Pressure-sensitive pad with calibration unit

Abstract

A pressure sensitive pad configured to be placed under a mattress and configured to identify the presence or absence of a threshold weight on the mattress. The pressure sensitive pad comprises a signal generator, a sensing area, at least one variable matching resistor, and a microcontroller unit (MCU). The signal generator is configured to generate an electrical signal. The sensing area has a sensing resistor having a first resistance that varies in response to pressure on the sensing area. The variable matching resistor is disposed in series with the sensing resistor and the signal generator and has a second resistance. The MCU has an analog-to-digital converter (CAN) configured to receive and measure a portion of the signal at an electrical junction between the sensing resistor and the at least one variable matching resistor, and the MCU is configured to control the second resistance of the at least one variable matching resistor.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 216,600, filed June 30, 2021, which is incorporated herein in its entirety.

[0002] The present invention, in some embodiments thereof, relates to an under-mattress pressure sensing pad, and particularly (but not exclusively) to the calibration of an under-mattress pressure sensing pad for use with various mattresses. [Background technology]

[0003] An under-mattress pressure-sensitive pad is a pad that is placed under a mattress to detect the presence of an object (such as a person) on the mattress. The pad is configured for use with a variety of mattresses having different shapes and weights and to consistently detect the presence of an object having a particular weight on the mattress. One example of such a pad is described in U.S. Patent No. 10,357,197 to Smith, Jr. et al.

[0004] The under-mattress pressure-sensing pad includes a sensing zone that is placed under the mattress. The sensing zone includes a conductive material, such as a wire, that has a resistance that decreases when pressure is applied. This allows a resistance measurement circuit, such as a voltage divider, to be used in conjunction with an analog-to-digital converter (ADC) of a microcontroller (MCU) to detect the presence of an object on the mattress. The variable resistance material may include one or more of a conductive film (e.g., a film called Velostat and manufactured by 3M, or any similar material), a conductive foam, a strain gauge that generates a variable resistance when deformed, and a conductive ink. Summary of the Invention [Problem to be solved by the invention]

[0005] The inventors have found that for some variable resistance materials, such as inks, the manufacturing process varies greatly. Changing aspects such as the thickness of the ink applied, the time the ink is cured, and the temperature during the curing process all affect the final product. These variables result in sensor materials with drastically changing final resistance response curves.

[0006] 1 shows an electrical diagram of a pad 100 known in the prior art. Pad 100 has a sensing zone with a variable resistor (hereinafter referred to as the "sensing resistor") with a resistance RSENSE that decreases as pressure on the sensing zone increases.

[0007] The pad 100 further comprises a signal generator 102, a resistor with a fixed resistance RPOT, and a microcontroller unit (MCU) 104 with an analog-to-digital converter (ADC) 106. The sensing zone is connected at one end to an electrical junction 108 and at the other end to ground 100. A resistor with resistance RPOT is connected at one end to the signal generator 102 and at the other end to the electrical junction 108. Finally, the electrical junction 108 is also connected to the ADC 106 of the MCU 104. Thus, the circuit forms a voltage divider, where the voltage from the signal generator 102 is applied to two resistors in series (a resistor with a fixed resistance RPOT and a variable sensing resistor with resistance RSENSE), and this voltage is measured by the MCU at the junction 108 between the two resistors.

[0008] Thus, once a signal is generated by the signal generator, it travels through a resistor with a fixed resistance RPOT to junction 108. From junction 108, the signal travels through a sensing zone (RSENSE) to ground 110 and also to the ADC 106.

[0009] The voltage of the portion of the signal that reaches the ADC and is measured by the ADC depends on RSENSE, which in turn depends on the pressure applied to the sensing zone. Thus, the voltage measured by the ADC is indicative of the pressure on the sensing zone and can be processed to determine the presence or absence of an object on the sensing zone.

[0010] In a sample of 10 ink-based pads randomly drawn from a population of 400 manufactured by the inventors, no-load resistance values ​​(resistance of the sensing zone with no weight on top) were found to range from 10 kOhms to 98 kOhms. This variation is problematic when a voltage divider with a matching resistor (RPOT in FIG. 1) is used to measure the resistance of the pressure-sensitive material in the sensing zone. The value of this matching resistor must be selected to complement the resistance of the sensing material in the sensing zone to provide an optimal operating range for the system. The substantial variation in the resistance of the sensing zone creates problems for mass production of under-mattress pressure-sensitive pads, since the resistance of the matching resistor RPOT must match the resistance value of the sensing zone. Since the resistance of the sensing zone varies substantially, the selection of the resistance of the matching resistor RPOT may not allow the optimal operating range of all sensing zones.

[0011] To illustrate this challenge, FIG. 2 shows sample response curves for two worst-case scenario pads from a sample of pads. The orange horizontal line 120 represents the maximum ADC voltage reading the microcontroller can reach. Curves 122 and 124 are the response curves for a low resistance pad (unloaded resistance of 50k ohms) and a high resistance pad (unloaded resistance of 120k ohms), respectively. As can be seen below, the voltage of the portion of the signal that reaches the ADC decreases as more weight is placed on the sensing zone and RSENSE decreases. The further the response curve deviates from this line, the less sensitive the pad is. In this case, the low resistance pad (curve 122) is about 50% less sensitive than the high resistance pad (curve 124).

