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Saving Output Data with Sensors

In this section, you will learn how to:

  • Use sensors to save the results of your EMPro project calculation
  • Choose the correct sensor to use depending on the type of data you want to save
  • Mesh voxel objects

Sensors are objects that save data during a simulation. Any type of data that can be saved in EMPro is saved with a sensor. The type of data that is saved by a sensor is dependent on the sensor type, as well as the specific data that is requested within various sensor editors. There are various types of sensors that are available within EMPro, including:

  • Port sensors
  • Near Field sensors, including:
    • Point sensors
    • Surface sensors
    • Rectangular sensors
    • Planar sensors
    • Solid Part sensors
    • Solid Box sensors
  • Far Zone sensors
  • Specific Absorption Rate (SAR) sensors
  • Hearing Aid Compatibility (HAC) sensors

Result objects are generated based on the sensor objects that are defined in the project. After a sensor has been placed, an editor is used to define its characteristics based on the output data. Each type of sensor has its own respective editor window. This section details the process of adding sensors into an EMPro project and requesting specific results with each type of sensor.

Sensor Tools

In general, sensors added to the project within the Sensor Tools dialog (with the exception of Port and SAR sensors). There are two ways to open this dialog. The first is to choose Sensor Tools from the drop-down list in the Geometry workspace window. The second is to right-click on the Sensors branch of the Project Tree and select the branch that corresponds to the desired sensor type.

Note

Port sensors are added by setting a component property. For more information on how to add a Port sensor, refer to Adding a New Component.
 
SAR sensors are added by double-clicking on the sensor in the Project Tree. For more information on SAR sensors, refer to SAR Sensors.

Sensor Tools menu

The following sections detail the process of adding each type of sensor within Sensor Tools, defining its associated characteristics, and requesting the desired output data to be calculated by the sensor.

Port Sensors

A Port sensor saves near-zone voltage and current data at the location of a circuit component. Port sensors are automatically added to the project when a circuit component is added and the This Component Is A Port property box is checked in the Properties tab of the Circuit Component Properties dialog.

Each Port sensor can have a different source resistance. For more information on specifying the source resistance, refer to Specifying the Source Resistance.

S-Parameter, Group Delay, VSWR and Reflection Coefficient Calculations

When S-parameter computation is enabled in the Simulations workspace window, Port sensors will also save data used to compute S-parameters, Group Delay, VSWR, and reflection coefficient. When multiple Active Feeds are used in the simulation, S-parameters will be computed at each Port sensor with respect to each active feed.

Note

S-parameters at each Port sensor are calculated using the characteristic impedance retrieved from the circuit component definitions of that port and the active feed. For more information on defining S-Parameter calculations with single or multiple ports, refer to S-parameters Simulation Setup.

Near Field Sensors

Near Field sensors are used to save time-domain and/or frequency-domain near zone field quantities at specific points within the bounds of calculation space. In general, field data is retrieved using the Point, Surface, or Solid sensors, and hearing aid field values are recorded using the Hearing Aid Compatibility (HAC) sensor. Solid sensor results can be viewed as 2-D plots, and both Solid and Surface sensors can be viewed as 3-D field sequences (excluding Point sensor data).

Field Retrieval

The field quantities of X -, Y -, and Z -directed electric (E) and magnetic (H) fields may be saved at a specific point, across a surface, or throughout a volume with a Point sensor, Surface sensor, or Solid sensor, respectively. Additionally, X -, Y -, and Z -directed current density (J) may be collected with any of these sensors. Current densities are determined by multiplying the conductivity of the material at the specified location by the electric field in the given direction. When a PEC material is present, the current density will be computed by the loop of magnetic fields surrounding that cell edge. Thus, the current density only includes the conduction current. When a near-zone source is used as the input, the total field values are available. With an incident Plane Wave input, the scattered and total electric and magnetic fields may be saved in addition to the total current density.

