EP3534673B1 - Induction hob and method for operating an induction hob - Google Patents

Description

• The present invention relates generally to the field of induction hobs. More specifically, the present invention is related to an induction hob comprising a power circuit in which the functionality of a current transducer is replaced by arithmetic functionality provided by a control entity.
BACKGROUND OF THE INVENTION
• Induction hobs for preparing food are well known in prior art. Induction hobs typically comprise at least one induction coil placed below a hob plate in order to heat a piece of cookware. For controlling the induction hob, values regarding the peak current flowing through the induction coil and power factor indicating the load of the induction coil (dependent of the position of the piece of cookware, the material of the piece of cookware etc.) are required.
• Common induction hobs comprise a current transducer based on which peak current flowing through the induction coil and a power factor can be determined. However, the usage of a current transducer is disadvantageous because the total costs and footprint of the power circuit board are increased.
• The document EP 3030041 A1 discloses a cooking hob and method, wherein the control entity is configured to provide and/ or receive resonance capacitor information , applying a discrete mathematical transformation to the resonance capacitor information thereby obtaining modified resonance capacitor information, said control entity being further configured to calculate first and second electrical information related to the induction coil, based on said modified resonance capacitor information.
• The document EP 2334142 A1 discloses an inductive heating device.
• The document EP 1528839 A1 discloses an induction heating cooker and method for operating the same.
SUMMARY OF THE INVENTION
• It is an objective of the embodiments of the invention to provide an induction hob, which is improved with respect to costs and footprint of the power circuit board. The objective is solved by the features of the independent claims. Preferred embodiments are given in the dependent claims. If not explicitly indicated otherwise, embodiments of the invention can be freely combined with each other.
• According to an aspect, the invention relates to an induction hob comprising a circuitry for powering at least one induction coil. The circuitry comprises a power circuit portion with at least one switching element adapted to provide pulsed electric power to said induction coil and an oscillating circuit portion comprising at least one resonance capacitor which is associated with a resonance capacitor voltage, said induction coil being electrically coupled with said power circuit portion and said oscillating circuit portion. Said induction hob comprises a control entity, said control entity being configured to provide and/or receive sampled resonance capacitor voltage values, applying a discrete mathematical transformation to the sampled resonance capacitor voltage values thereby obtaining modified resonance capacitor voltage information. Said control entity is further configured to calculate information regarding the amplitude of the electric current provided through said induction coil and information regarding the phase delay between the electric current provided through said induction coil and the voltage applied to the induction coil based on said modified resonance capacitor voltage information.
• Said induction hob is advantageous because the functionality of the current transducer can be replaced by a mathematical approach, said mathematical approach taking available information of the power circuit of the induction hob. Said control entity is configured to calculate peak current value and phase delay based on said available information. Thereby, the total costs and footprint of the power circuit can be reduced, specifically when using existing resources (e.g. microprocessor etc.) for calculating said values.
• According to embodiments, said resonance capacitor voltage is indicative for a voltage provided at a circuit node located between a pair of capacitors included in said oscillating circuit portion. Said circuit node may be used for electrically coupling the induction coil with the oscillating circuit portion. One capacitor of said pair of capacitors extends between said circuit node and supply voltage wherein the other capacitor of said pair of capacitors extends between said circuit node and ground.
• According to embodiments, said resonance capacitor voltage is obtained using a sensing circuit portion comprising a voltage divider. Said voltage divider may be formed by two or more resistors which allow the measurement of resonance capacitor voltage.
• According to embodiments, said information regarding the amplitude of the electric current provided through said induction coil and/or said information regarding the phase delay between the electric current provided through said induction coil and the voltage applied to the induction coil is calculated without considering information indicative for a voltage provided at a circuit node located between a pair of switching elements. In other words, only voltage information of a node included in the oscillating circuit portion but no voltage information of a node included in the power circuit portion is required. Thereby estimation inaccuracies can be reduced.
• According to embodiments, said discrete mathematical transformation is a discrete cosine transformation. Based on the discrete cosine transformation, a sinusoidal wave is developed based on which the maximum and minimum values of resonance capacitor voltage are calculated. So, in other words, the maximum and minimum values are not directly established from the sampled resonance capacitor voltage but based on the sampled values of the resonance capacitor voltage, a sinusoidal wave is determined that fits to the acquired samples of resonance capacitor voltage. Thereby, the influence of noise can be significantly reduced.
• According to embodiments, said discrete mathematical transformation is based on sine and cosine reference convolution signals, said sine and cosine reference convolution signals being calculated based on information regarding the frequency of the electric current provided through said induction coil and the sampling frequency based on which the sampled resonance capacitor voltage is obtained. Said sine and cosine reference convolution signals form reference signals for establishing the sinusoidal wave based on the sampled values of the resonance capacitor voltage.
• According to embodiments, said sine and cosine reference convolution signals are based on the following formulas: $$\omega =2\cdot \pi \cdot \mathit{inFreq}$$

  

  $$\cos \mathit{REF}\left(k\right)=\cos \left(\left(k-1\right)\cdot \omega \cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}\right);$$

  

  $$\cos \mathit{REF}\mathit{ACC}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\cos \mathrm{\hspace{0.17em}}\mathit{REF}}\left(i\right)+\cos \mathrm{\hspace{0.17em}}\mathit{REF}\left(k\right);$$

  

  $$\sin \mathit{REF}\left(k\right)=\sin \left(\left(k-1\right)\cdot \omega \cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}\right);$$

  

  $$\sin REF\mathrm{\hspace{0.17em}}\mathit{ACC}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\sin \mathit{REF}}\left(i\right)+\sin \mathit{REF}\left(k\right);$$

