Machine tools feed drives are mostly made of linear motors, ball screws or rack and pinion drives. The drive technology is selected according to the required travel length, dynamics, payload, positioning accuracy, price... For medium and large size heavy duty milling machines, the electronically preloaded rack and pinion drive solution is commonly used, at least in one of the axis which hanaies the longest travel in combination with a high load. As opposed to the linear and ball screw drives, which concentrated most of the attention of the research community, the rack and pinion drives have seldom been studied in the literature. However, the rack and pinion feed drive system have several specificities that should be well understood to properly adjust the axis controller. First, two motors are used to generate the electronic preload that avoids the existing clearance between the pinions and rack, which generates the undesirable backlash effect. The preload level should be adjusted to fulfil the precision requirements without increasing the power consumption unnecessarily. Second, a specific proportional-integral controller is used to maintain the specified preload during the machine movements. Third, both motors have their own current and velocity control loops, but they are generally using a Master-Slave configuration for the position loop. Finally, compared to ball screws or linear drives, there are much more parameters to adjust for the regulation of a rack and pinion drive. The objective of this study is to analyze the influence of the feed drive control parameters on the closed-loop response using measured Frequency Response Functions on a rack and pinion drive. Linear Fractional Transformations are used to simulate the linear components of the multi-input multi-output system. The considered inputs are the both motors torque and the generated external perturbations at the machine tooltip. Meanwhile, the outputs are measured by the rotary encoders of both motors, the linear encoder and an acceleration sensor located at the machine tooltip. Experimental tests are carried out on a heavy duty machine with an electronically preloaded rack and pinion drive. The effect of the electronic preload on the dynamical response is measured. Then, knowing the position and velocity loops control structure, the effect of the feed drive parameter of the controllers can be simulated at the tooltip.
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