WO2019078841A1

WO2019078841A1 - Fluid level sensors

Info

Publication number: WO2019078841A1
Application number: PCT/US2017/057152
Authority: WO
Inventors: Kevin ROURKE; John MCNEILLY
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2017-10-18
Filing date: 2017-10-18
Publication date: 2019-04-25

Abstract

A fluid level sensor is disclosed. The fluid level sensor may comprise a first electrical contact; and a second electrical contact forming a capacitive coupling with the first electrical contact. A capacitance between the first electrical contact and the second electrical contact is variable in response to a force imparted to the fluid level sensor by an excitation source. A method and a system are also disclosed.

Description

FLUID LEVEL SENSORS

Background

[0001 ] It may be intended to determine a level of fluid, such as print agent, in a fluid container, such as a print agent cartridge. In some examples, a capacitance of circuitry in such a fluid container can be used as an indicator of such a fluid level. A capacitance may change if a first plate of a capacitor is moved relative to a second plate of a capacitor. If the moving plate is submerged in a liquid, then the change in capacitance will be different to the change in capacitance if the moving plate is in air.

Brief Description of the Drawings

[0002] Examples will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0003] Figure 1 is a simplified schematic of an example of a fluid level sensor;

[0004] Figure 2 is a simplified schematic of an example of a fluid level sensor in a replaceable print apparatus component;

[0005] Figure 3 is a simplified schematic of a further example of a fluid level sensor;

[0006] Figure 4 is a simplified schematic of a further example of a fluid level sensor;

[0007] Figure 5 is a simplified schematic of an example of a fluid level sensor mounted to a surface;

[0008] Figure 6 is a simplified schematic of an example of a fluid level sensor and a wall;

[0009] Figure 7 is a flowchart of an example of a method of detecting a change in capacitance in response to application of a force;

[0010] Figure 8 is a flowchart of a further example of a method of detecting a change in capacitance in response to application of a force; and

[001 1 ] Figure 9 is a simplified schematic of an example of a print apparatus. Detailed Description

[0012] A device is disclosed which can be used to determine a level of fluid in a fluid container. The device, which may be considered to be a fluid level sensor, may be installed into a fluid container, such as a print agent container, which may be used in a printing apparatus. The fluid level sensor may function in such a way that its behaviour inside a fluid container can be examined and analysed from outside the fluid container, without a physical (e.g. wired) connection between the fluid level sensor and a device or circuitry analysing the sensor's behaviour.

[0013] Figure 1 is a simplified schematic of an example of a fluid level sensor 100. The sensor 100 may, in some examples, be used in a print agent container to determine a level of print agent in the container. For example, the fluid level sensor 100 may be installed in print agent container which forms at least part of an ink cartridge for use in a printing apparatus.

[0014] The fluid level sensor 100 comprises a first electrical contact 102 and a second electrical contact 104 forming a capacitive coupling with the first electrical contact 102. A capacitance between the first electrical contact 102 and the second electrical contact 104 is variable in response to a force imparted to the fluid level sensor 100 by an excitation source 106. The first and second electrical contacts 102, 104 may form part of a circuit, and the capacitance of the circuit may be variable in response to the imparted force. The fluid level sensor 100, or circuitry associated therewith, may include other electronic components which are not discussed herein, but which may be in electrical communication with the first and second electrical contacts 102, 104.

[0015] In some examples, the fluid level sensor 100 may be installed in a replaceable print apparatus component, such as a print agent container. The first electrical contact 102 may be disposed outside such a replaceable print apparatus component and the second electrical contact 104 may be disposed within the replaceable print apparatus component. In other examples, the positions of the first and second electrical contacts 102, 104 may be reversed (i.e. the second electrical contact may be disposed outside the replaceable print apparatus component and the first electrical contact may be disposed within the replaceable print apparatus component). The electrical contact which is disposed outside the replaceable print apparatus component (e.g. the first electrical contact 102) may, for example, be disposed on an outer surface of a wall or housing of the replaceable print apparatus component. In some examples, the first electrical contact 102 which is disposed outside the replaceable print apparatus component may be remote from (e.g. spaced apart or separated from) the replaceable print apparatus component. However, the first electrical contact 102 outside the replaceable print apparatus component is close enough to the second electrical contact 104 to maintain a capacitive coupling between the first and second electrical contacts 102, 104. As discussed below, the second electrical contact 104 may be movable relative to the first electrical contact 102. In some examples, in order to maintain a capacitive coupling between the electrical contacts, the second electrical contact 104 may, in its resting position, be positioned within approximately 4 millimetres of the first electrical contact 102.