  

  wherein
• infreq is the frequency of the current flowing through the induction coil;
• k is an index indicating a certain sample value;
• cosREF ACC and sinREF ACC are accumulators; and
• Δt_SAMPLE,s is the time span between two consecutive sampled values represented in seconds [s].
• According to embodiments, said sine and cosine reference convolution signals are based on the following formulas: $$\mathit{averageCOS}=\frac{\cos \mathrm{\hspace{0.17em}}\mathit{REF}\mathit{ACC}}{\mathit{bufferlength}};$$

  

  $$\mathit{averageSIN}=\frac{\sin \mathrm{\hspace{0.17em}}\mathit{REF}\mathit{ACC}}{\mathit{bufferlength}};$$

  

  $$\cos \mathit{REF}\left(k\right)=\mathit{cosREF}\left(k-1\right)-\mathit{averageCOS};$$

  

  $$\sin \mathit{REF}\left(k\right)=\mathit{sinREF}\left(k-1\right)-\mathit{averageSIN};$$

  

  wherein
  bufferlength is calculated based on the following formula: $$\mathit{bufferlength}=\mathit{trunc}\left(\frac{1}{\mathit{inFreq}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}}\right);$$

  

  wherein inFreq is the frequency of the electric current provided through said induction coil.
• According to embodiments, fitting signals are calculated as follows: $$\mathit{accCos}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\cos \mathit{REF}\left(i\right)\cdot \mathit{inputSample}\left(i\right)+\cos \mathit{REF}\left(k\right)\cdot \mathit{inputSample}\left(k\right);}$$

  

  $$\mathit{accSin}\left(k\right)={\displaystyle \sum _1^{i=k-1}\sin \mathit{REF}\left(i\right)\cdot \mathit{inputSample}\left(i\right)+\sin \mathit{REF}\left(k\right)\cdot \mathit{inputSample}\left(k\right);}$$

  

  $$\mathit{accAverageRec}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\mathit{inputSample}\left(i\right)+\mathit{inputSample}\left(k\right).}$$

  
• According to embodiments, intermediate signal values are calculated as follows: $$\cos \mathit{EST}=2\cdot \mathit{accCos}\cdot \mathit{inFreq}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s};$$

  

  $$\sin \mathit{EST}=2\cdot \mathit{accSin}\cdot \mathit{inFreq}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s};$$

  

  $$\mathit{avgEst}=\frac{\mathit{accAverageRec}}{\mathit{bufferLength}}.$$

  
• The avgEst value can be corrected considering the BIAS due to the leakage current that supply the resonance capacitor also in absence of power generation. This increases the precision of acquired data and at the end the estimated values.
• According to embodiments, peak value, maximum and minimum values of resonance capacitor voltage and phase delay value are calculated as follows: $$\mathit{peakValue}=\sqrt{\mathrm{<figure-callout\; id="2"\; label="cos\; Est"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">cos</figure-callout>}{\mathit{Est}}^{}{\mathit{}}^2+\mathrm{<figure-callout\; id="2"\; label="sin\; Est"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">sin</figure-callout>}{\mathit{Est}}^{}{\mathit{}}^2};$$

  

  $$\mathit{VcMax}=\mathit{avgEst}+\mathit{peakValue};$$

  

  $$\mathit{VcMin}=\mathit{avgEst}-\mathit{peakValue};$$

  

  $$\mathit{phaseDelay}\mathit{Vc}={\tan }^{-1}\left(\frac{\mathit{sinEst}}{\mathit{cosEst}}\right).$$

  
• According to embodiments, the peak value of the electric current provided through said induction coil is calculated based on the following formula: $$\mathit{Ic}=2\mathit{\pi ƒ}\left(\mathit{VcMax}-\mathit{VcMin}\right)\cdot \mathit{Cres};$$

  

  wherein Cres is the capacity value of the resonance capacitor and f is the switching frequency applied to the induction coil.
• Consider that the following compensation could be added or not, depending on the desired reliability level or precision that would be reached in the estimation/reconstruction method. This is an option and could be applied or not.
• According to embodiments, a compensated phase delay between the electric current provided through said induction coil and the voltage applied to the induction coil is calculated based on the following compensation formulas: $$\sin \mathit{Comp}=\frac{2\pi \left(\mathit{cosEst}\cdot \mathrm{<figure-callout\; id="4"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(4\mathit{\pi K}\right)-\mathit{cosEst}+\mathit{sinEst}\cdot \mathrm{<figure-callout\; id="4"\; label="sin"\; filenames="imgf0001.png"\; state="\{\{state\}\}">sin</figure-callout>}\left(4\mathit{\pi K}\right)+4\pi \cdot \mathit{sinEst}\cdot \mathrm{<figure-callout\; id="4"\; label="K\; cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">K</figure-callout>}\right)}{\cos };$$

  

  $$\begin{array}{l}\cos \mathit{Comp}\\ =-\left(\frac{2\pi \left(\mathit{sinEst}+\mathit{cosEst}\cdot \mathrm{<figure-callout\; id="4"\; label="sin"\; filenames="imgf0001.png"\; state="\{\{state\}\}">sin</figure-callout>}\left(4\mathit{\pi K}\right)-\mathit{sinEst}\cdot \mathit{<figure-callout\; id="4"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(4\mathit{\pi K}\right)-4\pi \cdot \mathit{cosEst}\cdot \mathrm{<figure-callout\; id="4"\; label="K\; cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">K</figure-callout>}\right)}{\cos }\right);\end{array}$$

  

  wherein K is a compensation coefficient calculated as: $$K=\mathit{numSample}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}\cdot \mathit{inFreq}.$$

