Vehicle mounted spraying systems incorporating a boom that extends laterally both sides of a vehicle such as a tractor are commonly used to spray agricultural crops with liquid based products such as fertilisers or other chemicals. Typically, these spraying systems are mounted to the rear of the vehicle which will also carry a tank containing the liquid that is to be sprayed. To ensure that the correct amount of liquid is sprayed, the spraying system is configured so that a given flow rate is dispensed from a plurality of sprayers located along the arm at a predetermined height above the surface to be sprayed. Often these vehicle mounted spraying systems will incorporate a height adjustment capability to allow the overall height of the boom to be adjusted as desired. Booms vary in size, with typical wing tip to wing tip lengths being 90 feet (27 m), 120 feet (37 m) and 150 feet (46 m).
Whilst such a system is adequate over flat country, where the surface to be sprayed is undulating or sloped simple height adjustment of the boom relative to the vehicle is not sufficient as the ground to the right of the vehicle may be elevated with respect the ground surface to the left of the vehicle. Throughout the specification when referring to a side of the vehicle this is defined in accordance with a view taken towards the rear of the vehicle. To overcome this problem, the boom is divided into separate articulated arms or wings each of which are independently adjustable by hydraulic rams which function to raise or lower the booms in accordance with a control signal provided by ultrasonic distance sensors located on each wing. These distance sensors measure the distance between the wing and the ground surface. In this manner, either the left or right wing of the boom may be automatically raised or lowered as required.
However, there are a number of disadvantages with this approach. As the wings extend for relatively large distances (15-25 m) from the vehicle they are mounted to a central rigid support structure which itself is resiliently mounted to the vehicle. This resilient mounting includes a combination of springs, shock absorbers and pendulums so as to absorb severe twisting and movement shocks and provide some mechanical self levelling of the boom (see for example FIG. 1B). This mounting also provides approximately ±10° of travel in the roll direction which functions to absorb the significant stresses that the central support structure would otherwise encounter if it were to be rigidly mounted to the vehicle.
Unfortunately, the effect of this resilient mounting is to greatly reduce the stability of the wing height control as, for example, raising the left wing to compensate for a raised ground profile in this region will in fact cause the right wing to pivot upwardly due to the torque imparted on the entire boom by the redistribution of weight on the left hand side. This will then result in a control signal being sent to the right hand side to lower the right wing, thereby leading to a potential instability. Eventually, the boom will reach equilibrium but only after a delay of approximately one to three seconds during which time the sprayed liquid will not be dispensed in the correct amounts over the ground.
Another significant disadvantage of these prior art vehicle mounted spray systems occurs when the vehicle encounters a local undulation in the surface such as a rock or a rut in the ground that causes the vehicle to rapidly change lateral slope angle. In extreme circumstances, this could cause the tip of a wing to impact the ground as the speed of response of the ultrasonic distance sensors located on the wings is not rapid enough to prevent this occurring. Even in the case where an impact is avoided, the raising of the wing to avoid the impact will cause the raising of the opposed wing as discussed earlier, once again resulting in a time of instability of the spraying system.
What is needed is a vehicle mounted spraying system capable of improved stability control.