Laser Land Leveller

    Laser Leveller, Laser Bucket Scrapper, Laser Machine Control, laser Grading Machine all these terms refers to one class of equipments. Locally they call it Computer Godi, Computer manja/Karaha in india. This Machine first introduced in 1997 by Trimble Navigations through its distributer SPLTech Pvt ltd. Now these machines are produced by number of companies and sold in india. There are few local manufacturers too.

Why we Need Laser Leveller?
(3 Year ago i was interviewed by retired BARC scientist, i explained him it but after 15 minutes of explaination he didnt got anything but the nontech HR understood everything then he explained the same now somehow he was still confused but nodded all because of his ego) 
Our eye cant tell us everything right always, sometimes we cant judge two ground levels relative height just by looking. Similar to the magnetic hill in ladakh there are regions with small variation around us which can be observed when they filled with water. Here we need some instrument to measure actual relative height.

How It works ?
Laser Reference Plane, yes this is the key behind the whole technique. We create a virtual disc of light with a Rotating Laser beam. The disc of light is made perfectly horizontal by adjusting the position of Light source by electronic system with spirit levels. Here we are laser so the light beam is perfectly straight and narrow with no or negligible spread. So we have perfect light disc of light with 4-5mm vertical width  


Laser Plane Visualisation of Virtual Light Plane
This light plane intensity decreases with distance from the center and usable upto 500mtr (4/5miliWatt 650nm Source)

Now you have a reference level all across your field now at any point if you want measure relative height of ground, you can easily calculate it from the distance from the lightplane. Now if you want a machine to do it, the machine should have some sensor to detect vertical height of the light disc.

Components:

1. Transmitter:
Mostly they use a semiconductor laser of 650nm which rotates at a programbale speed og 300/600 RPM. The Laser source i.e. the diode is placed on special mechanical assembly which is shock compensated and automatically balanced with electronically sensed spirit levels(Newer manufacturers are using Acceleration/Gyration sensors). The diode throws light upwards which is then partially reflected by a prism. (This mechanism helps the transmitter for vertical measurements required for civil construction workers, vertical beam is useless for grading application) 

The beam width of most transmitters is 5 to 7mm.

The transmitter is powered by easy replacable rechargeable batteries of 4000mAH which lasts for atleast 4/5 working hours.

2. Laser receiver

Laser receiver is assembly of Photodiode array, amplifier comparator signal conditioners and Logic Unit. The photodiodes are mounted in a vertical strip each set of photo diode in same vertcal position are grouped together. This unit is most important part of the system.
Although we are using laser but if you compare 5 mili watt laser at 400 meter distance from source compared to normal diffused daylight is not distinguishable. We cant use high intensity laser beam for user safety.

DeadBand/Threshold band: This is the minimum detectable change in height through the receiver. The level of accuracy depends on this. Some of the manufacturers provide a programmable Deadband for their instruments.

Protocol: Most of the receivers use CAN for communication between The Control Box and receiver. Some use 4-20mA current loop for signals.

3. Control Box:

This component is a medium sized box attached to the vehicle just beside the operator seat. This component is also acts as junction box and control panel.
A display attached to the box for monitoring of proper function of the instrument. (there are high probability of improper function during operation due to natural causes like wind so operator need to monitor it although the system is automatic.

Data logger and Sensors for Agriculture

 There are vast number of companies producing data-loggers for agriculture. The top most provider is campbell scientific the loggers are the best in performance supports all types of sensors but they are very costly and operation is not user friendly.

Sensors come in different electrical designs, for example Supply Voltage / excitation voltage: some of the sensors work on 3 volt supply some others on 5 or 12v some sensors support 5 to 12v . Loggers are installed in remote fields and expected to work unattended for one or two years. So the power consumption is important which is affected by Current Consumption of the sensors. Normally moisture sensors use 10-16mA. So the sensors are switched off by the logger program i.e. called Power Cycling. Some users unaware about this facility in loggers the connect the supply lines together which means all sensors powered up at a time consuming more power.we are not reading all sensors at a time, its one by one so power should be so. Some digital sensors Hibernate(sleep mode) themselves when there is no read signal. These type of digital sensors specify two different power consumption in spec sheet 1.Read time Current, 2.Sleep/Standby Current. Sensor Ouput Voltage  Range of analog sensors vary from 0-1v 0-2.5v 0-3v 0-5v. The data logger should support all these, if the logger has no option for small voltage ranges then the logger readings may achieve less accuracy. Sensor Output Proportionality also different for analog sensors some sensors come with internal voltage reference so the output is independent of Supply voltage and in some cases its proportional to input Supply Voltage. In digital sensors the voltage level is always in TTL level so these factors does not affect.

Portable Crop Health Sensor

      Remote sensing uses images from satellite or hovercrafts using visible IR NIR and multispectral cameras, these methods are costly and involves a lot of mathematical calculation to retrieve the useful information from the images. Well, the most useful parameter for agriculture is the NDVI.

Normalized Difference Vegetative Index (NDVI) is a ratio of reflectance of two spectrums, RED/Visible and NIR.

NIR=NIR reflectance, VIS=Reflectance in Visible spectra

0r

NIR=NIR reflectance, VIS=Reflectance in Red

Why NDVI is useful ?

\Healthy plants with enough nitrogen i.e. enough cholrophyll do photosynthesis when they are exposed to visible or PAR, so they absorb the radiation, where as the spectra above 700 nm is not absorbed nor transmitted by Healthy leaves, so there is a steep increase in reflectance above this frequency, although there are other factors which may affect reflectivity of IR but these factors dominate all others.

