Can you get more out of your centre pivots?

With increasing world wide pressure on freshwater resources it is the responsibility of those within the agricultural sector to produce more while using less water. One way of doing this is to increase the efficiency of irrigation equipment.


It certainly is possible to fine-tune the operation of your irrigation equipment to not only optimise watering and thereby improve crop yield, but you can decrease running costs by saving electricity.


Very few centre pivots run on completely flat ground. If the highest point of your field is 10 meters higher than the lowest, you can have a pressure difference of up to 1 bar (14.5 psi) between those points. This being the case you have to ask yourself two questions: Do you always have the minimum required pressure in your end section when it is over the higher part of the field? Secondly, are you wasting electricity when the end section is in the low area, and pressure is above the desired value? When you use pump manifold pressure, or even pivot point pressure as setpoint for your VSD PID, you may be maintaining a constant pressure at the point of measure, but what is the pressure throughout all the spans?



On the right we have an actual example of what the pressures look like during three revolutions on a sloping field when using pump manifold pressure as VSD setpoint. While the pressure at the pump manifold remain constant, the end section pressure fall below minimum operational pressure over the higher areas on the field.







One way to overcome this problem is to set the VSD to deliver a higher constant pressure. Though this blanket method may solve the first problem of low pressures, it increases the severity of the second problem - wasting electricity whenever the pivot moves over a lower area.


Solution? Use your end section pressure as PID setpoint. Let your pump only work as hard as is needed to maintain minimum pressure. Nothing more, nothing less.



Charts 1 and 2 show the difference between using manifold vs. end section pressures as VSD setpoints. The challenge at the first instance is to make sure end section pressure does not drop below minimum required operating pressure. Power usage is sub-optimal during the peaks when the pump is essentially working against the sprinkler regulators - electricity being wasted.








In the second instance we can see the pressure being maintained where it matters most, by dynamically changing the pressure at the pump as the current pivot elevation requires. This means no sprinkler under performance due to low pressures (higher yield) and no unnecessary power usage to fight the sprinkler regulators (lower electricity bills.)






How do you get the end section pressure signal to your pump station? At AgriSense we have highly cost effective and robust solutions for this and other challenges. We make use of very robust, low cost and low power radio links to transmit the data needed to optimise your system. The power output of our units are below 17dBm (50mW) which keeps it well below the scope of licensed transmissions. But don't let the low transmitting power fool you - ranges of up to 20kms can be reached under ideal conditions.


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