[0016] The excitation source 106 may generate a force to act on the fluid level sensor 100. In some examples, the force generated by the excitation source 106 may be considered to be a stimulus. The stimulus may, for example, excite the fluid level sensor 100 or, more particularly, an electrical contact 102, 104 of the fluid contact, as is discussed in greater detail below. The first electrical contact 102 may, in some examples, be moveable relative to the second electrical contact 104. The force generated by the excitation source 106 may cause the first and second electrical contacts 102, 104 to move relative to one another. Movement of the first electrical contact 102 relative to the second electrical contact 104 as a result of the force from the excitation source 106 may cause the capacitance between the electrically conductive elements to vary.

[0017] Figure 2 is a simplified schematic of an example of a fluid level sensor 200 and a replaceable print apparatus component 210. According to the example shown in Figure 2, the first electrical contact 102 is disposed outside the replaceable print apparatus component 210 and the second electrical contact 104 is disposed within the replaceable print apparatus component. The fluid level sensor 200 may further comprise a third electrical contact 202 disposed outside the replaceable print apparatus component 210 and a fourth electrical contact 204 capacitively coupled to the third electrical contact, and disposed within the replaceable print apparatus component. The first and second electrical contacts 102, 104 are capacitively coupled to one another through a housing or wall 212 of the replaceable print apparatus component 210, and the third and fourth electrical contacts 202, 204 are capacitively coupled to one another through the wall 212. As in the example discussed above, the third electrical contact 202 may, in some examples, be disposed on an outer surface of the wall 212 of the replaceable print apparatus component 210.

[0018] In some examples, those electrical contacts disposed within the replaceable print apparatus component 210 (e.g. the second and fourth electrical contacts 104, 204 in the above examples), may be disposed on or connected to an inner surface of a wall or housing of the replaceable print apparatus component (e.g. the inner surface of the wall 212 on which the first and third electrically conductive elements 102, 202 are disposed).

[0019] The electrical contacts 104, 204 disposed within the replaceable print apparatus component 210 may exhibit distinct resonant behaviours. In some examples, the second electrical contact 104 and the fourth electrical contact 204 may have distinct resonant frequencies. For example, the second electrical contact 104 may have a first resonant frequency and the fourth electrical contact 204 may have a second resonant frequency which is different to the first resonant frequency. In some examples, the second and fourth electrical contacts 104, 204 may be in electrical communication with one another, for example via circuitry within the replaceable print apparatus component 210. The fluid level sensor 100 itself may form part of the circuitry, and/or may form the electrical connection between the electrical contacts 104, 204.

[0020] According to some examples, the first and second electrical contacts 102, 104 may be considered to be plates of a variable capacitor. A capacitance of the variable capacitor may be variable in response to the force applied to the variable capacitor. For example, a force may be applied to one of the electrical contacts, and this may cause a change in the capacitance of the variable capacitor.

[0021 ] An example of fluid level sensor 300 is shown in Figure 3. In the example shown in Figure 3, the fluid level sensor 300 is generally rectangular. In other examples, however, as is apparent from the examples below, the device may have a different general shape.

[0022] The fluid level sensor 300 may, in some examples, include, a mounting portion 302 for mounting the device to a surface, such as the inner surface of a wall or housing (e.g. the wall 212) of a replaceable print apparatus component 210, such as a print agent cartridge. The fluid level sensor 300 may include an electrically conductive element, such as the second electrical contact 104. In some examples, the mounting portion 302 may be located at or near to a first, proximal end of the device 300 and the second electrical contact 104 may be located at or near to a second, distal end of the device.

