Roll Your Own Servo Tester

So you want to test the range of your servos.  Who needs a servo tester/driver?  If you have a MCU, a POT and some wire, you can make one.

Servo Tester Recipe:

1. Fez MCU (code written for NETMF)
2. POT or TrimPot
3. Some jumper wire
4. Servo to test
5. Breadboard
6. 5v-7v battery to power servo.

Hook your pot, servo and wires up as shown.  Pick a free PWM port on your FEZ.  Also pick a free Analog port.  We will use the analog port to read relative pot level.  We will scale the AIn port reading to match the Ms (microsecond) range we want to see for the 0-180 deg range of the servo.  Now just add the following code:

public static void ServoTester()
{
    // Config pin and scale for reading POT voltage.
    AnalogIn ai = new AnalogIn(AnalogIn.Pin.Ain5);
    FEZ_Components.Button but = new FEZ_Components.Button(FEZ_Pin.Digital.LDR);

    // Initially, set your scale a bit beyond servo limits.
    // Kick range numbers down till buzzing stops at each end.
    uint FREQ = 20 * 1000 * 1000; // 20ms Freq.
    int lower = 690 * 1000;
    int upper = 2422 * 1000;
    int neutral = (upper - lower) / 2;
    Debug.Print("Lower: " + lower + " Upper: " + upper + " Neutral: " + neutral);
    ai.SetLinearScale(lower, upper);

    // Set servo line pwm.
    PWM servo = new PWM(PWM.Pin.PWM5);

    uint min = uint.MaxValue;
    uint max = 0;
    while (but.GetState() != FEZ_Components.Button.ButtonState.Pressed)
    {
        // Read pot and set servo.
        uint newPos = (uint)ai.Read();
        if (newPos < min)
        {
            min = newPos;
            Debug.Print("Min: " + min + " Max: " + max);
        }
        if (newPos > max)
        {
            max = newPos;
            Debug.Print("Min: " + min + " Max: " + max);
        }
        servo.SetPulse(FREQ, newPos);
    }
}

Turn everything on.  As you turn the POT back and forth, the value will be converted to a scaled High-Time that we use for SetPulse().  The servo should turn CCW or CW to the value.  Set your linear scale to be a bit outside the edges of your servo’s limits at first.  Get limits from its’ datasheet.  Many common servos are in 1-2ms range with neutral at 1.5ms.  However, I am testing a Medium sized servo from SparkFun.  It is a DGServo from China, so it seems to have different limits.  However, using this simple test rig, I found the limits are around .69-2.43ms.  You know you hit the servos’ edge when it starts buzzing.  It should be able to hold max positions without buzzing.  After you find the values, you may want to back them off a little more to add a small margin.  Going past the limits will drain more current and heat up servo needlessly.

That’s it.  Hope that may be useful.