• 3-DOF Servo Arm v1: Code

    Jamie Ashton • 04/21/2025 at 16:05 • 0 comments

    Overview:

    This is the code for v1 of the arm, in which it is manually controlled by 3 potentiometers. This code was written in C++ in Mbed Studio as I was using the STM Nucleo-F401RE microcontroller.

    Servo Class:

    To begin with I searched for an Mbed servo library, ideally one similar to the Arduino library. I found this one by Simon Ford https://os.mbed.com/teams/Biorobotics-Project/code/Servo/ which works very well. 

    Potentiometer Class

    I then added a basic potentiometer class with basic functionality, I am planning to refine this later especially when I have more going on. 

    class Potentiometer //Begin potentiometer class definition
{
private: //Private data member declaration
 AnalogIn inputSignal; //Declaration of AnalogIn object
 float VDD; //Float variable to speficy the value of VDD (3.3 V for the Nucleo-64)
public: // Public declarations

//Constructor - user provided pin name assigned to AnalogIn. VDD is also provided to determine maximum measurable voltage
 Potentiometer(PinName pin, float v) : inputSignal(pin), VDD(v) {}
 float amplitudeVolts(void) //Public member function to measure the amplitude in volts
 {
 return (inputSignal.read()*VDD); //Scales the 0.0-1.0 value by VDD to read the input in volts
 }

 float amplitudeNorm(void) //Public member function to measure the normalised amplitude
 {
 return inputSignal.read(); //Returns the ADC value normalised to range 0.0 - 1.0
 }
};

    This is a class from my Microcontroller Engineering II module, studied earlier this year, that I reused.

    Pot-Servo Class

    As the arm was going to be using minimum 3 servos (we'll get to the end-effector later) I decided to write a class that combined the functionality of the Servo class and the Potentiometer class - PotServo. This allowed me to instantiate each joint and its control as a single object.

    class PotServo {
    private:
        Potentiometer pot;
        Servo servo;

    public:
        PotServo(PinName potPin, float vdd, PinName servoPin)
        : pot(potPin, vdd), servo(servoPin) {}

    void update() {
        float position = pot.amplitudeNorm();
        servo.write(position);
    }
};

    Main Loop

    I only had access to 2 potentiometers, so for the time being I tested the code with just 2 servos.

    int main()
{
    PotServo servo1(A0, 3.3, PA_7);
    PotServo servo2(A1, 3.3, PB_6);

    while(true){
        servo1.update();
        servo2.update();
    }   
}

  • 3-DOF Servo Arm v1: Schematic & PCB

    Jamie Ashton • 04/21/2025 at 15:52 • 0 comments

    Overview

    The first version of the arm will simply be three servos controlled by 3 potentiometers, this allows full manual control over base, shoulder and elbow rotation. This can be controlled with any microcontroller really, but for the schematic i've used the Arduino Nano, as its size is convenient for the PCB. When prototyping this circuit I used both the Arduino Nano and the STM NUCLEO-F401RE. The external power source is necessary as the board itself cannot drive all 3 servos simultaneously. 

    Schematic

    PCB Design

    This was my first time using KiCad, so I kept the design simple as a starting project.

    3D-View