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Sensors 2012, 12, 9884-9912; doi:10.3390/s120709884
sensors
ISSN 1424-8220
www.mdpi.com/journal/sensors
Review
Foot Plantar Pressure Measurement System: A Review
Abdul Hadi Abdul Razak
1,2,
*, Aladin Zayegh
1
, Rezaul K. Begg
3
and Yufridin Wahab
4
1
School of Engineering and Science, Victoria University, Melbourne, VIC 3032, Australia;
E-Mail: Aladin.Zayegh@vu.edu.au
2
Faculty of Electrical Engineering, Universiti Teknologi MARA, Shah Alam 40000, Malaysia
3
School of Sport and Exercise Science (SES) and Institute of Sport, Exercise and Active Living (ISEAL),
Victoria University, Melbourne, VIC 3032, Australia; E-Mail: Rezaul.Begg@vu.edu.au
4
Centre for Industrial Collaboration, School of Microelectronic Engineering, Universiti Malaysia Perlis,
Arau 02600, Malaysia; E-Mail: yufridin@unimap.edu.my
* Author to whom correspondence should be addressed; E-Mail: hadi@ieee.org;
Tel.: +61-3-9919-5047; Fax: +61-3-9919-4908.
Received: 15 May 2012; in revised form: 27 June 2012 / Accepted: 3 July 2012 /
Published: 23 July 2012
Abstract: Foot plantar pressure is the pressure field that acts between the foot and the
support surface during everyday locomotor activities. Information derived from such
pressure measures is important in gait and posture research for diagnosing lower limb
problems, footwear design, sport biomechanics, injury prevention and other applications.
This paper reviews foot plantar sensors characteristics as reported in the literature in
addition to foot plantar pressure measurement systems applied to a variety of research
problems. Strengths and limitations of current systems are discussed and a wireless foot
plantar pressure system is proposed suitable for measuring high pressure distributions
under the foot with high accuracy and reliability. The novel system is based on highly
linear pressure sensors with no hysteresis.
Keywords: foot plantar pressure; pressure sensor; wireless system
OPEN ACCESS
Sensors 2012, 12 9885
1. Introduction
The development of miniature, lightweight, and energy efficient circuit solutions for healthcare
sensor applications is an increasingly important research focus given the rapid technological advances
in healthcare monitoring equipment, microfabrication processes and wireless communication. One area
that has attracted considerable attention by researchers in biomedical and sport related applications is
the analysis of foot plantar pressure distributions to reveal the interface pressure between the foot
plantar surface and the shoe sole. Typical applications are footwear design [1], sports performance
analysis and injury prevention [2], improvement in balance control [3], and diagnosing disease [4].
More recently innovative applications have also been made to human identification [5], biometric [6],
monitoring posture allocation [7] and rehabilitation support systems [8–10]. Based on this research it
is clear that techniques capable of accurately and efficiently measuring foot pressure are crucial to
further developments.
The plantar pressure systems available on the market or in research laboratories vary in sensor
configuration to meet different application requirements. Typically the configuration is one of three
types: pressure distribution platforms, imaging technologies with sophisticated image processing
software and in-shoe systems. In designing plantar pressure measurement devices the key requirements
are spatial resolution, sampling frequency, accuracy, sensitivity and calibration [11]. These requirements
will be discussed in detail later.
In-shoe foot plantar sensors have paved the way to better efficiency, flexibility, mobility and
reduced cost measurement systems. For the system to be mobile and wearable for monitoring activities
of daily life, the system should be wireless with low power consumption. Wireless in-shoe foot plantar
measurement systems have potential application to data transfer communication systems, miniaturized
biomedical sensors and other uses. For compact, low cost devices for short-range wireless applications
an on-chip antenna is a practical solution. On-chip antenna implementation is feasible with the
assistance of rapid scaling of low cost complementary metal-oxide-semiconductor (CMOS) technology.
The feasibility of creating circuits and systems to operate at lower frequency bands and subsequently
reducing the antenna size using on-chip antennas has been discussed [12,13].
This review will first summarize the existing methods for measuring foot plantar pressure and the
advantages and disadvantages of a range of commercial pressure sensors used in published research.
Subsequently, the discussion will introduce a micro-electromechanical (MEMS) pressure sensor that
has considerably enhanced performance characteristics. Finally various solutions presented by
researchers to measure foot plantar pressure using in-shoe system will be reviewed. The review
critically examines the devices used in measuring systems, such as sensors, processing units and
wireless transmitters. The paper compares the compactness, power consumption, number of sensors
and placements of sensors used in published systems and we propose a new system, the MEMS sensor.
The MEMS sensor will interface with a wireless data acquisition (DAQ) unit, which is a full-custom
design using CMOS technology. This novel solution will be on a single chip making it highly compact
and low in power consumption.
The paper is divided into eight sections. Section 2 presents the requirements for plantar pressure
measurement systems. The foot plantar pressure measurement environment will be discussed in
Section 3. Section 4 will describe the application requirements of foot plantar sensors. Section 5
Sensors 2012, 12 9886
documents the commercial foot plantar pressure measurement sensors in detail. The wireless foot
plantar pressure systems will be reviewed in Section 6. Section 7 presents our proposed new approach
to recording foot plantar pressure and the system‟s design. Finally, Section 8 discusses the suitability
of the proposed system and conclusions.
2. Needs for Plantar Pressure Measurement
Feet provide the primary surface of interaction with the environment during locomotion. Thus, it is
important to diagnose foot problems at an early stage for injury prevention, risk management and general
wellbeing. One approach to measuring foot health, widely used in various applications, is examining foot
plantar pressure characteristics. It is, therefore, important that accurate and reliable foot plantar pressure
measurement systems are developed. One of the earliest applications of plantar pressure was the
evaluation of footwear. Lavery et al. [14] in 1997 determined the effectiveness of therapeutic and athletic
shoes with and without viscoelastic insoles using the mean peak plantar pressure as the evaluation
parameter. Since then there have been many other studies of foot pressure measurement; for example,
Mueller [15] applied plantar pressure to the design of footwear for people without impairments (i.e., the
general public). Furthermore, Praet and Louwerens [16] and Queen et al. [17] found that the most
effective method for reducing the pressure beneath a neuropathic forefoot is using rocker bottom shoes
and claimed the rocker would decrease pressure under the first and fifth ray (metatarsal head). The
metatarsal heads are often the site of ulceration in patients with cavovarus deformity. Queen et al.
indicated that future shoe design for the prevention of metatarsal stress fractures should be gender
specific due to differences in plantar loading between men and women.
With regard to applications involving disease diagnosis, many researchers have focused on foot
ulceration problems due to diabetes that can result in excessive foot plantar pressures in specific areas
under the foot. It is estimated that diabetes mellitus accounts for over $1 billion per year in medical
expenses in the United States alone [18]. Diabetes is now considered an epidemic and, according to
some reports, the number of affected patients is expected to increase from 171 million in 2000 to
366 million in 2030 [19]. Improvement in balance is considered important both in sports and
biomedical applications. Notable applications in sport are soccer balance training [20] and forefoot
loading during running [21]. With respect to healthcare, pressure distributions can be related to gait
instability in the elderly and other balance impaired individuals and foot plantar pressure information
can be used for improving balance in the elderly [22]. Based on the above discussion, it is crucial to
devise techniques capable of accurately and efficiently measuring foot pressure.
3. Foot Plantar Pressure Measurement Environments
There are a variety of plantar pressure measurement systems but in general they can be classified
into one of two types: platform systems and in-shoe systems.
3.1. Platform Systems
Platform systems are constructed from a flat, rigid array of pressure sensing elements arranged in a
matrix configuration and embedded in the floor to allow normal gait. Platform systems can be used for
Sensors 2012, 12 9887
both static and dynamic studies but are generally restricted to research laboratories. One advantage is that
a platform is easy to use because it is stationary and flat but has the disadvantage that the patient requires
familiarization to ensure natural gait. Furthermore, it is important for the foot to contact the centre of the
sensing area for an accurate reading [23]. Limitations include: space, indoor measurement, and patient‟s
ability to make contact with the platform, Figures 1 and 2 show a platform-based sensor [24,25].
Figure 1. A platform-based foot plantar pressure sensor emed
by Novel [24].
Figure 2. A platform based foot plantar pressure sensor by Zebris Medical GmbH [25].
3.2. In-Shoe Systems
In-shoe sensors are flexible and embedded in the shoe such that measurements reflect the interface
between the foot and the shoe. The system is flexible making it portable which allows a wider variety
of studies with different gait tasks, footwear designs, and terrains [23].
Figure 3. An in-shoe based foot plantar pressure sensor by Pedar
Novel [24].
Sensors 2012, 12 9888
Figure 4. An in-shoe based foot plantar pressure sensor F-Scan
System by Tekscan [26].
They are, therefore, highly recommended [11,23] for studying orthotics and footwear design
but there is the possibility of the sensor slipping. Sensors should be suitably secured to prevent
slippage and ensure reliable results. A further limitation is that the spatial resolution of the data is low
compared to platform systems due to fewer sensors [11,23]. Figures 3 and 4 illustrate in-shoe based
systems [24,26].
4. Requirement of Foot Plantar Sensors
In taking any biomechanical measurements, devices must be optimized for the specific application
to ensure that readings are accurate. Detailed analysis must be thoroughly undertaken prior to any
measurements and for foot plantar system two main considerations must be met; the target
implementation requirements and the sensor requirements.
4.1. Target Implementation Requirements
Real-time measurement of natural gait parameters requires that sensors should be mobile,
untethered, can be placed in the shoe sole, and can sample effectively in the target environment. The
main requirements of such sensors are as follows:
(1) Very Mobile: To make a sensor mobile, it must be light and of small overall size [27,28], the
suggested shoe mounted device should be 300 g or less.
(2) Limited Cabling: A foot plantar system should have limited wiring, wireless is ideal. This is to
ensure comfortable, safe and natural gait [28].
(3) Shoe and Sensor Placement: To be located in the shoe sole the sensor must be thin, flexible [29]
and light [27]. It is reported that a shoe attachment of mass 300 g or less does not affect gait
significantly [27]. Shu et al. [30] mentioned that the sole of foot can be divided into 15 areas:
heel (area 1–3), midfoot (area 4–5), metatarsal (area 6–10), and toe (area 11–15), as illustrated
in Figure 5. These areas support most of the body weight and are adjusted by the body‟s
balance; therefore, ideally the 15 sensors are necessary to cover most of the body weight
changes based on the Figure 5 anatomy.
