Patent Publication Number: US-10308035-B2

Title: Fluid level sensor and related methods

Description:
BACKGROUND 
     Accurate ink level sensing in ink supply reservoirs for various types of inkjet printers is desirable for a number of reasons. For example, sensing the correct level of ink and providing a corresponding indication of the amount of ink left in A fluid cartridge allows printer users to prepare to replace depleted ink cartridges. Accurate ink level indications also help to avoid wasting ink, since inaccurate ink level indications often result in the premature replacement of ink cartridges that still contain ink. In addition, printing systems can use ink level sensing to trigger certain actions that help prevent low quality prints that might result from inadequate supply levels. 
     While there are a number of techniques available for determining the level of fluid in a reservoir, or a fluidic chamber, various challenges remain related to their accuracy and cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a fluid ejection device embodied as an inkjet printing system suitable for incorporating a fluid level sensor, according to an embodiment; 
         FIG. 2  shows a bottom view of one end of a TIJ printhead having a single fluid slot formed in a silicon die substrate, according to an embodiment; 
         FIG. 3  shows a cross-sectional view of an example fluid drop generator, according to an embodiment; 
         FIG. 4  shows partial top and side views of a MEMS structure in different stages as ink is retracted over the sensor plate during a priming operation, according to an embodiment; 
         FIG. 5  shows an example of a high level block diagram of an ink level sensor circuit, according to an embodiment; 
         FIG. 6  shows a range select circuit, according to an embodiment; 
         FIG. 7  shows an ink level sensor as a black box element, according to an embodiment; 
         FIG. 8  shows a dry response curve, a wet response curve; and a difference curve over a range of input stimulus, according to an embodiment; 
         FIG. 9  shows a weak dry response curve, a weak wet response curve, and a weak difference curve, according to an embodiment; 
         FIGS. 10A-10C  show examples of process and environmental variations affecting weak wet and dry response curves, according to an embodiment; 
         FIG. 11  overlays the wet-dry difference signals from  FIGS. 10A-10C  and shows the difference plotted against the stimulus, illustrating shifts caused by process and environment, according to an embodiment; 
         FIG. 12  shows difference signal curves based on response instead of on stimulus, according to an embodiment; 
         FIGS. 13 and 14  show flowcharts of example methods of sensing a fluid level, according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Overview of Problem and Solution 
     As noted above, there are a number of techniques available for determining the level of fluid in a reservoir or fluidic chamber. For example, prisms have been used to reflect or refract light beams in ink cartridges to generate electrical and/or user-viewable ink level indications. Backpressure indicators are another way to determine fluid levels in a reservoir. Some printing systems count the number of drops ejected from inkjet print cartridges as a way of determining ink levels. Still other techniques use the electrical conductivity of the fluid as a level indicator in printing systems. Challenges remain, however, regarding improving the accuracy and cost of fluid level sensing systems and techniques. 
     Embodiments of the present disclosure provide a fluid level sensor and related methods that improve on prior ink level sensing techniques. The disclosed sensor and methods include a MEMS structure with fluidic elements, a sensor circuit, and a biasing technique to bias the circuit at an optimum operating point. The operating point at which the circuit is biased enables a maximum output difference signal between a dry ink condition (i.e., no ink present) and a wet ink condition (i.e., ink present). The sensor circuit includes a sensor plate in a fluidic channel. Backpressure exerted on the ink in the channel (e.g., while spitting or priming) retracts the ink from a nozzle and pulls it back through the channel over the sensor plate, exposing the plate to air. The circuit includes a current source to supply a current to the sensor plate and induce a voltage response across the plate. The voltage response measured across the plate provides an indication of whether the plate is wet (i.e., indicating ink is present in the fluidic channel) or dry (i.e., indicating air is present in the fluidic channel). The biasing technique employs an algorithm to bias the current source at an optimum point where the amount of current supplied to the sensor plate induces a maximum differential voltage response across the sensor plate between the wet and dry plate conditions in weak signal conditions. 
