I am making a project based on Industrial automation using arduino in which I have to check whether the object on my circular conveyor belt is faulty or not. For that I used a 12 Volt DC motor 200 RPM and controlled its speed using arduino (PWM) and TIP122 and potentiometer. Also I am making an arrangement placing an IR sensor so that when the object is detected the conveyor stops but the thing is when the object is detected by the sensor it takes time to turn off the motor and also the speed of the motor is not constant and even if I set it at a specific value, it get changed at will. How can I control the speed of the motor properly and the motor should be low immediately when the object is detected by the IR sensor. Please help its urgent.

 

Speed of a motor cannot be precisely controlled by voltage variation.

Why not use a stepper motor so that the positioning of the object on the conveyor belt is precise?

 

To maintain a constant speed you need some feedback of the actual speed to the control circuit.
To stop the motor quickly disconnect the power and then short circuit the motor. This works as a brake.

 

Is this a school/college project?

 

I'm with @Ramussons with this, is to use a stepper if you want to avoid a brake or using F.B. from the motor.
They also have the advantage of holding torque, even when off.
You can also get mains (AC) driven steppers that do not need any kind of controller, the down side is they are fixed speed to the frequency of the supply, 72rpm on 60hz.
But if necessary, gearing could be employed, they run at relitively low rpm, but stop/start 'instantly'.
Max.

 

Thanks for your suggestions. I used relay to turn off my motor when IR sensor detects the object and I connected the potentiometer directly to the supply and the relay is breaking the circuit perfectly, hence stopping the motor instantly.

 

Is this a school/college project?

This is my college project.