But in all healthy plants these change does not happen reflection is relatively same or color specific, so this can be used to detect healthy crops.

 

Plant Reflection Spectra

Portable NDVI Sensor

Although this parameter was developed by remote sensing experts for their use, it is now used in handheld sensors too. These are categorised in two groups

• Active NDVI Sensor
• Passive NDVI Sensor

Passive NDVI Sensors

Well these passive sensors are similar to radiometric spectral survey, instead using single spectra as in radiometric survey they use two bands which have centers at the priority spectrums i.e. Red and NIR. The most popular sensor of this type is the NDVI bracket using two sensors 1. PAR sensor, 2.Pyrometric IR sensor.

NDVI Bracket marketed by HOBO Dataloggers

NDVI bracket uses two pairs of Sensors One pair for measuring the incoming solar radiance in both the spectras and the second pair two monitor the reflected light intensity from the crops in both spectras.

Active NDVI Sensor

These sensors does not use solar radiance for measurement instead they use their own light sources, Some of them use polychromatic light source (Holland scientific) and some use monochromatic light source. They use a pulse modulated light to reduce effect of drift due to change in solar irradiance. 

GreenSeeker Ntech(Acquired By Trimble Navigations)

They use two set of LEDs for Red(660nm) and NIR (750nm now shifted to 770nm)which are pulsed one by one and measure the reflectivity displayed in a numeric display(in new portable sensor) or acquired by a attached datalogger, they have also CAN supported sensors for use in automated variable rate fertiliser spray)

Portable NDVI Sensor

NDVI Sensor

NDVI sensor CM-1000 from Spectrum technologies

Plant Pen/NDVI Pen from Qubit systems

Crop sensor from Holland scientific

Holland scientific uses relatively differnt technique than the others, it is similar to differnce between dispersive spectrometer and Non-disperssive spectrometry. All others use the active light source with monochromatic output, where as holland scientific uses polychromatic source, which is more similar to solar irradiance simulating a normal experimental condition.(assuming monochromatic source may change normal photosynthetic response)

RapidScan CS 45

Crop circle ACS-470

Agleader OPTRX

OptRx uses a red-edge light wave to scan plants. Most competitive products use only a red wavelength, which has been shown in studies to be non-responsive at high plant density. The red-edge lightwave stays responsive to health-stress on plants at later growth stages than other competitive products.

OptRx crop sensors use a single algorithm application. Trials have been conducted in multiple states that have shown positive results of using a single algorithm in comparison to multiple algorithms.

Other Useful Spectras:

PIC from cirrus digital systems

from the above picture one can guess How a multispectral camera can be used to extract lot of information for plant phenotyping.

Except this there are some other optical methods like fluorescence as the  chlorophylls exhibit a fluorescence effect, absorbing in 440 nm(blue) and emitting back in 600-650nm(red), which is known as chlorophyll fluorescence or plant fluorescence. for analysis of phtosynthesis process this is most useful, Kautsky effect can be observed with this method.

For phenotyping inside lab fluorescence imaging is the best and accurate, other cheaper alternatinve is fluorescence Sensor like FMS

Source:Use of optical Sensors... Oklahoma state University

NB: am an instrument engineer, all those reviews and declarations is based on my knowledge as a engineer and physicist , am not a biologist or plant physiologist, if you have any suggestion please send me mail or add a comment below in the comment section thank you.

Ref 

Use of Optical Sensorsto Monitor Plant Nitrogen Statusin Horticultural Plants

Comparison Of Active And Passive Spectral Sensors In Discriminating Biomass Parameters And Nitrogen Status In Wheat Cultivars

Soil moisture Sensors

     Measuring soil moisture is an essential thing in water management and agricultural research(Plant phenomics e.g. drought resistance) works. Although there are several methods but a few of them accepted for research purpose.

The Low-cost methods all depend on the electrical characteristics, the simpler ones are just Capacitive sensors which measures the capacitive reactance formed by Soil as a dielectric medium. 

What are the existing sensor techniques ?

Most of them use the change in dielectric property of soil except the tensiometer and neutron sensor.
The tensiometer reads the pressure change by water evaporation from surrounding soil, and a gauge shows the pressure change interpreted or scaled as water potential.

The neutron gauge uses the neutron scattering power of water, the sensors contains some radioactive substance and radiates neutron particles if there is enough water in surrounding soil then it will be moderated and scattered by the hydrogen in water molecule. Although this sensors is most accurate for radioactive nature of the sensor makes it outofmarket, buyer have to get special permission from the government for this.

Rest of the sensors use the dielectric property of water so the overall dielectric property of soil, but the technique to read the dielectric property is different for each type of sensors.

Resistive Sensor
Capacitive Sensor
Time Domain reflectometry
Frequency Domain Reflectometry
Terawave sensing

Again the capactive sensors have different classes

Low Frequency i.e. 100-200KHz
High Frequency Sensors 60-80MHz
PCB based Capacitive Probe
Annular Ring type Probe (Gopher/MicroGopher/Redlight Green light)
Fringe Capacitance sensor

and the method of capacitance measurement

• Making a capacitance controlled oscillator and measure the frequency
• Using the Capacitor for Rectification of a HF wave and the output signal voltage is measured.
• Using the fringe effect on capacitance

the soil salinity affects a lot in low frequency capacitance measurement and high frequency measurement is least affected. and this makes the low frequency sensors obsolete now the most popular sensors are the 80MHz PCB Probe type sensors(Example Decagon SH100)

TDR Probes from different manufaturers

TDR150, TDR35 from specmeters
TDR200 from campbellscientific
Delta-T Theta Probe, ML3 Probe
HydraSense CampbellSci