To get our models to flip"right themselves", we want to add actuation control to them, control their actuation and tweak the this control until we obtain our desired flip a flipping motion. We have an optimization algorithm take care of the tweaking for us. The algorithm will tweak During each step of the optimization, the algorithm tweaks the control in some intelligent way, then we 'll run a forward dynamic simulation with the given tweaked controls, and we 'll see finally check if the cat flipshas flipped. The algorithm tweaking continues to tweak the control until we decide that the cat model has achieved a flip is achieved.

This procedure of algorithmically tweaking control inputs, running a forward dynamic simulation, then judging how well the model/simulation achieved some objective is called dynamic optimization. It is the way that biomechanists would prefer to study human movement (see Sky Higher and Miller, 2012 for just two of many exampleexamples) , and is commonly viewed as the alternative to tracking simulations. The models used for studying such motions are complex, and so it takes a long time to run a dynamic optimization (days to weeks). For cat flipping, however, an optimization takes only an hour or two. Thus, it's an ideal problem for getting one's feet wet with first learning dynamic optimization.

Code Organization

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First, we present an overview of the 2 source files we use for the optimization. The file optimize.cpp creates an executable that (mostly just) creates the necessary objects , and runs the optimization. The code block below shows only 4 displays the four most important lines from this file, the most important lines, that which show how we connect the various pieces of the optimization. Look at the next code block, a skeleton of the second file, FlippinFelinesOptimizerSystem.h, to see . It is where we define the FlippinFelinesOptimizerTool and FlippinFelinesOptimizerSystem classes.

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As you can see, our optimizer system must be a subclass of SimTK::OptimizerSystem. We override its objectiveFunc() function, which is then called by the SimTK::Optimizer as part of its algorithm. It's in objectiveFunc() that we (1) convert the optimizer's tweaks to optimization parameters into changes in the actuation control of the model, (2) run a forward dynamic simulation of the cat's fall, and (3) evaluate the simulation by assigning a value for our objective function, f, which is what we calledon the main Flippin' Felines page. See SimTK::OptimizerSystem reference and SimTK::Optimizer reference for more information.

A:

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optimize.cpp:

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the executable that ties the pieces together

Here, we show the complete optimize.cpp. The user must provide one command line argument, which is the name of an XML file that defines an FlippinFelinesOptimizerTool object (we'll get to this in the next section). A serialization is a representation of an object or objects in data files (which, in our case, is a plain text file in XML format). See this page for more information about C++ command line arguments.

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void constructProperties()
{
    constructProperty_results_directory("results");
    constructProperty_model_filename("flippinfelines_*FILL THIS IN*.osim");
    constructProperty_num_optim_spline_points(20);
    constructProperty_anterior_legs_down_weight(1.0);
    constructProperty_posterior_legs_down_weight(1.0);
    constructProperty_sagittal_symmetry_weight(1.0);
    constructProperty_use_coordinate_limit_forces(true);
    constructProperty_initial_parameters_filename("");
}

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Sample Input

Below is what the XML serialization of this class looks like. This is exactly what we meant by the optimizer_setup_file.xml above. This is a simplified version of what's generated by the executable optimize_input_template, which is generated from optimize_input_template.cpp. We've given you this file, but we do not discuss it in this tutorial.

<?xml version="1.0" encoding="UTF-8" ?>
<OpenSimDocument Version="30000">
    <FlippinFelinesOptimizerTool>
        <!--Directory in which to save optimization log and results-->
        <results_directory>results</results_directory>
        <!--Specifies path to model file, WITH .osim extension-->
        <model_filename>flippinfelines_*FILL THIS IN*.osim</model_filename>
        <!--Number of points being optimized in each spline function. Constant across all splines. If an initial_parameters_filename is provided, the functions specified in that file must have the correct number of points. We do not error-check for this.-->
        <num_optim_spline_points>20</num_optim_spline_points>
        <!--Adds terms to the objective to minimize final value of (roll - Pi) and related speeds-->
        <anterior_legs_down_weight>1</anterior_legs_down_weight>
        <!--Adds terms to the objective to minimize final value of (twist - 0) and related speeds-->
        <posterior_legs_down_weight>1</posterior_legs_down_weight>
        <!--Adds a term to the objective to minimize final value of (hunch + 2 * pitch)-->
        <sagittal_symmetry_weight>1</sagittal_symmetry_weight>
        <!--TRUE: use coordinate limit forces, FALSE: ignore coordinate limit forces-->
        <use_coordinate_limit_forces>true</use_coordinate_limit_forces>
        <!--File containing FunctionSet of SimmSpline's used to initialize optimization parameters. If not provided, initial parameters are all 0.0, and this element must be DELETED from the XML file (cannot just leave it blank). The name of each function must be identical to that of the actuator it is for. x values are ignored. The time values that are actually used in the simulation are equally spaced from t = 0 to t = 1.0 s, and there should be as many points in each function as given by the num_optim_spline_points property. y values should be nondimensional and between -1 and 1 (negative values normalized by minControl if minControl is negative; otherwise the value is normalized by maxControl). NOTE that the output optimized splines are NOT NONDIMENSIONAL. Be careful, we do not do any error checking.-->
        <initial_parameters_filename>initial_parameters.xml</initial_parameters_filename>
    </FlippinFelinesOptimizerTool>
</OpenSimDocument> 

C: FlippinFelinesOptimizerSystem

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Enjoy playing around with the code. We hope you feel you've learned something useful!