     Advantages of the disclosed fluid level sensor and related methods include a high tolerance to contamination from debris left behind in the MEMS structure (e.g., fluidic channels and ink chambers) that enables accurate indications between wet and dry conditions. The sensor cost is controlled because of its use of circuitry and MEMS structures placed onto an existing thermal ink jet print head. The size of the circuitry is such that it can be placed in the space of a few ink-jet nozzles. 
     In one embodiment, a fluid level sensor includes a sensor circuit having a sensor plate and a current source. The fluid level sensor also includes an algorithm having processor-executable instructions to bias the current source such that current applied to the sensor plate from the current source induces a maximum difference in response voltage between a dry sensor plate condition and a wet sensor plate condition. 
     In one embodiment, a fluid level sensor includes a current source and a DAC (digital-to-analog convertor) to convert an input code into a bias voltage for the current source. The sensor also includes a sensor plate and a switch to apply current from the current source to the sensor plate. A measurement module determines a wet or dry sensor plate condition by comparing a response voltage on the sensor plate to a threshold. 
     In another embodiment, a method of sensing a fluid level includes applying stimulus voltage to a sensor circuit in wet and dry conditions. The stimulus voltage has a range from a minimum to a maximum voltage. The method includes measuring a wet response and a dry response over the stimulus range. A difference response between the wet and dry responses is determined, and a peak difference is located in the difference response. The method then determines a peak stimulus voltage that corresponds to the peak difference. 
     In another embodiment, a method of sensing a fluid level includes biasing a current source such that a current will induce a maximum voltage variation across a sensor plate between a wet sensor plate condition and a dry sensor plate condition. The method also includes applying the current to the sensor plate, sampling a response voltage across the sensor plate, comparing the response voltage to a threshold voltage, and determining the dry sensor plate condition based on the comparing. 
     Illustrative Embodiments 
       FIG. 1  illustrates a fluid ejection device embodied as an inkjet printing system  100  suitable for implementing a fluid level sensor and methods as disclosed herein, according to an embodiment of the disclosure. In this embodiment, a fluid ejection assembly is disclosed as a fluid drop jetting printhead  114 . Inkjet printing system  100  includes an inkjet printhead assembly  102 , an ink supply assembly  104 , a mounting assembly  106 , a media transport assembly  108 , an electronic printer controller  110 , and at least one power supply  112  that provides power to the various electrical components of inkjet printing system  100 . Inkjet printhead assembly  102  includes at least one fluid ejection assembly  114  (printhead  114 ) that ejects drops of ink through a plurality of orifices or nozzles  116  toward a print medium  118  so as to print onto print media  118 . Print media  118  can be any type of suitable sheet or roll material, such as paper, card stock, transparencies, polyester, plywood, foam board, fabric, canvas, and the like. Nozzles  116  are typically arranged in one or more columns or arrays such that properly sequenced ejection of ink from nozzles  116  causes characters, symbols, and/or other graphics or images to be printed on print media  118  as inkjet printhead assembly  102  and print media  118  are moved relative to each other. 
     Ink supply assembly  104  supplies fluid ink to printhead assembly  102  and includes a reservoir  120  for storing ink. Ink flows from reservoir  120  to inkjet printhead assembly  102 . Ink supply assembly  104  and inkjet printhead assembly  102  can form either a one-way ink delivery system or a recirculating ink delivery system. In a one-way ink delivery system, substantially all of the ink supplied to inkjet printhead assembly  102  is consumed during printing. In a recirculating ink delivery system, however, only a portion of the ink supplied to printhead assembly  102  is consumed during printing. Ink not consumed during printing is returned to ink supply assembly  104 . 
