GB2448350A

GB2448350A - PLL type resolver utilizing transducer excitation signal

Info

Publication number: GB2448350A
Application number: GB0707052A
Authority: GB
Inventors: Lazhar Ben-Brahim; Mohieddine Benammar; Mohd A Alhamadi; Nasser A Al-Emadi; Mohamed Al-Hitmi
Original assignee: Qatar University
Current assignee: Qatar University
Priority date: 2007-04-12
Filing date: 2007-04-12
Publication date: 2008-10-15
Anticipated expiration: 2027-04-12
Also published as: GB0707052D0; GB2448350B

Abstract

The present invention relates to a resolver analog/digital converter apparatus. The resolver apparatus determines an angle C as an estimate of actual angle R , from sin( R ) and cos ( R ) signals. The converter makes use of the excitation signal of the transducer to determine the sine and cosine values. The resolver includes a phase shifter 40 that produces a cos( l t) signal from the excitation sin( l t) signal. A sawtooth generator produces a signal l t from the sin ( l t) signal, and a PPL scheme produces an angle C as an estimate of angle R using sin( R ), cos( R ), sin( l t), cos( l t) and l t signals. Such a PLL closed-loop method does not use voltage controlled oscillator, low pass filter, counter, D/A Converters and lookup tables.

Description

1 2448350 PLL TYPE RESOLVER TO 360 DEGREES LINEAR

CONVERTER APPARATUS

NON-PATENT REFERENCES: 1. Catalog of Admotec "Understanding Resolvers and Resolver-to-Digital Conversion" htto:/Iwww.admotec.comll'T02.Dcff, 1998 PATENT REFERENCES: 1. PLL Type RcsolverfDigital Converter JP60079221-05-07-1985 2. Resolver To Digital Converter USOO5 162798-11-10-1992 3. Compensated High Speed PLL Circuit WO 2006/030335 23-03-2006

BACKGROUND OF THE INVENTION

The present invention relates to R/D converter circuit applied to resolvers or other sinusoidal quadrature encoders. Resolvers are reliable angle transducers especially suited to hostile environments. These devices are widely used in various positioning applications including robots, machine tools, aircrafts, radars and satellite antennas.

Resolvers resemble small electric motors and are essentially rotary transformers designed so the coefficient of coupling between rotor and stator wind ings varies with the shaft angle (see the catalog of Admotec "Undersianding Resolvers and Resolver-to -Digital Conversion" http:llwww.admotec.com/Tf02.pdf, 1998 (nonpatent literature 1)). According to this document, the rotor winding of the resolver is used as primary and is supplied with a sinusoidal excitation voltage, = Aex sinwt (1) As a result the two windings located, at right angles, in the stator produce amplitude-modulated AC signals, one with the sine and the other with the cosine of shaft angle, 9. If the angular velocity (dO /dt) of the rotor is much smaller than w, a condition usually fulfilled because of the relatively elevated excitation or carrier frequency (typically a few kHz), the stator waveforms of the resolver are given by: Vc(t,O)=aA cosOsina,t (2) Vs(t,9)=aAasinOsincot (3) where a is a constant representing the transformation ratio between stator and rotor windings. These signals and modules, shown in FIG. 1, are indispensable elements for the processing of position and/or speed information out of the resolver. The converters used to convert the transducer signals into a measure of the angle are normally referred to as Resolver-to-Digital converters (R/D). The modules shown in FIG. 1 are groundwork for the proposed RID converter device. In this invention, the new resolver converter method takes advantage of the available sinusoidal excitation signal in (I) used to operate the resolver. This excitation signal is optimally used to replace Look-Up-Table (LUT) and associated Voltage Controlled Oscillator (VCO), counter and D/A converters (DACs) used in conventional PLL-type converters. By doing this, the implementation of the proposed resolver-to-converter scheme is greatly simplified when compared to conventional schemes.

CONVENTIONAL PLL METHOD

Commercial resolver converters are built mainly around the successful PLL tracking method (see patent literature 3,2, and 3). This PLL technique is based upon the use of the resolver output signals in a closed ioop arrangement. In this setup an estimated angle, !J', is made to track the shaft angle, 9, as shown in FIG. 2. The technique requires the determination of the sine and cosine of the estimated angle, usually from a LUT addressed with an up/down counter. A Phase Detector (PD), composed of two multipliers the outputs of which are subtracted in order to calculate the error signal sin(9. : sin(9 -= sin Gcos!t' -sin!J'cosG (4) At steady state, the P1 controller guarantees the convergence of the estimated angle towards the true angle, 9 by ensuring sin(94') 9!PO.

