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| 2006 Tutorials |
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| Sunday, October 15, 2006 1:30 p.m. to 5:30 p.m. Control/Design Interactions in Precision
Machines There are increasing requirements on precision and speed in the industry today. In order to meet the required performance criteria, it is no longer sufficient to close a control loop around a plant. Instead, the dynamics of the machine needs to be modeled well along with the disturbances the machine is subjected to. A controller can then be designed – such a controller is governed by some fundamental constitutive laws and may or may not be capable of delivering the performance required. In case performance requirements are not met, the mechanical and control designers work in an iterative fashion to come with a design that meets specifications – repeatability is used as the figure of merit. This course is targeted towards the practicing mechanical and control designers, and introduces them to the basics of control/design interactions. Modal analysis is introduced to obtain natural frequencies and mode shapes which are then used to create transfer functions to model nominal input/output behavior. Disturbances such as ground vibrations, payload motions etc. are modeled as dynamic systems subject to white noise inputs. The constitutive laws governing a control system design are then introduced. Issues behind collocation and non-collocation of actuators and sensors are discussed, along with design limitations imposed by non-minimum phase plants. These limitations are illustrated through two case studies. One case study is a disk drive actuator design – the various parameters available to the mechanical designer and the limitations on the controls designer are studied. Plant and disturbance models are set up for the disk drive actuator and the effect of design choices on final performance is illustrated through simulations. The other case study is a stage design with a mirror for interferometry – the choice of feedback (encoders on the stage motors vs. laser interferometry) is studied and performance limits illustrated through simulation. In many cases mechanical design problems such as passive vibration isolation can be introduced as control/filtering problems where the design of the controller in effect leads to the design of optimal passive mount (stiffness and damping). Practical techniques to get around some of these limitations using innovative mechanical designs (dynamic vibration absorbers, constrained layer damping) are also presented. Control performance is modeled through frequency response analysis and the repeatability of the precision machine under closed-loop control is derived. Prerequisites: Course attendees need to have a fundamental background in dynamic systems. Most concepts will be developed based on second-order mass-spring damper systems. Frequency responses will be introduced and used as the fundamental tool to develop the concepts in the course, and it will help to have an introductory course in control systems that covers the basics of root-locus techniques and Bode plots.
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