comparison pressure measurement options

7/30/2019 Comparison Pressure Measurement Options

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Comparison of Interface Pressure Measurement Options 2

Table of Contents

COMPARISON OF INTERFACE PRESSURE MEASUREMENT OPTIONS ................................................................................. 3

SENSING PRESSURE ................................................................................................................................................................... 3

INTERFACE PRESSURE SENSING TECHNOLOGIES ............................................................................................................................... 3

Load Cell .......................................................................................................................................................................... 4

Pressure Indicating Film .................................................................................................................................................. 5

Tactile Pressure Sensor ................................................................................................................................................... 6

Tactile Force Sensor ........................................................................................................................................................ 8

COMPARISON OF MEASUREMENTS FROM DIFFERENT PRESSURE SENSING TECHNOLOGIES....................................................................... 9

Load Cell .......................................................................................................................................................................... 9

Pressure Indicating Film ................................................................................................................................................ 10

Tactile Pressure Sensor ................................................................................................................................................. 11

Data from Three Technologies ...................................................................................................................................... 13

PRESSURE SENSING APPLICATIONS ............................................................................................................................................. 13

Fuel Cell Stack and Battery Plates ................................................................................................................................. 13

Tire Tread ...................................................................................................................................................................... 14

Chemical Mechanical Polishing (CMP) .......................................................................................................................... 15

Body Measurement in a Seat ........................................................................................................................................ 16Nip or Roller Measurement ........................................................................................................................................... 17

CONCLUSION ......................................................................................................................................................................... 18


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COMPARISON OF INTERFACE PRESSURE MEASUREMENTOPTIONSSENSING PRESSURE

An increasingly competitive global marketplace means that design engineers must efficiently deliver a high qualityproduct. Countless emerging technologies impact the design process and engineers must practice due diligence toensure analysis tools meet their applications requirements. This paper focuses specifically on technology for interfaceforce and pressure measurement between two surfaces and includes a review of technology composition and dataoutput. This paper will also examine capabilities driven by form factor, precision and environment that influenceselection criteria of interface force and pressure sensors.

Pressure is measured in a variety of applications, ranging from medical research to product development. Some keyproblems that engineers are trying to resolve are:

Identifying failure mode of a product and/or design Hot Spot Understanding force distribution between two load bearing surfaces Verifying proper sealing or snap fit in product design Calibrating manufacturing equipment (nips, surface processes, or spray patterns) Verifying alignment of pressure rollers Evaluating effects of different manufacturing processes on product quality Benchmarking competitive product

The more data available in these applications, the better a researcher can assess the situation and identify a resolution.

INTERFACE PRESSURE SENSING TECHNOLOGIESPressure is the measurement of force over an area. The formula is commonly written as:

P =F/AWhen two objects are held in contact, they exert force on each other. Therefore, the average interface pressure is thetotal force divided by the interface area. When this interface pressure is not uniformly distributed, localized interfacepressure measurement is necessary to find concentrations of peak pressure. Even between relatively flat surfaces, onefinds the interface pressure distribution is often non-uniform with localized areas of peak pressure. Interface pressuremeasurement sensing technologies can identify the location and magnitude of these peak pressures, or visualizepressure gradients across the interface. When trying to measure interface pressure, measurement devices shouldminimally impact the contact pressure profile of the items being measured.

Three technologies to consider when trying to measure force or interface pressures are:1. Load cells2. Pressure indicating film3. Tactile Pressure Mapping systems

While there is some overlap with the information each sensing technology provides, they each present a unique value inthe problem-solving process. The sensors ability to match the interface with the surfaces applying the load becomescritical as the shape of the target becomes increasingly abnormal.


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Load CellThe force or pressure sensor that most researchers and engineers are familiar with is a load cell. They can use a

variety of technologies to sense loads. Strain gauges, piezoelectric elements, and variable capacitance are among themethods in high use. Depending on the force applied and the mechanics of the application, multiple form factors ofload cells are utilized. Compression, S- or Z- beam, and shear beam load cells all rely on strain gauges measuring themechanical deformation of a beam to quantify the force of the load applied.

One of the most reliable load cells is a compression canister (Figure 1), which utilizes a full bridge of strain gauges thatare bonded to load-bearing structures.

Figure 1: Inside View of Loa d Cell

When force is applied to the load cell, the bearings deform and a mechanical stress is placed on the strain gaugeschanging the resistance of the output signal. After calibration, the output voltage correlates to the force applied to theload.

Because of the durability of the bearing structure, the reliability and accuracy of the device remain resilient through veryhigh forces applied to the load cell. The strain gauges can measure forces that put the bearings in tension, in additionto compression. The form factor of the load cell yields a bulky device. When used in tight quarters, this bulkinessmeans that the load cell can be difficult to position in the manner desired for collecting relevant data. Furthermore, theload cell can only output the total force applied to the bearing and cannot show distribution of pressures on a surface.In some instances, researchers can use multiple load cells to measure forces over multiple regions of a contactinterface.

Bearing Structure

Strain Gauges


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Figure 2: Pressure Measurem ent u sing Mult i p le Load Cells

In Figure 2, an interface or contact area is shown broken into four quadrants with a unique load cell measuring eacharea. This configuration provides more detail on the force distribution across the surface, giving the average pressurefor each zone. The more load cells used, the finer the granularity of pressure distribution that can be measured. Asmore load cells are added to the array, the data collection apparatus can become very expensive and cumbersome to

use. Furthermore, the size of the load cells themselves limits the density of measurement points. If localized pressuredistribution is different between load cell locations, the pressure spike will be missed and the gradient will be averagedby the neighboring load cells. In addition, as load cells are typically embedded into the structure, the introduction ofmany load cells could affect the structural integrity of the device; thereby, altering or skewing the results. In mostinstances where force applied to an area is inconsistent, the load cell can show the total force but cannot identifylocalized spikes in pressure.

Load cells are commonly used with testing equipment like an Instron system. A mechanical load is applied to an object(either compression or tension), and the load cell can accurately measure the forces experienced over a broad range.

Pressure Indicating FilmPressure indicating film is used to measure interface pressures between two surfaces. Under a layer of polyester, acolor developing material is layered next to tiny microcapsules, which are designed to break under different pressures.

When pressure is applied to the film, the microcapsules rupture, distributing ink where pressure is applied. As moreforce is applied to a location, more microcapsules are broken, increasing the intensity of color on the film. The resultis an image of the force applied across the sensing area. Typically, high sensitivity film for low-pressure measurementsutilize two sheets of polyester to keep the color developing material and microcapsules separate until the measurementis taken. High-pressure films usually have all components on one sheet.

F igure 3 : Componen ts o f Pressure Ind ica t in g F i lm

LCLC

LCLC


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This consumable film brings some unique features to applications. Its flexibility and thinness allow the film to be usedin a broad range of applications to capture images of pressure profiles. The film can be cut in unique shapes to fit intoapplications with obstructions. Because there are no attached electronics, the film can be fed through rollers to obtainthe pressure distribution with no concern of crushing wires or expensive electronics. An optional scanner and softwaresolution provides the ability to run a digital analysis of a scanned sheet.

The nature of the film will only provide the peak pressure between interfaces during a measurement. This has obvious

limitations when trying to measure dynamic applications. Static measurements can also be affected, as there is often aspike force on the film as the two interfaces are initially brought together. An example is the measurement of pressuredistribution between an engine block and its head. As each fastener is tightened to secure the two halves together, thefastener can relax the stress on a neighbor. When measuring this distribution dynamically, one will only observe thepeak pressure during the tightening process. As the resting pressure between the interfaces is not measureable,designers can overestimate the actual final pressure when using pressure indicating film.

Tactile Pressure SensorA common type of tactile pressure sensor consists of a unique piezoresistive material sandwiched between two pieces offlexible polyester, with printed silver conductors on each half. The result is an extremely thin 0.004 (0.1 mm) sensor,

which can be manipulated to fit in a broad range of applications with minimal disturbance to the behavior of the systembeing tested.