[0023] A further example of a fluid level sensor 400 for sensing a fluid level in a fluid container is shown in Figure 4. In the example shown in Figure 4, the fluid level sensor 400 is generally L-shaped. The fluid level sensor 400 includes a mounting portion 402 for mounting the device to a surface, such as the inner surface of a wall or housing (e.g. the wall 212) of a fluid container, such as the replaceable print apparatus component 210. The mounting portion may include an aperture, or multiple apertures 404, for receiving a peg, a screw or the like, for attaching the device to a surface. The device 400 includes two electrical contacts, corresponding to the contacts 104, 204 shown in Figure 2. In the arrangement of Figure 4, however, each element is positioned on a limb, or arm. For example, the second electrical contact 104 may be located on a first arm 406 and the fourth electrical contact 204 may be located on a second arm 408. In some examples, the mounting portion 402 may be located at a portion of the device from which the first arm 406 and the second arm 408 extend. In general, the arrangement of the device 400 may be such that, in use, the electrical contacts 104 and 204 are disposed at different depths within a fluid container. [0024] In other examples, the device may be of a shape and/or configuration other than those shown in Figures 3 and 4. In general, the device 300, 400 may be formed from any material which allows suitable vibration characteristics to be designed. In other words, any material may be used which exhibits a strong vibratory response to a stimulus. Using a metal may further enable an electrical connection to be formed through the device. In some examples, the device 300, 400 may be formed from 301/302 stainless steel. Such a material has strong chemical resistance characteristics. A ferritic material, such as ferritic steel allows for magnetic excitation of the device, as discussed below.

[0025] Figure 5 is a simplified schematic of an example of a fluid level sensor, such as the fluid level sensor 300, mounted to a surface 502 of a wall 504 of a fluid container. The fluid level sensor 300 is mounted to the wall 504 at the mounting portion 302. The fluid level sensor 300, in this example, is substantially planar. The device 300 may, for example, be formed from sheet metal, such as aluminium, stainless steel, or the like. The second electrical contact 104 is located at the distal end of the device 300, and is spaced apart from the wall 504. The distal end of the device 300, including the second electrical contact 104, is moveable relative to the wall 504. In this way, the second electrical contact 104 may move in a direction shown by the arrow A, for example in an oscillatory or vibratory manner, when a force is applied to the fluid level sensor 300 by the excitation source 106. Thus, in some examples, the second electrical contact 104 may comprise a vibrational element. The force applied to the fluid level sensor 300, and/or to the second electrical contact 104 by the excitation source 106 is to cause the second electrical contact to move relative to the wall 504 (e.g. to oscillate towards and away from the wall). In other words, the second electrical contact 104 may be moveable relative to the first electrical contact 102, and the second electrical contact may be to move in response to the force imparted by the excitation source.

[0026] Electrical contacts are provided on an outer surface 506 of the wall 504. In the example shown in Figure 5, the first electrical contact 102 is provided at a position substantially aligned with the second electrical contact 104 of the device 300. In this way, the first and second electrical contacts 102, 104 are capacitively coupled to one another through the wall 504 and through a gap between the wall and the second electrical contact. The proximal end of the device 300 is in electrical communication with an electrically conductive element 508 formed on the outer surface 506 of the wall 504. In some examples, the proximal end (e.g. the mounting portion 302) of the device may be capacitively coupled to the electrically conductive element 508, as indicated in Figure 5. In other examples, a physical electrical connection may exist between the electrically conductive element 508 and the proximal end of the device 300, for example via mounting elements used to mount the device 300 to the wall 504.

[0027] The first electrical contact 102 and the electrically conductive element 508 may be electrically connected, for example capacitively or physically, with other components or circuitry 510. The circuitry 510 may include processing circuitry, for example for measuring capacitances. In some examples, an air gap and an associated capacitance may be introduced between the electrically conductive element 508 and the circuitry 510, and between the first electrical contact 102 and the circuitry 510. As noted above, the replaceable print apparatus component 100 may, in some examples, comprise a print agent container, such as an ink cartridge. The ink cartridge may be mounted in a carriage, operable to move the ink cartridge over a substrate to be printed. The first electrical contact 102 and the electrically conductive element 508 may be electrically connected to electrical contacts of the carriage and/or to circuitry associated with a printing apparatus.