(4) Low Cost: The sensor must be affordable for general application [28] to benefit from
inexpensive, mass-produced electronics components combined with novel sensor solutions.
(5) Low Power Consumption: It should exhibit low power consumption such that energy from a
small battery is sufficient for collecting and recording the required data.
Sensors 2012, 12 9889
Figure 5. Foot anatomical areas [30].
4.2. Plantar Pressure Sensor Requirements
The key specifications for sensor performance include: linearity, hysteresis, sensing size, pressure
range and temperature sensitivity [27,29,31]. Brief discussion of these is important as a basis for the
selection of a sensor for specific applications.
(1) Hysteresis: Hysteresis can be determined by observing the output signal when the sensor is
loaded and unloaded. When the applied pressure is increased by loading or decreased by
unloading, two different responses are observed (Figures 6 and 7).
Figure 6. Hysteresis caused by loading and unloading a pressure sensor usually measured
at the 50% pressure range. Adopted from [32].
Pressure
100%50%
F.S.
Sensor Output
Midscale
Hysteresis
Sensors 2012, 12 9890
Figure 7. Negligible hysteresis of MEMS-based pressure sensor [29].
(2) Linearity: The response of the sensor to the applied pressure, when plotted, will show the
linearity figure of merit, i.e., how straight the plotted line is. Linearity indicates how simple or
complicated the signal processing circuitry will be, a highly linear response requires very simple
signal processing circuitry and vice versa, a linear pressure sensor is, therefore, preferred.
(3) Temperature Sensitivity: Sensors may produce different pressure readings as the ambient
temperature changes. This may be due to the materials that are part of the sensor body as they
respond differently to temperature change. A sensor with low temperature sensitivity in the
20 °C to 37 °C range is preferred [29].
(4) Pressure Range: The pressure range is the key specification for a pressure sensor. As different
applications require different operating pressures application-specific sensor development is
normally adopted in the design. Maximum pressure is the upper limit that the pressure sensor
can measure and vice versa. It is also important to note that burst pressure is the maximum
pressure that the sensor can withstand before breakage as opposed to maximum pressure. Foot
plantar pressure values of up to 1,900 kPa are typically reported in the literature but an extreme
pressure of up to 3 MPa has been documented by Urry [31]. One of the foot plantar pressure
sensor designs considers 3 MPa as burst pressure value, for comparison when a healthy person
of 75 kg body mass is standing on only one forefoot, if pressure is evenly distributed, the
interfacial pressure for every 31.2 mm
2
foot plantar area approximates 2.3 MPa [33].
(5) Sensing Area of the Sensor: Size and placement of the sensor are also critical, as shown in Figure 8.
As a large sensor may underestimate the peak pressure and it is suggested that a minimum
sensor of 5 mm × 5 mm should be used, whereas sensors smaller than this must be designed as
array sensors.
(6) Operating Frequency: It is recommended [31] that to measure foot plantar pressure precisely
for running activities the sensors must be capable of sampling at 200 Hz. This frequency is
generally considered sufficient for sampling most everyday gait activities.
(7) Creep and Repeatability: Creep is the deformation of material under elevated temperature and
static stress. It directly relates to the time dependent permanent deformation of materials when
Sensors 2012, 12 9891
subjected to a constant load or stress [34], as in Figure 9. Low creep sensors are one of the key
requirements in foot pressure measurement. Repeatability refers to the ability to produce
reliable result even after long period of time [29]. High cyclic loads may cause deformation or
fatigue [34]. Repeatability problems can be eliminated if the sensor exhibits no creep or
deformation over repetitive or high cyclic loads.
Figure 8. Effect of sensor sizing and placement.
M
The area of least
pressure
A wrong placement
of sensor
A bigger sensor
that may
underestimate
pressure due to
averaging effect
The best placement
of sensor (M)
The area of
maximum pressure
Figure 9. Example of erroneous readings due to sensor creep of Pedar
Insole. The curve
is the error reading by the sensor and plotted line is the correct pressure values. Modified
from [35].
Time (hours)
21
Total Force (N)
30
1100
1200
1300
1400
5. Commercial Foot Plantar Pressure Measurement Sensors
There are several pressure sensors available on the market. Such sensor technologies utilize
capacitive sensors, resistive sensors, piezoelectric sensors and piezoresistive sensors. These sensors
provide electrical signal output (either voltage or current) that is proportional to the measured pressure.
The required key specifications for a pressure sensor in terms of sensor performance include linearity,
hysteresis, temperature sensitivity, sensing size and pressure range. The most common pressure
sensors are capacitive sensors, resistive sensors, piezoelectric sensor and piezoresistive sensor.
Sensors 2012, 12 9892
5.1. Capacitive Sensors
The sensor consists of two conductive electrically charged plates separated by a dielectric elastic
layer. Once a pressure is applied the dielectric elastic layer bends, which shortens the distance between
the two plates resulting in a voltage change proportional to the applied pressure [11,31]. Figure 10
shows the capacitive sensor construction. Commercial products based on this system are the emed
platform systems (Novel, Germany) [24] and Pedar
in-shoe systems (Novel, Germany) [24].
Figure 10. Capacitive pressure sensor construction [36].
5.2. Resistive Sensors
Force-Sensing Resistor (FSR) is a good example of the resistive sensor. When pressure is applied
the sensor measures the resistance of conductive foam between two electrodes. The current through the
resistive sensor increases as the conductive layer changes (i.e., decreases resistance) under pressure.
