Why has been there much research in the area of torque estimation procedures and torque observers when the load torque on a motor can be measured by measuring the current flowing through the coils of the motor, especially in the case of dc motors?

 

I am not sure I understand.

There are applications where torque measurement is vital IE... A steel mill or any mill that uses tension reels. They can also measure tension using a three roll bridle and if they know the roll diameters and the wrap angle using load cells they can calculate it

 

Because coils' current isn't proportional directly to load's torque/force due to dynamics, clogging, viscous damping, magnetic saturation, etc.