in the details

I bring this up becasue I saw you were looking at toggle switches.

If you mount your switches thru the door instead of inside the enclosure you need to make sure they have the same NEMA rating as your enclosure. Also, avoid making any holes in the top of your enclosure, coming thru the bottom will lead to less problems later (think which way water runs).