GB2446059A

GB2446059A - Measuring load on a spring

Info

Publication number: GB2446059A
Application number: GB0801283A
Authority: GB
Inventors: Jonathan Michael Schaffer
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-01-25
Filing date: 2008-01-23
Publication date: 2008-07-30
Also published as: WO2008090338A1; GB0801283D0; GB0701393D0

Abstract

The displacement of a spring 6 or load applied to spring 6 is sensed by an electric circuit 7 measuring a change in inductance L. The electric circuit 7 comprises a Colpitts oscillator circuit. Pulses produced at a resonant frequency are counted as an indication of the spring's 6 inductance (fig 8). Spring 6 can comprise a helical, coaxial double (21, 22, fig. 5), flat or coach spring (28, 29, fig. 6). Embodiments of the invention include sensing the weight of someone on a trampoline (fig.8) or the height a person jumps (fig. 7), the load on a spring balance (50, fig, 13) or strain gauge, the direction a platform (54, fig. 15) or joystick (62, fig 16) moves, the load a crane lifts (fig. 14) or the actions of a punch ball (34, fig. 9) or punch bag (37, fig. 9).

Description

MEASURING LOAD

This disclosure relates to the measurement of load and to useful applications derived from such measurement.

When a load is applied to a spring, a displacement results. In the case of a perfect elastic solid, the displacement is exactly proportional to the load applied and dependent on its Young's Modulus. To a first order of magnitude and for loads for which the spring is designed, the displacement of practical helical metallic springs or of coach springs is proportional to the load applied.

The methods and apparatus described herein derive from the realisation by the present inventors that a spring if connected in an electrical circuit will have an inductance that varies with changes in its geometry resulting from a displacement caused by a load applied to the spring, so that determining the inductance provides a measure of displacement and hence of load.

In accordance with a first aspect of this disclosure, displacement of a spring, or alternatively the load applied thereto is measured by sensing the change of inductance of the spring as it is displaced or as load is applied thereto.

According to a second and alternative aspect of this disclosure, apparatus for determining a displacement of a spring or of a load applied to a spring comprises electric circuit components coupled to the said spring and adapted to sense the inductance of the spring, change of said sensed inductance implying displacement of the spring due to an applied load.

The method and apparatus may be embodied in a system for the straight forward measurement of a load, as in weighing, for example in a spring balance, or in a load cell, in which, where space allows, a spring sensor as disclosed herein which senses changes in inductance of the spring may replace a strain gauge sensing changes in electrical resistance of a member such as a metal foil.

However, and as explained below with respect to a number of embodiments, particular benefits arise in apparatus, such as trampolines or joysticks in which one or more springs are present as an inherent component of the apparatus. Application of the techniques described herein allows useful results to be derived, which can then be used in other applications, such as for safety or regulation.

In particular embodiments disclosed hereinbelow: The spring is connected as an inductor L in an electrical circuit including capacitance C, and the resonant frequency of the LC circuit is sensed to provide a measure of the inductance L of the spring. The electric circuit comprises a Colpitts oscillator circuit. The Colpitts oscillator circuit produces pulses at the resonant frequency, and is coupled to a counter to count the number of the said pulses occurring in a set period as an indication of the inductance of the spring during the said period. The spring comprises a plate spring, a flat spring or a coach spring, preferably a pair of opposed coach springs. The spring comprises a helical spring, preferably a coaxial double spring.

In one embodiment the spring comprises a spring coupling a trampoline mat to a trampoline frame. The frequency of movement of the trampoline mat is determined by sensing changing inductance of a trampoline spring at sample intervals to thereby give an indication of the weight of a trampoline user. The height jumped by a trampoline user is determined by sensing changing inductance of a trampoline spring at sample intervals and detennining the length of time during a cycle of movement of the trampoline sheet during which the sensed inductance is essentially constant.

In a preferred embodiment, these teachings are applied to three or more springs around the circumference of a trampoline linked to a computer. This allows the computer to determine if a user of the trampoline strays from a central safe zone of the trampoline, or is over a specified safe weight limit for the trampoline, or if more than one user at a time is using the trampoline. Visible or audible alarms can be given if speflc circumstances warrant this.

