I want to tune pid controller by using genetic algorithm...

hello friends.... I have simulink model to control the aircraft longitudinal motion. In that model i was used PID Controller. Here i required to tune PID controller by using Genetic algorithm. For that purpose , i want to get error as a input to genetic and required to get output as gain values such as Kp,Ki, Kd.

so i need to create a function in simulink model..

I dont know how to get error as a input to the function and getting gain values .. anybody know pls explain to me..

if u can explain with some example..

thanks... with regards suresh