PLC?
My first winter project is on my new high tunnel (greenhouse) High
temperature control by ventilation is critical for good growing. I will be ventilating by using a rollup side curtain. I plan to automate the process by using a small 12 volt DC gear motor and wench. I've settled on my mechanical design and plan to order parts tomorrow. My query is on control. KISS is the order of the day here. I plan on two thermostats - Hi and Lo. Also two limit switches - full open and full closed. The control is simple - if you make Hi; roll up for a few seconds and go to delay timer. If you make Lo unroll a few seconds and go to delay timer. Stop on limit switches. Repeat 24X7. I'm thinking this is PLC territory. I know nothing about them. Is there an inexpensive simple to program unit that runs on 12 volt DC? Alternatively, I can see a way to do it with a timer that continuously does a few seconds on then several minutes off continuously. Karl |
PLC?
"Karl Townsend" fired this volley in
anews.com: I'm thinking this is PLC territory. I know nothing about them. Is there an inexpensive simple to program unit that runs on 12 volt DC? Alternatively, I can see a way to do it with a timer that continuously does a few seconds on then several minutes off continuously. I think you'll find you don't need any electronics, at all. Most wenches can think (albeit, just a little) on their own. And all have the requisite strength to open and close a roll-up blind. If the wench can't do the job, fire her and get another. If you're still not pleased, there are all sorts of tiny PLCs out there with most of the requirements a wench could satisfy. LLoyd |
PLC?
There are some of the little (micro, Pico, Nano) PLC's that have a LCD
display and some keys used for configuration. That way you don't need to get programming software, cables, etc. http://www.softwarecorp.com/FX2N14.htm http://www.ab.com/programmablecontro...ontroller.html http://www.automationdirect.com/adc/...nits/D0-05DR-D The Automation Direct PLC looks like it would take programming software and cable. You can get a couple of 12VDC relays to control your gear motor. RogerN |
PLC?
http://www.automationdirect.com/adc/...nits/D0-05DR-D
The Automation Direct PLC looks like it would take programming software and cable. You can get a couple of 12VDC relays to control your gear motor. RogerN I like the price of this one best, its the cheapest. And, Automation Direct has always been good to me. I see the software is free for 100 words max. I have no clue, would this be enough for my application? Room to spare? Karl |
PLC?
"Karl Townsend" wrote:
I like the price of this one best, its the cheapest. And, Automation Direct has always been good to me. I see the software is free for 100 words max. I have no clue, would this be enough for my application? Room to spare? I'm thinking it would. Download the manual and start coding. The documentation should tell you how to count the words you are using. You might want to look he http://www.bb-elec.com/product_multi...TrailType=Main I never got a chance to use one since I the thing I needed done was easy enough to wire up using dicrete relays but it was looking like quite a deal at the time. Wes -- "Dr. Dobb's Journal of Computer Calisthenics and Orthodontia, Running Light without Overbyte". |
PLC?
"Karl Townsend" wrote in message anews.com... http://www.automationdirect.com/adc/...nits/D0-05DR-D The Automation Direct PLC looks like it would take programming software and cable. You can get a couple of 12VDC relays to control your gear motor. RogerN I like the price of this one best, its the cheapest. And, Automation Direct has always been good to me. I see the software is free for 100 words max. I have no clue, would this be enough for my application? Room to spare? Karl I think it would be plenty, not sure if they consider each bit instruction as a word or not. You want open for a couple of seconds if not on open limit and high temp. You want close for a couple of seconds in not close limit and low temp. Another timer could reset the open and close timers if still hot/cold after XX minutes. Maybe if it's hot, opened for a couple of seconds, maybe 15 minutes later if it's still hot open for a couple more seconds. I'm sure you could get it going in less than 100 words, probably in less than 20 words. Some of those other PLC's had analog inputs. If you were to use a temperature sensor to an analog input you wouldn't need the thermostats. A thermistor could possibly be used but may need an amplifier to get better resolution. RogerN |
PLC?
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What is "high tunnel greenhouse "? was PLC?
