WO2019078838A1

WO2019078838A1 - Frequency-domain computations of vibration damping

Info

Publication number: WO2019078838A1
Application number: PCT/US2017/057137
Authority: WO
Inventors: John Rossi; Erik D. Ness; Toby COWGER; Rodney Chew Yong TAY
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2017-10-18
Filing date: 2017-10-18
Publication date: 2019-04-25

Abstract

In some examples, a controller is to receive measurement data acquired by a vibration-based sensor provided in a fluid supply, convert the measurement data from to a frequency domain, and compute, in the frequency domain, a metric representing damping of vibration of the vibration-based sensor, based on a first value representing a signal component of the measurement data and a second value representing a noise component of the measurement data.

Description

FREQUENCY-DOMAIN COMPUTATIONS OF VIBRATION DAMPING Background

[0001 ] A printing system can include a printhead that has nozzles to dispense printing fluid to a target. In a two-dimensional (2D) printing system, the target is a print medium, such as a paper or another type of substrate onto which print images can be formed. Examples of 2D printing systems include inkjet printing systems that are able to dispense droplets of inks. In a three-dimensional (3D) printing system, the target can be a layer or multiple layers of build material deposited to form a 3D object

Brief Description of the Drawings

[0002] Some implementations of the present disclosure are described with respect to the following figures.

[0003] Fig. 1 is a block diagram of a system that includes a fluid supply and a controller for determining a level of fluid in the fluid supply, according to some examples.

[0004] Fig. 2 is a schematic diagram of a fluid supply and a circuit board, according to further examples.

[0005] Fig. 3 is a flow diagram of a process of determining a level of fluid or presence of a vibration-based sensor, according to additional examples.

[0006] Fig. 4 is a flow diagram of a process of determining a level of fluid or presence of a vibration-based sensor, according to further examples.

[0007] Fig. 5 is a block diagram of an apparatus including a controller according to other examples.

[0008] Fig. 6 is a block diagram of a printing system according to further examples. [0009] Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. The figures are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings provide examples and/or implementations consistent with the description; however, the description is not limited to the examples and/or implementations provided in the drawings.

Detailed Description

[0010] In the present disclosure, use of the term "a," "an", or "the" is intended to include the plural forms as well, unless the context clearly indicates otherwise. Also, the term "includes," "including," "comprises," "comprising," "have," or "having" when used in this disclosure specifies the presence of the stated elements, but do not preclude the presence or addition of other elements.

[001 1 ] A printing system can receive a printing fluid supply, or alternatively, multiple printing fluid supplies, that contain printing fluid(s) for use in printing onto a target.

[0012] A printing system can be a two-dimensional (2D) or three-dimensional (3D) printing system. A 2D printing system dispenses printing fluid, such as ink, to form images on print media, such as paper media or other types of print media. A 3D printing system forms a 3D object by depositing successive layers of build material. Printing agents dispensed from the 3D printing system can include ink, as well as agents used to fuse powders of a layer of build material, detail a layer of build material (such as by defining edges or shapes of the layer of build material), and so forth.

[0013] Sensors can be used to detect a level of printing fluid in a printing fluid supply. In some examples, such sensors can include electronic sensors (such as in the form of chips), optical sensors, and so forth. The sensors can add some amount of cost to the printing fluid supplies, such as due to the cost of the parts for the sensors, and the cost of adding the sensors to printing fluid supplies during manufacture of the printing fluid supplies. [0014] In accordance with some implementations of the present disclosure, a vibration-based sensor (or alternatively, multiple vibration-based sensors) can be used in a fluid supply, such as a printing fluid supply for use in a printing system, or another type of fluid supply in another type of system. In some examples, a vibration-based sensor can include a tuning fork. More generally, a vibration-based sensor has a portion (a "vibration element") that is free to vibrate in response to an input stimulus.

[0015] A measurement device is able to measure a parameter of the vibration element of the vibration-based sensor. The measured parameter exhibits different characteristics depending upon whether the vibration element is immersed in a fluid, or not immersed in a fluid.