[0028] A force may be applied to the second electrical contact 104, to the fluid level sensor and/or to the entire replaceable print apparatus component 100. The type of force applied may depend upon the nature of the excitation source 106 generating the force. In some examples, the excitation source 106 may comprise a source selected from a group comprising: a magnetic field generator an acoustic wave generator, a vibrating motor and a piezoelectric element. A magnetic field generator may, for example, comprise a permanent magnet or an electromagnet. In some examples, the excitation source 106 may comprise an electromagnet, such as an electromagnetic coil, which may be controlled by control circuitry. An acoustic wave generator may, for example, comprise a device, such as a woofer, to generate an acoustic radiation force (e.g. a low frequency vibration) to be imparted to the fluid level sensor. A vibrating motor or a piezoelectric element may, for example, comprise a device to generate a vibrational force to be imparted to the fluid level sensor, thereby to cause the second electrical contact 104 to vibrate. The excitation source 106 may, for example, be located within a printing apparatus within which the replaceable print apparatus component 210 is installed. In examples in which the excitation source 106 comprises a magnetic field source, the at least part of the arm or arms of the fluid level sensor, or the electrical contacts 104, 204 may comprise (e.g. may be formed from, or include) magnetic or ferritic material.

[0029] In some examples, the force generated by the excitation source 106 may comprise an electromagnetic pulse or force applied to the second electrical contact 104, the fluid level sensor 300 and/or the replaceable print apparatus component 210. For example, a magnetic field may be generated for a short period of time, in order to attract the distal end of the fluid level sensor 300 in a direction away from the wall 504. Once the magnetic field is deactivated, the distal end (i.e. the end at which the second electrical contact 104 is located) is released from the magnetic field, causing it to return to its original, resting, position.

[0030] After the force has been applied to the second electrical contact 104 of the fluid level sensor, the distal end of the fluid level sensor may undergo oscillatory motion as it returns to its resting position. In other words, the second electrical contact 104 may oscillate towards and away from the wall 504 in the direction of the arrow A in Figure 5, for example in a vibratory manner. As such, the portion of the device capable of moving relative to the wall 504 may be considered to be a vibrational member.

[0031 ] The example shown in Figure 5 relates to a fluid level sensor 300 having a single vibrational member (i.e. the second electrical contact 104). In other examples, the fluid level sensor may include additional vibrational members which may move (e.g. with oscillatory motion) relative to a wall of the replaceable print apparatus component or fluid container in which the fluid level sensor is located. For example, the fluid level sensor 400 shown in Figure 4 has two vibrational members, namely the second electrical contact 104 located on the first arm 406, and the fourth electrical contact 204 located on the second arm 408. Proximal ends of the first and second arms 406, 408 may be connected to the wall of fluid container, and the distal ends of the arms may be free to move relative to the wall, for example in the manner described above with reference to Figure 5. The force generated by the excitation source 106 may, for example, cause both free arms of the fluid level sensor 400 to oscillate relative to the wall.

[0032] Figure 6 is a simplified schematic showing an example of the fluid level sensor 400 and a wall 504 of the replaceable print apparatus component or fluid container. The position of the fluid level sensor 400 in its mounted configuration on the wall is shown by the outline 602 and dashed lines indicate the positions of the electrical contacts 104, 204 when the device is mounted on the wall 504. The positions of the first electrical contact 102 and the third electrical contact 202 on the outer surface of the wall 504 are indicated. In this example, the mounting portion 402 of the fluid level sensor 400 may not form a direct electrical connection with a contact (e.g. electrical contact 508) formed on the outer surface of the wall 504 in a position aligned with the mounting portion. Instead, a capacitive connection is formed between the first electrical contact 102 outside the fluid container and the second electrical contact 104 of the fluid level sensor 400 (i.e. through the wall 504). A capacitive connection is also formed between the third electrical contact 202 outside the fluid container and the fourth electrical contact 204 of the device 400 (i.e. through the wall 504). The first electrical contact 102 and the third electrical contact 202 may be electrically connected, for example capacitively or physically, with other components or circuitry, such as circuitry 510.

[0033] As noted above, when a force is applied to the fluid level sensor 100, 300, 400 or to a fluid container in which the fluid level sensor is disposed, the free end or ends (i.e. the second electrical contact 104 and/or the fourth electrical contact 204) may be caused to oscillate or vibrate relative to the wall 504. As the electrical contacts 104, 204 move relative to the wall 504 (and, therefore, relative to the first and third electrical contacts 102, 202 outside the fluid container), the capacitance of the circuit, and particularly of each capacitive coupling, is caused to vary. By measuring the variation in the capacitance resulting from the movement of the electrical contacts 104, 204, it is possible to determine whether each of the electrical contacts is submerged in a substance in the fluid container. If the electrical contacts 104, 204 are caused to vibrate in air, then they will vibrate at their resonant frequency. However, if the electrically conductive elements 104, 204 are submerged in a substance, such as print agent, then they will not vibrate at their resonant frequency, or they will vibrate, but with heavy damping. Thus, if the capacitance changes at a rate corresponding to the resonant frequency of a particular electrically conductive element 104, 204, then it may be determined that the electrically conductive element is oscillating in air and, therefore, is not submerged in a liquid, such as print agent.