Hey Karl,
A local farmer here won an award very recently for his "high tunnel greenhouse" farming, and here you are doing the same stuff. Local media didn't attempt to explain what is special about it. We buy "retail" from their farm, and I don't see anything different about either the crop or the greenhouses they have, other than maybe an extended season. Lots of agriculture around here under plastic "glass", maybe several hundred acres. Not as much as in the Leamington area, known as the Ontario Sun Belt, where that would be in the thousands of acres I would think. So, in a few short words, can you tell me what a "high tunnel greenhouse" is, and why it is special anyway? Take care. Brian Lawson, Bothwell, Ontario. ps....lot's of older and "NOS" PLCs available on the internet auctions to try. I'd like to give some a go, but the availability of the programmers required is not so easy to find at "that looks like fun" prices. XXXXXXXXXXXXXXXXXXXX On Sun, 6 Dec 2009 19:31:56 -0600, "Karl Townsend" wrote: My first winter project is on my new high tunnel (greenhouse) High temperature control by ventilation is critical for good growing. I will be ventilating by using a rollup side curtain. I plan to automate the process by using a small 12 volt DC gear motor and wench. I've settled on my mechanical design and plan to order parts tomorrow. My query is on control. KISS is the order of the day here. I plan on two thermostats - Hi and Lo. Also two limit switches - full open and full closed. The control is simple - if you make Hi; roll up for a few seconds and go to delay timer. If you make Lo unroll a few seconds and go to delay timer. Stop on limit switches. Repeat 24X7. I'm thinking this is PLC territory. I know nothing about them. Is there an inexpensive simple to program unit that runs on 12 volt DC? Alternatively, I can see a way to do it with a timer that continuously does a few seconds on then several minutes off continuously. Karl |
What is "high tunnel greenhouse "? was PLC?
"Brian Lawson" wrote in message ... Hey Karl, A local farmer here won an award very recently for his "high tunnel greenhouse" farming, and here you are doing the same stuff. Local media didn't attempt to explain what is special about it. We buy "retail" from their farm, and I don't see anything different about either the crop or the greenhouses they have, other than maybe an extended season. Lots of agriculture around here under plastic "glass", maybe several hundred acres. Not as much as in the Leamington area, known as the Ontario Sun Belt, where that would be in the thousands of acres I would think. So, in a few short words, can you tell me what a "high tunnel greenhouse" is, and why it is special anyway? Think simple greenhouse exept $/sq.ft. must be lower. Usually no to limited electric. Most important crops are planted in soil. The aim is to produce a crop such as tomato, etc. that harvests much earlier in the season. Karl |
PLC?
On Mon, 07 Dec 2009 09:05:28 -0600, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote: Tim Wescott fired this volley in news:xumdnd- : You must lead a boring (or very odd) life if most of your requirements for a wench can be satisfied by a PLC! Hell! PLC's can cook and clean, and fetch a beer. For the other things, one can find a 'temporary wench'. LLoyd Reminds me of the 3F Rule. If it flies, floats, or f----, rent it. Told to my cousin at a dermatologist's conference by an old thrice divorced attendee. Pete Keillor |
PLC?
Thanks for all the links. I surfed them and they all seem to violate KISS
and cost too much. I've been surfing the web for days on this subject and just came from a growers conference. Nobody (growers) there was using automatic ventilation except the university types. So far the simplest unit I found costs $700 each (need four per house) and fails if 110 Volt isn't there. I got the mechanical part designed at under $50 per unit that would run days on a deep cycle battery. Control is my weak subject so I'll spend way more time figuring on this. Karl |
PLC?
--For a good time you might want to mess with a Basic Stamp.
Building the circuit and understanding the process will make it much more fun. -- "Steamboat Ed" Haas : Currently broke and Hacking the Trailing Edge! : looking for a job... www.nmpproducts.com ---Decks a-wash in a sea of words--- |
PLC?
On Mon, 07 Dec 2009 18:39:57 +0000, steamer wrote:
--For a good time you might want to mess with a Basic Stamp. Building the circuit and understanding the process will make it much more fun. There are a number of advantages and disadvantages to this (I was going to suggest it as a last resort). - per-piece price will be lower, if you can find a home for it in an existing enclosure. - It'll be way more versatile than a PLC - It may be harder to wrap your brain around than PLC programming (although maybe not -- relay logic is good if you already do relay logic, but if you're new to it Basic may be better). - It'll be easier to put a display on it if you want. In the final analysis, though, my knee-jerk reaction is to avoid PLCs 'cause I've never used one, yet for this project I'd give it serious thought. -- www.wescottdesign.com |
PLC?
On Sun, 06 Dec 2009 19:31:56 -0600, Karl Townsend wrote:
My first winter project is on my new high tunnel (greenhouse) High temperature control by ventilation is critical for good growing. I will be ventilating by using a rollup side curtain. I plan to automate the process by using a small 12 volt DC gear motor and wench. I've settled on my mechanical design and plan to order parts tomorrow. My query is on control. KISS is the order of the day here. I plan on two thermostats - Hi and Lo. Also two limit switches - full open and full closed. The control is simple - if you make Hi; roll up for a few seconds and go to delay timer. If you make Lo unroll a few seconds and go to delay timer. Stop on limit switches. Repeat 24X7. I'm thinking this is PLC territory. I know nothing about them. Is there an inexpensive simple to program unit that runs on 12 volt DC? Alternatively, I can see a way to do it with a timer that continuously does a few seconds on then several minutes off continuously. Karl Your control rule sounds like it could result in continual oscillation given the right combination of hysteresis, outside temperature, and inside temperature ('continual' in this case being up - wait - down - wait - repeat). Can you find a dead time that will let these things cycle continuously without draining the batteries, yet will still give you quick enough response when the sun comes out from the clouds or a cold front blows in? Instead of a fast motion and then a wait, you could use smaller motors geared down more, and just let them grind things open or shut really slowly. This definitely increases the delay and therefore the potential for oscillation without a fancier control rule -- but then you don't have to struggle with the 'wait' logic. -- www.wescottdesign.com |
PLC?