[0016] In some implementations of the present disclosure, a controller performs various tasks to allow the controller to determine a level of a fluid in a fluid supply. The tasks include receiving measurement data acquired by the vibration-based sensor provided in the fluid supply, converting the measurement data from a first domain (e.g., a time domain, a spatial domain, etc.) to a frequency domain.

Additionally, the tasks include computing, in the frequency domain, a metric representing damping of vibration of the vibration-based sensor, based on a first value (e.g., a power spectral density) representing a signal component of the measurement data and a second value (e.g., a power spectral density) representing a noise component of the measurement data.

[0017] Although reference is made to fluid supplies for use in printing systems in some examples, it is noted that techniques or mechanisms of the present disclosure are applicable to other types of fluid dispensing systems used in non-printing applications that are able to dispense fluids through nozzles or other fluid outlets. Examples of such other types of fluid dispensing systems include those used in fluid sensing systems, medical systems, vehicles, fluid flow control systems, and so forth.

[0018] Fig. 1 is a block diagram of an example system 100 that includes a fluid supply 102 and a controller 104. In some examples, the system 100 can be a printing system, and the fluid supply 102 can be a printing fluid supply. In other examples, the system 100 can be a fluid dispensing system used in a non-printing application.

[0019] The controller 104 can include a hardware processing circuit, such as any or some combination of the following: a microprocessor, a core of a multi-core microprocessor, a microcontroller, a programmable gate array, a programmable integrated circuit device, or any other type of hardware processing circuit.

Alternatively, the controller 104 can include a combination of a hardware processing circuit and machine-readable instructions executable on the hardware processing circuit.

[0020] The fluid supply 102 can be in the form of a cartridge, a printbar, or any other supply in the form of a tank, box, and so forth, to store fluid. In Fig. 1 , the fluid supply 102 includes a fluid reservoir 103 contained within a housing 105 of the fluid supply 102. The fluid reservoir 103 holds a fluid that can be dispensed through an outlet 106 (or alternatively, multiple outlets 106) along a direction indicated by arrow 108. An outlet can also be referred to as a nozzle. Although Fig. 1 shows dispensing of fluid downwardly from the fluid supply 102, it is noted that in other example, fluid can be dispensed from the fluid supply 102 in a different direction.

[0021 ] The fluid supply 102 can be removably mounted in the system 100. In such examples, the system 100 can be provided to an end user without the fluid supply 102. Once the end user receives the system 100, the end user can install the fluid supply 102 in the system 100. When the fluid in the fluid supply 102 is depleted as a result of use, the end user can remove the fluid supply from the system 100 for replacement with another fluid supply.

[0022] In other examples, the fluid supply 102 is fixedly attached in the system 100. In such latter examples, fluid in the fluid supply 102 can be replenished by injecting fluid into the fluid supply 102. [0023] The system 100 has a fluid supply mounting structure 1 10, onto which the fluid supply 102 can be installed. In some examples, the mounting structure 1 10 includes a carriage that is movable within the system 100 to move the mounted fluid supply 102 to different locations for dispensing fluid onto a target at those locations. In a 3D printing operation, the target can include a 3D object that is formed with successive layers. In a 2D printing operation, the target can include a print medium, such as paper, plastic, and so forth. In non-printing applications, the target can refer to any object or location onto or toward which fluid is to be dispensed.

[0024] In accordance with some examples of the present disclosure, the fluid supply 102 includes a vibration-based sensor 1 12 that can be used to detect a level of fluid in the fluid reservoir 103. The vibration-based sensor 1 12 is electrically connected over a link 1 14 to the controller 104. The link 1 14 can include an electrical conductor (or a set of electrical conductors). Alternatively, the link 1 14 can include a wireless link, and the vibration-based sensor 1 12 can communicate wirelessly with the controller 104 over the wireless link.

[0025] The controller 104 can perform various tasks, including a frequency- domain vibration damping metric computation task 1 16. The frequency-domain vibration damping metric computation task computes a metric that represents damping of the vibration-based sensor 1 12.