     In one embodiment, ink supply assembly  104  supplies ink under positive pressure through an ink conditioning assembly  105  to inkjet printhead assembly  102  via an interface connection, such as a supply tube. Ink supply assembly  104  includes, for example, a reservoir  120 , pumps and pressure regulators (not specifically illustrated). Reservoir  120  may be removed, replaced; and/or refilled. Conditioning in the ink conditioning assembly  105  may include filtering, pre-heating, pressure surge absorption, and degassing. Ink is drawn under negative pressure from the printhead assembly  102  to the ink supply assembly  104 . The pressure difference between the inlet and outlet to the printhead assembly  102  is selected to achieve the correct backpressure at the nozzles  116 , and is usually a negative pressure between negative 1″ and negative 10″ of H2O. However, as the ink supply (e.g., in reservoir  120 ) nears its end of life, the backpressure exerted during printing or priming operations increases. The increased backpressure is strong enough to retract the ink meniscus from the nozzle  116  and back through the fluidic channel of the MEMS structure. In one embodiment, printhead  114  includes an ink level sensor  206  ( FIG. 2 ) that uses the increased backpressure and retracted meniscus to provide an accurate ink level indication toward the end of life of the ink supply. 
     Mounting assembly  106  positions inkjet printhead assembly  102  relative to media transport assembly  108 , and media transport assembly  108  positions print media  118  relative to inkjet printhead assembly  102 . Thus, a print zone  122  is defined adjacent to nozzles  116  in an area between inkjet printhead assembly  102  and print media  118 . In one embodiment, inkjet printhead assembly  102  is a scanning type printhead assembly. As such, mounting assembly  106  includes a carriage for moving inkjet printhead assembly  102  relative to media transport assembly  108  to scan print media  118 . In another embodiment, inkjet printhead assembly  102  is a non-scanning type printhead assembly. As such, mounting assembly  106  fixes inkjet printhead assembly  102  at a prescribed position relative to media transport assembly  108  while media transport assembly  108  positions print media  118  relative to inkjet printhead assembly  102 . 
     Electronic printer controller  110  typically includes a processor, firmware, software, one or more memory components including volatile and no-volatile memory components, and other printer electronics for communicating with and controlling inkjet printhead assembly  102 , mounting assembly  106 , and media transport assembly  108 . Electronic controller  110  receives data  124  from a host system, such as a computer, and temporarily stores data  124  in a memory. Typically, data  124  is sent to inkjet printing system  100  along an electronic, infrared, optical, or other information transfer path. Data  124  represents, for example, a document and/or file to be printed. As such, data  124  forms a print job for inkjet printing system  100  and includes one or more print job commands and/or command parameters. 
     In one embodiment, electronic printer controller  110  controls inkjet printhead assembly  102  for ejection of ink drops from nozzles  116 . Thus, electronic controller  110  defines a pattern of ejected ink drops that form characters, symbols, and/or other graphics or images on print media  118 . The pattern of ejected ink drops is determined by the print job commands and/or command parameters from data  124 . In one embodiment, electronic controller  110  includes a biasing algorithm  126  having executable instructions to execute on controller  110 . The biasing algorithm  126  executes to control the ink level sensor  206  ( FIG. 2 ) and to determine an optimum operating/bias point that produces a maximum voltage response difference from the sensor  206  between a wet condition (i.e., when ink is present) and a dry condition (when air is present). Electronic controller  110  additionally includes a measurement module  128  having executable instructions to execute on controller  110 . After an optimum bias point is determined, measurement module  128  executes to initiate a measurement cycle that controls the ink level sensor  206  and determines an ink level based on a measured time period during which a dry condition persists in a fluidic channel of the MEMS structure. 
     In the described embodiments, inkjet printing system  100  is a drop-on-demand thermal inkjet printing system with a thermal inkjet (TIJ) printhead  114  suitable for implementing an ink level sensor as disclosed herein. In one implementation, inkjet printhead assembly  102  includes a single TIJ printhead  114 . In another implementation, inkjet printhead assembly  102  includes a wide array of TIJ printheads  114 . While the fabrication processes associated with TIJ printheads are well suited to the integration of the disclosed ink level sensor, other printhead types such as a piezoelectric printhead can also implement such an ink level sensor. Thus, the disclosed ink level sensor is not limited to implementation in a TIJ printhead  114 . 
