ROM144 Element Description

ROM144 represents a 2-D or 3-D reduced order model of a coupled electrostatic-structural system. The element fully couples the electromechanical domains and represents a reduced order model suitable for use in finite element analysis as well as electromechanical circuit simulations. The element has ten modal degrees of freedom relating modal forces and modal displacements (EMF), ten voltage degrees of freedom relating electrical current and potential (VOLT) and, optionally, 10 master nodes relating nodal forces to nodal displacements (UX). Only nine of the 10 modal degrees of freedom and five of the 10 voltage degrees of freedom are actually used. The element is suitable for simulating the electromechanical response of micro-electromechanical devices (MEMS) such as clamped beams, micromirror actuators, and RF switches.

The element is derived from a series of uncoupled structural and electrostatic domain simulations using the electrostatic elements (such as PLANE121, SOLID122, SOLID123, and INFIN111) and structural elements which are compatible with electrostatic elements. The ROM144 element represents a complicated flexible structure whose nodes move mainly in one direction either X, Y or Z referred to the global Cartesian axes. For instance, torsional systems with angles less than ten degree or flexible bending of cantilevers or membranes obey those restrictions (pressure sensors, cantilever for AF microscopy, RF filter). Geometrical nonlinearities caused by stress stiffening or initial prestress are considered as well as multiple conductor systems. See ROM144 in the Mechanical APDL Theory Reference for more details about this element.

Figure 144.1:  ROM144 Schematic

ROM144 Input Data

The element is defined by 20 (KEYOPT(1) = 0) or 30 nodes (KEYOPT(1) = 1). A reduced order model file filename.rom and the appropriate polynomial coefficients for the strain energy and capacitance functions stored in jobname_ijk.pcs must be available in the working directory. Furthermore, the model database filename.db and the reduced solution file (.rdsp) generated by the Use Pass are required to perform an Expansion Pass.

Real constant number 1 (R1) is the element identification number (ID). It is automatically created by the circuit builder (see Using the Circuit Builder in the Low-Frequency Electromagnetic Analysis Guide), and is not required input for analysis purposes. The element supports nodal forces F and displacements D applied at ROM master nodes (21 to 30). The UX degree of freedom must be chosen independent from the physical direction of the original master node. Electrode current and voltage can be applied only to the first five active voltage nodes (11–15). Modal displacements may be set by the EMF degree of freedom using the D command. Element loads defined in the Generation Pass may be scaled and superimposed by the RMLVSCALE command. ROM144 can be attached to other finite elements such as COMBIN14 and COMBIN40 at the master DOF. The "reaction force" for the modal displacement degree of freedom (EMF) is a modal force, labeled CURT, and should be used when defining the solution convergence criteria (CNVTOL command). The "reaction force" for the electric potential degree of freedom (VOLT) is current, labeled AMPS. The element is compatible with the electric circuit elements CIRCU124 and CIRCU125 and the electromechanical transducer element TRANS126.

Modal damping ratios may be altered by the RMMRANGE command. Save the ROM database before using the changed data in the Use Pass.

A summary of the element input is given in "ROM144 Input Summary".

ROM144 Output Data

The solution output associated with the element is in two forms:

Table 144.2: ROM Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table in the Element Reference for more information. The following notation is used in Table 144.2: ROM Item and Sequence Numbers:

ROM144 Assumptions and Restrictions

ROM144 Product Restrictions

There are no product-specific restrictions for this element.