Westbay Auditor
Software Review of Shielded Enclosure EMC Performance
Westbay Auditor collects the mechanical properties of your equipment enclosure, ranging from material properties to panel openings, along with the details of the emc (electromagnetic compatibility) specification and known interference sources. An Audit process then automatically assesses the possible emissions performance of the shielded enclosure compared with the specification, and produces an audit report.
Overview
The physical enclosure of an item of electronic equipment has a primary function of assembly and protection of the circuitry within, and also of protection of the user from dangerous voltages. The enclosure however can also play an important role in ensuring the equipment meets electromagnetic compatibility (emc) requirements.
The assessment of the emc capabilities of an enclosure unfortunately is not an exact science. It is more a combination of approximated theories and good design practice, and is frequently administered by an emc engineer. Emc is often poorly understood by electronic engineers, and poses an even greater challenge for mechanical design engineers, who may be involved in enclosure design.
Westbay Auditor provides an alternative approach, by providing a program to gather an enclosure's mechanical characteristics, to automatically apply emc theories on shielding, leakage from apertures etc, and to automatically apply qualitative assessments. The latter are based on sets of rules built into the program, but accessible and editable by the user.
A rigorous approach to review of the emc features of the equipment is enforced.
Westbay Auditor produces an isometric view of the enclosure, with a representation of features such as cooling vents, display openings, access panels etc. All enclosure views can be be displayed, using rotate left and right buttons, and flip up/flip down buttons.
A wide range of parameters can be set for the enclosure; the materials definition form is shown above, where the material properties itself are set, along with details of the surface finish. Shielding effectiveness is affected by material conductivity and permeability. Interfaces with panels are affected by surface finish properties.
The flow of operation of Westbay Auditor. After definition of mechanical features and emc requirements, the Audit Process reviews each feature against theoretical performance, and qualitative rules. Textual summaries are made of each of the fourteen audit stages, which can be viewed on-screen, and also written to a text file for importation into other applications.
The following features are supported by Westbay Auditor:
(Click on the highlighted link above for more details of the
features).
 |
The Enclosure: definition of rectangular enclosure by overall dimensions and material thickness; by material properties (a user-editable materials library is provided); by material thickness. Each seam of the enclosure can be defined by its construction, as solid, soldered or screwed . |
|
Apertures: rectangular or circular openings in the enclosure walls can be placed on any surface, either singly, or in arrays. Apertures may also be protected by a shielded window, or a honeycomb mesh, whose shielding performance is user-defined. The aperture can also be protected with a waveguide tube, whilst a display can have an internal shielded sub-enclosure. |
|
Access Panels: openings which are covered by removable panels can be defined on any surface. The method of fixing, including fastener spacing is definable, along with material and thickness. An emi gasket can be fitted between the panel and the enclosure itself. |
|
Panel Mounting Components: Openings into the enclosure which are used for mounting components such as indicator lamps, fuses etc can be defined. Although these are usually small, they can act as 'hidden' openings where plastic-bodied conponents are used. |
|
Conductor Interfaces: the details of conductors entering the equipment can be logged. These effectively form part of the enclosure design, in that shielded cables must be correctly terminated at the enclosure wall. Filters are also often mounted at or close to the enclosure wall, and the Auditor can examine any identified internal noise sources (such as clock harmonics), and advise on possible filter values to achieve fixed levels of attenuation, based on the assumption that the internal noise source couples to cables exiting the equipment, causing interference currents. |
Field
Definition Features
(Click on the
highlighted link above for more details of the Field Definition
Features).
|
Emissions Specification: external field limits which the equipment must meet can be defined, in terms of dBuV/m versus frequency. Some common specifications are included, and the user can create his own limits. These are used in the Audit Process to compare calculated leakage of identified noise sources with the field limit. |
|
Internal Noise Sources: the level of internally generated fields may be known, from actual measurements on the system, or from estimates calculated from loop areas on pcbs. These can be logged by the Auditor, and are used in the Audit process to estimate the magnitude of fields outside the enclosure. |
The internal noise sources definition form. A set of noise frequencies and field levels can be stored, and used in other projects. For example, a pcb which has been characterised can be used in conjunction with another enclosure project. Similarly, several noise files can be merged together into a new project.
Calculation Tools
The Audit Process makes use of several shielding analyses to calculate the basic shielding effectiveness, leakage from an aperture, cavity resonant frequencies, and attenuation due to a waveguide. These calculations are also made available directly to the user as a set of tools, allowing spot calculations at a particular frequency, and also shielding level versus frequency graphs. These can be saved subsequently as a .BMP file for use in other applications, or sent to the printer.
The Aperture Leakage form, which allows the user to estimate the
attenuation an opening in the enclosure offers to a radiated field.
The
Audit Process applies this calculation automatically to every identified
opening in the enclosure, including gaps created along panel edges for
example.
The
Audit Process
(Click on the highlighted link above for
more details of the Audit Process).
|
1 Summarises the project name, the emissions specification levels, any identified noise signals. Compares the noise levels with the emissions specification, and states whether these exceed the specification before the shielding is taken into account. |
|
2 Summarises the enclosure size, material and finish. |
|
3 Provides the shielding level of the enclosure walls, at a range of frequencies, and calculates the first sixteen resonant frequencies. |
|
4 Calculates the external field levels caused by leakage through the enclosure walls of identified sources. (The external field will normally be very low). |
|
5 Examines the leakage due to the enclosure seams, and calculates the frequencies where the attenuation is expected to be 40dB, and 0dB. No leakage is reported if the seam is solid or gasketed. |
|
6 Examines the enclosure seams again, and compares the actual external field level with the emissions specification. |
|
7 Summarises the apertures, and calculates the 40dB and 0dB attenuation frequencies. Additional shielding components are not taken into account at this stage. |
|
8 Examines the apertures again, and compares the external field with the emissions specification, and states the margin. Additional shielding components are not taken into account at this stage. |
|
9 Examines the apertures again, and takes into account any additional shielding components such as shielded windows. External field levels are again compared with the specification |
|
10 Summarises the access panels, their properties, description, and fixing means. If the panel is fixed by fasteners, without a gasket, a narrow aperture is assumed to exist between each fastener. In this case the frequencies for 40dB and 0dB attenuation are calculated. |
|
11 Compares the panel edge leakage fields with the emissions specification, and states the margin. |
|
12 Summarises the panel mounting components. Where these produce an aperture, the 40dB and 0dB frequencies are stated. |
|
13 Compares the panel mounting component leakage fields with the emissions specification, and states the margin. |
|
14 Summarises the conductor interface properties. |
The Audit Report can be written to a text file for import into other documents, or viewed stage by stage on the screen.
Audit Commentary
A powerful feature of the Auditor is the ability to create 'qualitative' commentaries. This allows the sort of comment an 'emc expert' might make to be inserted into the Audit file, by simply commenting that an aperture above a certain size for example, "is big enough to pose a potential problem". Standard conditions are provided, but the user can edit these, and create his own comments.
Westbay Auditor Lite can be downloaded free of charge.
System Requirements
Windows 95 or later. 2Mb RAM, 3.5Mb Hard Disk space.
Westbay Auditor is a trademark of Westbay Technology Ltd
Windows and
Windows 95 are trademarks of Microsoft Corporation