       FIG. 2  shows a bottom view of one end of a TIJ printhead  114  having a single fluid slot  200  formed in a silicon die substrate  202 , according to an embodiment of the disclosure. Although printhead  114  is shown with a single fluid slot  200 , the principles discussed herein are not limited in their application to a printhead with just one slot  200 . Rather, other printhead configurations are also possible, such as printheads with two or more fluid slots, or printheads that use various sized holes to bring ink to fluidic channels and chambers. The fluid slot  200  is an elongated slot formed in the substrate  202  that is in fluid communication with a fluid supply, such as a fluid reservoir  120 . Fluid slot  200  has fluid drop generators  300  arranged along both sides of the slot that include fluid chambers  204  and nozzles  116 . Substrate  202  underlies a chamber layer having fluid chambers  204  and a nozzle layer having nozzles  116  formed therein, as discussed below with respect to  FIG. 3 . However, for the purpose of illustration, the chamber layer and nozzle layer in  FIG. 2  are assumed to be transparent in order to show the underlying substrate  202 . Therefore, chambers  204  and nozzles  116  in  FIG. 2  are illustrated using dashed lines. 
     In addition to drop generators  300  arranged along the sides of the slot  200 , the TIJ printhead  114  includes one or more fluid (ink) level sensors  206 . A fluid level sensor  206  generally includes a MEMS structure and an integrated sensor circuit  208 . A MEMS structure includes, for example, fluid slot  200 , fluidic channels  210 , fluid chambers  204  and nozzles  116 . A sensor circuit  208  includes a sensor plate  212  located on the floor of a fluidic channel  210 , and other circuitry  214 . The other circuitry  214  includes, for example, a current source, a buffer amplifier, a DAC (digital-to-analog convertor), an ADC (analog-to-digital convertor), and measurement circuitry. The sensor plate  212  is a metal plate formed, for example, of tantalum. Portions of the other circuitry  214 , such as the ADC and measurement circuitry, may not all be in one location on substrate  202 , but instead may be distributed on substrate  202  in different locations. The fluid sensor  206  and sensor circuit  208  are discussed in greater detail below with respect to  FIGS. 4 and 5 . 
       FIG. 3  shows a cross-sectional view of an example fluid drop generator  300 , according to an embodiment of the disclosure. Each drop generator  300  includes a nozzle  116 , a fluid chamber  204 , and a firing element  302  disposed in the fluid chamber  204 . Nozzles  116  are formed in nozzle layer  310  and are generally arranged to form nozzle columns along the sides of the fluid slot  200 . Firing element  302  is a thermal resistor formed of a metal plate (e.g., tantalum-aluminum, TaAl) on an insulating layer  304  (e.g., polysilicon glass, PSG) on a top surface of the silicon substrate  202 . A passivation layer  306  over the firing element  302  protects the firing element from ink in chamber  204  and acts as a mechanical passivation or protective cavitation barrier structure to absorb the shock of collapsing vapor bubbles. A chamber layer  308  has walls and chambers  204  that separate the substrate  202  from the nozzle layer  310 . 
     During printing, a fluid drop is ejected from a chamber  204  through a corresponding nozzle  116 , and the chamber  204  is then refilled with fluid circulating from fluid slot  200 . More specifically, an electric current is passed through a resistor firing element  302  resulting in rapid heating of the element. A thin layer of fluid adjacent to the passivation layer  306  that covers firing element  302  is superheated and vaporizes, creating a vapor bubble in the corresponding firing chamber  204 . The rapidly expanding vapor bubble forces a fluid drop out of the corresponding nozzle  116 . When the heating element cools, the vapor bubble quickly collapses, drawing more fluid from fluid slot  200  into the firing chamber  204  in preparation for ejecting another drop from the nozzle  116 . 
