The Optimisation of A High Speed Servomechanism
Robert Rayner, 2010
The problem of mechanism design is one of the fundamental, classical engineering problems. The ability to create mechanical devices capable of performing specific tasks and to follow desired motion paths, lies at the heart of the discipline. Numerous different methods of addressing this problem have been developed over the years. The literature shows that extensive work has been carried out over the years to develop methods of optimising mechanism kinematics, generating mechanism designs which can accurately follow given output paths.
When a mechanism is actuated at high speeds, a range of problems are encountered as physical imbalances in the mechanism design are excited, leading to the induction of high frequency harmonic content in its output motion. This harmonic content can greatly limit the maximum operating speed of a mechanism, resulting in loss of motional accuracy and even physical failure of the mechanism. The vast majority of mechanism synthesis methods consider only mechanism kinematics with mechanism dynamics rarely given much consideration, if any at all. The literature indicated that a link exists between the amount of harmonic content present in the output motion of a mechanism and the peak-to-peak magnitude of the variation in drive torque needed to generate that motion.
A prototype servo-mechanical test rig was provided by the project's industrial sponsor company, ITCM Ltd. The test rig possessed a multi-link mechanism, nicknamed the Woodpecker mechanism, which had been observed to have an output motion rich in harmonic content. This mechanism was modelled and used as a test case for development work. A reconfigurable test rig was also designed and built. Using this test rig, it was possible to assemble and actuate a wide variety of mechanism designs without re-fabricating parts.
The work described in this thesis details the development of two contrasting approaches of optimising the performance of servomechanisms for operation at high speeds. The output motions of these mechanisms will possess low levels of harmonic content.
The first method was entitled the Cam Function Generation Method. This method is applicable for use with existing servomechanisms. Using this method, a variable velocity demand signal was generated. This demand signal takes into account and compensates for the dynamic characteristics of the system. The resultant motion was simulated to yield reductions in peak-to-peak magnitude of drive torque magnitude. The method was applied to the prototype test rig. The experimental results confirmed the simulated results.
The second approach tackles the problem of synthesising new mechanism designs taking into account mechanism dynamics and kinematic to generate mechanisms which possess not only good motional accuracy but also good high speed operating characteristics. A mechanism synthesis tool called SWORDS was used to perform the kinematic synthesis whilst a dynamic simulation program entitled DYSIM was used to perform the dynamic analysis using inverse dynamics. The process was applied to the Woodpecker mechanism. A series of three superior alternative mechanism designs were synthesised and a hierarchy of effectiveness between them was identified.
Dynamic models of the reconfigurable test rig, configured in the form of the seed mechanism and the three alternative mechanism designs were created. Inverse dynamic analysis revealed a change in performance hierarchy compared to that identified during the synthesis process. The seed mechanism and the alternative mechanisms were assembled and actuated using a velocity step demand signal to analyse their dynamic qualities. The experimental results verified the simulation results with the same hierarchy of performance being observed. The Cam Function Generation Method was applied to the seed mechanism and the dynamically most superior alternative mechanism design. It was possible to observe that the alternative mechanisms possessed considerably better dynamic characteristics than the seed mechanism. The Cam Function Generation method was applied to the most superior alternative mechanism design. Additional dynamic improvements were observed.used to characterize the surge pressure and volume overshoot across a range of test conditions, the characterized surge pressure being within 6% of the measured values.