John K. Jackman

Modeling recirculating conveyors with blocking

Abstract Recirculating conveyors provide both a delivery mechanism and a buffer for queueing behavior. Although these systems are commonly found in flexible manufacturing systems and assembly systems, accurate analytical models for performance evaluation have not been available. We present a new analytical model for such systems that provides virtually the same information as obtained from simulation models. This model uses processing times and travel times having an exponential distribution and allows for blocking and recirculation of loads and can be extended to large conveying systems with multiple servers.

A simulation-based work order release mechanism for a flexible manufacturing system

A simulation model and real-time interface module for an operational FMS (flexible manufacturing system) facility were developed to evaluate work order release sequences on a real-time basis. Unlike most simulation studies, the evaluation is based on the transient behavior of the system and not steady-state performance. The time window in which a work order is predicted to be completed is considered in order to determine if a particular work order sequence meets due date requirements set forth by the MRP (materials requirement planning) system. In addition, results of the simulation can be used to schedule the building of fixtures and the loading of raw material. The model was successfully interfaced with the real-time control database so that initial conditions could be determined. This information is essential when looking at the transient behavior. Simulation results provide analysts with information to make improvements in the short-term schedule, resulting in better work order release decisions.<<ETX>>

Evaluating assembly sequences for automatic assembly systems

Selecting the assembly sequence and associated automatic assembly stations necessary to assemble a candidate product design is a critical step in the life cycle of a product. We present a method that determines the best sequence and set of stations using cost and performance measures. The method uses a multi-echelon optimization procedure based on simulated annealing. We show how simulated annealing can be used for such problems and present examples in which we determine the optimal assembly sequence for a product in an automated manufacturing environment.

Form Error Estimation Using Spatial Statistics

Form error estimation is an essential step in the assessment of product geometry created through one or more manufacturing processes. We present a new method using spatial statistics to estimate form error. Using large sets of uniform sample points measured from five common machined surfaces, we compare the form error estimates using individual points and fitted surfaces obtained through spatial statistical methods. The results show that spatial statistics can provide more accurate estimates of form error under certain conditions.

Probe orientation for coordinate measuring machine systems using design models

Abstract The rapidly increasing usage of coordinate measuring machines in computer-integrated manufacturing environments underscores the need for a more science-based approach to inspection than the current ad hoc approach now in place. In this context we address one element of the inspection process, namely, the location of sample points. We present a method for evaluating the location of sample points using the workpiece model from the design. Accessibility analysis is used to determine possible interference between the workpiece model and probe model.

Understanding student pathways in context-rich problems

This paper describes the ways that students’ problem-solving behaviors evolve when solving multi-faceted, context-rich problems within a web-based learning environment. During the semester, groups of two or three students worked on five physics problems that required drawing on more than one concept and, hence, could not be readily solved with simple “plug-and-chug” strategies. The problems were presented to students in a data-rich, online problem-based learning environment that tracked which information items were selected by students as they attempted to solve the problem. The students also completed a variety of tasks, like entering an initial qualitative analysis of the problem into an online form. Students were not constrained to complete these tasks in any specific order. As they gained more experience in solving context-rich physics problems, student groups showed some progression towards expert-like behavior as they completed qualitative analysis earlier and were more selective in their perusal of informational resources. However, there was room for more improvement as approximately half of the groups still completed the qualitative analysis task towards the end of the problem-solving process rather than at the beginning of the task when it would have been most useful to their work.

An icon-based approach to system control development

The implementation of an icon-based manufacturing message specification (MMS) messaging system for system control in a manufacturing automation protocol (MAP environment) is described. The system was designed with a four-layer hierarchy. The top layer acts as a user interface and essentially provides a data manipulation service. The next layer invokes the appropriate firmware calls to manage the communication process. The third layer consists of the firmware that handles the actual transmission of data across the network. The lowest level is the actual network hardware. Twelve MMS services were implemented in order to provide the capability to carry out a typical control session. The system was tested using software that simulates the external communications of an MMS virtual manufacturing device. The LabVIEW MAP environment used imposed at least two limitations on the operation of the system. First, LabVIEW makes no provision for data structures; therefore, each parameter had to be passed from the MMS layer to the code VI layer as a variable of a specific data type. Second, LabVIEW provides no convenient way to maintain global variables throughout a diagram. >

A Shannon sampling approach to form error estimation

Abstract Form error estimation is a critical step in verifying that a product satisfies design specifications. It is demonstrated how Shannon sampling can be used to reconstruct surface profiles if they are treated as band-limited signals. To demonstrate its use in form estimation, a Shannon sampling series is applied to flatness error estimation for uniform sample points measured from five common machined surfaces. The results indicate that Shannon sampling provides superior performance over using points directly for estimation.

A graphical methodology for simulating communication networks

A novel approach for simulating communication networks is discussed and illustrated with two examples of a token-passing network. The method is based on a graphical representation of the network consisting of three components: topology, nodes, and protocols. The topology and nodes are drawn to a network specification. The protocols are drawn by using Petri nets with some extended features. A simulation model can be automatically generated from this representation. Results of the simulation are compared to those of an analytical model, showing excellent agreement. >

Feasibility of Automatic Detection of Surface Cracks in Wind Turbine Blades

Cracks on the surface of a wind turbine blade (WTB) can be a sign of current or future damage to the underlying structure depending on the severity of the cracks. We investigated a new method for automatically detecting surface cracks based on image processing techniques. The method was evaluated by varying crack parameters and our method parameters. Identifying and quantifying cracks as small as hair thickness is possible with this technique. Orientation of a crack did not affect the results. The effects of uneven background illumination (present in images captured on-tower) were significantly reduced by optimizing the threshold value for the Canny edge detection method. The accuracy of quantifying a crack was increased by processing an image with both the Sobel and Canny edge detection methods and then combining the results to reduce background noise.