Electronic – How to record a pressure map


There are some Dr. Scholl's commercials showing their "foot-doctor-o-matic" station that measures the pressure your feet put down on a mat then tells you what insoles to buy. How does the thing actually work; what sort of component is able to translate an array of force across a plane into electronic signals? If the thing cheats and uses temperature, the question still stands; how can I record an array of pressures across a plane?

Best Answer

Pressure sensors arranged in a matrix to produce pressure related voltages which are scanned, digitised and analysed. Some pages on the site below say "pizeoelectric" pressure sensing and some say "resistive". Both are feasible.

  • A compressible material which varies in resistance between top and bottom surfaces could be arranged in a matrix with each point scanned and measured.
    Think in terms of semiconductor strain gauges, although not necessarily arranged in the usual bridge manner.

  • A piezo element will produce a pressure related static voltage which can be scanned and measured.

SPI Sensor productrs Inc make a wide range of surface pressure indicating sensors.

For a typical foot pad suitable for two feet at once, one system, The GoTec foot mapping sensor has the following specification:

  • Technology Resistive
    Number of Sensors 2,304 (Arranged in a 48 x 48 Matrix)
    Spatial Resolution 0.31 in. (8 mm)
    Sensing Area 15 in. x 15 in. (38 cm x 38 cm)
    Data Resolution 12-bit
    Pressure Range 0.72 to 30 PSI (0.05 to 2.10 kg/cm²)
    Data Acquisition Frequency 180 Hz
    Accuracy ±10%
    Platform Height 0.25 in. (0.63 cm)
    Weight 9.25 lbs. (4.2 kg)
    Operating System Windows® XP / 7 Connection Method USB / Wireless Operating Temp. Range 5°F to 86°F (-15°C to 30°C)

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The products shown below closely match your description.
Whether Scholls use the same method is unknown but it seems likely.

Their systems are based in some and maybe in all cases on piezoelectric pressure sensing elements (although "resistive" is mentioned) which are arranged in a matrix and then scanned and digitized by standard techniques.

Their Tactilus equestrics saddle fitting sensor is said to use piezo pressure measurement

  • The Tactilus Equestrics® sensor is a matrix-based tactile surface sensor that works by the principle of piezoresistance. Tiny sensing cells cover the entire surface area of the sensor “skin” allowing for discrete spot pressure analysis at any point in the contact region. The Tactilus Equestrics® sensor provides real-time data showing precisely where the pressure points occurs between the saddle and the horse.

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Specification: Note the use of the term "Piezoresistive" which places some doubt on whether the resistive and pizeoxxx systems are in fact the same.

Technology Piezoresistive Pressure Range 0 - 5 PSI (0 - 0.35 kg/cm²) Grid Size 18 x 22 split mat Sensing Points 388 Total Sensing Area 29.5 x 17.5 in. (75.9 x 44.5 cm) Scan Speed Up to 10 hertz Thickness 30 mils (0.76 mm) Accuracy ±10%

"sensor products inc" advertise a range of tactile pressure indicating films.

Foot plate sensor

Sew-in piezoeresistive sensing material.
Unfortunately the page related to this image was dead :-(

Eeontex conductive fabric used for pressure sensor construction - data sheets here

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Maxim - demystifying PR sensors](http://www.maxim-ic.com/app-notes/index.mvp/id/871)

  • Abstract: Monocrystalline silicon pressure sensors have come into wide use in recent years. Though manufactured with semiconductor technology, they also operate on the resistive principle. The resistance change in a monocrystalline semiconductor (a piezoelectric effect) is substantially higher than that in standard strain gauges, whose resistance changes with geometrical changes in the structure. Conductivity in a doped semiconductor is influenced by a change (compression or stretching of the crystal grid) that can be produced by an extremely small mechanical deformation. Using a signal conditioning integrated circuit to temperature compensate and amplify the signal offers superior performance over discrete circuits.

PE & PR sensors U.Dayton.

PE & PR Effects - COMSOL demo - Flash so can't copy live image. Worth a peek.

Silicon PR sensors useful.

PR design and optimisation somewhat off topic but useful and good references.

Northwestern edu - piezoresistive sensors 36 pages. Textbook chapter. Good.

Wikipedia - piezoresistive effect

"Body mapping":

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