Test Strategies of an Intelligent Instrument

Copyright 6th IFAC Symposium on Cost Oriented Automation Berlin, Germany, 2001

TEST STRATEGIES OF AN INTELLIGENT INSTRUMENT

Philippe JEANJEAN, Dr Jean-Luc NOIZETTE , Dr. Jean-Marc THIRIET

Centre de Recherche en Automatique de Nancy, Universite Henri Poincare - Nancy 1, ESSTIN, 2 rue Jean Lamour F-54519 Vandoeuvre-!es-Nancy cedex Email: {Name}@esstin.uhp-nancyfr

ABSTRACT. Intelligent instruments, sensors or actuators, differ from usual instruments because of a more complex architecture, allowing them to increase significantly their performances, and to integrate a multitude of new services. These evolutions, due to an increasing use of embedded electronics, make manufacture much more difficult. Therefore, the setting of a test strategy becomes necessary, and the purpose is to validate, throughout the product life cycle, the instrument operation. This paper describes the major test strategies adapted to the instrument and provides a methodology to assess each of them in order to make manufacturer's final decision easier. KEYWORDS : intelligent instrument, test strategies, test criteria, product manufacturing, validation.

INTRODUCTION The specificity of an intelligent instrument can be summarized by its internal capacity of processing, associated with communication functions. The implementation of these functionalities requires generally complex electronic components integrated within an electronic card. The goal of this paper is to review the various strategies of test, which allow to validate manufacturing, to guarantee functionalities and to ensure the maintenance of these electronic systems. First, we consider the concept of intelligent instrument, mainly the sensor, followed by a part dedicated to the "electronic test", then we detail the physical internal structure of an intelligent sensor and highlight the elements to be validated. Finally, we present the various strategies of test according to their performance and applicability.

2

DEFINITION OF THE INTELLIGENT SENSOR CONCEPT

The intelligent instruments problematic aims at improving sensors characteristics, by adding "intelligence" or a kind of capacity to adapt the sensor to its environment thanks to both abilities to get a perception of the sensor environment and the

possibility to elaborate a decision as a numerical processing. 2.1 Problematic In order to reply to the following objective: "How exploiting in the best way data collected on a process, these data being representative of both the state of the process and products flows?" the process engineers have worked for the resolution of two problems: how producing CREDIBLE MEASUREMENTS, which give way to a reliable failure origin diagnosis, from pertinent information, without systematically suing the sensor? how sending back this information, in the most versatile way? Progresses in related domains such as microelectronic and micro-computer, which led to the replacement of data analogue processing by digital processing, have given way to developments concerning both intelligent measurement (intelligent sensor aspect) and communication (network aspect). Kleinschmidt proposes a distinction between smart and intelligent, as applied to the so called equipment (Kleinschmidt and Schmidt, 1991)

(Morel, and lung, 1997). Hierarchy

Sensors &

Actuators

Fig. I: "Smart Sensor" and "Intelligent Sensor" gy

In order to clarify the vocabulary, the terminology "smart instrument" (sensor or actuator) refers to a stand alone instrument possessing the functionalities described in the following, while the terminology intelligent equipment refers to the equipment being part of an AS (Automation System) and contributing to its "intelligence" (fig. I).

Fig. 2: CMMS approach The distribution of intelligence in APS through intelligent instruments must first of all call for a standardization of the functions of these instruments, in order to provide them with the ability to cooperate. One means by which this standardization can be achieved is through a modeling of intelligent instruments.

The research work achieved in this field was concentrated on making a census of services to be offered by intelligent instruments and an attempt to reach a functional definition for these instruments. The results of their reflections have been gathered in two books on the subject (Robert, et ai, 1993; Staroswiecki, and Bayart, 1994). At the same time, a reflection on the evolution of automation production systems (APS) had been undertaken. The increasing of industrial competitiveness and the demand of customers for consistent production quality, have led control engineers to reorganize automation production systems. This reorganization has been facilitated due to the services offered by intelligent instruments. Indeed, part of the reorganization was based on the distribution of intelligence within the APS (Automation Production System) and an extensive use of fieldbuses. The intelligent instrument functionalities and services coupled with fieldbuses allow a direct connection of sensors, actuators and controllers, creating decentralized control loops to off load central computers of burden control functions. Automation systems conforming to this type of configuration may be called Distributed Intelligence Base Automation Systems (DIBAS). Distributed intelligence and the use of fie1dbuses have also allowed to envisage a better integration of the three distinct islands of an APS (Automation Production System), i.e.: the Control island the Maintenance island the Technical Management island While in classical APS these islands are managed independently, the use of fieldbuses and intelligent instrument functionalities and services allow a propagation of consistent and coherent information among the three islands, thus tending towards their integration: the CMMS (Control, Maintenance and technical Management Service) concept (fig. 2)

