DOI: 10.5281/zenodo.20809345

Parallel Mirror Steer-by-Wire

An open-source research platform for bilateral embodied human–robot interaction.

Developed at the MOME Robotics Studio, this repository documents an experimental steer-by-wire system that enables two physically separated BLDC motors to exchange motion through real-time wireless communication and local torque control.

The platform supports research into embodied interaction, haptic reciprocity, adaptive robotics, and Research through Design methodologies.

Makerbase MKS ESP32 FOC Mega, single-motor board used by the current prototype. Product image supplied by the project owner from the AliExpress listing.

Abstract

This repository documents a research prototype for a wireless bilateral motor interface. Two BLDC motor nodes exchange angular position data over ESP-NOW and generate a mutual spring-like resistance through local torque control. Rather than implementing rigid master-slave tracking, the system explores a softer form of steer-by-wire coupling in which motion on one side is physically felt on the other.

The project sits at the intersection of rapid prototyping, interaction design, and rehabilitation-oriented haptic research. Its main contribution is a working proof of concept for a lightweight, mechanically unlinked, mutually perceivable motor interaction using low-cost embedded hardware.

Research Context

The prototype was developed as part of a broader investigation into bilateral physical feedback systems. A key motivation was to build an experimental platform that can be iterated quickly and used to study how resistance, compliance, and reciprocity can be designed into small-scale motorized interfaces.

Instead of treating the system as a finished device, this repository presents it as a research instrument: a functional platform for testing haptic behavior, control strategies, and possible future applications in assistive or rehabilitation contexts.

SimpleFOC Project Lineage

This project is built with the open-source Arduino SimpleFOC library and began from the control concept demonstrated in SimpleFOC's official Haptics — Steer by Wire example. That example connects two locally controlled BLDC motors with a virtual spring: each motor receives a torque-voltage request proportional to the angular difference between the two shafts.

The present prototype preserves that bilateral virtual-spring principle but adapts it substantially:

SimpleFOC sample	This project
two motors controlled by one MCU	one ESP32 controller per motor
both angles available locally	angles and velocities exchanged over ESP-NOW
direct proportional virtual spring	deadband, nonlinear spring, damping, integral correction, and progressive end-stop
demonstration-level torque-voltage control	measured-current, speed, and software thermal foldback
no distributed link state	protocol versioning, peer synchronization, timeout handling, and latched handoff faults

SimpleFOC remains the motor-control foundation: it provides sensor integration, FOC commutation, the three-PWM driver abstraction, and torque-voltage control. The wireless protocol, bilateral state machine, safety limits, and tactile profiles are project-specific extensions.

System Overview

Each node in the system:

• runs a local FOC control loop on an ESP32
• reads shaft position from an AS5048A magnetic angle sensor
• sends the measured angle to the other node over ESP-NOW
• computes the difference between local and remote angle
• converts that difference into a softened spring response
• falls back to zero torque if communication is lost

This produces a bilateral coupling effect that feels like shared resistance rather than exact positional locking.

Current Prototype Status

• validated ESP32 FOC board pin mapping
• stable open-loop motor test completed
• working ESP-NOW communication between two boards
• bilateral mirror behavior running in the protocol-7 handoff firmware
• suitable as a research prototype, not yet as a deployable device

Hardware Platform

The current prototype uses the single-motor Makerbase MKS ESP32 FOC Mega shown above. This is not the earlier MKS ESP32 FOC board version previously pictured in the repository. The Mega board can be identified by its Makerbase ESP32 FOC Mega silkscreen, USB-C connector, single three-phase motor terminal, and two white sensor connectors.

Main Components

• 2x Makerbase MKS ESP32 FOC Mega single-motor driver board
• 2x iPower GM3506 hollow-shaft brushless gimbal motor, 24N/22P
• 2x AS5048A magnetic encoder fitted to the GM3506 motors
• 2x separate protected 3S LiPo battery
• wiring, connectors, and programming interface

Motor and Encoder

The tested motor is the iPower GM3506 brushless gimbal motor with AS5048A encoder, not a generic high-speed BLDC motor. Its 24N/22P construction has 11 rotor pole pairs, which is why the firmware uses POLE_PAIRS=11.