Samplings of near field data may be saved by specifying Sampling Time Range in any of the near field sensor definition windows. Near field data will be collected in specific planes of the geometry during the EMPro calculation at every interval specified within the definition. A field file containing the electric and magnetic fields and the current will be created for each timestep specified. For example, setting an entry beginning at timestep 100, ending at timestep 1000, with an increment of 100 will create 10 field files which may be viewed as a movie after the EMPro calculation is performed.

Note

Be aware of the number of field slices to save, as they can store enormous amounts of data. Single field files may contain megabytes of data depending on the number of cells in the specified plane.

Point Sensors

A Point Sensor is positioned at a specific point-location in the simulation space, and can be defined by the location of a specific vertex in a part object or by a Cartesian 3-D expression. The sensor records data as it occurs at the specified point in space.

Point sensors record data by means of field interpolation or geometric "snapping". When using the interpolated sampling method, the field components are interpolated to the exact location of the point sensor. This is performed by linear interpolation among the surrounding eight appropriate field value sample points (e.g., when measuring Ex, the eight surrounding X -directed edge centers are used for the interpolation, and when measuring Hx , the eight surrounding X -directed cell face center points are used for the interpolation). When using the snapped sampling method, the location of the point sensor is snapped to the nearest E-grid cell vertex. Field components for snapped point sensors come from the cell whose lowest-index corner is defined by the snapped location of the sensor. The sensor location is thus dependent on the grid definition.

Point Sensor Properties

To define a Point Sensor, open the point sensor properties dialog under Sensor Tools. In the Location tab, define the sensor location manually by typing in its coordinates, or automatically by clicking on the intended location in the simulation space with the Selection tool. In the Properties tab, enter the name of the sensor, select the desired Point Sensor Definition, and choose the sampling method as described above.

Point Sensor properties dialog

Point Sensor Definition Editor

The Point Sensor Definition Editor window is used to assign definitions associated with a Point Sensor.

To access the editor, double-click on an existing Point Sensor Definition in the Definitions: Sensor Data Definitions branch of the Project Tree. If no point sensor definition is present, right-click on this branch and select New Point Sensor Definition.

In the Fields to Save region of the editor, select the desired point sensor output data to save:

  • E: Electric Field Intensity time
  • H: Magnetic Field Intensity vs time
  • B: Magnetic Flux Density vs time
  • J: Current Density vs time
  • Scattered E: Scattered Electric Field vs time
  • Scattered H: Scattered Magnetic Field vs time
  • Scattered B: Scattered Magnetic Induction Field vs time

Note

Scattered field values can be retrieved only if a Gaussian beam or a scattered field plane wave external excitation is used to excite the simulation.

Point Sensor Definition Editor

Define the Sampling Time Range by entering the Start Time and End Time, or by simply checking Start of Simulation and End of Simulation to automatically assign the sampling time range to these values. Choose a Sampling Interval to indicate how often data is saved within this time range.

Surface Sensors

Surface sensors collect data on one or more faces of a geometric object in the simulation space. Like Point Sensors, they can be interpolated or mesh-snapped.

There are three types of surface sensors in EMPro:

  • Sensor On Part Surface
  • Rectangular Sensor
  • Planar Sensor

Note

Refer to the Surface Sensor Definition Editor section to reference the output data that can be retrieved by a surface sensor after it has been created within Sensor Tools.

Sensor on Part Surface Properties

To define a Sensor On Part Surface, select the object in the simulation space that the sensor will be attached to by clicking on it in the Pick Model tab. In the Pick Faces tab, select the specific face to attach the surface sensor. Finally, in the Properties tab, and assign the new sensor a Name, Definition and Sampling Method.

Note

Definition, as mentioned here and in the following two sensor descriptions, refers to definitions stored in the Definitions: Sensor Data Definitions branch of the Project Tree.

Sensor on Part Surface properties dialog

Rectangular Sensor Properties

Define a Rectangular Sensor by first using the Orientation tab to choose the plane in which the rectangle is defined. Then, use the Rectangle tab to define the rectangle's two opposite corners. Finally, under the Properties tab, assign the sensor a Name, Definition and Sampling Method.

Note

For an explanation of the Orientation tab, refer to the section Specify Orientation Tab.