       FIG. 4  shows partial top and side views of a MEMS structure in different stages as ink is retracted over the sensor plate during a priming operation, according to an embodiment of the disclosure. As noted above, a fluid level sensor  206  generally includes a MEMS structure having a fluidic channel  210 , a fluid chamber  204  and a dedicated sensor nozzle  116 . A fluid level sensor  206  also includes a sensor circuit  208  with a sensor plate  212  located on the floor of a fluidic channel  210 . The sensor circuit  208  operates to detect the presence or absence of fluid (ink) in the fluidic channel during a priming operation. As the ink supply in reservoir  120  nears its end of life, the backpressure exerted during printing or priming operations becomes strong enough to retract the ink meniscus from the nozzle  116  and back through the fluidic channel  210 , exposing the sensor plate  212  to air.  FIG. 4( a )  shows a normal state where ink  400  fills the chamber  204  and forms an ink meniscus  402  within the nozzle  116 . In this state, the sensor plate  212  is in a wet condition as it is covered with the ink that fills the fluidic channel  210 . During a priming operation, or a normal ink drop ejection printing operation, a backpressure is exerted on the ink in the fluidic channel  210  which retracts the ink meniscus  402  from the nozzle and pulls it back within the channel as shown in  FIG. 4( b ) . As the ink supply in reservoir  120  nears its end of life, this backpressure increases, as does the time it takes for the ink to flow back into the channel  210  and nozzle  116 . As shown in  FIG. 4( c ) , the increased backpressure pulls the ink meniscus far enough back into the channel  210  that the sensor plate  212  is exposed to air drawn in through nozzle  116 . As discussed below, the sensor circuit  208  uses the exposed sensor plate  212  to determine an accurate ink level near the end of life of the ink supply. 
       FIG. 5  shows an example of a high level block diagram of a fluid level sensor circuit  208 , according to an embodiment of the disclosure. The sensor circuit  208  includes a DAC (digital-to-analog convertor)  500 , an input S&amp;H (sample and hold element)  502 , a current source  504 , a sensor plate  212 , a switch  506 , an output S&amp;H  508 , an ADC (analog-to-digital convertor)  510 , a state machine  512 , a clock  514 , and a number of registers such as registers 0xD0-0xD6,  516 . Operation of the sensor circuit  208  begins with configuring (i.e., biasing) the current source  504  with the DAC  500  and input S&amp;H  502  while switch  506  is closed to short out the sensor plate  212 . The biasing algorithm  126 , discussed in greater detail below, executes on controller  110  to determine a stimulus (input code) to apply to register 0xD2 that yields an optimumbias voltage from the DAC  500  with which to bias the current source  504 . 
     After the current source  504  is biased, the measurement module  128  executes on controller  110  and initiates a fluid level measurement cycle during which it controls the sensor circuit  208  through state machine  512 . When it is time to measure, the state machine  512  coordinates the measurement by stepping the circuit  208  through several stages that prepare the circuit, take the measurements, and return the circuit to idle. In a first step, the state machine  512  initiates a priming event. The priming event spits or ejects ink from the nozzle  116  to clear the nozzle and chamber  204  of ink, and creates a backpressure spike in the fluidic channel  210 . The state machine  512  then provides a delay period. The delay period is variable, but typically lasts on the order of between 2 and 32 microseconds. After the delay, a first circuit preparation step opens switch  506 , applying current from the current source  504  to the sensor plate  212 . The applied current charges the plate capacitance and induces a voltage response across the plate. 
     Note that the current supplied from the currentsource  504  is based on the following relationship:
 
 I α( V   gs   −V   t ) 2  
 
where Vgs is the bias voltage from the DAC  500 . Vgs is the gate-to-source voltage and Vt is the gate threshold voltage of a current-producing transistor of the current source  504 . Current source  504  includes a range select circuit, shown generally in  FIG. 6 , that enables the voltage from the DAC  500  to be applied to one of three current-producing transistors  600 ,  602 ,  604 , that produce current for the ranges 1×, 10× and 100×. Once a transistor is selected to produce current, the voltage from the DAC  500  is applied at the gate of the selected transistor which determines the amount of current supplied by current source  504 .
 
     In a second circuit preparation step, the state machine  512  opens the switch  506  and provides a second delay period, which again lasts on the order of between 2 and 32 microseconds. After the second delay, the state machine  512  causes the output S&amp;H element  508  to sample (i.e., measure) the analog response voltage at the sensor plate  212  and to hold it. The state machine  512  then initiates a conversion through ADC  510  that converts the sampled analog response voltage to a digital value that is stored in a register, 0xD6. The register holds the digital response voltage until the measurement module  128  reads the register. The circuit  208  is then put in an idle mode until another measurement cycle is initiated. 
