Wiring and PCB Design
Overview
Electronic integration is one of the most error-prone aspects of robot system integration. This section covers wiring best practices, connector selection, and custom PCB design fundamentals.
Wiring Fundamentals
Wire Gauge (AWG)
AWG (American Wire Gauge) is the standard for wire thickness. Lower numbers mean thicker wire.
| AWG | Diameter (mm) | Current Capacity (A) | Resistance (mΩ/m) | Typical Use |
|---|---|---|---|---|
| 10 | 2.59 | 30 | 3.3 | Battery main, large motors |
| 12 | 2.05 | 20 | 5.2 | Motor power |
| 14 | 1.63 | 15 | 8.3 | Medium power devices |
| 16 | 1.29 | 10 | 13.2 | Servo power |
| 18 | 1.02 | 7 | 21.0 | Small motors, sensor power |
| 20 | 0.81 | 5 | 33.3 | Sensor signal lines |
| 22 | 0.64 | 3 | 53.0 | Low-power signals |
| 24 | 0.51 | 2 | 84.2 | I2C/SPI signal lines |
| 26 | 0.40 | 1.3 | 134 | Fine signal lines |
| 28 | 0.32 | 0.8 | 213 | Ultra-fine signal lines |
Wire Selection Principle:
\[I_{wire\_rating} \geq 1.5 \times I_{max\_load}\]
Leave 50% margin to prevent overheating.
Wire Types
| Type | Features | Application |
|---|---|---|
| Silicone wire | Flexible, heat resistant (-60~200°C) | Robot joints, motor wiring |
| PVC wire | Cheap, stiff | Fixed routing |
| Teflon wire | Heat and chemical resistant | Harsh environments |
| Ribbon cable (FFC/FPC) | Flat, flexible | Displays, tight spaces |
| Twisted pair | Noise resistant | CAN Bus, differential signals |
| Shielded cable | Electromagnetic shielding | Analog signals, encoders |
Color Coding
Standard color conventions (not mandatory, but strongly recommended):
| Color | Use |
|---|---|
| Red | Positive power (VCC/V+) |
| Black | Ground (GND) |
| Yellow | Signal (PWM/Signal) |
| Green | Communication (CAN-H/TX) |
| Blue | Communication (CAN-L/RX) |
| White | Communication (SDA/MOSI) |
| Orange | Spare/Secondary power |
Labels: Label both ends of every wire with source and destination.
Connector Selection
Power Connectors
| Connector | Current Rating | Typical Voltage | Features | Use |
|---|---|---|---|---|
| XT60 | 60A | High voltage | High current, latch | Battery main connector |
| XT30 | 30A | Medium voltage | Medium current | Motor branch |
| DC barrel | 1-5A | 5-24V | Simple | Dev board power |
| Anderson PP | 15-180A | High voltage | Industrial grade | Large robots |
| Deans (T-plug) | 40A | High voltage | Common in RC | Small robots |
Signal Connectors
| Connector | Pitch | Pin Count | Features | Use |
|---|---|---|---|---|
| JST-XH | 2.5mm | 2-16 | Latching | Battery balance, sensors |
| JST-PH | 2.0mm | 2-16 | Small | Small sensors |
| JST-SH | 1.0mm | 2-10 | Micro | QWIIC (I2C) |
| Molex Micro-Fit | 3.0mm | 2-24 | Industrial grade | Motor drivers |
| DuPont 2.54mm | 2.54mm | Any | Breadboard friendly | Prototyping |
| Hirose DF13 | 1.25mm | 2-15 | Reliable, compact | Pixhawk flight controller |
Waterproof Connectors
| Connector | Protection Rating | Pin Count | Use |
|---|---|---|---|
| M8 circular | IP67 | 3-8 | Industrial sensors |
| M12 circular | IP67 | 4-12 | EtherCAT, industrial |
| IP67 USB | IP67 | 4 | Outdoor cameras |
Quick-Disconnect Design
Robots need quick disassembly for maintenance:
- Use latching connectors for joint harnesses (do not solder permanently)
- Use standard connectors between modules (M8/M12)
- Each removable module has its own wiring harness
Strain Relief
The most common failure mode for connectors is cable pulling causing solder/crimp joint fracture.
Locations that must have strain relief:
- Connector exit points
- Cables passing through panels
- Cables near robot joints
- Any location where pulling may occur
Methods:
| Method | Application |
|---|---|
| Heat shrink + adhesive | Connector tail |
| Cable ties | Cable crossing points |
| P-clips | Mounting to frame |
| Cable chain | Linear motion cables |
| Spiral wrap | Rotating joints |
Crimping Tools
Strongly recommended to use proper crimping tools rather than soldering connector terminals:
| Tool | Compatible Connectors | Price Range |
|---|---|---|
| Engineer PA-09 | JST-XH/PH | ~$30 |
| Engineer PA-20 | JST-SH, small terminals | ~$30 |
| IWISS SN-28B | DuPont 2.54mm | ~$20 |
| Molex hand crimper | Molex Micro-Fit | ~$40 |
| XT60 soldering | XT60 (requires soldering) | Soldering iron |
Crimping vs. Soldering:
- Crimping: Reliable, repeatable, professional
- Soldering: Flexible but inconsistent, heat may damage wire
- Industrial standards require crimping (mandatory in aerospace)
Custom PCB Design
When to Design a Custom PCB
- Too many messy wires (>20 fly wires -> consider PCB)
- Specific power distribution needs
- Signal conditioning circuits needed (amplification, filtering)
- Mass production requirements
- Strict size/weight constraints
KiCad Workflow
KiCad is the most popular open-source PCB design software.
