Non-minimum phase difficulties prevent the use of input-output feedback linearization. To avoid this problem earlier work proposed the use of synthetic outputs in order to place zeros in prescribed locations under a condition of static equivalence. The objective here is also to introduce a synthetic output but this is defined through the use of perturbations aimed at yielding maximum relative degree. Resetting of the initial conditions allows design freedom for closing the gap between the behaviour of the synthetic and actual output. (c) 2006 Elsevier Ltd. All rights reserved.