  
• According to embodiments, a new compensated phase delay and a new compensated peak value of the electric current provided through said induction coil could be computed based on the following formula: $$\mathit{peakValueComp}=\sqrt{\mathrm{<figure-callout\; id="2"\; label="cos\; Comp"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">cos</figure-callout>}{\mathit{Comp}}^{}{\mathit{}}^2+\mathrm{<figure-callout\; id="2"\; label="sin\; Comp"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">sin</figure-callout>}{\mathit{Comp}}^{}{\mathit{}}^2};$$

  

  $$\mathit{VcMaxComp}=\mathit{avgEst}+\mathit{peakValueComp};$$

  

  $$\mathit{VcMinComp}=\mathit{avgEst}-\mathit{peakValueComp};$$

  

  $$\mathit{phaseDelayVcVomp}={\tan }^{-1}\frac{\sin \mathit{Comp}}{\cos \mathit{Comp}}.$$

  

  and $$\mathit{IcComp}=2\mathit{\pi ƒ}\left(\mathit{VcMaxComp}-\mathit{VcMinComp}\right)\cdot \mathit{Cres};$$

  
• According to embodiments, an estimated average power value is calculated based on the following formula: $$\mathit{avgPowerEst}=\frac{{\displaystyle {\sum }_{k=1}^{\mathit{nSamples}}\mathit{Vmain}\left(k\right)\cdot {\mathit{Ic}}_{\mathit{Est}}\left(k\right)\cdot \mathit{PhM}\_\mathit{<figure-callout\; id="2"\; label="Est\; k"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">Est</figure-callout>}\left(k\right)\left(\right)}}{2\ast \mathit{MainLineFreq}};$$

  

  wherein Vmain is the rectified value of main line voltage at sample k, I_cEst is the estimated value (compensated or not) of coil current obtained by the resonance capacitor analysis, PhM_{_}Est is the phase delay (compensated or not) obtained by the resonance capacitor analysis and MainLineFreq is the frequency of AC main line voltage applied to the induction hob.
• According to embodiments, the induction hob comprises no current transducer electrically coupled with the induction coil, wherein information regarding the amplitude of the electric current provided through said induction coil and information regarding the phase delay between the electric current provided through said induction coil and the voltage applied to the induction coil are provided by an algorithm calculating maximum and minimum peak values of resonance capacitor voltage by reconstructing a sinusoidal wave based on sampled values of said resonance capacitor voltage.
• According to a further aspect, the invention relates to a method for operating an induction hob. The induction hob comprises a circuitry for powering at least one induction coil. The circuitry comprises a power circuit portion with at least one switching element adapted to provide pulsed electric power to said induction coil and an oscillating circuit portion comprising at least one resonance capacitor which is associated with a resonance capacitor voltage, said induction coil being electrically coupled with said power circuit portion and said oscillating circuit portion. Said induction hob comprises a control entity performing the steps of:
• providing and/or receiving sampled resonance capacitor voltage values;
• applying a discrete mathematical transformation to the sampled resonance capacitor voltage values thereby obtaining modified resonance capacitor voltage information;
• calculating information regarding the amplitude of the electric current provided through said induction coil and information regarding the phase delay between the electric current provided through said induction coil and the voltage applied to the induction coil based on said modified resonance capacitor voltage information.
• The term "essentially" or "approximately" as used in the invention means deviations from the exact value by +/- 10%, preferably by +/- 5% and/or deviations in the form of changes that are insignificant for the function.
BRIEF DESCRIPTION OF THE DRAWINGS
• The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

Fig. 1

shows an example embodiment of a schematic power circuit of a state-of-the-art induction hob;

Fig. 2

shows an example embodiment of a schematic power circuit of an induction hob according to the present invention;