Figure 4: Construction of a Tacti le Pressure Sensor

The silver traces provide a conductive track for the scanning electronics to transmit a signal through the piezoresistiveink. As pressure is applied to the sensing area, the resistance of the ink changes and the scanning electronics collectsthe analog data. The analog data is converted to a digital signal, which is then transmitted to a PC.

F igure 5 : Componen ts in a Tac t i le Pressure Mapp ing Sys tem

Data Capture &Visualization Software

Data AcquisitionElectronics

Tactile ArrayPressure Sensor


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Figure 5 shows the three components that make up a tactile pressure mapping system: the sensor, the scanningelectronics and the software. When no force is applied to the sensor, the resistance of the ink is on the order of Mega-Ohms (M). As more force is applied, the output resistance drops to as low as 10 k. The output, expressed asconductance (1/R), is linearized to correlate with the load applied, and converted to digital counts or raw values on ascale of 0-255. The typical linearity error is


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The software that accompanies the tactile pressure sensor provides multiple tools for analysis. A real time windowshows the activity in the sensor area, which allows the user to see the impact of adjustments made. In dynamicapplications, a movie of the pressure distribution can be recorded for frame-by-frame analysis. The software cangenerate various types of graphs showing force, pressure, contact area, and other parameters versus time or position onthe sensor. The software can also identify the center of force for the contact region.

The tactile pressure sensor requires more work to calibrate properly as the end user must calibrate for the specific

application. The stiffness or compliance of the interface material has an effect on the pressure distribution and onlocalized peak pressures. Therefore, the sensor should be calibrated against materials very similar to those in theapplication. Additionally, if the pressure sensitive elements are under a constant load for a prolonged duration of time,the ink will continue to compress, changing the resistance, creating a drift in the pressure measurement. The outputincrease is between 0%-3% per log time, which can be compensated for with proper calibration. Over time, sections ofthe sensor, which have been exposed to higher forces, can start to plastically deform and exhibit different sensitivity. Aperiodic equilibration will correct the inconsistency. A device called an equilibrator applies a uniform pressure acrossthe sensor face. While this uniform pressure is applied, the software can create scale factors to normalize the output ofeach individual sensel. High pressure, complex contours and high temperatures will all have an impact on the overalllifespan of a tactile pressure sensor. The same scanning electronics can be used with a replacement sensor once asensor has reached the end of its lifecycle.

Tactile Force SensorFor basic applications, a tactile force sensor can be used. The construction of the force sensor is similar to a pressuresensor but instead of a matrix of sensing traces, the ink uniformly covers an area to measure the total force applied tothat space as shown in Figure 8. This sensor cannot map the pressure distribution but the resistance change correlatesto the force over the entire area. Since only one voltage is being measured for the entire sensing area, the dataacquisition electronics are much simpler and compact. Furthermore, the output can easily be connected to customelectronics to integrate force data with other measurements. The sensing area is available in a variety of shapes. Thethinness and flexibility of the force sensor allows it to be mounted in a broad range of hard-to-fit applications.

Figure 8: Tacti le Force Sensor

The tactile force sensor enjoys many of the benefits as the tactile pressure sensors as they utilize the same technology.Therefore, similar considerations must be taken when using the force sensor. Application specific calibration, drift, andlifespan are all factors still to consider with this sensor. However, since there is one sensing area, equilibration is notnecessary to ensure consistent data collection.


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Like a load cell, a tactile force sensor will only provide information of the total force applied to the sensing area. Asdescribed earlier, form factor and performance are the differentiators between the tactile force sensor and the load cell.

Available in a variety of form factors -- some of which can be cut to shape tactile pressure and force sensors can beapplied to a broad range of applications. Due to their flexibility, forces can be measured against tires, door seals, bodyparts, rollers, and numerous other surfaces. These sensors can provide a wealth of information about the pressure andforce between two contacting surfaces.

