ODEResults

Results of ODE integration

Since R2023b

Description

ODEResults objects store the solution information from the integration of an ode object. The solve and solutionFcn object functions of ode can return ODEResults objects.

You can use the Time and Solution properties of an ODEResults object to extract or plot the integration results.

• If events are being tracked (that is, the ode object has a nonempty EventDefinition property), then the EventTime, EventSolution, and EventIndex properties contain information related to events that triggered during the integration. See odeEvent for more information on detecting events during the solution process.

• If sensitivity analysis is performed (that is, the ode object has a nonempty Sensitivity property), then the Sensitivity property contains partial derivative values of the equations taken with respect to parameters. If events are also being tracked, then the EventSensitivity property contains sensitivity values at the time of each event. See odeSensitivity for more information on performing sensitivity analysis.

All properties of ODEResults objects are read-only. You can index into the properties to extract their values with the syntax data = obj.property.

Creation

Use solve or solutionFcn to integrate an ode object and create an ODEResults object. For example, S = solve(F,0,10) creates an ODEResults object S.

Properties

Standard Results

Time — Evaluation points

row vector

This property is read-only.

Evaluation points, returned as a row vector. Typically, the independent variable in an ODE problem is time, but the independent variable can represent other quantities as well.

• If you solve the ODE problem using a time range with two elements [t0 tf], then Time contains the internal step values chosen by the solver.

• If you solve the ODE problem using a vector of times [t0 t1 t2 ... tf], then Time contains the same values.

Solution — Solution at evaluation points

matrix

This property is read-only.

Solution at evaluation points, returned as a matrix. Each row Solution(i,:) is the complete solution for one component, and each column Solution(:,j) is the solution of all components at the corresponding evaluation point Time(j).

Event Results

EventTime — Time when events triggered

row vector

This property is read-only.

Time when events triggered, returned as a row vector. EventSolution(:,j) contains the solution of all variables at time EventTime(j), and EventIndex(j) indicates which event triggered.

EventSolution — Solution at time of events

matrix

This property is read-only.

Solution at time of events, returned as a matrix. EventSolution has a number of columns equal to numel(EventTime), and the same number of rows as Solution. EventSolution(:,j) contains the solution of all variables at time EventTime(j), and EventIndex(j) indicates which event triggered.

EventIndex — Index of triggered events

row vector

This property is read-only.

Index of triggered events, returned as a row vector. EventSolution(:,j) contains the solution of all variables at time EventTime(j), and EventIndex(j) indicates which event triggered.

Sensitivity Analysis

Sensitivity — Sensitivity analysis results

array

This property is read-only.

Sensitivity analysis results, returned as an array. Sensitivity has a number of columns equal to the number of parameters being analyzed, and the same number of rows as Solution. The number of pages in the array is equal to the number of time steps.

Consider an ODE system of the form

$$\begin{array}{cc}\frac{d{y}_i}{dt}=f_i\left(t,y,p\right),& i=1,2,\mathrm{...},n\end{array}\text{}\text{}\text{}$$

The parameters p can be treated as independent variables to obtain the sensitivity functions

$$\begin{array}{ccc}{u}_{ij}=\frac{\partial y_i}{\partial p_j},& i=1,2,\mathrm{...},n,& j=1,2,\mathrm{...},m\end{array}$$

The sensitivity analysis results contain these partial derivative values of the equations taken with respect to the parameters. You can control aspects of the sensitivity analysis by setting properties of the odeSensitivity object assigned to the Senstivity property of the ode object.

EventSensitivity — Sensitivity values at time of events

array

This property is read-only.

Sensitivity values at time of events, returned as an array. EventSensitivity has a number of columns equal to the number of parameters being analyzed, and the same number of rows as Solution. The number of pages in the array is equal to the number of events, numel(EventTime).

Examples

Solve Single ODE

Create an empty ode object, and then specify values for the ODEFcn and InitialValue properties.

F = ode;
F.ODEFcn = @(t,y) 2*t;
F.InitialValue = 0;

Check which solver is selected for the problem, and then solve the equation over the time range [0 10].

F.SelectedSolver

ans = 
  SolverID enumeration

    ode45

sol = solve(F,0,10)

sol = 
  ODEResults with properties:

        Time: [0 0.2500 0.5000 0.7500 1 1.2500 1.5000 1.7500 2 2.2500 2.5000 2.7500 3 3.2500 3.5000 3.7500 4 4.2500 4.5000 4.7500 5 5.2500 5.5000 5.7500 6 6.2500 6.5000 6.7500 7 7.2500 7.5000 7.7500 8 8.2500 8.5000 8.7500 9 9.2500 9.5000 9.7500 10]
    Solution: [0 0.0625 0.2500 0.5625 1.0000 1.5625 2.2500 3.0625 4 5.0625 6.2500 7.5625 9 10.5625 12.2500 14.0625 16 18.0625 20.2500 22.5625 25 27.5625 30.2500 33.0625 36 39.0625 42.2500 45.5625 49 52.5625 56.2500 60.0625 64 68.0625 ... ] (1x41 double)

Plot the results.

plot(sol.Time,sol.Solution,"-o")

Interpolate ODE Solution Outside of Original Interval

Create an ode object for the equations @(t,y) [y(2); 1000*(1-y(1)^2)*y(2)-y(1)] with the initial conditions [2 0]. Specify the solver as "stiff".

