The command pidtool(B, 'pid') launches the PID Tuner app and designs a PID controller for plant B. However, plant B is actually the closed-loop feedback system with your improperly configured PID controller embedded. The configuration of your PID controller is improper because the degree of the numerator exceeds the degree of the denominator.
By the way, please note that pidtool has been removed. Instead, use pidTuner with this syntax: pidTuner(Plant, Cbase) to launch the app and compare the performance of the auto-tuned PID controller to that of the baseline controller (Cbase).
% parameters
kp = 350;
ki = 300;
kd = 50;
m = 1;
b = 10;
k = 20;
% baseline controller
numC = [kd kp ki];
denC = [1 0];
Cbase = tf(numC, denC)
% plant tranfer function
numP = 1;
denP = [m b k];
Plant = tf(numP, denP)
% launch the app and make comparison to Cbase
pidtool(Plant, Cbase)