[0026] The controller 104 also performance a fluid level determining task 1 18 that can be used to determine a level of fluid in the fluid reservoir 103, based on the damping metric computed by the frequency-domain vibration damping metric computation task 1 16.

[0027] Fig. 2 is a schematic diagram of an example arrangement that includes a fluid supply 202 and a circuit board 204, according to further examples. The fluid supply 202 and the circuit board 204 can be provided in a fluid dispensing system. In the view of Fig. 2, the fluid supply 202 and the circuit board 204 are separated from one another (i.e., they are not connected). In operation, the circuit board 204 is connected to the fluid supply 202 (as explained further below). [0028] The fluid supply 202 shown in Fig. 2 is in the form of a cartridge. An outer housing 204 of the fluid supply 202 defines an inner chamber that stores a fluid. A portion of the outer housing 204 is cut away (as designated at 206) to show a vibration element 208 (e.g., a tuning fork or any other element that can vibrate responsive to a mechanical stimulus). The vibration element 208 can be formed of a metal or another type of electrically conductive material.

[0029] The vibration element 208 is inside a fluid reservoir 210 defined inside the housing 204 of the fluid supply 202. The vibration element 208 has a first end portion 208-1 attached to a wall of the housing 204 or to another fixed structure inside the fluid supply 202. The vibration element 208 further includes a second end portion 208-2 that is free to vibrate.

[0030] In some examples of the present disclosure, vibration of the vibration element 208 (more specifically, vibration of the second end portion 208-2) is caused by movement of the fluid supply 202. The movement of the fluid supply 202 provides a mechanical stimulus that induces vibration of the vibration element 208.

[0031 ] As shown in Fig. 2, the fluid supply 202 can be attached to a moveable carriage 212. The moveable carriage 212 can be moved during operation of a system, such as a printing system or other type of fluid dispensing system.

[0032] There can be two types of mechanical stimulus for causing vibration of the vibration element 208. A first type of mechanical stimulus is an impulse stimulus, which is produced by a single movement of the carriage 212. This single movement of the carriage 212 produces an impulse that results in vibration of the vibration element 208 at its resonance frequency.

[0033] A second type of mechanical stimulus is a fixed frequency stimulus produced by moving the carriage 212 back and forth in an oscillating fashion at a specified frequency. In such examples, the vibration element 208 vibrates at the oscillating frequency of the input stimulus caused by the oscillating movement of the carriage in 212. [0034] In some examples, electrically conductive pads 216 (formed of a metal or another type of electrically conductive material) can be placed on an outer surface 218 of the housing 204. The electrically conductive pads 216 and the vibration element 208 can form the plates of a capacitor, where the dielectric between the plates of the capacitor include a layer of the housing 204 and the material inside the fluid reservoir 210. The material inside the fluid reservoir 210 can be a fluid, air, or a combination of a fluid and air. The capacitance of this capacitor formed using the electrically conductive pads 216 and the vibration element 208 can be measured by a capacitance measurement device, which can be provided externally of the fluid supply 202 in some examples.

[0035] Although two electrically conductive pads 216 are shown in Fig. 2, it is noted in other examples, just one electrically conductive pad or more than two electrically conductive pads can be used for measuring capacitance.

[0036] The electrically conductive pads 216 are electrically connected over traces 220 to an electrical connector 222, which can also be arranged on an outer surface (e.g., top surface) of the housing 204. The electrical connector 222 allows for an electrical connection to be made between the fluid supply 202 and the circuit board 204. The circuit board 204 has a corresponding connector (not shown) that can mate with the connector 222.