Your control rule sounds like it could result in continual oscillation given the right combination of hysteresis, outside temperature, and inside temperature ('continual' in this case being up - wait - down - wait - repeat). Can you find a dead time that will let these things cycle continuously without draining the batteries, yet will still give you quick enough response when the sun comes out from the clouds or a cold front blows in? Instead of a fast motion and then a wait, you could use smaller motors geared down more, and just let them grind things open or shut really slowly. This definitely increases the delay and therefore the potential for oscillation without a fancier control rule -- but then you don't have to struggle with the 'wait' logic. You described the exact issue. Mechanically, I've got it geared down to raise/lower 12"/minute with a total travel of 60 inches. Gets hard to gear it much slower. My "plan A" is two stats so you have a dead band, then play with % run to get response without oscillation. I suppose if you knew exactly what percent on/off was optimum you could just gear it for 100% on. I'm intrigued with this analog input option on a PLC. I should be able to use a higher percent on when farther from temp target. The high end computer systems you see on university research houses do this. Karl |
PLC?
On Mon, 07 Dec 2009 13:44:03 -0600, F. George McDuffee wrote:
On Mon, 07 Dec 2009 13:15:54 -0600, Tim Wescott wrote: On Sun, 06 Dec 2009 19:31:56 -0600, Karl Townsend wrote: My first winter project is on my new high tunnel (greenhouse) High temperature control by ventilation is critical for good growing. I will be ventilating by using a rollup side curtain. I plan to automate the process by using a small 12 volt DC gear motor and wench. I've settled on my mechanical design and plan to order parts tomorrow. My query is on control. KISS is the order of the day here. I plan on two thermostats - Hi and Lo. Also two limit switches - full open and full closed. The control is simple - if you make Hi; roll up for a few seconds and go to delay timer. If you make Lo unroll a few seconds and go to delay timer. Stop on limit switches. Repeat 24X7. I'm thinking this is PLC territory. I know nothing about them. Is there an inexpensive simple to program unit that runs on 12 volt DC? Alternatively, I can see a way to do it with a timer that continuously does a few seconds on then several minutes off continuously. Karl Your control rule sounds like it could result in continual oscillation given the right combination of hysteresis, outside temperature, and inside temperature ('continual' in this case being up - wait - down - wait - repeat). Can you find a dead time that will let these things cycle continuously without draining the batteries, yet will still give you quick enough response when the sun comes out from the clouds or a cold front blows in? Instead of a fast motion and then a wait, you could use smaller motors geared down more, and just let them grind things open or shut really slowly. This definitely increases the delay and therefore the potential for oscillation without a fancier control rule -- but then you don't have to struggle with the 'wait' logic. ======= This is one of the reasons I suggested using a PC as controller in the links. "Fuzzy logic," to minimize system oscillation/hunting, is relatively easy to implement and the other parameters such as minimum open/closed time are simple to set/vary. http://www.sciencedirect.com/science? _ob=ArticleURL&_udi=B6V05-45MVNXH-5&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_doc anchor=&view=c&_searchStrId=1126063724&_rerunOrigi n=google&_acct=C000050221&_version=1&_urlVersion=0 &_userid=10&md5=03104bbde30e71fc4b9bbb365222f94 2 http://portal.acm.org/citation.cfm?id=603724 http://www.actahort.org/members/show...knrarnr=406_46 Another reason is the ability for easy expansion to simultaneously control many more inputs, for example not watering during full sun [if that is important]. Inexpensive UPS are now available to maintain the control PC in cases of a power outage. google on "fuzzy logic" "process control" greenhouse for c 4.7k hits In over twenty years of professional life closing control loops I have yet to personally see an instance where fuzzy logic was really necessary. Fuzzy logic is designed as a way to encode an intuitive understanding of a problem and it's solution into hardware and software, when you have no way of getting a solid mathematical grasp on things. But you still have to know enough control theory to verify the resulting -- highly nonlinear -- controller. If you don't do the verification (and inevitable reengineering) then you're just using fuzzy logic as a ready source of mumbo-jumbo to intellectually intimidate the customer while you get paid lots of money to screw things up. Not, mind you, that I have anything against fuzzy logic :-). -- www.wescottdesign.com |
PLC?