[0034] Movement of the electrically conductive elements 104, 204 or, more generally, of the arms of the fluid level sensor 300, 400, may be caused by the force generated by the excitation source 106, as discussed above. In some examples, the excitation source 106 may be to generate an impulse to be applied to the fluid level sensor 300, 400 or to the replaceable print apparatus component 210. An impulse may be considered to be a temporary or momentary force which causes the free ends of the arms of the fluid level sensor 300, 400 to oscillate or vibrate. By measuring the change in capacitance immediately after (or soon after) the impulse has been applied, it may be possible to determine whether either of the arms (and, therefore, either of the electrical contacts 104, 204) is vibrating at its resonant frequency.

[0035] Another way of causing movement of the arms of the fluid level sensor 300, 400 is to cause movement of the arms at a defined driving frequency. In some examples, the excitation source 106 may generate a series of force pulses at a defined rate (i.e. having a defined frequency). In other words, the excitation source 106 may generate a cyclic force, to cause cyclic movement of the arms of the fluid level sensor 300, 400. The force pulses may cause the arms of the fluid level sensor 300, 400 to oscillate or vibrate at the defined frequency. In an example, an electromagnet may be repeatedly activated and deactivated at a driving frequency so as to cause a magnetic field to act on the fluid level sensor 300, 400, and therefore cause oscillatory movement of the arm or arms of the device, at the driving frequency.

[0036] If a change in capacitance detected in response to the application of the cyclic force at a driving frequency indicates a variation at the same driving frequency, then it may be determined that the fluid level sensor is present within the replaceable print apparatus component 210, and functioning as intended (e.g. that the free ends of the arms of the fluid level sensor are able to move and vibrate relative to the wall 504).

[0037] In some examples, other methods for causing cyclic movement of the arms of the fluid level sensor 300, 400 may be implemented. In the event that the other methods for causing cyclic movement fail to function as intended, then the method described above (i.e. causing cyclic movement using the excitation source 106) may be used. In some examples, methods for causing cyclic movement of the arms of the fluid level sensor may cause the intended movement if the arms of the fluid level sensor are submerged in a substance (e.g. print agent), but may not cause the intended movement if the arms are in air. The cyclic force pulses generated by the excitation source 106 may cause cyclic movement of the arms regardless of the substance in which they are submerged (e.g. regardless of whether the arms are in air or liquid). In other words, the excitation source 106 may be used as an

alternative or backup to generate cyclic movement of the arms of the fluid level sensor.

[0038] In some examples, a replaceable print apparatus component, such as a print agent cartridge for depositing print agent in a printing apparatus, may comprise elements of the fluid level sensors 300, 400 discussed above. In some examples, a replaceable print apparatus component may comprise a fluid container. The fluid container may house fluid, such as print agent. The replaceable print apparatus component may comprise a variable capacitor having a first electrically conductive element disposed within the fluid container and a second electrically conductive element disposed outside the fluid container. The first electrically conductive element may comprise the first electrical contact 102, and the second electrically conductive element may comprise the second electrical contact 104 discussed above. A capacitance between the first electrically conductive element and the second electrically conductive element may be variable in response to a stimulus received by the first electrically conductive element from outside the fluid container. The stimulus may comprise a force received, for example, from the excitation source 106.

[0039] Figure 7 is a flowchart of an example of a method 700 for detecting a change in capacitance in response to application of a force. The method 700 comprises, at block 702, generating a force from a force generation device. The force generation device may comprise the excitation source 106. At block 704, the method 700 comprises applying the force to a first electrically conductive element of a variable capacitor of a circuit. The first electrically conductive element may comprise the first electrical contact 102. The first electrically conductive element 102 may, in some examples, be considered to form part of a variable capacitor along with the second electrically conductive element 104, as discussed above. The method comprises, at block 706, detecting a change in capacitance in the circuit in response to the application of the force to the first electrically conductive element 102. The change in capacitance may, in some examples, be indicative of a substance within which the first electrically conductive element is submerged. For example, the change in capacitance, or the rate of change of the capacitance may be different for an arm of a fluid level sensor submerged in print agent, than for an arm of a fluid level sensor in air. Thus, the method 700 may be considered to be a method for determining a fluid level.

