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This brief proposes a new estimation technique for nonlinear systems subject to dead-zone input nonlinearities. Using the fact that the dead-zone input nonlinearity can be modeled as a line input function with a bounded disturbance term then, by appropriate selection of a system input, we show that the simultaneous estimation of the dead-zone parameters with the unmeasured system states is possible under some mild conditions. The proposed observation can be applied to both symmetric and non-symmetric dead-zone input with a possible extension to time-varying dead-zone characteristics. To show the usefulness of the developed results, two practical examples including a process system and a dc-motor subject to a nonlinear friction are studied.