%% Digital Servo Control of a Hard-Disk Drive
% This demo shows how to use the Control System Toolbox to design  a digital
% servo controller for a disk drive read/write head.
%
% For details about the system and model, see Chapter 14 of "Digital Control of
% Dynamic Systems," by Franklin, Powell, and Workman.
%
% Author: P. Gahinet 8/2000
% Copyright 1986-2002 The MathWorks, Inc. 
% $Revision: 1.22 $  $Date: 2002/04/19 02:48:47 $

%%
% Draw a picture of the system to be modeled.

diskdemo_aux(1)

%%
% The head-disk assembly (HDA) and actuators are modeled by a 10th-order
% transfer function including two rigid-body modes and the first four
% resonances. 
%  
% The model input is the current ic driving the voice coil motor, and the output
% is the position error signal (PES, in % of track width). The model also
% includes a small delay.

diskdemo_aux(2)

%%
% The coupling coefficients, damping, and natural frequencies (in Hz) for the
% dominant flexible modes are listed above.  Given this data, construct a
% nominal model of the head assembly:

load diskdemo
Gr = tf(1e6,[1 12.5 0],'inputdelay',1e-5);
Gf1 = tf(w1*[a1 b1*w1],[1 2*z1*w1 w1^2]); % first  resonance
Gf2 = tf(w2*[a2 b2*w2],[1 2*z2*w2 w2^2]); % second resonance
Gf3 = tf(w3*[a3 b3*w3],[1 2*z3*w3 w3^2]); % third  resonance
Gf4 = tf(w4*[a4 b4*w4],[1 2*z4*w4 w4^2]); % fourth resonance
G = Gr * (ss(Gf1) + Gf2 + Gf3 + Gf4);     % convert to state space for accuracy

diskdemo_aux(3)

%%
% Plot the Bode response of the head assembly model:

cla reset
set(G,'inputname','ic','outputname','PES')
bode(G)
title('Bode diagram of the head assembly model');

diskdemo_aux(4)
%%
% Servo control is used to keep the read/write head "on track."   The servo
% controller C(z) is digital and designed to maintain the PES (offset from the
% track center) close to zero.
%  
% The disturbance considered here is a step variation d in the input current ic.
% Your task is to design a digital compensator C(z) with adequate disturbance
% rejection performance.

diskdemo_aux(5)

%%
% The sampling time for the digital servo is Ts = 7e-5 sec (14.2 kHz).
%  
% Realistic design specs are listed above.

diskdemo_aux(6)

%%
% Since the servo controller is digital, you can perform the design in the 
% discrete domain.  To this effect, discretize the HDA model using C2D and the
% zero-order hold (ZOH) method:

cla reset
Ts = 7e-5;
Gd = c2d(G,Ts);
bode(G,'b',Gd,'r') % compare with the continuous-time model
title('Continuous (blue) and discretized (red) HDA models');

diskdemo_aux(7)

%%
% Now to the compensator design.  Start with a pure integrator 1/(z-1) to ensure
% zero steady-state error, plot the root locus of the open-loop model Gd*C, and
% zoom around z=1 using the Zoom In option under the Tools menu.

C = tf(1,[1 -1],Ts);
rlocus(Gd*C)

diskdemo_aux(8)

%%
% Because of the two poles at z=1, the servo loop is unstable for all positive
% gains.  To stabilize the feedback loop, first add a pair of zeros near z=1.

C = C * zpk([.963,.963],-0.706,1,Ts);
rlocus(Gd*C)

diskdemo_aux(10)

%%
% Next adjust the loop gain by clicking on the locus and dragging the black
% square inside the unit circle.  The loop gain is displayed in the data marker.
% A gain of approximately 50 stabilizes the loop (set C1 = 50*C).

C1 = 50 * C;

%%
% Now simulate the closed-loop response to a step disturbance in current.  The
% disturbance is smoothly rejected, but the PES is too large (head deviates from
% track center by 45% of track width).

cl_step = feedback(Gd,C1);
step(cl_step)
title('Rejection of a step disturbance (PES = position error)')

diskdemo_aux(11)

%%
% Next look at the open-loop Bode response and the stability margins.  The gain
% at 100 Hz is only 15 dB (vs. spec of 20 dB) and the gain margin is only 7dB,
% so increasing the loop gain is not an option.

margin(Gd*C1)

diskdemo_aux(12)

%%
% To make room for higher low-frequency gain, add a notch filter near the 4000
% Hz resonance.
%  
% Note: The SISO Design Tool is ideal for interactively tuning these compensator
% parameters.  See the web version of this demo for a live showcase.

w0 = 4e3 * 2*pi;                                 % notch frequency in rad/sec
notch = tf([1 2*0.06*w0 w0^2],[1 2*w0 w0^2]);    % continuous-time notch
notchd = c2d(notch,Ts,'matched');                % discrete-time notch
C2 = C1 * notchd;

bode(notchd)
title('Discrete-time notch filter');

diskdemo_aux(13)

%%
% You can now safely double the loop gain.
% The resulting stability margins and gain at 100 Hz are within specs.

C2 = 2 * C2;
margin(Gd * C2)

diskdemo_aux(14)

%%
% Step disturbance rejection has also greatly improved.  The PES now stays
% below 20% of the track width.

cl_step1 = feedback(Gd,C1);
cl_step2 = feedback(Gd,C2);
step(cl_step1,'r--',cl_step2,'b')
title('2nd-order compensator C1 (red) vs. 4th-order compensator C2 (blue)')

%%
% Check if the 3dB peak gain spec on   T = Gd*C/(1+Gd*C)   (closed-loop 
% sensitivity) is met:
%  
%  >> bodemag(feedback(Gd*C,1))
%  
% To see the peak value, right-click on the axis and choose the Peak Response
% option under the Characteristics menu, then hold the mouse over the blue
% marker, or just click on it.

Gd = c2d(G,Ts);
Ts = 7e-5;
        
T = feedback(Gd*C2,1);
bodemag(T)
title('Peak response of closed-loop sensitivity T(s)')

diskdemo_aux(15)

%%
% Finally let's analyze the robustness to variations in the damping and natural
% frequencies of the 2nd and 3rd flexible modes.  Generate an array of 16 models
% corresponding to all combinations of extremal values of z2,w2,z3,w3:

[z2,w2,z3,w3] = ndgrid([.5*z2,1.5*z2],[.9*w2,1.1*w2],[.5*z3,1.5*z3],[.8*w3,1.2*w3]);
for j=1:16,  
    Gf21(:,:,j) = tf(w2(j)*[a2 b2*w2(j)] , [1 2*z2(j)*w2(j) w2(j)^2]);
    Gf31(:,:,j) = tf(w3(j)*[a3 b3*w3(j)] , [1 2*z3(j)*w3(j) w3(j)^2]);
end
G1 = Gr * (ss(Gf1) + Gf21 + Gf31 + Gf4);

diskdemo_aux(16)

%%
% Discretize these 16 models at once and see how the parameter variations
% affect the open-loop response. Note: You can click on any curve to
% identify the underlying model.

Gd = c2d(G1,Ts);
bode(Gd*C2)

title('Open-loop response - Monte Carlo analysis') 

diskdemo_aux(17)

%%
% Plot the step disturbance rejection performance for these 16 models:
%  
%  >> step(feedback(Gd,C2))
%  
% All 16 responses are nearly identical: our servo design is robust!

step(feedback(Gd,C2))
title('Step disturbance rejection - Monte Carlo analysis')