FSRs are made of a conductive polymer that changes resistance with force, applying force causes
conductive particles to touch increasing the current through the sensors [11,31]. Figure 11 shows the
resistive sensor construction and commercial products based on this principle are MatScan
platform
systems (Tekscan, USA) [26] and F-Scan
in-shoe systems (Tekscan, USA) [26].
Figure 11. Resistive pressure sensor construction [37].
5.3. Piezoelectric Sensors
The sensor produces an electric field (voltage) in response to pressure. Piezoelectric devices have
high impedance and therefore susceptible to excessive electrical interference that leads to an
unacceptable signal-to-noise ratio.
Sensors 2012, 12 9893
The most suitable material for clinically oriented body pressure measurement is polyvinylidene
fluoride (PVDF) because it is flexible, thin and deformable [11,31]. Figure 12 shows the piezoelectric
sensor construction. Commercial products based on this system are Measurement Specialties, USA [38]
and PCB Piezotronics, Inc., USA [39].
Figure 12. Piezoelectric pressure sensor construction [39].
Applied Pressure
Output Voltage
Piezoelectric
Element
Electrodes
5.4. Piezoresistive Sensors
This sensor is made of semiconductor material. In Piezoresistive material the bulk resistivity is
influenced by the force or pressure applied, when the sensor is unloaded resistivity is high and when
force is applied resistance decreases [11]. Figure 13 shows the piezoresistive sensor construction.
When there is pressure on the piezoelectric element (quartz crystal) it produces electric charges from
its surface. These charges create voltage proportional to the applied force. Commercial products based
on this system are FlexiForce
(Tekscan, USA) [26] and ParoTec (Paromed, Germany) [40].
Figure 13. Piezoresistive pressure sensor construction [41].
The requirements of the pressure sensor for the specific application are low hysteresis, linearity of
output, and pressure range [27,29,31]. A recommended pressure range for gait analysis (walking) is
approximately 1,000 kPa [31] but for sports the pressure range should be larger due to the nature of the
movements. There are a number of commercially available foot-pressure sensors on the market but
they generally do not fulfil the requirements of many biomechanical applications due to specification
and performance limitations. The limitations include but not limited to, the specified hysteresis [29],
pressure span and physical sensor dimensions [42].
Sensors 2012, 12 9894
In comparison with traditional foot plantar pressure sensors such as capacitive sensors, resistive
sensors, piezoelectric sensors and piezoresistive sensors the MEMS pressure sensors have many
advantages. For example, easy communication with electrical elements in semiconductor chips, small
size, lower power consumption, low cost, increased reliability and higher precision. To provide a better
alternative developments of a specifically designed miniature foot pressure sensor based on MEMS
technology have been explored. In response to the needs of such sensors, Wahab et al. [43]
successfully designed, fabricated and tested a miniature foot pressure sensor based on MEMS
technology that can be inserted in the insole of a shoe [43]. As reported by Wahab et al. [43]
significant performance enhancements have been achieved, for example, the sensor is small, has
high-pressure range measurement capability, and excellent linearity both at low and high pressures and
possesses negligible hysteresis.
Currently available in-shoe pressure sensor parameters are compared to Wahab et al. [43] in
Table 1. Sensors from Vista Medical, Novel and Tekscan show some performance limitations as they
are made of sheets of polymer or elastomer leading to issues such as repeatability, hysteresis, creep
and non-linearity of the sensor output [29]. In addition to the above limitations some sensors (e.g.,
Parotec) have limited pressure range and relatively large dimensions. Figure 14 demonstrates the
linearity of the MEMS based pressure sensor and the fabricated sensor as displayed in Figure 15.
Table 1. Commercially available in-shoe pressure sensors compared to Wahab et al. sensor.
Vista Medical
[44]
Tekscan
[26]
Novel
[24]
Parotec
[45]
Textile Sensor
[30]
Wahab et al.
[43]
Sensor Size 2 mm thick 0.15 mm thick 1.9 mm thick
~4 cm2
(hydrocell)
Not Specified 2 mm thick
Number of
Sensor
128 (in shoe) 960 (insole) 99 (insole) 24 (insole) 6 (insole) 15 (insole)
Range (kPa) 260 1,034 1,200 625 800 3,000
Frequency
(Hz)
Not Specified 500 Not Specified 250 100 200
Hysteresis Not Specified 24% <7%
0.05% at
20 Ncm–2
Not Specified Negligible
Figure 14. Graph demonstrating highly linear output voltage vs. pressure relationship.
Sensors 2012, 12 9895
Figure 15. Picture of fabricated sensors.
6. Recent Trends in Foot Plantar Pressure Measurement
Trends in biomedical monitoring are toward using real-time and in-situ measurement of normal
daily life parameters to keep pace with a fast-changing and demanding scientific environment. Gait
analysis researchers are focusing on designing systems for uninterrupted measurement of real life
parameters which is important in understanding the effect of daily activities on health. The ideal
system to achieve this would be mobile, un-tethered, placed in the shoe sole and able to measure
effectively in the targeted environment.