Fig. 3

shows sampling values of resonance capacitor voltage Vc over a whole period; and

Fig. 4

shows an equivalent circuit of a power circuit according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
• The present invention will now be described more fully with reference to the accompanying drawings, in which example embodiments are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Throughout the following description similar reference numerals have been used to denote similar elements, parts, items or features, when applicable.
• Fig. 1 shows a schematic diagram of a power circuit 1 of a state-of-the-art induction hob. The power circuit 1 comprises an input stage 2. Said input stage 2 may be coupled with AC mains, e.g. 230V AC mains. Said input stage 2 may be adapted to rectify and/or filter the AC mains voltage. Specifically, the input stage 2 may comprise a rectification bridge. In addition, the power circuit 1 may comprise a coil driver entity 3. The coil driver entity 3 may be adapted to control one or more switching elements 4, 5. Said switching elements 4, 5 may be electrically coupled with said input stage 2 in order to receive rectified AC voltage. In addition, said coil driver entity 3 may be electrically coupled with control inputs of said switching elements 4, 5 in order to be able to provide pulsed electrical power to an induction coil 6. Said switching elements 4, 5 may be, for example, IGBTs. The IGBTs may be integrated in a power circuit portion 7, said power circuit portion 7 being configured as a half-bridge converter.
• Between said power circuit portion 7 and said induction coil 6, a current transducer 8 is provided. Said current transducer 8 may be adapted to provide information regarding the peak value of the electric current provided through the induction coil 6 (in the following also referred to as coil current) and the power factor. More in detail, the coil current Ic may flow through the current transducer 8. Thereby, the current transducer 8 is able to measure/determine the peak value of the coil current Ic and the power factor. The current transducer 8 may be electrically coupled with a circuit node 7a of the power circuit portion 7 which is arranged between the pair of switching elements 4, 5.
• At the opposite side of the current transducer 8, the induction coil 6 is coupled with an oscillating circuit portion. Said oscillating circuit portion 9 may comprise a pair of capacitors 9.1, 9.2, said capacitors 9.1, 9.2 forming together with the inductivity of the induction coil 6 an electrical resonant or quasi-resonant circuit which enables an oscillating excitation of the induction coil 6. The induction coil 6 may be coupled with a circuit node 9a being arranged between said pair of capacitors 9.1, 9.2. Said capacitors 9.1, 9.2 are in the following referred to as resonance capacitors.
• Said current transducer 8 may be electrically coupled with a control entity 10 for providing information regarding the peak value of the coil current and the power factor to said control entity 10. Based on said information, the control entity 10 controls the switching elements 4, 5 of the power circuit portion 7.
• Fig. 2 shows a schematic diagram of a power circuit 1a of an induction hob according to the present invention. The basic structure of the power circuit 1a is similar to the structure of the power circuit 1. Therefore, in the following only differences of the power circuit 1a with respect to power circuit 1 are explained. Apart from that, the features described before do also apply to the embodiment of Fig. 2.
• The first main difference to the power circuit 1 is that the power circuit 1a does not comprise a current transducer 8. More in detail, the induction coil 6 is directly coupled with the circuit node 7.1 provided between the pair of switching elements 4, 5. A further difference is the voltage divider 11 which is electrically coupled with the circuit node 9a of the oscillating circuit portion 9. In order to be able to replace the functionality of the current transducer 8, the control entity 10 is configured to gather information regarding the peak value of the coil current and the power factor/phase delay based on a mathematical algorithm. More in detail, the control entity 10 may receive certain information available at the power circuit 1a, e.g. information correlated/associated with the voltage of the circuit node 9a. The wording "information correlated/associated with a voltage" may refer to the case that a voltage is tapped at a certain node (e.g. node 9a) thereby said information being the voltage value at said node. However, the wording "information correlated/associated with a voltage" may alternatively be indicative for said voltage at said node, but may be derived by an arithmetic operation based on other parameters or, for example, derived by the voltage divider 11.
• In the following, the implementation of calculating information regarding the peak value of the coil current and an estimated power value is described in detail.
• The inventor(s) found out that an estimation of coil current Ic, phase delay between the electric current and the voltage applied to the induction coil and the electric power applied to the induction coil can be obtained from an analysis of resonance capacitor voltage, i.e. without considering an equivalent local coil/pot model including an estimation of an equivalent load series resistance and an equivalent load series inductance.
• The algorithmic implementation (and not based on a current transducer) of providing information regarding the peak value of the coil current and the estimated power value can be obtained based on several information available at the power circuit 1a or derivable from information available at the power circuit 1a.
• In the arithmetic implementation, information regarding the resonance capacitor voltage Vc is gathered. Preferably, said information is obtained by the voltage divider 11. Usually, the resonance capacitor voltage Vc is quite close to a sinusoid. However, in order to reduce the impact of noise, the maximum and minimum values of resonance capacitor voltage Vc are, preferably, not obtained directly by a peak determination entity or peak determination function but resonance capacitor voltage Vc is sampled and a best fitting method is applied on said samples in order to obtain a "best fitted" sinusoidal wave that fits the acquired samples of resonance capacitor voltage Vc.
• Fig. 3 shows an example of sampled values of Vc related to a switching frequency of switching elements 4, 5 of 40kHz.
• In order to increase noise immunity and thereby obtain a high reliability level, a discrete mathematical transformation is applied to the sampled values of Vc. For example, said discrete mathematical transformation is a discrete cosine transformation (DCT). However, also other discrete mathematical transformations can be used. Considering that DCT is derived from the Fourier Transformation, other methodologies are for instance the Wavelet filtering, the Fourier Transform (Regular, Fast or other typologies) and related mathematical variations.
• The resonance capacitor voltage Vc is sampled, for example, with a frequency of 1MHz or more. Also lower frequencies may be possible, for instance in the range of 100kHz to 1MHz, specifically 500kHz or 250kHz. Said sampling may be obtained periodically, for example with a frequency of 10kHz. As a result of said sampling, a complete period of resonance capacitor voltage Vc is stored in a storage entity, e.g. RAM. Said stored samples of resonance capacitor voltage Vc are elaborated in order to obtain upper-mentioned estimated values of coil current Ic, phase delay between the electric current and the voltage applied to the induction coil and the electrical power applied to the induction coil 6.
• For applying said discrete mathematical transformation to the sampled values of Vc, sine and cosine reference convolution signals are computed based on the following formulas: $$\omega =2\cdot \pi \cdot \mathit{inFreq};$$

  

  $$\cos \mathit{REF}\left(k\right)=\cos \left(\left(k-1\right)\cdot \omega \cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}\right);$$

  

  $$\cos \mathit{REF}\mathit{ACC}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\cos \mathit{REF}\left(i\right)+\cos \mathit{REF}\left(k\right);}$$

  

  $$\sin \mathit{REF}\left(k\right)=\sin \left(\left(k-1\right)\cdot \omega \cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}\right);$$

  

  $$\sin REF\mathrm{\hspace{0.17em}}\mathit{ACC}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\sin \mathit{REF}}\left(i\right)+\sin \mathit{REF}\left(k\right);$$

  

  wherein
• infreq is the frequency of the current flowing through the induction coil;
• Δt_SAMPLE,s is the time span between two consecutive sampling values represented in seconds [s];
• k is an index indicating a certain sample value; and cosREF ACC and sinREF ACC are accumulators.
• The total number of samples referring to one period of resonance capacitor voltage Vc can be calculated as follows: $$\mathit{bufferlength}=\mathit{trunc}\left(\frac{1}{\mathit{inFreq}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}}\right);$$

