PCBot: a Minimalist Robot Designed for Swarm Applications

Authors: Jingxian Wang, Michael Rubenstein · IROS 2022

Licences

Open Access
Paper: CC BY 4.0

Everything below draws on the full paper under its CC BY 4.0 licence; its figure is credited to the authors.

TL;DR

PCBot moves on an orbital shake table by switching a magnet with a coil made of PCB traces: five parts, 18 g, under 20 s to assemble, about 66 m of travel per 70 mAh charge.

Why it matters

Building the actuator into the PCB moves most of the work into automated PCB fabrication, leaving five parts to assemble by hand: a 3D-printed chassis, the PCB, a battery, a screw as steel core and a rubber-faced magnet. The shared table is a steel plate driven by motors, which the authors describe as simple to manufacture and scale up.

Problem

Swarm size is limited by each robot's complexity, cost and assembly time: the paper cites SWARM-BOT and e-puck swarms of under 100 robots, and Kilobot's five minutes of manual assembly per robot capping swarms at about 1,000. Robots with very simple actuators often move complexity into the environment, such as the magnet-embedded table HoverBots need.

Proposed solution

PCBot sits on a steel orbital shake table and moves by choosing when to stick to or slip on the surface. Its only actuator is a bi-stable solenoid: a coil made of traces in the multi-layer PCB moves a rubber-faced neodymium magnet between an 'attached' state (magnet holds the table, robot pivots around it) and a 'detached' state (robot slides by inertia), drawing current only for the ≈1 ms switch. A polarized overhead light and a polarizer-covered light sensor give orientation through a software phase-locked loop, and the coil spiral is shaped by numerically minimising energy per unit of magnet force.

Key insights

A coil made from PCB traces can act as the actuator, so it is produced in the automated PCB process instead of by winding wire or making electro-permanent magnets.
A bi-stable magnet needs power only for the ≈1 ms state change, not for the whole detached period, which lasts tens of milliseconds to seconds.
Moving the actuation into a shared orbital table lets each robot be very simple; timing when it sticks and slips gives motion in any 2-D direction.
Spinning passively with the table under a polarized light source gives the robot its orientation without a gyroscope.
Material choice sets the friction gap: chassis on HDPE about 0.12, Neoprene rubber 0.45 kinetic and 0.67 static, giving about 73 mN of friction attached versus 29 mN detached.

Main results

Robot size 48 × 42 × 14 mm, mass 18 g, centre of mass 5.2 mm from the rotation axis; five major components, assembled in under 20 seconds.
Straight-line motion with a 100 ms magnet lift per cycle: 47 mm per 10 actuation cycles on average, with standard deviations of 8° in direction and 7 mm in distance. The text describes 36 such experiments, while the Fig. 11 caption refers to 30.
With a 50 ms lift (30 experiments): 21 mm per 10 cycles, with standard deviations of 4° and 5 mm, so direction became more accurate and distance less.
Energy: 18 mW when idle (mostly the microcontroller) and 68 mJ per actuation cycle; the 70 mAh battery supports about 14,000 cycles, about 66 m at 4.7 mm per cycle, or over 140 m at more than 10 mm per step with over 200 ms detached.
The optimised coil shape saves 40% energy compared with a simple 0.7 mm-wide coil in the idealised analysis, and 70% in the actual PCBot board, where vias and nearby components degrade the simple coil.
Chosen design point: I0 = 5 A, steel core to table 7.5 mm, magnet height 1.85 mm attached and 2.90 mm detached, with at least 0.1 mm margin; table at 115 ± 2 rpm with a 10.7 mm orbital radius.

Limitations

PCBot currently only moves; it lacks the inter-robot sensing and communication a full swarm robot needs, and IR transceivers are planned.
There is no position feedback, and step length varies, mainly with uneven friction across the HDPE sheet, so the rectangular path did not close at step 60.
Actuation dominates power use: moving once per second could push actuation power above 500 mW, because a PCB coil has about ten times fewer turns and ten times higher resistance than a wire coil.
The robot depends on a dedicated environment: a steel orbital shake table with an HDPE coating and a polarized overhead light.
Experiments use a single prototype robot; operation of a swarm is not tested.

Field impact

Finalist for the IROS 2022 Best Paper Award on Robot Mechanisms and Design (sponsored by ROBOTIS) and on the IROS 2022 Best Paper Award and ABB Best Student Paper Award combined finalist list; 2 citations. The paper does not link code or design files.

Abstract

Complexity, cost, and power requirements for the actuation of individual robots can play a large factor in limiting the size of robotic swarms. Here we present PCBot, a minimalist robot that can precisely move on an orbital shake table using a bi-stable solenoid actuator built directly into its PCB. This allows the actuator to be built as part of the automated PCB manufacturing process, greatly reducing the impact it has on manual assembly. Thanks to this novel actuator design, PCBot has merely five major components and can be assembled in under 20 seconds, potentially enabling them to be easily mass-manufactured. Here we present the electro-magnetic and mechanical design of PCBot. Additionally, a prototype robot is used to demonstrate its ability to move in a straight line as well as follow given paths.

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