But to achieve a desired output with minimum effect of disturbances ,I Make it a close loop with unity feedback and it works very well
Well, you think it works well but it doesn't in reality. If you want a precision control system you need much more gain (and probably integral and maybe some differential) error amplification after the summer (called G below): -
And, if you considered an expanded picture of what a good control loop has to deal with you'd draw this (featuring load effects and "other nasties"): -