[0037] A controller 224 (which is similar to the controller 104 of Fig. 1 ) can be mounted on the circuit board 204. The controller 224 can receive measurement data corresponding to output signals from the electrically conductive pads 216, which represent a capacitance between the electrically conductive pads and the vibration element 208. The signals from the electrically conductive pads 216 can be analog signals (e.g., alternating current or AC signals). In some examples, the controller 224 can include an analog-to-digital (ADC) converter to convert the analog signals from the fluid supply 202 to digital data that can be processed by the controller 224. In other examples, an ADC converter external of the controller 224 can be provided on the circuit board 204. [0038] The controller 224 receives measurement data acquired by a vibration- based sensor that includes the vibration element 208 and the electrically conductive pads 216. Receiving measurement data acquired by the vibration-based sensor can refer to receiving measurement data directly as output by the vibration-based sensor (e.g., analog signals of the electrically conductive pads 216 output by the connector 222) or measurement data produced by an ADC converter or other intermediate device that converts between the analog signals of the vibration-based sensor and converted signals (e.g., digital signals) that is received by the controller 224.

[0039] The vibration element 208 has a damping characteristic that changes depending upon whether or not the vibration element 208 is immersed in a fluid or not immersed in a fluid. More specifically, when the vibration element 208 is immersed in a fluid, then a vibration of the vibration element 208 is damped by a greater extent than if the vibration element 208 were not immersed in a fluid (i.e., the vibration element 208 vibrates in air inside the fluid reservoir 210 of the fluid supply 202, such as when the fluid has been depleted inside the fluid supply 202).

[0040] The amount of damping of the vibration element 208 refers to how quickly vibration of the vibration element 208 is reduced. By computing a damping metric that represents damping of the vibration of the vibration-based sensor, the controller 224 is able to determine the level of fluid in the fluid supply 202.

[0041 ] Although Figs. 1 and 2 show fluid supplies with just one vibration-based sensor, it is noted that in other examples, a fluid supply can include multiple vibration-based sensors, and the controller (104 in Fig. 1 or 224 in Fig. 2) can receive measurement data from the multiple vibration-based sensors to determine damping metrics for the respective multiple vibration-based sensors, and to determine a fluid level in the fluid supply based on the damping metrics.

[0042] Fig. 3 is a flow diagram of a process 300 of determining a fluid level in a fluid supply (e.g., the fluid supply 102 or 202), in accordance with some examples. The process can be performed by a controller (e.g., the controller 104 or 224). [0043] The process 300 receives (at 302) measurement data acquired by a vibration-based sensor in the fluid supply. The process 300 converts (at 304) the measurement data from a first domain (e.g., time domain, spatial domain, etc.) to a frequency domain. This conversion can be performed by applying a discrete Fourier transform (DFT) on the measurement data. For example, a fast Fourier transform (FFT) can be used to compute the DFT on the measurement data. Note that the measurement data that is converted is a digitized form of the measurement data acquired by the vibration-based sensor.

[0044] The process 300 further computes (at 306), in the frequency domain, a metric representing damping of vibration of the vibration-based sensor, based on a first value representing a signal component of the measurement data and a second value representing a noise component of the measurement data. The computation of such a damping metric is discussed in further detail below.

[0045] The process 300 outputs (at 308), based on the computed metric, an indication of a level of fluid in the fluid supply or presence of the vibration-based sensor.

[0046] Fig. 4 depicts an example process 400 for computing a damping metric (referred to as a "figure of merit") that represents a strength of damping, according to further examples. The following assumes that a fluid supply includes two vibration- based sensors, where the two vibration-based sensors include respective damping elements that vibrate at different resonance frequencies. Although reference is made to an example with two vibration-based sensors, it is noted that in other examples, the technique described can be applied to a fluid supply which just one vibration-based sensor or to a fluid supply that has more than two vibration-based sensors.

[0047] The process 400 defines (at 402) a first target frequency FREQ1 for the first vibration-based sensor, and a second target frequency FREQ2 for the second vibration-based sensor. In examples where the stimulus for vibrating the vibration- based sensors is an impulse stimulus, then the target frequency FREQ1 is the resonance frequency of the first vibration-based sensor, and the target frequency FREQ2 is the resonance frequency of the second vibration-based sensor. The resonance frequencies of the first and second vibration-based sensors can be the same or can be different.

