Robotic Arm Controller PCB

Background

The goal of the project was a controller board for the 6-DOF robotic arm: one board that connects the PC over USB to the servos' half-duplex serial bus at 1 Mbps and distributes 12 V to six servo ports, mounted in a bay in the arm's base. It replaces an off-the-shelf servo bus adapter and separate power wiring. Revision 0.4 is released for fabrication with every part assembled by the manufacturer. It has not been fabricated yet.

Design Requirements

• USB linkUSB-C connection to the PC.
• Servo busHalf-duplex TTL serial bus at 1 Mbps, where the PC does not receive its own transmitted packets.
• Servo portsSix 3-pin ports: 1 GND, 2 +12 V, 3 DATA.
• Power12 V at up to 8.4 A worst-case arm hold, fused, reverse-polarity protected and switched.
• Mounting96 × 40 mm, mounted component side down on four M3 bosses in the base bay.
• AssemblyEvery part assembled by the manufacturer, through-hole connectors included.

Schematic

• USB to serialCH343G, rated to 6 Mbps; chosen over the CH340C because the manufacturer's datasheet directs 1 Mbps one-way traffic to the CH343. USBLC6-2SC6 ESD protection on the USB lines, 5.1 kΩ CC resistors, XC6206 3.3 V regulator.
• Half-duplex busAn SN74LVC1G126 drives the shared data line and an SN74LVC1G125 gates the receive path. One transmit-enable net drives both, in opposite senses. An SN74LVC1G14 generates it from TX with a timed RC (2.2 kΩ, 100 pF C0G, BAT54WS), so the PC does not receive its own packets.
• ReceiveMCP6561 comparator at a 1.5 V threshold with hysteresis; 33 Ω series resistor per port; bidirectional ESD diode on the data line.
• 12 V path12 A Littelfuse 451 SMD fuse (the 451 series is derated to 75 % for continuous use; worst-case hold is 8.4 A). Two AOD403 P-channel MOSFETs for reverse-polarity blocking and on/off switching with a 10 V gate zener, an SMBJ15CA TVS and a 1000 µF bulk capacitor.

Layout

• Power copperPower nets as copper pours. The narrowest 12 V section, measured from the routed board, is 7.1 mm of 1 oz copper. No vias in the current path.
• Signal routingSignal nets routed by a Python autorouter over an unbroken bottom ground plane.
• ConnectorsUSB-C, four servo ports, the switch and the 12 V terminal on one long edge.
• ToolsSchematic and layout generated from Python scripts in KiCad 10, with symbols and footprints from the KiCad libraries.

Verification and Manufacturing

• Design checksEvery semiconductor pinout and every polar part checked against its datasheet; 0 unconnected pads.
• FitBoard STEP placed in the Fusion 360 arm assembly and collision-checked in the base.
• Revision 0.3 to 0.4The blade-fuse holder became an SMD fuse and an unused fallback header was removed, so the manufacturer assembles every part.
• Assembly filesChecked on JLCPCB's live quote and placement viewer: corrected the BOM format, an out-of-stock connector, through-hole part centroids and per-part rotations.
• Part costThe 13 fee-carrying parts surveyed against JLCPCB's basic and preferred libraries; four diodes moved to equivalent preferred parts.

Result

A two-layer controller board released for fabrication: ERC and DRC clean with schematic parity, fit-checked in the arm's CAD, and assembly files checked in the manufacturer's placement viewer, with the assembled-board quote reduced from $82.28 to $66.55.

Next Steps

• FabricationFabrication and assembly at JLCPCB.
• Bring-upBefore the arm: pin 1 checked with a meter before any servo is connected, the data line on a scope at 1 Mbps, the 12 V rail during a hard stop, and port temperature under load.