Customizable pressure sensor array

A pressure sensor array can be used to record a pressure distribution in gait analysis and/or tactile sensing applications. The pressure sensor array can include a piezo-resistive material and a uniform distribution of a plurality of flexible circuits. Each of the plurality of flexible circuits comprise at least one wire connecting an internal portion of a respective flexible circuit to a common port. A device housing the pressure sensor array can be customized to a size and used for a gait analysis and/or tactile sensing application. The arrangement of the wiring permits partial sensors to be used as part of the pressure sensor array during the gait analysis and/or tactile sensing application.

TECHNICAL FIELD

This disclosure generally relates to pressure sensing and, more specifically, to a pressure sensor array that can be customized to different sizes without sacrificing pressure sensing ability.

BACKGROUND

Slip and falls are a leading cause of preventable workplace accidents. These preventable workplace accidents may be reduced with a better understanding of the biomechanics of the human gait under workplace conditions that can be learned by gait analysis. Camera-based assessment systems are commonly used for gait analysis, but workplace environments have poor lighting and multiple visual barriers making camera-based assessment systems impractical. An alternate solution involves continuous monitoring of pressure using an array of pressure sensors (or “pressure sensor array”, a group of pressure sensors, which can be deployed in a certain geometry pattern and used for recording pressure signals) placed in a shoe insole. While such pressure sensor arrays can be used for continuous monitoring of gait information in most workplace conditions, these pressure sensor arrays do not allow for customization of the insole size, since trimming the insole destroys the pressure sensor arrays. For pressure sensor arrays to be used in shoes for gait analysis, insoles must be made to fit different foot sizes, which increases the cost of manufacturing, making the use of pressure sensor arrays cost prohibitive in monitoring gait analysis in workplace slip and fall conditions.

SUMMARY

In an aspect, a system that includes a device housing a pressure sensor array is described. The pressure sensor array includes a piezo-resistive material and a uniform distribution of a plurality of flexible circuits. Each of the plurality of flexible circuits includes at least one wire connected to a common port. The device and the pressure sensor array are customizable to different sizes for gait analysis and/or tactile sensing applications. Each of the at least one wire from each of the plurality of flexible circuits is positioned to ensure that each of the plurality of flexible circuits is connected to the common port even when customized so that every one of the plurality of flexible circuits remaining on the sensor array as full flexible circuits and partial flexible circuits is usable after the pressure sensor array is customized.

In another aspect, a method that uses the device housing the pressure sensor array is described. The device housing the pressure sensor array can be customized to a size. The pressure sensor array includes a piezo-resistive material and a uniform distribution of a plurality of flexible circuits, and each of the plurality of flexible circuits includes at least one wire connecting an internal portion of a respective flexible circuit to a common port. The customized pressure sensor array can be used for a gait analysis and/or tactile sensing application.

DETAILED DESCRIPTION

This disclosure describes a customizable pressure sensor array that can be customized to different sizes without sacrificing pressure sensing ability (for example, a “trimmable pressure sensor array” that can be altered, cut, trimmed, or the like, to different sizes). Such trimmable pressure sensor arrays that can be cut into different sizes without losing sensing resolution are particularly well suited for gait analysis and/or other tactile sensing applications. In an example gait analysis application, a shoe insole (or insert, bottom, or the like) can be used with a pressure sensor array in a certain geometry within the shoe insole. Such pressure sensor arrays can be used for continuous monitoring of gait information in most workplace conditions; however, traditional pressure sensor arrays do not allow for customization of the insole size, since trimming the insole destroys the pressure sensor arrays. The customizable pressure sensor arrays of the present disclosure can be fit inside customizable insoles (or insert, bottom, or the like—any part of the shoe that can be customized to different sizes), which can be customized for different foot sizes.

