EXCELlence in Robot Programming for Coating Applications

Impressive gains have been made in thermal spray coatings through material development and coating parameter selection. Nevertheless, it is usually up to shop personnel to develop a robot program to achieve the final coating. Coating programs are often developed using trial and error. Some motion and deposit analysis programs are available [1], [2]. However, they can be costly and require a level of expertise that is generally not available to the typical coating shop. This paper discusses how Microsoft Excel 2003 software can be used as an aid for off-line development of a robot program for coating applications. It starts with a discussion of the Normal Distribution characteristic of thermal spray patterns. It then goes through development of an example using standard Excel function and chart tools for interactive feedback of the coating layers. Also included is a discussion on the use of the program for sensitivity analysis regarding changes to the spray pattern. Introduction Typical coating specifications provide details on the powder chemistry, powder morphology, powder feed rate, gas flow rates, gun configuration, gun traverse speed and gun stand-off distance. But little if any information is provided to the coating technician regarding motions for the gun. This is understandable for coatings applied manually, using a hand-held gun. For such cases it is up to the operator to establish the motion pattern. With the advent of robotic gun manipulators, the coating engineer can specify the gun motions for optimum results. This paper has two objectives: 1) to illustrate a technique for motion program development using tools available in Excel software and 2) to illustrate the cost-benefits of a well-engineered motion pattern. A sample program is developed to illustrate the technique and the benefits. Procedure Our starting point is to assume a Normal Distribution for the coating section. Values for the Normal Distribution are determined from coating tests. A mathematical representation of the object being coated is developed, a series of potential coating paths defined and trial paths evaluated. A representative section is developed using the Excel Chart Wizard. This allows for evaluation of the coating and determination of compliance to the specification. Characterization of Spray Pattern For the sample case, coating on a plate from multiple passes of the thermal spray gun results in a cross-section of the type illustrated in Fig. 1. The data in this figure is from vacuum plasma coating, but similar distributions have been observed in other thermal spray operations, including atmospheric plasma spray, flame spray and HVOF operations. If the coating follows a Normal Distribution characteristic, the coating thickness (ordinate) can be defined as shown in the following equation: