Patent Publication Number: US-2021190574-A1

Title: Fluid level sensors

Description:
BACKGROUND 
     The vibrational behavior of vibrating elements may be used in some examples to detect the presence and/or level of a liquid. For example, a vibrational element such as a ‘tuning fork’ may vibrate at a resonant, or ‘natural’ frequency in air, but in liquid such vibration may be damped. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting examples will now be described with reference to the accompanying drawings, in which: 
         FIGS. 1 to 3  and  FIGS. 4A-4C  are examples of fluid level sensors; 
         FIG. 5  is an example of a replaceable print apparatus component comprising a fluid level sensor; 
         FIG. 6A  is an example of a print agent container comprising a fluid level sensor; and 
         FIG. 6B  is an equivalent circuit to the apparatus shown in  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION 
     In some examples, a fluid level may be detected using a vibrating element. For example, such a vibrating element may be positioned within a fluid reservoir. In some examples herein, the fluid reservoir is a print agent container for use in printing. For example, in inkjet printing, a print cartridge may contain a supply of ink which is used to form text and images on a substrate. The level of ink in the print cartridge may be used to indicate to the user when the ink is about to run out, or to estimate the rate at which ink is being dispensed. 
       FIG. 1  is an example of a fluid level sensor  100  comprising a vibrational paddle  102  wherein an edge  104  of the vibrational paddle  102  comprises a fluid surface contact point  106  having a peaked profile. As shown in the example, this may comprise a narrowing to a point. 
     In use, the fluid level sensor  100  may be arranged in a fluid container such that the fluid surface contact point  106  is oriented lowermost. In this way, as a liquid level falls, the fluid surface contact point  106  will be uncovered last. By providing a fluid surface contact point  106  which has a peaked profile, the influence of tilt on the level at which the vibrating paddle is uncovered will be minimized. The point also provides a more distinct state change than a flattened edge and can help to dissipate any foam on the surface of the liquid. 
     In some examples, the paddle  102  may be substantially rectangular, and the peak may be formed in a long or short edge of such a rectangular paddle  102 . 
     In some examples, the fluid level sensor  100  may be for use in a print agent reservoir, for example a print cartridge. The dimensions of the fluid level sensor may be selected accordingly. 
     In some examples, the fluid level sensor  100  may have a resonant or ‘natural’ vibrational frequency on the order of 10 to 100 Hz. This is within the range of frequencies that may be readily fabricated using stainless steel flat springs (which may be a suitable material choice for the fluid level sensor  100 ) with dimensions suitable for inclusion in print apparatus components, and detection apparatus which may be used in detecting the movement of the sensor  100  (for example, analogue to digital converters) which are sensitive to this range are readily available. In addition, it may be noted that fluid level sensors  100  with higher resonant frequencies have lower displacement for the same quantity of input energy and therefore the movement of a fluid level sensor  100  becomes more difficult to detect with increasing resonant frequency. Moreover, when seeking to accurately characterize an oscillation, higher frequencies are associated with higher sampling rates. Higher sampling rates in turn consume greater monitoring and processing resource. 
     The lower end of the frequency range may be associated with the size of the fluid level sensor  100  (which may in turn be limited by the size of a container, which as noted above may in some examples be a print apparatus component such as a print agent cartridge). Thus, with different processing, material and/or size constraints, different frequency ranges may be appropriate. 
     In some examples, frequencies around national power supply frequencies (for example, around 50 Hz and 60 Hz in most countries) may be avoided, as this can result in a false reading due to the power supply signal. 
     In some examples, the fluid level sensor  100  may comprise a stamped spring like component, and/or may be substantially planar. For example, the fluid level sensor  100  may comprise a stamped spring plate. Providing such a planar sensor  100  may be of use in certain fluid containers (such as print cartridges) which are constrained in one direction. In other examples, the fluid level sensor  100  may comprise a coiled spring or the like. In some examples, the sensor  100  may be fabricated from a material which is selected for vibrational characteristics and/or corrosion resistance. In some examples this may comprise a metal, for example stainless steel, or a plastic, or the like. 