Making a single-sized insole housing a customizable pressure sensor array can decrease the cost of manufacturing, making the use of pressure sensor arrays no longer cost prohibitive in performing gait analysis in workplace slip and fall conditions. The customizable pressure sensor array is not limited to applications in customizable shoe insoles. For example, the customizable pressure sensor array can be housed within any type of body molding housing that can mold to any portion of a user's body and used in a tactile sensing application. The term “user” is synonymous with the terms “subject” and “patient” and refers to a bipedal animal, like a human.

FIG.1illustrates an example of a system100that includes a customizable pressure sensor array102enclosed in a customizable housing101. The customizable housing101can be made of any material that can hold the pressure sensor array102therein. For example, the customizable housing101may be any type of body molding material that can mold to any portion of a user's body, but still be customized to size (e.g., a fabric, a polymer, and/or the like). As another example, the customizable housing101can be a shoe insole (or insert, bottom, or the like—any part of the shoe that can be customized to different sizes). The customizable pressure sensor array102can include a pressure sensitive material (like a piezo-resistive material202or any other material that has a variable resistance that varies proportionally with an applied pressure) and one or more circuit layers (circuit layer A201aand/or circuit layer B201b, as illustrated). The materials used to construct the pressure sensitive material and the corresponding circuits are easily trimmed to ensure that the pressure sensor array102can be easily customized for different sizes to which the housing101is trimmed. The pressure sensitive material can be consistent such that a pressure sensitivity of any point on the material is similar to other points on the material. The corresponding circuits are designated to make sure that after being customized into different sizes, the sensor array can maintain its original resolution (the resolution of the pressure sensor array refers to the number of pressure sensors in a unit area of the pressure sensor array).

In the example shown inFIG.2, the pressure sensitive material is represented as piezo-resistive material202. The one or more circuit layers are represented as circuit layer A201aand circuit layer B201b, which sandwich the piezo-resistive layer202. However, the piezo-resistive layer202may be contacted by only one of circuit layer A201aand circuit layer B201b. For example, circuit layer A201amay be sandwiched by the piezo-resistive layer202and circuit layer B201b. In another example, circuit layer B201bcan be sandwiched by circuit layer A201aand the piezo-resistive layer. In still another example, circuit layer A201aand circuit layer B201bcan be next to one another and each contacting the piezo-resistive material202. Circuit layer A201aand/or circuit layer B201bcan be placed in numerous other arrangements with the piezo-resistive layer202. One of the circuit layers (e.g., circuit layer A201a) can connect the piezo-resistive material202to a source voltage via a fixed resistance, while the other of the circuit layers (e.g., circuit layer B201b) can connect the piezo-resistive material202to a ground electronic level.

Each of the one or more circuit layers (e.g., circuit layer A201aand/or circuit layer B) can include a uniform distribution of a plurality of flexible circuits. The one or more circuit layers (e.g., circuit layer A201aand/or circuit layer B) can be individually flexible to create a plurality of flexible circuits that are wired to a common port. As an example, the one or more circuit layers can include one or more electrical components deposited onto flexible substrates (e.g., as a flexible printed circuit board or PCB). The one or more electrical components can include pads (e.g., copper pads) and wires. The wires can connect the pads to a common port.

FIG.3is an example of circuit layer A201aand/or circuit layer B201b, which each include an equal number of uniformly distributed pads301a-1. Circuit layer A201aand/or circuit layer B201B can include a plurality of pads (representing electrical components)301a-1. Twelve pads are shown inFIG.3, but this number is not limiting and is instead only exemplary. Each of the pads301a-1is connected to at least one wire302a-1(each of302a-1is shown as a single wire, but may each represent a plurality of wires). The wires302a-1can be positioned relative to the respective flexible circuit (or pad)301a-1to ensure that each of the plurality of flexible circuits301a-1is connected to the common port, even when customized so that every one of the plurality of plurality of flexible circuits301a-1remaining on the customized pressure sensor array as full flexible circuits and partial flexible circuits is still usable after the pressure sensor array is customized Notably, the pressure sensor array retains its original pressure sensing resolution after being customized to any number of different sizes due to the uniform distribution of flexible circuits on the one or more circuit layers (e.g., circuit layer A201aand/or circuit layer B). As an example, each wire302a-1can be connected to an internal portion of the respective flexible circuit (internal meaning away from or opposite to an edge of the housing101). The wires302a-1connect the pads301a-1to the common port303and transmit data from the pads301a-1to the common port303, which can connect to signal processing circuitry (shown, for example, inFIG.8).

