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3.11 The Curve Editor
The Curve Editor is accessed either from the Tools/Curve Editor menu or by pressing the corresponding button on the toolbar. It can also be accessed using the F4 shortcut. The Curve Editor gives an overview of all the time-dependent parameters in the model. FEBioStudio allows the user to define the time dependency explicitly through the use of so-called load controllers. In an FEBio analysis, the load controller is evaluated at the simulated time and usually multiplies the value of the associated model parameter. The Curve Editor allows user to create load controllers, associate them with one or more model parameters, and edit the load controllers.
In the left panel, the model tree is presented, but only parameters that can be associated with a load controller will be listed here. Select a parameter here in order to see the load controller, if any, currently associated with this parameter. At the top of this panel, a Filter selection is available that allows you to show the parameters of only a specific type of model component, e.g. boundary conditions, material parameters, etc. This filter can be helpful in finding a specific parameter.
The right panel shows the current load controller associated with the parameter selected in the left panel. At the top, the load controller can be selected that will be associated with this parameter. Clicking the New button allows you to add a new load controller and associate it with the parameter. Depending on the type of load controller selected, either the controller's properties or a graphical representation of the load controller will be shown in the right panel.
One particularly useful type of load controller is the load curve, which consists of time,value pairs that will be interpolated. FEBio Studio often associates a default load curve to certain parameters of boundary conditions and loads. This default load curve ramps up the value from zero to one over the unit time interval. However, all curves can be modified using the tools available at the bottom of the Curve Editor. The curve's data points are represented as dots on the view. These data points can be selected by clicking on them with the left mouse button. They can also be moved by click dragging them. The current (time, value) pair of the selected point is displayed on the toolbar at the bottom of the view. This toolbar offers the following features.




The toolbar at the top of the Curve Editor provides additional tools for modifying the active curve.








The current view can be zoomed in or out, either by using the zoom buttons at the bottom of the view, or using the mouse wheel. When you scroll the mouse wheel while hovering over one of the axes, the graph will only zoom in that axis' direction. Depressing the left mouse button while moving the mouse, pans the view.
At the top of the curve view, you can see two drop-down lists. The first list allows the user to set the curve type which defines the interpolation mode for the currently displayed curve. The choices are as follows:
- linear: use a linear interpolation between the curve points
- step: use a constant interpolation between the curve points. The value of the curve is defined by the value of the point closest and to the right of a particular ordinate.
- smooth: A cubic polynomial is fitted through the data points resulting in a smooth interpolation.
- cubic spline: A cubic spline is fitted through the data points.
- control points: The data points define the control points of a spline.
- Approximate: An approximation of the control points interpolation.
- smooth step: A cubic polynomial is fitted between each pairs of data points. The slope at the end points is zero.
The second list displays the extend mode options. The extend mode defines the value of the curve outside its defined range. The choices are:
- Constant: the value is clamped to the range of the curve as defined by the first and last point.
- Extrapolate: the value is extrapolated from the end-points of the curve.
- Repeat: the curve is repeated on either end of the curve's domain
- Repeat offset: same as repeat, except that the curve is offset by the end-point values.
If the previous tools are not sufficient to describe the evolution of the load curve in detail, the Equation Editor can be used (Figure 3.12↑). This tool is accessed from the toolbar and allows the user to enter a mathematical function of time. Use the symbol to reference time. This function will be evaluated and discretized to generate a set of points that interpolate the function approximately.