COMPARISON OF MEASUREMENTS FROM DIFFERENT PRESSURESENSING TECHNOLOGIESTo analyze the capabilities and limitations of load cells, pressure sensing film, and tactile pressure sensors, an identicalload was applied by an Instron

machine to all sensing technologies. For the comparison, the load is applied via a 2 x

2 aluminum plate interface. 800 lbs (363 kg) of force was applied to the sensor and then increased to 1600 lbs (726kg).

Load CellWith the support of data collection software and an analog-to-digital interface, accurate force data can be measured onthe force applied by the plate. Figure 9 shows the results of data collected at 5 Hz for a duration of 100 seconds. Theresulting force vs. time profile demonstrates increasing force for 20 seconds, constant force for 10 seconds, and then asecond increase in force.

F igure 9 : Ou tpu t f rom Load Ce l l

The load cell used for data collection can measure up to 22,480 lbs (10,200 kg). The small bumps along the curve area result of the low analog-to-digital converter resolution. Aside from the resolution issue, the data shown is linear and

very consistent. Even though the input card A/D resolution limits the precision of the data, the measurement is still veryaccurate across a wide range of forces High precision over a broad spectrum of forces is one of the strengths of thistechnology. To determine the average pressure applied, the user divides the reported force by the area of the plateapplying the force, to determine the two plateaus to have 200 (1.38 MPa) and 400psi (2.76 MPa), respectively. For acontrolled experiment, the adjustment calculation is straightforward; however, for unique shapes, where the contactarea is not known, or non-flat surfaces, the pressure estimate is less accurate.


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Pressure Indicating FilmThe pressure indicating film was only used to measure the 800 lbs load applied to the interface. To collect data, theSuper Low Pressure (LLW) film was used, which is rated for 71-355 psi (0.5 to 2.5 MPa), but there are many otherpressure ranges available. The resulting peak pressure is shown in Figure 10.

Figure 10: Photo of Su per Low Pressure (LLW)

The resulting image from the pressure indicating film only indicate peak pressures, so there is no data reflecting thedynamic nature of the pressure applied. In this experiment only 800 lbs (363 kg) was applied to the film. If theInstron were to also apply a 1600 lbs (726 kg) load to the film, the higher measurement would have covered up the800 lbs (363 kg) reading. This can be problematic if a measurement application applies a spike force on the filmbefore the two interfaces reach a resting pressure.

The film was scanned and the data reviewed in film analyzing software. The resulting evaluation, shown in Figure 10,provides key metrics such as overall load, loaded area, peak pressure, average pressure, and Prescale Effective Ratepercentage. The Prescale Effective Rate percentage provides an assessment of the quality of the data collected and is anindication of the accuracy of the data in the report.

F igure 11: F i lm Ana ly t ic s


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With a Prescale Effective Rate percentage around 80%, the data provided is suspect. The reported load, 3901N (877lbs), and average pressure, of 1.52 MPa (220 psi), represent approximately a 10% error from the load actually appliedby the Instron.

The simplicity of the pressure indicating film provides versatility to collect interface pressure in a variety of applications.However, this simplicity means that only peak pressures can be recorded, resulting in loss of information regarding thedynamic nature of the applied pressure.

Tactile Pressure SensorA variety of tactile pressure sensor form factors are available, providing options for sensing area, spatial resolution, andpressure range. Because of the density of sensing elements, a wealth of data is collected.

Figure 12: Pressure Mappin g System D isplay

Figure 12 shows the image generated by the tactile pressure mapping system software. The first image is pixelated

because this is the raw data collected from the sensor. Each pixel represents the data from an individual sensel in thesensor. The second image demonstrates the softwares ability to interpolate, average, and contour the data in the image;creating a clearer representation of the object being measured. The algorithms used for interpolation have no impacton the softwares data analysis, only the image produced. This maintains accurate data while providing clearer imagesfor review. In addition to the color representing the pressure distribution, the image plots two icons on the screen: a

white box that identifies the location of the peak pressure, and a grey-white diamond that identifies the center of force inthe contact area.


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Figure 13: Screen Shot o f Softw are

The software provides multiple ways to view and analyze the data. Just under 100 seconds of data was recorded as theInstron applied various forces. The graph at the bottom of Figure 13 shows a plot of the total force applied versus time.Collected sensor frames can be synchronized with video to enhance the evaluation of data collection.

