An Expert System for SEM Fault Diagnosis

1. The Task

The objective was to determine if expert system technology could tackle the task of fault diagnosis in the modern scanning electron microscope (SEM) and to provide a prototype system for the specific needs of LEO Electron Microscopy Ltd.

2. The Justification

SEM fault diagnosis suffers from two major problems, namely scarcity of domain expertise and geographical dispersion between instrument manufacturers and the customers. Initial diagnosis is conducted by telephone conversations with ambiguities in verbal descriptions of SEM images likely to cause misdiagnosis. Improving customer service while minimising costs is critical.

3. The Solution

The solution is twofold, requiring a system which provides the service engineer with the knowledge of the leading service engineers and accurate information directly from the faulty instrument. The former lends itself to a knowledge-based approach given the long history of diagnostic applications. The latter requires some mechanism for remotely controlling an SEM. A complete system requires a combination of both.


4. The Implementation

The Expert System

Having gathered diagnostic knowledge from interviews with service engineers and analysis of service manuals and case histories, the results were formalised in a rule-based representation. We decided to build an expert system from scratch in order to maintain sufficient flexibility to meet the requirements of this application, opting for the Win-Prolog development environment (from Logic Programming Associates) due to its versatility, functionality, tight Windows integration and cost. Software design has progressed from demonstration prototype (to provide proof of concept) to research prototype (increased knowledge base and a graphical user interface) and is currently at the field prototype stage.

A Sample Rule

filamentsaturation :
if suspect 'filament current'
action null
request 'Check filament current setting - answer Y if set to 1st or 2nd peak, N if was not set properly but is now OK, O if unable to saturate'
positive exonerate 'filament current'
negative treated 'filament current'
otherwise suspect 'misaligned filament' with high and suspect 'EHT set' with medium and suspect 'gun' with medium
purpose 'Determine whether the filament is correctly saturated'
justify_positive 'Filament is set to appropriate peak and so the beam should be sufficiently intense for imaging'
justify_negative 'An unsaturated filament will not provide a sufficiently defined beam and so imaging would be degraded'
justify_otherwise 'Inability to saturate suggests either a badly aligned filament or a physical fault preventing proper filament heating'
zone all
author 'NHM Caldwell'
source 'D Leary & M Palmer'
creation_date '21-Sep-95'.

Picture of FIRST A.I.D.

Remote SEM Control

The modern SEM is a fully computer-controlled instrument with the control software executing as a standard application in the Microsoft Windows environment on a minimally augmented PC. Like any other PC, such a system (PC-SEM) can be connected to a network to share hardware, software and information resources. The SEM software is designed to cooperate with other software packages such as image analysis or instrumental accessory control systems and several programmer-level interfaces exist.

Picture of a LEO 435VP SEM

By building a CGI (Common Gateway Interface) script to utilise one of these interfaces and a set of HTML pages to provide a user interface, CUED produced a platform-independent generic remote control package for the SEM. With the package resident on a web-server, our SEM could be controlled simply using a Web browser, with the remote user able to grab images, inspect and set instrument parrameters.

Since then, this work has been transferred to LEO and the industrial strength version is now available for free download from their site.

The NetSEM Remote Control Package

Remote Diagnosis

The remaining objective was to connect the expert system to the remote control package. Fortunately Spyglass Inc. defined a standard by which applications could control web browsers and to greater or lesser extents the principal browsers have followed this specification. For Windows versions, Netscape Navigator uses Dynamic Data Exchange protocols to permit two-way interactions between the browser and other applications. By making extensive use of the DDE support of LPA's Win-Prolog we were able to achieve the critical tasks of starting and closing Netscape, loading Web pages and files, determining if a download was successful and sending requests to the CGI script. Hence FIRST A.I.D. can acquire images on the user's behalf, conduct remote tests on an instrument via the Internet and in some cases correct instrument problems, and provide online help in the ubiquitous hypertext of the Web.

Diagram of FIRST A.I.D. Architecture

5. The Future

Currently FIRST A.I.D. covers a wide variety of instrument, accessory and software problems in a little under two hundred rules. Evaluation of the system is proceeding at LEO Cambridge with final modifications being made. All being well, it should see active service in the near future.

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