On Mon, 07 Dec 2009 13:56:56 -0600, Karl Townsend wrote:
Your control rule sounds like it could result in continual oscillation given the right combination of hysteresis, outside temperature, and inside temperature ('continual' in this case being up - wait - down - wait - repeat). Can you find a dead time that will let these things cycle continuously without draining the batteries, yet will still give you quick enough response when the sun comes out from the clouds or a cold front blows in? Instead of a fast motion and then a wait, you could use smaller motors geared down more, and just let them grind things open or shut really slowly. This definitely increases the delay and therefore the potential for oscillation without a fancier control rule -- but then you don't have to struggle with the 'wait' logic. You described the exact issue. Mechanically, I've got it geared down to raise/lower 12"/minute with a total travel of 60 inches. Gets hard to gear it much slower. My "plan A" is two stats so you have a dead band, then play with % run to get response without oscillation. I suppose if you knew exactly what percent on/off was optimum you could just gear it for 100% on. If only... I suspect that the optimum percentage will vary depending on the difference between inside and outside temperature. You're essentially setting up a temperature control system that has a constant heat input (from the sun), and variable heat shedding (the vents). But the amount of heat shedding that you get at any one moment is dependent not only on the amount that the vents are open, but on the outside temperature and (I suspect) on the amount of wind. So the interior temperature delta that you'll see for 1" of opening will be much bigger on a windy deep-winter day than it will on a mild day in the spring. This points to either needing to change your tuning manually with the season, needing to have an external thermometer that you use to select a control rule, some sort of adaptive controller, a heavily compromised control rule that guarantees stability on bad days at the cost of being really sluggish on good days, or just a happy acceptance of the fact that your motors are always moving, but your greenhouses are always at a good temperature and generating money. I'm intrigued with this analog input option on a PLC. I should be able to use a higher percent on when farther from temp target. The high end computer systems you see on university research houses do this. If you want to go there, let me suggest _right now_ that you forget the PLC approach and lean toward using a PC or a Basic Stamp -- or find a PLC that uses a real programming language. Unless you want to spend your time working around the limitations of your controller instead of solving your problem, you want to start out with a controller that is way more capable than you think you need -- because with control loops, you _always_ end up needing more smarts in the controller than you thought when you started. I still have this problem, and I've been doing it for 20 years. If you look at your controller's capabilities and say "that's about right" then you're severely undershooting your goal. This is actually an odd case, because while I'm thinking of this as if you were a client, I wouldn't necessarily take my own advice if I were doing it myself. Were I doing this for myself I'd start with the simple route that you have proposed or something close, see how it works, and be ready to improve on it next winter. You'll learn so much about the dependability of your mechanical arrangement, the sufficiency of your batteries, plus all the stuff that no one has anticipated even thinking about yet. But if you were paying me to get it working I wouldn't do that. Instead I'd either walk or I'd get all complex (and expensive) on you. Why? Because when you engineer a system that shows promise but doesn't make the customer money, they don't remember the joy of your low-ball bill -- they just remember that you built them a system that didn't @#$% work! -- www.wescottdesign.com |
PLC?
--For a good time you might want to mess with a Basic Stamp. Building
the circuit and understanding the process will make it much more fun. There are a number of advantages and disadvantages to this (I was going to suggest it as a last resort). - per-piece price will be lower, if you can find a home for it in an existing enclosure. - It'll be way more versatile than a PLC - It may be harder to wrap your brain around than PLC programming (although maybe not -- relay logic is good if you already do relay logic, but if you're new to it Basic may be better). - It'll be easier to put a display on it if you want. In the final analysis, though, my knee-jerk reaction is to avoid PLCs 'cause I've never used one, yet for this project I'd give it serious thought. I actually hate PLCs, never got on with ladder logic very well. I'm much more comfortable with computer programming. Are we talking about a PIC chip here, like a 16f628A/16f877A? Would either you or Steamer be willing to get me started with what hardware to order? And stuff to start reading? Karl |
PLC?
"Karl Townsend" fired this volley in
anews.com: I'm intrigued with this analog input option on a PLC. I should be able to use a higher percent on when farther from temp target. The high end computer systems you see on university research houses do this. You're describing a PID (proportional-integral-derivative) control system, which, despite being described as "fuzzy logic", is not -- it's a software emulation of an analog, variable gain, proportional output system. Most decent PLCs have freely-available PID loop software available for them. If you use one of the sorts that allows easy user programming (usually via both ladder logic and a crippled sort of BASIC) you can easily set one up for PID control. You'll need to modify the curtain controller from simply on/off to a position-sensing controller to make best use of such a system. I'm not sure they're in the price range you'd like to spend (although pretty cheap), but the TriLogic PLCs are both capable and easy to program with free software that comes with them. I have a three NC machines (dedicated automated machinery for manufacturing fireworks) operating under control of the TriLogic boards, and have not a single breakdown in seven years of operation (totalling 21 controller-years, 40-60 hours a week, year-round). LLoyd |
PLC?
