No, I meant there were 2 designs you can use.
1) You use one SRF in the middle, and 2 IR on 2 sides. You set a fixed distance to SRF, it's not a digital. If the real distance is longer than the fixed distance (in your program), you accelerate you robot, if shorter, you deccelerate.
The 2 IR is used to turn the robot, if the left IR is ON, turn the robot to left, because the object is on the left of your robot, and you have to follow it.
2) If you got 2 SRF, you put it on 2 sides, and no need the middle one. You do the same as in the first case. But you now get clearer information about the object that you have to follow. You may keep the fixed distance to follow the robot or a range distance as you want to.
|____1___|_____|________|
|________|_2_|________|
|________|_|________|
|_______| |____3_|
|____| |____|
|__| |__|
The 2 SRF will be showed like this. If the object is some where in this range, you may control your robot with ease to follow.
IF the object is at 1 (the SRF1 detect the object, the SRF2 doesn't, and you know the distance), you turn the robot to left and move forward quickly. But if it is at 2, and the distance is good enough (in the range you wanna), slowly move forward to the robot until it is close enough to the object. If the object is at 3, it means the object is so close to the robot, and it's in the right. So, slowly, move back and turn right.
I give here an example, you may creat your control rules as complex as you want, because to this 2 SRF, you get more information than the first case.
You may use GP2D12 in stead of SRF, but GP2D12 give you a shorter range (20 - 80cm). SRF give you 3cm - 3m range of detection.