In another embodiment, a platform is mounted on springs from a fixed support in a hexapod configuration, changes in induction of the individual springs providing an indication of displacement of the platform with six degrees of freedom. In a further embodiment, a joystick, which may be an aircraft joystick, a games controller or manipulator for computer aided design, is provided with a spring sensor of the kind disclosed herein for each of one or more of its degrees of freedom, and preferably all six of its degrees of freedom, displacement of the joystick in such one or more degrees of freedom being sensed by sensing changes in inductances of the said spring sensors.

In other embodiments: A spring sensor is mounted to a punch ball or punch bag to analyse actions of a boxer. Spring sensors are employed to analyse skills of a sportsman in a ball sport. A spring sensor is fitted to a float to provide warning of a person falling into a body of water by sensing displacement of the spring resulting from waves affecting the float.

A spring sensor provides a warning should a crane seek to lift a load that is too heavy to be safe. The spring sensor may also weigh individual loads and the cumulative load lifted, for example, into a ship lying alongside the crane in a dock.

Particular embodiments are described hereinbelow, by way of example only, with reference to the accompanying drawings, in which:-Fig. 1 is a perspective view of a trampoline; Fig. 2 shows a circuit diagram for an electric circuit for sensing the inductance of a spring; Figs. 3 and 4 show circuit diagrams for alternative circuits to that of Fig. 2; Fig. 5 shows a coaxial double spring; Fig. 6 shows a pair of opposed coach springs; Figs. 7 and 8 show experimental pulse counts obtained using the circuit of Fig. 2, being indicative of the vaiying inductance of a helical trampoline spring, and obtained during use of the trampoline; Fig. 9 shows a punch ball and a punch bag each employing the teachings of this

disclosure;

Fig. 10 shows an application of the teachings of this disclosure to a device for giving a warning of a person falling into a body of water; Fig. II shows an application of the teachings of this disclosure to apparatus for analysing the ability of a sportsman in a ball sport; Fig. 12 shows an alternative application of the teachings of this disclosure to apparatus for analysing the ability of a sportsman in a ball sport; Fig. 13 shows a spring balance in which the teachings of this disclosure are employed; S Fig. 14 shows a crane in which the teachings of this disclosure are employed; Fig. 15 is a schematic illustration of how the teachings of this disclosure may be applied to displacement of a platform with six degrees of freedom; and Fig. 16 is a schematic illustration of how the teachings of this disclosure may be applied to a joystick.

Referring first to Fig. 1, a trampoline 1 comprises a trampoline mat 2 suspended by a plurality of springs, here helical springs 3, from a trampoline frame 4 supported by respective leg structures 5 from the ground or a floor.

Fig. 2 shows a spring 6, such as one of the springs 3 shown in Fig. 1, connected in an electric circuit 7 for sensing the inductance of spring 6. One end 8 of spring 6 is connected to ground. The other end 9 is connected via a shielded cable 10, which is chosen to be as short as feasible, to a coupling capacitor 11 connected to base 12 of a bipolar junction NPN transitor 13. Emitter 14 of transistor 12 is connected via resistor Ri and capacitor C2, in parallel to each other, to ground. A further capacitor Cl is connected between base 12 and emitter 14 of transistor 13. Base 12 of transistor 13 is connected via resistor R2 to DC voltage supply V, while collector 15 of transistor 13 is also connected to voltage supply V via a further resistor R3. Output 16 is coupled to collector 15.

The circuit illustrated in Fig. 2 is a typical Colpitts oscillator circuit which will provide a pulsed output at output 16 with a frequencyf given by f= 1/(2irJ[L *(j *C2/(CJ + C2)]) (1) where L is the inductance of spring 6. The pulsed output passes to a counter 17 which counts the number of pulses in the output within a predetermined sampling interval to provide an indication of frequency j and which may be coupled to a computer or microprocessor 18. The values for the resistive and capacitive circuit components given in Fig. 2 are suitable for a spring 6 with an inductance of around 10H.

Colpitts oscillator circuits with a common base topology as in Fig. 3 or a common collector topology as in Fig. 4 may also be used. The frequency f of the circuit in each of these cases will be given by formula (1) above. The bipolar transistor may be replaced by a JFET in any of these circuit topologies.

