EP1859226A1

EP1859226A1 - Signal processing and position determining apparatus and methods

Info

Publication number: EP1859226A1
Application number: EP06704955A
Authority: EP
Inventors: Reza Faculty of Eng. & Ind. Scie. HOSEINNEZHAD; Peter Pacifica Group Techn. Pty Ltd. HARDING
Original assignee: Rabit Joint Venture Ltd
Current assignee: Rabit Joint Venture Ltd
Priority date: 2005-03-04
Filing date: 2006-03-03
Publication date: 2007-11-28
Also published as: WO2006092026A1; JP2008532017A; CN101147041A; CA2599693A1; KR20070116850A

Abstract

Signal processing apparatus, e.g. a resolver-to-digital converter (30), converts an analogue input, e.g. sinusoidal signals such as resolver signals Vsin,Vcos, into a digital output, e.g. an angular position and angular speed for a rotor. A first processing unit (31), e.g. a quadrature encoder, monitors the input at a first resolution to produce a first tracking error e1 between the digital output and the analogue input, and a second processing unit (32), which e.g. samples the input signals and e.g. applies a sine cosine function or an arctan function, monitors the input at a second resolution higher than the first resolution to produce a second tracking error e2 between the digital output and analogue input. The apparatus determines the digital output based on the first or second tracking error, e.g. through processing with a filter (33), and includes a switch (34) for switching between the two errors so as to promote tracking stability. Switching may for example be based on a comparison of the first error signal e1 with a threshold value, e.g. a maximum error value.

Description

Signal Processing and Position Determining Apparatus and Methods

This patent application claims priority from Australian Provisional Patent Application No. 2005901050 filed on 4 March 2005, the contents of which are incorporated herein in their entirety by reference.

The present invention relates to apparatus and methods for use in determining the position of a monitored object. It has applicability for example in the detection of the angular position and/or velocity of a shaft, e.g. the rotor of an electric motor, although it has broader application also. The present invention may be especially useful in vehicle control systems, including in control-by-wire systems, such as brake-by-wire systems.

When using an electric motor as an actuator in a control system, e.g. to actuate the calliper of a wheel brake, it can be important to obtain feedback on the position and/or speed of the motor rotor during activation, so that the actuator can be precisely controlled.

Typically, a resolver is used to monitor rotor position. The resolver has itself a rotor that couples with the monitored rotor and includes a coil within it. The resolver also includes a pair of stationary orthogonal coils mounted adjacent the resolver rotor, which couple with the resolver rotor coil to output sine and cosine signals respectively as the rotor coil rotates. As the sine and cosine signals are out of phase, measurement of both amplitudes can provide an absolute determination of rotor angle at any point in time.

The present invention aims to provide novel signal processing and position determining apparatus and methods that in their various embodiments provide a number of advantages and that may for example be used to in association with a resolver or the like to provide a digital position signal.

Viewed from one aspect, the present invention provides a signal processing apparatus for converting an analogue input indicative of the position of a monitored object into a digital output, the digital output tracking the input, the apparatus including a processing unit for: monitoring said input at a first resolution for producing a first tracking error between said digital output and said input; monitoring said input at a second resolution, higher than said first resolution, for producing a second tracking error between said digital output and said input; and determining said digital output based on said first or second tracking error, said processing unit switching between the use of said first and second tracking errors for providing said digital output so as to promote tracking stability.

The present invention may therefore in one embodiment provide a converter for converting the analogue output of a resolver into digital angular position and velocity values. These values may then be used for feedback control or for monitoring purposes. The present invention is not however limited to such systems, and may be applied to many other situations.

The present invention can provide a converter that is stable at high speeds and accelerations, and that is highly accurate. The second error signal may be obtained by sampling the input a number of times per period, so that a position signal of high resolution may be extracted. However, the apparatus also tracks divergence of the output digital signal with respect to the input signal, and when this is too high, e.g. tending to an unstable situation in which angle lock is lost, the apparatus is able to switch to the first error signal which is of a lower resolution and which provides a more discrete output that is less likely to be affected by loss of lock.

Lock might for example be lost due to high rotor speeds or accelerations that the resolver or high-resolution sampler portion of the converter are unable to handle, and/or due to noise, temperature drifts, phase shifts or amplitude unevenness or the like.

In these cases, the apparatus may use the lower resolution signal until the situation stabilises, thereby allowing the apparatus to switch back to the high-resolution tracking.

The input is preferably a sinusoidal input, and preferably consists of a sine and a cosine signal, e.g. as provided by a resolver. The present invention may also however be used to convert other inputs, e.g. a single sine signal or more than two signals or signals having different phase relations, e.g. other than 90 degrees, e.g. between 0 and 180 degrees. For example, if object position sensing is carried out using a synchro, which is similar to a resolver but uses three windings, then the apparatus may receive three sinusoidal signals, each offset from one another by 120 degrees. The present invention may also be used with other signal generators, e.g. with a multipole resolver, an encoder, such as an optical encoder, or with an inductosyn™ transducer. The latter provides the same type of signals as a resolver, but detects linear motion directly and utilises an energised scale made from a printed circuit trace having a regular waveform pattern and a slider that runs along the scale and includes a pair of printed circuit traces parallel to and of the same pitch as the scale pattern, but with one slider trace shifted a quarter of a pitch to the other. The processing unit may include a conditioning unit for conditioning the input signal, e.g. so as to provide a square wave input, for determining the first tracking error. This could for example comprise a Schmidt trigger for each input signal. Conditioning, e.g. the use of square waves, can facilitate a coarse determination of the input signal state, whilst providing robustness against noise and the like.

As the processing unit is only to monitor the analogue signals at a low resolution for the first tracking error, it may for example merely identify the state of the input in terms of the particular half, quarter or the like of the measuring period of the object's movement, e.g. in which of four quarters of a sinusoidal period the rotor is angled. The processing unit may for example monitor, e.g. count, a zero-crossing or rising and/or falling edges of a conditioned, e.g. square wave, input to determine the position of the monitored object.

In a particularly preferred embodiment, the processing unit includes a quadrature encoder. A quadrature encoder determines one of four positions of the monitored object based on a pair of square waves separated in phase by 90 degrees. The encoder determines the "1" or "0" state of the two waves to determine which of four positions in a repeating cycle the object is in, e.g. in which 90 degree sector of a rotation a rotor is in.

A quadrature encoder is a particularly robust form of measurement that gives the overall tracking converter stability where it might otherwise lose lock.

In order to provide the first error signal, the digital output of the apparatus may be subtracted from the quadrature encoder output.

In one embodiment, the processing unit monitors the analogue input with a high degree of resolution by determining the object position based on an inverse tangent function. Thus, a quotient of sine and cosine signals from a resolver may be subject to an inverse tangent function in order to obtain the positional information, e.g. the angle of rotation of a motor rotor. The result may then be compared with the current digital output of the apparatus in order to provide the tracking error.

In a particularly preferred form of the present invention, the processing unit receives sine and cosine input signals from the monitored object, e.g. from a resolver, and determines the second tracking error according to:

sin(ø - θ) = sin0 cos <9 - cos<9sin θ (1 )

where θ is the object position, e.g. rotor angle, and θ is the digital output.

Switching between the two tracking errors may be carried out in any suitable manner so as to promote stability to the overall converter.

In one preferred embodiment, the processing unit switches from the second to the first error signal based on the first error signal, e.g. when the first error signal is above a threshold value. The threshold value may be e.g. 90 degrees, and is intended to prevent the error from exceeding the maximum error value of the first error signal, e.g. pi/2 for a quadrature encoder as the first processing unit. Thus, the error signal is prevented from becoming unstable.

Other switching regimes may also be used. For example, the apparatus may compare the second error signal against a threshold value, or an absolute difference between the outputs of the two processing units. It may use a combination of the two error signals and the current digital output to determine whether or not to switch. It may monitor the rate of change of the error signals.

The processing of the first or second tracking error in order to provide the digital output may take any suitable form, and may utilise a suitable filter element. The filter element may be configured to provide both a position and velocity output. In one preferred form, it may provide a type-2 servo loop, and may include two integrating elements.

It may include an integrator for integrating the supplied error signal to produce an error signal that can act on a tracking counter to increment or decrement the digital output signal. A suitable transfer function would be a Pl observer cascaded with an integrator, which can remove steady-state output error in response to constant ramp inputs. The transfer function might be for example of the form G(s) = Ki (K₂+ 1/s). Other transfer functions are also possible, including e.g. an extended Kalman filter or a Luenberger filter. The transfer function may for example take the form: G(s) = (K₁S² + K₂S + K₃)/s².

As said, the monitored object may be a shaft, and the digital output may be an angular position of the shaft, or the monitored object may be a slider and the digital output will provide a linear movement of the slider along a scale. Velocities may also be output.

As also said, the signal generator/object monitor may be a resolver, a synchro or a linear encoder, e.g. an inductosyn™ transducer. It might also be an optical encoder.

The present invention may be used in many different situations. For example, it may be used in the feedback control of a motor, e.g. an electric or hydraulic motor, or in the sensing of a position, e.g. as an angle or position sensor. Applications include vehicle management and control systems, e.g. avionic, naval or automobile controls, and may be used in control-by-wire systems, e.g. brake control systems, steering systems or gear-changing systems. It may also be used in robotics or in machinery control, e.g. the numeric control of machine tools.

The present invention is particularly useful in safety critical situations, as the present invention facilitates the construction of a stable and robust position monitor. The present invention extends to an electric motor control including apparatus according to any of the above embodiments.

The present invention also extends to a vehicle control system including apparatus according to any of the above embodiments.

The present invention further extends to a method of converting an analogue input indicative of the position of a monitored object into a digital positional output, the digital output tracking the input, the method including: monitoring the input at a first resolution to produce a first tracking error between the digital output and the input; monitoring the input at a second resolution higher than the first resolution to produce a second tracking error between the digital output and the input; determining the digital output based on the first or second tracking error; and switching between the first and second tracking errors so as to promote tracking stability.

The present invention also extends to software for controlling a digital signal processor to convert an analogue input indicative of the position of a monitored object into a digital positional output, the digital output tracking the input, the software including: a component for monitoring the input at a first resolution to produce a first tracking error between the digital output and the input; a component for monitoring the input at a second resolution higher than the first resolution to produce a second tracking error between the digital output and the input; and a determination component for determining said digital output based on said first or second tracking error, said determination component switching between said first and second tracking errors so as to promote tracking stability.

The software may be provided on any suitable storage media, and the present invention extends to storage media having the software thereon. It also extends to hardware and firmware in which the software code is embedded, and to digital signal processor chips incorporating code for carrying out the present invention, e.g. a DSP chip.

The present invention may also be implemented electronically, without the use of software. For example, the switches, filters, integrators, signal multiplications and additions, and the like, may all be implemented using resistors, capacitors, operational amplifiers, and the like. Application Specific

Integrated Circuits (ASICs), Programmable Logic Devices (PLDs), and

Programmable Gate Arrays (PGAs) may also be used. The resulting electronics may be a hybrid system, using both digital and analogue components.

Viewed from a further aspect, the present invention provides signal processing apparatus for providing a digital angle output based upon an input signal indicative of the sensed position of a monitored object, the digital output of the apparatus tracking the input signal and the apparatus including switching means for causing the digital angle output to be based either on the output of a quadrature encoder or the output of a sampling means that samples the input signal. Viewed from another aspect, the present invention provides a resolver- to-digital converter for providing a digital angle from the analogue sinusoidal output of a resolver, the converter including a quadrature encoder for determining a first angle and for providing a first error signal based on the first angle and the converter output, an observer for sampling the resolver output and for providing a second error signal based on the sampled resolver output and the converter output, a filter for determining the converter output based on the first or second error signal, and a switch for switching the filter input from the second error signal to the first error signal when it is determined that the converter is losing lock on the resolver output. Viewed from a further aspect, the present invention provides an angle tracking observer for monitoring the output signals of a resolver and for providing a digital rotor angle of the resolver, wherein the observer includes a sampler for sampling the resolver output signals and a quadrature encoder for monitoring the resolver output signals, and wherein the observer determines a digital angle for the rotor based on an output of the quadrature encoder rather than the sampler when the digital angle diverges from the rotor angle indicated by the resolver output.

Viewed from a still further aspect, the present invention provides signal processing apparatus for receiving a sinusoidal input indicative of the position of a monitored object and for providing an output signal indicative of a position of the monitored object, the apparatus including: a first error determining component for receiving the sinusoidal input and for determining a first positional signal for the object at a first resolution, for receiving the apparatus output, and for determining a first error signal based on the first positional signal and the apparatus output; a second error determining component for receiving the sinusoidal input and for determining a second positional signal for the object at a second resolution higher than the first resolution, for receiving the apparatus output, and for determining a second error signal based on the second positional signal and the apparatus output; a positional determining component for providing the output signal based on the first or second error function; and a switching component for providing the first or second error signal to the positional determining component, said switching component switching to the first error signal when the output signal is tending to diverge.

Viewed from another aspect, the present invention provides signal processing apparatus for converting an analogue input indicative of the position of a monitored object into a digital positional output, the digital output tracking the input, the apparatus including: a first processing unit for monitoring said input at a first resolution and for producing a first tracking error between said digital output and said input; a second processing unit for monitoring said input at a second resolution higher than said first resolution and for producing a second tracking error between said digital output and said input; a third processing unit for determining said digital output based on said first or second tracking error; and a switching unit for supplying said first or second tracking error to said third processing unit, said switching unit switching between said first and second tracking errors so as to promote tracking stability.

It should be noted that any one of the aspects mentioned above may include any of the features mentioned in relation to any of the other aspects mentioned above, as appropriate. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. It is to be understood that the particularity of the drawings does not supersede the generality of the preceding description of the invention, and that the invention may take other forms to those described by the drawings. In the drawings:

Figure 1 is a schematic diagram of a motor control system to which processing apparatus in accordance with an embodiment of the present invention may be applied;

Figure 2 is a schematic diagram of a resolver; Figure 3 is a schematic block diagram of processing apparatus in accordance with an embodiment of the present invention;

Figure 4 is a schematic block diagram of one type of processing apparatus that embodies the structure of the Fig. 3 apparatus; Figure 5 is a diagram of one method of implementing the apparatus of

Fig. 4;

Figure 6 is a schematic block diagram of another processing apparatus embodying the structure of the Fig. 3 apparatus;

Figures 7a to 7d are graphs of simulation results for a first input signal scenario; and

Figures 8a to 8d are graphs of simulation results for a second input scenario.

Referring to Fig. 1 , a motor control system 1 includes an electric motor 2 having a shaft 3. The motor 2 is controlled by a controller 4, which receives command signals C and energises the motor 2 at a suitable voltage and for a suitable time so as to provide a commanded action.

Such systems are used in many different applications and environments. They may for example be used to provide actuators in vehicle control systems, and may be used in vehicle braking systems. Thus, in one embodiment, the motor control system may be used in a calliper brake assembly of a brake-by-wire system, with the electric motor 2 activating a pair of brake pads 5 to grip a brake disc 6. The shaft 3 may for example engage the pads 5 via reduction gearing and a rotational-to-rectilinear motion mechanism, e.g. a ballscrew. In this braking embodiment, the controller 4 receives command signals from a remote electronic control unit 7 of a vehicle management system, and energises the motor 2 at a suitable voltage and for a suitable time so as to provide the commanded braking action.

The electronic control unit (ECU) 7 will issue brake commands in accordance with driver indications and driving conditions. Thus, it receives signals from various sensors 8 relating to the vehicle state and various sensors

9 relating to the driver's intentions, and outputs command signals to the various brake assemblies 1 of the vehicle accordingly. The vehicle sensors 8 may include wheel speed sensors, accelerometers, engine sensors, yaw sensors, exhaust gas sensors, and the like, whilst driver indicator/demand sensors (human machine interaction (HMI) sensors) may include brake or throttle pedal sensors and steering wheel sensors. For example, a brake pedal sensor may provide a signal proportional to the driver's demand for braking, e.g. as determined by the amount of movement of the pedal or the force exerted by the driver's foot on the pedal.

The ECU 7 may control vehicle braking in accordance with a number of different braking regimes. It may merely follow a driver's indications or may provide some intervention, e.g. to provide anti-lock braking (ABS), traction control (TC), vehicle stability, or panic braking control.

The electronic control unit 7 and brake calliper assembly 1 may communicate in any suitable manner, e.g. they may be individually connected together, and/or may connect together via one or more communications buses. In order to ensure that the motor 2 is operated accurately, the controller 4 controls the motor 2 in a feedback manner, and the motor 2 includes a resolver 11 on its shaft 3 to provide feedback signals 12 indicative of the angular position θ and angular speed ω of the motor shaft 3. The controller 4 can then alter the energisation of the motor 2 to ensure that the commanded action, e.g. a braking action, is properly achieved. A converter 13, e.g. in the controller 4, coverts the analogue output of the resolver 11 into a digital input for the controller 4.

The resolver 11 is shown schematically in Fig. 2. It comprises a rotor 20 having a coil 21 therein, and a pair of stator coils 22 and 23 that are orthogonal to one another and that couple with the rotor coil 21. Energisation of the rotor coil 20 with an AC signal V_ref produces a signal of the same frequency in each of the stator coils 22 and 23, but the strength of the coupling changes in a sinusoidal manner as the rotor 20 rotates, so that the output signal from each stator coil 22,23 is amplitude-modulated with a sinusoidal envelope. As the stator coils are orthogonal to one another, the envelopes of the two stator coils are separate from one another by 90 degrees, so that the stator coil 22 produces a sine signal envelope with respect to rotor position and the stator coil 23 produces a cosine signal envelope.

Accordingly, the resolver 11 provides a sin(θ) signal V_3Jn and a cos(θ) signal V_COs, and, by measuring the two signals at any point in time, allows the absolute angular position θ of the rotor to be obtained. The rate of change of rotor angle θ can also be determined from the two signals so as to provide the rotor's angular velocity ω.

The present invention relates to the processing of position signals, such as those provided by the resolver 11 , and the conversion of analogue position signals to digital signals. It can thus be applied to the converter 13 of a motor control system to provide a digital rotary angle θ and a digital angular speed ώ from the analogue inputs 12, although the invention has broader application also to any appropriate analogue to digital processing of position signals, e.g. in the fields of position sensors and actuators in general.

The general structure of one embodiment of the present invention is shown in Fig.3, in the form of a resolver-to-digital converter 30 that determines a digital angular position θ and a digital angular speed ώ from the resolver analogue sinusoidal input of V_Si_n and V_cos by tracking the digital output to the analogue input. It can thus be termed an angle-tracking observer.

The converter 30 includes a first processing unit 31 that monitors the resolver outputs with a first (low) resolution and produces a first tracking error ei between the actual rotor angle θ and the output digital angle θ . It also includes a second processing unit 32 that monitors the resolver outputs with a second (higher) resolution and produces a second tracking error e₂ between the actual rotor angle θ and the output digital angle θ .

A filter 33 receives the first or second error signal ei or e₂ and is configured to provide a suitable transfer function so that a digital rotor angle θ and/or a digital rotor angular speed ώ can be derived from the error signal. These digital signals may then be used in the feedback control of the motor 2 as provided by the controller 4.

A switch 34 connects either the first or second error signal ei or e₂ with the filter 33, and is activated by a switch control 35 that switches between the two error signals ei and e₂ so as to ensure that the output of the converter remains stable. Thus, the switch control 35 may receive either or both of the digital outputs, and either or both of the tracking errors, and determines from these whether the digital output is losing its lock and whether the output is tending to drift or diverge from the input. Generally, the switch control 35 supplies the filter 33 with the second tracking error β₂ so as to provide a high-resolution determination of rotor angle and speed. However, when it is determined that the digital output is losing lock, the switch control 35 supplies the filter 33 with the lower resolution tracking error e-_\ . This allows the converter output to remain stable, as the lower resolution processing unit will be less affected by noise, sensor drift and the like, and so will provide a more robust output than the second higher resolution processing unit.

The first processing unit may for example determine position by monitoring discrete events of the input signal or a processed or shaped version of the input signal, e.g. zero crossings or edge events, such as leading or trailing edges of a waveform, whilst the second processing unit may monitor the input signals by sampling them at a number of points in their cycles, e.g. including at a number of points between the discrete events. A more specific example of the apparatus of Fig. 3 is shown in the converter 40 of Fig. 4. In this case, the first processing unit includes a quadrature encoder 41a. This encoder receives the pair of resolver outputs and conditions them into a pair of square waves that are separated in phase by 90 degrees. The encoder 41a then determines the position of the motor shaft 3 as being within one of four quadrants of a complete period of rotation depending on the amplitude on each input signal channel, e.g. by monitoring the rising and/or falling edges of the square wave pulses.

Thus, the pair of square waves can have amplitude pairs "1 ,1", "1 ,0", "0,1", or "0,0", depending on the quadrant in which the rotor angle lies. The encoder 41a thus determines the amplitude pair and from this can determine the rotor angle within pi/2 (and also the direction of rotor motion by monitoring the sequence in which the amplitude pairs are received). The encoder 41a outputs the detected rotor angle quadrant, and a comparator 41b subtracts the current digital output angle θ from this to provide the first tracking error signal G₁.

Thus, the first tracking error is based on: - θ (2)

where N_qUad is the quadrant number, and [.] means round downwards to the nearest integer.

The second processing unit includes a pair of multipliers 42a and 42b that multiply the two resolver input signals V_Sj_n and V_cos respectively by the cosine and sine of the digital output angle θ , as determined by cos and sin generators 42c and 42d. The results of the two multipliers are then sent to a comparator 42e to provide the second tracking error e₂.

Thus, the second tracking error e₂ is obtained from the trigonometric identity: θ - θ J= sin θ cos θ - cos θ sin θ (3)

It will be noted that when sin(6> - θ) is small, sin(# - #)« θ - θ , and so, as sin(<9 - θ)→ 0 , then Θ -Θ → O also. Furthermore, the closed-loop system will be almost linear for small errors.

The converter 40 also includes filter elements 43a and 43b that take one or the other of the error signals e-i or e₂ and provide the digital angle position θ and the digital angular speed ώ for the rotor. The filter elements include a Pl observer 43a cascaded with an integrator 43b, which is able to remove the steady-state output error in response to constant ramp inputs. The transfer function G(s) may be for example:

G(s) = Ki(K₂ + 1/s) (4)

Other transfer functions are also possible however, e.g. an extended

Kalman filter or a Luenberger observer. It may for example be:

G(S) = (K₁S² H- K₂S H- K₃Vs² (5) • The switching between the two error signals ei and e₂ is determined in the present example by the absolute value of the error signal ei from the quadrature processing unit 41 a,41 b. Thus, block 45a takes the absolute value of the error signal ei and this is compared with a threshold value M in a comparator 45b. The result is then passed to block 45c, which determines the sign of the result, and outputs a "0" if the comparator output is less than zero, and a "1" otherwise. The switch 44 passes the error signal e-_\ if it receives a control signal of "1" and passes the error signal β2 if it receives a control signal of "0". Accordingly, tracking will normally occur using a high-resolution system formed by the second processing unit and the filter, so as to provide an accurate monitoring of the resolver rotor 20 and so the motor shaft 2. However, when the error signal between the analogue input and the digital output (as determined by the quadrature encoder) is greater than a set value M (e.g. 90 degrees), it is determined that the output is losing lock and to counter this, tracking is continued using the quadrature encoder. As soon as lock returns, the converter resumes tracking based on the high resolution processing unit. Thus, the converter output is a hybrid of the quadrature and sampling systems. The second processing unit provides a higher resolution tracking of the motor shaft angle θ than does the first processing unit. The first processing unit however is more robust and stable than the second processing unit, and, by combining the two, the present embodiment allows for a converter that is overall both accurate and stable in its tracking, thereby enabling good control of the motor 2 by the controller 6. The switching unit 44 need not only be operated based on a comparison of the first tracking error e-_\ with a threshold value, and any other suitable monitoring of the output signals may occur to determine stability and divergence of the output. For example, the second tracking error or an absolute difference between the two errors may be compared with a threshold. Fig. 5 shows a similar converter 50 to that of Fig. 4, but showing various hardware components. In this embodiment, a pair of multiplying DACs 52a and

52b multiply the analogue inputs V_Sm and V_cos by digitally generated sin(#) and cosψj functions respectively and provide their outputs to a summing amplifier 52c, the output of which passes through a demodulator 52d to provide the error signal e₂.

The error signal e₂ passes through the switch 54 to a digital filter 53a that provides a transfer function such as in (4) or (5) above, and the output of this filter passes to an integrator 53b (which removes lag error associated with the motor shaft 3 rotating at a constant angular velocity). The output of the integrator 53b passes to a voltage-controlled oscillator 53c to generate a constant frequency to track the input. This controls an up-down counter 53d to evaluate polarity and to count up for forward rotation and down for reverse rotation (and thereby provide a second integrator).

The quadrature encoder 51a includes a pair of Schmidt triggers 51b to provide a pair of square waves, and a counter 51c that determines the rotor angle with an accuracy of pi/2 from the two square wave input channels, and passes its output to a summing amplifier 51 d to provide the error signal ei by subtraction of the digital error signal θ . This error signal ei is also used to control the switch 54, so that when the error signal ei exceeds a certain threshold, the switch 54 passes the error signal ei rather than the error signal e₂.

As can be seen, the operation of the converter 50 is similar to that of a phase-locked loop, which generates a local frequency and adjusts it to track an input signal frequency.

Figure 6 shows another embodiment of the present invention, in which a converter 60 includes a quadrature converter 61a and comparator 61 b in the same manner as in Fig. 4, but in which the high resolution processing is carried out by a block 62a that applies an inverse tangent function to a quotient of the sine and cosine input signals. Thus, block 62a outputs the rotor angle θ in accordance with:

(6) where arctan2 is a four-quarter arctangent function. This signal is output to comparator 62b, so that the switch 64 passes either the error signal ei from the quadrature encoder or the error signal e₂ from the arctan encoder to the filter elements 63a and 63b to thereby determine a digital angular position and angular velocity for the motor shaft 3. In all of the above cases, the converter provides a stable and accurate digital output that faithfully tracks the signal input, thereby ensuring accurate feedback control.

Two simulations are now described with reference to Figs. 7a-7d and Figs. 8a-8d.

In the first case of Figs. 7a-7d, the real angular position varies with a high constant acceleration of α=500 rad/sec², whilst in the second case of Figs. 8a- 8d, the real angular position variation has a sinusoidal pattern given by θ(t)=200πsin(0.4πt), with angular acceleration also reaching a maximum of 500 rad/sec². In both cases the simulations were run for 80 seconds, and, for noise effect, a white noise bounded within [-0.1 ,0.1] was added to the pure sine and cosine signals of the simulated resolver output.

Figures 7a,7b and 8a,8b graph the outputs of known observers, whilst Figures 7c,7d and 8c,8d graph the output of an observer in accordance with the form of the Fig. 4 embodiment when using a transfer function of:

2S,' + 211. + 915 ₍₇₎

S

The two known types of observer were (i) a basic resolver converter of the form of Fig. 4 and with the transfer function (7), but without the quadrature encoder or the switching unit, and (ii) a Kalman filtering observer.

In the first case, both the basic resolver converter and Kalman filter are unstable, as seen in Figs. 7a and 7b, whereas, as seen in Fig. 7c, the estimator in accordance with the Fig.4 form is stable and its estimation error converges to zero following a transient response (Fig. 7c is plotted only over [0,5] seconds for clarity).

Fig. 7d shows the switching mode, in which a binary "zero" is when the high resolution sampler is used, and a binary "one" is when the quadrature encoder count is used to tune the output.

In the second case, again, both the basic resolver converter and the Kalman filter were unstable, as seen in Figs. 8a and 8b, whereas the position estimation error of the present Fig. 4 embodiment is again stable, as shown in Fig. 8c (which is plotted only over [0,10] seconds for clarity). In this case, although the estimation error does not asymptotically approach zero, it is bounded between ± 2 degrees.

The switching mode is shown in Fig. 8d, in a similar manner to that of Fig. 7d, although, for clarity, the plot is shown only between [9.4,9.5] seconds. Although the above discussion has focussed on the motor control of a calliper brake, it will be appreciated that the present invention has applicability to many other systems, including for example other vehicle control systems, such as steering-by-wire control systems and by-wire gear change control systems. It will be appreciated that the present invention also has broad applicability outside of vehicle control and motor control, and may relate to systems in general that require accurate conversion of analogue positional information into digital information. It is especially useful in safety critical situations, as converters in accordance with the present invention facilitate accurate and stable digital outputs.

The present invention may be used with actuators, such as electric or hydraulic motors, so as to provide feedback control. It may also be used with sensors, e.g. angular sensors, so as to provide positional information with regard to a monitored object. The present invention may for example be used in vehicle control systems, e.g. aerospace control systems or automobile control systems. It may also be used in e.g. robotics, military systems and in the control of machinery in general, e.g. in the control of numerically controlled machine tools.

As well as being used with the output of a resolver, the present invention may also be applied to the output of other sensors, including for example a synchro, which is similar to a resolver, but has three windings. Both resolvers and sychros provide rotational information, but may also provide information as to linear motion e.g. through the use of a ballscrew connection. Further, the present invention may be used with direct linear motion sensors, e.g. an inductosyn™ device, which includes a scale made from a printed circuit trace which is energised with a reference voltage, and a slider which runs along the scale, and has a pair of traces of the same pitch as the scale, but with one trace separated from the other by a 90 degree shift. The output of the two slider scales is then similar to that of a resolver. The present invention may also be used with encoders, e.g. optical sinusoidal encoders that shine light through optical gratings on a disc.

Although the described embodiments show switching of error signals from a first pair of processing units to a third processing unit, the system can have other equivalent structures, e.g. in which positional signals are switched and errors are determined based on the switched positional signals.

The present invention may be used with many types of analogue input, and for example although preferable, the signals do not need to be sinusoidal, and could instead be any suitable signal for allowing positional information to be resolved. Nor do the input signals need to be 90 degrees out of phase, and other phase relations may also be used.

The present invention may be implemented in any suitable manner, e.g. as hardware elements, encoded within a digital signal processor (DSP) chip, and/or provided in software. It is to be understood that various alterations, additions and/or modifications may be made to the parts previously described without departing from the ambit of the present invention, and that, in the light of the above teachings, the present invention may be implemented in a variety of manners in software, firmware and/or hardware, as would be understood by the skilled person.

Claims

1. Signal processing apparatus for converting an analogue input indicative of the position of a monitored object into a digital positional output, the digital output tracking the sinusoidal input, the apparatus including a processing unit for: monitoring said input at a first resolution for producing a first tracking error between said digital output and said input; monitoring said input at a second resolution, higher than said first resolution, for producing a second tracking error between said digital output and said input; and determining said digital output based on said first or second tracking error, said processing unit switching between the use of said first and second tracking errors for providing said digital output so as to promote tracking stability.

2. The apparatus of claim 1 , wherein said input is a sinusoidal input.

3. The apparatus of claim 1 or 2, wherein said input includes a sine and a cosine signal.

4. The apparatus of claim 1 , 2 or 3, wherein said processing unit monitors at said first resolution by monitoring discrete events of the input, and monitors at said second resolution by sampling the input.

5. The apparatus of any preceding claim, wherein said processing unit includes a conditioning unit for conditioning said input so as to provide a square wave input for monitoring at said first resolution.

6. The apparatus of claim 5, wherein said processing unit monitors rising and/or falling edges of said square wave input to monitor the position of said object.

7. The apparatus of any preceding claim, wherein said processing unit includes a quadrature encoder for determining said first positional signal based on a pair of phase separated input signals.

8. The apparatus of any preceding claim, wherein said processing unit monitors at said second resolution based on an inverse tangent function of a pair of phase-separated sinusoidal input signals from said monitored object.

9. The apparatus of any one of claims 1 to 7, wherein said processing unit receives sine and cosine input signals from said monitored object, and determines said second tracking error according to:

sin θ

where θ is the object position, and θ is the digital output.

10. The apparatus of any preceding claim, wherein said processing unit includes a first processing unit for outputting said first error signal, a second processing unit for outputting said second error signal, a third processing signal for determining said digital output based on said first or second error signal, and a switching unit for switching said first or second tracking error to said third processing unit.

11. The apparatus of any preceding claim, wherein said processing unit uses said first error signal to provide said digital output when said first error signal is above a threshold value.

12. The apparatus of any preceding claim, wherein said processing unit includes an integrator for integrating the first or second error signal to produce said digital output signal.

13. The apparatus of any preceding claim, wherein said processing unit provides a type-2 servo loop.

14. The apparatus of any preceding claim, wherein said processing unit determines a velocity of said monitored object from the first or second error signal.

15. The apparatus of any preceding claim, wherein said monitored object is a shaft, and wherein the digital output is an angular position of said shaft.

16. The apparatus of any preceding claim, wherein said monitored object is a slider on a scale.

17. The apparatus of any preceding claim, wherein the input is from a resolver.

18. The apparatus of any one of claims 1 to 16, wherein the input is from an encoder.

19. The apparatus of any one of claims 1 to 16, wherein said input is from a synchro.

20. The apparatus of any one of claims 1 to 16, wherein said input is from a linear position sensor that includes a reference scale of patterned wire energised with a reference voltage, and a slider which runs along the scale and has a pair of patterned wires of the same pitch as the reference scale pattern, but with one patterned wire separated from the other by a 90 degree shift.

21. A motor control including apparatus according to any preceding claim.

22. A position sensor including apparatus according to any one of claims 1 to 20.

23. A vehicle control system including apparatus according to any preceding claim.

24. A method of converting an analogue input indicative of the position of a monitored object into a digital positional output, the digital output tracking the input, the method including: monitoring said input at a first resolution to produce a first tracking error between said digital output and said input; monitoring said input at a second resolution higher than said first resolution to produce a second tracking error between said digital output and said input; determining said digital output based on said first or second tracking error; and switching between said first and second tracking errors so as to promote tracking stability.

25. Software for controlling a digital signal processor to convert an analogue input indicative of the position of a monitored object into a digital positional output, the digital output tracking the input, the software including: a component for monitoring said input at a first resolution to produce a first tracking error between said digital output and said input; a component for monitoring said input at a second resolution higher than said first resolution to produce a second tracking error between said digital output and said input; and a determination component for determining said digital output based on said first or second tracking error, said determination component switching between said first and second tracking errors so as to promote tracking stability.

26. Storage media including the software of claim 25 thereon.

27. Signal processing apparatus for providing a digital angle output based upon an input signal indicative of the sensed position of a monitored object, the digital output of the apparatus tracking the input signal and the apparatus including switching means for causing the digital angle output to be based either on the output of a quadrature encoder or the output of a sampling means that samples the input signal.

28. A resolver-to-digital converter for providing a digital angle from the analogue sinusoidal output of a resolver, the converter including a quadrature encoder for determining a first angle and for providing a first error signal based on the first angle and the converter output, an observer for sampling the resolver output and for providing a second error signal based on the sampled resolver output and the converter output, a filter for determining the converter output based on the first or second error signal, and a switch for switching the filter input from the second error signal to the first error signal when it is determined that the converter is losing lock on the resolver output.

29. An angle tracking observer for monitoring the output signals of a resolver and for providing a digital rotor angle of the resolver, wherein the observer includes a sampler for sampling the resolver output signals and a quadrature encoder for monitoring the resolver output signals, and wherein the observer determines a digital angle for the rotor based on an output of the quadrature encoder rather than the sampler when the digital angle diverges from the rotor angle indicated by the resolver output.

30. Signal processing apparatus for receiving a sinusoidal input indicative of the position of a monitored object and for providing an output signal indicative of a position of the monitored object, the apparatus including: a first error determining component for receiving the sinusoidal input and for determining a first positional signal for the object at a first resolution, for receiving the apparatus output, and for determining a first error signal based on the first positional signal and the apparatus output; a second error determining component for receiving the sinusoidal input and for determining a second positional signal for the object at a second resolution higher than the first resolution, for receiving the apparatus output, and for determining a second error signal based on the second positional signal and the apparatus output; a positional determining component for providing said output signal based on said first or second error function; and a switching component for providing said first or second error signal to said positional determining component, said switching component switching to said first error signal when the output signal is tending to diverge.

31. Signal processing apparatus for converting an analogue input indicative of the position of a monitored object into a digital positional output, the digital output tracking the input, the apparatus including: a first processing unit for monitoring said input at a first resolution and for producing a first tracking error between said digital output and said input; a second processing unit for monitoring said input at a second resolution higher than said first resolution and for producing a second tracking error between said digital output and said input; a third processing unit for determining said digital output based on said first or second tracking error; and a switching unit for supplying said first or second tracking error to said third processing unit, said switching unit switching between said first and second tracking errors so as to promote tracking stability.