Index exceeds the number of array elements (1) PID controller

% set up motor with initial conditions and properties
motor.timeConstant = 5/60; % [min]
motor.gain = 0.2; % [rpm/V]

% transfer function for the motor
s = tf('s');
TFmotor_OL = motor.gain / (motor.timeConstant*s +1);

% set up PID
PID.Pgain = 4;              % proportional gain
PID.Igain = 80;             % internal gain
PID.Dgain = 1;              % derivative gain

% create the controller
controller = pid(PID.Pgain, PID.Igain, PID.Dgain);

% connect the controller and motor
TFmotor_CL = feedback(controller*TFmotor_OL, 1);

% plot open-loop response
step(TFmotor_OL, 2)

% add time constant for derivative
tf = 0;

% plot closed-loop response
hold on                     % keep open-loop plot
step(TFmotor_CL, 2)         % plot closed-loop response
legend('show')              % show legend

It says the error is occuring in the line defining 's'. I have no idea why or how to solve it, any suggestions?

Answer 1

[Mathieu NOE]

hi

it works for me (R 2020a)

maybe you have an older matlab version

try the other syntax for tf :

% set up motor with initial conditions and properties
motor.timeConstant = 5/60; % [min]
motor.gain = 0.2; % [rpm/V]

% transfer function for the motor
% s = tf('s');
% TFmotor_OL = motor.gain / (motor.timeConstant*s +1);

 % alternate method : sys = tf(numerator,denominator)
 TFmotor_OL = tf([0 motor.gain],[motor.timeConstant  1]);
 
% set up PID
PID.Pgain = 4;              % proportional gain
PID.Igain = 80;             % internal gain
PID.Dgain = 1;              % derivative gain
% create the controller
controller = pid(PID.Pgain, PID.Igain, PID.Dgain);
% connect the controller and motor
TFmotor_CL = feedback(controller*TFmotor_OL, 1);
% plot open-loop response
step(TFmotor_OL, 2)
% add time constant for derivative
tf = 0;
% plot closed-loop response
hold on                     % keep open-loop plot
step(TFmotor_CL, 2)         % plot closed-loop response
legend('show')              % show legend

[Lucy Beksinska]

Okay I'll try that, I'm using R2020b. Thanks!