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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 curves. A load curve is simply an interpolated function of (time, value) pairs. The value will have a meaning that depends on the parameter associated with the curve. For example, if the load curve describes a nodal displacement, the value of the curve is the actual displacement and has the length units used in the design of the geometry. If the load curve describes a material parameter, then the value is the time evolution of that parameter.
It is important to note that many features that allow you to define a load curve also allow you to define a scale factor. In that case, the actual value of the parameter is the scale factor multiplied with the load curve value. For example, when prescribing the displacement, the scale factor is defined in the properties dialog when creating the prescribed constraint. The scale factor is not taken into account in the Curve Editor.
At the top of the curve editor's overview panel, the selection filter can be found. This filter allows you to show the load curves 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.
Most load curves have a default linear shape that ramps up the value from zero to one. 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.
When this button is toggled, each click on the curve view will add a new node. Note that you don't to click this button to add nodes. Nodes can also be added by shift click with the left mouse button on the curve view.
This button will delete the node that is currently selected in the curve view.
The snap-to-grid option will allow you to move a node on the intersections of the grid lines.
Zooms the curve view out so that all nodes are visible within the bounds of the curve view.
The toolbar at the top of the Curve Editor provides additional tools for modifying the active curve.
This will load data from a file. The file must be a simple text file with one line of data for each point. On each line, specify the time-load value pair delimited by a space. You can enter as many lines as you want.
Save the active load curve to a text file.
Copy the curve data to the clipboard. This allows the curve data to be pasted in another application that supports clipboard operations.
Store the load curve data so it can be pasted to another curve.
Paste the curve data that was copied to the active load curve.
Undo the last change to the active load curve.
Redo the last change that was undone.
Open the equation editor where load curve data can be generated via a mathematical equation (see below).
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 spline is fitted through the data points.
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.