N20 Servo

A CH32V003-powered small size closed-loop actuator

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Table of Contents

• 1. Introduction
• 2. Credits
• 3. Technical Specifications
• 4. PCB & Hardware Revisions
• 5. CAD & Mechanical Design
• 6. Firmware & PlatformIO Environments
• 7. Additional Tools
• 8. Changelog

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1. Introduction

The N20 Servo is an affordable, high-performance(?) closed-loop actuator system built around the WCH CH32V003 RISC-V 32-bit microcontroller. The project was created from my dissatisfaction of the SG90 servo performance: lots of backlash and weak torque, and the failed attempt when trying and modify the servo with the DIY solution (link in Credits) but found it rather hard to properly install the very thin metal wiper and secure the potentiometer body onto the metal surface of the motor. Therefore, I set my goal on creating a better alternative.

This project contains the firmware, CAD assemblies, PCB views, and debugging utilities required to build, test, and tune.
If there's anything wrong, you can raise an issue and let me know. Thank you for your help.

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2. Credits

This project is designed, developed, and maintained by: imdolce (aka dolce_st, hovietkien49).

Important

Code base development is assisted by OpenAI Codex. On my side, I try to read and comprehend the functions used on this chip or the whole CH32 family in general, as it is my first time working with this chip.

Other sources and ideas:

• DIY Servo Modding Guide: How to convert an N20 gear motor into a servo using SG90 parts — by JANIUL HAQ
• Angle Sensor Adapter: N20-Servo-Module (3D print + N20 motor + angle sensor = servo motor) — by jkugalde
• N20 DC Motor CAD Model: N20 DC Gear Motor | GrabCAD Library — by RamBros 3D
• SG90 CAD Model: SG90 Micro Servo 9g | GrabCAD Library — by Mattheus Frasson

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3. Technical Specifications

The following table outlines the hardware and software specifications of the N20 Servo:

Parameter	Specification	Notes
Microcontroller	WCH CH32V003J4M6 (RISC-V 32-bit Core @ 48 MHz)	Dirt cheap 5 cents MCU.
Operating voltage	5V	Shared power bus with motor driver.
Actuator	N20 Geared DC Motor	High torque, compact size.
H-bridge chip	RZ78xx, SA8301, DRV8210, DRV8837	RZ78xx driver series include: RZ7888, RZ7889, RZ7899. Currently the RZ7888 and SA8301 are used because it's cheap.
H-Bridge Output	Dual-PWM control on PC1 and PC2	Drives motor direction and speed using TIM2.
PWM Frequency	2.0 kHz	Adjustable via MOTOR_PWM_HZ in config.
Position Feedback	SV01A103AEA01R00 10K (or similar clones) angle position detector (Wiper input pin PA2)	Read via ADC1 Channel 0, 10-bit resolution..
Traverse range	Up to 330° (-165° to +165° symmetrical) mechanical travel	To make the servo safely accomodate within the wiper track of the angle detector, the traverse angle is limited to 320° (-160° to +160° symmetrical).
Input Signal	50 Hz RC Servo PWM, PC4	Captured via TIM1 Channel 4 (1000µs to 2000µs, 0° angle at 1500µs).
Polling rate	500 Hz (2 ms)	Position feedback and control loop.
Telemetry UART	921600 baud on PD5 (UART1 Remap 2 TX)	CSV telemetry output for live tuning.

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4. PCB & Hardware Revisions

Tip

You can view & get the PCB schematic and designs in this repository, or visit my post on OSHWLAB (coming soon).

Important

It is not recommended to order PCB of the initial version 1D2 (1Д2), as it requires good soldering skill to install components on both sides and wiring up the angle detector with bodge wires. The 1D3 (1Д3) version aims to resolve both issues. More details will be added as soon as I can get my hands on the new 1Д3 PCB.

1. Initial version 1D2 (1Д2)

The electronics are designed to fit directly on top of the N20 motor. The enclosure is designed specifically for the first revision 1Д2

View	Imagew	Description
PCB View		Bare PCB appearance, top and bottom view.
Top populated view		Fully populated top view of the PCB.
Bottom populated view		Partially populated bottom view of the PCB, bottom 2 22uF capacitors are removed due to their interference with the motor below.
Angle sensor and adapter collet		Top and bottom view of the angle sensor, with the adapter collet installed, the standalone collet (second to the left side) the 3D printed custom fixture to easily print the adapter collets (white rectangle on the left side)
Test setup		Consists of: Partially assembled N20 Servo assembly (to make space for the Outcoming UART TX telemetry), WEMOS D1 Mini for PWM output, WCH-LinkE for UART telemtry reading and MCU programming.

2. Second version 1D3 (1Д3)

In this new version, it aims to resolve the difficulty of populating the PCB and wiring the angle detector. Currently I have not tested the design myself, so documentation is limited. There are 5 different PCB designs, 1Д3-1/2/3/4 incorporates 4 different designs of 4 different motor drivers, it was made to avoid ordering multiple PCB revisions. The remaining 4 designs 1Д3-1, 1Д3-2, 1Д3-3, 1Д3-4 are standalone boards, using the SA8301, RZ7888, DRV8210DRLR, DRV8837DSGR respectively to each design.

View	Image	Description
PCB View		Coming soon.
Top populated view		Coming soon.
Bottom populated view		Coming soon.

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5. CAD & Mechanical Design

The enclosure is designed using Onshape. The link is included here: CAD Design

Important

Use 4x M1.4x6 Self-tap SHCS and a 1.3mm Allen key or T4 Torx screwdriver bit to close and secure the top cover. The same screw size will persist across all future releases.

Isometric View	Front View
Side View	Rear View
Top Down View

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6. Firmware & PlatformIO Environments

The firmware is written in C, using WCH Standard Peripheral Library (StdPeriphLib) aka noneos-sdk. The project is managed using PlatformIO.

Build & Uploading

To compile and upload the default firmware environment using a WCH-LinkE programmer, run:

platformio run -e genericCH32V003J4M6_servo_control -t upload

Or click the Upload button

PlatformIO Environments

The environment for this project is defined in platformio.ini:

Environment Name	Description	Build Configurations
genericCH32V003J4M6_servo_control	Production Closed-Loop Servo: Primary operational environment. Telemetry output is disabled to prevent UART TX blocks and ensure minimum control loop jitter.	TEST_MODE_SERVO_CONTROL SERVO_CONTROL_TELEMETRY_ENABLE=0
genericCH32V003J4M6_servo_control_debug	Debug Closed-Loop Servo: Enables high-speed CSV telemetry output via UART for visualization and tuning.	TEST_MODE_SERVO_CONTROL SERVO_CONTROL_TELEMETRY_ENABLE=1
genericCH32V003J4M6	Motor Sweep Test: Periodically drives the motor from limit to limit. Useful for verifying gear engagement, mechanical tolerances, and limits.	TEST_MODE_MOTOR_SWEEP
genericCH32V003J4M6_adc_only	ADC Calibrator: Reads the position potentiometer raw values and displays computed angles. Essential for finding POT_ADC_AT_NEG160 and POT_ADC_AT_POS160 parameters.	TEST_MODE_ADC_ONLY
genericCH32V003J4M6_rc_input_capture	PWM Signal Capture Monitor: Measures the pulse width and period of incoming RC PWM signals without driving the motor. Used for verifying receiver compatibility.	TEST_MODE_RC_INPUT_CAPTURE

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7. Additional Tools

To facilitate tuning and development, the project includes desktop tools located in the tools/ folder. Precompiled executables are included alongside their source Python implementations. Refer to Tools guide

1. Serial Dashboard

• File: tools/python/serial_dashboard.py (Executable: tools/serial_dashboard.exe)
• Description: Reads the high-speed UART telemetry stream and plots real-time curves showing command target vs. measured position, error metrics, motor duty cycle, and signal validity.
• Usage:

  python tools/python/serial_dashboard.py --port COM5 --baud 921600

• Test / Demo Modes: (This will be removed in the next release).

  python tools/python/serial_dashboard.py --demo --demo-mode servo
  python tools/python/serial_dashboard.py --self-test

  Or use the included executeable.

2. Servo Configurator

• File: tools/python/servo_configurator.py (Executable: tools/servo_configurator.exe)
• Description: A graphical user interface using Tkinter for inspecting and editing the configuration header include/servo_config.h. It automatically checks configuration boundaries, performs self-tests, and saves a timestamped backup before modifying the file.
• Usage:

  python tools/python/servo_configurator.py

  Or use the included executeable.

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8. Changelog

• v1.1.0: Added precompiled executable for serial_dashboard.py and servo_configurator.py, released new PCB version 1Д3-1/2/3/4 and its standalone variants.
• v1.0.0: Initial release, no documentation available..