F = ode(ODEFcn=@(t,y) [y(2); 1000*(1-y(1)^2)*y(2)-y(1)],InitialValue=[2 0],Solver="stiff");

Create a function handle that can evaluate the solution of the problem in the interval [0 1000] by using the solutionFcn method.

• Specify Extension="on" to enable the function handle to evaluate the solution at times outside of the original interval [0 1000].

• Specify OutputVariables=1 so that the function handle interpolates only the first solution variable.

• Specify two outputs to also return the integration results in the original interval.

[fh,S] = solutionFcn(F,0,1000,Extension="on",OutputVariables=1)

fh = function_handle with value:
    @ode.solutionFcn/interpolate

S = 
  ODEResults with properties:

        Time: [0 1.4606e-05 2.9212e-05 4.3818e-05 1.1010e-04 1.7639e-04 2.4267e-04 3.0896e-04 4.5006e-04 5.9116e-04 7.3226e-04 8.7336e-04 0.0010 0.0012 0.0013 0.0015 0.0017 0.0018 0.0021 0.0024 0.0027 0.0030 0.0033 0.0044 0.0055 ... ] (1x227 double)
    Solution: [2x227 double]

Extract and plot the integration results for the first solution variable in S, and then use the function handle fh to also evaluate the solution in the extended interval [1000 3000]. Plot the extended solution in red.

plot(S.Time,S.Solution(1,:),"-o")
t = linspace(1000,3000);
ys = fh(t);
hold on
plot(t,ys,"r-o")
hold off

Examine Parameter Sensitivity

Solve an ODE system with two equations and two parameters, and perform sensitivity analysis on the parameters.

Create an ode object to represent this system of equations.

$$\begin{array}{l}\frac{{\mathrm{dy}}_1}{\mathrm{dt}}={\mathit{p}}_1{\mathit{y}}_1-{\mathit{y}}_2\\ \frac{{\mathrm{dy}}_2}{\mathrm{dt}}={-\mathit{p}}_2{\mathit{y}}_2\end{array}$$

Specify the initial conditions as ${\mathit{y}}_1\left(0\right)=2$ and ${\mathit{y}}_2\left(0\right)=3$, and parameter values of ${\mathit{p}}_1=0.05$ and ${\mathit{p}}_2=1.5$. To enable sensitivity analysis of the parameters, set the Sensitivity property of the ode object to an odeSensitivity object.

p = [0.05 1.5];
F = ode(ODEFcn=@(t,y,p) [p(1)*y(1)-y(2); -p(2)*y(2)], ...
        InitialValue=[2 3], ...
        Parameters=p, ...
        Sensitivity=odeSensitivity)

F = 
  ode with properties:

   Problem definition
               ODEFcn: @(t,y,p)[p(1)*y(1)-y(2);-p(2)*y(2)]
           Parameters: [0.0500 1.5000]
          InitialTime: 0
         InitialValue: [2 3]
          Sensitivity: [1x1 odeSensitivity]
         EquationType: standard

   Solver properties
    AbsoluteTolerance: 1.0000e-06
    RelativeTolerance: 1.0000e-03
               Solver: auto
       SelectedSolver: cvodesnonstiff

  Show all properties

Because the equations are nonstiff and sensitivity analysis is enabled, the ode object automatically chooses the cvodesnonstiff solver for this problem.

Solve the ODE over the time interval [0 5], and plot the solution for each component.

S = solve(F,0,5)

S = 
  ODEResults with properties:

           Time: [0 2.9540e-09 2.9543e-05 2.2465e-04 4.1976e-04 0.0024 0.0080 0.0137 0.0245 0.0353 0.0611 0.0869 0.1499 0.2129 0.3169 0.4208 0.5248 0.7204 0.9161 1.1118 1.3075 1.5031 1.6988 1.8945 2.0901 2.2858 2.4815 2.6772 2.8728 ... ] (1x40 double)
       Solution: [2x40 double]
    Sensitivity: [2x2x40 double]

plot(S.Time,S.Solution(1,:),"-o",S.Time,S.Solution(2,:),"-o")
legend("y1","y2")

The values in S.Sensitivity are partial derivatives of the equations with respect to the parameters. To examine the effects of the parameter values during the integration, plot the sensitivity values.

figure
hold on
plot(S.Time,squeeze(S.Sensitivity(1,1,:)),"-o")
plot(S.Time,squeeze(S.Sensitivity(1,2,:)),"-o")
plot(S.Time,squeeze(S.Sensitivity(2,1,:)),"-o")
plot(S.Time,squeeze(S.Sensitivity(2,2,:)),"-o")
legend("p1,eq1","p2,eq1","p1,eq2","p2,eq2")
hold off