  
• Thereby, the "trunc"-function provides the integer part of the division result.
• The final reference convolution signals with the compensation of the average are: $$\mathit{averageCOS}=\frac{\cos \mathrm{\hspace{0.17em}}\mathit{REF}\mathit{ACC}}{\mathit{bufferlength}};$$

  

  $$\mathit{averageSIN}=\frac{\sin \mathrm{\hspace{0.17em}}\mathit{REF}\mathit{ACC}}{\mathit{bufferlength}};$$

  

  $$\mathit{cosREF}\left(k\right)=\mathit{cosREF}\left(k-1\right)-\mathit{averageCOS}$$

  

  $$\mathit{sinREF}\left(k\right)=\mathit{sinREF}\left(k-1\right)-\mathit{averageSIN}$$

  
• Said reference convolution signals could be computed every half of main line voltage period. For example, if the main line period is 50 Hz, the half main line voltage period is 10ms. The reference convolution signals can be computed every half of main line voltage period because the switching frequency usually changes every half of main line voltage period.
• Inside the half of main line voltage period (e.g. 10ms), the interesting signals are updated every 100µs, for example.
• Based on the acquired samples of resonance capacitor voltage Vc, in the following referred to as inputSample, and based on the convolution signals described before, fitting signals are computed based on the following formulas: $$\mathit{accCos}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\cos \mathit{REF}\left(i\right)\cdot \mathit{inputSample}\left(i\right)+\cos \mathit{REF}\left(k\right)\cdot \mathit{inputSample}\left(k\right);}$$

  

  $$\mathit{accSin}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\mathit{sinREF}\left(i\right)\cdot \mathit{inputSample}\left(i\right)+\mathit{sinREF}\left(k\right)\cdot \mathit{inputSample}\left(k\right);}$$

  

  $$\mathit{accAverageRec}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\mathit{inputSample}\left(i\right)+\mathit{inputSample}\left(k\right)};$$

  
• Based on the results of formulas 11 to 13, intermediate signal values are calculated as follows: $$\cos \mathit{EST}=2\cdot \mathit{accCos}\cdot \mathit{inFreq}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s};$$

  

  $$\sin \mathit{EST}=2\cdot \mathit{accSin}\cdot \mathit{inFreq}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s};$$

  

  $$\mathit{avgEst}=\frac{\mathit{accAverageRec}}{\mathit{bufferLength}};$$

  
• The average estimation (avgEst) value can be corrected considering the BIAS due to the leakage current that supply the resonance capacitor also in absence of power generation. This increases the precision of acquired data and at the end the estimated values. According to a test case, at a line voltage of 230V, this value is around 18.5V.
• The peak, maximum and minimum values of resonance capacitor voltage Vc and the phase delay of Vc can be calculated based on the following formulas: $$\mathit{peakValue}=\sqrt{\mathrm{<figure-callout\; id="2"\; label="cos\; Est"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">cos</figure-callout>}{\mathit{Est}}^{}{\mathit{}}^2+\mathrm{<figure-callout\; id="2"\; label="sin\; Est"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">sin</figure-callout>}{\mathit{Est}}^{}{\mathit{}}^2};$$

  

  $$\mathit{VcMax}=\mathit{avgEst}+\mathit{peakValue};$$

  

  $$\mathit{VcMin}=\mathit{avgEst}-\mathit{peakValue};$$

  

  $$\mathit{phaseDelayVc}={\tan }^{-1}\left(\frac{\mathit{sinEst}}{\mathit{cosEst}}\right);$$

  
• Said phase delay value phaseDelay Vc is the delay between the generated voltage to the coil and the envelop of resonance capacitor voltage reconstructed with the fast sampling acquisition procedure.
• Fig. 4 shows an equivalent circuit covering the power circuit portion 7, the induction coil 6 and the oscillating circuit 9 of the power circuit 1a according to Fig. 2. The induction coil 6 is replaced by a load representation modelled by R_s and L_s. The values of R_s and L_s depend on the applied frequency, the temperature, the material of the piece of cookware placed on the induction coil and the position of the piece of cookware with respect to the induction coil 6. Based on said equivalent circuit, the coil current I_c can be reconstructed using the following model equations: $$I_1=I_{\mathit{cL}}+I_{\mathit{cH}};$$

  

  $$\frac{{\mathit{dV}}_c}{\mathit{dt}}=\frac{I_1}{2C_{\mathit{res},2}};$$

  

  $$I_C=I_1=2C_{\mathit{res},2}\cdot \frac{{\mathit{dV}}_c}{\mathit{dt}};$$

  
• Based on the last formula, coil current I_c can be reconstructed as follows: $$I_C=2\mathit{\pi ƒ}\left(V_{C,\mathit{max}}-V_{C,\mathit{min}}\right)C_{\mathit{res},2};$$

  

