In this paper, the stiffness of a 3-PSP spatial parallelmanipulator is investigated. Unlike traditional stiffness analysis, themoving platformis assumed to be flexible. Two analytical methods are used in finding the robot stiffness. In the firstmethod, robot ismodeled as lumped systemand principle of virtual work is used. In the second method, the robot is modeled as a distributed system and strain energy of robotmain components as well as Castigliano's theoremare used. Force analysis is also presented and reaction forces at the joints aswell as internal forces/moments are obtained. For each of the main robot components, a matrix called Wrench Compliant Module Jacobian, WCMJ, is introduced. Thesematriceswill allowmapping the applied externalwrench on themoving platform to corresponding reaction forces for the corresponding compliant module. All analysis is presented using invariant form. To evaluate accuracy of the two methods, finite element analysis is used. Finally, using the distributed method, maximum and minimum eigenvalues of the stiffness matrix are obtained and values of kinematic stiffness index are presented.

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