Use Callback Function to Restart Integration of Bouncing Ball

Consider a ball thrown upward with an initial velocity $\mathrm{dy}/\mathrm{dt}$. The ball is subject to acceleration due to gravity aimed downward, so its acceleration is

$$\frac{d^{2}y}{d{t}^2}=-g.$$

Rewriting the equation as a first-order system of equations with the substitutions ${\mathit{y}}_1=\mathit{y}$ and ${\mathit{y}}_2=\frac{\mathrm{dy}}{\mathrm{dt}}$ yields

$$\begin{array}{l}{y}_1^{\prime }=y_2\\ y_2^{\prime }=-g.\end{array}$$

Solve the equations for the position ${\mathit{y}}_1$ and velocity ${\mathit{y}}_2$ of the ball over time.

Define Equations and Initial Conditions

Create a function handle for the first-order system of equations that accepts two inputs for (t,y). Use the value $\mathit{g}=9.8$ for the acceleration due to gravity.

dydt = @(t,y) [y(2); -9.8];

Next, create a vector with the initial conditions. The ball starts at position ${\mathit{y}}_1=3$ at $\mathit{t}=0$ as it is thrown upward with initial velocity ${\mathit{y}}_2=20$.

y0 = [3 20];

Model Ball Bounces as Events

The ball initially travels upward until the force due to gravity causes it to change direction and head back down to the ground. If you solve the equations without more consideration, then the ball falls back downward forever without striking the ground. Instead, you can use an event function to detect when the position of the ball goes to zero where the ground is located. Because the solution component ${\mathit{y}}_1=\mathit{y}$ is the position of the ball, the event function tracks the value of ${\mathit{y}}_1$ so that an event triggers whenever ${\mathit{y}}_1=0$.

Create a function handle for the event function that accepts two inputs for (t,y).

bounceEvent = @(t,y) y(1);

When the ball strikes the ground, its direction changes again as it heads back upwards with a new (smaller) initial velocity. To model this situation, use a callback function along with the event function. When an event triggers, the ODE solver invokes the callback function. The callback function resets the position and initial velocity of the ball so that the integration can restart with the correct initial conditions. When an event occurs, the callback function sets the position ${\mathit{y}}_1=0$ and attenuates the velocity by a factor of $0.9$ while reversing its direction back upward. Define a callback function that performs these actions.

function [stop,y] = bounceResponse(t,y)
stop = false;
y(1) = 0;
y(2) = -0.9*y(2);
end

(The callback function is included as a local function at the end of the example.)

Create an odeEvent object to represent the bouncing ball events. Specify Direction="descending" so that only events where the position is decreasing are detected. Also, specify Response="callback" so that the solver invokes the callback function when an event occurs.

E = odeEvent(EventFcn=bounceEvent, ...
             Direction="descending", ...
             Response="callback", ...
             CallbackFcn=@bounceResponse)

E = 
  odeEvent with properties:

       EventFcn: @(t,y)y(1)
      Direction: descending
       Response: callback
    CallbackFcn: @bounceResponse

Solve Equations

Create an ode object for the problem, specifying the equations dydt, initial conditions y0, and events E as property values.

F = ode(ODEFcn=dydt,InitialValue=y0,EventDefinition=E);

Integrate the equations over the time interval [0 30] by using the solve method. Specify Refine=8 to generate 8 points per step. The resulting object has properties for the time and solution, and because events are being tracked, the object also displays properties related to the events that triggered during the integration.

S = solve(F,0,30,Refine=8)

S = 
  ODEResults with properties:

             Time: [0 0.0038 0.0075 0.0113 0.0151 0.0188 0.0226 0.0264 0.0301 0.0490 0.0678 0.0867 0.1055 0.1243 0.1432 0.1620 0.1809 0.2751 0.3692 0.4634 0.5576 0.6518 0.7460 0.8402 0.9344 1.3094 1.6844 2.0594 2.4344 2.8094 3.1844 ... ] (1x468 double)
         Solution: [2x468 double]
        EventTime: [4.2265 8.1607 11.7015 14.8882 17.7563 20.3375 22.6606 24.7514 26.6331 28.3267 29.8509]
    EventSolution: [2x11 double]
       EventIndex: [1 1 1 1 1 1 1 1 1 1 1]

Plot Results

Plot the position ${\mathit{y}}_1$ of the ball over time, marking the initial position with a green circle and events with red circles.

plot(S.Time,S.Solution(1,:),"--")
hold on
plot(S.EventTime,S.EventSolution(1,:),"ro")
plot(0,y0(1),"go")
hold off
ylim([0 25])
xlabel("Time")
ylabel("Position y_1")

Local Functions

function [stop,y] = bounceResponse(t,y)
stop = false;
y(1) = 0;
y(2) = -0.9*y(2);
end

Version History

Introduced in R2023b