     Where such a fluid level sensor  100  is provided in a print apparatus component, it may be fixed to the component at a mounting point (e.g. the widened end of the sensor  100  shown in the Figure, although such a widened end may not be shown in all embodiments), whereas a distal portion of the vibrational paddle  102  may be free to move. The nature of the movement of the vibrational paddle  102  may be used to determine a fluid level. In particular, when subjected to a stimulus, the movement of the paddle  102  may indicate if the paddle is in fluid or in air. 
     A stimulus applied may take various forms. For example, an impulse, or sudden force, may be applied by causing a moving component containing the fluid level sensor  100  to rapidly decelerate, for example by stopping a carriage housing the component (which may be a print apparatus component) suddenly, or by causing the carriage to knock against a stopping member. In other examples, an external device, such as an electromagnet, may be used to generate an impulse force, for example by generating a magnetic field to act on the vibrational paddle  102  of the fluid level sensor  100  then removing the magnetic field, to cause the vibrational paddle  102  to oscillate as it returns to a resting position. 
     Another way of causing movement of the vibrational paddle  102  is to cause movement of the fluid level sensor  100  at a defined driving frequency. In some examples, a direction of movement of a print apparatus container containing a sensor  100  may be rapidly and repeatedly be reversed. Such movement may be referred to as cyclic movement. For example, a mechanism for causing a carriage to move within a printing apparatus may cause a fluid container such as a print agent cartridge to move backwards and forwards, for example along a track, at a defined frequency. Fluid, such as print agent within a fluid container, may be caused to slosh from one side of the fluid container to an opposite side of the fluid container at the same defined frequency. The moving liquid may cause the paddle  102  of the fluid level sensor  100  to oscillate at the same frequency. 
     The position of the vibrational paddle  102  may in some examples be detectable capacitively. For example, the vibrational paddle  102  may function as (if the fluid level sensor  100  is conductive), or may comprise, a capacitive plate, and another capacitive plate may be mounted on a surface of the print apparatus component and the capacitance may be monitored via a circuit (which may for example be provided as part of the print apparatus). For example, an impulse may be applied and the capacitance of the circuit comprising a sensor  100  may change at a rate corresponding to a characteristic wavelength of fluid with the container if the fluid level sensor  100  is submerged in fluid, or at the resonant or natural frequency of the fluid level sensor  100  if the vibrational paddle  102  is in air. 
       FIG. 2  shows an example of a two limb fluid level sensor  200 , in which the limbs are mounted at relatively offset angles. In this example, a first substantially horizontal arm  202  and a second substantially vertical arm  204  is provided. While in this example, the arms are substantially orthogonal to one another, this may not be the case in all examples. However, it may be noted that the illustrated design results in efficient nesting in raw material, for example if the sensor  200  is formed using a stamping process. Each of the arms  202 ,  204  comprises a distal portion  206   a ,  206   b , and a proximal portion  208   a ,  208   b . In each case, the proximal portion  208   a ,  208   b  has a first solid surface area per unit length and the distal portion  206   a ,  206   b  has a second solid surface area per unit length, wherein the second solid surface area is greater than the first solid surface area. In other words, the distal portions present a greater surface area against which fluid may act per unit length, and therefore provide vibrational paddles  210   a ,  210   b . In this example, the paddles  210  are formed with fluid contact points  106  having peaked profiles as described above in relation to  FIG. 1 , but this need not be the case in all examples. 
     In the illustrated example, the horizontal arm  202  achieves the reduction in surface area by having a narrow section forming the proximal portion  208   a  (i.e., the proximal portion has a first width and the distal portion has a second width, the second width being greater than the first width), whereas the vertical arm  204  achieves this reduction in surface area by having a cut-out section formed in the proximal portion  208   b . In other examples, a region of relatively high flexibility may be provided by selecting a more flexible material for a region, or there may be no region of increased flexibility. It may be noted that a region of increased flexibility may be provided in conjunction with, or separate from, other features of a fluid level sensor  100 ,  200 . 
