Patent Description:
The ultrasound sensors, such as flow sensors, suffer from zero point drift as a function temperature and ageing. The ultra sound sensors typically work at a resonance. However, aging changes the resonance frequency and lowers quality of resonance. For example in household water flow meters and heating, ventilation, and air conditioning (HVAC) air flow meters, the operation time is more than ten years.

There have been several attempts to overcome both the drift and the aging. Sensors have been carefully chosen and mechanical design has been kept as precise as possible. As perfectly matched sensors as possible and symmetrical structures that have tight manufacturing tolerance requirements are used for minimizing the problem, but the compensation still leaves room for improvement. Additionally, they increase manufacturing throughput time, complexity and cost.

Temperature compensation and calibration in different temperatures have also been tried but they do not actually solve the problem either.

Document <CIT> discloses an ultrasound flow sensor apparatus wherein a frequency controller receiving a feedback from an ultrasound transmitter and lock a frequency of the ultrasound output to driving frequency of the ultrasound transmitter in order to control the frequency of the ultrasound output by the ultrasound transmitter. Hence, an improvement would be welcome.

The present invention seeks to provide a novel ultrasound sensor apparatus and a novel ultrasound transmission method.

Furthermore, words "comprising" and "including" should be understood as not limiting the described embodiments to consist of only those features that have been mentioned and such embodiments may also contain features/structures that have not been specifically mentioned. All combinations of the embodiments are considered possible if their combination does not lead to structural or logical contradiction.

It should be noted that while Figures illustrate various embodiments, they are simplified diagrams that only show some structures and/or functional entities. The connections shown in the Figures may refer to logical or physical connections. It is apparent to a person skilled in the art that the described apparatus may also comprise other functions and structures than those described in Figures and text. It should be appreciated that details of some functions, structures, and the signalling used for measurement and/or controlling are irrelevant to the actual invention. Therefore, they need not be discussed in more detail here.

<FIG> illustrates an example of a measurement of flowing fluid. The flow of fluid has a speed v which may be constant with respect to time or it may vary as function of time. The fluid may flow in a tube, for example.

The fluid is flowable matter. The fluid may be gas or liquid. If the fluid is liquid, the fluid may be composed of only one liquid or it may be a mixture of two or more different liquids. Alternatively, if the fluid is gas, the fluid may be composed of only one gas or it may be a mixture of two or more different gases. Alternatively the fluid may be a mixture of at least one liquid and at least one gas. The gas may be composed of only one gas or it may be a mixture of two or more gases. Furthermore, the fluid may be suspension or emulsion, for example.

The ultrasound is sound in a frequency range in which a lowest frequency is higher than the highest audible frequency that a human being can hear. The ultrasound is often defined to be a sound with one or more frequencies above <NUM>. The range of ultrasound may be within a frequency range <NUM> - <NUM>, for example.

In the example of <FIG>, an ultrasound sensor arrangement may comprise a transmitter <NUM> and two receivers R1 and R2. The ultrasound transmitter <NUM> transmits ultrasound through the flowing fluid towards the receivers R1 and R2 in order to measure the speed v of the fluid. A phase difference Δθ between the ultrasound signals detected by the receivers R1 and R2 can be mathematically expressed as: <MAT> where k is wavenumber of the ultrasound and can be expressed as k = <MAT>, where f is frequency of the ultrasound, M is Mach number expressing relation v/c between the speed of sound c and the speed of the flowing fluid v, X1 is distance between the ultrasound transmitter <NUM> and the first receiver R1, X2 is a distance between the ultrasound transmitter <NUM> and the second receiver R2. In this example, the ultrasound is considered to have only one frequency f and thus only one wavenumber k.

If the distances X1 and X2, D1 and D2 are the same, X1 = X2 and D1 = D2, the difference of the square roots multiplied by the wavenumber k, <MAT>, which may be called an error term, can be eliminated and becomes thus <NUM>. But if that condition is not met, which happens in practice because of assembly inaccuracy and deformations which in turn are caused temperature variation. A result is a zero offset error caused by temperature variation to the measurement performed by the transmitter <NUM> and the receivers R1 and R2. Namely, the speed of sound c is a function of temperature T, and the dependence may be expressed to be at least approximately: <MAT> where K denotes Kelvin in Kelvin scale.

Because the wavenumber k depends on the speed c of sound <MAT>, the phase difference also depends on the temperature T if the symmetry condition is not met. In the prior art, the temperature T of the fluid has been tried to keep constant (see patent document <CIT>) to overcome the technical problem. The temperature of the fluid cannot be kept constant in real life. However, this cumbersome solution of keeping the temperature of the fluid constant may be made unnecessary when the problem of the ultrasound measurement is solved in a manner disclosed in this document, although the solution of keeping temperature of the fluid constant may be used together with what is taught in this document.

<FIG> illustrates an example of the ultrasound sensor apparatus. The ultrasound sensor apparatus <NUM> comprises at least one ultrasound transmitter <NUM> and a frequency controller <NUM>. The at least one ultrasound transmitter outputs ultrasound into fluid. The frequency controller <NUM> receives information on temperature of the fluid and controls the frequency of the ultrasound output by the at least one ultrasound transmitter <NUM> as a function of the temperature of the fluid.