"Lloyd E. Sponenburgh" lloydspinsidemindspring.com fired this volley in
. 3.70: Most decent PLCs have freely-available PID loop software available for them. oops! Forgot to say, the TriLogic boards have some user-devised, free PID samples to play with. LLoyd |
PLC?
On Mon, 07 Dec 2009 15:08:44 -0600, Karl Townsend wrote:
--For a good time you might want to mess with a Basic Stamp. Building the circuit and understanding the process will make it much more fun. There are a number of advantages and disadvantages to this (I was going to suggest it as a last resort). - per-piece price will be lower, if you can find a home for it in an existing enclosure. - It'll be way more versatile than a PLC - It may be harder to wrap your brain around than PLC programming (although maybe not -- relay logic is good if you already do relay logic, but if you're new to it Basic may be better). - It'll be easier to put a display on it if you want. In the final analysis, though, my knee-jerk reaction is to avoid PLCs 'cause I've never used one, yet for this project I'd give it serious thought. I actually hate PLCs, never got on with ladder logic very well. I'm much more comfortable with computer programming. Are we talking about a PIC chip here, like a 16f628A/16f877A? Would either you or Steamer be willing to get me started with what hardware to order? And stuff to start reading? Karl Do you want to program in Basic, assembly, or C? For Basic, you want to get a Basic Stamp -- these have a PIC on them, but it's hidden behind a layer of programming, so what you see is Basic. They're sold by Parallax, who is not pushing their "propeller" chip, which I can't find it in my heart to recommend because it uses a proprietary compiler, and those always end up burning you in the end. For assembly or C you (probably) want to get a PicKit (SP?) designed by Microchip. They have what you need to get a basic computer up an running, and Microchip at least used to have a free C compiler for some of their computers. I wouldn't recommend the C or assembly route unless you have some serious programming experience under your belt. C and assembly are each the programming equivalent of a manual lathe -- you can do anything possible with them, but you have a lot to learn before you can start doing anything useful with them. _Any_ of these routes will get you a board that only drives logic levels out, and will leave you responsible for all of the surrounding drive electronics to turn it into motor commands and protection to keep it from going up in smoke during the first lightning storm. A PLC is much more likely to have some useful drive capability built in. Look to the Parallax website and SparkFun electronics for neato modules that you can assemble into a working system. Used wisely, you should be able to get away without having to design a board, at least for the prototype system. -- www.wescottdesign.com |
PLC?
....
If you want to go there, let me suggest _right now_ that you forget the PLC approach and lean toward using a PC or a Basic Stamp -- or find a PLC that uses a real programming language. Unless you want to spend your time working around the limitations of your controller instead of solving your problem, you want to start out with a controller that is way more capable than you think you need -- because with control loops, you _always_ end up needing more smarts in the controller than you thought when you started. I still have this problem, and I've been doing it for 20 years. If you look at your controller's capabilities and say "that's about right" then you're severely undershooting your goal. This is actually an odd case, because while I'm thinking of this as if you were a client, I wouldn't necessarily take my own advice if I were doing it myself. Were I doing this for myself I'd start with the simple route that you have proposed or something close, see how it works, and be ready to improve on it next winter. You'll learn so much about the dependability of your mechanical arrangement, the sufficiency of your batteries, plus all the stuff that no one has anticipated even thinking about yet. But if you were paying me to get it working I wouldn't do that. Instead I'd either walk or I'd get all complex (and expensive) on you. Why? Because when you engineer a system that shows promise but doesn't make the customer money, they don't remember the joy of your low-ball bill -- they just remember that you built them a system that didn't @#$% work! Your last paragraph explains why a gear motor on a thermostat is going for $700 a copy. its going to take some fool willing to have a few failures to come up with a $100 solution. OK, I'm sold on trying this basic stamp approach. Sure could use some help on what items to purchase. I don't mind spending a few (several) weeks learning a new subject. Karl |
PLC?
Karl Townsend wrote:
... But if you were paying me to get it working I wouldn't do that. Instead I'd either walk or I'd get all complex (and expensive) on you. Why? Because when you engineer a system that shows promise but doesn't make the customer money, they don't remember the joy of your low-ball bill -- they just remember that you built them a system that didn't @#$% work! Your last paragraph explains why a gear motor on a thermostat is going for $700 a copy. its going to take some fool willing to have a few failures to come up with a $100 solution. OK, I'm sold on trying this basic stamp approach. Sure could use some help on what items to purchase. I don't mind spending a few (several) weeks learning a new subject. Karl There are a bunch of tiny little control processors on the market today. For what you want to do, the BASIC stamp would probably be the best choice. Start with Wiki for a good overviwew: http://en.wikipedia.org/wiki/BASIC_Stamp More involved: http://www.parallax.com/tabid/295/Default.aspx Best place to get started with the Stamp would be the Activity Kit. (that's probably gonna wrap) http://www.parallax.com/Store/Microc...me,ProductName Example Projects: http://www.rentron.com/BasicStamp.htm Relay control example: http://www.rentron.com/pc-relay.htm http://www.emesystems.com/BS2index.htm Development and info downloads: (including Stamp and application manuals) http://webspace.webring.com/people/a...downloads.html |
PLC?