     The measurement module  128  compares the digitized response voltage to an R detect  threshold to determine if the sensor plate is in a dry condition. If the measured response exceeds R detect  then the dry condition is present. Otherwise the wet condition is present. (Calculation of the R detect  threshold is discussed below). Detecting a dry condition indicates that the backpressure has pulled the ink in the fluidic channel  210  back far enough to expose the sensor plate  212  to air. Through additional measurement cycles, the length of time that the dry condition persists (i.e., while the sensor plate is exposed to air) is measured and used to interpolate the magnitude of backpressure creating the dry condition. Since the backpressure increases predictably toward the end of the life of the ink supply, an accurate determination of the ink level can then be made. 
     As noted above, the biasing algorithm  126  executes on controller  110  to determine an optimum bias voltage from the DAC  500  with which to bias the current source  504 . The biasing algorithm  126  controls the fluid level sensor  206  (i.e., the sensor circuit  208  and MEMS structure) while determining the bias voltage. From the perspective of the biasing algorithm  126 , as shown in  FIG. 7 , the fluid level sensor  206  is a black box element that receives an input or stimulus and provides an output or response. An input voltage is set using a 0-255 (8-bit) number (input code) applied to register 0xD2 of sensor circuit  208 . The input number or code in register 0xD2 is a stimulus that is applied to the DAC  500 , and the analog voltage output from the DAC is the stimulus multiplied by 10 mV. Therefore, the range of analog bias voltage from the DAC  500  that is available for biasing the current source  504  is 0-2.55V. The output or response from the sensor circuit  208  is a digital code stored in an 8-bit register 0xD6. 
     The biasing algorithm uses the stimulus-response relationship of the sensor circuit  208  between input codes and output codes to provide an optimum output delta signal (i.e., a maximum response voltage) between when the sensor plate  212  is wet (i.e., when ink is present in MEMS fluidic channel  210  and covers the plate) and when the sensor plate  212  is dry (i.e., when ink has been pulled out of the MEMS fluidic channel  210  and air surrounds the plate). As shown in  FIG. 8 , when the stimulus (input codes) is swept from its minimum to its maximum pre-charge voltage count (i.e., 0-255; S min  to S max ), the response (output codes) generate response waveforms that progress through three distinct regions: Off. Active and Saturated. Together, the three regions form the shape of a lazy “S”.  FIG. 8  shows a dry response curve  800 , a wet response curve  802 , and a difference curve  804  that indicates the difference between the wet and dry response curves over the range of input stimulus. The  FIG. 8  response curves depict favorable conditions where the responses are strong. In general, the largest signal delta (i.e., largest difference response curve) occurs between the case where the sensor plate  212  is fully wet with a full channel of ink, and the case where the sensor plate  212  is fully dry with full contact with air in the channel. 
     Although the response curves vary between the presence and absence of fluid/ink (i.e., between wet and dry conditions), the amount of variance is stronger when there is little or no contamination present in the MEMS structure, such as conductive debris and ink residue. Therefore, the response is initially strong as shown by the strong response curves in  FIG. 8 . However, over time the MEMS structure may become contaminated with ink residue in the fluidic channels and chambers, and the dry response in particular will degrade and become closer to the wet response. Contamination causes conduction in the dry case that makes the dry response weak, which results in a weak difference between the dry and wet response.  FIG. 9  shows weak dry  900 , wet  902 , and difference  904  response curves where unfavorable conditions such as contamination in the MEMS structure have degraded the responses. As can be seen in  FIG. 9 , the difference between the weak wet and weak dry response curves is much less than the difference shown in the strong response curves of  FIG. 8 . The strong difference curve  804  shown in  FIG. 8  provides a strong distinction between a wet and dry condition that can be readily evaluated. However, under weak response conditions, finding a distinction between wet and dry conditions is more challenging because of the weak difference. The biasing algorithm  126  finds the optimum point of difference in the weak response difference curve  904  (i.e., shown in  FIG. 9 ) where fluid/ink level measurements will provide the maximum response between wet and dry conditions. 