     It may be noted that the length of such a proximal portion  208 , or the amount of material cut-out therefrom, will have an effect on the vibrational behaviour of that arm  202 ,  204 . Providing portions of increased flexibility (for example by providing a region of reduced surface area) may increase the displacement caused by a particular stimulus and/or increase the decay period, and therefore increase signal strength. By providing a cut-out rather than a narrowed portion, there may be increased handling robustness in manufacture as the proximal portion  208  is supported on both sides. In addition, this may assist in reducing torsional and/or longitudinal cross talk. 
     It may be noted that, by providing a plurality of vibrational paddles  210   a ,  210   b , the fluid level may be sensed at various heights. For example, it may be determined when the paddle  210   a  on the horizontal arm  202  becomes uncovered a fluid level in a container containing the sensor  200  reduces and, subsequently, it may be determined when the lower paddle  210   b  on the vertical arm  204  is uncovered. In some examples, detection may be carried out using a capacitive sensor, wherein the paddles  210  provide first plates of a capacitive sensor and a second plate is mounted, for example on the interior or exterior of a housing of a fluid container. The movement of the paddles  210  may be sensed for example capacitively (with the paddles each acting as a plate of a capacitor), inductively or in some other way. 
     Again, in this example, the fluid level sensor  200  may comprise a stamped spring plate. By stamping the fluid level sensor  200 , it may be formed without requiring joints, hinges or the like. 
     In this example, the fluid sensor  200  comprises a mounting point  212 , which in this example comprises a plurality of fixing points  214   a ,  214   b ,  214   c . By providing a plurality of fixing points  214 , the position of the sensor  200  within a container may be readily constrained. It may be noted that, in this example, the fixing points  214  are relatively spaced to provide a solid region therebetween. This solid region may allow the fitting of additional components. In some examples, this may allow a suitably sized vacuum cup for a “pick and place” operation to engage with the mounting point  212 , for example during manufacture. 
     In this example, it may be noted that the horizontal arm  202  overreaches the vertical arm  204  and the mounting point  212 . This in effect allows the horizontal arm to be longer without increasing the overall footprint of the sensor  200 . The width of the continuous material forming the link between the arms  202 ,  204  may be selected so as to provide an intended frequency. It may be noted that the width may be altered without changing the outer envelope of the sensor  200 . 
     In this example, it may be noted that vibrational paddles  210   a ,  210   b  formed in the distal portions of the fluid level sensor  200  comprise rounded corners. This may assist in reducing damage to any other components which the fluid level sensor  200  may come into contact with. 
     In this example, the arms  202 ,  204  each comprise a plurality of attachment points  216   a - d  for mounting a removable mass. For example, the mass may comprise a clamshell mass. Adding a mass to a particular attachment point  216   a - d  may change the resonant behaviour of the sensor  200 . By providing attachment points  216   a - d  in such positions, the repeatable placement of an added mass is facilitated. In this example, attachment points  216   a - d  comprise notches formed on the edges of the arms  202 ,  204 . However, in other examples, these may comprise any feature which allows a mass to be attached at a particular location, for example comprising a cut-out, protrusion or the like. 
     In one example, the thickness of each arm  202 ,  204  may be around 0.5 to 2 mm. The length of each arm  202 ,  204  may be on the order of 2 to 5 mm, or up to around a few centimetres. These values may be selected with a view to the robustness and natural or resonant frequency of the sensor  200 . It may be noted that stiffness is a function of the thickness of each arm  202 ,  204  to the third power but a function of the width of the each arm  202 ,  204  to the first power, meaning that varying the width may ‘fine tune’ vibrational performance. 
     An example of a sensor  200  comprising two such masses  300   a ,  300   b  is shown in  FIG. 3 . 
     In this example, it may be the case that the arms  202 ,  204  can exhibit substantially the same resonant frequency, or different resonant frequencies, depending on the placement of the mass  300 . For example, if the mass  300  is mounted at the inner position, the resonant frequency of both arms may be around 30 to 40 Hz, whereas if the mass  300  is mounted at the outer position on each arm (as is shown in Figures), that arm may exhibit vibrational resonance at between 20 and 30 Hz. In some examples, the masses  300  may for example be around 0.2 to 0.5 g. 
     For example, this may allow selection of a first arm to vibrate at a first frequency and the second arm to vibrate at a second frequency, but these frequencies may be switched to the alternative arm between different instances of the sensors  200 . 
