903.【Design】SkyEdge — Power Line & Transmission Tower Inspection Drone
📷 CMOS × Lens × Stand-Off Distance: Designing the Trade-Off
In the previous article, we defined the per-flight V–I budget for the SkyEdge inspection drone.
In this article, we address the factors that directly determine inspection quality:
the relationship between CMOS sensor, lens, and shooting distance, and we make explicit the design decisions required to produce images that can actually be measured.
❌ 1. Typical failure modes in inspection imaging
Common mistakes in power-line inspection include:
- Selecting a high-resolution CMOS, but pairing it with an inadequate lens
- Widening the field of view until defects collapse to less than one pixel
- Prioritizing safety distance so strongly that the image becomes unmeasurable
The root cause is simple:
CMOS, lens, and distance are being decided independently.
🔄 2. The fundamental trade-off relationship
Whether an inspection image is valid is determined by three parameters:
- CMOS resolution (pixel pitch)
- Lens focal length (field of view)
- Shooting distance
These interact according to the following relationship:
Minimum resolvable feature on the object ≈
(shooting distance × pixel pitch) / effective focal length
This relationship dominates everything that follows.
🧱 3. SkyEdge baseline assumptions (revisited)
| Item | Assumption |
|---|---|
| Target | Transmission lines & towers |
| Shooting distance | 5–30 m |
| Typical defects | Insulator cracks / loose bolts / corrosion |
| Required resolution | ≤ 0.5 mm @ 10 m |
| Sensor | 1-inch-class CMOS |
| Resolution | 20–24 MP |
🔍 4. Constraints on the CMOS sensor
4.1 Pixel pitch
For 20–24 MP sensors in the 1-inch class:
- Pixel pitch is realistically ~2.4–2.8 µm
Pushing below this range leads to:
- Reduced sensitivity
- Degraded SNR
- Poor performance in real inspection scenes (backlight, shadows)
For inspection, raw resolution is not free.
🔭 5. Selecting the lens focal length
5.1 If you go too wide
- Defects collapse into 1–2 pixels
- Effective resolution drops after distortion correction
- Temporal comparison becomes unstable
5.2 If you go too telephoto
- Field coverage becomes too narrow
- Reproducible flight becomes harder
- Tracking and stabilization demands increase sharply
🎯 6. SkyEdge’s chosen operating point (numerically fixed)
Recommended visible-light camera optics
| Item | Value |
|---|---|
| Focal length (35 mm equiv.) | 50–70 mm |
| Actual focal length (1”) | ~18–25 mm |
| Horizontal FOV | ~25–35° |
| Lens distortion | ≤ 1% (with calibration) |
| F-number | F4–F5.6 |
The key point is intentional:
A slightly narrow field of view is optimal for inspection.
📐 7. Verifying achievable resolution (example)
Conditions
- Shooting distance: 10 m
- Actual focal length: 20 mm
- Pixel pitch: 2.5 µm
Resulting object-space resolution
- Approximately 0.4–0.5 mm per pixel
This satisfies the fixed requirement of 0.5 mm @ 10 m.
🚫 8. Why a global shutter is mandatory
In real inspection flight conditions:
- Propeller-induced vibration
- Micro-oscillation during hovering
- Low-speed translational motion
Using a rolling shutter under these conditions causes:
- Breakdown of geometry after distortion correction
- Loss of reliable frame-to-frame differencing
For inspection use:
Geometric integrity matters more than anything else.
📏 9. Treating shooting distance as a control variable
In SkyEdge, shooting distance is not:
- An operator guideline
- A loose operational rule
It is treated as a controlled variable.
- Distance sensors actively maintain 5–30 m
- If distance deviates, a “capture invalid” flag is set
- Images are not forced when conditions are wrong
The system logs not:
“Was an image taken?”
but rather
“Were the conditions valid for measurement?”
🧠 10. Why this design creates differentiation
Many inspection drones:
- Install high-resolution CMOS sensors
- Choose field of view for visual convenience
- Leave distance management to the pilot
SkyEdge instead:
- Starts from defect size
- Back-calculates CMOS, lens, and distance
- Fixes all three simultaneously as specifications
Not occasionally good images,
but consistently measurable images—every time.
That is the differentiation.
🧩 11. Summary
CMOS, lens, and shooting distance cannot be optimized independently.
For power-line and tower inspection, you must back-calculate from:
- Defect size
- Safety distance
- Reproducibility requirements
and fix all three parameters together.
SkyEdge embeds this directly into the design
as numerical specifications, not preferences.
This article includes conceptual design elements,
but is grounded in realistic optical design and inspection requirements.