The signal processing circuitry can receive signals from each of the wires302a-1and from each of circuit layer A201aand circuit layer B201band determine the resistance of the piezo-resistive layer between portions of circuit layer A201aand circuit layer B. Based on the resistance, the signal processing circuitry can determine the pressure experienced by the different portions of the piezo-resistive layer202. The signal processing circuit can receive many inputs from many individual circuits.FIG.4shows a single circuit400(a circuit in this sense is a portion of circuit layer A401a(or a pad), a portion of circuit layer B401b(or a pad opposed to the pad in circuit layer A401a), and a portion of the piezo-resistive layer402. The piezo-resistive material within the piezo-resistive layer402can be modeled as a variable resistance (Rs403) that can vary in a manner proportional (or otherwise related) to the pressure applied to the piezo-resistive material.

A portion of circuit layer B401bcan connect the portion of the piezo-resistive material402to the ground electronic level. A portion of circuit layer A401acan connect the portion of the piezo-resistive material402to a source voltage (Vcc406) via a fixed resistor (Rf407). A voltage divider circuit (represented by Vcc406, Rf407, and MUX404, but may include additional components) and an analog to digital convertor (ADC405) can be used to measure the voltage drop on the pressure sensor. The resistance of the individual pressure sensor can be measured with the following Equation:

Rsensor=Vsensor⁢⁢RfixedVcc-Vsensor,
where Rsensor is the resistance of the portion of piezo-resistive material402(represented as Rs403inFIG.4), Rfixed (represented as Rf407inFIG.4) is the resistance of the fixed resistor that is used to build the voltage divider circuit, Vsensor is the voltage drop on the pressure sensor, which could be measured by ADC405, and Vcc406is the source voltage. By controlling the MUX404, all pressure sensors in the array can be scanned and a pressure map can be acquired.

In view of the foregoing structural and functional features described above, example methods that can be performed by the system100will be better appreciated with reference toFIGS.5-6. While, for the purposes of simplicity of explanation, the example methods ofFIGS.5-6are shown and described as executing serially, the present examples are not limited by the illustrated order, as some actions could in other examples occur in different orders or concurrently from that shown and described herein. Moreover, it is not necessary that all described actions be performed to implement a method. One or more aspects of the methods can be stored in one or more non-transitory computer-readable media and executed by one or more processing resources, such as described herein.

FIG.5illustrates a method500for using a customizable pressure sensor array in a customizable device housing (for gait analysis or tactile sensing). At502, a customizable device housing (e.g., housing101) a customizable pressure sensor array (e.g., pressure sensor array102) can be scaled (e.g., by trimming or any other type of scaling mechanism) to a size. For example, the size can be configured for a particular user. In other words, users of different sizes can each scale the device housing to the appropriate size for individual use of the device housing. At504, the scaled pressure sensor array can be used by the particular user for a gait analysis application and/or a tactile sensing application.

FIG.6illustrates a method600for determining a pressure for each portion of a customizable pressure sensor array (e.g., a portion is shown inFIG.4). At602, a common port303can be connected to at least one multiplexer404for signal processing. At604, a resistance (Rs403) can be calculated for each portion of the pressure sensor array (as described with respect toFIG.4). At606, a pressure can be determined for each portion of the pressure sensor array based on the resistance of each portion of the pressure sensor array. The pressure can be displayed in a pressure map, for example.

Experimental

The following experiment shows the design and use of a customizable pressure sensor array (also referred to as a “customizable pressure sensor array”) that meets the demand of being trimmed to different sizes while maintaining a sensing resolution so that accuracy is not affected by the trimming. The customizable pressure sensor array includes a piezo-resistive fabric and a plurality of flexible circuits, which are uniformly distributed. Wires on the flexible circuits were designed to ensure that all of the full/partial sensors remaining on the pressure sensor array after trimming could still be used for sensing.