....
Best place to get started with the Stamp would be the Activity Kit. (that's probably gonna wrap) http://www.parallax.com/Store/Microc...me,ProductName .... I just found an Xmas present request. I'll work through this and other simple projects before even thinking of the task at hand. Thanks for the advice Karl |
PLC?
Tim Wescott wrote:
In the final analysis, though, my knee-jerk reaction is to avoid PLCs 'cause I've never used one, yet for this project I'd give it serious thought. Plc's are great. I had a couple 600 ton St Lawrence forming presses with about 22 control relays, 5 cycleflex counters or timers that would not stay running. Took a class at Allen Bradley on programming a plc and in 43 rungs of logic, I replaced that mess and kept us from shutting down a GM assembly plant. Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller |
PLC?
This looks simple enough to do with two time delay and a few regular
relays. Ladder logic makes more sense to me if I draw it as states and transitions. jsw |
PLC?
What's that Lassie? You say that Karl Townsend fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue by Sun, 6 Dec 2009 19:31:56 -0600: I plan to automate the process by using a small 12 volt DC gear motor and wench. I've settled on my mechanical design and plan to order parts tomorrow. You can make an analog-ish controller to do what you want. I had good results using two small audio amplifiers as a servo drive to power a small gearmotor. For your project I would stick to relay control of the motor though. If you got two standard thermostats, and a few relays, I think you would have it. Look at :http://www.redrok.com/electron.htm for some ideas. If you could rig a potentiometer with the curtain, then you could have proportional control. That is, the hotter it is, the more open the curtain is. With the right gain, it should hold temperature well. -- Dan H. northshore MA. |
PLC?
"Karl Townsend" wrote in message anews.com... ... Best place to get started with the Stamp would be the Activity Kit. (that's probably gonna wrap) http://www.parallax.com/Store/Microc...me,ProductName ... I just found an Xmas present request. I'll work through this and other simple projects before even thinking of the task at hand. Thanks for the advice Karl This place has some Basic Stamp kits for a pretty good discount. http://www.kitsusa.net/phpstore/html...Kits-17-1.html Yesterday I ordered a Boe Bot (based on Parallax Board of Education kit) for my son for Christmas. I first started to recommend a Basic Stamp to you for a control but I thought by the time you connected it to drivers for the old lady... err wench it was getting sort of complicated unless you want to fiddle with electronics. Basic Stamp and temperature measurement is easy enough but what about the control of the motor? Do you have a winch control? A couple of relays or a solid state H bridge? Don't forget to protect the Basic stamp from the inductive kickback if you use electromagnetic relays. I kind of like the idea of something like the Gordos or Opto22 solid state optically isolated relay modules to drive the final electromagnetic relays (understand I don't know what kind of currents your motor will draw). Of course if you're into playing with electronics you could get some logic level power mosfets that would probably do the job. Many of the Stamp kits have applications for sensing temperature. I would think the goal for your greenhouse would be to position curtain so that you have zero rate of change with zero temperature error and adjust to try to keep the rate of change of temperature proportional to the error in temperature. That way, if it works out right, as you approach zero temperature error, you would approach zero rate of change (curtain would be adjusted to hold temp). Perhaps as part of the program you could have it record how much adjust time of the curtains accumulated over a day. Try to tune the algorithm for best temperature control with minimal winch run time. RogerN |
PLC?
"Karl Townsend" wrote
My first winter project is on my new high tunnel (greenhouse) High temperature control by ventilation is critical for good growing. I will be ventilating by using a rollup side curtain. I plan to automate the process by using a small 12 volt DC gear motor and wench. I've settled on my mechanical design and plan to order parts tomorrow. My query is on control. KISS is the order of the day here. I plan on two thermostats - Hi and Lo. Also two limit switches - full open and full closed. The control is simple - if you make Hi; roll up for a few seconds and go to delay timer. If you make Lo unroll a few seconds and go to delay timer. Stop on limit switches. Repeat 24X7. I'm thinking this is PLC territory. I know nothing about them. Is there an inexpensive simple to program unit that runs on 12 volt DC? Alternatively, I can see a way to do it with a timer that continuously does a few seconds on then several minutes off continuously. If your goal is KISS, skip the PLC and try something like this: http://search.digikey.com/scripts/Dk...&name=Z2695-ND all the control logic is built in. the alarm can be used to revers the direction of the motor. Lots of manufacturers out there with similar options. |
PLC?
