ServoTester

Here is a really small side project that I developed while working on something else…

I had to test a RC speed controller that I wanted to use to control an electric car window motor, and for the 100th time I was facing the same dilemma: find 8 batteries for my remote control, dismount the RX part from the quadcopter and use that, OR grab the Arduino and write quickly some code to generate the corresponding signals ? Neither of which was actually particularly handy…

So I finally decided to build a small stand alone servo tester :

The hard part was to avoid trying to squeeze too many features in it, as I kept thinking that having an LCD screen showing the exact pulse width in microsecs would be useful, etc. ….

I finally managed to convince myself that “less is more” and the final design simply has a micro-controller (what else than the trustworthy AtTiny85 !?  🙂 )  and a big knob (that I salvaged from a radio).

Here’s some sort of schema, though that’s probably an overstatement 🙂

Servo Tester Schema

Servo Tester

A, yes, sorry there is also a small capacitor to avoid noise from the motor affecting the MCU (see the lesson I’ve learned here, the HARD way !) and a LED, to give some basic visual feedback, at least when the device is operating.

And here is the simple and clean soldering at the back (I’m becoming quite good at this, it’s almost like a craft: initially how to position the parts to minimize the distances and then how to re-use existing connections to minimize extra soldering… 🙂 ), I probably should one day make the leap to design proper PCBs…

Servo Tester Back

And as usual, here’s the code that runs on the AtTiny85.

I contemplated the idea of using AVR C code directly, but it’s so much easier to use Arduino and the existing libraries…

One remark before you look at the code: you might find it unnecessarily complex… The reason is that either because the potentiometer is of low quality or maybe that’s normal with these mechanical devices, the readings can be VERY noisy, especially while it’s moving ! So half of the code simply tries to filter that out…

/*
* trandi 15 June 2013
* This could have been MUCH simpler, if it wasn't for a noisy and bad potentiomenter,
* and hence the need to filter the input data.
*/

#include <SoftwareServo.h>

#define PIN_SERVO 0
#define PIN_POT 2
#define PULSE_MIN 600 // in microsecs
#define PULSE_MAX 2400 // in microsecs
#define READINGS_COUNT 10

SoftwareServo servo;
long potValue = 0;
int readings[READINGS_COUNT] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
byte readingsPos = 0;
unsigned long nextReadingDue = 0;

void setup(){
servo.attach(PIN_SERVO);
servo.setMinimumPulse(PULSE_MIN);
servo.setMaximumPulse(PULSE_MAX);
}

void loop(){
// read the analog pin once every millisec, and store the data until we have enough to filter
unsigned long currentTime = micros();
if(currentTime > nextReadingDue){
readings[readingsPos] = analogRead(PIN_POT);
readingsPos ++;

nextReadingDue = currentTime + 1000;
}

// once every READINGS_COUNT readings, clean the data and update the servo
// also filter using weighted moving average to avoid excessive jittering due to poor potentiometer
if(readingsPos > READINGS_COUNT){
potValue = (9 * potValue + getCleanReading())/ 10;

servo.write(map(potValue, 0, 1023, 0, 180));

readingsPos = 0;
}

// good to call this as often as we can, so that we don't miss the moment when the signal has to go up or down
SoftwareServo::refresh();
}

// reads several analog values and tries to do some smart filtering
int getCleanReading(){
int reading, minReading=9999, maxReading=0;
long result = 0;

for(byte i=0; i<READINGS_COUNT; i++){
readings[i] = analogRead(PIN_POT);
result += readings[i];

if(readings[i] < minReading){
minReading = readings[i];
}
if(readings[i] > maxReading){
maxReading = readings[i];
}
}

// reaturn the average after eliminating min and max readings
return (result - minReading - maxReading) / (READINGS_COUNT - 2);
}