Spring 6 need not be a simple helical spring 3 as in Fig. 1. In order to connect a simple helical spring such as spring 3, electrical connections would need to be made to both ends of the spring. It is inconvenient to connect a wire from external circuitry to the end of spring 3 looped through an eyelet 19 in edge band 20 of trampoline mat 2 as this end of the spring is in constant movement when the trampoline is in use. Trailing wires from an eyelet 19 would be unsightly. To overcome this problem two adjacent springs may be electrically connected at their inner ends by connecting eyelet 19 of one to eyelet 19 of the adjacent spring. At their outer ends, one spring is connected to ground, while the other spring is connected to coupling capacitor 11 of the circuit of Fig, 2. The electrical components and associated apparatus may be mounted on frame 4 or leg structure 5. In this arrangement, the outer spring ends must be electrically insulated from trampoline frame 4. In effect, the two adjacent springs function as a single spring sensor.

In a preferred arrangement, shown in Fig, 5, rather than a single helical spring, each of springs 3 comprises a coaxial double spring formed from two springs 21, 22 that are connected both mechanically and electrically at 23 adjacent one end. End 23 is mounted to eyelet 19. Their respective other ends 24, 25 are mechanically coupled to trampoline frame 4 but isolated electrically from it. One of ends 24 and 25 forms end 8 of spring 6 connected to ground. The other of ends 24 and 25 serves as other end 9 of spring 6 and is connected to coupling capacitor 11.

We prefer a Colpitts oscillator circuit in the above described trampoline system, since the count at counter 17 may be sampled during successive intervals of as little as 5 milliseconds to provide an effectively Continuous indication of the varying inductance of the trampoline spring and thus of the load therein, allowing useful information to be derived as explained in more detail below.

However, in general, the inductance of a spring, which need not be a helical spring, but may be a flat, plate or coach spring, or a pair of opposed coach springs, as shown in Fig, 6, can be measured in any resonant electric circuit involving the inductance L of the spring and a capacitance C. In general the resonant frequencyf will be given by f i/(2r'J[L *çj (2) The inductance L of the spring varies with deformation of the spring with load, so that a spring may be calibrated in terms of applied load against the sensed frequencyf regardless of the nature of the spring. The resonant circuit is preferably stimulated electrically or magnetically to an electromagnetic oscillation. Preferably, the stimulation happens at the resonance frequency of the circuit. An amplifier may inject energy into the resonant circuit via a feedback loop. Rather than a direct count being made of pulses, as in the Colpitts oscillator circuit arrangement disclosed above a frequency-to-voltage converter may be used, with a digital signal corresponding to the resultant voltage, which can then be inputted to a computer, being derived via an analogue-to-digital converter. In alternative arrangements, an oscillatory circuit may be stimulated at any frequency, the inductance L then being indicated by measuring the amplitude and/or the phase of forced oscillation.

Turning now to Figs. 7 and 8 which show actual experimental pulse counts obtained using a Colpitts oscillator circuit with a single helical trampoline spring, the counts being sampled in 5 millisecond intervals and plotted as sample counts against time in seconds, it will be seen that in both Figures the count, and hence the inductance of the spring, and hence the deformation of the spring, and hence the load on the spring, is varying in an approximately periodic fashion. This corresponds to the use of the trampoline.

The plot of count against time in Fig. 7 shows periods in each trampoline cycle where for an interval t, the count, and hence the inductance of the spring, and hence the deformation of the spring, and hence the load on the spring, is substantially constant. This corresponds to the intervals in each trampoline cycle during which the user no longer makes contact with the trampoline mat as theyjump. Thus at time ti the jumper left the trampoline mat, while at time:2, they hit the mat again. The interval I from II to:2 in each trampoline cycle is a measure of the heigbt Ii of the jump in that trampoline cycle. The height h can be shown to be related to the airtime interval t as follows: h=g*12/8 (3) where g is the acceleration due to gravity. Thus the height h of successive bounced jumps may be determined by computer 18 linked to counter 17. Should height h exceed a preset safety level, computer 18 may be arranged to provide a warning indication via a warning indicator 26 (Fig. 2).

The plot shown in Fig. 8, in contrast, has no flat periods, showing that the user remains in contact with the trampoline mat, but starting from a standing position and building up a bounce. The frequency of the trampoline oscillations shown in Fig, 8 as the user induces energy (amplitude of trampoline movement) preparatory to jumping, is related to the weight of the user. Sensing of that trampoline frequency by the computer 18 enables the computer to tell whether the user is too heavy or too light for the rated trampoline, and allows it to give a safety warning indication via indicator 26 (Fig. 2).