For that your parts would be 1 solid state relay to turn motor ON/OFF and
1 electromechanical 4PDT relay motor direction UP/DOWN (2 Poles) and the other 2 poles to jump the limit switches (UP jumps down L/S and DOWN jumps up L/S). Or if you have and would prefer to use 4 SSR's that should work great too. Will a basic stamp power a 4PDT relay coil directly? I haven't checked, but I assume it won't. If it will, do you set up the controller to sink the relay? Or can it also source? Karl |
PLC?
"Karl Townsend" wrote in message anews.com... For that your parts would be 1 solid state relay to turn motor ON/OFF and 1 electromechanical 4PDT relay motor direction UP/DOWN (2 Poles) and the other 2 poles to jump the limit switches (UP jumps down L/S and DOWN jumps up L/S). Or if you have and would prefer to use 4 SSR's that should work great too. Will a basic stamp power a 4PDT relay coil directly? I haven't checked, but I assume it won't. If it will, do you set up the controller to sink the relay? Or can it also source? Karl IIRC about all the controller can drive would be an LED, works with SSR's but would need a driver transistor for a 4PDT relay coil. I kinda like TIP120 transistors, something like 3A output capable with a current gain of at least 1000. A ~1K Ohm resistor, TIP120 NPN Darlington transistor and 1N4001 diode should make a nice relay drive... or a lower power SSR could do the job. IIRC the Basic Stamp outputs can sink or source. I think they can handle a little more current sinking than sourcing, though it's been years since I played with them. RogerN |
PLC?
"RogerN" fired this volley in
m: Perhaps as part of the program you could have it record how much adjust time of the curtains accumulated over a day. Try to tune the algorithm for best temperature control with minimal winch run time. Roger, a decent PID algorithm accommodates changes in conditions, and "learns" the lead/lag characteristics of the system. Most of the even most-basic PID controllers (ready made) have that capability. LLoyd |
PLC?
Karl Townsend wrote:
Would either you or Steamer be willing to get me started with what hardware to order? And stuff to start reading? --Parallax has a kit consisting of everything you'll need to get started and to get a solid understanding of what's going on. The book's called "What's a Microcontroller" and it comes with a kit of parts that are very well thought out. I had a UC Extension class called "Gizmology" that used this material and it was a real eye-opener; I've used it to make a pneumatic tapdancing robot (pretty primitive but it worked!) and lately I've used it to drive my calliope. --Reason I like the Stamp: they've got the most comprehensive manual and it assumes you know NOTHING when you start out; something sadly lacking from all the other systems I've encountered.. --You'll want to start he http://www.parallax.com/Default.aspx?tabid=362 Ping me off list ifyawanna know more. And they have a forum at: http://forums.parallax.com/forums/ -- "Steamboat Ed" Haas : Currently broke and Hacking the Trailing Edge! : looking for a job... www.nmpproducts.com ---Decks a-wash in a sea of words--- |
PLC?
Would either you or Steamer be willing to get me started with what hardware to order? And stuff to start reading? --Parallax has a kit consisting of everything you'll need to get started and to get a solid understanding of what's going on. The book's called "What's a Microcontroller" and it comes with a kit of parts that are very well thought out. I had a UC Extension class called "Gizmology" that used this material and it was a real eye-opener; I've used it to make a pneumatic tapdancing robot (pretty primitive but it worked!) and lately I've used it to drive my calliope. --Reason I like the Stamp: they've got the most comprehensive manual and it assumes you know NOTHING when you start out; something sadly lacking from all the other systems I've encountered.. --You'll want to start he http://www.parallax.com/Default.aspx?tabid=362 Ping me off list ifyawanna know more. And they have a forum at: http://forums.parallax.com/forums/ -- "Steamboat Ed" Haas : Currently broke and Hacking the Trailing Edge! : looking for a job... www.nmpproducts.com ---Decks a-wash in a sea of words--- Thanks for the advice. I ordered their starter kit and plan to spend several weeks with it right after Christmas. If these can play a calliope they must be "good stuff Maynard" Karl |
PLC?
