Quad-Spider: The 4-legged bluetooth Spider

System Specifications

Domain Embedded Robotics & Wireless Control

Actuation 12-DOF (12× SG90 Servos, 3-DOF per Leg)

Chassis & Power Symmetrical Acrylic Frame, 11.1V LiPo + Buck Converter (5-6V)

Microcontroller & Comms Arduino, HC-05 Bluetooth Module (SoftwareSerial)

User Interface Custom Android App (MIT App Inventor)

Introduction: Sharpening the Axe Before the Swing

In the spirit of the age-old lumberjack analogy, this project wasn’t about rushing to build a robot. Instead, it was a meticulously pre-planned engineering experiment, focusing not only on functionality but on modularity, control theory, and real-time human-robot interaction.

Over the summer, I built Quad-Spider, a Bluetooth-controlled quadruped robot that responds to text-based motion commands from a custom Android application created using MIT App Inventor. This article is a deep dive into how I went from concept to prototype, and how I iterated across hardware limitations, software design, and control stability.

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Mechanical and Electrical Framework

Robot Chassis & Motion

• Leg Configuration: 4 legs × 3 DOF = 12 degrees of freedom
• Servos: 12 × SG90 servo motors (PWM-controlled)
• Structure: Lightweight acrylic chassis, designed for symmetry and center-of-mass alignment
• Power Source: 11.1V 2200mAh LiPo Battery regulated via a buck converter to 5–6V for stable servo operation
  Spyder model

Pin Mapping

• Servo Control Pins: Digital Pins 2–13 (1 per servo)
• Bluetooth TX/RX: Pins 10 (RX), 11 (TX) via SoftwareSerial
• Arduino Board: Arduino Uno or Nano

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Bluetooth Communication Architecture

flowchart LR
    subgraph UI["Mobile Controller"]
        A[Android MIT App] -->|Bluetooth SPP Packet| B[HC-05 Wireless Module]
    end
    subgraph MCU["Embedded Processing"]
        B -->|UART 9600 Baud| C[Arduino MCU SoftwareSerial]
        C -->|Command Parser| D[Gait State Engine]
    end
    subgraph Output["Actuation"]
        D -->|12x PWM Channels| E[12x SG90 Micro Servos]
    end

Module: HC-05 Bluetooth Serial Module

• Baud Rate: 9600
• Voltage Divider: 8kΩ & 12kΩ resistors on RX line for 5V → 3.3V compatibility
• Communication Protocol: UART serial interpreted by Arduino and parsed via custom logic

Command Structure

Simple line-based protocol:

"w 5\n" // Walk forward 5 steps
"s 1\n" // Sit once
"k 3\n" // Shake leg gesture 3 times

Each command consists of a keyword and step count, terminated with \n.

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Software Stack on Arduino

Yes, you read that right… Instead of a brain for high-end compute, we decided to use this one instead.

Core Libraries

• Servo.h for PWM control
• FlexiTimer2.h for periodic timed control of all 12 servos
• SoftwareSerial.h for Bluetooth handling

Servo Timing & Synchronization

• FlexiTimer2 used at 20ms intervals (50Hz PWM)
• Smooth motion via step-by-step interpolation
• Avoided servo jitter and overloads with staggered updates

Command Parsing Logic

Dropped SerialCommand in favor of memory-efficient manual parsing:

if (BTSerial.available()) {
    char c = BTSerial.read();
    if (c == '\n') {
        parseCommand(buffer);
        buffer = "";
    } else {
        buffer += c;
    }
}

This allows precise one-line-at-a-time parsing with low memory overhead.

The Custom App – Built with MIT App Inventor

Features

• Clean UI with command buttons (Walk, Sit, Wave, etc.)
• Input box for custom step count
• Real-time Bluetooth response display via .ReceiveText
• Handles newline-based protocol and displays the latest status line

Workflow

• On button click: send "cmd step\n" string
• Use .BytesAvailableToReceive and .ReceiveText(-1)
• Split at \n, select last item from list
• Display in label or debugger box

Design Thinking & Challenges

Constraint-Aware Coding

• 2KB SRAM limit on Arduino Uno
• Lean memory buffers without dynamic allocations
• Pin mapping optimized to utilize available digital outputs without conflict

Power Management

• Buck converter regulated to ~6V output
• Logic rails isolated from high-draw servo power to prevent brownouts and resets

Gesture Sequencing

• Each gait and gesture defined as target joint angle arrays
• Step interpolation applied across transitions to reduce mechanical shock

Demo & Results

Visit the GitHub repository for:

• Arduino firmware
• MIT App Inventor .aia source
• Wiring schematics
• Kinematic calculations and gait sequence maps

Conclusion

This project demonstrates coordinated 12-DOF multi-leg actuation, wireless command parsing, and real-time mobile interfacing under resource-constrained embedded hardware.

• GitHub Repository: Spider_Quad