As early as the 1990s, Zhu et al. [46] developed a system for measuring the pressure distribution
beneath the foot using seven force-sensitive resistors (FSR) and they used it to differentiate pressure
between walking and shuffling [47]. In 1995, Hausdorff et al. [48] built a footswitch system capable of
detecting temporal gait parameters using two FSR sensors. Later, in 1997, Cleveland Medical Devices
Inc. [49] created an in-shoe wireless system which could measure time of foot contact, the weight on
each foot and the centre of pressure (COP) of each foot. The system used a set of thick-film force
sensors and since then there has been further development of in-shoe pressure sensor systems. In this
paper, the focus is on the current development of the system.
6.1. Wired Systems Application
Over the past two years there has been increasing interest in developing in-shoe foot plantar
pressure systems and recently there have been applications to plantar pressure using both wired and
wireless systems. In 2011, a paper employed dynamic plantar pressure for human identification using a
FlexiForce
(Tekscan, USA) in-sole pressure sensor [5]. They compared the pressure at different
positions of key points then identified and classified them using a support vector machine (SVM)
running on a PC. The system uses wire to transfer data from the sensor to a data acquisition card on a
PC (Figure 16) and it is reported that the system has 96% identification accuracy.
Yamakawa et al. [6] also proposed their own biometric identification in-shoe system based on both
feet pressure change and reported that the system could recognize over 90% of the test subjects. The
system used F-scan (Nitta Corp, Japan) as the pressure sensor (see Figure 17).
Another innovative application is an in-shoe system to measure triaxial stress in high-heeled
shoes [50]. The paper investigated the distribution of contact pressure and sheer stress simultaneously
in high-heeled shoes utilizing five in-shoe triaxial force transducers commercialized by Anhui June
Sport, China.
Sensors 2012, 12 9896
Figure 16. Identification based on dynamic plantar pressure in-shoe system [5].
Figure 17. Biometric identification based on foot pressure pattern changes. Modified from [6].
Control
Equipment
Sensor
Sheet
Shear stresses can cause blisters, callosities and trophic ulcers. The size of transducer is 17 mm × 18
mm × 10 mm and has 870 kPa full scale pressure range. In the system the transducer in mounted under
the hallux, the first, second and fourth metatarsal heads as well as the heel as shown in Figure 18. As
can be seen from the figure, peak sheer stress occurs at the second metatarsal; this type of information
can be useful for future high-heeled shoe design.
Figure 18. Mounted transducer in high-heeled shoe [50].
Work undertaken by Healy et al. [51] claimed that their in-shoe system has a better repeatability
compared to other commercially available systems. The sensor is practically similar to F-Scan
(Tekscan, USA) which uses resistive force sensor.
Sensors 2012, 12 9897
The system is named “WalkinSense” and consists of a data acquisition and processing unit and
eight individual sensors. It appears that only the sensor part is their own development, the rest of the
system is similar to F-Scan
(Tekscan, USA) hardware and software. The location of the sensors is
illustrated in Figure 19, whilst the WalkinSense® System is shown in Figure 20.
Figure 19. WalkinSense
sensor placement [51].
Figure 20. WalkinSense
system [51].
All of the works described above [5,6,50,51] have a common feature that is wired to the processing
unit or PC. All of them have certain benefits but for a wired system the major limitation is application in
everyday monitoring. The wired system may encumber the test subject causing trip hazards or even a
fall, and it can also affect the normal gait patterns. Therefore, it is recommended to make the system
mobile for everyday usage, and the system must adapt to a wireless system. Research presented
in [50,51] developed their own transducers but seems to have limited number of sensor placement. As
mentioned in section IV, pressure recording from 15 locations is regarded as ideal for gait analysis.
Papers [5,6] on the other hand used off-the-shelf sensors, which has also limitations as highlighted in
Section 5.
6.2. General Wireless Systems Application
As mentioned earlier, there is a need for a system that can provide a wireless, real-time and reliable
result to measure foot plantar pressure. There have been quite a few works undertaken in both research
and commercial platforms that focused on developing a more mobile method of measuring foot
pressure. Shoe based systems have been increasing, in terms of the number of publications and
Sensors 2012, 12 9898
commercial products, due to shrinking in size of sensors, processing unit communication device and
data storage. Further, the obvious reason for its usefulness is that such a system could measure the
pressure distribution directly beneath the foot.
The work undertaken by Bamberg et al. 2008 [27] from Massachusetts Institute of Technology
(MIT), had been mentioned in a large number of the papers that were reviewed. The main reason why
Bamberg et al. have received so much attention in the literature is that they had come up with arguably
the most complete wireless in-shoe system for gait analysis to date. They called it GaitShoe. In their
system, the sensors included three orthogonal accelerometers, three orthogonal gyroscopes, four force
sensors, two bidirectional bend sensors, two dynamic pressure sensors and electric field height sensors.
The devices were capable of detecting heel-strike, estimating foot orientation and position and toe-off.
The microcontroller (Silicon Laboratories), RF Monolithic (as the transceiver), antenna and power
supply were also attached to the shoes. Figure 21 displays the GaitShoe with all the hardware mounted
on the shoes.
In 2009, Benocci et al. [52] from University of Bologna, Italy developed a wireless system for gait
and posture analysis. The wearable system utilised 24 hydrocells (by Paromed) to measure the plantar
pressure and inertial measurement unit (IMU) in each shoe insole. The IMU integrated a 3-axes
accelerometer and a digital 3-axes gyroscope. To control the system, Texas Instrument MPS430
microcontroller was implemented and Bluetooth acted as the transceiver. The collected data from the
sensor allowed the user to recognize walking phases such as swing and stance, step and stride duration,
double support and single duration. Figure 22 shows Benocci et al. wireless gait shoe.
