M Howarth

Industrial application of fuzzy systems: adaptive fuzzy control of solder paste stencil printing

Abstract This paper presents an adaptive fuzzy control algorithm for the control of the solder paste stencil printing stage of surface mount printed circuit board (PCB) assembly. The proposed method of automatic solder paste stencil printing consists of four blocks: fuzzy feature extraction, defect classification of paste deposits, adaptive fuzzy rule-based model identification and subsequently an optimal control action for the stencil printing process. Experimental results are presented to illustrate the capability of the algorithm.

Learning manipulative skills with ART

The research reported in this paper is related to the creation of self-adapting robots that are capable of learning manipulative skills online. The investigation includes the design of a novel neural network controller (NNC), which is based on the adaptive resonance theory (ART) and a dynamic knowledge base, whose knowledge is regulated by specific assembly operations. A force/torque (F/T) sensor was attached to the robots wrist and this was the only information available to the NNC during the assembly operations, since the precise location of the components was unknown. The knowledge is enhanced online, based on the success in predicting the motion that reduces the constraint forces. The results demonstrate the generalisation capability of the NNC by learning the assembly of different part geometries using the same initial knowledge base. The learning time for a complete new operation was achieved in approximately 1 minute.

Experimental design with fuzzy levels

A novel technique is introduced for planning experimental design employing fuzzy rule-based systems. The significant aspect of the proposed experimental design with fuzzy levels (EDFL) is assigning a membership function for each level of variable factors. Consequently, the design matrix and observed responses can be represented in a set of fuzzy rules. Several examples are presented to clarify the proposed idea and the results are compared with the conventional Taguchi methodology. We have specifically planned an L18 orthogonal array EDFL for the solder paste printing stage of surface mount printed circuit board assembly to provide a model for the process and to optimize the selection of variable factors.

Adaptive fuzzy control of solder paste printing: the identification of deposit defects

This paper presents the use of fuzzy logic to implement feature extraction and defect classification of an adaptive fuzzy control algorithm for the control of the solder paste stencil printing stage of surface mount printed circuit board (PCB) assembly. Experimental results are presented to illustrate the capability of the algorithm.

A geometrically validated approach to autonomous robotic assembly

The paper discusses the employment of different sources of information to support robotic assembly operations. During component interaction, part of the wrist sensed force and torque information is found to be geometrically dependent. This enables the real-time sensorial data retrieved from the assembly scene to be combined with the information on the geometry of the component and the history of the insertion itself. As a result, an intelligent control architecture is developed to perform simple peg-hole assembly operations emphasising the aspects which relate to learning an appropriate state-action mapping without requiring an a priori defined set of manipulative skills. A real time peg in hole experiment involving a PUMA 761 industrial manipulator is detailed to support the theoretical results.

An adaptive learning approach to control contact force in assembly

Robotic assembly operations can be performed by specifying an exact model of the operation. However, the uncertainties involved during assembly make it difficult to conceive such a model. In these cases, the use of a connectionist model can be advantageous. In this paper the design of a neural network controller (NNC) based on unsupervised learning is presented. The NNC consists of two stages, adaptation and decision. The first stage based on adaptive resonance theory (ART) classifies and recognises all the contact force patterns, whereas the other stage selects the appropriate arm motion direction. Initial results on the implementation of the NNC, using a 6-DOF PUMA robot with a wrist force/torque (F/T) sensor, demonstrate its ability to learn new or novel contact force patterns fast. If previously learned force patterns are encountered these are accessed directly otherwise memory space is allocated to them without forgetting past events, hence creating a stable system.

Orthogonal fuzzy rule-based systems: selection of optimum rules

In this paper, the concept of orthogonal fuzzy rule-based systems is introduced. Orthogonal rules are an extension to the definition of orthogonal vectors when the vectors are vectors of membership functions in the antecedent part of rules. The number and combination of rules in a fuzzy rule-based system will be optimised by applying orthogonal rules. The number of rules, and subsequently the complexity of the fuzzy rule-based systems, are directly associated with the number of input variables and distinguishable membership functions for each individual input variable. A subset of rules can be used if it is known which subset provides closer behaviour to the case when all rules are used. Orthogonal fuzzy rule-based systems are proposed as a judgment as to whether the optimal rules are selected. The application of orthogonal fuzzy rules becomes essential when fuzzy rule-based systems containing many inputs are used. An illustrative example is presented to create a model for the solder paste printing stage of surface mount tech-nology.

An investigation into the printing characteristics and mechanical dynamics of advanced squeegee mechanisms

The new breed of solder paste deposition systems, such as the DEK ProFlow, incorporate a contained solder paste chamber enabling independent control of machine parameters, pressure, down force and print speed. Previous work by Ekere et al. (1994) and Monaghan et al. (1991-2) have modelled conventional squeegee systems, but only part of this work is applicable to the new mechanism. Practical experimentation has been employed, both to gain an appreciation of the printing characteristics in general and to serve as reference for purely analytical models. Flow visualisation experiments have provided useful indications of flow dynamics within the print head, highlighting dominant flow patterns, velocity profiles and other phenomena such as evidence of air entrapment or cavitation. The FIDAP CFD package is used to perform detailed analysis, providing measures of data not practically obtainable through physical measurement. Pressure, shear and temperature profiles are obtained for a range of operating conditions and their influence on ink/solder paste transfer can be evaluated, leading to the identification of operational limits. Results from the FIDAP model compare very well with the experimental work and have identified the critical areas of the squeegee mechanism. This analysis is now being used to refine the process, to extend its application to lower viscosity materials and to incorporate simpler control systems than have been found to be necessary with conventional squeegee mechanisms (Lofti and Howarth, 1998; Zhuang et al., 1997).

An intelligent closed-loop control of solder paste stencil printing

We have introduced an automated learning closed-loop control scheme for the solder paste stencil printing stage of SMT. It reduces the number of board failures due to inadequate regulation of the printing parameters. Learning is used to compensate for a lack of a priori design information by exploiting empirical information that is gained experimentally.

Production and evaluation of textured polymer sheets as optical pressure-sensing interfaces

Abstract This paper evaluates some polymer materials as pressure-sensing interfaces in a novel optical pressure-mapping device. The device relies on frustrated total internal reflection in a flat glass plate whereby the incident angle is limited such that the light normally cannot escape. However, when a textured polymer interface is pressed against one face the total internal reflection is frustrated by the high optical density polymer and light escapes at the tiny contacts between the glass and the flattened peaks of the polymers textured surface. Viewed through the glass, each contact point becomes a bright spot. The brightness is proportional to the pressure exerted, thus the device converts pressure to brightness. Using a CCD-camera to observe the contact patch, computer image analysis produces coloured pressure-contour maps which identify small regions of abnormally high pressure under a loaded body. Applications at Nottingham Trent University (NTU) include the detection of: (i) actual squeegee pressure during screen printing; (ii) breast-cancer lumps; and (iii) manufacturing defects in remoulded passenger-aircraft tyres. The pressure ranges vary enormously according to the application, with the complication that some loads are dynamic. Hence several interfaces have been developed using different polymers and surface topographies, the experimental results are being reported in this paper.