[0048] In some examples, FREQ1 and FREQ2 can be in the form of a frequency range, where FREQ1 represents a first range of frequencies, and FREQ2 represents a second range of frequencies. A range of frequencies can take in to account the tolerance of vibration of each of the vibration-based sensors. For example, FREQ1 can be centered at the expected resonance frequency of the first vibration-based sensor, and a range can be defined on both sides of the expected resonance frequency that takes into account the tolerance of vibration of the first vibration- based sensor. FREQ1 can thus be expressed as

EXPECTED_RESONANCE_FREQ_1 ± X Hertz (Hz), where X represents a frequency tolerance band. FREQ2 can be defined in similar fashion.

[0049] If the stimulus to induce vibration of the vibration-based sensors is a fixed frequency stimulus (e.g., due to oscillating motion of the fluid supply), then FREQ1 and FREQ2 will have the same frequency value or same frequency range, i.e., the frequency value or range of frequencies corresponding to the frequency of oscillation of the motion of the fluid supply.

[0050] The process 400 further defines (at 404) a noise window. The noise window specifies the frequencies at which noise is expected to occur. Noise can refer to the portion of the measurement data at frequencies that are outside of the range of frequencies expected for the signal component of the measurement data. The signal component of the measurement data corresponds to the portion of the measurement data that is caused by the vibration of the vibration-based sensor.

[0051 ] In some examples, the noise window can include a range of low

frequencies that corresponds to low-frequency noise. In other examples, the noise window can include a range of other frequencies at which noise is expected to occur. The noise window excludes frequencies of the signal component of the measurement data for the first vibration-based sensor, selected harmonics of the signal component of the measurement data for the first vibration-based sensor, the signal component of the measurement data for the second vibration-based sensor, and selected harmonics of the signal component of the measurement data for the second vibration-based sensor. In other words, the frequencies of such components are not part of the noise window.

[0052] Each harmonic of a signal component occurs at a corresponding frequency. For each harmonic frequency, a range of frequencies can be defined around the harmonic frequency, such as ±M Hz around the harmonic frequency.

[0053] The process 400 processes (at 406) the measurement data. The processing of the measurement data can include the following tasks, for example.

[0054] As part of the processing, the process 400 can remove a DC offset of the measurement data. Also, the processing can include applying a Hanning window or other type of windowing function to remove endpoint distortion effects. In addition, the processing can pad the measurement data with zeros at the end to improve bin resolution.

[0055] The processing also includes applying a DFT to the foregoing processed measurement data (e.g., after removal of the DC offset, application of a windowing function, and padding of the measurement data), to convert the measurement data from a first domain to measurement data in the frequency domain. In some examples, the DFT can involve computing an FFT on the processed measurement data. The processing further includes computing a power spectral density (PSD) parameter based on the measurement data in the frequency domain. For example, the PSD parameter can be computed as follows:

PSD=ABS(FFT(measurement data))². (Eq. 1 )

[0056] In Eq. 1 , FFT(measurement data) represents the computation of an FFT on the processed measurement data, and ABS() is a function, applied on FFT(measurement data), that returns the modulus or complex amplitude of a complex number corresponding to FFT(measurement data).

[0057] The power spectral density represents the distribution of power into frequency components composing the signal represented by the measurement data in the frequency domain. The power spectral density includes power values at respective different frequencies.

[0058] The process 400 next determines (at 408) an oscillation frequency of each of the vibration-based sensors. The process 400 makes this determination by finding a frequency of largest peak (of PSD values) within a signal window. In other words, the process 400 attempt to find a first largest PSD value (from multiple PSD values) within the target FREQ1 range, and find a second largest PSD value (from multiple PSD values) within the target FREQ2 range. The frequency at which the largest PSD value occurs within the target FREQ1 range is the oscillation frequency of the first vibration-based sensor, and the frequency at which the largest PSD value occurs within the target FREQ2 range is the oscillation frequency of the second vibration-based sensor.

