EXPERT SYSTEMS

KNOWLEDGE BASED COMPUTER SYSTEM FOR BRAODCAST TRANSMITTER FOR FAULT DIAGNOSIS
R.K. SINGH, Director(E0, All India Radio&Doordarshan, Staff Training Institute(T), Delhi.

ABSTRACT
This paper is based on the experience gained while developing Expert System for broadcast transmitters in A.I.R. Network. The necessity of Expert System was felt to meet the challenge of clearing faults in transmitter in the absence of transmitter expert in minimum time and ensuring safety of the personnel and system in the process. The Expert System is an intelligent computer program capable of performing at the level of transmitter experts and the paper deals with its design philosophy and methodology. All the elements of decision making process of transmitter expert have been incorporated in four modules, namely, Knowledge Base, Inference Engine, Explanation and Knowledge Acquisition modules. Decision trees for various complaints have been developed and used as main source for building Knowledge Base. Field trial runs of the Expert System have been carried out and found most encouraging.
KEYWORDS : TRANSMITTER ES, FAUTL DIAGNOSIS, BROADCASTING

1. INTRODUCTION

Radio Broadcasting has retained its importance as it most conveniently establishes vital mass link between the government and the people. A long interruption in broadcast service may create serious confusion in the minds of people. Modern generation high power transmitters are now used for broadcasting but being costly, very often standby transmitters are not provided and broadcast engineers strive to run the services with minimum breakdowns.Reliable operation of high power transmitter is a complex technical process. Even with advanced technology, faults do occur in transmitters for reasons like failure/ageing of components, transients, dust, mismatch in circuit etc. Very often the faults are of random and unpredictable nature and develop quickly. The faults are to be diagnosed and cleared fast without experimentations, which call for the services of a transmitter expert.

The acquisition of expertise takes a long time. Its transfer is an involved and difficult process and the transmitter experts may not have the time or patience to transfer the related knowledge. Further,the operating staff working at remotely located high power transmitters may not have the requisite expertise to deal with the faults and arrival of expert for the purpose may take time. Hence the need was felt to develop an Expert System and provide it at the transmitter which could serve as an expert and provide valuable help in fault clearing.

2. BROADCAST TRANSMITTER - A DESCRIPTION

Circuitry of a modern broadcast transmitter typically consists of Radio Frequency (RF) Chain, Audio Frequency (AF) Chain, Control Circuit, Power Supply System, Cooling System, Protection System and Supervision System as shown in fig 1.

        In the RF Chain, a high frequency carrier of rated power is generated in steps starting from a crystal oscillator upto the final Power Amplifier (PA) employing a very costly high power transmitting tube. In AF Chain, the audio signal is processed upto a desired level and used to modulate the RF carrier. The modulated carrier is fed to radiating mass through a feeder line after necessary matching. Control Circuit uses digital circuitry to achieve sequential operation of transmitter starting typically from Auxiliary Supply ON to Ventillation ON, Filament ON, Bias ON, LT ON, HT ON and finally Modulation Release. The Control Circuit also ensures protection of vital components and operating personnel against circuit disturbances. The Power Supply System provides the required DC voltages to various stages using solid state rectifiers. The main HV DC Supply is provided by using Thyristor Converter Unit. Protection System ensures protection of costly tubes and components by using devices like Crowbar, UV Detector, APD Monitor etc. Supervision System displays the status of functioning of various stages and occurance of primary complaints like tripping of MCB, abnormal door interlock, abnormal water temperature alongwith values or status of critical parameters through meters and lamps.

The complaints or faults of the transmitter are mainly based on the status of functioning of various stages and on the inferences derived from the primary complaints. They are typically classified as under:
(1) AUX NOT THROUGH (2) VENT NOT THROUGH (3) FILAMENT NOT THROUGH (4) BIAS NOT THROUGH (5) HT NOT THROUGH (6) HT NOT RAISING (7) AUDIO NOT THROUGH (8) RF POWER NOT THROUGH.