2.2 Functionalities or services ofan intelligent sensor The user needs an "ideal sensor" which possesses the following properties (Robert, et ai, 1993; Thomesse, et ai, 1995): to be credible (Delmas, 1992), that means the sensor must be able to recognize and to signal its actual state (correct or not correct), to supply measurement with a date, to be interoperable: which means the ability for components to be able to co-operate for a particular application. This implies that the same communication standards have to be employed for the components in order to allow the exchange of information. It also means that the two components comply to a common information interpretation (Thomesse, et ai, 1995), to be inter-workable: this characteristic, not easy to differentiate trom the former one, concerns more particularly the equipment as a whole constituted of what is normalized (like the interoperability) and the application processes (resources for the application processes executions), the required service is the one awaited. to be interchangeable: an equipment of brand X can be replaced by an equipment of brand Y without altering the initial schedules, to preserve an ascending compatibility. From that, we may say that, as a complement to the traditional "measure" functionality expected for a normal sensor, an intelligent sensor adds new ones relative to the validation, the configuration and more particularly the communication.

.i.-:!...3 J

3

THE ELECTRONIC TESTS

The electronic test can be applied during the three essential stages of the life cycle of a product: manufacture, exploitation, and maintenance. The deployed test strategies will not take the same form according to their applicability.

strategy also specifies the estimated performances of the test in terms of effectiveness and costs. The test strategies can be divided in three essential categories characterized mainly by the means of test implemented, the performances in terms of detected faults and their field of application. (Tegethoff, and Chen., 1994)

3.1 Test during manufacturing (Vinnakota, 1998)

4.1 Hardware test

The manufacturing test of electronic cards constitutes a major stake for the companies which want to ensure for their products a perfect quality of manufacturing, while reducing the times to market and the manufacturing costs. Two essential approaches are indicated: the fault model-based test whose aim consists in detecting defects at the origin of the failure or the abnormal operation of a subset, the benchmark or specification-based test which allows to check conformity with performances or specifications announced, presented as sales arguments.

Electrical test. These strategies enable to check some electrical parameters of the sensor elements by using traditional external instruments. One distinguishes: in-circuit test, which consists in reaching each equipotential of the electronic system by the intermediary of an interface called "bed-of-nails" (Loveday, 1995). and thus to test individually each component and each track, the flying probe test uses the same philosophy as the in-circuit test, but the access to equipotential is carried out this time by motorized probes, generally two or four, the structural tests, via specific electrical measurements (consumption of the integrated circuits, measurement of the induced magnetic field, etc), validate the internal structure of the components, the boundary scan belongs to the category of the internal test. The principle consists in adding logic designs to a component (digital integrated circuit) in order to reach its pins by means of a scan bus. This technique facilitates the interconnection test between the component and the PCB (Printed Circuit Board).

3.2 Test during exploitation (Robert, et al. 1997) The purpose of this category of test, more commonly called" Technological Validation It, is to validate functional measurement by characterizing the state of correct operation of the sensor. Increasing credibility of measurement resulting from this test is a contribution to the intelligence of the sensor. The validation applies to the material components of the sensor (hardware) and rises from strategies of self-test, self-diagnosis and self-monitoring.

3.3 Test during maintenance The maintenance test is mainly applied to products having known an abnormal operation in the course of exploitation. The goal consists then in detecting the defective element for its repair. This procedure, called "diagnosis", often supplements the traditional tests, whose result does not always enable to locate the defect with precision. The intelligent instruments can support repair by bringing out important information, thanks to their functions of self-diagnosis associated with the concept of history.

4

THE TEST STRATEGIES

A test strategy describes the methods and the means to ensure the checking of the product, taking into account the manufacturer's initial requirements. The

Tests by inspection. First, these strategies consist in examining the card using inspection instruments (i.e., optical instruments), then comparing the observations of all the components with a model of a card considered to be correct (golden card). the automated optical inspection by camera makes possible the detection of non-electrical defects related to the manufacturing process (rupture of tracks, missing component, etc.) the inspection by laser measures the height of the elements of the card and thus detects primarily the absence or the bad orientation of the components, the inspection by x-rays is limited to check the quality of the soldered joints and the tracks of the card. Unlike other techniques, the x-rays method can be used in spite of potential problems of access. 4.2 Software test System functions can also be realized with a software implementation. The program is stored in a memory and carried out by a processor.

