I m using a QRD1114 for my robot border line detection. The sensor has 50us response time
No. the detector has a fall time of 50µs under ideal conditions, which is when the emitter is cut out completely (and instantly) and when the coupling between emitter and detector is as weak as possible, while still being able to "rise" the detector, so that the least amount of charge carriers has to be deplenished - not in the actual use you want to put it to.
However, I don't think it's the reaction time of your detector that ruins your day, even if it was ten times what you think.
You might even need to up the current in the emitter, depending on the actual reflectance of the arena/border "colors" and the distance between the sensor and the floor - 100..150 Ohm might get you a cleaner response.
but when I actually put it at the bottom of my robot, the robot rush out of the border line in a speed of 80cm/sec and stop at the border line at 70cm/sec speed.
That does not compute?!
If it stop at the border line, it will be at 0.0cm/s, so what exactly do you mean??
To paraphrase an old Clash hit "Would it stay, or would it go?"
The border line is 5cm in width.
5/70=71ms,5/80=62ms. So that the actual response time should be in between 71ms to 62ms.
I'm really not sure what you mean here (either), but if it hits the first edge of the 5cm wide border at 80cm/s and you want it to stop within this 5cm, you have...
80cm/s means that it moves 5cm in 62.5ms.
To stop, your detector should detect the first edge and then the motor will have the remainder of the 62.5ms to decellerate the 'bot to a stand-still.
So, if the detector does need 500µs to react worst case - a far better question is whether you can stop the motors (and with them, the entire 'bot) in 60ms.
My circuit is connected in this way:
[Snip long link]
Anyone know how to connect can speed up the response time?
Put the sensor over both light and dark material, as used in the arena (remember that IR doesn't see things light/dark as the human eye!) and measure the center tap voltage (the voltage at where you'll connect it to the input pin). Then post the results and I'll help you calculate a better detector resistance - you may need to redo the measurements with another value of detector resistance, to calculate the optimum value, so keep the setup.
The rule of thumb is, the faster reaction you want, the lower impedance you need, as lower impedances will deplenish the carriers faster. You have to make up for that with more power to the emitter, so change the emitter resistor to 100 Ohm before measuring.