The graph in Figure 13 plots two data series: the pressure sensor data and the analog input from the load cell. The left

vertical axis defines the force, and it correlates to the output of the tactile pressure sensor. The right axis defines theload cell reading. This load cell is connected to the system via an analog-to-digital converter. In the experiment, forcefrom the Instron was applied to the tactile pressure sensor and load cell simultaneously. This is possible because of thetactile pressure sensors thinness. The tactile pressure sensor was placed directly on top of the load cell and was non-intrusive when taking a measurement. The software used the input from the load cell to calibrate the tactile pressuresensor, creating an accurate measurement. This is reflected in the overlap of the two curves in Figure 13. Thisconfiguration leverages the strengths of load cells and tactile pressure sensors to provide the user with a complete andaccurate analysis. In this experiment, the tactile pressure sensor reports force data within 3% of the actual force beingapplied.

In addition to identifying the center of force, the software can show the peak pressures of each of the sensels, track thetrajectory of the center of force, view the data in a 3-D graph, and view the force value of each sensel. Data can bereviewed in Microsoft Excel or exported in ASCII format for in-depth analysis. Graph options can be adjusted to showobject area, contact area, force, and pressure over time, frame, or position in line/histogram graphs.


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Data from Three TechnologiesWhen measuring interface pressure, load cells provide the most reliable data, but the size and number of load cellsrestrict the range of applications in which they can be used. In addition, total load is easily reported but getting finegranularity of pressure distribution can be limited due to the size of the load cell. Pressure indicating film can bemanipulated to fit into a variety of applications, but the data has less accuracy and cannot report the range of forcesapplied in dynamic applications. Tactile pressure sensors can provide detailed dynamic measurements of interfacepressure with minimal impact on system dynamics. The sensing elements need to be properly calibrated to provideaccurate data, but the resulting measurements provide the most in depth analysis of interface system dynamics.

Depending on the information needed and the physical constraints of the system being measured, load cells, pressureindicating film, and tactile pressure sensors each have advantages and constraints for providing accurate andmeaningful data. Understanding how these strengths and limitations influence an application is critical.

PRESSURE SENSING APPLICATIONSMeasuring the forces between two surfaces is critical in solving many problems involving product design and quality,manufacturing processes, failure mode identification, and system behavior. This section reviews some commonapplications and shows the information that a tactile pressure mapping system can provide when trying to optimize a

solution.

Fuel Cell Stack and Battery PlatesUniform contact pressure over a large area of dissimilar materials is an important factor in the performance of both fuelcells and some battery assemblies. In fuel cells, adjacent thin plates are stacked up to separate flows of hydrogen orhydrocarbons and oxygen. These plates must be in good contact with each other for efficient performance. Seals at theedge of the stack prevent leakage of chemicals from one region to another and from the interior to the environment. Inbatteries, consistent contact over large plate surface areas supports low internal resistance.

Designers try to optimize clamp assembly configuration to get a uniform pressure distribution between the clamps. Theconsistency of pressure between the two plates has a correlation to the electrical performance of the cell in operation.

A tactile pressure sensor can be placed between the plates of two different clamping configurations (standard andreinforced) to compare pressures.

F igure 14: Tac t i le Pressure Da ta o f Forces Betw een S tandard C lampin g and Re in forced C lam ping

A) Standard Clamp B) Reinforced Clamp


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Figure 14 shows the pressure distribution on the fuel cell with the original clamp configuration and the reinforcedconfiguration. While the reinforced clamping costs more to implement, designers can see it improve the distribution ofpressure across the plate face. Using the tactile pressure measurement system software, an analysis can be run on asection of the plate face near the clamping. A box is drawn around the area of interest, and the graphs are generated.The traces in the graph in Figure 15 correlate to the color of the boxes in Figure 14.