3. EXPERT SYSTEM FOR BROADCAST TRANSMITTER - A METHODOLOGY DESIGN

Development of ES involves interactive transformation of domain specific knowledge of transmitter expert into machine processable form. The transmitter experts are usually non computer professionals and so the development of ES has to be a joint venture of transmitter expert and computer expert. The process becomes inherently slow and it is also not feasible to associate both the experts for a long timeand hence it was decided to develop the ES in two stages.


        In the first stage all the elements of the decision making process of transmitter expert have been crystalised as shown in fig. 2. Thereafter a control flexible structure known as Expert System Shell as shown in fig. 3 has been developed by the computer expert comprising four basic modules, namely, Knowledge Base(KB), Inference Engine, Explanation Module and Knowledge Acquisition Module incorporating all the elements of the decision making process of transmitter expert.Kowledge Base of the shell is empty which is filled up by transmitter expert in second stage.

The Shell is built up by eliciting the information relating to the process and logic of the fault diagnosis from the transmitter expert interactively. While filling up the KB, the need of providing additional facilities like graphics, pictures etc. was felt in the data collection mode and at the time of fault confirmation, and the shell was modified accordingly. Interaction between the two experts continued till the desired level of ES could be realised.
4. CONCEPTUALISATION OF THE EXPERT SYSTEM (ES)

The ES should be able to diagnose the faults like a human expert and its performance level should be higher than the concerted efforts of many experts. The ES has been conceptualised as under.

i) Analysing in detail how the transmitter expert proceeds with his thought process while clearing
the faults. During this analysis, the elements of the thought process and the required domain
specific knowledge got crystalised.

ii) Breaking the entire fault clearing process into basic steps.

iii) Designing a user friendly computer program which proceeds towards fault diagnosis in a way
similar to that adopted by the transmitter expert.

After developing the ES, its field trial runs were conducted successfully and its results utilised to remove the deficencies of the ES. Feedback received during the discussions with experts and users about the evaluation and utility of the ES has been used to refine the design of ES further to make it more effective and versatile.

4.1 DESIGN CONSIDERTATIONS:

Following aspects have been taken into account during the development of the Expert System.

All the elements of the decision making process of the transmitter expert has been incorporated in the ES. The Shell should be simple to enable the transmitter expert build the ES directly without training in AI programing or the assistance of a computer engineer.
KB structure of the shell has been designed to support knowledge frames of complaints, hypotheses descriptions, parameter descriptions, string and user defined LISP functions. The complaints have been defined as narrowly and specifically as possible.

It has been made to contain user defined or system generated prompt for data collection, standard I/O functions for display, printout and data entry and slots for filling explanations and help. It also has the facility to attach weightages to individual observations and threshold limits for conclusions.
Inference Engine has been so designed that inference procedures are independent of the structure and domain of KB.

Powerful forward, backward and random branching search strategies supported by efficient heuristics have been incorporated in the Inference Engine.

The shell has been made to act as a tool that assists in eliciting the knowledge from transmitter expert. It supports a window based menu driven knowledge acquisition scheme with which the KB has been built up effectively.

The KB has been so structured that it is possible to modify, update or delete any portion of KB without affecting the structure of entire program.

The ES has been developed to perform at the level of transmitter expert and be able to explain its reasoning after fault diagnosis. Additional useful information about possible emergency arrangements to run the transmitter temporarily if required has been incorporated.

The power of ES depends on the quality and quantity of domain specific knowledge.Hence the KB has been made exhaustive and precise.

has been made to contain ON-LINE HELP and answers to WHY as well as the Case sheet to store data collected and inferences made. It contains explanations covering intermediate results, justifications for conclusions, path of investigations etc.

Necessary safety precautions to be taken while collecting data have been incorporated into the KB.

