The Study On A New Type Of Fast Tool Servo And Its Performance

This thesis presents a new combined fast tool servo developed for cutting optical free-form surfaces in a diamond turning machine.This fast tool servo consists of a long stroke fast tool servo and a micro stroke fast tool servo.It possesses features of high accuracy,long stroke and high bandwidth.We choose the voice coil motor to drive the long stroke fast tool servo which can obtain a stroke of±1mm.And choose the piezoelectric stack to drive the micro stroke fast tool servo which can gain a stroke of 15μm.We add the lever to the FTS which provide the tool enough force to cut the work-piece.By using the high revolution laser interferometer and capacitive sensor enable the FTS measure the linear displacement of the tool.We choose the LA28-22-000A linear voice coil motor with a stroke of±5.72mm, peak force of 266.89N and continuous stall force of 82.74N.The moving coil of the motor has a weight of only 230g.The guide mechanism is made of 00cr17Ni4cu4Nb stainless steel which can separate the magnetic field.This steel with tensile strength of 1262MPa and allowable stress of 688MPa can reduce the disturbance of the external factors.We design a curved beam of rectangular section as the guide mechanism of the long stroke FTS.The flexure mechanism has a factor of safety of 1.5 with respect to endurance limits when displaced±1mm.The stiffness of the flexure is 101.27N/mm in the y direction and 61.2N/mm in the x direction.We strengthen the motor's force twice by a lever to ensure the flexure gain a stroke of±1mm.The lever is made of LY12 aluminum with allowable stress of 240MPA.The allowable maximum rotary angle of the lever is 0.009324 which is larger than the actual rotary angle of 0.007143.In the long stroke FTS,a high accuracy laser interferometer is selected to measure the displacement of the tool and to provide feedback for the controller.This interferometer provide of a range of±1mm, revolution of 10nm and linearity of±2.03%.The PICMA(PI Ceramic Monolithic Actuator) is selected to drive the micro stroke FTS which is manufactured using a cofiring technology.This advanced process allow for multilayer designs which have individual layer thicknesses of just 20 to 100μm.Hence the PICMA actuator requires nominal voltages of only 20 to 120V.It is superior to conventional multilayer actuator in industrial applications and high-endurance situations,where it show substantially longer lifetimes both in static and dynamic operation.The mode we select to drive the micro stroke FTS is P-887.50 with stroke of 15μm±10%,peak force of 1750N and stiffness of 100N/μm.The dimension of the actuator is 7mm×7mm×18mm.We select the angle flexure hinge as the guide mechanism of micro stroke FTS.The angle flexure hinge is design to a symmetric construction which can eliminate the disturbance of micro stroke FTS to long stroke FTS.The stiffness of this flexure is 28.59N/μm in the y direction,and 406.44N/μm in the x direction.We select the SPNS1100 single channel capacitance sensor to measure the stroke of the micro stroke FTS and provide feedback for the controller which is manufactured by QUEENSGATE in English.Employing Queensgates' innovative capacitive displacement measurement technology,the SPNS1100 provides nanometer measurement capability in low cost package.It provides a revolution of 0.0077FS%,and linearity of 0.08%,range of 240μm and bandwidth of 35 KHz.It can provide a revolution of 20nm even the sensor works at 35 KHz.In this thesis,we first make certain that we select large deformation flexure hinge and micro deformation flexure hinge as the guide mechanism.According to the performance requirement of FTS system,we design the entire structure of the combined FTS,and developed its 3D model by CATIA.To machine optical free-form surfaces by diamond turning,the cutting tool must move synchronously with the spindle rotation(the bandwidth of the FTS must be at least 2 times larger than the frequency of the spindle),this demands FTS for extremely high acceleration. Therefore,besides trying to decrease the mass and inertia of every part,we select the moving coil motor to drive the long stroke FTS,and select to piezoelectric stack to drive the micro stroke FTS.We design the lever with aluminum.This thesis introduces the operating principle of the voice coil motor and the piezoelectric stack,and explains some performance parameter of them,then sum up the application of these actuators in FTS.After that,we introduce the operating principle and performance of capacitance sensor and laser interference.The dynamic model of the FTS is formulated which lays the foundation of control system,and get the dynamic parameters by use of Lagrange's equation.Considering the FTS as lumped mass spring system,the dynamic model has been developed.The analytic solution of displacement step response has been given by use of Laplace transform method,and we analyze the change rule of the response when we change the control signal.Then we gain the phase margin and magnitude margin which both are infinite by MATLAB.So the system is stable.This thesis also conducts the structure finite element analysis on the large deformation flexure and the base.Through static analysis,analyzes the performance of the flexure when we change some parameters.The model analysis of the large deformation flexure and the base are carried out,which analyzes the first to the eighth modes of resonance of the flexure and the base,and then reveals the weak parts of those components.The first mode of resonance of the flexure and the base are 54.086Hz and 130.60Hz,respectively,so the bandwidth meets our demand.