CAPE-OPEN UPDATE, Volume 10

CAPE-OPEN UPDATE is a publication of the CAPE-OPEN Laboratories Network (CO-LaN), a non-profit consortium for the development of the CAPE-OPEN standard.

TECHNICAL ARTICLES

The IPOPT Interface to CAPE-OPEN (click here to access full paper)

Yi-dong Lang and Lorenz T. Biegler, Carnegie Mellon University

Computer aided decision making often requires the solution of optimization problems to ensure systematic improvement of objectives. The most general form of this optimization problem is MINLP (Mixed Integer Nonlinear Programming). Efficient and effective solvers are essential for solution of optimization problems. IPOPT is a recently developed and well-tested advanced NLP solver. Moreover, CAPE-OPEN recently issued a standard definition of “what should be done” to interface MINLP or NLP models to solvers. It is expected that prospective commercial vendors will offer CAPE-OPEN compliant modeling systems to implement “how to do it”. This report details the interface that makes IPOPT CAPE-OPEN compliant. That is, IPOPT can now obtain all required information from any CAPE-OPEN compliant MINLP modeling system, solve the problem iteratively and find the optimal solution. As a result, the CAPE-OPEN community can take advantage of IPOPT to solve its own optimization models easier and faster.

This report briefly describes the IPOPT algorithm and focus on the need for calculation of exact first and second derivatives through the CAPE-OPEN standard. This is a key point to fully exploit the performance of IPOPT. Moreover, IPOPT is well encapsulated and has only two interface ports. One connects to MINLPSystem created by MINLPSolverManager. Another connects to the MINLP modeling system. Therefore, as noted in the report, it is relatively easy to make IPOPT CAPE-OPEN compliant. Finally, to demonstrate CO compliance, the CO-Tester is used as a pseudo MINLP modeling system and the wrapper from CO-LaN makes the demonstration straightforward. It is shown that IPOPT successfully solves an NLP problem formulated with the GUI in the CO-Tester, by communicating iteratively with the pseudo MINLP modeling system in the CO-Tester.

The information presented here for wrapping IPOPT within CAPE-OPEN is also useful as a reference for other groups that would like to implement similar tasks with other software packages. Also, we note that while this work is based on MS COM as middleware and the CO-Tester as MINLP vendor, IPOPT remains CAPE-OPEN compliant as long as the methods in IPOPT interface obtain the required information from an MINLP vendor with either MS COM or CORBA as middleware.

Mark Matzopoulos, Process Systems Enterprise (PSE)  

The gPROMS CAPE-OPEN Unit Object (gO:CAPE-OPEN) plug has recently been successfully tested and demonstrated executing a gPROMS model within SimSci Esscor’s PRO/II and other steady-state flowsheeting packages.

gO:CAPE-OPEN is one of the gPROMS Objects, the family of products that enable gPROMS models to execute within a variety of engineering software environments such as Computational Fluid Dynamic packages and the Mathworks MATLAB and Simulink environments. gO:CAPE-OPEN allows a gPROMS model to run inside any CAPE-OPEN compliant process simulator, and has been successfully tested on a large-scale (50,000 equations) rate-based distillation column with Aspen Technology’s Aspen Plus in the past.

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