Robot platform

PCB Design Documentation

Overview

The robot-platform mainboard integrates the ESP32-S3 MCU, motor drivers, power management, sensor interfaces, and the PBC-34 payload connector on a single 4-layer PCB.

Board Specifications

Layer Stack

Power Architecture

Battery Pack(s) ─── XT30 ──→ Q8 P-MOS ──→ VBAT Bus (7.4V)
                              (reverse      │
                              protection)   │
                    ┌──────────────────────────┼──────────────────────┐
                    │                          │                      │
                    ▼                          ▼                      ▼
              MP2359 Buck              MP1584EN Buck            [Safety Relay]
              5V / 1.2A                5V / 5A                       │
              (ESP32, sensors)              │                    Coil supply:
                    │               ┌──────┼──────┐           +5V_ESP → SW1b NC
                    ▼               │      │      │           → +5V_ESTOP_COIL
              AP2112K-3.3        Q6a/Q6b   Q3                 → RLY1 coil
              3.3V / 600mA       P-MOS SW  P-MOS SW                  │
              (logic, I2C)          │         │                      ▼
                                    ▼         ▼               Motor DRV8833
                              +5V_PI_SW   +5V_PAYLOAD         (7.4V / 10A max)
                              (Pi5, 3A)   (Payload, 2A)
                              GPIO45 ctrl  MCP23017 ctrl

Supervisory Circuits

• TPS3813 (U10): External watchdog, 1.6s timeout. GPIO46 toggles WDI; if firmware hangs, RST pulls ESP32 EN low → hardware reset. Independent of software watchdog.
• Pi5 Power Switch (Q6a/Q6b/Q7): Dual Si2301CDS P-MOS (parallel, 5A total) with AO3400A N-MOS driver. GPIO45 HIGH = Pi5 ON. Default OFF at boot (R34 pull-down). Standard/Industrial SKU only (DNP on Basic).
• E-stop Safety Path (SW1b → RLY1): The E-stop button has dual NC contact pairs. NC2 (SW1b) is in series with the safety relay coil +5V supply. Pressing E-stop physically breaks the coil circuit → relay de-energizes → motor power cut. This is a pure hardware interlock — zero firmware dependency, compliant with ISO 13850. R17 (Q5 gate pull-up) also connects to +5V_ESTOP_COIL so the N-MOS driver loses gate drive simultaneously.

PBC-34 Payload Bus Connector

2×17 pin header (2.54mm pitch), keyed with polarization notch.

ESD Protection

• TVS diodes on all external-facing pins (PBC-34, USB, motor connectors)
• ESD rating: IEC 61000-4-2, ±8kV contact / ±15kV air
• Specific parts: PESD5V0S1BB (single-line) on signal pins

Design Rules

• Power traces: 2mm minimum for motor current paths
• Keep analog (ADC, encoder) traces away from PWM/motor traces
• Ground plane stitching vias every 5mm along board edges
• Motor driver thermal pads connected to internal ground plane
• Decoupling capacitors placed within 3mm of IC power pins
• USB differential pairs: 90Ω impedance controlled
• BNO055 IMU: 100nF + 10µF bulk cap on VDD (C4 + C17, per datasheet)

MCP23017 Interrupt (TP1)

INTA is active-low open-drain with 10k pull-up (R36), routed to test pad TP1. Mirror mode combines both ports onto INTA. Current firmware uses 100ms polling.

To enable interrupt-driven operation (future rev or user modification):

1. Bridge TP1 to an available ESP32 GPIO via bodge wire
2. Configure GPIO as input with pull-up, falling-edge interrupt
3. In ISR: read MCP23017 INTCAP register to identify source (payload detect, DRV8833 fault, GPIO change)
4. Reduces I2C bus load from continuous polling to event-driven

Status Display Panel (Optional Add-on)

1.3" SH1106 128×64 OLED with 3 integrated tactile buttons (UP/DOWN/OK). Connects via existing I2C bus (address 0x3C) + MCP23017 button inputs.

No PCB modification required. Module mounts on chassis top panel via M2 standoffs, connects through 4-pin GH1.25 (I2C) + 3-pin GH1.25 (buttons) to mainboard headers.

I2C bus load: +10pF capacitance (SH1106 typical). Existing 2.2kΩ pull-ups remain adequate for 400kHz with total bus capacitance < 250pF.

Industrial SKU Motor Driver Strategy

The mainboard PCB uses DRV8833 (dual H-bridge, 1.2A/ch) for Basic and Standard SKUs. The Industrial SKU requires BTS7960 (43A half-bridge) for 550 motors at 12V.

Approach A: External Module (Initial Batch)

CAN Bus Termination (R9, 120Ω)

R9 is a 120Ω CAN bus termination resistor, shipped as DNP (Do Not Populate) on Basic and Standard SKUs (which have no CAN bus).

For Industrial SKU: R9 MUST be populated (soldered) when:

• The robot is at one end of the CAN bus (most single-robot setups)
• The Payload device does NOT provide its own termination

Do NOT populate R9 when:

• The robot sits in the middle of a multi-node CAN chain
• Both ends of the bus are already terminated by other nodes

CAN bus requires exactly two 120Ω terminations — one at each physical end of the bus. Measure 60Ω between CAN-H and CAN-L (bus powered off) to confirm correct termination.

Recommended for first production run (< 100 units):

• Mainboard PCB unchanged — no BTS7960 footprint on-board
• Motor power output via 4× XT30 connectors (2-pin, 30A rated)
• PWM/EN signals routed to GH1.25-4P connector (PWM_H, PWM_L, VCC, GND)
• Off-the-shelf BTS7960 module (¥18/ea, 2 modules for 4 motors)
• Mounting: M3 standoffs on chassis plate, 100mm motor wires

Advantages: No PCB re-spin, fast iteration, replaceable if damaged.

Approach B: Integrated PCB (Volume Production)

For production run > 500 units:

• New KiCad variant: robot-platform-industrial.kicad_pcb
• Replace DRV8833 area with 4× BTS7960 half-bridge ICs
• Add ACS712-05B current sense on each motor phase
• Requires: larger board (140×100mm), heatsink mounting holes, thermal vias under driver pads
• Internal ground plane extended for thermal relief
• Estimated NRE: ¥3,000 (4-layer prototype + stencil)

Decision Criteria

Current decision: Approach A for initial batch (Q2 2026). Evaluate Approach B after 100-unit production feedback.

Manufacturing Notes

• Gerber format: RS-274X
• Drill file: Excellon format
• Pick and place: centroid + rotation per component
• All resistors/capacitors 0402 minimum (0603 preferred for hand assembly)
• QFN packages require solder paste stencil (0.12mm thickness)