       FIGS. 10A-10C  show examples of weak dry response curves  1000  and weak wet response curves  1002  and their variations in response to differences in process and environmental conditions, such as manufacturing process, supply voltage and temperature (PV&amp;T), according to an embodiment of the disclosure.  FIG. 10A  shows example curves over input stimulus ranges 1×, 10× and 100×, respectively, with worst (W) case processing conditions, a 5.5 volt supply, and 15 degrees centigrade temperature (referenced in FIGs. as “W; 5.5V; 15 C”).  FIG. 10B  shows example curves over input stimulus ranges 1×, 10× and 100×, respectively, with best case (B) processing conditions, a 4.5 volt supply, and 110 degrees centigrade temperature (referenced in FIGs. as “B; 4.5V; 110 C”).  FIG. 10C  shows example curves over input stimulus ranges 1×, 10× and 100×, respectively, with typical (T) processing conditions, a 5.0 volt supply, and 60 degrees centigrade temperature (referenced in FIGs. as “T; 5.0V; 60 C”). In some cases, the active regions of the response curves change in slope due to variations in PV&amp;T. In other cases, the active regions of the response curves shift their placement, starting earlier or later in the off region. The dry and wet response curves in  FIGS. 10A-10C  show such variations in slopes and starting points that can result from varying PV&amp;T conditions. The difference curves  1004  in  FIGS. 10A-10C  show the difference between the wet and dry response curves over the range of input stimulus and over variations in PV&amp;T conditions. 
       FIG. 11  shows the difference between the dry response and wet response plotted against the stimulus, according to an embodiment of the disclosure. The difference curves  1004  shown in  FIG. 10  are overlayed to form  FIG. 11 . The intention is to illustrate that the height of the peak of the difference curves, the slope of the approach and decay of the curves, and the placement of the center of the stimulus axis along the curves, all vary across PV&amp;T. 
       FIG. 12  shows an example of composite difference curves  1200  plotted against the wet response, according to an embodiment of the disclosure. By shifting the basis of the difference curves to response, instead of stimulus, a measure of isolation from PV&amp;T differences is achieved. The biasing algorithm  126  finds a solution where the optimum difference point is located in the weak difference case that provides a maximum ink level measurement response between wet and dry conditions. Therefore, the solution should be tolerant to such variations in PV&amp;T, as well as provide as large a margin as possible. Accordingly, as shown in  FIG. 12 , a large amount of the PV&amp;T variance can be removed by viewing the difference curve  1004  as a function of the wet response curve  1002 , instead of as a function of the input stimulus. This is because there is a large variation in output value for a given stimulus over process, voltage and temperature (PV&amp;T). However, the difference between the dry condition (no ink) and the wet condition (ink present) does not vary as much over PV&amp;T, so using this difference subtracts off much of the PV&amp;T-induced variation. The composite of the difference curves encompasses the area formed by overlaying many difference curves determined across all process and environmental (PV&amp;T) conditions. Thus, the region above the composite difference represents viable signal response area that is independent of PV&amp;T conditions. The center of the composite difference represents the location where ink level measurements should be made in order to achieve a peak response (R peak ) that maximizes the voltage response between a dry condition and a wet condition. The location of the R peak  response is expressed as a percentage of the span between the minimum and maximum wet response, R min  and R max . Thus, the location of R peak  on the composite difference curve  1200  is called R pd % . In addition, during a measurement cycle, the height of the peak of the composite difference curve  1200  at location R pd %  represents the minimum difference expected (as a percentage of the span between R min , and R max ) when the dry condition is present, and can be called D min % . 
     The biasing algorithm  126  determines an input stimulus value S peak , that produces the peak response R peak  located on the composite difference curve  1200  at Rpd %. The algorithm inputs a minimum stimulus (S min ) at register 0xD2 and samples the response in register 0xD6. The algorithm also inputs a maximum stimulus (S max ) at register 0xD2 and samples the response in register 0xD6. These two values in register 0xD6 are the extremes of response, R min  and R max  respectively. The peak response value R peak  can then be calculated as follows:
 
 R   peak   =R   min +( R   pd % *( R   max   −R   min ))
 
     The corresponding stimulus value, S peak , can then be found by a variety of approaches. The stimulus can, for example, be swept from S min  to S max , stopping when the response reaches R peak . Another approach is to use a binary search. The stimulus value S peak  that produces the peak response R peak  is the input code applied to register 0xD2 to optimally bias the current source  504  in sensor circuit  208  such that a maximum response can be measured across the sensor plate  212  between a dry plate condition and a wet plate condition. 
