IDENTIFICATION OF PARAMETERS, SENSORS AND SENSOR LOCATIONS
3.1 Parameters influencing plant growth
Environmental factors such as light, temperature, water, and soil moisture greatly influence plant growth and geographic distribution. These factors determine the suitability of a crop for a particular location, cropping pattern, management practices, and levels of inputs needed. A crop performs best and is least costly to produce if it is grown under the most favorable environmental conditions. To maximize the production of any crop, it is important to understand how these environmental factors affect plant growth and development
3.1.1 Temperature effect on plant growth
Most plants are injured by temperatures at or slightly below freezing point. Tropical or subtropical plants may be killed or damaged at temperatures above freezing point but below 100C. This type of injury is called chilling injury. When the temperature rises too high, heat destruction of the chloroplast result in cell death. This occurs in the range (45-550C). In tomatoes, fruits exposed on vines to high temperatures and high solar radiation can reach 120-125ºF (49-52ºC). If green fruits are exposed at these temperatures for an hour or more, they become sunburned; and ripe fruits become scalded. This is referred to as a heat stress
3.1.1.1 Air temperature
Each kind of crop has a range called optimum air temperature range in which it grows and develop rapidly. For most crops the optimum functional efficiency occurs mostly between 55 and 75ºF (12 and 24ºC). Plants are classified according to their optimum air temperature range as cool season and warm season crop. Those that grow and develop below 650F are called cool season and those that perform above 650F are warm season.
3.1.1.2 Soil temperature
Soil temperature has a dramatic effect on seed germination, root development and nutrient absorption by root. In general the higher the temperature, the faster these processes occur.
3.1.2 Light
Light in plant is used for producing food through photosynthesis. Light intensity is the major factors governing the rate of photosynthesis. The amount of light received by plants in particular region is affected of the incident (incoming) light and the length of the day. Crops such as corn, cucurbits, legumes, potato, and sweet potato require a relatively high level of light for proper plant growth while onions, asparagus, carrot, celery, lettuce and spinach can grow satisfactorily with lower light intensity
Some plants change their growth in response to day length and exhibit photoperiods. One important plant response to day length in some plants is flowering. Some plants flower when a specific day length minimum has been passed. Short day plants flower rapidly when the days get shorter and long-day plants flower fast when days get longer. Plants that are not affected by day length are called day neutral plants. These plants can flower under any light period.
Plants capture the energy in light using a green pigment called chlorophyll. A very precise number of photons at specific wavelengths (near 680 nm) are required to split a water molecule (H2O) within the green leaf, which releases oxygen (O2), and provides chemical energy to continue the long biochemical process to produce more complex molecules such as carbohydrates. Carbon dioxide from the air and water from within the leaf combine to produce oxygen and photosynthesis
3.1.3 Humidity
The ideal level for humidity depends on the type of crop to be grown. Plants from tropical climates will need a higher humidity than those native to arid regions. Most greenhouse crops favor relative humidity between 60% and 80% RH.
Plants require a higher humidity when the temperature is high and a lower humidity when it is low. This helps to moderate the transpiration rate of the plant i.e. when it is hot the plant will tend to transpire heavily so by making the RH high this will tend to reduce transpiration and delay the point at which the plant starts to wilt. The converse is also true i.e. when temperatures are low then RH should be reduced. At low temperatures the plant will tend to reduce transpiration (with consequently reduced growth) but by reducing the RH this will tend to raise transpiration back up again.
3.1.3.1 Low humidity effect
Typically in hot dry weather with vents wide open
a. Plants close stomata to try to stop wilting, therefore they stop absorbing CO2
b. Plants become dry and leaves become small and hard and may develop dry tip burn
c. Wilting will occur if condition is severe.
3.1.3.2 High humidity effect
Usually at night in winter when vents are closed the humidity will built up
a. Condensation from the roof causes fungal diseases
b. Mineral deficiency appears due to low sap movement
3.1.4 Effect of soil moisture on plant growth
Water is required by all living organism. Plants can be stressed by lack of moisture as well as excess of moisture. Water deficit is developed in plants when the rate of transpiration exceeded that of water absorption.
3.2 Identification of parameters
Based on their effect on the growth of plants, the parameters identified for the monitoring and control of the greenhouse are:
1. Temperature
2. Humidity
3. Soil moisture
4. Light
3.3 General sensor selection criteria
The selection of sensors for the measurement of the parameters in the greenhouse is based on the ability of the sensor to provide a digital output directly to the control and monitoring systems of the greenhouse. This is because digital or IC sensors reduces the cost of developing signal conditioning circuit, reduces development time and provide superior signal output because the noise effect due to the absence of wires is totally eliminated. All the sensors are therefore going to be selected based on their ability to satisfy the requirement of sensing the parameters of the greenhouse.