Item	Value used by this project
Motor model	iPower GM3506 hollow-shaft brushless gimbal motor
Winding / rotor configuration	24N/22P (11 pole pairs)
Encoder	ams OSRAM AS5048A absolute magnetic rotary encoder
Encoder interface	14-bit SPI, read at 1 MHz
Project supply	protected 3S LiPo per node
Product supply class	2-3S
Product listing load point	12 V, 1 A
Product listing internal resistance	5.6 Ohm ±5%

The product values above identify the selected motor variant; they are not independent measurements by this project. See the iPower GM3506 product page and the AS5048A datasheet.

Validated Board Information

Item	Value / mapping	Notes
Board	Makerbase MKS ESP32 FOC Mega	single-motor hardware used by the current prototype
MCU	ESP32-WROOM-32E	onboard module
Input marking	12-24 V DC	the project is validated only with protected 3S packs
Gate driver	IR2104	3 half-bridge drivers
MOSFET stage	30V / 30A	board-level power stage
Current sense	INA240A2	active phase-current foldback and telemetry
Motor output	A / B / C	3-phase BLDC connection

Pinout Used by the Current Sketch

Function	Pin
PWM_A	GPIO32
PWM_B	GPIO33
PWM_C	GPIO25
EN_PIN	GPIO12
SPI_SCK	GPIO18
SPI_MISO	GPIO19
SPI_MOSI	GPIO23
SPI_CS	GPIO5
CURRENT_A	GPIO39
CURRENT_B	GPIO36
VIN_STARTUP	GPIO13

Power Note

The current prototype is validated with two separate protected 3S LiPo packs. The handoff build and its limits have not been validated outside that setup.

Control Strategy

The current implementation uses a simple but effective haptic coupling model:

1. local shaft angle is sampled continuously
2. angle data is transmitted to the peer node at about 500 Hz
3. the angular difference is measured locally
4. a deadband removes small offsets around zero
5. the remaining error is shaped through a nonlinear spring curve
6. the resulting value is applied in torque-voltage mode

Key runtime parameters in the current sketch:

Parameter	Value	Meaning
CHANNEL	1	fixed communication channel
SEND_US	2000	about 500 Hz send rate
TIMEOUT_MS	250	link timeout threshold
power-stage ceiling	7.0 V	board-level voltage ceiling
handoff command ceiling	2.50 V	boost spring output ceiling
boost soft/hard current	0.52/0.70 A	fast current foldback envelope
sustained thermal current	0.45/0.60 A	thermal derating envelope
deadband	0.025 rad	no-resistance zone around zero
PWM carrier	40 kHz	synchronized sine PWM

Software

MKS ESP32 FOC Mega port

See the firmware documentation for the purpose and safe bring-up order of every included sketch.

The staged MKS port is in firmware/MKS_Bilateral_Link_Test/. It uses the validated M0 mapping (PWM 32/33/25, enable 12) and AS5048A SPI mapping (SCK 18, MISO 19, MOSI 23, CS 5). The first-stage firmware exchanges validated, unwrapped encoder positions over ESP-NOW while keeping both motor drivers disabled.

The autonomous bilateral controller is in firmware/MKS_Parallel_Mirror/. Both nodes load their stored FOC calibration, establish the ESP-NOW link, zero their local coordinates, and arm automatically after startup. No USB host or serial command is required during normal operation. A real link timeout disables the local power stage. Development builds can recover automatically; the published handoff build latches a runtime link fault until both nodes are fully power-cycled.

The current prototype uses a 7 V power-stage ceiling with a 2.50 V handoff command ceiling. Position error, shaft speed, measured phase current, and an estimated sustained thermal load are handled with continuous foldback.

Development builds provide gentle, normal, strong, and boost profiles. The protocol-7 handoff build locks both nodes to the tested boost profile, disables calibration and profile tuning commands, waits 5 seconds before synchronized arming, stops after 10 minutes, and latches off after a runtime link fault.

For standalone testing, disconnect USB from both boards and cycle both external motor supplies. Keep both mechanisms free during the approximately 5 s automatic startup interval. Follow the supervised operation guide.

Onboard RGB LED Hardware Erratum

Macro photographs confirm that the part between the two sensor connectors is a flat four-pad 3528 addressable RGB LED. Makerbase's schematic identifies it as XL-3528RGBW-WS2812B on GPIO2. Electrical probing found a PCB footprint error on both tested boards: the package's top-left VDD pad follows GPIO2, while its top-right DI pad is tied to 3.3 V. The data and supply nets are therefore swapped, so the installed LED cannot be controlled in software. The firmware therefore does not use the onboard LED. Repair requires lifting and cross-wiring the two upper LED leads or fitting an external indicator.

