Working with VRML Sensors

Add VRML Sensors to Virtual Worlds

This section describes how to interface a Simulink® block diagram to sensors in a virtual reality scene. It also describes how to programmatically input signals from the virtual world into a simulation model.

Virtual reality scenes can contain sensors, nodes able to generate events and output values depending on time, user navigation, and actions and distance changes in the scene. These nodes add interactivity to the virtual world. You can use Simulink 3D Animation™ functions to read sensor field values into simulation models and control simulation based on the user interaction with the virtual scene.

You can define the following VRML sensors in the scene:

CylinderSensorMaps pointer motion (for example, a mouse or wand) into a rotation on an invisible cylinder that is aligned with the y-axis of the local coordinate system.
PlaneSensorMaps pointing device motion into two-dimensional translation in a plane parallel to the z=0 plane of the local coordinate system.
ProximitySensorGenerates events when the viewer enters, exits, and moves within a region in space (defined by a box).
SphereSensorMaps pointing device motion into spherical rotation about the origin of the local coordinate system.
TimeSensorGenerates events as time passes.
TouchSensorTracks the location and state of the pointing device and detects when you point at geometry contained by the TouchSensor node parent group.
VisibilitySensorDetects visibility changes of a rectangular box as you navigate the world.

Interactive Mode

Interactive mode allows clients to modify a remote virtual world via events from sensor nodes defined in the virtual world. Interactive mode is useful when a virtual world includes a sensor.

Interactive mode is disabled by default on clients. You can enable (or later disable) interactive mode on a client via context menu in the Web Viewer or by pressing the I key shortcut.

You can disable interactive mode for a particular virtual world on the host computer. For details, see the ClientUpdates property, using vrworld/get or vrworld/set.

Read VRML Sensor Values

To read a value of a readable VRML field (either exposedField or eventOut), first synchronize that field with the vrnode/sync method. After synchronization, each time the field changes in the scene, the field value updates on the host. You can then read the value of the field with the vrnode/getfield method or directly access the field value using the dot notation.

Reading VRML Sensor Values Example

The virtual scene for the Magnetic Levitation ModelMagnetic Levitation Model example, maglev.wrl, contains a PlaneSensor (with the DEF name 'Grab_Sensor'). The PlaneSensor is attached to the ball geometry to register your attempts to move the ball up or down when grabbing it using the mouse. The example uses the sensor fields minPosition and maxPosition to restrict movement in other directions. You can use the output of the sensor translation field as the new setpoint for the ball position controller. You can read the sensor output value into a MATLAB® variable setpoint with the following:

% create the vrworld object and open the world
wh = vrworld('maglev.wrl');

% get the node handle
nh = vrnode(wh, 'Grab_Sensor');

% synchronize the translation field
sync(nh, 'translation', 'on');

% 3 alternative ways to read the synchronized field value
setpoint = getfield(nh, 'translation');
setpoint = nh.translation;
setpoint = wh.Grab_Sensor.translation;
To use the setpoint value in a Simulink model, you can write an S-function or an MATLAB Function block that reads the sensor output periodically. For an example of such an S-function:

  1. Right-click the VR Sensor Reader block of Magnetic Levitation ModelMagnetic Levitation Model (vrmaglev) model and select Mask > Look Under Mask.

    The vrmaglev/VR Sensor Reader model displays. This model contains the vrextin block, which is an S-function block. The vrextin S-function synchronizes the sensor field in the setup method and periodically reads its value in the mdlUpdate method.

  2. To examine the S-function parameters, right-click vrextin and select S-Function Parameters.

    The parameters defined in the mask supply the sample time, virtual world, and the node/field to read.

Note the following about the vrextin S-function:

  • Instead of setting its own block outputs, the vrextin S-function sets the value of the adjacent Constant block value_holder. This setting makes the VR Sensor Reader block compatible with Simulink Coder™ code generation so that the model can run on Simulink Coder targets.

  • The signal loop between user action (grabbing the ball to a desired position using a mouse) closes through the associated Simulink model vrmaglev. As a result, grabbing the ball to a new position works only when the model is running and when the model sets the blue selection method switch to the virtual reality sensor signal path. To experience the behavior of the PlaneSensor using the virtual scene only, save the maglev.wrl file under a new name and remove the comment symbol (#) to enable the last line of this file. This action activates direct routing of sensor output to a ball translation. You can then experiment with the newly created scene instead of the original maglev.wrl world.

    ROUTE Grab_Sensor.translation_changed TO Ball.translation
  • You can use this method to input information from all VRML node fields of the type exposedField or eventOut, not only a Sensor eventOut field. See VRML Data Class Types for more information about VRML Data class types.

  • For fields of class exposedField, you can use an alternate name using the field name with the suffix, _changed. For example, translation and translation_changed are alternate names for requesting the translation field value of the above Grab_Sensor node.

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