     In examples in which the arms  202 ,  204  have different natural or resonant frequencies (be that by placement of the mass  300  or due to the form and/or materials thereof), the response of the arms  202 ,  204  may be distinguishable even when the arms  202 ,  204  are connected in series to a single sensing circuit, as the response will have characteristics of both frequencies. In other examples, the arms  202 ,  204  may have the same response, and the strength of the response signal could be utilized to determine if just one or both arms  202 ,  204  was responding at its resonant frequency. In other examples, each arm  202 ,  204  may be monitored individually. 
     While examples utilizing one and two vibrating elements have been described above, in principle any number of vibrating elements could be provided. 
       FIGS. 4A-C  provide examples of alternative designs for a fluid level sensor, and parts in common with the sensor  200  of  FIGS. 2 and 3  are labelled with like numbers. 
       FIG. 4A  shows another example of a fluid level sensor  400 , in this case comprising two orthogonal arms  402 ,  404 . It may be noted that, in this example, the narrow proximal portion of the arms are relatively long, which may alter the vibrational characteristics, as well as the strength of the component. This is one example of how a design may be tailored to a particular use case. 
       FIG. 4B  shows an example of a ‘Z-shaped’ sensor  406 , comprising a vibrating arm  408  and a fixed plate  410 , which in use of the sensor  406  within a print apparatus component may be fixed at a predetermined distance from a second capacitive plate, to which it may be capacitively coupled. The second capacitive plate may be electrically coupled to a sensing apparatus (which may for example be provided as part of the print apparatus), which may sense the changes in capacitance associated with a change in the dielectric there between from liquid when submerged in a print agent to air when the print agent level falls below the level of the fixed plate  410 . The tapered shape of the fixed plate  410  may assist in discerning dropping fluid levels. The motion of the end of the vibrating arm  408  may be sensed as described above, for example capacitively. 
       FIG. 4C  shows an example of a sensor  412  comprising relatively short arms. Shorter arms provide increased overall robustness and reduce chance of entanglement of sensor  412  during manufacture (for example within vibratory feeder or similar), as another sensor  412  can no longer fit into this hole. By providing shorter arms, the vibrational frequency may increase, reducing a minimum sampling rate to correctly characterize the movement of the sensor. Masses may optionally be fitted at the attachment points  216   a ,  216   b.    
     It may be appreciated that the illustrated examples provides just some examples of the design options available, and many variations on these designs or combinations of features of different designs could be made. 
     In particular, sensors may have any number of oscillating members, which may be orientated in any direction. Sensors may comprise any or any combinations of: a region of increased flexibility, a peaked fluid contact point, an arm which ‘over reaches’ a contact point, rounded corners, any number of mass attachment points, and the like. Any of these features may be provided in the absence of any other feature. 
     In some examples, sensors may be bent such that any mounting point and the vibrating paddle are not aligned, for example such that when a mounting point is fixed to a wall of a print apparatus component, a vibrational paddle may be spaced therefrom and free to move. In other examples, the mounting point may provide a ‘platform’ which provides a spacing between a paddle and a wall. 
       FIG. 5  is an example of a replaceable print apparatus component  500  comprising a fluid container  502  and a fluid level sensor  504  disposed within the fluid container  502  comprising first and second vibrational arms  506 ,  508 , wherein the first and second arms  506 ,  508  extend in different directions, are disposed at different fluid depths within the container and are associated with respective first and second resonant or vibrational behaviors. In some examples, the first and second resonant behavior may be the same—in other words, the first and second resonant behavior may comprise substantially the same resonant frequency, signal strength (displacement in response to a stimulus) and/or decay rate of vibration. In other examples, the first and second resonant/vibrational behavior may be the same—in other words, the first and second resonant/vibrational behavior may comprise substantially different frequency, signal strength and/or decay rate of resonance. 
     In some examples, as a shown in the Figure, the first and second arm  506 ,  508  extend from a common origin. However, this may not be the case in all examples. In addition, as a shown in the Figure, the first arm comprises a longitudinally extending arm and the second arm comprises a laterally extending arm, but this need not be the case in all examples. 