The height and number of jumps, the amplitudes of bounces and the weight of the user together provide an indication of the energy expended by the user during a session on the trampoline.

As each successive bounce of the trampoline is detected as a varying count with a periodicity, computer 18 may be linked to a light system 27 (Fig. 2) adapted to provide a light show in synchronism with the trampoline bounce. The light system may be arranged to change colour depending upon the detected height h of successive jumps.

If three or more spaced springs of the set supporting the trampoline mat 2 from trampoline frame 4 are arranged as spring sensors to have their inductance sensed, as described above, then further useful results can be achieved. Preferably such spring sensors are disposed at equally spaced positions around the mat 2. By comparing the outputs from the three sensors, a vectorial analysis may readily be carried out by a single computer 18 to which respective counters 17 associated with the different springs are coupled, so that not only height of jump (as explained above with reference to Fig. 7), but also position of take-off and landing can be calculated. If the User is straying from a central safety zone of the trampoline mat and is in danger of falling off the trampoline, computer 18 may cause a warning indication to be generated by warning indicator 26. When the User is jumping awkwardly (two-footed take-off or landing rather than take-off or landing with both feet together in the approved manner) this will be detected from the varying counts as two-slightly spaced changes at ti and:2. If the user lands on their back or seat, this will also be noticeable in the varying counts, and the vectorial analysis will detect an apparent wide area of landing. When such conditions are detected, a warning signal can readily be generated from warning indicator 26.

As noted above, this disclosure is not limited to helical springs. Fig. 6 shows a pair of opposed coach or leaf springs 28, 29 coupled mechanically and electrically at one end by an electrically conductive connection 30, and connected mechanically via an electrical isolator 31 at their other ends. The said other ends are coupled to respective electric connectors 32 and 33. It will be appreciated that each of springs 28 and 29 may comprise a set of superposed leaves. When the opposed coach spring structure of Fig. 6 is coupled into a sensing electrical circuit, such as that of Fig. 2, as spring 6, one such connector 32 or 33 will serve as end 8 of spring 6 connected to ground, while the other of connectors 32 and 33 will serve as end 9 of spring 6 connected to coupling capacitor 11.

The teachings of this disclosure are not limited to trampolines. Spring sensors as described herein may be employed for a wide variety of other utilities.

Thus, referring to Fig. 9, a punch ball 34 may be mounted between floor and ceiling by respective helical springs 35, 36. A punch bag 37 may be mounted by means of a flat spring 38 from either ceiling, as here, or floor. In each of these case, by sensing the varying inductance of the spring or springs, the number of punches, frequency or speed of punching, force of punches and accumulated energy use of the boxer using the punch ball or punch bag may all be calculated.

Fig. 10 shows a float 39 in a body of water 40. The float is tethered to the bottom of water body 40 by a tether 41 that includes a helical spring 42. If someone falls into water 40, this will generate waves W resulting in a rapidly varying tension in tether 41 as the waves pass the float. If spring 42 is coupled to an electrical circuit such as that of Fig. 2 to sense variations in its inductance caused by varying tension on the spring, a warning may be generated when wave-caused variations are sensed.

The teachings of the present disclosure may be applied to an analysis of a sportsman's ability in a ball sport. A net 43 (Fig. Ii) is positioned to receive golf-balls driven into it, tennis-balls hit into it, or footballs kicked into it. Net 43 is mounted by springs 44 from a frame 45. With three or more spaced springs 44 serving as spring sensors, as described above for a trampoline, a computer 18 to which the spring sensors are coupled may sense the force and direction of a ball striking the net, as well as the position at which the ball strikes the net, in a similar way to the analysis of the jumping of a trampoline user. Given this information, computer 18 can perform a ballistic analysis to determine where the ball would have struck the ground but for net 43. Thus, for example, the apparatus of Fig. II allows an analysis of a golfer's driving ability or of the accuracy of a tennis player's serve.

Similarly, with a swingball, such as ball 46 (either a tennis-ball or a football) suspended by a rope 47 and spring 48 from a stand 49, as shown in Fig. 12, the power with which a tennis player strikes the tennis-ball or the power with which a footballer strikes the football may be determined from the varying inductance of spring 48.