Customizable Pressure Sensor Array Design

A pressure sensor array includes a plurality of pressure sensors made of a pressure sensor material and corresponding circuits with wires for transmitting data. To be customizable into different sizes according to the requirement of different users, the following requirements should be met:(1) The pressure sensitive material and corresponding circuits should be made of materials that are easily trimmed to ensure that the pressure sensor array can be trimmed into different sizes.(2) The pressure sensitive material should be consistent, so that the pressure sensitivity of any point on the pressure sensitive material is similar.(3) The corresponding circuits should be designed to ensure that after being cut into different sizes, the pressure sensor array could maintain its original resolution.
Pressure Sensitive Material

A commercially available piezo-resistive fabric material made by EeonTex™ was used for pressure sensitive material. Similar to normal fabric materials, the piezo-resistive fabric material is thin (with a thickness of 0.8 mm), light weight (with a weight of 170 g/m2), flexible, and easily trimmed.

Corresponding Circuit Design

The circuit material of the pressure senor array is a flexible Printed Circuit Board (PCB). The flexible PCB includes a flexible, lightweight polymer (like polyimide) as the laminate material. The flexible PCB is also very thin (around 0.15 mm), so the size of the PCB is easily customizable. The customizable pressure sensor array uses the flexible PCB on the top and the bottom of the pressure sensitive material. Copper pads and wires are distributed on the flexible PCB meeting the following requirements.

(1) The copper pads should be distributed uniformly. Since the location, shape and size of a pressure sensor are determined by the corresponding copper pads on the top and bottom layers of the sensor array, distributing the copper pads uniformly could make sure that the pressure sensors on the sensor array are uniformly distributed.

(2) The wires should be designed to ensure that all the full/partial sensors remaining on the sensor array are connected to a common connection after the sensor array is trimmed into different sizes.

Pressure Sensor Array

The pressure sensor array was built with a three-layer design. The top and bottom layer each includes a plurality of corresponding circuits distributed uniformly (example of a top layer top layer shown inFIG.7). For different applications, there might be different requirements on sensor array resolution and sensor shape and size, which could be met by configuring the distribution density, shape and size of the copper pads on the top and bottom layers. The middle layer is made of piezo-resistive material, the resistance of which is not stable but related to the applied pressure and can be modeled as a variable resistor. Therefore, through measuring the resistance of the piezo-resistive material, the force applied on it could be estimated. An example of the mechanism of measuring the resistance of the piezo-resistive material in a single pressure sensor is shown in the schematic diagram ofFIG.4. The copper pads on the bottom layer connected the piezo-resistive material to the ground electronic level, and the copper pads on the top lay connected the piezo-resistive material to the source voltage (Vcc) via a fixed resistor.

A resistance of the pressure sensor could be measured by the following Equation:

Rsensor=Vsensor⁢⁢RfixedVcc-Vsensor,
where Rsensoris the resistance of the piezo-resistive material covered by the pair of copper pads on the top and bottom layer; Rfixedis the resistance of the fixed resistor which is used to build a voltage divider circuit, Vsensoris the voltage dropped on the pressure sensor; and Vccis the source voltage of the voltage divider circuit.

In this Example (shown schematically inFIG.7), thirty-six (36) pressure sensors were distributed uniformly on the square sensor array. To customize the sensor array into different sizes, the user only needs to trim the sensor array. In addition, the placement of the wires with respect to the corresponding circuits can make sure that all the sensors left on the array could be connected with a common port (e.g., a FPC port), no matter to what size the pressure sensor is customized. To make the wire design more efficient, top and bottom layers of the sensor array both used two layers: one layer for distributing the copper pads, and the other layer is for routing the wires.

