README.md

🤖 Mini VibroBot — Single-Motor BLE Micro Robot

A stamp-sized BLE-controlled vibrobot that steers using only one reversible ERM motor — no wheels, no servos, no gears.

How It Works · Math · Hardware · Build · Firmware

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What Is This?

Most robots use multiple motors, wheels, or servos to steer. This project does it with one motor and pure software.

A single coin ERM (Eccentric Rotating Mass) motor — the flat pancake type from phone vibration alerts — is mounted at a 20-30° tilt on a chassis the size of a postage stamp. The tilt is critical: it converts vertical vibration into a directional horizontal force. By controlling when the motor spins clockwise vs counterclockwise, and for how long, the robot can:

• Move forward (rapid CW↔CCW switching)
• Curve right (sustained CW)
• Curve left (sustained CCW)
• Stop (motor off)

All controlled wirelessly over BLE 5.0 from a phone.

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📐 How It Works

The Stick-Slip Principle

When the ERM motor spins, the eccentric mass creates a rotating centrifugal force. Because the motor is tilted at an angle (30–45°), this force has two components:

    Coin ERM Motor (tilted 20-30° above chassis)
         _____
        /  ○  \   ← coin motor (flat pancake, 10mm dia)
       /________\
          ╱ ← tilt angle 20-30°
         ╱
━━━━━━━━╱━━━━━━━━━━  ← chassis (2cm × 2cm)
●   ●   ●   ●   ●   ← pen nib contacts (pentagon)

Video_._Main_Explainer_se.mp4

Phase	What Happens
Slip	Horizontal force > static friction → robot slides forward
Stick	Force drops below friction → robot holds position
Net result	Robot crawls forward one tiny step per vibration cycle

Steering Without a Second Motor

This is the key insight. When the ERM spins clockwise, the rotating force vector sweeps in one direction, creating a yaw torque bias — the robot curves right. Reverse to counterclockwise and the yaw torque mirrors — robot curves left.

Rapid CW↔CCW switching cancels the yaw torques out → straight line.

Sustained CW  →  curves RIGHT  ↷
Sustained CCW →  curves LEFT   ↶
Rapid CW/CCW  →  STRAIGHT      ↑

Validated by: SimoBot (4.76g, 20mm diameter — peer reviewed), Pro Know single-motor robot (ESP-based, open source)

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🔬 The Physics

Simple Version

The eccentric mass spins in a circle. That spinning creates a centrifugal force that pushes the robot. The tilt angle decides which direction the push goes.

Full Mathematical Treatment

1. Centrifugal Force

$$F_c = m_e \cdot \omega^2 \cdot r$$

Where:

• $m_e$ = eccentric mass (kg)
• $\omega = \frac{2\pi \cdot \text{RPM}}{60}$ = angular velocity (rad/s)
• $r$ = eccentricity (distance of mass from center, m)

For a 4mm ERM at ~12,000 RPM: $$\omega = \frac{2\pi \times 12000}{60} \approx 1257 \text{ rad/s}$$

2. Force Decomposition (Tilted Motor)

$$F_{\text{vertical}} = F_c \cdot \cos(\theta)$$ $$F_{\text{horizontal}} = F_c \cdot \sin(\theta)$$

At tilt angle $\theta = 25°$ (coin ERM optimal):

• $F_h = F_c \cdot \sin(25°) \approx 0.423 \cdot F_c$ (propulsion)
• $F_v = F_c \cdot \cos(25°) \approx 0.906 \cdot F_c$ (modulates normal force)

Note: Coin ERM vibrates primarily in the Z-axis when flat. The 20-30° tilt converts this into a usable horizontal propulsion component. Never mount flat — it kills directional control.

3. Stick-Slip Condition

Robot moves only when horizontal force exceeds static friction:

$$F_c \cdot \sin(\theta) > \mu_s \cdot (Mg - F_c \cdot \cos(\theta))$$

Solving for minimum ω:

$$\omega_{\min} = \sqrt{\frac{\mu_s \cdot Mg}{r(sin\theta + \mu_s \cos\theta) \cdot m_e}}$$

Physical meaning: Too slow = robot just sits there vibrating. Above $\omega_{\min}$ = robot moves.

4. Net Displacement Per Cycle

$$\Delta x = \frac{(F_h - \mu_k \cdot N) \cdot t_{\text{slip}}^2}{2M}$$

Where $N = Mg - F_c\cos(\theta)$ is the reduced normal force during slip phase.

5. Optimal CW↔CCW Switching Frequency

The motor needs time to reach full speed before reversing:

$$f_{\text{opt}} = \frac{1}{2\tau_m}$$

where $\tau_m \approx 50\text{–}100\text{ ms}$ for a 4mm ERM, giving:

$$f_{\text{opt}} \approx 2\text{–}4 \text{ Hz} \quad (250\text{–}500\text{ ms per phase})$$

Confirmed by SimoBot: 500ms CW / 500ms CCW produced straight-line motion.

6. Yaw Torque (Steering)

$$\tau_{\text{yaw}} = F_c \cdot d_{\text{offset}}$$

Where $d_{\text{offset}}$ is lateral distance from motor to chassis centroid. Reversing $\omega$ direction reverses $\tau_{\text{yaw}}$ sign → opposite curve direction.

