Pulse Width Modulation For Newbies

I was recently asked to describe Pulse Width Modulation to a non-farming audience. My instinct was to send them back to what we’d already written about the topic on Sprayers101, here and here. But on reviewing the material, I soon realized that most of our posts assume a certain amount of basic knowledge and understanding. What about people who are new to the business, or just curious? Not that helpful. 

This is the first in a series of articles that cover off topics which may be too basic for many, but are nonetheless important for others. More to come. And suggestions welcome.

Sprayers are used to apply crop protection agents to fields, and as with all crop inputs, it’s important to apply the correct dose.  For boom sprayers, dose is a product of the swath width, the sprayer travel speed, and the flow rate of spray liquid through the nozzles. Of these three factors, swath width is taken as constant, whereas travel speed and flow rate are variable. If travel speed changes, flow rate also needs to change to maintain the target application rate.

The vast majority of nozzles come in fixed sizes. As a consequence, the only way to change their flow is with spray pressure. In a modern sprayer, a rate controller takes care of the math and the adjustments.  For example, if the sprayer speeds up, it will need to deliver more liquid to keep the same application volume per acre. The rate controller knows the swath width and senses travel speed and liquid flow rate. If the travel speed increases, the rate controller causes the spray pressure to increase until the flow rate sensor shows that the flow is enough to maintain the target application rate.

The problem with this approach is that sprayer nozzles are very sensitive to spray pressure. Too slow a speed, and the lower pressure will cause the spray pattern to deteriorate, resulting in poor coverage. Too fast a speed and the spray will become too fine, creating drift problems. As a result, traditional sprayer operators have to stay within a very specific, narrow speed range. This may not always be possible if, for example, the terrain is hilly or the soil is wet.

One solution to this problem is to control flow rate differently.  A fairly new way to do it is with Pulse-Width Modulation (PWM). This is a fancy term that describes a well established way that liquid flow rates are controlled in a number of other tasks such as fuel injection or hydraulic oil systems.

With PWM, each nozzle body is equipped with an electronic solenoid (shut-off valve). The valve turns on and off ten or more times every second, creating an intermittent, pulsed spray. The proportion of time that the valve is open, called the pulse width or duty cycle, can be electronically controlled.

For example, each nozzle can operate at its full rated flow (100% duty cycle) or a fraction of its flow (say 20% duty cycle). Duty cycle is proportional to the flow rate of the nozzle. At 20% duty cycle, the nozzle delivers about one fifth of the flow compared to 100%. The pulses are so quick that it doesn’t affect overall coverage or droplet size. With this system, as a sprayer speeds up or slows down, the duty cycle changes automatically to match the flow rate requirements calculated by the rate controller.

What does this mean in practice? For one, the sprayer no longer relies on a pressure change to influence the nozzle flow rate because duty cycle has taken over that job. In fact, the operator can set the pressure to whatever is necessary for best coverage or best drift control, whatever is most important. A change in travel speed caused by a hill or a low spot doesn’t affect pressure any more. The end result is a spray application that is not only more accurate, but also more consistent over varying conditions.

Drift control is easier with a PWM system. A common way to reduce drift is to make the spray coarser, and this can be achieved with lower spray pressure. But lowering the pressure results in less liquid flow, and the operator has to slow the sprayer down if the same application rate is to be maintained. With a PWM system, the operator simply lowers the pressure. The system makes up for the lower flow by internally increasing duty cycle, allowing the same travel speed to continue.

An added side benefit with a PWM system is that it provides opportunities for site-specific management of application rates. Parts of the field needing less or more product can receive what they need. All the operator does is change the rate, via duty cycle, according to a prescription map.

A further bonus is the highly resolved sectional control that can be achieved. With any wide agricultural implement, overlaps are inevitable. With an advanced version of a PWM system, individual nozzle solenoids can be shut off or turned back on as required, thereby preventing double applications at these overlaps.

In short, PWM systems give operators much more control over their spray operation. And that’s good for everybody.