[0059] The process 400 uses (at 410) the determined oscillation frequency of each respective vibration-based sensor to determine a signal window and harmonics windows for the respective vibration-based sensor. The signal window refers to a window of frequencies of the signal component of the measurement data produced by the respective vibration-based sensor, and the harmonic windows refer to the frequency ranges corresponding to the harmonic frequencies of the signal component of the measurement data produced by the respective vibration-based sensor.

[0060] A first signal window (for the first vibration-based sensor), represented as SIGNAL_WINDOW_l, includes a first range of frequencies centered at the determined oscillation frequency (represented as MEAS_FREQ_1) of the first vibration-based sensor with a tolerance band around the measured oscillation frequency of the first vibration-based sensor. [0061 ] The second signal window (for the first vibration-based sensor), represented as SIGNAL_WINDOW_2, includes a second range of frequencies centered at the determined oscillation frequency (represented as MEAS_FREQ_2) of the second vibration-based sensor with a tolerance band around the measured oscillation frequency of the second vibration-based sensor.

[0062] The harmonic window for a first harmonic of the signal component for the first vibration-based sensor is computed as follows:

HARM_WIND_1_1 = MEAS_FREQ_1² + HARMONIC_TOLERANCE. (Eq. 2)

[0063] The harmonic window for a second harmonic of the signal component for the first vibration-based sensor is computed as follows:

HARM_WIND_2_1 = MEAS_FREQ_1³ + HARMONIC_TOLERANCE. (Eq. 3)

[0064] The harmonic window for the n^th harmonic of the signal component for the first vibration-based sensor is computed as follows:

HARM_WIND_n_l = MEAS_FREQ_lⁿ⁺¹ + HARMONIC_TOLERANCE. (Eq. 4)

[0065] The harmonic windows of the signal component for the second vibration- based sensor can be computed in similar fashion, where the n^th harmonic of the signal component for the second vibration-based sensor is computed as follows:

HARM_WIND_n_2 = MEAS_FREQ_2ⁿ⁺¹ + HARMONIC_TOLERANCE. (Eq. 5)

[0066] The process 400 then computes (at 412) the actual noise window based on the signal windows and harmonics windows computed (at 410). The noise window includes any frequencies other than the high pass frequency band stop (the high frequencies outside the low frequencies to be filtered out by a low-pass filter to remove or attenuate low frequency noise), the first signal window, the second signal window, and the harmonic windows of a selected number (n > 1 ) of harmonics of the signal components for the first and second vibration-based sensors. [0067] More specifically, the actual noise window (NOISE_WIND) includes frequencies computed as follows:

FULL_SPECTRUM - HPF_BAND_STOP - SIGNAL_WINDOW_l - SIGNAL_WINDOW_2 - HARM_WIND_1_1 - ... H ARM_WI N D_n_ 1 - HARM_WIND_1_2 - ... - H ARM_WI N D_n_2.

(Eq. 6)

[0068] In the above equation, FULL_SPECTRUM represents the full spectrum of frequencies, and HPF_BAND_STOP represents the high pass frequency band stop.

[0069] The process 400 next computes (at 414) a figure of merit (which is a form of a damping metric), for each of the vibration-based sensors. The figure of merit for the first vibration-based sensor is represented as FIGURE_OF_MERITl, and the figure of merit for the second vibration-based sensor is represented as FIGURE_OF_MERIT2. The parameter FIGURE_OF_MERITj, where j = 1 or 2, is computed as:

F I GU RE_0 F_M E RI Tj

= 10^■ log₁₀(SUM(PSD(SIGNAL_WINDOWJ))/SUM(PSD(NOISE_WIND)).

(Eq. 7)

[0070] More generally, the figure of merit for vibration-based sensor j is based on a ratio of the signal power, SUM(PSD(SIGNAL_WINDOWj), to the noise power

SUM(PSD(NOISE_WIND). The sum performed in Eq. 7 is over the frequency bins of the FFT (those bins that are within each signal window for the respective vibration- based sensor). The value PSD(SIGNAL_WINDOW_j) is the power spectral density of the signal component at the frequencies of SIGNAL_WINDOW_j, and the value of PSD(NOISE_WIND) is the power spectral density of the noise component at the frequencies of NOISE_WIND.