Battery voltage is sampled before Wi-Fi starts because the board routes VIN measurement to ESP32 GPIO13 (ADC2). Continuous battery measurement is not reliable while ESP-NOW is active.

See docs/prototype-checklist.md for the remaining work before extended human testing.

Main sketch:

• firmware/MKS_Parallel_Mirror/MKS_Parallel_Mirror.ino

Main libraries:

• SimpleFOC 2.4.0
• esp_now
• WiFi
• esp_wifi

Setup

Requirements

• Arduino IDE or Arduino CLI
• Espressif ESP32 Arduino core 3.3.10
• Simple FOC 2.4.0
• board target ESP32 Dev Module (esp32:esp32:esp32)

The complete versioned build, wiring, commissioning, and acceptance procedure is in the reproduction guide.

Wiring

• connect the BLDC motor to the A / B / C outputs
• connect the AS5048A sensor to the SPI pins defined in the sketch
• verify power and ground wiring carefully before startup

MAC Address Configuration

The sketch contains both validated MAC addresses and identifies node A or B automatically:

static const uint8_t MAC_A[6]={0x30,0xC9,0x22,0x5F,0x5D,0x1C};
static const uint8_t MAC_B[6]={0x30,0xC9,0x22,0x5F,0x5D,0x24};

First Power-Up Checklist

• run MKS_Bilateral_Link_Test before enabling either motor
• confirm both AS5048A encoders report valid, continuous angles
• check current-sense offsets with MKS_Current_Sense_Diagnostic
• verify the board-specific MAC addresses and electrical-zero calibration
• confirm that a link timeout disables both power stages
• use the supervised handoff procedure only after the staged checks pass

Parameter Tuning

The main tactile parameters are:

• each ControlProfile entry's voltageLimit and springK
• frictionComp, positionI, and the integral limits
• viscousDamping and couplingDamping
• DEAD_BAND for center free-play
• SOFTEN_EXP for softening the near-zero response
• SEND_US for communication update frequency
• TIMEOUT_MS for disengagement speed after link loss

The published protocol-7 handoff build has HANDOFF_MODE=true; it locks both nodes to the validated boost profile and disables serial tuning commands. Parameter changes belong in a development build and require repeated current, thermal, fault-recovery, and tactile validation.

Prototype Limitations

• the current implementation is still a research prototype
• current sensing limits the voltage-mode torque request but has not yet been checked against an external calibrated current instrument
• there is no full fault management, emergency stop layer, or production safety envelope
• mechanical integration and long-duration reliability testing are still future work

Safety Notes

• this is a high-current motor controller board, so initial tests should always be mechanically secured
• incorrect phase order, sensor wiring, or an overly high voltage ceiling can cause vibration, heating, or sudden motion
• do not treat the current prototype as a clinically validated or deployment-ready system

Repository Contents

• handoff firmware: firmware/MKS_Parallel_Mirror/MKS_Parallel_Mirror.ino
• documentation index: docs/README.md
• firmware guide: firmware/README.md
• reproduction guide: docs/reproducibility.md
• supervised operation guide: docs/HANDOFF.md
• Hungarian operator translation: docs/HANDOFF-HU.md
• development summary: docs/development-log.md
• prototype readiness checklist: docs/prototype-checklist.md
• planned custom controller: hardware/parallel-foc-rev-a/README.md
• historical first-generation firmware: firmware/legacy/README.md
• board reference image: docs/board-overview.png

Future Directions

• complete instrumented thermal and externally calibrated current tests
• add continuous battery and motor-temperature monitoring
• add a hardware emergency stop, fuse, guarding, and mechanical travel limits
• complete the custom controller schematic and PCB layout
• define a repeatable user-test protocol and mechanical acceptance test
• continue control refinement using logged, repeatable comparisons

Project Status

This repository documents a working proof of concept. At its current stage, the project demonstrates that a bilateral, wireless, mutually perceivable motor mirror connection can be implemented in an ESP32 + SimpleFOC environment using a lightweight embedded architecture.