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My Motion-Controlled Miniature Turret

April 9, 2012

I needed a break from work (real work), so I decided to spend an hour or two playing with microcontrollers again.

I’ve been itching to build a sentry gun, however, I’m still in the process of getting comfortable with opencv, face detection, pattern recognition, etc.

In the meantime, I decided this was the perfect opportunity to experiment with my 3-axis accelerometer and a couple servos.

A while back, I posted an entry about my homemade Nintendo powerglove using flex sensors. This time, I want to attach an accelerometer to that same glove and use my hand to control a little missile turret.

First, I ghetto-rigged this miniature pan/tilt turret using two micro servos, cardboard, screws, and duct tape. I probably shouldn’t be using cardboard, but this is just a prototype.

Next, I connected my ADXL335 3-axis accelerometer’s X,Y,Z terminals to analog pins 1-3 on my arduino. I plugged ground into ground and power into the 3.3V source (5V is too high).

I serial.printed the readings while playing with the accelerometer to find out what the range of the readings were. This is what I got:

Min Max
X 268 409
Y 263 402
Z 277 412

X was the result of tilting the accelerometer left and right.
Y was the result of tilting the accelerometer forward and back.
Z reflected the orientation of the accelerometer whether it was upright or upside down.

Now, the servos accept a digital value from 0 to 180 for a full 180 degree range.
So, how do get the arduino to convert accelerometer readings to servo outputs? Exactly the same way you derive the well-known Fahrenheit/Celsius conversion formula. In case you forgot: (skip if you already know it)

How to Derive the Fahrenheit to Celsius Conversion Formula

You need two anchor points for each Celsius and Fahrenheit. We’ll use the water freezing and boiling temperatures.

freezes boils
C 0 100
F 32 212

Now, we represent these values in slope-intercept form:
y = mx + b
and we’ll say y is Fahrenheit.
We get 32 = 0m + b
and 212 = 100m + b
b = 32 and plugging that into the second equation, we derive that m = 9/5
Therefore, F = (9/5)*C + 32

So given the data:
268 = 0m + b
b = 268
409 = 180m + 268
141 = 180m
m = 141/180 = 47/60
Therefore: xAcc = xServo*(47/60)+268

263 = b
402 = 180m + 263
134 = 180m
m = 67/90
Therefore: yAcc = yServo*(67/90) + 263

And that’s all there is to it!

Onto the code:

#include <Servo.h>
Servo servo1;
Servo servo2;
const int xpin = A3; 
const int ypin = A2; 
const int zpin = A1; 
int pos1 = 90;
int pos2 = 90;

void setup()
void loop()
  pos1 = (analogRead(xpin) - 268) * (int)(60 / 47);
  pos2 = (analogRead(ypin) - 263) * (int)(90 / 67);

void moveto(Servo whichservo,int position){

Adjusting the delay value to a lesser integer makes it more responsive but I raised the delay because I naturally shake a lot and the turret looks like it’s spazzing out.

So what is next? I need to mount a little water pistol and detect humans using the openCV library. If and when I do that, I will do a follow-up.

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