7. Power Budget

Source	Calculation	Value
Battery energy	80mAh × 3.7V	296 mWh
HT7737C boost	3.7V → 5V at ~85% eff	~4.25V output
CP6208 drop	1.0–1.6V at 100mA	Motor gets ~2.6–3.4V
Motor current	~80–120mA	—
BL602 BLE	~15–20mA	—
Total draw	~100–140mA peak	—
Runtime estimate	80mAh ÷ 120mA	~40 min

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🔧 Hardware

Bill of Materials

Component	Part	Purpose
MCU	Ai-WB2-M1-I (BL602)	BLE 5.0 + WiFi brain
H-Bridge	CP6208DTR	Motor CW/CCW control
Motor	Coin ERM (10mm dia, 3V)	Stick-slip locomotion, low noise
Boost	HT7737C SOT-89-3	3.7V → 5V for motor
Battery	80mAh 20-30C LiPo	Power
Contacts	5× ballpoint pen nibs	Pentagon ground contact
Cap	100µF	Motor spike buffer
Cap	0.1µF	CP6208 bypass

Circuit Connections

LiPo (+) ──→ HT7737C IN ──→ 5V OUT ──→ CP6208 VCC
LiPo (+) ──→ BL602 Pin3 (VCC 3.3V via LDO)
LiPo (-) ──→ Common GND

BL602 Pin19 (IO3) ──10kΩ──GND
                  └──────────→ CP6208 AIN
BL602 Pin26 (IO1) ──10kΩ──GND  
                  └──────────→ CP6208 BIN

CP6208 AOUT ──→ Motor (+)
CP6208 BOUT ──→ Motor (-)

Motor Control Truth Table

IO3 (AIN)	IO1 (BIN)	Motor	Robot
HIGH	LOW	CW	Curves Right
LOW	HIGH	CCW	Curves Left
Rapid toggle	Rapid toggle	Oscillating	Straight
LOW	LOW	Stop	Stop

Chassis Design

    Top view (20mm × 20mm)
    ┌─────────────────┐
    │   [BL602 PCB]   │
    │   ___           │
    │  /○ \  ← coin   │
    │ /____\ motor    │
    │  ╱ tilted 25°   │
    └─────────────────┘

    Side view — motor tilt is everything
    ┌──────┐
    │ coin │ ← 10mm dia, 3mm thick
    └──┬───┘
       │ ← tilted 20-30° from horizontal
    ───┴──────────────  chassis
    ●  ●  ●  ●  ●      pen nib pentagon

    Bottom view — pentagon nib contacts
         ●
       ●   ●
       ●   ●
    (omnidirectional low-friction steel balls)

Critical mounting note: Coin ERM must be tilted 20-30° off horizontal. Flat mounting = only vertical vibration = no steering, weak locomotion. The tilt is the entire physics basis of the project.

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🏗 Build Guide

Phase 1 — Bench Test (Current Stage)

• BL602 module communication via UART confirmed
• AT firmware responding (BLE advertising confirmed)
• CP6208 wired and tested
• Custom GPIO firmware flashed
• Motor CW/CCW bench tested
• Locomotion direction verified

Phase 2 — Robot Assembly

• Pentagon pen nib contacts attached
• ERM motor mounted at 35° angle
• HT7737C boost circuit integrated
• LiPo connected
• Chassis weight measured

Phase 3 — BLE Control

• Custom BLE firmware flashed
• Phone app / web BLE controller built
• Forward/Left/Right/Stop commands working
• Switching frequency tuned for surface

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💻 Firmware

Planned Control Protocol (BLE)

Command byte → Motor action
0x46 ('F') → Forward (rapid CW/CCW 250ms intervals)
0x52 ('R') → Right (sustained CW)
0x4C ('L') → Left (sustained CCW)
0x53 ('S') → Stop

GPIO Control Logic (Pseudo)

void forward() {
    while(moving) {
        gpio_write(IO3, HIGH); gpio_write(IO1, LOW);  // CW
        delay(250);
        gpio_write(IO3, LOW);  gpio_write(IO1, HIGH); // CCW
        delay(250);
    }
}

void right() {
    gpio_write(IO3, HIGH);
    gpio_write(IO1, LOW);  // Sustained CW
}

void left() {
    gpio_write(IO3, LOW);
    gpio_write(IO1, HIGH); // Sustained CCW
}

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📚 References

1. SimoBot: "SimoBot: An underactuated miniature robot driven by a single motor" — 4.76g, 20mm, peer-reviewed validation of this exact concept
2. Pro Know Robot — Open source ESP-based single-motor vibrobot using DRV8212 + pager motor
3. PufferFace Robot (PFR) — Soft-body vibrobot using ERM for pipeline navigation
4. Hexbug Nano — Commercial bristlebot (unidirectional, for comparison)

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👤 Author

Ishant Jaiswal — B.Tech Robotics & Automation, 3rd Year
GitHub: @Ishu1519

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📄 License

MIT License — build it, improve it, share it.