  wherein
• ƒ is the frequency of the AC-current provided to the induction coil;
• V _C,max is the maximum value of the voltage provided at circuit node 9a;
• V _C,min is the minimum value of the voltage provided at circuit node 9a; and
• C _{res ,2} is the capacitor value of a resonance capacitor included in said oscillating circuit.
• Similar to the coil current estimation process, it is possible to define a procedure for estimating the phase delay.
• The estimation of phase delay can be performed based on the following facts:
• the resonance capacitor voltage has a phase delay of 90° with respect to the current through the resonance capacitor;
• the current through the resonance capacitor is a portion of the total coil current, specifically one half of the total coil current in case of identical values of C _{res ,1} and C _{res ,2};
• the knowledge of phase delay between the pulse width modulated (PWM) signal (which is present at circuit node 7a between the switching elements 4 as indicated in Figure 1) and maximum of V_cRes is linked to the value of phase delay between Vc and Ic.
• In order to obtain an improved phase delay estimation, based on formula 20, a compensation procedure is introduced considering that the number of samples stored in the storage entity is an integer number and for this reason, the information stored in the storage entity does not reflect an entire period of resonance capacitor voltage Vc.
• Consider that the following compensation could be added or not, depending on the desired reliability level or precision that would be reached in the estimation/reconstruction method. So, in other words, this option and could be applied or not.
• Preferably, said compensation is done every time, the coil current I_c and phase delay are computed, i.e. according to the upper-mentioned timing regime, every 100µs (or more). Said compensation is made based on the following set of formulas: $$\sin \mathrm{\hspace{0.17em}}\mathit{Comp}=\frac{2\pi \left(\mathit{cosEst}\cdot \mathrm{<figure-callout\; id="4"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(4\mathit{\pi K}\right)-\mathit{cosEst}+\mathit{sinEst}\cdot \mathrm{<figure-callout\; id="4"\; label="sin"\; filenames="imgf0001.png"\; state="\{\{state\}\}">sin</figure-callout>}\left(4\mathit{\pi K}\right)+4\pi \cdot \mathit{sinEst}\cdot K\right)}{\mathrm{<figure-callout\; id="4"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(4\mathit{\pi K}\right)+8{\mathrm{<figure-callout\; id="2"\; label="K"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">K</figure-callout>}}^2{\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\pi </figure-callout>}}^2-1};$$

  

  $$\cos \mathrm{\hspace{0.17em}}\mathit{Comp}=-\left(\frac{2\pi \left(\mathit{sinEst}+\mathit{cosEst}\cdot \mathrm{<figure-callout\; id="4"\; label="sin"\; filenames="imgf0001.png"\; state="\{\{state\}\}">sin</figure-callout>}\left(4\mathit{\pi K}\right)-\mathit{<figure-callout\; id="4"\; label="sinEst\; \cdot \; cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">sinEst</figure-callout>}\cdot \mathit{cos}\mathit{}\left(4\mathit{\pi K}\right)-4\pi \cdot \mathit{cosEst}\cdot K\right)}{\mathrm{<figure-callout\; id="4"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(4\mathit{\pi K}\right)+8{\mathrm{<figure-callout\; id="2"\; label="K"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">K</figure-callout>}}^2{\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">\pi </figure-callout>}}^2-1}\right);$$

  
• Thereby, sinComp is the estimated sine value, cosComp is the estimated cosine value and K is a compensation coefficient calculated as follows: $$K=\mathit{numSample}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}\cdot \mathit{inFreq};$$

  
• Said compensation coefficient is always lower than zero.
• Based on formulas 25 and 26, a new compensated phase delay and a new compensated peak value of the electric current provided through said induction coil can be calculated as follows: $$\mathit{peakValueComp}=\sqrt{\mathrm{<figure-callout\; id="2"\; label="cos\; Comp"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">cos</figure-callout>}\mathrm{\hspace{0.17em}}{\mathit{Comp}}^{}{\mathit{}}^2+\mathrm{<figure-callout\; id="2"\; label="sin\; Comp"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">sin</figure-callout>}\mathrm{\hspace{0.17em}}{\mathit{Comp}}^{}{\mathit{}}^2};$$

  

  $$\mathit{VcMaxComp}=\mathit{avgEst}+\mathit{peakValueComp};$$

  

  $$\mathit{VcMinComp}=\mathit{avgEst}-\mathit{PeakValueComp};$$

  

  $$\mathit{phaseDelayVcVomp}={\tan }^{-1}\frac{\sin \mathrm{\hspace{0.17em}}\mathit{Comp}}{\cos \mathrm{\hspace{0.17em}}\mathit{Comp}}.$$

  

  $$\mathit{IcComp}=2\mathit{\pi ƒ}\left(\mathit{VcMaxComp}-\mathit{VcMinComp}\right)\cdot \mathit{Cres};$$

  
• The electric power provided through the induction coil is estimated considering the already estimated values of coil current Ic and phase delay. Said estimated power value is calculated as an average based on the entire half of main line voltage and can be calculated considering a sum of instantaneous values based on the following formula: $$\mathit{avgPowerEst}=\frac{{\displaystyle {\sum }_{k=1}^{\mathit{nSample}}\mathit{Vmain}\left(k\right)\cdot {\mathit{Ic}}_{\mathit{Est}\left(k\right)}\cdot \mathit{PhM}\_\mathit{<figure-callout\; id="2"\; label="Est\; k"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">Est</figure-callout>}\left(k\right)\left(\right)}}{2\ast \mathit{MainLineFreq}};$$

  

  wherein Vmain is the rectified value of main line voltage at sample k, I_cEst is the estimated value (compensated or not) of coil current obtained by the resonance capacitor analysis at sample k, PhM_{_}Est is the phase delay (compensated or not) obtained by the resonance capacitor analysis and MainLineFreq is the frequency of AC main line voltage applied to the induction hob.
• The estimated power value could be calculated considering, for instance, only a portion of the half of main line voltage period. For instance, the portion of period near the zero crossing of main line rectified voltage value could be skipped in the calculation of the average power value.
• It should be noted that the description and drawings merely illustrate the principles of the proposed induction hob. Those skilled in the art will be able to implement various arrangements that, although not explicitly described or shown herein, embody the principles of the invention as claimed. and
List of reference numerals

1, 1a

power circuit

2

input stage

3

coil driver entity

4

switching element

5

switching element

6

induction coil

7

power circuit portion

7a

circuit node

8

current transducer

9

oscillating circuit portion

9a

circuit node

9.1

capacitor

9.2

capacitor

10

control entity

11

voltage divider

Ic

coil current

Vc

resonance capacitor voltage

Claims (15)