Figure 21. Shoe-integrated wireless sensor system, GaitShoe [27], showing all the
hardware components.
Figure 22. A wireless systems for gait and posture analysis based on pressure insoles and
inertial measurement units [52].
Sensors 2012, 12 9899
Ming Young Biomedical Corp., Taiwan published a state-of-the-art digital textile sensor for
measuring gait analysis [53]. Four dome shaped sensors were knitted on each sock. The dome shape
sensors were able to record spatio-temporal plantar pressure patterns which were used to calculate the
centre of pressure (COP) excursions. Five clip type sensors were sewn to the pant to record lower limb
movement. The system was reported to measure the duration of stride cycles and left/right steps,
cadence, walking speed, and COP. The microcontroller (Texas Instrument MPS430) and Bluetooth
were attached to the wearer‟s belt. Figure 23 portrays the digital textile sensors in action.
Figure 23. A wireless gait analysis system by digital textile sensors [53].
Shu et al. [30] developed an in-shoe plantar pressure measurement and analysis system based on
fabric pressure sensing array in collaboration with Hong Kong Research Institute of Textiles and
Apparel Ltd. The sensors used were textile fabric sensor array, which is soft, light and has high
pressure sensitivity. The sensors were connected with a soft polymeric board through conductive yarns
and integrated into the insole.
Sensors were attached to six locations in the insole, as shown in Figure 24. The microcontroller
PIC18F452 and the Bluetooth module were attached to the ankle of the patient. The system could
interface with desktop, laptop and smart phone and was able to calculate parameters such as mean
pressure, peak pressure, COP and shift speed of COP. The results were presented for both static and
dynamic measurement conditions. Figure 25 shows the in-shoe plantar pressure measurement and
analysis system based on fabric pressure sensing array.
Figure 24. Fabric pressure sensing array indicating sensor placement [30].
Sensors 2012, 12 9900
Figure 25. In-shoe plantar pressure measurement and analysis system based on fabric
pressure sensing array [30].
The developments of wearable wireless sensor system for measuring foot plantar pressure have
been encouraging. There is no doubt about their application potentials, especially the biomechanics
communities. Nearly all use off-the-shelf sensors, microprocessors and wireless transmitters, so the
end product is bulky and not comfortable to wear by the patients. The digital textile sensors by
Chang-Ming et al. [53] are small and really flexible but it is wired to a Bluetooth based transmitter
device at the belt. Both Benocci et al. [52] and Lin Shu et al. [30] used Bluetooth modules to attach to
the ankle. Even more uncomfortable would be the system proposed by Bamberg et al. [27] where
the whole sensor and wireless communication tools are attached to the top of the shoe. Although
Bamberg et al. has developed the in-shoe gait analysis system but the system is not wearable for daily
activities monitoring.
6.3. Major Application Areas
6.3.1. Rehabilitation Applications
Wireless foot plantar systems have been applied to a number of areas including rehabilitations,
sports and daily life gait monitoring. For example, Crosbie and Nicol [54] indicated that as part of
rehabilitation procedure of patients with spinal cord injury and diabetes, it is quite useful to measure
the efforts exerted by lower limbs such as plantar surface pressure/force distributions and the contact
sensation with the ground. This information is essential in providing better rehabilitation strategies.
Recent publications on wireless systems for rehabilitation applications include work by Neaga et al. [8]
for monitoring the progressive loading of lower limb in post-traumatic rehabilitation, by Wada et al. [9]
for rehabilitation support system and by Edgar et al. [10] on wearable shoe for rehabilitation of stroke
patients. Neaga et al. used F-Scan
(Tekscan, USA) for the sensor, microcontroller based data
acquisition and RF transceiver for wireless communication. The system indicates to the user excessive
loading of the lower limb through LED indicators. Figure 26 presents the prototype shoe and the
prototype hub. Wada et al. developed their system named “GaitGuide”. The system used sensors units
(gyro sensor, acceleration sensor, ultrasonic sensor and pressure sensor), a wireless module, an
electronic tag to collect data from the shoe and display gait information. The “GaitGuide” could collect
the gait information in the form of step length, step width and pressure. The gait information can be used
Sensors 2012, 12 9901
to design a specific rehabilitation program for a particular patient. The “GaitGuide” prototype is
displayed in Figure 27. Edgar et al. [10] indicated that their system can classify patient‟s recovery from
stroke posture. They claim that the system had 99% accuracy in the classification. The system applied a
microcontroller (Texas Instrument, USA), a Bluetooth module (Roving Network, USA), accelerometer
sensor and in-sole pressure sensor. Figure 28 shows components of the developed prototype.
Figure 26. Neaga et al. [8] microcontroller board and shoe prototype.
Figure 27. “GaitGuide” shoe prototype. Modified from [9].
Pressure Sensors
Gyro Sensor
Acceleration Sensor
Wireless Transmitter
Ultrasonic Sensor
Receiver
Ultrasonic Sensor
Transmitter
Sensors 2012, 12 9902
Figure 28. Edgar et al. [10] shoe prototype.
Pressure Sensors
Insole Connector
Insole Connector
Battery
Shoe Board
Bluetooth Module
Insole Connector
Microcontroller
Accelerometer
These published researches [8–10] are all wireless, and all are designed to assist patients with
mobility problems. Figures 26–28, however, indicate that these bulky electronics may not be suitable
for monitoring recovery in post-traumatic patients. Another point worth noting is that these three
systems used commercial foot plantar pressure sensors, and the limitations of commercial sensors have
been highlighted in Section 5.