4.2 FRAMEWORK OF KNOWLEDGE BASE INFERENCE ENGINE FOR TRANSMITTER

The transmitter is sequentially switched ON from cold state to full power. After studying the nature of manifestations of faults, majority of them have been linked to the status of functioning of various stages of the transmitter and listed as complaints. The complaints related to transmitter running with distortion or less output power have also been listed. Yet another set of faults, based on inferences derived from abnonmal parameter status, have been accommodated. A particular fault may be due to any one of the large number of specific reasons, which have been compiled and listed as hypotheses. Which hypothesis should be considered first depends on the individual transmitter. A transmitter, for example, may be working with slight mismatching of radiating antenna or its power supply parameters may be unstable or a little abnormal and these may be resulting in occurance of some faults. While tackling faults, a large number of parameters like meters, lamps, relays,etc.,associated with the hypothesis under consideration, have to be examined or evaluated by applying rules. Subsequently inferences are drawn leading to confirmation or rejection of that hypothesis. All the parameters information and rules have been incorporated in the KB.

The strategy of filling up of the KB has been as under. After stating the problem it was so diagrammed that all its aspects got illustrated as simply as possible. The most common diagram used in working out such problems is the "decision tree". It is very useful and effective as it enables one visualize all factors that must be considered in reaching the decision and to check how one consideration leads to another. Accordingly the required number of decision trees were prepared by transmitter experts and utilised as the source for filling various elements of the KB. Structure wise, important elements of the KB are described in brief as under.

The description of complaints comprise its name, a set of action commands and explanation cum justification. The actions are a set of rules, assertions, function calls or input/output statements etc. The hypotheses part has exactly the same structure as complaints. Description of parameters consists of its name , type of variable, its range, prompt message to collect the data,picture file name,help message to assist in answering the querry for data value, explanation for collecting data and action commands to handle their possible values. The vocabulary of rules and action commands may contain vocabulary realted to names of components, devices, sub systems etc. There may be multiple worded proper nouns also and the system should be able to keep track of additional information supplied. User is allowed to define new LISP functions. These details are filled up by the transmitter expert using his knowledge, experience, heuristics and most important the decision tree. In totality the complete Expert System for the Broadcast Transmitter can be modelled as shown in fig. 5

5. CONCLUSION:
Expert System for a broadcast transmitter has proved to be of great help in clearing fault reliably and taking care of the safety of personnel and equipment. At stations the operating staff may have just enough knowledge to run the transmitter but do not possess the required reasoning power for fault clearing. Provision of a user friendly ES at such a station would be most appropriate. Use of this ES is most relevant at those stations where the redundancy in the transmitter system is very low. While tackling complicated faults even the transmitter experts consult circuit diagrams and proceed inferencially, which takes time. Because of its speed, the ES is able to diagnose the fault faster and so even the experts prefer to use it at times. The ES is found very effective in tutoring the technical staff for acquiring knowledge about fault clearing and generating useful database of transmitter faults.

The refinement of KB of the ES is a continuous process. It can be modified, updated and deleted without affecting the structure of the program. In the later stage of development, the abnormality associated with deteriorating values of a parameter has been treated as a complaint and accommodated, thereby taking care of those incipient faults which take a little time before causing breakdown. Further, experts of a particular type of transmitter at various stations may join hands in building a powerful ES for that transmitter.

ON-LINE ES can also be employed in the transmitter.Such an ES will make the system complex and more vulnerable because of increased interfaces etc. The OFF-LINE ES can as well be instantly used where the operating staff forms an important link between the transmitter and the ES. Ultimately what matters is the clearing of the fault which may involve replacement of components and not alone the diagnosis. However, an ON-LINE ES may be especially useful in dealing with those incipient faults which take a little time before resulting in breakdown.

REFERENCES

Reference Manuals-300 kW MW BBC Transmitter day.

Reference Manual-"DIAGNOS" ESS 3.0 developed by CIRA, ER & DC Thiruvananthapuram, India.

A comprehensive guide to AI and Expert Systems by Robert I Levene, Diane E Drang, Barry Edelson.

Artificial Intelligence- Promise & Performance by Alian Bonnet.

AI & Es Development by David W Rolston.