In theory, it would be necessary to test the software thanks to the techniques well-known in the dataprocessing field, whose philosophy is very close to the electronic test. Thus, a data-processing test consists in seeking, on the one hand, the programming errors (analogy with the errors of manufacture concerning electronic cards) and, on the other hand, conformity with its specifications (expected performances of the system). The techniques used are various and complete: the marginal test allows to locate defects by carrying out the software in particular conditions supposed to reveal bugs more easily. This technique is gradually replaced by the formal test (formal proof), whose goal is not only to validate the program but also to prove that it is exact. That consists in checking that, being given an initial set of inputs, the program finishes and that the provided results are conform to the specifications (Xanthakis, et ai, 2000) In practice, as an exhaustive test will be very timeconsuming, we will make the assumption that the algorithm is conform to these specifications. Therefore, it will not be useful to validate the operations, but only to check if the version of the memorized program is the expected one. For that, a simple and reliable technique consists in calculating the check number of the memory to be validated. The check number can be calculated by employing a CRC method (Cyclic Redundancy Check). This validation is generally incorporated within a incircuit test.

4.3 Functional test Functional test allows the validation of the system nominal functionality by simulating the interactions of the system with its environment. To implement this strategy, two techniques can be applied. The first one requires complex external test instruments and can be used for any kind of electronic systems. The second one is called BIST (Built-In Self-Test) and belongs to the internal tests category (Rajski, 1997). The purpose of this method is to integrate the elements necessary to an embedded test directly within the component, so as to validate its functionality. The result of the test is represented by a logical state located on the "BIST output" of the component. This test does not require test instruments and concerns only digital integrated circuits.

5

CHOICE OF A STRATEGY

The essential stage of a test development consists in choosing the optimal strategy in relation to the constraints mentioned in the schedule of conditions.

For that, many criteria (Jeanjean and Noizette, 200 I) and must be taken into account: criteria relating to the technologies employed within the sensor, criteria relating to the effectiveness of the test and to the various induced costs. The connection between the strategies, the technological requirements, the performances and the costs constitute the key to obtain a validated product, in adequacy with quality constraints specified in the ISO 900x standard.

5.1 Definition of the criteria Technological criteria. The final choice of the strategy strongly depends on the technological options taken during the design of intelligent instruments. Thus, the tests "Boundary Scan" or BIST are applicable only if the instrument integrates components using respectively "Boundary Scan" or BIST technologies. The other techniques are particularly subjected to the conditions of accessibility to the elements to be tested. Table 1 specifies the compatibility between the strategies of test and the various technological constraints. Table I Technological constraints Test Strategies'

'" E PC .~ c AC 0 u DC ;;; 5l, MC

g AR

f-

F-P

Func

X

Bs

Bist

SITUC

Y

Y

Y

Y

Y

Y

N

N

N

Y

Y

Y

Y

Y

Y

N

N

Y

y

Y

Y

Y

Y

Y

0

0

y

Las AOI

Y

Y

Y

Y

Y

Y

0

0

y

N

N

N

Y

Y

Y

N

N

N

each

electric

N

N

N

mccha

0

'0 ME c .c

I-C

cquipot

VO

vi-

vi-

connector

sual

sua I

-nical

'Strategies: I-C (In-Circuit), F-P (Flying Probe), Func (Functional), Las (Laser), AOI (Optical), X (X Ray), BS (Boundary Scan), Bist (Built in Self Test), Struc( Structural). lTechnological Constraints: PC (Passive component), AC (Activc Component), DC (Digital Component), MC (Mixed Signal Component), ME (Mechanical Element), and AR (Access Required). Results: Y (Yes), N (No), 0 (Optional).