[0012] The challenge of using a voltage divider in a sensing circuit is the inherent effect of the signal shifting away from the ADC maximum (or Vref) value. In the voltage divider equation, as shown in Figure 1, it is desirable for Vout (the voltage measured by the ADC) to be as close as possible to Vin (the voltage of the signal generated by the signal generator) at the calibration time in order to maintain the maximum operating range of readings at the ADC. Unfortunately, in standard ADC settings, Vref=Vin. This means that the maximum Vout that can be obtained for any value of RSENSE is determined by RPOT.

[0013]

number

[0014] As the value of RSENSE (the sensor material) decreases for a given RPOT, Vout begins to approach zero, which can be observed in Figure 2 as the response curve 122 of the low resistance pad shifts lower on the graph than the response curve 124 of the high resistance pad.

[0015] Making RPOT lower helps bring the response curve back up, but it can only be adjusted down to a certain extent. As RPOT approaches zero ohms, the pad's battery drain increases during normal operation, and choosing RPOT to be zero completely negates the effect of the voltage divider.

[0016] To solve the above problems found by the inventors of the present invention, in some embodiments of the present invention, the matching resistor RPOT in the voltage divider is a variable potentiometer controlled by the MCU. This allows the matching resistor to be adjusted during pad calibration to select a desired value that complements the actual resistor of the sensing zone. This helps to mitigate the change in the sensor material of the pad by creating an adaptive circuit.

[0017] Additionally, in some embodiments of the present invention, Vref is set to be the desired voltage, which shifts the "ADC Max" line seen in Figure 1 downward toward the pad's response curve. As will be further shown below, this shift allows the adaptation circuitry to have a larger operating range and dynamically adjust the pad's response curve.

[0018] Therefore, an aspect of some embodiments of the present invention relates to a pressure sensitive pad configured to be placed under a mattress and configured to identify the presence or absence of a threshold weight on the mattress. The pressure sensitive pad comprises a signal generator, a sensing zone, at least one variable matching resistor, and a microcontroller unit (MCU). The signal generator is configured to generate an electrical signal. The sensing zone has a sensing resistor with a first resistance that varies in response to pressure applied to the sensing zone. The at least one variable matching resistor is in series with the sensing resistor and the signal generator and has a second resistance. The MCU has an analog-to-digital converter (ADC) configured to receive and measure a portion of the signal at an electrical junction between the sensing resistor and the at least one variable matching resistor. The MCU is configured to control the second resistance of the at least one variable matching resistor. The sensing zone is configured to be connected to an electrical ground. The pressure sensitive pad is configured to be calibrated for any mattress by placing the sensing zone under the mattress without any additional weight on the mattress, generating an electrical signal via the signal generator, automatically measuring the portion of the electrical signal that reaches the ADC by the ADC, and automatically varying, via the microcontroller unit, a second resistance of the at least one variable matching resistor until a desired value of the second resistance is found such that the portion of the signal that reaches the ADC is within a predetermined range of a desired predetermined voltage. In post-calibration operation, the microcontroller unit is configured to set the second resistance to a desired value, and the signal generator is configured to generate an electrical signal in a regular manner, the electrical signal being configured to travel via the at least one variable matching resistor to the electrical junction, from the electrical junction to the ADC, and from the electrical junction to electrical ground via the sensing resistor, such that the portion of the signal that reaches the ADC is dependent on the first resistance of the sensing resistor, and the portion of the signal that reaches the ADC can be processed to identify the presence or absence of a threshold weight on the sensing zone.

[0019] In a variant, the at least one variable matching resistor comprises two variable matching resistors in series, each of the two variable matching resistors having a corresponding second resistance independently controlled by the microcontroller unit. The MCU is configured to automatically change the second resistance of the at least one variable matching resistor by: changing a first portion of the second resistance to reach a first desired value, whereby the portion of the signal reaching the ADC has a voltage within a predetermined coarse range of the desired voltage, while keeping the second portion of the first resistance fixed; keeping the first portion of the second resistance fixed at the first desired value, and changing a second portion of the second resistance to reach a second desired value, whereby the portion of the signal reaching the ADC has a voltage within a predetermined range of the desired voltage, the predetermined range being smaller than the predetermined coarse range. In operation, the microcontroller unit is configured to set the first portion of the second resistance to the first desired value and set the second portion of the second resistance to the second desired value.

[0020] In another variation, the pressure sensitive pad further comprises a third resistor in series with the at least one variable resistor, the third matched resistor having a fixed resistance.

[0021] In yet another variation, in operation, the MCU is further configured to determine whether the calibration is valid by: (i) starting a period; (ii) at the end of the period, performing a first check to determine whether the measured voltage at the ADC during the post-calibration period was always sufficiently above a predetermined desired voltage; (iii) if the check in step (ii) is positive, increasing the first desired value and repeating all steps from step (i); (iv) if the check in step (iii) is negative, at the end of the period, performing a second check to determine whether the measured voltage at the ADC during the post-calibration period was always sufficiently close to 0V; (v) if the check in step (iv) is positive, decreasing the first desired value and repeating all steps from step (i); and (vi) if the check in step (iv) is negative, determining that the calibration was accurate.