[0040] In some examples, generating the force (block 702) may comprise generating a magnetic pulse to interact with the first electrically conductive element. Such a magnetic pulse may cause a momentary attraction of the first electrically conductive element (e.g. the first electrical contact 102) of the fluid level sensor away from its resting position. Once the magnetic pulse has ended, the first electrically conductive element may oscillate back to its resting position. In air, the oscillations may be at a resonant frequency of the first electrically conductive element, while in some other substances, the first electrically conductive element may oscillate at some other frequency, they may vibrate at the resonant frequency, but may be heavily damped, or they may not vibrate at all (e.g. the oscillations may be rapidly damped).

[0041 ] In some examples, generating the force (block 702) may comprise generating a plurality of magnetic pulses at a defined frequency. For example, the magnetic pulses may cause the first electrically conductive element to oscillate at the defined frequency. In this way, cyclic motion of the first electrically conductive element may be caused, as discussed above.

[0042] The defined frequency may, in some examples, comprise a resonant frequency of the first electrically conductive element. In this way, the

oscillation/vibration of the electrically conductive element(s) may be amplified.

[0043] Figure 8 is a flowchart of an example of a method 800 for detecting a change in capacitance in response to application of a force. The method 800 may comprise blocks from Figure 7. The method 800 may, in some examples, comprise determining, from the detected change in capacitance, a state of the first electrically conductive element. As noted above, the state to be determined may, in some examples, include whether or not the first electrically conductive element is submerged in a liquid. From a determination of whether or not the first electrically conductive element is submerged in a liquid, such as print agent, it may be possible to determine an amount of liquid in the container.

[0044] Figure 9 is a simplified schematic of a system 900. The system 900 may, in some examples, comprise a printing apparatus or printing system. Such a printing apparatus may include a print agent cartridge containing print agent. The print agent cartridge may be to deposit print agent in a pattern onto a printable substrate.

[0045] The system 900 comprises a replaceable print apparatus component 210 having a first electrically conductive element 102 disposed outside the replaceable print apparatus component and a second electrically conductive element 104 disposed within the replaceable print apparatus component. The systems 900 comprises a force generator 106 to generate a force to be imparted to the second electrically conductive element 104. A capacitance between the first electrically conductive element 102 and the second electrically conductive element 104 is variable in response to the imparted force.

[0046] The replaceable print apparatus component 210 may, in some examples, comprise a print agent container. In such examples, the first and second electrically conductive elements 102, 104 may form part of a fluid level sensor, such as the sensor 100, 300, 400 discussed above. The fluid level sensor may be associated with (e.g. installed on/within) the print agent container and used to determine an amount of print agent within the container.

[0047] In some examples, the replaceable print apparatus component 210 may comprise third and fourth electrically conductive elements (e.g. third and fourth electrical contacts 202, 204). Together, the first, second, third and fourth electrically conductive elements 102, 104, 202, 204 may form part of a fluid level sensor such as the sensor 400 (e.g. having two arms) discussed above.

[0048] The force generator 106 may, in some examples, be to generate an electromagnetic force. For example, the force generator may comprise an

electromagnetic coil. The force generator 106 may function in a manner similar to the excitation source 106 discussed above. For example, the force generator may impart a force to the second electrically conductive element 104, causing the second electrically conductive element to move relative to the first electrically conductive element 102, thereby varying the capacitance between them.

[0049] Within the system 900, the replaceable print apparatus component 210 may, in some examples, be mounted on a carriage. The carriage may carry the

replaceable print apparatus component 210 between various positions in the system, for example along a track. For example, in a printing apparatus, the carriage may carry the print agent container from one side of the printable substrate to an opposite side of the printable substrate, so that print agent may be deposited over a printable area of the substrate. In such a system, the force generator 106 may be positioned at one side of the printable substrate (e.g. near to an end of the track).