[0071 ] A higher value of the figure of merit indicates less damping, which means that the vibration element of the vibration-based sensor is not within a fluid. A lower value of the figure of merit indicates more damping, which means that the vibration element of the vibration-based sensor is within a fluid. For example, if the value of the figure of merit is greater than a threshold, then that indicates that the vibration element is not within a fluid, which indicates that the fluid in the fluid supply has dropped below a respective level.

[0072] The first and second vibration-based sensors can be positioned at different elevations within the fluid supply so that they can detect different fluid levels. For example, the first vibration-based sensor is positioned at a first elevation for detecting whether fluid is above or below a first level within the fluid supply, while the second vibration-based sensor is positioned at a different second elevation for detecting whether fluid is above or below a different second level within the fluid supply.

[0073] In other examples, the first and second vibration-based sensors can be positioned at the same elevation in the fluid supply, and the figures of merit computed for the first and second vibration-based sensors can be aggregated (e.g., averaged) to detect the level of a fluid in the fluid supply.

[0074] A similar technique can be applied to detect presence of a vibration-based sensor in a fluid supply. The figure of merit of a vibration-based sensor can be compared to a specified sensor present threshold, where the threshold can be defined based on empirical characterization of historical measurement data collected from the vibration-based sensor. If the figure of merit is greater than the specified sensor present threshold, then that indicates that a vibration-based sensor is present in the fluid supply. If the figure of merit is not greater than the specified sensor present threshold, then that indicates that a vibration-based sensor is not present in the fluid supply.

[0075] Fig. 5 is a block diagram of an apparatus 500 that includes a controller 502 to perform various tasks. The tasks performed by the controller 502 include a measurement data receiving task 504 to receive measurement data acquired by a vibration-based sensor provided in a fluid supply. The tasks further include a measurement data converting task 506 to convert the measurement data from to a frequency domain. The tasks additionally include a damping metric computing task 508 to compute, in the frequency domain, a metric representing damping of vibration of the vibration-based sensor, based on a first value representing a signal component of the measurement data and a second value representing a noise component of the measurement data.

[0076] Fig. 6 is a block diagram of a printing system 600 that includes a fluid supply mounting structure 602 to receive a fluid supply comprising a vibration-based sensor, and a controller 604 to perform various tasks. The tasks of the controller 604 include a measurement data receiving task 606 to receive measurement data acquired by the vibration-based sensor, a measurement data converting task 608 to convert the measurement data from a first domain to a frequency domain, a damping metric computing task 610 to compute, in the frequency domain, a metric

representing damping of vibration of the vibration-based sensor, based on a first value representing a signal component of the measurement data and a second value representing a noise component of the measurement data, and a fluid level determining task 612 to determine a level of fluid in the fluid supply based on the computed metric.

[0077] In some examples, the tasks performed by the controller 502 (Fig. 5) or the controller 604 (Fig. 6) can be performed by machine-readable instructions executable on a hardware processing circuit of the controller.

[0078] The machine-readable instructions can be stored in a non-transitory computer-readable or machine-readable storage medium. The storage medium can include any or some combination of the following: a semiconductor memory device such as a dynamic or static random access memory (a DRAM or SRAM), an erasable and programmable read-only memory (EPROM), an electrically erasable and programmable read-only memory (EEPROM) and flash memory; a magnetic disk such as a fixed, floppy and removable disk; another magnetic medium including tape; an optical medium such as a compact disk (CD) or a digital video disk (DVD); or another type of storage device. Note that the instructions discussed above can be provided on one computer-readable or machine-readable storage medium, or alternatively, can be provided on multiple computer-readable or machine-readable storage media distributed in a large system having possibly plural nodes. Such computer-readable or machine-readable storage medium or media is (are) considered to be part of an article (or article of manufacture). An article or article of manufacture can refer to any manufactured single component or multiple

components. The storage medium or media can be located either in the machine running the machine-readable instructions, or located at a remote site from which machine-readable instructions can be downloaded over a network for execution.