FULL MAIL ADDRESS :- R.K. SINGH, Tel(off): +91-11-723909

				DIRECTOR(E), Staff Training Institute(T), (Res.) : +91-11-6250375
				All India Radio & Doordarshan, Fax : +91-11-7233909
				Kingsway,

Delhi-110009, INDIA

ALL INDIA RADIO
STAFF TRAINING INSTITUTE (TECHNICAL)

'XTR300ES' - An Expert System for 300 kiloWatt Medium Wave
High Power Transmitter in All India Radio Network

ABSTRACT

The paper titled "Knowledge Based Computer System for Broadcast Transmitter Fault Diagnosis" submitted by AIR in the ABU Engineering Committee meeting 1997 has already dealt about the design philosophy and methodology used for the development of expert system for broadcast transmitters. This paper deals with the design strategies adopted for building the 'Knowledge Base' (KB) of XTR 300 ES. The main group of elements in the KB are descriptions of complaints, descriptions of confirmatory tests of hypotheses, descriptions of various parameters used/evaluated in the previous two descriptions. The 'Think Tank' of the ES has been developed by incorporating the "Rules" framed by a group of transmitter experts using 'Decision Trees' prepared for various complaints of the transmitter. The paper also presents results of Field trials of ES which show that many complicated faults in the transmitter have been confirmed in a very short time. In normal course, Engineer on duty would have taken much longer time in diagnosing these complicated faults.

Introduction

All India Radio has large number of high power transmitters in its network. Generally these transmitters are located at remote places and manned by only 2 or 3 operating personnel. Although these transmitters are employed with advanced technology, still faults do occur in it for reasons like failure and aging of components, occurrence of transients and surges, dust, overheating of components, dry soldering, sparking in cubicles and many other reasons.

300 kW MW BBC (Brown Boveri Corporations) High Power Transmitter is one of the modern broadcast transmitter in AIR network having state of art of digital control circuit employing TTL logic which ensures sequential switching of transmitter, protection of vital and costly components and safety of operating personnel. Important features of this transmitter includes complete solid state circuitry in RF and audio chain except for one transmitting tube in Power Amplifier (PA) and two transmitting tubes in modulator, condensed vapour cooling system, crowbar protection system, thyristor controlled power supply system, veritron unit for safety of very costly power amplifier transmitting tube, filter wall for protecting digital circuit from RF, pre-corrector in audio chain to improve the efficiency of the transmitter, SF6 anode breaker for HT supply etc.

Over the years it has been observed that operating personnel are either unable to diagnose the fault or take very long time to clear it. The faults encountered in the transmitter are of random and unpredictable nature. Therefore the services of transmitter expert are inevitable to clear the fault so that broadcast service could be maintained. Generally due to various reasons the arrival of the expert at the transmitting station may take long time which may further increase the duration of breakdown time in the transmitter.

The expert system 'XTR300ES' provides valuable assistance to the operating personnel in diagnosing and clearing the faults in the transmitter in a very short time.

Design Considerations for the development of Knowledge Base (KB) for the Expert System (ES) XTR300ES.

1All the elements of the "Decision making process" of the transmitter expert must be incorporated in the structure of the KB to enable it to perform at the level of transmitter expert. This implies ability to explain the line of reasoning used to arrive at a solution as well as flexibility which permits incremental building up of the KB.

2After observing symptoms of the faulty transmitter the individual transmitter expert may adopt different reasoning process based on their experience and judgmental abilities. Each expert may apply different heuristics. The "Think Tank" of the ES should incorporate the best possible reasoning of the transmitter expert or a group of experts.
Components of the circuitry of the high power broadcast transmitter are spread over a large area in various cubicles. One of the main hurdles encountered while diagnosing the fault is locating specific parameters such as resistance, condenser, coil, lamp, LED, connections at some terminal blocks, contactor, switches etc. Often this increases the break down period. Therefore necessary 'HELP' in the form of instructions, circuit diagrams, pictures etc. must be provided in the KB.
The ES should not only confirm the fault in the transmitter but also provide possible emergency arrangement to maintain the broadcast service. If required, additional useful information like availability of spares etc. should also be incorporated in the KB.

5 During fault diagnoses the transmitter expert collects values of some parameters such as status of lamps, voltages and other symptoms. The operating personnel may raise queries about justification of collection of data values, relevance of various possible states, related inferences to the different values of the parameter, etc. Answer to these queries should be part of the KB.