6.4. Sport Applications
Another application that relies on a wireless system is sport application. Noteworthy mentions are
research by Salpavaara et al. [55] and Holleczek et al. [56]. These two papers employed innovative
new sensors for their application using custom made laminated capacitive sensor matrix and textile
pressure sensors respectively. Salpavaara et al. system can be utilized to monitor the timing and
movement of the legs of the athlete during throwing, jumping and running in various sports events.
The obtained data can be used for improving sports coaching. They opt for javelin throwing for their
case study. In their case study they conclude that the timing of the steps, support and release phase has
a great importance in the performance and the pace of steps should increase towards the end of the
throw event. In their system they employ a capacitance-to-digital converter (Analog Device, USA),
microcontroller (Atmel, USA) and a Zigbee-compliant radio. Figure 29 shows the five sensors
placement. Holleczek et al. developed “SnowPro”, a wearable sport trainer, capable of supporting
snowboarders in improving their skill. The system is able to analyze the dynamics of the weight
distribution inside the boots. This type of information is essential for identifying the wrong weight
shifting techniques, which usually lead to painful crashes in snowboarding sport. The system gives
feedback to the user in real-time or after the activity about user performance and support user during
learning process. This system utilizes three integrated textile pressure sensors, six capacitance-to-digital
converters and a Bluetooth module. Figure 30 displays the final design by Holleczek et al. [56].
Sensors 2012, 12 9903
Figure 29. The five sensors placement of Salpavaara et al. designed system. Modified
from [55].
Sensors 1
Sensors 2
Sensors 3
Sensors 4
Sensors 5
Figure 30. The three sensors placement of Holleczek et al. sensor sock designed. (a) three
sensors placement; and (b) wearable system . Modified from [56].
Sensors 1
Sensors 2 Sensors 3
(a) (b)
The obvious deficiency in the system is the number of sensors used. Salpavaara et al. [55] used five
and Holleczek et al. [56] used only three. For many sport biomechanics applications this number may
not be adequate. Besides that, for sport applications the system should not obstruct the athlete‟s
movement. From Figure 30(b) it is apparent that it is not very practical. This is due to the fact that both
systems utilized off-the-shelf equipment that is usually very bulky.
6.4.1. Other Wireless Systems Application
Other wireless in-shoe foot plantar pressure system that can be highlighted are those proposed by
Saito et al. [57] and De Rossi et al. [58] which employ unique pressure sensors to measure plantar
pressure during daily human activity. Saito et al. device consists of a shoe insole with seven
pressure-sensitive conductive rubber (PSCR) sensors (Yokohama Image System, Japan), 10-bit
analog-to-digital converter and a RF wireless transmission unit. Each (PSCR) sensor is about
15 mm × 10 mm × 0.8 mm, and can measure pressure in the range 25–250 kPa. The seven sensors are
placed at heel, lateral midfoot, great toe, head of the first metatarsal, centre midfoot and centre forefoot
as portrayed in Figure 31. Figure 31 also displays the complete shoe with the power source, wireless
transmitter and pressure measurement unit.
Sensors 2012, 12 9904
Figure 31. The seven sensors placement and the complete shoe. The box inlet is the power
source, wireless transmitter and pressure measurement unit [57].
Saito et al. [57] system has several benefit over the other system, namely they didn‟t adopt a
processing/microcontroller unit attached to the shoe ensuring the electronic circuitry is kept small, and
their processing unit is at the receiving end of the system. This benefit also makes the power
consumption lower compared to other systems, thus the system is capable of monitoring up to 20 hours
with changing power source. On the contrary, the sensor has limited pressure range; the maximum the
transducer could sense is 250 kPa. A typical obese person can generate more than 500 kPa average
peak pressure for both men and women [59] and for sport application for instance during triple jump it
is reported that the maximum pressure can reach around 750 kPa to 1 MPa depends on the athletes [60].
De Rossi et al. [58] employed 64 silicone-covered opto-electronic pressure sensors array, four
16-channel 14-bit analog-to-digital converters, a microcontroller and a Bluetooth module. The sensors
have 12 mm × 12 mm × 5.5 mm dimension, maximum loading of 500 kPa without damaging the
sensor. Figure 32 shows the dimensions of the sensor. The transduction principle of the sensors is
demonstrated in Figure 33. When a load is applied to the top face, the cover causes a deformation, and
lowers silicone „curtain‟ which obstructs the light path from the LED to the photodiode. Therefore,
lower the light from the LED receiving at the photodiode producing lower voltage at the output of the
photodiode. Thus, it is inversely proportional the relationship between input force and output voltage.
The sensors also show no significant static hysteresis. Figure 34 plots the force vs. output voltage
characterization. Figure 35 depicts the complete insole pressure system and the system fitted inside
a shoe.
Figure 32. De Rossi et al. sensor dimension.
Sensors 2012, 12 9905
Figure 33. Working principle of De Rossi et al. sensor.
Figure 34. De Rossi et al. sensor characterization: Force vs. Output Voltage.
Sensor Output (V)
Load (N)
20 40 60
0
-0.25
-0.6
-0.95
-1.3
Figure 35. De Rossi et al. insole sensor system and the system fitted inside a shoe.