Rectangular Sensor Properties dialog

Planar Sensor Properties

The Planar Sensor uses a point and normal direction defined in the Orientation tab to define an entire plane (within the boundaries of the simulation space) to collect sensor data. Select the Properties tab and assign the sensor a Name, Definition and Sampling Method.

Planar Sensor Properties dialog

Surface Sensor Definition Editor

The Surface Sensor Definition Editor window is used to assign definitions associated with a Surface Sensor.

To access the editor, double-click on an existing Surface Sensor Definition in the Definitions: Sensor Data Definitions branch of the Project Tree. If no surface sensor definition is present, right-click on this branch and select New Surface Sensor Definition.

In the Fields to Save area of the editor, select the desired surface sensor output data to save:

  • E: Electric Field Intensity time
  • H: Magnetic Field Intensity vs time
  • B: Magnetic Flux Density vs time
  • J: Current Density vs time
  • Scattered E: Scattered Electric Field vs time
  • Scattered H: Scattered Magnetic Field vs time
  • Scattered B: Scattered Magnetic Induction Field vs time

Scattered field values can be retrieved only if a Gaussian beam or a scattered field plane wave external excitation is used to excite the simulation.

  • Steady E: Steady Electric Field vs time
  • Steady H: Steady Magnetic Field vs time
  • Steady B: Steady Magnetic Induction Field vs time
  • Steady J: Steady Current Density Field vs time
    Surface Sensor Definition Editor

Define the Sampling Time Range by entering the Start Time and End Time, or by simply checking Start Of Simulation and End Of Simulation to automatically assign the sampling time range to these values. Choose a Sampling Interval to indicate how often data is saved within this time range.

Solid Sensors

Solid sensors collect data by capturing mesh-snapped fields within a volumetric space (interpolated data is not available).

There are two types of solid sensors in EMPro:

  • Solid Part Sensor
  • Solid Box Sensor

Note

Refer to Solid Sensor Definition Editor to reference the output data that can be retrieved by a solid sensor after it has been created within Sensor Tools.

Solid Part Sensor Properties

A Solid Part Sensor simply assumes the shape of the part that is selected in the Pick Model tab. Assign the sensor a Name and Definition in the Properties Tab.

Note

Definition, as mentioned here and in the following sensor description, refers to definitions stored in the Definitions: Sensor Data Definitions branch of the Project Tree.

Solid Part Sensor Properties dialog

Solid Box Sensor Properties

A Solid Box Sensor assumes the shape of a 3-D box. This shape is dictated by the Origin location defined in the Orientation tab, and its farthest corner is defined in the Opposite Corner Tab. Assign the sensor a Name and Definition in the Properties Tab.

Note

Refer to the section Specify Orientation Tab for an explanation of the Orientation tab.

Solid Box Sensor Properties dialog

Solid Sensor Definition Editor

The Solid Sensor Definition Editor window is used to assign definitions associated with a Solid Sensor.

To access the editor, double-click on an existing Solid Sensor Definition in the Definitions: Sensor Data Definitions branch of the Project tree. If no solid sensor definition is present, right-click on this branch and select New Solid Sensor Definition.

In the Fields to Save area of the editor, select the desired solid sensor output data to save:

  • E: Electric Field Intensity time
  • H: Magnetic Field Intensity vs time
  • B: Magnetic Flux Density vs time
  • J: Current Density vs time
  • Scattered E: Scattered Electric Field vs time
  • Scattered H: Scattered Magnetic Field vs time
  • Scattered B: Scattered Magnetic Induction Field vs time

Scattered field values can be retrieved only if a Gaussian Beam or a scattered field Plane Wave external excitation is used to excite the simulation.

  • Steady E: Steady Electric Field vs time
  • Steady H: Steady Magnetic Field vs time
  • Steady B: Steady Magnetic Induction Field vs time
  • Steady J: Steady Current Density Field vs time
    Solid Sensor Definition Editor

Define the Sampling Time Range by entering the Start Time and End Time, or by simply checking Start of Simulation and End of Simulation to automatically assign the sampling time range to these values. Choose a Sampling Interval to indicate how often data is saved within this time range.