Structure of the Knowledge Base (KB)

The structure of the KB has been designed to meet all the design considerations mentioned above. Basically it is a process of abductive reasoning hypothesizing and testing. The main groups of elements in the KB are descriptions of complaints", "descriptions of hypotheses" and descriptions of various parameters used/evaluated in the previous two descriptions.

1The components of complaint descriptions are name of complaint, a set of action commands, and a descriptive explanation cum justification. The explanation includes relevance/importance of the complaint, how it is related to specific hypotheses, why some additional data is required, how they can influence the decision-making etc. The actions are set of rules, assertions, function calls or input/output statements etc.

An example of knowledge Frame of one complaint is as follows:
NAME OF COMPLAINT: [Ventilation Not Through]
ACTIONS : [Examine lamp – H33]
[If lamp-H33 is on then examine supply-ac-3-phase]
[Examine lamp-H63]
[If lamp-H63 is on then confirm FS33-to-FS48-tripped]
[Examine lamp-H86]
[If lamp-H86 is on then examine V-3—of-X100]
[If lamp-H33 is off then confirm ( rel K6 or control print YAE47 or YAE50 suspected faulty]
EXPLANATION : [When Command 'Vent On' is executed Lamp H-14 associated with button K-14 on Key Board turns on Green (continuous light) initially and if prerequisites are not met with in 1 sec or so, the interlock line is interrupted and back indication is not through and the lamp starts flashing.]

2The components of hypothesis description are exactly same as complaints. Each hypothesis is associated with a set of actions as well as explanations/justifications.

HYPOTHESIS NAME : [FILTERWALL-CONNECTIONS-C103-FAULTY]

ACTIONS : [If V-C103-filterwall is equal to 0 then confirm filterwall-connections-C103-faulty].

EXPLANATION : [When Input 220V AC & power supply units YAK11 and YAK12 for DC 24V and Battery supply 24V, all are ok i.e. FU21, FS31 & FS32 are ok and lamp H81 is still glowing, then either FU22/FU26 for Modulator switch board are tripped or relay K1 is de-energised. If FU22 & FU26 are also ok then relay K1 is deenergised otherwise indication H81 is false due to faulty points 42459(K82)41772.0 In that case 24V at C103 filter wall will appear and if not the connections to C103 from current bar 3.0)].

3A number of parameters is used in various rules and assertions etc. The fault diagnostic process involves collecting the values of particular parameters when they are needed. The parameter descriptions include type of variable, prompt message to collect the data, options for values, ranges of values, picture file name, help message to assist in answering the query for data value, explanation/ justification for collecting the data, and action commands to handle the various possible values of data.

An example of sample parameter frame is as follows:

PARAMETER NAME : [V-52472-filterwall]

Type : [user-given non-numeric]

Choices/Range : [0 24]

PROMPT : [What is the voltage at filter-wall terminal 52472]

PICFILE : (A103.BMP CUB4.BMP)

HELP :[ C 52472 at filter wall is feed through capacitor. It is located at Position 7 in C524 row.]

EXPLANATIONS :[ Normally 24V Battery supply is available at C52472 terminal and K91 is operated. H31 is extinguished. In case of break in battery supply line, fault in Relay K201 or in mains the voltage at 52472 is OV]

ACTIONS : [If V-52472-filterwall is equal to 0 then examine (Replace relay K0201) else confirm with figure ctrl-prn-YAY07-K91-faulty print. bmp.]

Building of Knowledge Base (KB) of expert system XTR300ES.

1This Rule Based System tends to become slow, as more rules are incorporated in its 'Think Tank'. To over come this draw back each complaint of the transmitter has been considered as independent 'domain'. The complaints or faults of the transmitter are mainly based on the status of functioning of various stages and on the inferences derived from the primary complaint. These have been classified as i) AUX-NOT-THROUGH ii) VENT-NOT-THROUGH iii) FILAMENT- NOT-THROUGH iv) BIAS-NOT-THROUGH v) HT-NOT-THROUGH vi) HT-NOT-RAISING vii) AUDIO-NOT-THROUGH viii) RF POWER NOT THROUGH

During fault diagnoses the collection of values of some parameters like voltage at terminals may take more time as it involves locating the terminals and taking measurements etc. Therefore during initial stages of diagnostic process the parameters like status of lamps, meter readings, status of contactors etc. are considered for confirming some hypotheses. The voltage measurement is considered only when it becomes unavoidable. Accordingly the "Decision Tree" has been prepared seperately for each complaint by a group of transmitter experts.
Next ACTIONS according to decisions trees were developed and incorporated in the KB after compilation.
. Thereafter other important details like HELP, WHY, circuit diagrams, pictures & explanations were developed and incorporated in the KB.