<FIG> illustrates an example of driving configuration of a transducer <NUM> of the ultrasound sensor apparatus <NUM>. The transducer <NUM>, which can be considered to comprise the transmitter <NUM> and potentially also the receiver R1, R2, may be tuned to operate at a resonance frequency by adjusting a direct current (DC) bias of the transducer <NUM> with a regulable DC voltage source <NUM>. The DC voltage source <NUM> may be coupled in parallel with an alternate current (AC) voltage source <NUM>. Alternatively the DC voltage source <NUM> may be coupled in series with the AC voltage source <NUM>. With the DC voltage source <NUM> tunes the transducer <NUM> to a desired resonance frequency. A person skilled in the art is familiar with the tuning of the transducer <NUM>, per se. The controller <NUM> comprises necessary equipment or electronics for obtaining the actual information or variable for the control.

In an embodiment, the frequency controller <NUM> may adjust relation between the frequency and the speed of the ultrasound in the fluid as a function of the temperature of the fluid. Because the speed c of the ultrasound increases with increasing temperature, also the wavelength becomes longer. That, in turn, causes the wavenumber k to become smaller. The relation S of the speed c and the wavenumber k is S = c/k and the relation S will increase with larger speed c and smaller wavenumber k. That is why the frequency controller <NUM> may increase the frequency such that the wavenumber k becomes larger. When the temperature T goes down, the frequency controller <NUM> may decrease the frequency such that the wavenumber k becomes smaller.

In an embodiment, the frequency controller <NUM> may keep relation between frequency and speed of the ultrasound in the fluid constant as a function of the temperature of the fluid. That is, when the ultrasound sensor apparatus <NUM> is calibrated at a certain temperature T<NUM> before the assembly, the controller <NUM> may keep the relation constant if or when the temperature T varies. If the temperature goes up, the speed of ultrasound c goes also up and hence the controller <NUM> also increases the frequency of the ultrasound output by the ultrasound transmitter <NUM>. Then the frequency controller <NUM> may adjust the frequency such that the wavenumber k becomes larger to make the relation S constant i.e. S = c<NUM>/k<NUM> = c<NUM>/k<NUM>, where c<NUM> and k<NUM> are the original speed and wavenumber, respectively, in an original condition and c<NUM> and k<NUM> are speed and wavenumber is temperature different from the original condition.

In an embodiment, the ultrasound sensor apparatus <NUM> may also comprise one or more ultrasound receivers R1, R2. The ultrasound sensor apparatus <NUM> may measure the temperature of the fluid based on one or more averages of time-of-flights of the ultrasound that has travelled through the fluid and is received by the one or more ultrasound receivers R1 and R2. The time-of-flight may be measured indirectly as phase shifts or by measuring the time-of-flights using a timer. In an embodiment, the ultrasound sensor apparatus <NUM> comprises a transceiver or transducer which both transmits and receives ultrasound signals.

In an embodiment, the ultrasound sensor apparatus <NUM> may comprise a temperature meter <NUM>, which measures the temperature of the fluid, and may feed the information on said temperature of the fluid to the frequency controller <NUM>. In an embodiment, the temperature meter <NUM> may comprise a semiconductor temperature sensor. In an embodiment, the temperature meter <NUM> may comprise a resistance temperature sensor, a pyrometer, a thermistor, a thermocouple, for example. Additionally or alternatively, the temperature meter <NUM> may be configured to detect temperature of the fluid based on an infrared measurement.

<FIG> illustrates the ultrasound sensor apparatus <NUM>. Since the ultrasound sensor apparatuses typically works in a resonance mode and the resonance frequency of the piezoelectric micro-machined ultrasonic (PMUT) or (capacitive micro-machined ultrasonic) CMUT sensor can be tuned by DC voltage, the transceiver resonances can be tuned to be same as the operation frequency. In other words, the sensor resonance is or the at least one ultrasound transmitter <NUM> locked to driving frequency that is controlled by the controller <NUM>. The resonance tuning can be used to alleviate or eliminate ageing and need for precise sensor matching.

In an embodiment, the frequency controller <NUM> may receive information on resonance or a change of the resonance of the ultrasound sensor apparatus <NUM>, the resonance or the change of the resonance being formed and/or stored as a function of time. The frequency controller <NUM> may then adjust the frequency of the ultrasound output by the at least one ultrasound transmitter <NUM> based on the resonance or the change and time defining the moment of the resonance or the change.

In an embodiment, the frequency controller <NUM> may estimate information on resonance or a change of the resonance of the ultrasound sensor apparatus <NUM>, the resonance or the change of the resonance being formed and/or stored as a function of time. The frequency controller <NUM> may then adjust the frequency of the ultrasound output by the at least one ultrasound transmitter <NUM> based on the resonance or the change and time defining the moment of the resonance or the change.