Complete Process:
graph LR
A[Schematic Design] --> B[Electrical Rules Check ERC]
B --> C[Assign Footprints]
C --> D[PCB Layout]
D --> E[Routing]
E --> F[Design Rules Check DRC]
F --> G[Generate Gerber Files]
G --> H[Send to Manufacturer]
H --> I[Soldering and Debugging]
Schematic Design
The schematic is the logical representation of the circuit. Key steps:
- Place components: Select or create symbols from libraries
- Wire connections: Connect pins with wires
- Add power symbols: VCC, GND, etc.
- Add decoupling capacitors: Place 100nF + 10μF near each IC
- Annotate: Component values, part numbers
PCB Layout and Routing
Layout Principles:
- Keep power components away from sensitive signals
- Place decoupling capacitors close to IC pins
- Place connectors at board edges
- Consider thermal management (widen high-current paths, copper pour)
Routing Rules (2-layer board reference):
| Trace Width | Current Capacity | Use |
|---|---|---|
| 0.2mm | ~0.3A | Signal traces |
| 0.5mm | ~1A | Low power |
| 1.0mm | ~2A | Medium power |
| 2.0mm | ~4A | Power traces |
| Copper pour | >5A | High current |
Empirical formula (outer layer, 1oz copper):
\[I_{max} \approx 0.048 \cdot \Delta T^{0.44} \cdot A^{0.725}\]
where \(A\) is the conductor cross-section area (mil²) and \(\Delta T\) is the allowable temperature rise (°C).
Common Circuit Modules
Voltage Divider
Used for voltage sensing (e.g., battery voltage measurement):
\[V_{out} = V_{in} \cdot \frac{R_2}{R_1 + R_2}\]
Example: 24V battery voltage -> 3.3V ADC range:
\[\frac{R_2}{R_1 + R_2} = \frac{3.3}{24} \approx 0.1375\]
Choose \(R_1 = 100k\Omega\), \(R_2 = 15.8k\Omega\).
Level Shifter
Interfacing 3.3V and 5V logic:
- Unidirectional: MOSFET + pull-up resistor (BSS138 approach)
- Bidirectional: TXB0108 (8-channel auto-direction detection)
- I2C specific: PCA9306 (bidirectional, supports different voltage buses)
Breakout Board
Distributes a single high-density connector to multiple subsystems:
┌── IMU (SPI)
[Main Board] ── [Breakout Board] ── Motor Driver (CAN)
├── Sensor (I2C)
└── GPS (UART)
PCB Prototyping
Manufacturers
| Manufacturer | Minimum Price | Lead Time | Features |
|---|---|---|---|
| JLCPCB | 5 pcs/$2 | 1-3 days | Best value |
| PCBWay | 5 pcs/$5 | 2-5 days | International shipping friendly |
| HQ PCB | 5 pcs/$3 | 1-3 days | KiCad plugin |
Prototyping Notes
- Minimum trace width/spacing: 0.15mm (standard process)
- Minimum hole diameter: 0.3mm
- Copper weight: 1oz (35μm) standard, 2oz for high current
- Surface finish: HASL (cheap), ENIG (better pads)
- Board thickness: 1.6mm standard
- Solder mask color: Green is cheapest
Soldering
Manual Soldering (prototyping):
- Iron temperature: 300-360°C
- Flux is essential
- 0402 and larger packages can be hand-soldered (with experience)
- QFP/TQFP requires drag-soldering technique
Reflow Soldering (small batch):
- Stencil + solder paste -> Place components -> Heat reflow
- Budget option: Solder paste + hot air gun / modified toaster oven
Wire Harness Design
Harness Diagram
For complex robots, a wire harness diagram is essential:
Battery ─[XT60]─ Power Distribution ─┬─[XT30]─ Motor Driver 1
├─[XT30]─ Motor Driver 2
├─[DC barrel]─ Jetson
└─[JST-XH]─ Sensor Board
Main Controller ─┬─[USB-C]─ Depth Camera
├─[M12]─── LiDAR
├─[SPI(JST-SH)]─ IMU
└─[CAN(Molex)]─── Motor Driver CAN Bus
Harness Labeling Standards
Each wire/harness should be labeled with:
- Number: W001, W002, ...
- Source: Module name + pin
- Destination: Module name + pin
- Wire: AWG + color
- Length: Measured + 10% margin
Electromagnetic Compatibility (EMC) Basics
Common Interference Sources
| Source | Frequency Range | Impact |
|---|---|---|
| BLDC motor PWM | 10-100 kHz | ADC noise |
| Switching power supply | 100 kHz - 1 MHz | Sensor interference |
| Digital signals | MHz range | RF interference |
Mitigation Measures
- Power filtering: Large electrolytic capacitors + high-frequency ceramic capacitors
- Ground separation: Connect analog and digital ground at a single point
- Signal shielding: Use shielded cable for sensitive signals
- Layout isolation: Physically separate high-power and low-signal areas
- Ferrite beads: Suppress high-frequency interference
\[Z_{ferrite}(f) = R(f) + jX(f)\]
At the target frequency range (typically 10-100 MHz), impedance increases significantly, attenuating interference.
References
- KiCad Official Tutorials: kicad.org
- JLCPCB: jlcpcb.com
- IPC-2221: Generic Standard on Printed Board Design
- Phil's Lab (YouTube): Practical PCB design tutorials