1. Induction hob comprising a circuitry (1a) for powering at least one induction coil (6), the circuitry (1a) comprising a power circuit portion (7) with at least one switching element (4, 5) adapted to provide pulsed electric power to said induction coil (6) and an oscillating circuit portion (9) comprising at least one resonance capacitor (9.1, 9.2) which is associated with a resonance capacitor voltage, said induction coil (6) being electrically coupled with said power circuit portion (7) and said oscillating circuit portion (9), wherein said induction hob comprises a control entity (10), said control entity (10) being configured to provide and/or receive resonance capacitor information, the information being sampled resonance capacitor voltage values, applying a discrete mathematical transformation to the resonance capacitor information thereby obtaining modified resonance capacitor information, said control entity (10) being further configured to calculate first and second electrical information related to the induction coil (6), the information being information regarding the amplitude of the electric current (Ic) provided through said induction coil (6) and information regarding the phase delay between the electric current (Ic) provided through said induction coil (6) and the voltage applied to the induction coil (6) based on said modified resonance capacitor information.
2. Induction hob according to claim 1, wherein said first and/or second electrical information related to the induction coil (6) is calculated without considering information indicative for a voltage provided at a circuit node located between a pair of switching elements (4, 5).
3. Induction hob according to claim 1 or 2, wherein said discrete mathematical transformation is a discrete cosine transformation.
4. Induction hob according to anyone of the preceding claims, wherein said discrete mathematical transformation is based on sine and cosine reference convolution signals, said sine and cosine reference convolution signals being calculated based on information regarding the frequency of the electric current (Ic) provided through said induction coil (6) and the sampling frequency based on which the sampled resonance capacitor voltage is obtained.
5. Induction hob according to claim 4, wherein said sine and cosine reference convolution signals are based on the following formulas: $$\omega =2\cdot \pi \cdot \mathit{inFreq}$$

   

   $$\cos \mathit{REF}\left(k\right)=\cos \left(\left(k-1\right)\cdot \omega \cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}\right);$$

   

   $$\cos \mathrm{\hspace{0.17em}}\mathit{REF}\mathit{ACC}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\cos \mathrm{\hspace{0.17em}}\mathit{REF}}\left(i\right)+\cos \mathrm{\hspace{0.17em}}\mathit{REF}\left(k\right);$$

   

   $$\sin \mathrm{\hspace{0.17em}}\mathit{REF}\left(k\right)=\sin \left(\left(k-1\right)\cdot \omega \cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}\right);$$

   

   $$\sin \mathrm{\hspace{0.17em}}\mathit{REF}\mathit{ACC}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\sin \mathrm{\hspace{0.17em}}\mathit{REF}\left(i\right)+\sin \mathrm{\hspace{0.17em}}\mathit{REF}}\left(k\right);$$

   

   wherein

   infreq is the frequency of the current flowing through the induction coil;

   k is an index indicating a certain sample value; cosREF ACC and sinREF ACC are accumulators; and

   Δt_SAMPLE,s is the time span between two consecutive sampling values.
6. Induction hob according to anyone of the preceding claims, said sine and cosine reference convolution signals are based on the following formulas: $$\mathit{averageCOS}=\frac{\cos \mathrm{\hspace{0.17em}}\mathit{REF}\mathit{ACC}}{\mathit{bufferlength}};$$

   

   $$\mathit{averageSIN}=\frac{\sin \mathrm{\hspace{0.17em}}\mathit{REF}\mathit{ACC}}{\mathit{bufferlength}};$$

   

   $$\mathit{cosREF}\left(k\right)=\mathit{cosREF}\left(k-1\right)-\mathit{averageCOS};$$

   

   $$\mathit{sinREF}\left(k\right)=\mathit{sinREF}\left(k-1\right)-\mathit{averageSIN};$$

   

   wherein
   bufferlength is calculated based on the following formula: $$\mathit{bufferlength}=\mathit{trunc}\left(\frac{1}{\mathit{inFreq}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}}\right);$$

   

   wherein inFreq is the frequency of the electric current (Ic) provided through said induction coil (6).
7. Induction hob according to claim 6, wherein fitting signals are calculated as follows: $$\mathit{accCos}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\cos \mathrm{\hspace{0.17em}}\mathit{REF}}\left(i\right)\cdot \mathit{inputSample}\left(i\right)+\cos \mathrm{\hspace{0.17em}}\mathit{REF}\left(k\right)\cdot \mathit{inputSample}\left(k\right);$$

   

   $$\mathit{accSin}\left(k\right)={\displaystyle \sum _1^{i=k-1}\sin \mathrm{\hspace{0.17em}}\mathit{REF}}\left(i\right)\cdot \mathit{inputSample}\left(i\right)+\sin \mathrm{\hspace{0.17em}}\mathit{REF}\left(k\right)\cdot \mathit{inputSample}\left(k\right);$$

   

   $$\mathit{accAverageRec}\left(k\right)={\displaystyle {\sum }_1^{i=k-1}\mathit{inputsample}\left(i\right)+\mathit{inputSample}\left(k\right);}$$

   
8. Induction hob according to claim 7, wherein intermediate signal values are calculated as follows: $$\cos \mathit{EST}=2\cdot \mathit{accCos}\cdot \mathit{inFreq}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s};$$

   

   $$\sin \mathit{EST}=2\cdot \mathit{accSin}\cdot \mathit{inFreq}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s};$$

   

   $$\mathit{avgEst}=\frac{\mathit{accAverageRec}}{\mathit{bufferLength}};$$

   
9. Induction hob according to anyone of the preceding claims, wherein maximum and minimum values of resonance capacitor voltage and phase delay value are calculated as follows: $$\mathit{peakValue}=\sqrt{\cos \mathrm{\hspace{0.17em}}{\mathit{Est}}^2+\sin \mathrm{\hspace{0.17em}}{\mathit{Est}}^2};$$