[0022] The MCU may be further configured to repeat all steps from step (i) after performing step (vi).

[0023] Another aspect of some embodiments of the present invention relates to a method for calibrating a pressure sensing pad configured to be placed under a mattress and configured to identify the presence or absence of a threshold weight on the mattress. The method includes (i) providing a microcontroller unit (MCU) having a signal generator configured to generate an electrical signal, a sensing zone having a sensing resistor with a first resistance that varies in response to pressure applied to the sensing zone, at least one variable matching resistor in series with the sensing resistor and the signal generator and having a second resistance, and an analog-to-digital converter (ADC) configured to receive and measure a portion of the signal at an electrical junction between the sensing resistor and the at least one variable matching resistor, the MCU being configured to control the second resistance of the at least one variable matching resistor, (ii) connecting the sensing zone to an electrical ground, (iii) placing the sensing zone under the mattress without placing additional weight on the mattress, (iv) generating an electrical signal via the signal generator, (v) automatically measuring by the ADC a portion of the electrical signal that reaches the ADC, and (vi) automatically varying via the microcontroller unit the second resistance of the at least one variable matching resistor until a desired value of the second resistance is found such that the portion of the signal that reaches the ADC is within a predetermined range of a desired predetermined voltage.

[0024] In a variant, the step of providing at least one variable matching resistor comprises providing two variable matching resistors in series, each of the two variable matching resistors having a corresponding second resistance independently controlled by the microcontroller unit. The step of automatically changing the second resistance of the at least one variable matching resistor comprises changing a first portion of the second resistance to reach a first desired value, whereby the portion of the signal reaching the ADC has a voltage within a predetermined coarse range of the desired voltage while keeping the second portion of the first resistance fixed, and keeping the first portion of the second resistance fixed at the first desired value and changing a second portion of the second resistance to reach a second desired value, whereby the portion of the signal reaching the ADC has a voltage within a predetermined range of the desired voltage, the predetermined range being smaller than the predetermined coarse range.

[0025] Yet another aspect of some embodiments of the present invention relates to a method for operating a pressure sensing pad configured to be placed under a mattress and configured to identify the presence or absence of a threshold weight on the mattress, the method comprising the steps of calibrating the pressure sensing pad as described above, setting the second resistance to a desired value via a microcontroller unit, generating an electrical signal configured to travel through at least one variable matching resistor to the electrical junction, from the electrical junction to the ADC, and from the electrical junction to electrical ground via the sensing resistor such that the portion of the signal that reaches the ADC is dependent on the first resistance of the sensing resistor, and processing the portion of the signal that reaches the ADC to identify the presence or absence of a threshold weight on the sensing zone.

[0026] In a variant, the step of providing at least one variable matching resistor comprises providing two variable matching resistors in series, each of the two variable matching resistors having a corresponding second resistance independently controlled by the microcontroller unit, the step of automatically changing the second resistance of the at least one variable matching resistor comprises changing a first portion of the second resistance to reach a first desired value, whereby the portion of the signal reaching the ADC has a voltage within a predetermined coarse range of the desired voltage while keeping the second portion of the first resistance fixed, keeping the first portion of the second resistance fixed at the first desired value and changing a second portion of the second resistance to reach a second desired value, whereby the portion of the signal reaching the ADC has a voltage within a predetermined range of the desired voltage, the predetermined range being smaller than the predetermined coarse range, and the step of setting the second resistance to a desired value via the microcontroller unit comprises setting the first portion of the second resistance to the first desired value and setting the second portion of the second resistance to the second desired value.

[0027] In another variation, the method further comprises the steps of determining whether the calibration is valid by: (a) starting a period; (b) at the end of the period performing a first check to determine whether the measured value of the voltage at the ADC during the post-calibration period was always sufficiently above a predetermined desired voltage; (c) if the check of step (b) is positive, increasing the first value and repeating all steps from step (a); (d) if the check of step (c) is negative, at the end of the period performing a second check to determine whether the measured value of the voltage at the ADC during the post-calibration period was always sufficiently close to 0V; (e) if the check of step (d) is positive, decreasing the desired value and repeating all steps from step (a); and (f) if the check of step (d) is negative, determining that the calibration was accurate.

