Mr. RB thanks for all your supports. I completely agree wity you- here the problem is not with the mechanical design. The problem is entirely related to the motor and driver.
Ok, I will look for the driver you have suggested. Before that, for your information, I want to say you one more thing- this problem happens to my system in both directions. That's to say, when the pedal is on its upward movement, if I try to pull the pedal up with my hand then also the motor fails to keep up the input speed. I am meaning that, in both directions, if I give any force
toward the motor's rotating direction, it completely fails to comply with the desired input speed. However, when the pedal is in the upward move, it's load performance (weight on the pedal) is quite nice.
Now the thing is, I have already bought 6 drivers (made in Korea) for my system. They were quite expensive for me and it would be quite difficult for me to change them now. I am attaching the driver's manual. Though it's written in all-Korean, I am sure you can get a clear idea from the schematics. Please take a look at it and say me if I have any way to solve my problem with these drivers.
Please let me know if you have any further query
Infinite Thanks for all your helps