3.3.1 Temperature measurement sensor selected
Careful attention is giving in selecting the sensor that can withstand the high and low temperature of the greenhouse as well as having a high sensitivity and reliability in a suitable range for crop cultivation. Sensor TL module which is a part of SENSEnuts development platform, comprises of a temperature and light sensor is selected for measuring the temperature of the greenhouse. Both the temperature and light sensors are accessed from I2C port of radio module. The temperature sensor on the module is 12-bit resolution digital sensor, which can detect a temperature change of as low as 0.06250C. The operating range of temperature sensor is from -25 to 800 C. the sensor is capable of working at an extremely low power.
3.3.2 Light measurement sensor selected
The ambient light sensor on the SENSEnuts TL module has a 16-bit resolution with an excellent UV/IR rejection capability. The operating range of light sensor is from 0 to 64k Lux. The sensor is capable of working at an extremely low power. The two sensors in the module are capable of generating software independent interrupt to update the microcontroller about a critical event
3.3.3 Soil moisture sensor selected
Soil moisture sensor-digital output from SUNROM electronics with model number 1282 is used to measure the moisture content of the soil. It obtains volumetric water content by measuring the dielectric constant of the media through the utilization of frequency domain technology. Since the dielectric constant of water is much higher than that of air or soil minerals, the dielectric constant of the soil is a sensitive measure of volumetric water content.
The sensor has a low power requirement and very high resolution. This gives the ability to make many measurements (i.e. hourly) over a long period of time with minimal battery usage. In addition, the sensors incorporate a high frequency oscillation, which allows the sensor to accurately measure soil moisture in any soil with minimal salinity and textural effects.
3.3.3.1 Soil moisture specifications
a. Working voltage: +5V
b. Reading output every 100ms, very past response
c. Output format: serial data at 9600 baud rate (8-bit data, no parity, 1 stop bit)
d. Dimensions : 18mm width 160mm length of total sensor length, sensing probe is 110mm long, wire is 2 meters
3.3.3.2 Soil moisture sensor pin-outs
a. Green / Tx-out (transmit output). Output serial data of 5V logic level, usually connected to RXD pin of the microcontroller/ RS232/ USB/ UART
b. Red/ 5V regulated 5V output
c. Black/Ground common ground
3.3.3.3 Microcontroller interfacing
It’s very easy to interface with microcontroller having UART. Configure your microcontroller to communicate at 9600 baud rate. Start reading incoming ASCII data. You can use this ASCII data to display on seven segments, LCD or convert to integer and do calculations as per project requirement. If you need integer values, you convert ASCII packet to Integer
3.3.4 Humidity sensor selected
For measurement of humidity SHT75 is selected. Fast response time, low power consumption and tolerance against moisture climate made SHT75 relative humidity and temperature sensor a perfect solution for greenhouse humidity measurement. A capacitive sensor element is used for measuring relative humidity while temperature is measured using band-gap.
3.4 Sensor placement considerations
Placement of sensors should be such that the whole area under concern is covered and sensors are placed in position and altitudes so as to measure the parameters without hindrance. For example, light sensors must be placed at heights so as to avoid blockage from plant leaves. Moisture sensors, on the other hand, must be placed close enough to the ground necessary for accurate measurements. Strong winds and water currents may dislocate the sensors from desired positions, so proper fixtures need to be mounted to support the nodes. The nodes are placed in IP-67 rated water-proof packaging to endure the field conditions.
3.5 Greenhouse geometry
The dimension of the greenhouse is 6m length by 3m width and the covering is made of glass material. The greenhouse has a total height of 2.7m. There are four windows on both sides of the greenhouse length for cross air ventilation and a hinged door of 85cm by 57cm as the entrance of the greenhouse. Two layers of potted plants one at 0.7m and another at 1.5m from the ground can be grown on the shelves available on both sides of the greenhouse.
3.6 Sensor locations in the greenhouse
Two layers of sensors with six sensor nodes in each layer are deployed in the greenhouse for the measurement of the parameters. The motes are 50cm from each side of the walls and 1m from each other. Each node is equipped with temperature, light, humidity and soil moisture sensors. The average of the readings from all the sensors is calculated in order to have mean value of the measured parameter as the representative of the real condition of the greenhouse.