[0028] In a variation, the method further comprises, after step (f), repeating all steps from step (a). [Brief description of the drawings]

[0029] [Figure 1] FIG. 1 is a circuit diagram of an under-mattress pressure sensing pad known in the prior art. [Diagram 2] FIG. 2 is a graph of the response curves of prior art under-mattress pressure sensitive pads having various unloaded resistance values ​​as found by the inventors. [Diagram 3] FIG. 3 is a circuit diagram of an under-mattress pressure sensing pad with an adaptive voltage divider having at least one variable resistor controlled by an MCU, according to some embodiments of the present invention. [Figure 4] FIG. 4 is a graph of response curves of a pressure sensing pad under a mattress with various no-load resistance values ​​and adaptive voltage dividers, according to some embodiments of the present invention. [Diagram 5] FIG. 5 is a graph illustrating a binary search for a predetermined desired voltage in an ADC using a single variable resistor in accordance with some embodiments of the present invention. [Figure 6] FIG. 6 is a graph illustrating a binary search for a predetermined desired voltage in an ADC using two variable resistors, according to some embodiments of the present invention. [Figure 7] FIG. 7 is a flowchart of a method for calibrating and using an under-mattress pressure sensing pad with an adaptive pressure divider according to some embodiments of the present invention. [Figure 8] FIG. 8 is a flowchart of a method for adjusting inaccurate calibration of a pressure sensing pad under a mattress according to some embodiments of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The present invention is often described herein in terms of example environments. Descriptions of these environments are provided to enable various features and embodiments of the present invention to be described in the context of example applications. After reading this description, it will become apparent to one of ordinary skill in the art how the present invention can be implemented in different and alternative environments.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this invention belongs.All patents, applications, published applications and other publications mentioned herein are incorporated by reference in their entirety.If the definitions set forth in this section are contrary to or otherwise inconsistent with the definitions set forth in the applications, published applications and other publications incorporated herein by reference, the definitions set forth in this document shall prevail over the definitions incorporated herein by reference.

[0032] FIG. 3 is a circuit diagram of an under-mattress pressure sensing pad 200 forming an adaptive voltage divider with at least one variable resistor controlled by an MCU according to some embodiments of the present invention.

[0033] The pressure sensitive pad 200 comprises a signal generator 202 , a sensing zone, at least one variable matching resistor, and a microcontroller unit (MCU) 204 having an analog to digital converter (ADC) 206 .

[0034] The signal generator 202 is configured to generate an electrical signal having a predetermined voltage (Vin). The sensing zone has a sensing resistor with a first resistance R that varies in response to pressure applied to the sensing zone. More specifically, the resistance R decreases as pressure on the sensing zone increases.

[0035] The variable matching resistors are in series with the sense resistor and the signal generator 202. Each variable matching resistor is controlled by the MCU 204. In an embodiment where there is one variable resistor, the variable matching resistor has a second resistance Rwiper0. In some embodiments of the invention, there are multiple variable matching resistors. In a non-limiting example, there are two variable matching resistors with second resistances Rwiper0 and Rwiper1, respectively. In some embodiments of the invention, there is also a fixed resistor with resistance R1. In other words, the matching resistor device 203 is placed in series with the sense resistor with a first resistance RSENSE. The matching resistor device 203 comprises one or more variable matching resistors and optionally one or more fixed resistors, which are mutually controlled by the MCU.

[0036] In a non-limiting example, for a signal generated by the signal generator and having a voltage of about 3.3V, the two variable matching resistors were selected to be 100k ohm potentiometer chips to allow an adjustment range of 0 to 200k ohms in 512 steps. The fixed resistor with resistance R1 was selected to be small (1k ohm) to allow the potentiometer to have a maximum effect on the circuit. It should be noted that the number, type, and resistance value of the variable matching resistors and the fixed resistors in the matching resistor device 203, as well as the voltage of the signal generated by the signal generator 202, can be changed according to different requirements of the pads without departing from the spirit of the present invention.

[0037] The potentiometer chips were connected to the MCU via a SPI data bus. This allows the microcontroller to adjust both potentiometers to any available increment with a single data command, thereby speeding up finding the exact value. This could also be accomplished using an up / down potentiometer chip, but would require more logic to bring the potentiometers to a known good state and would require longer search times to locate the exact value.

[0038] A microcontroller unit (MCU) 204 has an analog-to-digital converter (ADC) 206 configured to receive and measure a portion of the signal at an electrical junction 208 between the sense resistor and the matching resistor arrangement 203. The MCU 204 is configured to control a second resistance of the at least one variable matching resistor.

[0039] 7, a flowchart 700 of a method for calibrating and using the under-mattress pressure sensing pad with adaptive pressure divider of FIG. 1 is shown in accordance with some embodiments of the present invention.

[0040] At 702, a signal generator, a sensing zone, an MCU with an ADC, and a matching resistor arrangement are provided as described above.

[0041] At 704, a sensing zone is positioned under the mattress.

[0042] At 706, the sensing zone is electrically connected to an electrical ground.

[0043] At 708, an electrical signal having a predetermined voltage is generated via a signal generator.

[0044] At 710, the ADC automatically measures the portion of the electrical signal that reaches the ADC.

[0045] At 712, the resistance of all of the matching resistor devices is automatically changed by the MCU until a desired value for the matching resistor devices is found such that the portion of the signal reaching the ADC is within a predetermined range of the desired predetermined voltage.

[0046] In some embodiments of the present invention, the matching resistance device comprises a variable resistor, in which the resistance of the variable matching resistor is varied to bring the voltage of the signal at the ADC to a predetermined desired voltage within a desired range.

