METHOD AND DEVICE FOR THE REGISTRATION AND ANALYSIS OF FORCES ACTING ON AN INDIVIDUAL'S BODY PART
The invention relates to a method and device for registering and analysing distribution of forces on a subject's body part.
In physiotherapeutic studies there is a great need to obtain information on the temporal and positional distribution of forces on a subject's body part. Examples of such studies include e.g. dynamical studies relating to walking, running and jumping as well as studies relating to equilibrium. In these studies one has to measure forces acting on the sole and find out their distribution within the sole. The application areas include both studies relating to sport performance and purely medical studies. Other subjects of research include e.g. registering of the forces acting on the body when sitting or lying. This is important in the efforts to develop seats and bed cushions for those unable to move or sick to reduce their stress. Information obtained from the study of the sitting process could be used in the tailoring of the seat part of a wheelchair to conform with the user's measures. Likewise bedsores arising from prolonged stay in bed could be prevented. Other applications include e.g. measurement of pressure distribution needed in the fitting of a limb stump to a prosthesis whereby painful injuries due to chafes and strong point loads can be prevented through more accurate machining of the prosthesis. Another area where information on the pressure distribution is important is measurement of occlusion in connection with fitting of a denture or dental filling.
Transducers capable of measuring distribution of forces are available in the market. They are based either on a piezoelectric PVDF-film (PVDF=polyvinylidene fluoride),
piezoelectric crystal elements, resistive strain gauges or purely mechanical film switches. Piezoelectric transducer elements are sensitive and reliable though very expensive. A simple transducer plate made of a PVDF-fil and containing a few measuring points costs tens of thousands of marks. Resistive transducers require an excitation signal to produce a signal and consequently transducer plates based on them become complicated, expensive and sensitive to external disturbances. Mechanical switch matrices provide information only on the exceeding of a definite force limit. In many applications the aim is a transducer plate with tens of measuring points and attachable to uneven surfaces. Hence the suitable transducer plate must be flexible. The transducer plate must stand heavy stresses undamaged. In some applications it should be nearly disposable so that expensive solutions are out of question.
The publication EP 182764, Kari Kirjavainen, describes a so-called EMF-film (Electro Mechanical Film) which is a thin plastic film with very small gas bubbles whose surface has been provided with electric charge. By coating this kind of bubble film on both sides with a thin metal film one obtains a structure with a measurable voltage between the metal films when a variable force is exerted on the film (e.g. when the film is compressed). This film can be applied as a force, pressure, touch or limit switch transducer or a microphone. Correspondingly, when a voltage is applied to the film, it deforms and it can be employed as a loudspeaker or electrical actuator. The advantage of this film is higher sensitivity and low price compared to other similar products like the PVDF film, for example. The film needs no separate control voltage but the charge distribution in it establishes the biasing. Only a suitable current/voltage converter or charge amplifier is needed to measure the signal. The film can be
effectively protected against external disturbances. The film can be easily flexed and it can be stretched on convex surfaces, if need be.
The object of this invention is to provide a novel transducer combination suitable for registering distribution of forces on a subject's body part containing several transducer elements made of the above mentioned bubble film.
The object of the invention is also to provide a method and apparatus which enables one to register and analyse distribution of forces on a subject's body part.
The characteristics of the invention appear in the claims.
The combination of several transducer elements according to the invention, which according to one embodiment is a flexible plate, can in principle be made in several ways.
According to one embodiment the bubble film structure described above is divided into several sections by a suitable dielectric material so that both the plastic films containing bubbles and the metal films covering them of the adjacent sections are completely isolated from each other. This kind of matrix type transducer plate is preferably manufactured so that a bubble film covered with a metal film is cut into small pieces which are attached to each other. According to another alternative a transducer plate can be manufactured from a solid bubble film covered with a patterned electrode plane. One possibility is already in the extrusion stage to divide the plastic film containing bubbles into isolated sections and then coat this product with a metal film within the middle of the sections only so that the metal films of the adjacent sections will not be in contact with each other.
According to another embodiment the transducer plates can be manufactured so that individual finger-like transducer elements made of the metal coated bubble film are embedded in the hollows of a plate made of a flexible dielectric material. Transducer elements of arbitrary shape and suitable for the purpose may be cut from the thin metal coated film (thickness ca. 0.1 mm) and their area may vary from a few millimetres up to square meters. The surfaces need not be flat since the film may be flexed even onto spherical surfaces. The different transducer elements used in physiotherapic applications may have the same or different area.
