MEASURING LOAD

Background

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.

Summary

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 determining 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 spefic 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.

Brief Description of the Drawings

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 varying 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. 11 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;

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.

Description of Preferred Embodiments

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 Rl 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 frequency /given by

/= l/{2π* ^y l[L*Cl *C2/(Cl + C2)JJ (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 / 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 lOμH.

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 /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 frequency /will be given by

/= l/{2π* ^y l[L*C]} (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 frequency/ 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 they jump. Thus at time tl the jumper left the trampoline mat, while at time t2, they hit the mat again. The interval t from tl to t2 in each trampoline cycle is a measure of the height h 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*t-/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 takeoff 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 tl and t2. 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. 11) 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. 11 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 readout 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, in 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 industry 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 of a joystick.

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 _x j, C _x ?, C _x3 , C _x4 , C _y i, Cy ₂ , C _y3 , C _y4 , C _y5 , C _5O , C _zl and C ₂₂ . Of these, let us suppose that six of the springs (namely: C _x ], C _y i, C _z i, C _x3 , C _y3 and C _y5 ) 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 _x ] and Cχ ₃ . Displacement of the joystick in the y direction is obtained from the sum of the sensed displacements in C _y i and C _y3 . Displacement of the joystick in the z direction is given by the sensed displacement in C _zl . Rotation of the joystick about the x axis is obtained from the sensed displacement in C _yl less the sensed displacement in C _y3 . Rotation of the joystick about the y axis is obtained from the sensed displacement in C _xl less the sensed displacement in C _λ3 . Rotation of the joystick about the z axis is obtained from the sensed displacement in C _y5 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.