     As noted above, in a measurement cycle the measurement module  128  determines if the sensor plate  212  is in a dry condition by comparing the response voltage measured across the plate to an R detect  threshold. If the measured response exceeds R detect  then the dry condition is present. Otherwise the wet condition is present. The R detect  threshold is calculated by the following equation:
 
 R   detect   =R   peak +(( R   max   −R   min )*( D   min % /2))
 
     The minimum difference D min %  expected in the response voltage is split (i.e., divided by 2) to share the noise margin between the dry condition case and the wet condition case. 
       FIG. 13  shows a flowchart of an example method  1300  of sensing a fluid level, according to an embodiment of the disclosure. Method  1300  is associated with the embodiments discussed above with respect to  FIGS. 1-12 . Method  1300  begins at block  1302 , with applying stimulus voltage to a sensor circuit in wet and dry conditions. The applied stimulus voltage has a range from a minimum to a maximum voltage. At block  1304 , a wet response and a dry response are measured over the stimulus range. The measuring includes sampling voltage across a sensor plate in a fluid channel that contains fluid, and sampling voltage across a sensor plate in a fluid channel from which the fluid has been withdrawn by an applied backpressure. The method  1300  continues at block  1306  with finding a difference response between the wet and dry responses, and at block  1308  a peak difference in the difference response is located. At block  1310 , a peak stimulus that corresponds to the peak difference is determined. This step includes determining a wet response value that corresponds to the peak difference, and correlating the wet response value to the peak stimulus voltage. At block  1312  of method  1300 , a current source of the sensor circuit is biased using the peak stimulus, and at block  1314 , current from the current source is applied to the sensor plate. At block  1316 , a voltage response across the sensor plate is sampled. The sensor plate voltage is compared with a threshold voltage at block  1318  to determine a dry plate condition, and the time period over which the dry plate condition persists is measured at block  1320 . At block  1322  of method  1300 , a fluid level is determined based on the time period. 
       FIG. 14  shows a flowchart of another example method  1400  of sensing a fluid level, according to an embodiment of the disclosure. Method  1400  is associated with the embodiments discussed above with respect to  FIGS. 1-12 . Method  1400  begins at block  1402 , with biasing a current source such that current from the current source will induce a maximum voltage variation across a sensor plate between a wet sensor plate condition and a dry sensor plate condition. Biasing the current source includes determining an input bias voltage that produces the maximum voltage variation and applying the input bias voltage to a transistor gate of the current source. Finding the input bias voltage includes applying a range of stimulus to the current source from a minimum stimulus voltage to a maximum stimulus voltage for both the wet sensor plate condition and the dry sensor plate condition. Applying the stimulus includes applying an 8-bit number ranging from zero to 255 to a DAC, and providing the output from the DAC as the 8-bit number multiplied by an analog voltage (e.g., 1 mV, 10 mV, 100 mV). Finding the input bias voltage also includes determining a wet condition voltage response and a dry condition voltage response across the sensor plate over the range of stimulus, determining a difference response between the wet condition voltage response and the dry condition voltage response, determining a peak difference response from the difference response, and locating a peak stimulus voltage that produces the peak difference response. 
     At block  1404  of method  1400 , the current produced from the biased current source is applied to the sensor plate, and at block  1406  a response voltage across the sensor is sampled. The response voltage is compared with a threshold voltage at block  1408  to determine a dry plate condition as shown at block  1410 . At block  1412 , prior to the sampling, back pressure is applied to retract the meniscus from the nozzle and past the sensor plate within a fluidic channel. The back pressure is applied through priming the nozzle which creates a backpressure spike. At block  1414 , the length of time that the dry sensor plate condition continues is measured, and at block  1416  a fluid level in the reservoir is determined based on the length of time.