The teachings of the present disclosure may be applied to a simple spring balance 50 (Fig. 13) embodied, for example as an angler's balance or as a suitcase balance, to sense the inductance of spring 51 of the spring balance. Since the spring balance will have been previously calibrated by count of a counter 17 against weight, this will allow a digital read-out 52 of the weight, for example, of a fish caught by an angler or of a suitcase. The simple spring balance of Fig. 13 may be scaled up to a weighing system applied to a crane as shown in Fig. 14, m which a sensor spring 53 is used to sense the crane's load. This system may be used to weigh individual loads, or simply to serve as a warning system to generate a warning indication in case the crane attempts to lift a weight sensed by sensor spring 53 that is greater than is safe. The system may also provide an indication of cumulative load lifted by the crane, for example, into a ship lying alongside the crane in a dock. If the cumulative load approaches or exceeds the safe payload, a warning may be given.

Conventional load cells employ strain gauges to provide an indication of the applied load. Strain gauges sense changes in the resistance, usually of a metal foil, as a consequence of deformation thereof resulting from a displacement. Where space allows, a spring sensor of the kind disclosed herein, in which changes in the inductance of the spring as a result of displacement is sensed, may replace a strain gauge. Alternatively, the applied load may be determined from changes sensed in the inductance of a spring to which the load is applied.

In general, the teachings herein may be applied to any situation in science or industiy in which it is desired to measure displacement, force, weight or acceleration.

One such application is to an analysis of the movement of a platform or ofajoystick.

Referring to the schematic diagram of Fig. 15, a platform 54 is mounted by a hexapod arrangement of helical springs 55-60 above a base (stator) 61 solid with the ground.

Platform 54 is represented schematically as a rigid essentially triangular structure. Similarly, stator 61 is schematically represented as a rigid essentially triangular structure. It should be understood that, in practice, both the platform and the stator may take any convenient shape.

The diagram of Fig. 15 should be seen as entirely schematic, with practical platform and practical stator reduced to their essential triangles. Each apex of the essential triangle of one structure is shown joined by respective springs to two spaced apices of the essential triangle of the other structure. By sensing the momentary inductions of each of the six springs 55- 60, this will provide an indication of their momentary lengths, from which the coordinates of the platform 54 as a whole in x, y and z Cartesian co-ordinates relative to stator 61, as well as the angular orientation of platform 54 in the three angular degrees of freedom (yaw. pitch and roll) relative to stator 61 may be calculated by a computer.

The system illustrated schematically in Fig. 15 may have many uses from flight cabin simulators for training pilots to games. The six degrees of freedom sensed by the system of Fig, 15 may be applied in a controller for real or virtual reality games. The movement of platform 54 with six degrees of freedom may be applied to control of a real aircraft.

Fig. 16 shows an alternative application of the present teachings to a joystick 62 (which may be the joystick of an aircraft, or a games controller, or a manipulator for use in computer aided design), again shown schematically. The joystick illustrated in Fig. 16 has six degrees of freedom, but the teachings herein may be applied equally well to joysticks with fewer degrees of freedom. In Fig. 16, joystick 62 has a grip surface 63 and an extension 64, and is supported by twelve springs, here identified by the directions in which they are orientated relative to x, y and z axes as follows: C,1, C,, C,, C,, Cr1, C),2, Cy3, Cy4, C, C, C,, and C,. Of these, let us suppose that six of the springs (namely: C,,, Cr1, Cz, C,3, C3 and Cr5) comprise spring sensors as disclosed herein for which their respective displacements are sensed by sensing changes in their respective inductances. Displacement of the joystick in the x direction is obtained from the sum of the sensed displacements in C,i and C,. Displacement of the joystick in the y direction is obtained from the sum of the sensed displacements in C, and C13. Displacement of the joystick in the z direction is given by the sensed displacement in C,,. Rotation of the joystick about the x axis is obtained from the sensed displacement in C),, less the sensed displacement in C),. Rotation of the joystick about the y axis is obtained from the sensed displacement in C, less the sensed displacement in C,3. Rotation of the joystick about the z axis is obtained from the sensed displacement in C), less the displacement of the joystick in the y direction as calculated above.

The above examples of practical applications of the teachings herein will suggest to persons employing springs in different forms of apparatus many other applications in which sensing of the deformation of the spring, and hence of the load applied thereto, by sensing variations in the electrical inductance of the spring, will enable useful results to be derived.

