The paper deals with the problem of state observers design for electromechanical control plants with sensorless manipulators in conditions of significant parametric uncertainty. For the estimation of unmeasured state variables, the structure of reduced observers is proposed depending on the set of sensors located on the electrical actuators. Decomposition procedures have been developed for synthesizing piecewise linear control actions of observers that provide a given estimation accuracy. This method does not require additional identification of unknown parameters, since parametrically indefinite differential equations describing the dynamics of unmeasured variables are not used when constructing observers.