The pressure platform, or baropodometric platform, measures the vertical component of the force thanks to a sensor array and, knowing the active area of each sensor, allows knowing the pressure response as the average force on the area (P=F/A).
It is important that the sensors used have a high sampling frequency to minimise errors.
The sampling frequency is expressed in Hertz and shows how many times per second an analogue signal is measured and converted into digital form. Each signal measure corresponds to a stored number, called sample.
Acquiring at a low sampling frequency, we will obtain few output values, therefore an inaccurate and poorly detailed acquisition.
Acquiring with a higher sampling frequency, the recorded data will be more and therefore the acquired signal will be more and more like the original.
Performing an exam at a high sampling frequency allows obtaining a high number of samples, in order to reproduce in the most accurate possible way the acquired signal.
During sampling, errors may occur, but applying the Nyquist-Shannon theorem, we will know the ideal sampling frequency in order to obtain a reconstruction as complete as possible of the original signal from its samples.
With the last generation platforms, with sampling frequencies higher than 400 Hz, it is, therefore, possible to accurately study the movement under examination.
In the case of dynamic analysis, with a sufficiently high sampling frequency, it is possible to analyse the Gait Line variations (that represents the trend of the centre of pressures during the step), thus being able to recognise and distinguish an unstable subject from a stable one.
The pressure platform consists of sensors, that is of tools that can convert the input variable to be measured into an easier to work variable. The most used are the resistive and capacitive ones, that differ depending on the output size they provide.
The operating principle of resistive sensors is based on the intensity variation of the electrical current flow according to the pressure applied to the surface of the sensor itself. The current going through the resistive sensor rises under pressure because of the conductive layer changes (resistance decreases). When the actual contact area of the sensor increases because of pressure, the electrical conductivity increases, in an approximately linear manner, within a certain range of pressure.
As for capacitive sensors, their operating principle is based on the change in capacitance caused by the pressure applied to the surface of the sensor. The sensor consists of two conductive plates, electrically charged and separated by a dielectric layer. By applying a pressure on the sensor, the dielectric layer bends, thus shortening the distance between the two plates and generating a voltage variation proportional to the pressure applied.
The various sensors can be prone to problems that can affect their performance; the main being hysteresis, creep and saturation.
We have a hysteresis when the value of a magnitude depends, in addition to instantaneous values, on the previous values too. Therefore, the system reacts late to the applied strains and depends on the previous states.
The sensor is affected by creep when there is a continuous increase of the output response even when the input load is constant. It directly relates to the time-dependent deformation of the materials when subjected to a constant load.
Saturation is a problem that affects a sensor when the load variations, beyond a given threshold, tend to have a value closer to an extreme value, which is the highest value compatible with the system.
As for capacitive sensors, these problems are solved using more complex electronics with significant costs, yet limiting the overall size of the products and the signal sampling frequency.
For the resistive sensors, it is possible to compensate and solve these problems appropriately and with a low-cost maintenance thanks to the use of performing techniques, instruments and software.
A baropodometric platform is an easy-to-use tool, both for static and dynamic studies.
The subject under exam needs some time to become familiar with the tool, in order to perform a movement as natural as possible. In addition, it is important that the foot touches the sensitive area of the platform.
The exam carried out with a pressure platform allows to:
- objectively observe the phenomenon;
- quantify the main parameters of the phenomenon;
- store in a database a large number of each patient’s exam, to compare, even at a later time, the variations and possible improvements that have arisen, for example, after a rehabilitation treatment;
- carry out screening tests in order to prevent and treat various diseases on time.
The main parameters studied during with the pressure platform are:
- Ground reaction forces,
- Mass of the subject.