F igure 15: H is togram o f Two C lam ping So lu t i ons for a Fue l Ce l l

Figure 15 shows the average pressure applied to the plate as a function of the distance across the drawn box object.Each distance interval is one sensor column width (1 cm.) across the length of the sensor face. Here, the graph showsthe standard solution (in red) has a peak pressure near the clamp, but the pressure drops off toward the middle of theplate. The reinforced clamping (in green) applies a higher pressure across the middle of the plate.

Load cells can provide accurate measurement of the forces applied, but require multiple units and complexconfigurations to reflect the interface pressure distribution. In addition, it can be very difficult to mount the larger loadcells without influencing the dynamics of the system. Pressure indicating film can also be used in this application, butoften the high spike pressures captured on the film occur during the clamping process and the actual pressures inthese areas are lower when the cell is in operation. This can provide misleading information of the actual interface

pressures. The film must be replaced for each incremental measurement, making measuring adjustments timeconsuming. In addition, as the seals or gaskets relax over time, the interface pressure will change, which cannot becaptured by a static measurement.

Engineers use the tactile pressure system to optimize the clamping design for fuel cells. The dynamic measurement ofthe tactile pressure sensor allows researchers to obtain real-time feedback while adjusting clamps, which greatlysimplifies optimization of parameters.

Tire TreadUnderstanding tire behavior during motion is critical to designing a high performance tire. A ruggedized tactile

pressure sensor, with electronics in metal enclosures, sits on a steel plate, and can measure the pressure distribution ofa tire rolling across the sensor face.


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Figure 16: Tacti le Pressure Surfa ce Image of T ire Footprint

The point-of-contact data is analyzed in the tactile pressure measurement software to determine perimeter area, cross-sectional pressure profile, measure footprint length and width, and other configurable parameters. Tire manufacturersuse the data to:

Evaluate and compare tire designs and tread patterns Assess different materials and rubber compound formulations Conduct quality control measurements Perform competitive benchmarking Assess the impact of vehicle suspension

Load cells are often used in conjunction with tactile pressure sensors to control the force applied to the tire within aload frame. Alone, the force data does not provide much information for tread analysis. In lieu of pressure indicatingfilm, ink is sometimes applied to the tire tread in order to evaluate the footprint of a tire. Ink measurements have manyof the problems of pressure indicating film: the analysis is difficult and often inaccurate, it is difficult to collect datafrom a moving tire and only spatial dimensions can be measured. For the most comprehensive feedback of tirefunctionality, the tire can be rolled over the tactile pressure sensor while collecting dynamic measurements.

Chemical Mechanical Polishing (CMP)In semiconductors, Liquid Crystal Displays (LCDs), Printed Circuit Boards (PCBs), and other high-end electronicsmanufacturing, chemical machine polishing requires that an even polish be performed on parts to avoid adverselyaffecting subsequent manufacturing steps.

Measuring this process with a load cell results in an overview of the total force applied to the surface, but does notidentify the regions of the parts that experience excess force. The load cell output is shown in Figure 17.


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Figure 17: Load Cell Data Recorded durin g CMP Process

A thin tactile pressure sensor can be placed between a wafer and polishinghead to measure pressures applied to the product during the process, asshown in Figure 18.

Figure 18 (right) : Data Collec ted by Tacti le Pressure Sensor onWafer Surface During Polishing ProcessBecause the full process is recorded, different points in the process arescrutinized. Designers can then make adjustments or changes based onframe evaluation or by reviewing the peak pressures the tactile pressuresensor experiences. The same sensor is used in the commissioning processand then reused for scheduled reliability testing to identify worn parts orother issues affecting product quality. Designers can use the data from thetactile pressure sensor to improve yield and productivity for manufacturing.

Figure 19 (right) : Softw are Display of Body Pressureon Sea t Back and Cu sh ionHere, pressure indicating film can provide similar information about the peak

pressures across the face of the wafer. However, capturing the impact ofadjustments made to the polishing fixture is more cumbersome, as a new sheet offilm must be cut, placed, and analyzed for each measurement.

Here, a load cell can be used to calibrate the tension of the tool on the part, but itcannot evaluate the consistency of the process. Pressure indicating film can be usedfor evaluation, but tactile pressure sensors are often the sensor of choice because oftheir resistance to heat and dynamic measurement capabilities.