   

   $$\mathit{VcMax}=\mathit{avgEst}+\mathit{peakValue};$$

   

   $$\mathit{VcMin}=\mathit{avgEst}-\mathit{peakValue};$$

   

   $$phaseDelay\mathrm{\hspace{0.17em}}\mathit{Vc}={\tan }^{-1}\left(\frac{\mathit{sinEst}}{\mathit{cosEst}}\right).$$

   
10. Induction hob according to claim 9, wherein the peak value of the electric current (Ic) provided through said induction coil (6) is calculated based on the following formula: $$\mathit{Ic}=2\mathit{\pi ƒ}\left(\mathit{VcMax}-\mathit{VcMin}\right)\cdot \mathit{Cres};$$

    

    wherein Cres is the capacity value of the resonance capacitor (9.1, 9.2) and f is the switching frequency applied to the induction coil (6).
11. Induction hob according to anyone of claims 8 to 10, wherein a compensated phase delay between the electric current (Ic) provided through said induction coil (6) and the voltage applied to the induction coil (6) is calculated based on the following compensation formulas: $$\sin \mathrm{\hspace{0.17em}}\mathit{Comp}=\frac{2\pi \left(\mathit{cosEst}\cdot \cos \left(4\mathit{\pi K}\right)-\mathit{cosEst}+\mathit{sinEst}\cdot \sin \left(4\mathit{\pi K}\right)+4\pi \cdot \mathit{sinEst}\cdot K\right)}{\cos \left(4\mathit{\pi K}\right)+8K^2{\pi }^2-1};$$

    

    $$\begin{array}{l}\cos \mathrm{\hspace{0.17em}}\mathit{Comp}\\ =-\left(\frac{2\pi \left(\mathit{sinEst}+\mathit{cosEst}\cdot \sin \left(4\mathit{\pi K}\right)-\mathit{sinEst}\cdot \mathit{cos}\left(4\mathit{\pi K}\right)-4\pi \cdot \mathit{cosEst}\cdot K\right)}{\cos \left(4\mathit{\pi K}\right)+8K^2{\pi }^2-1};\right)\end{array}$$

    

    wherein K is a compensation coefficient calculated as: $$K=\mathit{numSample}\cdot {\mathit{\Delta t}}_{\mathit{SAMPLE},s}\cdot \mathit{inFreq};$$

    
12. Induction hob according to claim 11, wherein a compensated phase delay is computed based on the following formula: $$\mathit{peakValueComp}=\sqrt{\cos \mathrm{\hspace{0.17em}}{\mathit{Comp}}^2+\sin \mathrm{\hspace{0.17em}}{\mathit{Comp}}^2};$$

    

    $$\mathit{VcMaxComp}=\mathit{avgEst}+\mathit{peakValueComp};$$

    

    $$\mathit{VcMinComp}=\mathit{avgEst}-\mathit{peakValueComp};$$

    

    $$\mathit{phaseDelayVcVomp}={\tan }^{-1}\frac{\sin \mathrm{\hspace{0.17em}}\mathit{Comp}}{\cos \mathrm{\hspace{0.17em}}\mathit{Comp}};$$

    

    $$IcComp\mathrm{\hspace{0.17em}}=2\mathit{\pi ƒ}\left(\mathit{VcMaxComp}-\mathit{VcMinComp}\right)\cdot \mathit{Cres};$$

    
13. Induction hob according to claim 12, wherein an estimated average power value is calculated based on the following formula: $$\mathit{avgPowerEst}=\frac{{\displaystyle {\sum }_{k=1}^{\mathit{nSamples}}\mathit{Vmain}\left(k\right)\cdot {\mathit{Ic}}_{\mathit{Est}\left(k\right)}\cdot \mathit{PhM}\_\mathit{Est}\left(k\right)}}{2\ast \mathit{MainLineFreq}};$$

    

    wherein Vmain is the rectified value of main line voltage at sample k, I_cEst is the estimated value of compensated or uncompensated coil current obtained by the resonance capacitor analysis and PhM_{_}Est is the compensated or uncompensated phase delay obtained by the resonance capacitor analysis.
14. Induction hob according to anyone of the preceding claims, comprising no current transducer (8) electrically coupled with the induction coil (6), wherein said first and second electrical information related to the induction coil (6) are provided by an algorithm calculating maximum and minimum peak values of resonance capacitor voltage by reconstructing a sinusoidal wave based on sampled values of said resonance capacitor voltage.
15. Method for operating an induction hob, the induction hob comprising a circuitry (1a) for powering at least one induction coil (6), the circuitry (1a) comprising a power circuit portion (7) with at least one switching element (4, 5) adapted to provide pulsed electric power to said induction coil (6) and an oscillating circuit portion (9) comprising at least one resonance capacitor (9.1, 9.2) which is associated with a resonance capacitor voltage, said induction coil (6) being electrically coupled with said power circuit portion (7) and said oscillating circuit portion (9), wherein said induction hob comprises a control entity (10) performing the steps of:

    - providing and/or receiving resonance capacitor information, for example resonance capacitor voltage values;

    - applying a discrete mathematical transformation to the resonance capacitor information thereby obtaining modified resonance capacitor information,;

    - calculating first and second electrical information related to the induction coil (6), the first and second electrical information being information regarding the amplitude of the electric current (Ic) provided through said induction coil (6) and information regarding the phase delay between the electric current (Ic) provided through said induction coil (6) and the voltage applied to the induction coil (6) based on said modified resonance capacitor information.

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