All sprayers, whether self-propelled or tractor-drawn, experience variations in spraying speed. Spray operators often need to speed up or slow down as the conditions or situation requires. 

To maintain a constant application volume per hectare, the spray liquid flow rate must change in direct proportion to the spraying speed. Sprayers achieve this with a rate controller, which is standard on most sprayers. 

The rate controller uses four main pieces of information to ensure a constant application rate: 

• The user enters the width of the boom, section widths and nozzle spacing (metres) and the desired water rate in litres per hectare (L/ha); and 

• The sprayer provides spraying speed information (kilometres per hour, collected from a GPS signal or a radar or a wheel-based sensor) and liquid flowrate (L/minute, collected from a flow metre on the main sprayer pressure line). 

With conventional boom sprayers the flowrate to the nozzles is adjusted in response to changes in spraying speed by manipulating the pump pressure. While changing pressure can adjust flow rate, the change in pressure may change the droplet size at the nozzle.  

An alternative way of controlling the flowrate to the nozzle without changing pressure in the spray lines is to use pulse width modulation (PWM), where each nozzle is rapidly turned on and off many times each second. Pressure is kept constant in the line, with the nozzle being turned ‘on’ more frequently to increase flow rate or having more ‘off’ periods when the required nozzle output is less (i.e. the sprayer slows down).  

PWM allows for functions such as single nozzle section control, turn compensation, variable rate application and boom recirculation to be incorporated into the spraying system.