     In some examples, as has been shown above, the second arm may extend across the width of the first arm, or vice versa. This may assist in minimising material use whilst providing a relatively long lateral extension for a given footprint of the fluid level sensor  504 . 
     In some examples, the fluid level sensor  504  may comprise a ferromagnetic material, such that it can be excited by magnet (which may be an electromagnet, controlled to carry out a sensing operation). In some examples, at least a part thereof may be electrically conductive and/or capable of acting as a plate in a capacitor (for example, an end region of the arms  506 ,  508 ). 
     The fluid level sensor  504  may have any of the characteristics described above. For example, it may be relatively flat or planar, although in other examples the fluid level sensor  504  may comprise a spring or a coil. The fluid level sensor  504  may be a monolithic component i.e., formed of a single piece of material. In other examples the sensor  504  may be jointed although this results in additional manufacturing processes. The fluid level sensor  504  may comprise a flexible portion (e.g. a narrowed portion, or a cut-out portion), as is described above, to increase the displacement following stimulus thereof. 
     The sensor  504  may comprise rounded corners and/or may comprise a conductive material, as has also been described above. 
     In some examples, at least one of the first and second vibrational arms  506 ,  508  comprises a vibrational paddle wherein an edge of the vibrational paddle comprises a fluid surface contact point having a peaked profile, as has been described above. 
     In use, any of the fluid level sensors  100 ,  200 ,  400 ,  406 ,  412 , described herein may be placed within a print apparatus component  500 , for example a print agent container or a print agent cartridge. In order to determine a fluid level, a portion (for example the vibrational paddles  102 ,  210 ) of the sensor  100 ,  200 ,  400   406 ,  412  may act as a plate of a capacitor. A second plate of the capacitor may be provided on a housing of the print apparatus component  500 . In some examples, the fluid level sensor  100 ,  200 ,  400 ,  406 ,  412  may be provided inside the housing of a print apparatus component and the second plate of the capacitor may be provided outside the housing. In use, the first and second plate may be capacitively coupled. Vibration of the fluid level sensor  100 ,  200 ,  400 ,  406 ,  412  can be sensed as a changing capacitance of the circuit including the first and second capacitive plates. Therefore, for example, resonant behaviour (i.e. vibration at the resonant or natural frequency of the sensor or an arm thereof) may be detected as a variation in capacitance having a frequency which is at the resonant or natural frequency of a sensor  100 ,  200 ,  400 ,  406 ,  412 . Such resonant behaviour will be seen when the corresponding vibrational paddle  102 ,  210  is fully uncovered by liquid. Thus, it may be detected when the fluid in the container is below the paddle  102 ,  210 . Where a plurality of paddles  102 ,  210  are provided, the fluid level may be determined at various fluid depths. 
       FIG. 6A  is an example of a print agent container  600  comprising a fluid reservoir  602  and first  604  and second  606  resonator elements disposed within the fluid reservoir  602  at different fluid depths and which extend in different directions. The first resonator element  604  exhibits resonant behaviour (i.e. vibration at the resonant or natural frequency of the resonator element) when fluid is at a first depth in the reservoir  602  and second resonator element  606  exhibits resonant behaviour when fluid is at a second depth in the reservoir  602 , and the first and second resonator elements  604 ,  606  have a common mounting point. 
     The print agent container  600  comprises capacitive plates  608   a, b  provided on the outer housing thereof, each of which is associated with a connection terminal  610   a ,  610   b.    
       FIG. 6B  shows an equivalent circuit  612 , in which distal regions of the first and second resonator elements  604 ,  606  provide plates of two variable capacitors  614   a ,  614   b  (which may be electrically connected via the material of the sensor, or an electrical connection such as a wire provided thereon), with opposing plates being provided by the capacitive plates  608   a, b  provided on the outer housing of the container  600 . Sensor circuitry connected to the connection terminals  610   a ,  610   b  may query the sensor, for example following a stimulus, and may determine by measuring indications of the capacitance, whether a sensor is present, absent, and whether a vibrational paddle/distal portion thereof is uncovered or submerged. 
     While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above-mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example. 
     The word “comprising” does not exclude the presence of elements other than those listed in a claim, and “a” or “an” does not exclude a plurality. 
     The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.