4. Stiffness Evaluation: Collinear Stiffness Value Based Approach

Quality control of machines with heavy demands on accuracy and productivity, such as machine tools, coordinate measuring machines, and industrial robots includes stiffness testing as a regular stage [1, 2]. The testing methods are fixed in national and international standards, which quantitatively limit the minimal effective stiffness value. For example, the standard stiffness test for a universal lathe includes measuring radial and axial displacements of the main spindle caused by given active radial and axial force, respectively. For a given pair “force vs. linear displacement” or “torque vs. angle-of-rotation”, the stiffness value has a simple definition as the ratio of the absolute values of the force (torque) to the linear displacement (angle-of-rotation). The same approach presenting stiffness mapping [3] is regularly applied for stiffness control of the parallel kinematics machines (PKMs) such as hexapod-based and tripod-based machines [4–7].