[0047] In some embodiments of the present invention, the matching resistor device includes two variable resistors in series. One has a resistance Rwiper0 and the other has a resistance Rwiper1 (see FIG. 3). In such an embodiment, both Rwiper0 and Rwiper1 are changed independently of each other by the MCU to bring the voltage of the signal at the ADC to a predetermined desired voltage within a desired range. In some embodiments of the present invention, while keeping Rwiper1 fixed at an initial resistance value, Rwiper0 is changed until a value for Rwiper0 is found such that the voltage of the signal at the ADC is within a coarse range of the predetermined desired voltage. Then, Rwiper0 is maintained at the previously found value and Rwiper1 is fine-tuned to bring the voltage at the ADC closer to the predetermined desired voltage within a fine range of the predetermined desired voltage. The fine range is smaller than the coarse range. This allows better control of the matching voltage and the final value of Vout (the voltage measured by the ADC) closer to the desired value.

[0048] At 714, after the calibration is completed, the microcontroller unit is configured to set the second resistance of the matched resistor arrangement 203 (FIG. 3) to the value found during the calibration.

[0049] At 716, the signal generator is configured to regularly generate an electrical signal configured to travel through the matched resistor arrangement to the electrical junction, from the electrical junction to the ADC, and from the electrical junction to electrical ground through the sensing resistor, such that the portion of the signal that reaches the ADC is dependent on a first resistance of the sensing resistor.

[0050] At 718, the portion of the signal that reaches the ADC is processed to identify the presence or absence of a threshold weight on the sensing zone.

[0051] In some embodiments of the invention, at 720, the microcontroller unit is configured to check whether the calibration has been performed correctly or is valid after a period during which the sensing zone pad may have deteriorated. This check may be performed periodically or continuously during an initial period of operation of the pad. If some conditions are met that indicate that the calibration has been performed incorrectly (as will be further described below with reference to FIG. 8), the MCU is configured to change the second resistance of the matching resistor arrangement 203 (of FIG. 3) over time during use of the pad in order to make the calibration accurate.

[0052] FIG. 4 is a graph of response curves of a pressure sensing pad under a mattress with different no-load resistance values ​​and adaptive voltage dividers, according to some embodiments of the present invention.

[0053] The matching resistors in the voltage divider allow the matching resistors to be adjusted during the pad calibration stage to select the ideal value desired. This helps to mitigate the variation of the sensor material in the sensing zone by creating an adaptive circuit. This was applied to the same two pads used in Figure 2, resulting in the response curve found in Figure 4.

[0054] 4, it is easy to see that response curves 122 and 124 are nearly identical to one another. Also, curves 122 and 124 have both been pulled up closer to the ADC maximum reading line 120. This vertical shift is due, in part, to the dynamic resistance calibration described above.

[0055] The vertical shift is also assisted by lowering the MCU's reference voltage (ADC maximum voltage) to the desired level. For example, if the voltage of the signal generated by the signal generator is 3.3V, the selected reference voltage can be lowered to about 1V. Thus, the ADC maximum line is shifted downwards.

[0056] While this does not solve the problem alone, it does allow the dynamic resistance calibration of the adaptive voltage divider to have a larger operating range in order to dynamically adjust the response curve.

[0057] As an example, referring to the values ​​in Table 1, the voltage at the ADC is measured for different combinations of unloaded RSENSE and Rwiper0 (in an embodiment where there is a single matching resistor). If the goal is to achieve a Vref of 3.3V, which is the maximum voltage at the ADC when RSENSE is much higher than the matching resistor, these Vref values ​​are all concentrated in the upper left part of the table. Therefore, when RSENSE is above 30k ohms, Rwiper0 can only be changed between 1k ohms and 2k ohms in order for the response curve to rise to Vref and increase the sensitivity of the pad. A higher value of Rwiper0 cannot compensate for a lower value of unloaded RSENSE to shift the response curve to reach about 3.3V.

[0058] Instead, if the target Vref is around 1V, we can see from the table that the 1V value gives a better chance to reach the target voltage since it runs through the center of our table. Thus, even if RSENSE falls below 30k ohms (even down to 500 ohms), Rwiper0 can be increased to 100k ohms to bring the response curve up and closer to Vref, thus maintaining a higher sensitivity.

[0059] [Table 1]

[0060] 5 and 6 refer to a binary search for a predetermined desired voltage in an ADC. Fig. 5 is a graph showing a binary search for a predetermined desired voltage in an ADC using a single variable resistor, according to some embodiments of the present invention. Fig. 6 is a graph showing a binary search for a predetermined desired voltage in an ADC using two variable resistors, according to some embodiments of the present invention.

[0061] In some embodiments, one or more variable matching resistors are varied via a binary search technique to find a predetermined desired voltage at the ADC during calibration.

[0062] A binary search begins by comparing the middle element of the array to the target value. If the target value matches the element, its position in the array is returned. If the target value is less than the element, the search continues in the lower half of the array. If the target value is greater than the element, the search continues in the upper half of the array. By doing this, the binary search technique eliminates the half where the target value cannot exist at each iteration.

[0063] The above description of the binary search is applicable to searching for a desired voltage in an ADC of the present invention, except that potentiometer steps are used instead of a sorted array and the "target value" is the voltage returned by the ADC.