Hearing Aid Compatibility Sensors

Hearing Aid Compatibility (HAC) sensors gather data on a 5cm by 5cm arbitrarily-oriented rectangle in freespace. They are used to determine if a wireless device (such as a cellphone) will generate electrical and magnetic fields large enough to interfere with a hearing aid. In these cases, they are useful for evaluating the wearer's ability to adjust the position of the phone to a better location.

The HAC sensor is centered at the origin of the coordinate system described in the Geometry tab.

This sensor collects steady-state E and H fields at grid points near the HAC plane at each frequency of interest. These values can be then interpolated onto the plane at a user-defined spatial resolution.

HAC Sensor properties dialog

Far Zone Sensors

Far Zone Sensors are located at theoretical infinite distance from the simulation geometry. They are only available in the absence of PMC or periodic outer boundary conditions, or when more than one PEC boundary is used.

To create a Far Zone Sensor, under the Geometry tab, choose its coordinate system:

  • THETA/PHI
  • ALPHA/EPSILON
  • ELEVATION/AZIMUTH

Far zone sensors can be created with one of the following geometries:

  • A range of theta/alpha/elevation over a constant (single) phi/epsilon/azimuth
  • A range of theta/alpha/elevation over a range of phi/epsilon/azimuth
  • A range of phi/epsilon/azimuth over a constant (single) theta/alpha/elevation

The following figures demonstrate the transformation of the far zone sensor based on the defined geometry in the Theta/Phi coordinate system.

Several Far Zone sensor geometries

As seen under the Properties tab, EMPro has the ability to compute both a steady-state far-zone (near-to-far) and a transient far-zone transform. These two options are described below.

Steady-state Far-zone Transformations

Steady-state transformations are particularly advantageous because the calculation overhead is minimal. They do not require the definition of specific far-zone angles before the FDTD computation, since all patterns are computed in post-processing using data that is automatically stored by the EMPro calculation engine. Instead, the calculation saves the tangential electric and magnetic fields on the far-zone transformation surface at two timesteps, near the end of the calculation when the system should be in steady state. This sampling determines the complex tangential fields on the far-zone surface at the excitation frequency. These fields are then used in post-processing to provide radiation gain or bistatic scattering in any far-zone direction at any pattern increment. This saves considerable computer time and memory if many far-zone directions are required.

Additionally, the selection of a steady-state far-zone transformation computes the single frequency input impedance, total input power, radiated power, and antenna efficiency. All values computed require that the calculation has reached steady state.

Defining a Far Zone sensor

The far-zone gain is displayed in units of dBi. This is the number of decibels of gain of an antenna referenced to the zero dB gain of a free-space isotropic radiator. This value is calculated based on the net power available at the source voltage output. Directivity is not available.

Transient Far-Zone Transformations

The transient far-zone calculation should be used when broadband results are desired, since the steady-state transform is only performed for a single frequency. The broad frequency range, therefore, can be determined at a few points in space. An additional feature of the transient far-zone transformation is that the time-domain far-zone electric fields are also generated and may be plotted.

The transient far-zone calculation requires extra calculation time for each far-zone angle specified, and unlike steady-state far-zone transformations, all far-zone angles must be defined before running the calculation engine. The transient far-zone calculation is intended for use in cases where the far-zone results at a few points are desired since it is computationally intensive. This calculation may be desired in instances when far-zone time-domain fields are needed.

If detailed gain patterns versus angle are necessary, you may reduce calculation time by enabling only steady-state data collection for your sensor and specifying a DFT frequency for your simulation at each frequency you are interested in.

SAR Sensors

EMPro includes several features that fall under the category of biological applications. For compliance with regulations on field absorption in human tissue, the Specific Absorption Rates (SARs) can be computed and averaged. Detailed human body meshes are available for simulations related to effects on realistic heterogeneous models of the body. For some wireless applications, the Specific Anthropomorphic Mannequin (SAM) head is used in addition to the heterogeneous human head (see the figure below).

