PWFM Tapered Fan vs Full Flow Even Fan for Robot Swarm Sprayers: Which is Better?

I put the question to Grok AI specifically in the context of a real world WEED-IT Quadro autonomous Swarm Sprayer operating at 10 km/h compared to a detuned version. Here is the answer it came up with.

To determine which nozzle performs better for single-spot spraying with isolated nozzles, both with a 40° top angle, at 3 bar pressure, and a height of 70 cm, we need to compare a tapered fan nozzle operating on PWM at 50 Hz and 50% duty cycle to an even fan nozzle that is half the size of the tapered fan nozzle, operating at full flow (100% duty cycle). The key difference from your previous question is the "half its size" specification for the even fan nozzle, which I interpret as having half the flow rate of the tapered fan nozzle at the same pressure. Let's analyse this step-by-step.

Key Parameters

• Height: 70 cm
• Top angle: 40° (for both nozzles)
• Pressure: 3 bar
• Tapered fan nozzle: Operates on PWM at 50 Hz with 50% duty cycle
• Even fan nozzle: Operates at full flow (100% duty cycle) and has half the flow rate of the tapered fan nozzle at 3 bar
• Objective: Effective single-spot spraying, requiring precise coverage, uniform droplet distribution, and minimal overspray

Analysis

1. Nozzle Spray Patterns

Tapered fan nozzle: Produces a spray pattern with higher flow at the center, tapering off toward the edges. This is ideal for spot spraying, as it concentrates the spray on the target, reducing overspray.

Even fan nozzle: Produces a uniform spray pattern across the spray width, designed for even coverage over larger areas. For spot spraying, this may result in less liquid reaching the target spot and more being dispersed outside it, reducing precision.

2. Flow Rate Comparison

Since the even fan nozzle is "half the size" of the tapered fan nozzle, we assume it has half the flow rate at 3 bar. For example:

If the tapered fan nozzle is a standard size (e.g., TeeJet XR4003, rated at ~1.2 L/min at 3 bar), the even fan nozzle would be equivalent to a smaller size (e.g., XR40015, rated at ~0.6 L/min at 3 bar).

• Tapered fan with PWM: At 50% duty cycle, the effective flow rate is halved. For a 1.2 L/min tapered fan nozzle, this would be ~0.6 L/min.
• Even fan at full flow: With half the size, its flow rate is ~0.6 L/min (matching the tapered fan's effective flow rate under PWM).

Thus, both nozzles deliver approximately the same effective flow rate (~0.6 L/min), making the comparison hinge on spray pattern and control rather than volume.

3. Effect of PWM on Tapered Fan Nozzle

PWM at 50 Hz and 50% duty cycle means the tapered fan nozzle is open for 10 ms per 20 ms cycle, reducing the effective flow to 50% of its rated capacity.

PWM maintains droplet size and spray pattern similar to continuous flow at 3 bar, but the pulsing allows precise control, which is advantageous for spot spraying. It delivers short, controlled bursts, reducing over-application and improving targeting accuracy.

4. Spray Coverage at 70 cm

Both nozzles have a 40° top angle, so the spray width at 70 cm is: Spray width = 2 × 70 cm × tan(20°) ≈ 51 cm

• Tapered fan: Concentrates more liquid in the center of the 51 cm width, ideal for spot spraying a small, targeted area.
• Even fan: Distributes liquid evenly across the 51 cm width, which may waste product outside the target spot, reducing efficiency for spot spraying.

5. Droplet Size and Drift

At 3 bar, both nozzles (e.g., XR or AIXR types) typically produce medium to coarse droplets, suitable for spot spraying to minimise drift. (NOTE: we actually fit ARAG CFLD-XC4003's or 02's in the real world, further reducing drift - Dave)

PWM at 50% duty cycle generally maintains similar droplet sizes as continuous flow at the same pressure. Since both nozzles operate at 3 bar and produce comparable flow rates, their droplet size and drift characteristics should be similar.

The tapered fan's concentrated pattern may slightly reduce drift potential by focusing spray on the target, while the even fan's uniform distribution may increase off-target drift.

6. Suitability for Spot Spraying

Tapered fan with PWM: The concentrated spray pattern, combined with PWM's precise control, makes it highly effective for spot spraying. The pulsing reduces the risk of over-application, and the tapered pattern ensures more liquid reaches the target spot, minimising waste.

Even fan at full flow: Despite having the same effective flow rate, the even fan's uniform spray pattern is less suited for spot spraying. It disperses liquid across a wider area, reducing the amount applied to the target and increasing overspray.

Conclusion

The tapered fan nozzle with PWM at 50 Hz and 50% duty cycle is better for single-spot spraying at 70 cm height, 3 bar pressure, and a 40° top angle. Its concentrated spray pattern and PWM-controlled bursts provide superior precision and efficiency for targeting isolated spots, even though both nozzles deliver similar flow rates. The even fan nozzle, with its uniform distribution, is less effective for spot spraying due to increased overspray and reduced focus on the target.

Not surprisingly, the analysis is consistent with Rometron's WEED-IT White Paper and the performance to date of dozens of PWFM (Pulse Width Frequency Modulated) Swarm Sprayers already deployed.

Note that the WEED-IT White Paper (linked above) also deals with the issue of 'skip' which may be a concern for some, and how that is mitigated with Frequency Modulation.

I hope you will find this analysis interesting. If any questions arise, please feel free to call me anytime.

Thanks and regards,

Dave Young

1800 920 312