[0064] At the start of a calibration cycle, all variable matching resistors are set to their midpoints to establish a known good state and in preparation for the next step. The MCU then begins performing a binary search to establish a target ADC value of approximately 1V, or within some specified range of 1V (e.g., ±0.1V).

[0065] This process is performed for the first variable matching resistor to determine its desired resistance. Once this process is complete, additional variable matching resistors, if present in the circuit, are repeated to find their respective desired values.

[0066] A simpler single potentiometer implementation can be seen in Figure 5. At the start of the calibration cycle (designated by marker A1), the potentiometer is set to its midpoint (100k Ohms). A binary search is then performed while varying Rwiper0 to narrow the voltage measured by the ADC to the desired value of (0.9±0.1)V (i.e., within 0.1V of the desired value of 1V) in 8 steps. In this case, the binary search was completed in about 12ms.

[0067] In comparison, Figure 6 shows the signals from the MCU to the potentiometers during potentiometer calibration performed with dual variable matched resistors. Each signal pulse is the result of commanding each potentiometer to increase or decrease resistance according to the binary search technique described above. The single signal starts out as the sensor circuit is energized, increases sharply from 0V, reaches a positive value and plateaus at the positive value, at which point the calibration logic takes its reading for the binary search. The sensor circuit is then de-energized, and the signal decreases sharply from the positive value to 0V while the potentiometer is adjusted for the next step in the calibration process.

[0068] Again, the midpoints of both matching resistors are set, and using the timescale at the top of the figure, the Rwiper0 binary search begins at time 6.085 seconds and ends at time 6.094 seconds (a total of 9 ms). Rwiper0 is then kept at the value found in the first binary search, after which the Rwiper1 binary search begins at time 6.095 seconds and completes at time 6.105 seconds (another total of 10 ms).

[0069] At the end of this double potentiometer calibration value search, the voltage at the ADC voltage is 0.9947V within a 20ms period, just below the 1V target. In comparison to a single potentiometer search, it can be seen that implementing this system as a dual potentiometer configuration results in a more accurate final voltage, at the expense of a longer calibration time. This time is still so small that it cannot be observed by the end user, so the tradeoff is very good. In addition, the inclusion of a second potentiometer allows the circuit to tune to a much wider range of pressure response curves, accommodating a greater tolerance for variations created during the pad manufacturing process.

[0070] In preliminary tests performed by the inventors, incremental weights were placed directly on the pressure sensitive ink. When these tests were performed without the variable matching resistor of the present invention and without setting the ADC voltage to the desired target, the minimum weight that had to be applied before a measurable difference in the pad response was observed was 100 grams.

[0071] Running this exact test again using the new dual potentiometer circuit with a 1V target, we found that the minimum weight that had to be applied before seeing a measurable difference in the pad's response was now 1 gram.

[0072] This test measured the weight response of the pad material and the pad circuitry. This test demonstrated that the circuitry of the present invention has much better fidelity than previously used pad circuits.

[0073] FIG. 8 is a flowchart of a method for adjusting inaccurate calibration of a pressure sensing pad under a mattress according to some embodiments of the present invention.

[0074] The pad is intended to be calibrated before use with only the mattress over the sensing zone, however, field testing by the inventors has concluded that the pad may not be accurately calibrated before use or that users may forget to calibrate the pad before use.

[0075] For example, if staff calibrate the pad with a patient on the mattress prior to use, during use the pad may detect a very heavy mattress and miss the patient getting out of bed, whereas if staff calibrate the pad without a mattress prior to use, during use the weight of the mattress may be detected by the pad as the weight of the person against the pad and the pad may detect the patient getting out of bed even when the patient is on the bed.

[0076] In the new potentiometer-based calibration system, a new check allows for post-calibration determination of whether the initial calibration was accurate.

[0077] In the pad of the present invention, a mattress without a patient should generate a signal in the pad of a given voltage (e.g., 1V) at the ADC, while a mattress with a patient present should generate a signal in the pad that is close to (but not 0V) at the ADC.

[0078] This means that if readings above 1V are consistently measured at the ADC over a period of time, excess weight has been removed from the pad, possibly indicating a pad that has been mis-calibrated by someone being on the mattress during calibration.

[0079] On the other hand, if readings of approximately 0V are consistently measured at the ADC over a period of time, then too much weight has been applied to the pad, possibly indicating a pad that has been incorrectly calibrated by not placing the mattress over the sensing zone during calibration.

[0080] Since both of the above scenarios (voltages of about 1V or 0V) can legitimately occur for short periods of time, it is not desirable to adjust the variable matching resistor too much or too quickly after detecting a potential miscalibration as described above. For example, if the mattress is lifted when the sheets are changed, the voltage at the ADC may briefly exceed about 1V. Or, if there is weight on the mattress, such as another person sitting on the bed in addition to the patient, the voltage at the ADC may be 0.

[0081] To address this, in some embodiments of the present invention, after calibration is complete, if the above scenario occurs, the voltage of the variable matching resistor device is changed by one step every few minutes to change the voltage of the ADC and bring the voltage at the ADC into the desired range. During testing by the inventors, this has resulted in miscalibrated pads being fixed typically within 30 minutes after pressure has been applied or removed from the pads (based on the type of miscalibration that occurred).

[0082] Therefore, in 800, after calibration, the voltage at the ADC is measured several times over a period of time. The period of time may be several minutes, for example, 2 minutes, 3 minutes, 5 minutes, or 6 minutes. Note that any length of time period is within the scope of the present invention.

[0083] At 802, a check is made to determine whether the voltage measurement at the ADC during the period is sustainably sufficiently above a predetermined desired voltage (e.g., above 1V and above 0.1V). "Sustainably" may be defined as any selected percentage of the measurement selected a priori, such as 40% or more, 50% or more, etc. If so, the resistance of the matched resistor device is increased at 804 and a new period begins.

[0084] If the voltage measurement at the ADC during the time period is not sustainably sufficiently above the predetermined desired value at 802, then a new check is made at 806 to determine whether the voltage measurement at the ADC is sustainably close to 0V (e.g., less than 0.1V) during the time period. "Sustainably" may be defined as any selected percentage of the measurement selected a priori, such as 40% or more, 50% or more, etc. If so, the resistance of the matched resistor device is decreased at 808 and a new time period begins.

[0085] If the voltage measurement at the ADC during the period at 806 was not consistently close enough to 0V, then the calibration is determined to be accurate at 810 .

[0086] It should be noted that the use of sub-method 720 may not be limited to only periods following calibration or when user activity is detected. Indeed, over time, the variable resistance material of the pad's sensing zone will degrade and become less sensitive to pressure, which will result in measurements indicating the absence of a person from the mattress even when a person is on the mattress. Thus, sub-method 720 may be used periodically or continuously to assess the validity of the initial calibration and to correct for degradation of the material of the pad's sensing zone. The dotted arrow leading from step 810 back to step 800 illustrates an embodiment in which sub-method 720 is performed periodically or continuously, even after the calibration has been deemed accurate in a previous cycle.

[0087] While the present invention is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects, time measurements, and functionality described in one or more of the individual embodiments are not limited in applicability to the particular embodiment for which they are described, but instead may be applied alone or in various combinations to one or more of the other embodiments of the invention, regardless of whether such an embodiment is described and whether such features are presented as being part of the described embodiment. Thus, the breadth and scope of the present invention should not be limited by any of the exemplary embodiments described above.

Claims

1. A pressure sensitive pad configured to be placed under a mattress and configured to identify the presence or absence of a threshold weight on said mattress, a signal generator configured to generate an electrical signal; a sensing zone having a sensing resistor with a first resistance that varies in response to pressure applied to the sensing zone; at least one variable matching resistor in series with the sense resistor and the signal generator, the variable matching resistor having a second resistance; a microcontroller unit (MCU) having an analog-to-digital converter (ADC) configured to receive and measure a portion of the electrical signal at an electrical junction between the sense resistor and the at least one variable matching resistor, the MCU being configured to control the second resistance of the at least one variable matching resistor; the sensing zone is configured to be connected to an electrical ground; The pressure sensitive pad is Positioning the sensing zone beneath the mattress without adding additional weight to the mattress; generating an electrical signal via the signal generator; automatically measuring, by the ADC, a portion of the electrical signal that reaches the ADC; automatically varying, via the microcontroller unit, the second resistance of the at least one variable matching resistor until a desired value of the second resistance is found such that a portion of the electrical signal reaching the ADC is within a predetermined range of a desired predetermined voltage. and configured to be calibrated for any mattress by In post-calibration operation, the microcontroller unit is configured to set the second resistor to the desired value; a signal generator configured to periodically generate an electrical signal configured to travel through the at least one variable matching resistor to the electrical junction, from the electrical junction to the ADC, and from the electrical junction to the electrical ground through the sensing resistor, such that a portion of the electrical signal that reaches the ADC is dependent on the first resistance of the sensing resistor, and such that a portion of the electrical signal that reaches the ADC can be processed to identify the presence or absence of a threshold weight on the sensing zone.

2. the at least one variable matching resistor comprises two variable matching resistors in series, each of the two variable matching resistors having a corresponding second resistance independently controlled by the microcontroller unit; The MCU adjusts the second resistance of the at least one variable matching resistor to: modifying a first portion of the second resistor to reach a first desired value, whereby a portion of the electrical signal reaching the ADC has a voltage within a predetermined coarse range of a desired voltage, while keeping a second portion of the first resistor fixed; maintaining a first portion of the second resistor fixed at the first desired value and varying a second portion of the second resistor to reach a second desired value, whereby a portion of the electrical signal reaching the ADC has a voltage within a predetermined range of the desired voltage, the predetermined range being less than the predetermined coarse range. configured to change automatically, 2. The pressure-sensitive pad of claim 1, wherein in operation, the microcontroller unit is configured to set a first portion of the second resistance to the first desired value and to set a second portion of the second resistance to the second desired value.

3. 2. The pressure sensitive pad of claim 1, further comprising a third matched resistor in series with the at least one variable matched resistor, the third matched resistor having a fixed resistance.

