MATLAB: Need Instantaneous PID Value

controlMATLAB

I am using PID controller and the input is 3×1. The output I am getting is in 3×1 tf format. But I need it to be 3×1 double at a single time instant.

(I will use a for loop later in which T_x will vary. At this moment, I am initializing T_x as [0;0;0])

Following is the code:

T_x=[0;0;0];
T_d=[0;0;0]; %desired value
T_e=T_d-T_x;
C_PD = pid(100,0,10,100);
T_u=T_e*C_PD

Best Answer

"The output I am getting is in 3x1 tf format" - This is correct because as per definition 'pid' command creates a continuous-time pid controller in parallel form. Basically you get a transfer function as a controller. When you multiply a constant (your input) to this transfer function, you get another transfer function.

"I need it to be 3x1 double at a single time instant"- One way is to convert the output from s domain to time domain using ilaplace or you can use the file exchange function given here. You will get your outputs as function of time from which you can easily substitute and get value of outputs at a particular time instant.

Related Solutions

MATLAB: Numeric answer for PID command

In order to simulate the time response of a system using the MATLAB control system toolbox you can use the lsim function. Note it is not valid to multiply a transfer function by a time domain signal as you have done in your code.

I made an example following what I think you were trying to accomplish below

% define time domain reference (setpoint), measured output, and error
% signals
 % signals must have a row for each time value and a column for each channel
t = 0:10; % start at initial time equal to zero continue to 10
T_d = [0 0 0];
T_o = [0.05*(10-t);0.04*(10-t);0.03*(10-t)]'; % note transpose, columns of T_o are channels
T_e = T_d-T_o;
% define PID controller parameters
Kp = 100; % proportional gain
Ki = 0; % integral gain
Kd = 10; % derivative gain
Tf = 100; % filter time constant
c = pid(Kp,Ki,Kd,Tf); % individual pid controller
C = [c 0 0;0 c 0;0 0 c]; % identical pid controllers for each channel
% simulate response of pid controller to error signal
T_u = lsim(C,T_e,t);

MATLAB: Pid tunnig with non unity feedback

Try the following approach:

s=tf('s'); %needed to define transfer function in s domain
G=1/3*s+4;H=1/(0.1*s+1); %if feedback is not unity, then it has to be considered in open loop, as G*H
C=pidtune(G*H,'PID',1) 
sys=(C*G)/(1+C*G*H)
step(sys)

Best Answer

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MATLAB: Numeric answer for PID command

MATLAB: Pid tunnig with non unity feedback

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