Performances criteria and costs. These selection criteria are not related to an obligation with regard to the design of the product, but rather related to the manufacturer's will to get an optimal test. The ideal solution of test is often defined by the following assertion: "the maximum of defects covered at a tolerable cost". In addition to that, many other essential criteria must be considered:

A test should have the following qualities imperatively: to be able to detect any defect in the product to be tested, to be carried out in a minimum of time, in order not to slow down the whole production line, to be able to locate with precision any defect having been detected, to require the minimum of human and material resources during its development and exploitation. These qualities are represented respectively by the following criteria of performances: fault coverage, test time, diagnosis accuracy, and test cost. Fault coverage is a very theoretical concept supposed to represent the percentage of faults, which the test will detect compared to the potential total number of faults on the circuit to test. For the test incircuit, the concept of faults is rather simple, because it is in direct connection with the components, while in functional test a fault is a very abstract concept, as it is not easily quantifiable. Test time, which depends as much on the strategy used as on the product to be tested, is essential if the test is located on a production line. So, the functional test is known as being slow because of the complexity of measurements to realize. On the other hand, the time of in-circuit test, considered to be very short, will depend only on the number of equipotentials, the components number, the speed of the tester and the associated resources. Diagnosis accuracy will have more or less importance according to costs of product repair and according to its lifespan. Indeed, if the costs of repair are too significant, the rejection of the defective product is immediate and the interest of a diagnosis is no more useful. For a product, which has a relatively long cycle of life, the efforts must be focused on maintenance and a precise diagnosis of the defects becomes necessary. Test costs are divided into many categories (Ambler and Schuhmacher, 1999): development costs, exploitation costs, fixed costs, maintenance costs, recursive costs (which depend on the number of products manufactured), costs in human resources and material resources . The costs strongly depend on the three criteria of performances mentioned above, for instance: to minimize the time of test often requires the use of powerful, but expensive, instruments, to increase the coverage rate requires more complex tests to develop, generating an increase in the development costs, to improve the accuracy of diagnosis facilitates, on the one hand, the repair work and also the costs of maintenance, but requires on the other

hand more complex and consequently more expensive tests.

5.2 Comparison of the various strategies In the following part, a comparison of the various strategies is proposed. Let's emphasize the fact that the following data are coming from various articles and papers. It means that the information has to be taken into account with caution, and give only some indicative values.

Comparison according to previous criteria. Table 2 shows how strategies are more or less adapted to a product according to the criterion that must be optimized. For instance, the costs (initial and development) of an In-Situ strategy are average. In addition, the test time is low and the coverage rate and the diagnosis accuracy are high. But, the development is compelled by the need of electric access. Table 2. Strategies' comparison according to performance and cost criteria Test Strategies'

'"'5E "~

I-C

F-P

Func

I

~

~

CD

~

.,."" .,.

CR

ElT

0

'"-E

DA DC

.,. "" .,. .,. .,."" .,. .,."" .,. .,. "" ~

""

Las AOI

.,. "" "" .,."" ""

~

X

.,.

Bs

Hist

"" ""

~

Struct

'"

~

"" "" "" .,."" "" "" "" .,."" .,. '.,." .,."" .,."" .,. .,. "" '" ""

~

'Strategies: I-C (In-Circuit), F-P (Flying Probe), Func (Functional), Las (Laser), AOI (Optical), X (X Ray), BS (Boundary Scan), Bist (Built in SelfTcst), Struc (Structural). ' Performance criteria: I (Investment), CD (Cost Development), CR (Coverage Rate), lT (Test Time), DA (Diagnosis Accuracy), DC (Development Constraints). .,. (High), "" (Low), ~ (Average), .,.", (dcpends on the product).

Faults detected according to each strategy. Table 3 details, for each strategy, which kind of defect, in particular, is easily or badly detected. Table 3. Kind of faults detected Test Stratc!lies' I-C

'.!o

0

"

S U " Bc '0

F-P

.,. .,.

.,. .,. .,.

~

U Me

.,. .,.

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~

~

Func

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.,. .,. .,. .,. .,. .,. .,. .,. '" .,.'" "" .,. ~

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IStrategies: I-C (In-Circuit), F-P (Flying Probe), Func (Functional), Las (Laser), AOI (Optical), X (X Ray), BS (Boundary Scan), Bist (Built in Self Test), Struc (Structural). 'Performance criteria: 0 (Opens), S (Shorts), Be (Bad element), Me (Missing element), Bo (Bad orientation), Fc (Faulty component), Fs (Faulty system). l' (High), '" (Low), + (Average).

6

CONCLUSION

The intelligent instruments are composed of: some physical parts, such as the transducers, some analogue or digital electronic parts, such as the various sub parts for conditioning, processing and communication, and some software parts. Being able to test these various aspects of the intelligent instruments is a huge challenge. The optimal test strategy is not easy to determine and needs serious compromises between the various test objectives. It is now not so easy because of the variety of technologies used and the integration of these technologies, compared to the testing of electronic cards as it was achieved twenty years ago.

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