THIS issue features some papers, published in revised form, which were originally presented at the IFAC Symposium on Multivariable Systems held in Diisseldorf, Germany in October 1968. However, the issue begins with an editorial by Professor R. Tomovi6 about an approach that should be considered in analyzing complex man-machine control systems, perhaps the only type of multivariable system which was not considered at the DUsseldorf Symposium. The fact that systems involving man and biological controls require special consideration because of their unique features is illustrated in the first paper by Dr. F. A. Roberge which logically follows the editorial, and it is somewhat of an expository paper on Paradoxical Inhibition involving negative feedback in a non-linear manner.
The articles concerning biological systems are followed by a brief report on the Diisseldorf Symposium by P. C. Parks to introduce the papers selected from the symposium. Perhaps one of the outstanding features of the symposium itself was the extremely wide range of systems which were considered in the papers, both in theory and in actual application. In fact, the complex models which were used, and verified by simulation, to represent actual systems may be of interest to some theoreticians who have remarked that realistic models have not been discussed in the literature.
Of course, only a small sample of the papers included in the two preprint volumes can be included in this Journal, but even this small sample indicates the wide variety of topics which were presented. Thus, the first paper by H. Unbehauen concerns control of the live steam temperature in a complex boiler system. Industrial control is also the subject of the next paper by I. D. Gifford and J. R. Leigh in the design of a hot strip steel mill. Another practical control system in the communication field is presented in the following paper, by P. C. Parks and M. R. Miller, and it includes both the theoretical and practical problems of controlling the synchronization of pulse-coded modulation systems in stable modes. A more general paper on stability by S. Partovi and N. E. Nahi follows, but it does not concern any particular system. However, it does indicate a method based on modern stability theory for predicting the stability of practical systems which contain non-linear functions of several state variables.