How to achieve a discrete differential tracker with the S function

1、The form of tracking the differential are as follows:

v(k) is input，h、r、α are parameters and user-defined. 2、My code is follows: function[sys,x0,str,ts,simStateCompliance] = myTry_sfun(t,x,u,flag,h,r,a) switch flag, case 0, [sys,x0,str,ts,simStateCompliance] = mdlInitializeSizes; case 1, sys = mdlDerivatives(t,s,u); case 2, sys = mdlUpdate(t,x,u,h,r,a); case 3, sys = mdlOutputs(t,x,u); case 4, sys = mdlGetTimeOfNextVarHit(t,x,u); case 9. sys = mdlTerminate(t,x,u); otherwise DAStudio.error('Simulink:block:unhandledFlag',num2str(flag)); end function [sys,x0,str,ts,simStateCompliance] = mdlInitializeSizes sizes = simsizes; sizes.NumContStates = 0; sizes.NumDiscStates = 2; sizes.NumOutputs = 2; sizes.NumInputs = 1; sizes.DirFeedthrough = 0; sizes.NumSampleTimes = 1; sys = simsizes(sizes); x0 = [0,0]'; str = []; ts = [0,0]; simStateCompliance = 'UnknownSimState';

function sys = mdlDerivatives(t,x,u) sys = [];

function sys = mdlUpdate(t,x,u,h,r,a) % updata state variable x_1 = x(1)+ h*x(2); x_2 = x(2)-h*((r^2)*(sign(x(1)-u))*((abs(x(1)-u))^a) + r * x(2)); sys = [x_1;x_2]; function sys = mdlOutputs(t,x,u) % update Output sys = [x(1);x(2)]; function sys = mdlGetTimeOfNextVarHit(t,x,u) % This function is not necessary sampleTime = 0.001; sys = t + sampleTime;

function sys = mdlTerminate(t,x,u) sys = [];

function sys = mdlUpdate(t,x,u,h,r) % updata state variable x_1 = x(1)+ h*x(2); x_2 = x(2)-h*((r^2)*(sign(x(1)-u))*((abs(x(1)-u))^a) + r * x(2)); sys = [x_1;x_2]; function sys = mdlOutputs(t,x,u) % update Output sys = [x(1);x(2)]; function sys = mdlGetTimeOfNextVarHit(t,x,u) % This function is not necessary sampleTime = 0.001; sys = t + sampleTime;

function sys = mdlTerminate(t,x,u) sys = []; 3、My Simulink is follows:

4、My result is follows:

I can get x_2 but can’t get x_1. I want to know where I was wrong, or maybe you can give me an example for discrete system with S function as a reference. Thank you !