In an embodiment, the frequency controller <NUM> may have information on resonance or a change of the resonance of the ultrasound sensor apparatus <NUM> in its memory, the resonance or the change of the resonance being stored as a function of time. The frequency controller <NUM> may then the frequency of the ultrasound output by the at least one ultrasound transmitter <NUM> based on the resonance or the change and time defining the moment of the resonance or the change. The ultrasound sensor apparatus <NUM> may have the information in a table form in the memory. In an embodiment, the frequency controller <NUM> may have the information in its memory (see <FIG>). The information on the resonance or the change of the resonance as a function of time may be stored in the memory during manufacturing process, for example.

<FIG> illustrates a little bit more in details the ultrasound sensor apparatus <NUM>. Output from the transmitter <NUM> may be led to an amplifier <NUM>. The output of which may be fed to frequency multipliers <NUM> and <NUM>. The multiplier <NUM> may output a voltage or other electric signal based on a phase difference of the multiplied signals, which include a signal from the amplifier <NUM>, and a signal from the AC source <NUM> of a driving source <NUM> having <NUM>° phase shift. The <NUM>° phase shift may be performed by a phase shifter <NUM>, which may be analog or digital. This branch may be called Q-phase.

The multiplier <NUM> may also output a voltage based on a phase difference of the multiplied signals, which include a signal from the amplifier <NUM>, and a signal from the AC source <NUM> of a driving source <NUM>. This branch may be called I-phase, and this kind of IQ-phased signal processing may be called a quadrature detection. At least one of the outputs of the multipliers <NUM>, <NUM> is fed back to the frequency controller <NUM>. This kind of feedback may be considered a phase-lock-loop, and it may be used to control output frequency drift. The Q-phase signal may be used as measurement signal of the ultrasound apparatus <NUM> and it may with the I-phase signal be input to a data processing unit <NUM>, which include the frequency controller <NUM> although in <FIG> the two are drawn separately. The flow of the fluid can be calculated based on the I- and Q-signals in the controller <NUM>.

The data processing unit <NUM> may include or be in contact with a user interface <NUM>, which may present information on the ultrasound measurement of the fluid to a user. The user interface <NUM> may comprise a screen, a keyboard and/or a touch screen for inputting data to the ultrasound apparatus <NUM>. The data processing unit <NUM> may have a wired or wireless connection to a data network such as a local network and/or the Internet, for example, for data transfer and/or control of the ultrasound apparatus <NUM>.

<FIG> illustrates an example of the frequency controller <NUM> and/or the data processing unit <NUM>. The frequency controller <NUM> and/or the data processing unit <NUM> may comprise one or more processors <NUM>, and one or more memories <NUM> including computer program code. The one or more memories <NUM> and the computer program code may, with the one or more processors <NUM>, cause the frequency controller <NUM> and/or the data processing unit <NUM> at least to control the frequency of the ultrasound output by the at least one ultrasound transmitter <NUM> as a function of the temperature of the fluid.

What is explained above may have several technical advantages. Sensors' accuracy may be better. Requirement for a perfectly matched sensor may be avoided. Sensitivity may be good because the ultrasound sensor apparatus may be kept always in resonance, which may be critical for air coupled PMUTs and CMUTs, for example. Mechanical design may be non-critical. Need for calibration may be reduced. The zero offset correction may be performed during operation of the ultrasound sensor apparatus <NUM>, and it may be performed continuously, regularly or irregularly.

<FIG> is a flow chart of the transmitting method. In step <NUM>, ultrasound is output into fluid by at least one ultrasound transmitter <NUM> of an ultrasound sensor apparatus <NUM>.

In step <NUM> information on temperature of the fluid is received by a frequency controller <NUM> of the ultrasound sensor apparatus <NUM>.

In step <NUM>, the frequency of the ultrasound output by the at least one ultrasound transmitter <NUM> is controlled as a function of the temperature of the fluid by the frequency controller <NUM>.

In step <NUM>, which is optional, the temperature of the fluid may be measured by a temperature meter <NUM>; and the information on said temperature may be fed to the frequency controller <NUM> of the ultrasound sensor apparatus <NUM>.

The method shown in <FIG> may be implemented as a logic circuit solution or computer program. The computer program may be placed on a computer program distribution means for the distribution thereof. The computer program distribution means is readable by a data processing device, and it encodes the computer program commands, carries out the measurements and optionally controls the processes on the basis of the measurements.

The computer program may be distributed using a distribution medium which may be any medium readable by the controller. The medium may be a program storage medium, a memory, a software distribution package, or a compressed software package. In some cases, the distribution may be performed using at least one of the following: a near field communication signal, a short distance signal, and a telecommunications signal.

Claim 1:
An ultrasound flow sensor apparatus, wherein
the ultrasound flow sensor apparatus (<NUM>) comprises at least one ultrasound transmitter (<NUM>), which is configured to output ultrasound into flowing fluid,
wherein the ultrasound flow sensor apparatus (<NUM>) comprises a frequency controller (<NUM>), which is configured to receive information on temperature of the fluid and feedback from the at least one ultrasound transmitter (<NUM>), and lock a frequency of the ultrasound output to driving frequency of the at least one ultrasound transmitter (<NUM>) in order to control the frequency of the ultrasound output by the at least one ultrasound transmitter (<NUM>) as a function of the temperature of the fluid.