[0079] In the foregoing description, numerous details are set forth to provide an understanding of the subject disclosed herein. However, implementations may be practiced without some of these details. Other implementations may include modifications and variations from the details discussed above. It is intended that the appended claims cover such modifications and variations.

Claims

What is claimed is:

1 . An apparatus compris

a controller to:

receive measurement data acquired by a vibration-based sensor provided in a fluid supply;

convert the measurement data from to a frequency domain; and compute, in the frequency domain, a metric representing damping of vibration of the vibration-based sensor, based on a first value representing a signal component of the measurement data and a second value representing a noise component of the measurement data. 2. The apparatus of claim 1 , wherein the controller is to determine a level of fluid in the fluid supply based on the computed metric. 3. The apparatus of claim 1 , wherein the controller is to determine a presence of the vibration-based sensor in the fluid supply based on the computed metric. 4. The apparatus of claim 1 , wherein the controller is to:

determine an oscillation frequency of the vibration-based sensor; and determine a frequency window of the signal component using the determined oscillation frequency. 5. The apparatus of claim 4, wherein the controller is to determine the oscillation frequency of the vibration-based sensor by identifying a peak power spectral density value for the measurement data in the frequency domain within a target frequency range.

The apparatus of claim 4, wherein the controller is to:

determine a harmonics frequency window for a harmonic of the

component.

7. The apparatus of claim 6, wherein the signal window is based on the determined oscillation frequency and a tolerance band about the determined oscillation frequency. 8. The apparatus of claim 6, wherein the controller is to determine a noise frequency window in the frequency domain, the noise frequency window excluding the frequency window of the signal component and the harmonics frequency window, and

wherein the noise component is in the noise frequency window. 9. The apparatus of claim 8, wherein the controller is to apply a high-pass filter to remove low-frequency noise, and wherein the noise frequency window excludes the low-frequency noise. 10. The apparatus of claim 1 , wherein the first value representing the signal component of the measurement data is based on a power spectral density derived from the signal component of the measurement data, and

the second value representing the noise component of the measurement data is based on a power spectral density derived from the noise component of the measurement data.

1 1 . A printing system comprising:

a fluid supply mounting structure to receive a fluid supply comprising a vibration-based sensor; and

a controller to:

receive measurement data acquired by the vibration-based sensor; convert the measurement data from a first domain to a frequency domain;

compute, in the frequency domain, a metric representing damping of vibration of the vibration-based sensor, based on a first value representing a signal component of the measurement data and a second value representing a noise component of the measurement data; and

determine a level of fluid in the fluid supply based on the computed metric. 12. The printing system of claim 1 1 , further comprising:

the fluid supply, the fluid supply comprising:

a housing to store the fluid; and

the vibration-based sensor, the vibration-based sensor having a first portion affixed to the housing, and a second portion that is free to vibrate in response to a mechanical stimulus, wherein damping of a vibration of the second portion of the vibration-based sensor responsive to the mechanical stimulus is indicative of a level of the printing fluid in the housing. 13. The printing system of claim 1 1 , wherein the vibration of the vibration-based sensor is responsive to a mechanical stimulus, the mechanical stimulus comprising an impulse stimulus or fixed frequency stimulus.

14. A method comprising:

receiving, by a controller, measurement data acquired by a vibration-based sensor in a fluid supply;

converting, by the controller, the measurement data from a first domain to a frequency domain;

computing, by the controller in the frequency domain, a metric representing damping of vibration of the vibration-based sensor, based on a first value representing a signal component of the measurement data and a second value representing a noise component of the measurement data; and

outputting, by the controller based on the computed metric, an indication of a level of fluid in the fluid supply or presence of the vibration-based sensor. 15. The method of claim 14, wherein the vibration-based sensor comprises a tuning fork and a capacitance sensor to acquire the measurement data.