Body Measurement in a SeatMeasuring interface pressure between a persons body and a seat is very difficultbecause of the contours of the two objects and because these contours change as thesubject sinks into the seat. The body measurement tactile pressure sensor has aunique construction that provides minimal disturbance, conforms to the targetsbeing measured, and does not capture pressure artifacts from sheet tension(hammocking), only the forces applied to the loaded area.


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Very popular with automobile manufacturers and tier 1 suppliers, body tactile pressure sensors are used to record userexperience while driving. The data is used in competitive benchmarking, foam shape and stiffness adjustments,ergonomic improvements, and identifying high-pressure spots. Physicians, clinicians, and medical equipmentmanufacturers also find the data from body tactile pressure sensors invaluable.

Sometimes multiple load cells are placed in a seat, but this is typically used in presence sensing applications to identifythe presence or absence of a seated person. Pressure indicating film does not capture this information well, as the film

crinkles under the contours of the human form, resulting in nuisance streaks. The body measurement tactile pressuresensors unique construction conforms to complex, contoured and deformable support surfaces like seat cushions.Removing pressure artifacts from sheet tension provides the most accurate pressure data across the contours of thehuman form.

Nip or Roller MeasurementAny industry that manufactures products on rolls requires consistent pressure across the rollers to maintain productquality. These industries include steel, copper, film, paper, printing, packaging, and any materials that run through aconverting line. Nip rollers and pinch rollers need to be calibrated to hold tension on the line and maintain uniformthickness across products. System failures can lead to product derailment or deformities along the material surface.

The cost incurred because of down time and scrap material can be significant.

Tactile pressure sensors are excellent tools for measuring nip profiles, force, and pressure variations along a roll axis.The system measures and displays pinch roll nip static forces in real-time. For many large machines, nip contact isadjusted by setting hydraulic pressure at contact bearings located on the ends of the rollers. In the absence ofmeasurements, the relationship between hydraulic pressure and nip force is assumed linear, even though the machinelinkage and system hysteresis do not have linear behavior. Tactile pressure sensors measure nip force and width atlocations along the axis at various hydraulic pressures, providing real-time feedback to a PC. Crowning effectiveness, as

well as differences in pressures and contact width along the length of the roll, can be documented.

Figure 20: Screen Shot o f Ni p Roller Measuremen t

To measure roller pressure, the tactile pressure sensor has columns along the length of the roller. The system cansimultaneously provide total force, force applied to each column, nip width at each column, and force distribution.This data provides a process engineer or an operator with critical data for optimizing the settings on a machine. Wornparts can be identified early, avoiding unproductive and costly production runs. Because the tactile pressure sensor ispaired with a PC, measurements are saved by the software and referenced later for quality reviews or comparison toother identical machines in other plants, globally.


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Load cells can be used in this application, but the resolution is only as good as the number of unique load cells used.This can be cumbersome to apply. In addition, the load cells cannot identify the nip width. When pressure-sensitivefilm is run through the rollers, it can only identify peak pressures. Rollers must be secured and reopened to seemeasurements between adjustments, which can be very costly during production downtime. The tactile pressure sensorhas the ability to provide real-time feedback and remove guesswork from calibration of rollers, for quick and reliablemaintenance.

CONCLUSIONWhen trying to evaluate the forces and interface pressures, multiple data collection options are available. Form factor,precision, and environment all influence the solutions to be considered. The data provided by a tactile pressure sensorprovides a unique insight into a system or products performance, which is invaluable in system or process design.Leveraging a stronger understanding of pressure distribution yields positive results in a variety of applications inresearch, product development, manufacturing and quality control.

Visitwww.tekscan.comfor more information on these technologies and products:

Pressure Mapping Systems -http://www.tekscan.com/industrial-pressure-measurement

Pressure Indicating Film -http://www.tekscan.com/pressure-indicating-film.html

Force Sensors and Measurement Systems -http://www.tekscan.com/flexiforce.html
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