In summary, the conventional PLL technique requires: I. a Phase Detector (PD), 2. a Voltage Controlled Oscillator (VCO), 3. an Up-Down counter, 4. Digital-to-Analog Converters (DACs), and 5. a Look-Up-Table (LUT) for the digital measurement of the sine and cosine of the estimated angle.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus, preferably in the form of a resolver to analog or digital converter, for converting the amplitude modulated signals of the transducer into analog or digital signals representative of position and/or speed.

Conventional I'LL methods of R/D converters use LUT for their implementation.

Although the prior art system, PLL, thus described is capable of very high performance both at transient and at steady state, there are a number of aspects that can be improved. The major difficulty with this art is its complex implementation which requires mixed analog and digital circuitry. Furthermore1 the VCO and its associated components have a maximum operating frequency which, when combined with the resolution of the converter, defines a maximum resolver angular speed in which the converter can ensure that the estimated angle (!P) tracks the true angle (p).

In addition, VCOs operate linearly only over a limited range (see patent literature 2).

The objective of this invention is to simplify the PLL implementation by eliminating a number of components used in conventional PLL-type R/D converters. This invention uses the sinewave excitation signal, in equation (1), of the resolver to extract the sine and cosine of the estimated angle (sin( ) and cos( Yb)). This replaces the LUT, the VCO, the DACs and the up-down counter components used in conventional PLL.

Instead, an integrator, a comparator and sample and hold (SF1) components are used to realize the PLL circuit. The advantage of this invention is the simplicity of the method and the efficient use of available components in the resolver excitation circuitry. The added components (comparator, integrator, and SH) are low price devices and are available in the market. Beside this, the converter is simpler to design, tune and implement using either analog or digital circuitry. Thus, the use of the resolver excitation signal simplifies design and reduces the global cost of the resolver-converter device. Importantly, the present invention retains the good precision and performance of the resolver converter based on conventional PLL method.

FIG. 3 is the block diagram of the present invention for the resolver to linear converter. The converter (10) produces a signal that is linearly proportional to the shaft angle of the resolver (20) over 360 degrees. A generator (30) produces the necessary sinusoidal carrier excitation signal, sin(at), to the resolver. The resolver converts the mechanical angle 9 into analog modulated signals VAt, , V(t, 0) having sin(0) and cos(0) components as modulating signals respectively. The output signals V5(t,9) and V(t,0) obtained from the resolver are processed by a demodulator (50) to extract sin(0) and cos(0) signals. The phase shifter (40) produces a cos(*) signal from the sin(a*) signal; alternatively both cos(*) and sin(a*) signals may be generated simultaneously using a sine/cosine oscillator instead of the sinewave generator. These signals (cos(u.*) and sin(a*)) enable determination of cos(!P) and sin(), and hence serve as the analog equivalent of the LUT for the converter.

The converter (10) of FIG. 3 operates as follows. The signals sin($) and cos(0) obtained from the resolver signals after demodulation using (50) are input to the PD (60). The other two inputs of the PD are sin( and cos( values generated by the closed loop system. The error signal sin(O-generated by the PD is input to the P1 controller (70). Through this P1 controller, the negative feedback loop of the converter aims to reduce the error signal sin(0.W) to zero. This ensures that 0-P is small, and is ideally equal to zero. Thus, the measured resolver angle!F is a good approximation of 8. The output of the P1 controller is integrated by (80) to determine the angle F which is an estimate of the true angle 0. Feeding back sin(P) and cos() into the PD needs a LIJT in conventional methods. In this invention the required sin(P) and cos(P) values are read directly from the excitation signal (sin(a*)) generated by (20) and the associated cos(12*) signal provided by the phase shifter (40).

The signals cos(al) and sin(aX) serve as the analog equivalent of LUT for the converter. Receiving the angle!P from the feed-forward path, the extraction block diagram (90) uses two SH devices to read the corresponding values sin(!P) and cos() from the analog signals, sin(wt) and cos(a*), coming from the generator and phase shifter. The closed loop will continue to work through its P1 controller towards a zero error between the estimated angle P and actual angle 9.

