Hello I am new to the field of robotics but have some knowledge of raspberry pi, arduino, python. I want to make Robotic arm which can be used to find the centre of any disk. There may be disk of different diameter coming one after another on conveyor. I need to make hole at the center of disk using robotic arm. How can I do this ? What techniques and sensors I should use to implement the mechanical and electronic part. (I don't want to use camera and openCV). Thanks in advance.
In this case the precision is the most important factor.
industrial robots have typically a precision of 0.1 mm. If you are building one without in depth machine design and manufacturing experience, it is expected that the precision for your robot arm will not be as good.
Instead of a robot arm I would recommend a portal type structure. Much easier to build from COTS components.
If the disks are round (enough) you can use a distance sensor (laser or ultra sound depending in the material/coating you are trying to measure) at the side of the conveyor belt. As soon as the disk passes the sensor you can see how the radius is changing, if you know the velocity of your conveyor belt you can calculate the radius and then the center of the disk. Will be a bit trick if the conveyor belt stops and moves in the middl of a measurement but passing the disk in front of the sensor and knowing the velocity of the conveyor gives you enough measurments to eliminate noise.
Just wait until the center is under your portal tool on the conveyor belt (the disance to the sensor is constant) then move the tool to the center and drill.