Thus, suspension systems for motor vehicles such as cars (automobiles), lorries, trucks, vans, pick-ups, construction vehicles, motorcycles, train carriages (train cars), freight wagons (freight cars), and aircraft wheels all employ springs. The teachings of this invention may be applied to the investigation of, and monitoring of the suspension systems of any of the above vehicles.

The loading of the vehicle may be monitored.

A computer linked to such a vehicle suspension spring may be embodied as a so-called "black-box" for post-accident investigation in which a continuous looped record is kept, the oldest record being continuously overwritten by current data. Should an accident occur, the sensed inductance of the spring will determine how the suspension moved immediately before and during the accident. In the case of an aircraft undercarriage, the record may establish whether the undercarriage was correctly deployed, and whether the aircraft bounced on landing. In the case of a motor vehicle, the record will show if the vehicle was overloaded for the state of its suspension. Where the teachings are applied to suspension springs associated with different wheels, the comparison of records for different wheels can determine whether and how the vehicle was braked or how it swerved, and whether any wheels left the ground.

In the case of a freight carrying vehicle, the teachings of the present invention applied to its suspension springs enables its laden weight to be determined without recourse to a weighbridge, and a determination to be made as to whether the weight is evenly distributed.

In a motor vehicle, application of the teachings herein may give a warning when a suspension needs replacement.

The teachings herein may be applied to a feedback system in which the dampers of a vehicle suspension are automatically adjusted depending on the roughness of the surface over which the vehicle is travelling, or -over a longer term -depending upon wear in the suspension system that might lead to reduced stiffness of springs, and hence a tendency to increased suspension movement.

