In permanent magnet undulators, dimensional variations in the magnet blocks are responsible for displacing the magnets from their ideal positions. These displacements, which can be characterized in terms of the periodicity of the arrangement of the magnets, perturb the undulator field. It is shown that the deviations in the undulator field can be reduced to the desired level by adding two cost factors, based on the uniformity in periodicity, in the cost function used in the simulated annealing technique to determine the placement of the magnet blocks. ©1996 American Institute of Physics.