FIG. 4 shows a detailed block diagram of the extractor circuit (90). This circuit extracts sin(V') and cos() from the analog signals sin(o*) and cos(a*). A time-dependent angle 0* is derived from a sawtooth generator (94) that is synchronized by the resolver excitation signal sin(0*) provided by the sinewave generator (30). The value of the estimated angle W is compared to the amplitude of the signal 0* using the comparator (92). If the amplitude of a* is equal to the angle t' then the comparator (92) orders two sample and hold circuits (98), using the triggering signal TSH(t, 9), to read the sin(0*) and cos(o*) values which are equal to sin(V') and cos(!P) respectively at the sampling instant.

FIG. 5 shows the sawtooth generator (94) which consists of an integrator (942) that integrates a constant value (943). This integrator is reset by the zero crossing (941) of the sin(o.*) provided by the excitation generator as shown in FIG. 6.

FIG. 6 shows a saw-toothed waveform proportional to o* and synchronized with the zero crossing of sin(o*) signal.

FIG. 7 shows a P1 controller (70) which consists of a proportional regulator with a proportional gain Kp and an integrator with an integral gain Ki. The controller gains are tuned to obtain the best dynamic and steady state performances of the converter.

FIG. 8 shows waveforms of the input angle to the resolver made to change in an arbitrary fashion, the reference signals sin(o*), cos(a*) and o*, the transducer signals sin(9) and cos(. The estimated angle and sin(!P) and cos( values are tracking the true angle 9 and the reference signals sin(9) and cos(9) respectively at the triggering instant generated by TSH(tM. The triggering signal TSH(tM occurs as a result of a perfect match between the estimated angle 1' and the true angle 9. As a result a tracking is realized using the converter in FIG. 3. Note that the sampling at which the updating of the closed loop occurs is related to the frequency of the excitation signal, u=2tf As a consequence the value of this frequency will affect the dynamics of the estimation process; the higher the frequency the better the dynamic performance of the converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. I shows the principle of operation of the resolver with its input (excitation) and output signals FIG. 2 shows a block diagram of conventional PLL converter with VCO, LUT and DACs FIG. 3is a block diagram of the converter described in the present invention FIG. 4 shows the method used for the extraction of sin(') and cos( from the analog signals sin(a*) and cos(z*) FIG. 5 shows the block diagram of the sawtooth generator (94) FIG. 6 shows the inputs/outputs of sawtooth generator FIG. 7 shows the block diagram of the P1 controller FIG. 8 shows waveforms of the converter; the input angle 0 is made to change in an arbitrary fashion.

Claims

1. A PLL based resolver/converter apparatus for determining an angle!F' as an estimate of a true angle, e, from low frequency sin(9) and cos( signals, using the resolver excitation signal sin(o1), where L is the angular frequency of the sine signal and t is time, the converter comprising: A phase shifter that produces a cos(o*) signal from the generated sin(o*) signal; a sawtooth generator that produces a signal at from the generated sin(al) signal; and a closed loop PLL scheme that produces an angle Pas an estimate of the true angle Ousing sin(, cos(), sin(ot), cos(a*) and o* signals.

2. A converter as in claim 1, wherein: The high frequency signal sin(ut) is an excitation signal generated by an external generator. The low frequency signals sin(9) and cos($j are response signals received from an external device in response to the excitation signal and to a mechanical rotation angle 9.

3. A converter as in claim I, wherein the closed ioop PLL scheme comprises: A phase detector that produces a sin(9Y signal from the signals sin($, cos(G), sin(!P) and cos(; and a proportional and integral controller that ensures that the angle tracks the true angle t9, an integrator that integrates the output of the proportional and integral controller to determine the estimate angle; and a sine-cosine extractor circuit that uses the high frequency signals sin(0*) and cos(o.*) to derive the values of sin( and cos( necessary for the operation of the PLL converter.

4. A converter as in claim 3, wherein the sine-cosine extractor circuit comprises: A sawtooth generator that produces the o* signal from the sin(a*) signal, a comparator which compares the value Y' to the amplitude of the signal 0* in order to produce a triggering signal TSH(tM, sample and hold circuits triggered by TSH(t,O) to output the signals sin(P)sin(o*) and cos( V')cos(a*).