Claims

Claims 1. A method for providing an indication of the displacement of a

spring, or alternatively of the load applied thereto, by sensing the change of inductance of the spring as it is displaced or as load is applied thereto.
2. A method according to Claim I, wherein the spring is connected as an inductor L in an electrical circuit including capacitance C, and the resonant frequency of the LC circuit is sensed to provide a measure of the inductance L of the spring.
3. A method according to Claim 2, wherein the electrical circuit comprises a Colpitts oscillator circuit.
4. A method according to Claim 3, wherein the Colpitts oscillator circuit produces pulses at the resonant frequency, and is coupled to a counter to count the number of the said pulses occurring in a set period as an indication of the inductance of the spring during the said period.
5. A method according to any preceding Claim, wherein the spring comprises a spring coupling a trampoline mat to a trampoline frame; and wherein an indication of the frequency of movement of the trampoline mat, and hence of the weight of a User of the trampoline, is determined by sensing changing inductance of said spring at sample intervals.
6. A method according to any of Claims 1 to 4, wherein the spring comprises a spring coupling a trampoline mat to a trampoline frame; and wherein an indication of the height jumped by a trampoline user is determined by sensing changing inductance of a trampoline spring at sample intervals and determining the length of time during a cycle of movement of the trampoline sheet during which the sensed inductance is essentially constant.
7. A method according to any of Claims I to 4, applied simultaneously to at least three springs out of a plurality of springs disposed at spaced intervals around a trampoline and coupling a trampoline mat to a trampoline frame, thereby enabling a computer linked to said at least three springs to determine one or more of whether a User of the trampoline strays from a central safe zone of the trampoline, whether a User is over a specified safe weight limit for the trampoline, and whether more than one User at a time is using the trampoline.
8. Use of methods according to Claims 5 and 6 to provide an indication of cumulative energy expended by a User during a session on the trampoline.
9. A method according to any of Claims I to 4, wherein a platform is mounted on a plurality of springs from a fixed support in a hexapod configuration, and wherein the method is applied to said springs, whereby changes in induction of the springs provides an indication of displacement of the platform with six degrees of freedom.
10. A method according to any of Claims 1 to 4 applied to a joystick, preferably one of an aircraft joystick, a games controller and a manipulator for use in computer aided design, in which displacement of the joystick in at least one, preferably all, of its degrees of freedom displaces respective springs, whereby displacement of the joystick in such at least one degrees of freedom is sensed by sensing changes in inductances of the said respective springs.
11. Use of a method according to any of Claims 1 to 4 to analyse actions of a boxer via a spring mounted to one of a punch bail and a punch bag.
12. Use of a method according to any of Claims I to 4 to analyse actions of a sportsman in a ball sport via at least one spring coupled to at least one of a ball and a target for a ball for said ball sport.
13. Use of a method according to any of Claims I to 4 to detect waves in a body of water, thereby enabling the provision of a warning should an object such as a person fall into said body of water, via a spring mounted between a float and a fixed point, rapid displacement of said spring being indicative of waves.
14. Use of a method according to any of Claims I to 4 applied to a spring coupled to a crane to provide a warning should the crane seek to lift a load that is too heavy to be safe.
15. Use of a method according to any of Claims I to 4 applied to a spring coupled to a crane to provide both an indication of weight of individual loads and of cumulative load lifted into a container, preferably a ship lying alongside the crane in a dock.
16. Use of a method according to any of Claims I to 4 applied to springs forming part of the suspension system of a vehicle to provide an indication of at least one of loading of the vehicle, distribution of the vehicle's load, warning of the state of the suspension, and braking or control of the vehicle, or to provide automatic damper adjustment in the suspension system.
17. Apparatus for providing an indication of displacement of a spring or of load applied to a spring, the apparatus comprising electric circuit components coupled to the said spring and adapted to sense the inductance of the spring, change of said sensed inductance implying displacement of the spring due to an applied load.
18. Apparatus according to Claim 17, wherein the spring is connected as an inductor L in an electrical circuit including capacitance C, and the electric circuit is adapted to sense the resonant frequency of the LC Circuit to provide a measure of the inductance L of the spring.
19. Apparatus according to Claim 18, wherein the electric circuit comprises a Colpitts oscillator circuit.
20. Apparatus according to Claim 18, wherein the Colpitts oscillator circuit is coupled to a counter adapted to count the number of pulses produced by the Colpitts oscillator circuit at the resonant frequency occurring in a set period as an indication of the inductance of the spring during the said period.
21. Apparatus according to any of Claims 17 to 20, wherein the spring comprises a spring selected from the group comprising plate springs, flat springs, coach springs, a pair of opposed coach springs, helical springs, coaxial double helical springs, a spring of a spring balance, a spring coupled to a crane to measure loads lifted thereby, a spring coupled to boxing equipment selected from a punch bail and a punch bag, a spring coupling a float to a fixed point in a wave-detecting system, springs coupling a trampoline mat to a trampoline frame in a trampoline, a spring coupling a platform to a fixed support in a hexapod configuration, a spring coupling a joystick to a reference position and a spring forming part of a vehicle suspension system.
22. Apparatus according to any of Claims 17 to 20, comprising a trampoline mat coupled to a trampoline frame by a plurality of springs disposed at spaced intervals around the trampoline, respective said electric circuit components being coupled to three spaced said springs, and a computer coupled to said respective said electric circuit components; the computer being adapted to report changing inductance of at least one said spring at sample intervals.
23. Apparatus according to Claim 22, further comprising a light system controlled by said computer and arranged to provide a light show in synchronism with the trampoline bounce.
24. Apparatus according to Claim 22, further comprising a warning indicator controlled by said computer and adapted to provide a warning upon an unsafe condition of use of the trampoline being detected by said computer from the sensed displacements of said springs.
25. Apparatus according to any of Claims 17 to 20, comprising a platform mounted on springs from a fixed support in a hexapod configuration, respective said electric circuit components being coupled to individual said springs, whereby changes in induction of the individual springs provides an indication of displacement of the platform with six degrees of freedom.
26. Apparatus according to any of Claims 17 to 20, comprising a joystick provided with respective springs for each of at least one of its degrees of freedom, displacement of the joystick in such at least one or its degrees of freedom being sensed by sensing changes in inductances of the said springs.
27. Apparatus according to Claim 26, wherein said joystick is selected from the group comprising aircraft joysticks, games controllers and manipulators for use in computer aided design.
28. Apparatus according to one of Claims 17 to 20, comprising apparatus for analysing performance of a sportsman in a ball sport and including a target adapted to receive a said ball propelled by a said sportsman, at least one spring coupled to said target, respective said electric circuit components coupled to said at least one spring, and a computer coupled to said respective electric circuit components.
29. A method, substantially as hereinbefore described with reference to the accompanying drawings, for providing an indication of the displacement of a spring, or alternatively of the load applied thereto.
30. Apparatus for providing an indication of displacement of a spring or of load applied to a spring, the apparatus being substantially as hereinbefore described with reference to and as shown in the accompanying drawings.