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Design, Control & Automation of a Low Cost Hydroponic Irrigation Rig

MM4MPR - Individual Project 11/12 Chris Pearson
by

Chris Pearson

on 15 November 2012

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Transcript of Design, Control & Automation of a Low Cost Hydroponic Irrigation Rig

The System Plant Production-Hows it done? Plants require a lot of controlled variables to be healthy!
Some are easy... Temp, light, humidity (above plant variables)
Others are hard... nutrients, pH, correct water quantity (root zone variables)
There are currently, 2 main plant production systems SOIL or HYDROPONICS (SOIL LESS) More traditional Root zone variables
More stable
Harder to optimise Root zone variables
Less stable
Easier to optimise MM4MPR Individual Project 11/12
Chris Pearson "Design, control & automation of a low cost hydroponic irrigation rig" Event Control & Irrigation Regieme Partly automatic regieme used
Microcontrollers uses 16MHz crystal
1Hz interrupt controls soft timers Flow measurement Level sensor produces digital signal which operates run-off pump
I needed an analogue one! Discrete time filter (Thanks Prof. Garvey!) Continuous time: Re-arranged in discrete time: k=1... steady state response to step input is 1
Critically damped...no oscillation about step input
Mass => Need an idea of the systems natural frequency and this requires assumptions:

(1) How many times is the run-off pump going to be switched in an hour?
(2) How much run-off fluid is moved with each switch Main system objectives Maintain target EC
Maintain target pH
Maintain target leaching fraction. Peristaltic pump design All fluid is contained
Low maintenance
High accuracy, driven with bipolar stepper motors
Typically 20% occulsion is desirable
This design sees 17% Control Loops EC & pH: Irrigation: Straight proportional action
Setpoint must not be overshot Proportional and integral action Results Pumping Capabilities Flow estimation Irrigation Control Below 100 steps/s (0.5 rps), relationship very similar
Past this, distinct plateaus emerge as a result of changing fluid dynamic behavior.
Max pumping capability ~ 6.3 l/hr
Power = speed * torque Max pumping speed in the system set to 200 steps/s (1 rev/s) or 4.3 l/hr
According to the flow estimate of run-off at steady state, I am operating at ~10% of this = 0.43 l/hr
Therefore irrigating at 175% of what was initially assumed. ... hydroponic irrigation systems hold two important functions

(1) Replenish elements stored in the root zone.
(2) Provide mass flow of these elements to ensure uniform balance throughout the root zone Test 4 shows instability as the gains are set to highest. NB the non-linearity of this loop can lead to aggressive flick seen at the end of Test 4
Test 3 highlights stability at this operating region
'Spike' at about 30 mins in Test 3 is due to air blockages in flow restrictor
Gains must be set low enough to keep in phase with DTF Conclusion Non-linearity of irrigation loop means gains set for stability in one operating condition may not be stable in others. i.e if Q_plant is drastically different.
Taking logs of the set point and feedback ratio would help linearise the calculated 'error' and thus desensitise the controller output
Faster method for measuring the flow would yield faster and more accurate response for irrigation control
Peristaltic pumps offer a great solution to nutrient dosing and delivery
Overall the system shows promise, however further development is needed before I would be happy to grow my own tomatoes! If you look in the steady state region (past 80 mins) you see the ratio of flows has been met...more or less!
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