Based on the information in reference [58] the sensor design by De Rossi et al. has a number of
advantages. The advantages are the number of sensor placement nearly covers the whole surface of the
foot, the bulky electronics board is well hidden in the medial arch of the foot and the sensor has no
Sensors 2012, 12 9906
significant hysteresis. On the flipside, the sensors has a bad linearity at low and high pressure which
will require a more complex signal processing to ascertain a more accurate representation of the
pressure. Another downside is that the sensor has limited life expectancy, the sensor will damage if a
pressure exceeding 1 MPa is forced on it. The housing for the electronic board is made out of thin
PCB, which the authors mention that it is comfortable to wear.
From the review there is one common limitation in most of these systems, which is the wireless
transmitter/transceiver. If only the wireless transmitter/transceiver could be integrated and minimized
the size it could be inserted inside the insole of the shoe with the entire sensor. This would make the
shoe more wearable for daily life activities, and help with diagnosing foot problems.
7. Proposed Wireless DAQ Foot Plantar Pressure System
Based on the reviewed current in-shoe foot plantar systems, it seems there are some limitations that
could be improved. One area of the improvement could be development in the wireless data
acquisition (DAQ) for the in-shoe foot plantar pressure sensor system. So we propose to design and
implement miniaturised insole, low power and wearable wireless system using customized MEMS
sensors for measuring foot plantar pressure and interface it with wireless DAQ unit that can be also
slotted in the in-sole of the shoe. The research work requires a systematic understanding of different
types of wireless systems on chip, the requirement of MEMS pressure sensor for measuring foot
plantar pressure and realistic scenarios for their implementation and the application. The MEMS
pressure sensors have several advantages compared to others such as small in size, high pressure range,
linear and high reliability. The specific aim of the research is to design a wireless foot plantar pressure
measurement system. The transmitter must be compatible with the MEMS sensor, meet the
requirement of measuring foot plantar pressure analysis and wearable for in-shoe applications. The
receiver should be suitable for interfacing with data logger, desk-top or lap-top for further data
analysis. Figure 36 shows the block diagram of the proposed system.
Figure 36. Block diagram of the proposed system.
Smart Wireless
DAQ and
Control
Smart Wireless
DAQ Analysis
and Display
A wireless DAQ-IC which can be inserted in the insole of a shoe has been designed and simulated [61].
The system architecture is shown in Figure 37. In this design the first task was to transmit data from
only a single MEMS sensor. The layout design of the IC is displayed in Figure 38. The total chip size
of the design including pads is about 1 mm
2
.
Sensors 2012, 12 9907
Figure 37. Block diagram of system design.
MEMS
Sensor
ADC
External
Antenna
Voltage-Controlled
Oscillator
INTEGRATED CIRCUIT
FSK
Modulator
Oscillator
Figure 38. The layout design with padding.
1.06mm
1
.0
3
m m For further improvement of the system, we added more features based on our earlier design. First,
including an analog multiplexer (MUX) to ensure the single DAC chip can cater for all 15 sensors
based on our initial block diagram. Second, after the inclusion of the MUX a controlling unit is added
to the design (Figure 39) to control the whole system design making it a smart system and finally, we
integrated an on-chip antenna thus creating the whole wireless DAQ system in a single chip with only
the addition of the power supply. The new proposed design is depicted in Figure 39.
Figure 39. The new proposed system design block diagram.
MEMS Sensor
MUX ADC Power
Amplifier
On-Chip
Antenna
FSK
Modulator
Oscillator
Voltage-Controlled
Oscillator
INTEGRATED CIRCUIT
MEMS Sensor
MEMS Sensor
Timer/Digital Controller
Sensors 2012, 12 9908
8. Conclusions and Future Work
This paper has reviewed major foot plantar pressure measurement systems reported in the current
literature and available in the market. Firstly, it discussed the available plantar pressure sensors. Then, it
reviewed the latest research on wearable hardwired and wireless sensor systems for gait analysis and
discussed their limitations. Finally, it presented a proposed solution for wearable wireless sensor systems
for sensing foot plantar pressure. Initial IC design results show potential for some good results by the
proposed in-shoe foot plantar pressure measurement system. A power consumption of 19.53 mW was
achieved using charge redistribution successive approximation architecture as the DAQ and a ring VCO
as the FSK modulator. The total chip size of the design including pads is about 1 mm
2
. Analysis of the
full sensor node power consumption showed that this wireless DAQ is sufficient for the intended system
operations. These experimental results are encouraging and show that the system is feasible for
converting the sensor data to digital signals, and hence translating it to its frequency representation and
ready for transmitting. Although the design managed to minimise the dimension, among other significant
considerations is power consumption. Whilst 20 mW is moderately low, in today‟s microelectronics it is
still pretty high. Therefore, the system will be considered to redesign focusing on the power consumption
with a smaller CMOS design process before submitting to a fabrication foundry. The change in process
might also create a domino effect in terms of the output parameter so the new design might need to add
more features on the system such as including a power amplifier for increasing the signal power before
transmitting and redesign the VCO to meet the target ISM frequency band for transmission purposes.
Acknowledgments
This study was funded in part by Victoria University, Australia, Australian Government
Collaborative Research Networks (CRN) program, Malaysian Ministry of Higher Education (MOHE),
and Universiti Teknologi MARA (UiTM), Malaysia.
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