The Specific Anthropomorphic Mannequin (SAM) head

The Specific Absorption Rate, or SAR, is the unit of measure commonly used to determine the interaction of electromagnetic fields with human tissue. Most regulations involving devices producing electromagnetic fields must not exceed some exposure limits, typically defined in terms of the SAR averaged over a cubical volume of tissue.

Note

As an example, the IEEE sets exposure levels in terms of 1 g averaging volumes for most of the body, with a 10 g averaging volume applying to extremities such as the ears and fingers.

SAR is defined in terms of the room mean square (RMS) of the electric field magnitude by the relation

Where:

is the electrical conductivity in , and

is the material density (defined in in EMPro )

Since the FDTD grid defines the electric fields at the edges of the cells, a single SAR value is formed by summing and averaging the contributions of the 12 electric fields on the edges of the cells. The SAR is then referenced to the center of the FDTD cell.

In EMPro, the SAR is measured with the SAR sensor and may only be computed in normal dielectric materials. Frequency-dependent materials have a loss term formed by the imaginary part of the permittivity rather than simply by the conductivity, and are not supported for SAR calculations.

The SAR values are saved only in complete voxels (closed FDTD cells) where all 12 edges of the cell are lossy dielectric material (non-zero conductivity) with a non-zero density, therefore steady-state values for SAR and conduction currents will not exist in all planes. To exclude certain materials from a SAR calculation, simply leave the material density as zero. Saving the SAR in a plane of free-space will not produce any useful output as all values will be zero.

Note

For more information on voxels, refer to the section on Voxels.

Un-averaged SAR Calculation

Un-averaged SAR is measured in EMPro using the SAR Sensor. Note that most specifications which involve SAR limits are defined in terms of constant-mass regions, so they will require averaged SAR.

Averaged SAR Calculation

The averaged SAR calculation is more meaningful under most circumstances. This calculation is defined by regulations from organizations such as the IEEE and various government bodies. It is computed over cubical volumes of voxels where no face of the averaging volume is external to the body (and thus full of air or other non-tissue material). In certain cases, particularly at the surface of the body, the cubical volume rule can not be satisfied. In those situations, special rules exist for setting the SAR value in a given voxel. Refer to the IEEE published standards for regulating SAR calculations and setting SAR values in the Bibliography.

Note

Only one SAR averaging region can be defined per calculation run. Additional averaging can be performed as a post-processing step, given that sufficient un-averaged SAR was collected for the region of interest.

Averaged SAR is measured in EMPro using the SAR Averaging Sensor. There are several ways to compute average SAR values in EMPro, as shown in the SAR Averaging tab. One way is to save 1 gram or 10 gram average SAR regions over the Full Grid. During the calculation the averaged SAR values will be computed for all appropriate voxels. This process is time consuming, and since the 10 gram SAR is only applicable to the extremity tissues, it is not necessary to compute it for the entire geometry.

As an alternative to computing values over the entire grid, the EMPro interface also has a tool for computing the average SAR over a Box Region. If a subregion of the whole object is defined, then an option is available to allow all data outside of that subregion to be considered as free space. The figure below displays the Box Region dialog.

Another alternative is to select the Auto Subregion option. In this case, the Max/Min SAR Ratio is defined in decibels so that the requested 1 gram or 10 gram average is performed only where applicable, thus saving a great deal of calculation time. This quantity must be entered as a unitless ratio (amplitude) or in dBp (a decibel unit with suffix to indicate an absolute unit of electric power). For example, in a typical application, the extremity tissues would be identified by different material types from the body tissues, so indicating this value in the Max/Min SAR Ratio would isolate the calculation to that specific region. The figure below displays this dialog.

Requesting averaged SAR statistics in an automatic subregion

EMPro also offers tissue selection control under the Tissue Materials tab. You can compute averaged SAR for All Tissue Materials, or for Selected Extremity Tissue Materials. In choosing the latter option, a dialog box will appear with a list of available pre-defined materials to include in the calculation.

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