On Tue, 08 Dec 2009 09:23:49 -0600, Lloyd E. Sponenburgh wrote:
"RogerN" fired this volley in m: Perhaps as part of the program you could have it record how much adjust time of the curtains accumulated over a day. Try to tune the algorithm for best temperature control with minimal winch run time. Roger, a decent PID algorithm accommodates changes in conditions, Yup. and "learns" the lead/lag characteristics of the system. Not unless it's adaptive -- a plain jane PID algorithm isn't adaptive, but can be tuned to be more or less robust to changes in the characteristics of the system. Most of the even most-basic PID controllers (ready made) have that capability. The ones that I know of aren't adaptive -- they're autotune, which means they'll stimulate the system and come up with a tuning on operator command, but they won't do it all the time. I don't think things have changed much -- you don't see much hoopla about adaptive control anymore, and the definitive texts that I have read include cautions about when adaptive control won't work (like, blind). -- www.wescottdesign.com |
PLC?
On Sun, 06 Dec 2009 19:31:56 -0600, Karl Townsend wrote:
My first winter project is on my new high tunnel (greenhouse) High temperature control by ventilation is critical for good growing. I will be ventilating by using a rollup side curtain. I plan to automate the process by using a small 12 volt DC gear motor and wench. I've settled on my mechanical design and plan to order parts tomorrow. My query is on control. KISS is the order of the day here. I plan on two thermostats - Hi and Lo. Also two limit switches - full open and full closed. The control is simple - if you make Hi; roll up for a few seconds and go to delay timer. If you make Lo unroll a few seconds and go to delay timer. Stop on limit switches. Repeat 24X7. I'm thinking this is PLC territory. I know nothing about them. Is there an inexpensive simple to program unit that runs on 12 volt DC? Alternatively, I can see a way to do it with a timer that continuously does a few seconds on then several minutes off continuously. Karl The more I think about this the more I think you want the roll-up curtain to roll down over a tapered hole, so that when you're closed you don't get as much area change as when you're open -- because a given area change will be much more effective when it's cold and the curtains are closed up, and because once the curtains are mostly open a bit more isn't going to change much. Just a thought, but get it working first. -- www.wescottdesign.com |
What is "high tunnel greenhouse "? was PLC?
On Mon, 7 Dec 2009 10:09:26 -0600, "Karl Townsend"
wrote: Think simple greenhouse exept $/sq.ft. must be lower. Usually no to limited electric. Most important crops are planted in soil. The aim is to produce a crop such as tomato, etc. that harvests much earlier in the season. Karl Yabbut what's the difference between "High tunnel" and plain ordinary polytunnel greenhouses? Expiring minds want to know :-) Mark Rand RTFM |
PLC?
"Karl Townsend" wrote in message anews.com... Your control rule sounds like it could result in continual oscillation given the right combination of hysteresis, outside temperature, and inside temperature ('continual' in this case being up - wait - down - wait - repeat). Can you find a dead time that will let these things cycle continuously without draining the batteries, yet will still give you quick enough response when the sun comes out from the clouds or a cold front blows in? Instead of a fast motion and then a wait, you could use smaller motors geared down more, and just let them grind things open or shut really slowly. This definitely increases the delay and therefore the potential for oscillation without a fancier control rule -- but then you don't have to struggle with the 'wait' logic. You described the exact issue. Mechanically, I've got it geared down to raise/lower 12"/minute with a total travel of 60 inches. Gets hard to gear it much slower. My "plan A" is two stats so you have a dead band, then play with % run to get response without oscillation. I suppose if you knew exactly what percent on/off was optimum you could just gear it for 100% on. I'm intrigued with this analog input option on a PLC. I should be able to use a higher percent on when farther from temp target. The high end computer systems you see on university research houses do this. Karl A very cheap contol system would be 2 cheap snap acting home thermostats - 1 mounted high and 1 mounted low, although you might have to monkey with the location. You can still get units that come with an available proportional band adjustment, not much but usually 2-6 deg.F. Set 1 for cooling and 1 for heating. They boot up from power failure every time the power returns. Good luck Steve |
PLC?
Tim Wescott fired this volley in
: I don't think things have changed much -- you don't see much hoopla about adaptive control anymore, and the definitive texts that I have read include cautions about when adaptive control won't work (like, blind). I agree, except that many commercial PIDs now offer it as a user-selectable option. But you need to understand the difficulties it may impose. LLoyd |
What is "high tunnel greenhouse "? was PLC?
"Mark Rand" wrote in message ... On Mon, 7 Dec 2009 10:09:26 -0600, "Karl Townsend" wrote: Think simple greenhouse exept $/sq.ft. must be lower. Usually no to limited electric. Most important crops are planted in soil. The aim is to produce a crop such as tomato, etc. that harvests much earlier in the season. Karl Yabbut what's the difference between "High tunnel" and plain ordinary polytunnel greenhouses? Expiring minds want to know :-) Mark Rand RTFM If you're looking at the structure the difference is blurred. Few greenhouses use natural ventilation - large exhaust fans. most high tunnel have huge natural vent opening. The main difference is application. Greenhouse grow plant for resale in some sort of medium normally on shelves. High tunnel grows fruit and vegetables in the soil. Karl |
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