Signal Processing Circuit Design

A signal processing circuit (for example, the FPC connector, multiplexers, voltage divider circuit, ADC, MCU, power management unit, and data transmission unit as shown inFIG.8) measures the resistance of individual sensors within the sensor array and thereby estimate the force applied to the individual sensors. The common connector (a FPC connector) was used to electrically connect the pressure sensor array to the signal processing circuit. Multiplexers were used to connect all the pressure sensors to the voltage divider circuit, and then the voltage drop on each pressure sensor could be digitalized by an analog-to-digital convertor (ADC). For a sensor array, multiplexers are needed to connect all the pressure sensors on the sensor array to ADC. By controlling the multiplexer, all pressure sensors on the sensor array could be scanned one by one and then a pressure distribution map could be acquired. The measured data was sent out through the data transmission unit. The power management unit was used to supply suitable power to the components on circuit. The microcontroller unit (MCU) was used to control all the processes of the circuit.

Experimental Methods

Testing the Consistency of the Piezo-Resistive Material

A sheet of piezo-resistive material with a dimension of 30.5×33 cm was used. As shown inFIG.9, nine points on the margin and center areas of the sheet of piezo-resistive material were randomly selected for testing. A 130 kPa pressure was applied on each test point one by one. For each of the nine points, the resistance and the variation (the resistance difference of the test point with respect to the mean resistance of all nine points) were determined.

Testing the Influence of Trimming on the Customizable Pressure Sensor Array

The experiments test the influence of trimming on pressure spatial and temporal distribution patterns acquired by the customizable pressure sensor array. An insole-shaped customizable pressure sensor array was used to test the influence of trimming on the acquired pressure spatial and temporal distribution patterns (shown inFIG.10). To make the customizable pressure sensor array fit different foot sizes, a user need only to trim the sensor array along the white line corresponding to a respective foot size (shown inFIG.11). However, when cutting on the white lines, parts of several sensors on the edge of the customizable pressure sensor array would be cut off. However, wires on the customizable pressure sensor array connect all the full and partial sensors in the trimmed customizable pressure sensor array to the PFC port, ensuring the resolution of the customizable pressure sensor array is unchanged.

FIGS.12and13show details of the front and back of the signal processing circuit. The circuit board was designed in a small size (34×44×6 mm), which made it easy to be packaged in a normal insole.FIG.14shows a prototype of the customizable plantar pressure measurement system. In this prototype, all the circuits (i.e. signal processing circuit and customizable pressure sensor array) were packed into a normal insole.

Two gait parameters, gait cycle and cadence, were used to evaluate the customizable pressure sensor array. Experiments were designed to measure gait cycle and cadence respectively.

Gait cycle is defined as the time interval between two successive occurrences of one of the repetitive eight phases of walking (shown inFIG.15(A)-(H)). The gait phases could be detected based on plantar pressure distribution patterns. To distinguish the eight phases from plantar pressure distribution alone, the customizable pressure sensor array was designed with high resolution and sensitivity for the pressure change under foot.

Cadence is defined as the number of steps taken in a given time (e.g., steps per minute) and can be calculated with step cycle with the following Equation:

Cadence⁢⁢(stepsmin)=60⁢/⁢StepCycle⁡(s),
where StepCycle(s) is the duration time between the appearance of the same gait phase on both feet. For instance, time duration of the appearance of “initial contact” on the left foot and the next “initial contact” gait phase on the right foot is one step cycle. Since, trimming mainly influenced the sensors on the forefoot area, the cadence calculated with the “pre-swing” phase of each foot was used to evaluate the customizable pressure sensor array.

Two subjects with normal gait, foot size of 10.5 US and foot size 8.5 US, were involved in the experiment. The subject with a foot size of 10.5 US used the full size pressure sensor array. The other subject with a foot size of 8.5 US used a smaller pressure sensor array trimmed from the full size pressure sensor array. During the experiments, each subject wore the corresponding pair of insoles for plantar pressure recording.

