I haven't used the QTR sensors myself, but I noticed that the main difference between your code and the examples I found floating around is that you are calling readCalibrated(val) before readLine and the examples don't. I looked through the source for the libary and readLine calls readCalibrated in the first couple of lines anyway. Maybe pre-loading val is with the calibration values is causing the problem?
Also, are you passing the sensor over both the white and black surfaces during the calibration? A trick could be to get the bot to slowly spin on the spot during calibration, while sitting on the line, allowing the sensor to see both the line and the surrounding area.
Your sensors are actually circuit boards that process the sensor output and give you a easily understood and implemented signals. I have checked, and these boards include their bulk and decoupling capacitors.
One thing to improve is the grounding quality. To do that, you have to improve the impedance of the ground tracks. In low frequencies, the ground impedance is dominated by the resistance, however when the frequency goes high, inductance kicks in and dominates the impedance.
One way to make the ground impedance low at high frequencies is bypass capacitors that are put very close to the supply pins. Since you already have about 20 cm of wires, you cannot do much for the high frequency on your board. But at least you can isolate the noise from the board as noted below.
There are couple of things that come to my mind that will reduce overall noise:
Add a bulk capacitor:
Put an electrolytic capacitor that has a value of about 10uF to 220uF across GND and +5V. This will give you a cleaner power and room for inrush currents. Since capacitors have relatively low series resistance, they can give higher currents, of course limited with their storage. Also, connecting a 100nF ceramic capacitor with this bulk capacitor will filter out high frequency content and decouple the "sensor shield" from Arduino.
Add decoupling capacitors to sensors:
Sensors can include digital circuitry which can generate high frequency content, thanks to square waves. You cannot give these sensors low impedance power paths because of the already long wires, however, you can limit the high frequency content coming from these sensors to your board.
You can add some 100nF ceramic capacitors in-between Vcc and GND of these sensors and the high frequency content coming from the sensors will be shorted to GND with the help of the capacitors. In other words, you are going to decouple your sensors from your board. Make these capacitors as close as possible to power pins of each sensor. You can sort of use one capacitor in common for very close sensors.
Make the ground trace gain weight:
When a trace gets ticker, its resistance drops lower. Make the ground trace as thick as you can. Or better, as Andy aka suggested, make a ground fill, or a ground plane, depending on what your CAD software calls it.
Also, there is nothing pulling you from making the Vcc trace thicker and lower impedance.
One small detail is to twist those long cables so that they are all close to GND cable.
Best Answer
Within your main loop, you assign values to the
sensor[]array here:and then check the values here:
If you enter the first if statement (i.e., if all of the
sensor[]elements are0), then how would you ever enter the second if statement? You need some way to update the sensor values between the two. For example: