Patent Description:
Electrical devices such as motors are generally powered via a variable speed drive connected to a main. Classic voltage/frequency control laws are more and more replaced by sensorless control laws that are able to control both the speed and the torque of the electrical motor, without a mechanical speed or position sensor.

In the context of the present invention, "sensorless" does not refer to the complete absence of sensors but to the absence of some sensors, such as rotor speed or position sensors. It however generally relies on measurements of motor currents (or potentially of motor voltages).

Sensorless control of devices, in particular electric motors, may rely on extraction of information through signal injection, based on a measured variable (such as the current values). High-frequency signal injection consists in superimposing a high-frequency signal to the control signals of electric motors. The measured current response of the motor to this supplementary excitation is then extracted from the current measurement and additional signal processing allows to retrieve the speed or the position of the rotor at low or even zero speed.

The measured signal y(t) can be assumed to be of the form: <MAT> where:.

The unknown signals yi can then be used to estimate the position of the rotor or its speed, so as to determine the control signal to be applied to the motor.

Some known techniques may be used to retrieve the unknown signals yi:.

When using High-Frequency signal injection for sensorless control of electric motors, for example, the desired frequency can be obtained by band-pass filtering of the signal y(t) and by using an arctangent function to retrieve the position of the rotor, as detailed in the document <NPL>.

<NPL> proposes a technique for injecting an alternating carrier voltage for sensorless position estimation. The high frequency current components are measured using delta-sigma-modulators.

<CIT> discloses a monitoring circuit to detect a predetermined, periodic pattern in the output bit stream if a sigma-delta modulator.

<CIT> discloses a method involving fitting components of a motor control signal to a corresponding number of orthogonal pairs in a set of weighted orthogonal pairs. Frequency pairs providing a reduction in the mean squared error between current wave and the weighted orthogonal pairs that satisfy criteria are identified. Desired frequencies from the identified frequency pairs are compared with a harmonic model to determine a motor speed.

<CIT> provides a sigma-delta modulator (SDM) and associated system improving spectrum efficiency of wired interconnection. However, the known techniques suffer from a limited precision and require expensive fast Analog to Digital Converters ADCs, when ε is small, since it is imposed to measure at least two points per period ε, according to the Nyquist-Shannon theorem.

This lack of precision and cost problems apply, more generally, to the estimation of amplitudes of periodic signals in a measured signal.

There are therefore needs to improve the estimation of amplitudes of periodic components of a measured analog signal.

The invention proposes to use delta-sigma modulators without preprocessing the analog signal, to retrieve an accurate representation of the analog input while being less expensive than other ADCs of similar quality. In addition, galvanic insulation can easily be embedded.

According to the invention as claimed the control device comprises at least two filters, including a first filter and a second filter, the first filter is applied to the digital signal and to a first periodic signal, the second filter is applied to the digital signal and to a second periodic signal, and the first and second periodic signals are independent functions.

Therefore, several information can be extracted from the measured analog signal.

The first and second periodic signals are orthogonal.

This improves the accuracy of the estimation of the amplitudes.

According to some embodiments, the periodic signal or periodic signals is calculated by a computing unit of the control device based on a clock signal.

This enables to predefine the periodic signals and to increase the responsiveness of the method.

Alternatively, the periodic signal or periodic signals is/are received by the control device from an external device.

This enables to improve the flexibility of the method as different periodic signals can be used over time.

According to some embodiments, the control device is configured to control a variable speed drive in charge of an electric motor, the method comprises, before receiving the measured analog signal, controlling the variable speed drive to inject a motor voltage comprising a control law voltage and an additional voltage component. In that case, the external entity is the variable speed drive.

Therefore, signal injection can be performed to obtain additional information about the electric motor and to enhance the control law of the electric motor.

In complement, the additional voltage component can be a high-frequency component.

This enables to facilitate obtaining the additional information even at low-speed.

According to some embodiments, adapting the control law comprises estimating a rotor position and/or speed based on at least the amplitude signal.

This enables to improve the control of the motor position/speed by the variable speed drive.

According to some embodiments, the monitoring value calculated based on at least the amplitude signal can be:.

According to some embodiments, the method may further comprise comparing the calculated monitoring value with a preset threshold to decide to issue or not an alert.

Therefore, an abnormal situation can be automatically detected and signaled.

According to the invention as claimed, the first and second filters are Finite Impulse Response Filter, being a linear combination of basic blocks M, basic blocks M being iteratively defined by: <MAT> <MAT> where T is an integer multiple of the period of the periodic signals; and where f is a dummy function variable.

Further objects, aspects, effects and details of the invention are described in the following detailed description of a number of exemplary embodiments, with reference to the drawings.

By way of example only, the embodiments of the present disclosure will be described with reference to the accompanying drawings, wherein:.

<FIG> shows a system according to the invention as claimed.

On <FIG>, a control device <NUM> is included in a system comprising a variable speed drive, VSD, <NUM> and an electrical motor <NUM>, According to some embodiments, the control device <NUM> may be included in the VSD <NUM>. The control device <NUM> may also be used in any other context requiring signal processing of a measured signal to estimate amplitudes of periodic signals within the measured signal.

In <FIG> the amplitudes are used to adapt the control law of the electrical motor <NUM>. In particular, the unknown signals yi can be alternatively used to extract harmonics at known frequencies (for example in the context of Root Mean Square RMS value calculation, or Total Harmonic Distortion THDI calculation is sensors) so as to monitor these values.

No restriction is attached to the type of electrical motor, which can be for example a three-phase AC motor, such as a Synchronous Reluctance Motor, SynRM, a Permanent Magnet Synchronous Motor, PMSM, or an Induction machine also known as asynchronous motor.

The VSD <NUM> can be powered by a power source <NUM>.

The control device <NUM> may comprise a measuring unit <NUM>, such as current sensors, configured for measuring currents running through the electric motor <NUM>. On <FIG>, the VSD <NUM> is powered by a three-phase power source, for illustrative purposes only. In that context, the measuring unit <NUM> may measure currents on the three phases (or only two of them as the third one can be deduced from the two measured ones).

Alternatively, the measuring unit <NUM> may be an interface communicating with a measuring unit of the VSD <NUM>, when the VSD comprises such a measuring unit (current sensor) configured for measuring stator current(s). This enables to reduce the cost of the control device <NUM>, and to take advantage of the fact that VSDs are generally equipped with current measurement units.

The signal obtained by the measuring unit <NUM> is an analog signal.

The control device <NUM> further comprises a delta-sigma modulation unit <NUM> that is configured to apply a delta-sigma modulation to the analog signal obtained by the measuring unit <NUM>.

Regular Analog to Digital Converters, ADCs, other than delta-sigma modulators, sample a received analog signal at given instants and output representations of the instantaneous value of the signal over N bits. To sample a signal of a given frequency, the Nyquist-Shannon theorem imposes to have two samples per period, which is called the Nyquist rate.

Delta-sigma modulators are ADCs, which operate at higher frequencies but with a lower precision.

To obtain an accurate measurement from the output signal of the Delta-Sigma modulator (whose frequency is for example <NUM> and whose resolution is low, for example <NUM> bit), the measurement can be decimated by <NUM> for instance (oversampling ratio), to obtain a signal sampled at lower frequencies (<NUM> in the example).

Delta-sigma modulators yield a high-frequency <NUM>-bit signal (called bitstream), which is proportional in average to the analog input. To retrieve an accurate representation of the analog input, the average over N (oversampling ratio) samples has to be taken.

Delta-sigma modulators have the advantage to be less expensive than other ADCs of similar quality, and galvanic insulation can easily be embedded.

The principle of a delta-sigma modulation is well known and is not further described in the present application.

The control device <NUM> further comprises at least one estimator <NUM> connected at the output of the delta-sigma modulation unit <NUM>. On <FIG>, the control device <NUM> comprises n estimation units <NUM> to <NUM>. n, n being an integer equal to or greater than <NUM>.

The invention therefore proposes to directly connect the delta-sigma modulation unit <NUM> with the one or several estimation units <NUM> without preprocessing.

The estimation units <NUM> comprise computing capabilities or an electronic circuit that is/are configured to determine, based on a respective periodic signals s<NUM>(t), s<NUM>(t). sn(t), and based on the digital signal yΔΣ output by the delta-sigma modulation unit <NUM>, amplitude signals yi corresponding to the respective periodic signals si(t). For example, the estimators can be implemented either on an Application Specific Integrated Circuit, ASIC, a Field-Programmable Gate Array, FPGA, or a Digital Signal Processor, DSP.

The periodic signals si(t) are provided by the computing unit <NUM> or alternatively by an external entity. The computing unit <NUM> may compute the periodic signals si(t) based on a clock signal. The computing unit <NUM> may be integrated in the ASIC, FPGA or DSP mentioned above. Estimation units <NUM> are described hereafter. The estimation units <NUM> may be identical (but their outputs differ as they are fed with different periodic signals si(t)) or may be distinct.

The estimation units <NUM> comprise filters, such as Finite Impulse Response, FIR, filters, of order <NUM>, <NUM>, <NUM> or more. In what follows, there is detailed how to construct estimators of orders <NUM>, <NUM> and <NUM>, which basic building blocks are iterated sliding averages defined by recurrence as follows:<MAT><MAT> where ε' is an integer multiple of ε and f is a placeholder (or dummy) function variable.

A family of FIR filters used in the estimation units <NUM> may be:<MAT><MAT><MAT>.

More generally, the used FIR filter may be a linear combination of M<NUM>,. , Mk for which the sum of the coefficients is <NUM>. The coefficients do not depend on the periodic signals si(t).

The si(t), i varying between <NUM> and n, are orthogonal, for example orthogonal with respect to a scalar product over the set of <NUM>-periodic functions, i.e.: <MAT>.

The estimator ŷi for the yi up to O(εp) can be given by: <MAT>.

The order p of the filter allows to adjust the accuracy of the estimators (more accuracy when p is high).

The <MAT> can be applied directly to the output yΔΣ of the delta-sigma modulation unit <NUM>.

An alternative to the above estimator ŷi can be: <MAT>.

The same estimators in the different estimation units <NUM>-<NUM>. n can be used, provided that the si(t) are made orthogonal. The computing unit <NUM> is configured to make the si orthogonal, if they are originally not (for example, if they are received from an external entity and they are not orthogonal). Original signals si' are therefore processed to obtain orthogonal signals si and the above formula are applied to the orthogonal signals si. Once the estimations ŷi are obtained, the computing unit <NUM> may further be configured to calculate the estimations y'i in the original basis of signals si'.

The filters, applied to the signal yΔΣ instead of y, estimate ŷi such that ŷi = yi(t) + O(εp) + o(N-k) + O(N-(l+<NUM>)), where k is the order of the delta-sigma modulation unit <NUM>, N is the oversampling ratio of the delta-sigma modulation unit <NUM> and <MAT>, li being the class of the periodic signal si(t). The amplitudes yi can therefore be estimated by the estimation units <NUM>.

For example, in the context of signal injection for sensorless control of the motor <NUM> at low speed, the measured currents may be expressed as: <MAT> when considering a second order expansion, or <MAT> when considering a third order expansion.

The knowledge of y<NUM> and y<NUM> (and optionally of y<NUM>) may be used as parameters for the design of the VSD <NUM> control laws.

y<NUM>, y<NUM> (and y<NUM>) can be determined with accuracy using the low-cost delta-sigma modulation unit <NUM>, even if <NUM>/ε is larger than the effective Nyquist frequencies of the yi.

Once the amplitudes yi has or have been obtained, they are transferred to the control unit <NUM>. The control unit <NUM> is configured to:.

To this end, the control unit <NUM> may comprise a processor, a memory (RAM, ROM, flash memory, etc.) and an output interface for controlling the VSD <NUM> or for issuing and transmitting an alarm signal.

An example of sensorless control of a motor of the SynRM type, using signal injection, is described hereafter.

A model of the SynRM motor can be given by:<MAT><MAT><MAT> where <IMG>(θ) is a rotation function of angle theta in positive trigonometric direction.

The state of the motor is described by φSDQ, which is the vector of the stator flux in a rotor-oriented DQ frame, and θ, which is the angular position of the rotor.

The vector of stator voltages in a stationary αβ frame, noted uSαβ is the control input, while ω, the rotor speed, is a disturbance input, which is to be obtained to achieve a proper control of the SynRM motor.

When sensorless control is used, the sole available measurement is the vector of stator currents in the stationary αβ frame, noted ISαβ.

Parameters of the model are the resistance Rs of the stator and the matrix of inductances <MAT>.

When signal injection is used, the vector of stator voltages is: <MAT> where <MAT> is a high-frequency disturbance that is voluntarily superimposed on the control voltage vector uSαβ. The high-frequency disturbance in the voltage vector creates a high-frequency in the stator flux, which becomes <MAT> and in turn creates a disturbance in the measured current, which becomes:
<MAT>
where Ũ is the zero-mean primitive of ũ and the undisturbed variables follow the original model:<MAT><MAT><MAT>.

Estimating ISαβ and <IMG>(θ) (respectively corresponding to y<NUM> and y<NUM> mentioned above) using the invention detailed above, allows to retrieve theta from <IMG>(θ) = <IMG>(θ)L-<NUM><IMG>(-θ) and use it together with ISαβ to compute uSαβ in the control law.

<FIG> is a diagram showing the steps of a method according to the invention as claimed.

At a step <NUM>, the VSD <NUM> is controlled (by the control device <NUM> or by any other entity) to inject a motor voltage or motor voltages depending on a control law voltage and an additional voltage component, such as a high-frequency component for example.

At step <NUM>, the measuring unit <NUM> acquires an input signal such as measurements of the current (or currents for different phases) flowing through the motor <NUM>. The input signal is an analog signal.

At step <NUM>, the analog input signal y is processed by the delta-sigma modulation unit <NUM> to obtain the digital signal yΔΣ.

At step <NUM>, in parallel to step <NUM>, the one or several periodic signals si(t) is/are computed or received from an external entity.

At one or several steps <NUM>-<NUM>. n, each of the estimation units <NUM> estimates a signal yi based on the periodic signal received/computed at step <NUM> and based on the digital signal yΔΣ.

At step <NUM>, the control unit <NUM> adapts the control law of the VSD <NUM> and/or extracts harmonics at known frequencies to compute a monitoring value, for example to calculate RMS or THDI as explained above.

The calculation of RMS or THDI can be followed by an optional step <NUM> of comparison of the RMS or THDI value with a preset threshold, and an alarm can be generated based on the result of the comparison, as explained above.

Claim 1:
A method for estimating amplitudes of a periodic component in a measured signal, the measured signal corresponding to measurements of current flowing through an electric motor (<NUM>), wherein the method is performed by a control device (<NUM>) configured to control a variable speed drive (<NUM>) in charge of the electric motor (<NUM>),
wherein the control device (<NUM>) comprises at least two filters, including a first filter and a second filter, the method comprising the following consecutive operations:
- controlling (<NUM>) the variable speed drive (<NUM>) to inject a motor voltage comprising a control law voltage and an additional voltage component into the electric motor (<NUM>);
- receiving (<NUM>) said measured signal, the measured signal being analog;
- performing (<NUM>) a delta-sigma modulation on the received analog signal to obtain a digital signal;
- applying (<NUM>-<NUM>.n) the first filter to the product of the digital signal and a first periodic signal to estimate a first amplitude signal among said amplitudes, and
applying the second filter to the product of the digital signal and a second periodic signal to estimate a second amplitude signal among said amplitudes,
wherein the first and second periodic signals are independent orthogonal functions, and wherein the at least two filters are Finite Impulse Response Filters, being a linear combination of basic blocks M, basic blocks M being iteratively defined by: <MAT> <MAT>
where T is an integer multiple of the period of the periodic signals, and where f is a dummy function variable;
- adapting (<NUM>) a control law of the electric motor (<NUM>) based on at least the first and second amplitude signals, or calculating (<NUM>) a monitoring value based on at least the first and second amplitude signals.