For the experiment about gait cycle, each subject was asked to walk normally for five steps. During the experiment, one camera was used to tape the walking activities, from which eight gait phases of one gait cycle would be extracted. The other camera was used to record both the activity of the subject and the timestamp of the real-time plantar pressure data, which was used to realize the time synchronization between the taped activity video and plantar pressure data. After time synchronization, the plantar pressure distribution map of each posture in the taped video could be localized. In the experiment about cadence, each subject was asked to walk in three different cadences: 50, 60 and 70 beats per minute (BPM), respectively. A beep sound was used in the experiment to help the subject walk in the correct cadence. Before each experiment, the subject would walk with the beep sound for three minutes to be adaptive to that walking cadence. During the experiment, the subject would walk for 30 seconds with the beep sound. A rest time of 5 minutes was scheduled between experiments.

Experimental Results

Testing the Consistency of the Piezo-Resistive Material

The resistance (Ω) and variation (%) values for each of the nine test points (shown inFIG.9) are shown in TABLE 1 below.

As shown in TABLE 1, there were some differences in the pressure sensitivity between the nine test points. The largest resistance difference of these nine test points was 8.0%. Although the consistency of the material was not perfect, the material is sufficient for applications focusing on pressure distribution patterns, rather than accurate pressure values.

Testing the Influence of Trimming on the Customizable Pressure Sensor Array

FIG.15shows eight gait phases ((a)-(h)) of each subject and the corresponding plantar pressure distribution maps. For gait cycle, all of the eight gait phases were found in the tapped videos according to the standard postures shown inFIG.15. Then the plantar pressure distribution map corresponding to each gait phase was extracted. To evaluate the plantar pressure distribution map acquired by the customizable sensor array, a standard plantar pressure distribution map of each gait phase is necessary. For normal walking, the standard plantar pressure could be acquired by analyzing the posture in each gait phase. Taking the “Initial contact” phase for example, heel of the shaded foot inFIG.15starts to contact the ground. At this gait phase, plantar pressure of the shaded foot should concentrate on the heel area. On the contrary, plantar pressure of the contralateral foot should concentrate on the forefoot area. Finally, through comparing the measured plantar pressure distribution map with the standard plantar pressure distribution map, the performance of the customizable pressure sensor array could be evaluated.

Through comparing the standard plantar pressure distribution map with the pressure map acquired by the customizable pressure sensor array, it is obvious that the pressure distribution maps acquired with trimmed and non-trimmed sensor array are both correct for different gait phases. Taking the “Pre-swing” phase for example, only toes of the shaded foot touch the ground, and forefoot and heel of the contralateral foot starts to contact the ground. At this gait phase, there would be a little pressure on the toe area of the shaded foot. While on the contralateral foot, the pressure would be distributed on both forefoot and heel, but more pressure on heel. This is the same as the pressure distribution map acquired with both customizable sensor arrays: there is a little pressure on the right forefoot and a little pressure on the left forefoot, and more pressure is concentrated on the left heel. In addition, it is obvious that the trimmed sensors (in the toe area) are helpful to reveal the pressure distribution pattern.

For the experiment about cadence, the step cycle was calculated as the time difference between the “pre-swing” phase of one foot and the successive “pre-swing” phase of the other foot. Since the subject might need time to walk in a stable cadence, 10 successive steps in the middle (from 10 s to 20 s) of each experiment were used to calculate the mean step cycle and then cadence.

When the cadence was 50 BPM, the cadence was calculated to be 50.3 for the 8.5 US size and 49.1 for the 10.5 US size. When the cadence was 60 BPM, the cadence was calculated to be 59.0 for the 8.5 US size and 59.8 for the 10.5 US size. When the cadence was 70 BPM, the cadence was calculated to be 70.5 for the 8.5 US size and 69.6 for the 10.5 US size. The results show that both insole sizes could be used to extract cadence with high accuracy. The errors might be caused by the variance of walking cadence when the subjects were doing the experiment. The results indicate that trimming has no influence on the pressure spatial and temporal distribution patterns acquired by customizable pressure sensor array.

From the above description, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications are within the skill of one in the art and are intended to be covered by the appended claims.