[0050] The replaceable print apparatus component 210 may, in some examples, comprise a first replaceable print apparatus component, and the system may further comprise a second replaceable print apparatus component. In some examples, the system 900 may further include additional replaceable print apparatus component 210. For example, a printing apparatus may comprise four print agent containers, each container to contain print agent of a different colour. The first replaceable print apparatus component 210 may be arranged closer to the force generator 106 than the second replaceable print apparatus component, such that the force experienced by the first replaceable print apparatus component is greater than the force experienced by the second replaceable print apparatus component. In other words, a force (e.g. a temporary magnetic field) generated by the force generator 106 may affect fluid level sensors in both replaceable print apparatus components. However, the replaceable component which is closer to the force generator 106 will experience the force to a greater extent than the replaceable component located further away from the force generator. Thus, the arm or arms of a fluid level sensor in a print agent container nearer to the force generator will vibrate with an amplitude greater than that of the arm or arms of a fluid level sensor in a print agent container further away from the force generator.

[0051 ] A comparison of the relative amplitudes of the vibrations of the arms in the various print agent containers may reveal a gradual decrease in the amplitude of vibrations detected in the containers further away from the force generator. With this knowledge, the system may expect that a fluid level sensor in the container furthest from the force generator will have a relatively small vibration amplitude. If this is taken into account when detecting the vibrations (e.g. measuring the capacitance change), then the system can make sure that a particular print agent container having a fluid level sensor which exhibits a seemingly weak (i.e. low amplitude) response is not incorrectly categorised as broken (e.g. by the system interpreting the weak response as a broken fluid level sensor).

[0052] The system, method and apparatus disclosed herein may, in some examples, be used in print apparatus which uses a page-wide array of fluid containers having print nozzles (i.e. apertures through which print agent is deposited onto a printable substrate) rather than a print agent cartridge moved across the substrate by a carriage. In such a page-wide array, the print agent cartridge remains stationary with respect to the print apparatus, so the excitation source 106 may be used to apply a stimulus or force to a fluid level sensor in the print cartridge, as may not be possible to cause movement (e.g. vibration) using other methods.

[0053] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart.

[0054] 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.

[0055] The word "comprising" does not exclude the presence of elements other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims.

[0056] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.

Claims

1 . A fluid level sensor comprising:

a first electrical contact; and

a second electrical contact forming a capacitive coupling with the first electrical contact;

wherein a capacitance between the first electrical contact and the second electrical contact is variable in response to a force imparted to the fluid level sensor by an excitation source.

2. A fluid level sensor according to claim 1 , wherein the excitation source comprises a source selected from a group comprising: a magnetic field generator an acoustic wave generator, a vibrating motor and a piezoelectric element.

3. A fluid level sensor according to claim 1 , wherein the second electrical contact is moveable relative to the first electrical contact, and wherein the second electrical contact is to move in response to the force imparted by the excitation source.

4. A method comprising:

generating a force from a force generation device;

applying the force to a first electrically conductive element of a variable capacitor of a circuit; and

detecting a change in capacitance in the circuit in response to the application of the force to the first electrically conductive element.

5. A method according to claim 4, wherein generating the force comprises generating a magnetic pulse to interact with the first electrically conductive element.

6. A method according to claim 4, wherein generating the force comprises generating a plurality of magnetic pulses at a defined frequency.

7. A method according to claim 6, wherein the defined frequency comprises a resonant frequency of the first electrically conductive element.

8. A method according to claim 4, further comprising:

determining, from the detected change in capacitance, a state of the first electrically conductive element.

9. A method according to claim 8, wherein the state to be determined includes whether or not the first electrically conductive element is submerged in a liquid.

10. A system comprising:

a replaceable print apparatus component having a first electrically conductive element disposed outside the replaceable print apparatus component and a second electrically conductive element disposed within the replaceable print apparatus component; and

a force generator to generate a force to be imparted to the second electrically conductive element;

wherein a capacitance between the first electrically conductive element and the second electrically conductive element is variable in response to the imparted force.

1 1 . A system according to claim 10, wherein the force generator is to generate an electromagnetic force.

12. A system according to claim 1 1 , wherein the force generator comprises an electromagnetic coil.

13. A system according to claim 10, wherein the system comprises a printing apparatus.

14. A system according to claim 10, wherein the replaceable print apparatus component comprises a print agent container.

15. A system according to claim 10, wherein the replaceable print apparatus component comprises a first replaceable print apparatus component, and wherein the system further comprises:

a second replaceable print apparatus component;

wherein the first replaceable print apparatus component is arranged closer to the force generator than the second replaceable print apparatus component, such that the force experienced by the first replaceable print apparatus component is greater than the force experienced by the second replaceable print apparatus component.