Research On All-digital Resolver-to-digital Conversion

The servo control system is widely used in various industries because of high precision,fast dynamic response and high stability.The servo control system consists of multiple parts,of which the position feedback unit is the important part.The accuracy of the feedback position directly affects the performances of the servo control system.For the detection of position,sensors are mostly used.Among many position sensors,resolvers are applied more and more widely in the field of the position detection of rotating devices due to their advantages of high precision,high anti-interference and simple structure.Because the rotor position can not be got directly from the output of resolver,it is necessary to decode the output signals of resolver,which is called resolver-to-digital conversion.The hardware decoding requires an additional chip,and the cost is relatively high.Therefore,there are many studies on software decoding.Software decoding is also called all-digital resolver-to-digital conversion.Resolver is mainly composed of a stater and a rotor.There is an excitation winding which is supplied by sinusoidal power at the rotor.There are also two output windings whose magnetic axes are orthogonal at the stator.When the rotor rotates,the output windings generate modulated sine and cosine signal.According to the equivalent circuit of the resolver,the expressions of the output signals can be derived.Then the influence of amplitude error,phase error,offset error and harmonic error in the output signal on the calculation of rotor rotation angle is analyzied.In order to obtain rotor position,this paper proposes a geometric approximation method to perform resolver-to-digital conversion on the output signals of the resolver.This method is an open-loop all-digital resolver-to-digital conversion.First,the output signals of resolver are collected by oversampling technology to reduce the quantization error.Then they are demodulated by the frequency shift method.After acquiring the demodulated signals,the rotation angle of rotor is calculated based on the relationship that line length is nearly equal to arc length between two points in the trajectory circle of signals.And a compensation method is proposed to compensate the error of geometric approximation method.In order to verify the feasibility of geometric approximation,a simulation model is built for simulation analysis.In this paper,the closed-loop all-digital resolver-to-digital conversion and the open-loop all-digital resolver-to-digital conversion are simulated respectively.Three operating conditions of resolver including constant speed,uniform acceleration and speed step change are simulated.Through comparing and analyzing the simulation results of the open-loop all-digital resolverto-digital conversion and that of close-loop one,the advantages of geometric approximation method such as high precision and high response speed are demonstrated.Then an experimental platform is built to verify the geometric approximation.The construction of the experimental platform includes hardware design and software design.The hardware mainly includes the control unit,data acquisition circuit and amplifier circuit.The software mainly includes the program of system control and the host computer.According to the requirements of experiment,this paper selects chips of hardware circuits and designs corresponding circuits software programs.After the construction of experimental platform,the accuracy of the geometric approximation method is tested when the rotor of reslover rotates at different constant speeds.In the end,the calculation errors of the open-loop all-digital resolverto-digital conversion at different speeds do not exceed 0.009 rad,which proves the good performances of the geometric approximation method.