A Linear Ultrasonic Motor Driver Study In An Absolute Gravimeter

Studying the earth gravity field and its variations are of great significance in earthquake prediction,earth science,resources surveying and navigation guidance;the acceleration of gravity g is directly reflected of the earth gravity field information.These years,many technologies had advanced,such as frequency stabilized laser interference measuring technology,high precision digital acquisition and control technology,wide band and high precision vibration measurement technology and so on.As an important instrument of direct access to the gravitational acceleration,high precision absolute gravimeter's home-grown developing demands and applications demands is growing in every field.Linear ultrasonic motor is a new kind of driving mechanism.It will be used in high precision of the absolute gravimeter fall drive control as a new drive scheme.Traditional absolute gravimeters were applying high-precision trajectories of obtaining free fall acceleration to get the gravity value g,so the way of controlling its falling warehouse became a parameter of precision and stability in getting the g value.This also is one of key technologies of an absolute gravimeter developing.The falling driving modes of traditional laser interference absolute gravimeters can be divided into steel belt type,CAM type and bellows type.These types turn the rotary motions of servo motors into rectilinear motions in the vertical direction by drive control rotary motion by mechanical transmission,which will lead some problems in accuracy and size.Driving force of the linear ultrasonic motor(LUSM)is derived from the inverse piezoelectric effect of piezoelectric ceramics,and the LUSM is a structure which can transform microscopic friction movements to macro linear motions,by giving two columns with the voltage which has phase difference and amplitude difference and in the ultrasonic frequency spectrum on two piezoelectric ceramic pieces respectively.LUSM is based on standing wave propagation and particle's elliptical motion,and it is a new kind of device drivers,which commonly be used in precision platform control areas.The LUSM has many advantages,such as it has high precision,output rectilinear motions straightway,can self locked and has a simple mechanical structure,etc.The characteristics of raw materials,manufacturing technologies of ultrasonicmotor and nonlinear,which lead the LUSM driver must have high performance driving signals.Based on the working characteristics of the absolute gravimeter,this paper proposed that a linear ultrasonic motor driving the fall warehouse of an absolute gravimeter.Which applied a Cyclone IV FPGA(field programmable gate array)chip as the core,PC as control and a DDS(direct digital frequency synthesis technology)technology generating two columns of sine wave as a driver of a LUSM.This scheme directly output linear motion,and there is no movement conversing mechanism,which will be helpful to miniaturization the absolute gravimeter and reducing the vibration.After logic design and simulation,we a logic sequence diagram and digital drive signals,which proved the feasibility of the scheme.