It is the most practical way to manufacture a sole- transducer combination from separate transducer elements which are attached either right to the sole or to an insole of dielectric material in the shoe. To each foot e.g. 16 transducer elements can be attached. The surface of a seat-transducer combination may form a matrix with 16x16 (or 32x32) transducer elements. The sizes of the regions may be weighed in proportion to the areas. The regions most important for the measurement may have a denser arrangement of transducers. The overall area of the transducer may be of the order of 30 cm x 40 cm, for example. In principle it is possible to make matrix type transducer combinations of very high resolution from said bubble film but the required electronics (e.g. the signal amplifiers) for handling the large number of signals sets the limits.
A varying force acting on the surface of the transducer combination according to the invention produces in each transducer element a charge proportional to the force acting on it. The magnitude of the charge increases linearly with the force. The frequency band of the transducer element is several hundreds of kilohertzes and
the resonance frequency is about 170 kHz.
One embodiment of the invention relates to a method and apparatus for measuring distribution of forces on a subject's body part in ambulatory conditions in which case the stored signals can be handled at a later time and in a separate location. The electric signals from different transducer elements may be amplified, if need be, by buffer amplifiers on the transducer elements. The signals are stored either in a digital or analog memory. According to a preferred embodiment the amplified signals are taken to a so-called measuring card which transforms analog signals into digital form and stores them in the memory card from which they can be transferred e.g. via an RS- 232-interface into a programmed processor for processing.
According to another embodiment the method according to the invention can be performed in laboratory circumstances. In this case the signals converted into digital form are transferred directly through a cable (e.g. RS-232 interface) into a programmed processor for processing.
Suitable signal amplifiers are e.g. surface-mounting components with low current noise which operate in a current amplifying circuit and perform signal preprocessing (integration). The amplifiers are placed appropriately on the fringe of the transducer combination.
The measuring card must have a sufficient number of channels so that the signals from each transducer element can be processed separately. If necessary two or more cards may be used in parallel.
Suitable memory cards are e.g. the PCMCIA type memory cards in which case data transfer from the measuring
instrument into the analysing unit (computer) may be carried out by merely transferring the memory card.
If desired the signals fed into the computer may be visualised with a display terminal or a recorder. Furthermore, the programmed processor may calculate various values from the signals and compare them with one another.
Information obtained by the method and apparatus according to the invention may be widely utilized e.g. in physiotherapy like in the design of artificial limbs, seats, bed cushions, and orthopaedic supports. As another significant field of application we mention measurement of dental occlusion e.g. in connection with tooth filling or fitting of various kinds of dentures.
The invention will be now described in greater detail by means of the enclosed drawings in which
Fig. 1 shows a front view of a section of a transducer element, Fig. 2 shows a side view of the section of Fig. 1, Fig. 3 shows one embodiment of a transducer combination, Fig. 4 shows another embodiment of a transducer combination and Fig. 5 shows a solution for interfacing several transducer elements.
Figs 1 and 2 show the bubble film 11 of a transducer element 10 surrounded by a laminated layer 12. The reference number 11a in the figures denotes a bubble film wire. Reference number 13 denotes a connector and 14 a preamplifier.
Fig. 3 shows a transducer combination 20 of several
individual transducer elements 10 adapted to a sole 30. From the preamplifier 14 the signals go through a flat cable 15 into a data acquisition system 16 which in this solution is connected directly to a computer 17. Alternatively the data acquisition system 16 could have been connected to a portable memory card (ambulatory solution) .
Fig. 4 shows a transducer combination 20 applied in an artificial limb solution. The transducer combination 20 is a tubular arrangement which surrounds the surfaces of a limb stump 50. The artificial limb 40 (the artificial limb 40 is partly sectioned) in turn surrounds the transducer combination 20. Data are collected by separate pressure transducers 10 and taken through the preamplifier 14 and the cable 15 via the data acquisition system 16 into the memory card 18.
Fig. 5 shows in greater detail the connection of the separate transducers 10 to the preamplifier 14 by the connectors 13 and from there to the flat cable 15.
It is obvious to the specialist in the field that various embodiments of the inventions may vary within the scope of enclosed claims.