4. In operation, the MCU determines whether a calibration is valid by: (i) commencing the period; (ii) performing a first check at the end of said period to determine whether the measurement of the voltage at said ADC during a post-calibration period was consistently sufficiently above a predetermined desired voltage; (iii) if the check in step (ii) is positive, increasing the first desired value and repeating the entire process from step (i); (iv) if the check of step (iii) is negative, performing a second check at the end of said period to determine whether measurements of the voltage at said ADC during a post-calibration period were always sufficiently close to 0 V; (v) if the check of step (iv) is positive, decreasing said first desired value and repeating all steps from step (i); (vi) if the check in step (iv) is negative, determining that the calibration was accurate. The pressure sensitive pad of claim 1 , further configured to determine by:

5. The pressure-sensitive pad of claim 4 , wherein the MCU is configured to repeat all steps from step (i) onwards after performing step (vi).

6. 1. A method for calibrating a pressure sensing pad configured to be placed under a mattress and configured to identify the presence or absence of a threshold weight on the mattress, comprising: (i) a signal generator configured to generate an electrical signal; a sensing zone having a sensing resistor with a first resistance that varies in response to pressure applied to the sensing zone; at least one variable matching resistor in series with the sense resistor and the signal generator, the variable matching resistor having a second resistance; a microcontroller unit (MCU) having an analog-to-digital converter (ADC) configured to receive and measure a portion of the electrical signal at an electrical junction between the sense resistor and the at least one variable matching resistor, the MCU being configured to control the second resistance of the at least one variable matching resistor. Providing a step of (ii) connecting the sensing zone to an electrical ground; (iii) positioning the sensing zone beneath the mattress without adding any additional weight to the mattress; (iv) generating an electrical signal via the signal generator; (v) automatically measuring, by the ADC, the portion of the electrical signal that reaches the ADC; (vi) automatically varying, via the microcontroller unit, the second resistance of the at least one variable matching resistor until a desired value of the second resistance is found such that the portion of the electrical signal reaching the ADC is within a predetermined range of a desired predetermined voltage. The method includes:

7. providing at least one variable matching resistor includes providing two variable matching resistors in series, each of the two variable matching resistors having a corresponding second resistance independently controlled by the microcontroller unit; The step of automatically varying the second resistance of the at least one variable matching resistor comprises: modifying a first portion of the second resistor to reach a first desired value, whereby a portion of the electrical signal reaching the ADC has a voltage within a predetermined coarse range of the desired voltage, while keeping a second portion of the first resistor fixed; maintaining a first portion of the second resistor fixed at the first desired value and varying a second portion of the second resistor to reach a second desired value, whereby a portion of the electrical signal reaching the ADC has a voltage within a predetermined range of the desired voltage, the predetermined range being less than the predetermined coarse range. The method of claim 6, comprising:

8. 1. A method for operationally calibrating a pressure sensing pad configured to be placed under a mattress and configured to identify the presence or absence of a threshold weight on the mattress, comprising:

7. Calibrating the pressure sensitive pad according to the method of claim 6, setting, via the microcontroller unit, the second resistor to a desired value; generating an electrical signal configured to travel through the at least one variable matching resistor to the electrical junction, from the electrical junction to the ADC, and from the electrical junction to the electrical ground through the sense resistor, such that a portion of the electrical signal that reaches the ADC is dependent on the first resistance of the sense resistor; processing the portion of the electrical signal that reaches the ADC to identify the presence or absence of a threshold weight on the sensing zone; The method includes:

9. providing at least one variable matching resistor includes providing two variable matching resistors in series, each of the two variable matching resistors having a corresponding second resistance independently controlled by the microcontroller unit; The step of automatically varying the second resistance of the at least one variable matching resistor comprises: modifying a first portion of the second resistor to reach a first desired value, whereby a portion of the electrical signal reaching the ADC has a voltage within a predetermined coarse range of the desired voltage, while keeping a second portion of the first resistor fixed; maintaining a first portion of the second resistor fixed at the first desired value and varying a second portion of the second resistor to reach a second desired value, whereby a portion of the electrical signal reaching the ADC has a voltage within a predetermined range of the desired voltage, the predetermined range being less than the predetermined coarse range; 9. The method of claim 8, wherein setting, via the microcontroller unit, the second resistor to a desired value comprises setting a first portion of the second resistor to the first desired value and setting a second portion of the second resistor to the second desired value.

10. Check whether the calibration is valid (a) starting a period; (b) performing a first check at the end of the period to determine whether the measurement of the voltage at the ADC during the post-calibration period was consistently sufficiently above a predetermined desired voltage; (c) if the check of step (b) is positive, incrementing the first value and repeating the entire process from step (a); (d) if the check of step (c) is negative, performing a second check at the end of said period to determine whether the measurement of the voltage at said ADC during the post-calibration period was always sufficiently close to 0 V; (e) if the check of step (d) is affirmative, decreasing said desired value and repeating all steps from step (a); (f) if the check in step (d) is negative, determining that the calibration was accurate. The method of claim 8, further comprising determining by:

11. The method of claim 10, further comprising the step of repeating all steps from step (a) onwards after step (f).

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