Knowledge base for the complaint 'AUX-ON NOT THROUGH' has been shown in Fig. 1. Similar Knowledge Base was developed for other complaints too.

Field Trials and Evaluation of XTR300ES

The performance of the expert system 'XTR300ES' has been assessed by conducting field trials run on 300 kW MW BBC Transmitter at All India Radio, Lucknow, India. The operating personnel were asked to clear the faults with the help of 'XTR300ES' after stimulating the probable faults that could appear as well as the faults occurred earlier. A good number of field trials were carried out. Results of some field trials are given below :

S.No	Name of Complaint	No. of ParametersExamined	Name of Hypothesis confirmed	Time Taken (in Mins)	Remarks
1.	Filament NOT Through	6	Door contact faulty or Door Open	4	Actually contact of K-11 faulty. Change in KB required
2.	Bias Not Though	7	Thyristor Control interlock switches open	6	In A-89 unit GH402, NU300, KT 401, F11, F12, F13 K1, K2 K3, contacts are in series. Actually switches NV 300 was tripped
3.	Bias Not Through	8	Crow Bar Relay K303 operated	5	Contact of K303 was found open
4.	HT Not Through	9	Relay K-22 Faulty		Actually Relay K22 was not energised due to loose contact.
5.	Screen Grid Not Through	3	Contact of K15 faulty	3	Actually relay K16 was faulty. Contacts of K-15 & K-16 are in series. Correction in KB incorporated.
6.	Vent Not Through	6	FS switch Tripped	4	Actually veritron fan was not working FSI, FUI, F11 are in series in Varitron control ckt. to be incorporated
7.	Filament Not Through	10	FS41-48 tripped	5	Actually FS-47 was found tripped
8.	Vent Not Through	6	HP Air flow switch faulty	4	K-132, Air flow switch faulty
9.	Aux Not Through	8	FS61-67 & 72 in B. Mod tripped	6	FS-63 was found tripped
10.	BIAS Not Through	7	Change over switch faulty	6	S-70 was faulty
11.	Aux Not Through	5	Relay K201 or Battery supply faulty	5	K-201 was faulty
12.	BIAS Not Through	7	KM62, FS64, FS65, FS66, FS26 tripped	4	FS64 was tripped
13.	BIAS Not Through	7	A-89 control switches tripped	4	KT401 was found tripped
14.	Audio Not Through	4	Input Att. YAC07 in M-Rack faulty	3

Evaluations of field trails revealed that many complicated faults in the transmitter were confirmed by the ES in a very short time. In normal course the operating personnel would have taken much longer time in diagnosing these faults.

However in very few cases of field trails the ES did not confirm the exact fault in the transmitter although it could lead the operator very near to the solution. It was mainly due to the fact that initially the KB of 'XTR300ES' was developed by the transmitter experts using decision trees prepared with the help of transmitter manuals, circuit diagrams and also applying efficient heuristics. But there might be minor difference between actual circuitry of the transmitter and its circuit diagrams. Therefore necessary correction were incorporated in the KB, to suit the existing circuit condition.

Conclusion:

This expert system 'XTR300ES' is very user friendly. It is simple to operate and does not require any special knowledge of computer. Operating personnel including technicians have used this ES and found it very useful. It has also been found very effective in tutoring the technical staff at the transmitting station for acquiring knowledge of the 300kW MW BBC transmitter. This ES has the facility of saving and storing any transmitter fault, thus it can also be used for generating useful database on transmitter faults.

Encouraged by the success of 'XTR300ES', Expert System for other broadcast transmitters like 100kW MW, 50kW SW and 10 kW MW BEL transmitter have also been taken up.