An account of chronological developments in control of distributed parameter systems

Control systems arising in many engineering fields are often of distributed parameter type, which are modeled by partial differential equations. Decades of research have lead to a great deal of literature on distributed parameter systems scattered in a wide spectrum.Extensions of popular finite-dimensional techniques to infinite-dimensional systems as well as innovative infinite-dimensional specific control design approaches have been proposed. A comprehensive account of all the developments would probably require several volumes and is perhaps a very difficult task. In this paper, however, an attempt has been made to give a brief yet reasonably representative account of many of these developments in a chronological order. To make it accessible to a wide audience, mathematical descriptions have been completely avoided with the assumption that an interested reader can always find the mathematical details in the relevant references.

A New PWM Control Method for Three-Phase Autosequentially Commutated Current Source Inverters

A new current pulsewidth modulation (PWM) method is presented which uses the principle of creating zero three-phase currents at selected instants of time, through which the load current harmonic content can be controlled along with the magnitude of its fundamental content. This gives rise to reduction of motor torque ripples through the selection of suitable PWM patterns and a fast current control in the inverter by varying the pulsewidths of the PWM pattern. Under this new PWM mode of operation, the autosequentially commutated inverter (ASCI) circuit can be modified easily so that a higher number of pulses can be accomodated within a half-cycle, compared to the normal ASCI circuit. The experimental oscillograms verify the effectiveness of the new PWM method.

Evaluating new insecticides for the control of Helicoverpa spp. (Lepidoptera: Noctuidae) on capsicum and zucchini

The efficacy of insecticides in controlling Helicoverpa spp., predominantly H. armigera (Hubner), on capsicum and zucchini was tested in small plot trials. Indoxacarb, methoxyfenozide, spinosad, emamectin benzoate and novaluron provided control, as measured by the percentage of damaged fruit, equal to or better than standard treatments of methomyl or methomyl alternated with methamidophos on capsicum. The Helicoverpa nucleopolyhedrovirus gave control equivalent to the standard treatment, as did Bacillus thuringiensis aizawai, but B. thuringiensis kurstaki was ineffective. Helicoverpa armigera larvae were present in zucchini flowers but did little damage to the fruit. None of the insecticides significantly reduced the percentage of damaged zucchini fruit compared with the untreated control. Bifenthrin, spinosad, emamectin benzoate and methoxyfenozide were effective in controlling larvae in flowers, while methomyl, B. thuringiensis aizawai, B. thuringiensis kurstaki and novaluron were not effective. Data indicated that all the insecticides effectively controlled larvae of Diaphania indica (Saunders), cucumber moth, in the zucchini flowers. There has been a limited range of insecticides available to manage Helicoverpa spp. in these vegetable crops, but these trials demonstrate the effectiveness of a number of newer insecticides that could be used and that would be compatible with integrated pest management programs in the crops.

Productivity and health effects of anaplasmosis and babesiosis on Bos indicus cattle and their crosses, and the effects of differing intensity of tick control in Australia

Tick fever is an important disease of cattle where Rhipicephalus (Boophilus) microplus acts as a vector for the three causal organisms Babesia bovis, Babesia bigemina and Anaplasma marginale. Bos indicus cattle and their crosses are more resistant to the clinical effects of infection with B. bovis and B. bigemina than are Bos taurus cattle. Resistance is not complete, however, and herds of B. indicus-cross cattle are still at risk of babesiosis in environments where exposure to B. bovis is light in most years but occasionally high. The susceptibility of B. indicus cattle and their crosses to infection with A. marginale is similar to that of B. taurus cattle. In herds of B. indicus cattle and their crosses the infection rate of Babesia spp. and A. marginale is lowered because fewer ticks are likely to attach per day due to reduced numbers of ticks in the field (long-term effect on population, arising from high host resistance) and because a smaller proportion of ticks that do develop to feed on infected cattle will in turn be infected (due to lower parasitaemia). As a consequence, herds of B. indicus cattle are less likely than herds of B. taurus cattle to have high levels of population immunity to babesiosis or anaplasmosis. The effects of acaricide application on the probability of clinical disease due to anaplasmosis and babesiosis are unpredictable and dependent on the prevalence of infection in ticks and in cattle at the time of application. Attempting to manipulate population immunity through the toleration of specific threshold numbers of ticks with the aim of controlling tick fever is not reliable and the justification for acaricide application should be for the control of ticks rather than for tick fever. Vaccination of B. indicus cattle and their crosses is advisable in all areas where ticks exist, although vaccination against B. bigemina is probably not essential in pure B. indicus animals.