GB2169684A - Mechanical resonant system with controllable resonance - Google Patents

Abstract

The resilient means, which may be a mechanical spring or a gas spring, which provides the restoring force in a mechanical resonant system is arranged to be switchable between two or more states having differing rates of dependence of restoring force upon displacement from equilibrium. Thus a mechanical spring 11, 12 may be clamped at 13 to switch from the resilience of both parts 11, 12, acting together to the resilience of part 12 alone. By adjusting the time delay between switching, one obtains quasi resonance at any frequency within a range of frequencies. in an alternative system (Fig. 3) a first spring acts on the mass over the whole of its travel, while auxiliary springs act for only part of the travel. Systems using gas springs are described. The invention is applied to apparatus for extracting energy from waves on water. <IMAGE>

Description

SPECIFICATION Mechanical resonant system with controllable resonance This invention relates to a mechanical resonant system with controllable resonance and more particularly to such a system for use in the extraction of energy from waves on water.

It is known to provide an adjustable quasiresonance in a vibrating system by latching, that is by arresting the motion at the extreme amplitude and releasing it after a predetermined time. For example, a system with a natural resonant frequency of 0.2 Hz can be made to resonate at 0.1 Hz by arresting the motion for 2.5 seconds at each extreme amplitude.

This principle is applied in a device of the oscillating liquid column type for extracting energy from waves as described in British Patent No: 2088485B.

Arresting the motion of the moving mass in this way presents difficulties in that there is a discontinuity in the acceleration, that is there is infinite jerk; clamping of the mass can be difficult to effect, for example where this is of water; and there is a high harmonic content in the quasi-resonant motion.

According to the present invention there is provided a mechanical resonant system with controllable resonance comprising a moveable mass and resilient means for providing restoring force which acts upon the mass, the resilient means being switchable between two or more states having differing rates of the dependence of restoring force upon displacement from equilibrium, and means for effecting switching between the said states for periods of controlled duration and such that on switching there is no, or little substantially instantaneous change in force acting upon the mass.

Where the resilient means is a mechanical spring, the spring may operate in tension or compression or may operate partly in compression and partly in tension during oscillation of the mass. A mechanical spring can be switched between states as aforesaid by arresting and releasing the spring at any point between and spaced from its ends, for example by means of a releasable clamp.

Whilst clamping can occur at any moment during a cycle of oscillation, unclamping has to occur when the forces in the spring on each side of the clamp are equal otherwise energy is lost by surging in the springs. This can be effected by a one way latching mechanism which, when triggered, clamps a portion of the spring against movement in one direction but automatically releases when that portion of the spring moves in the opposite direction.

In another arrangement according to the invention, one spring is connected permanently to the mass and another spring or springs is or are engaged by the mass at an adjustable position at which the mass is displaced from equilibrium. The other spring is conveniently a compression spring.

Where the resilient means is a gas "spring", the oscillating mass acting to vary the volume of gas and the gas pressure providing the restoring force, the gas is conveniently contained in two or more chambers intercommunicating via valve means, whereby switching between different states of the resilient means is effected by opening and closing the said valve means. Such a system is analogous to a mechanical spring system but is more readily adapted for controlled switching. Thus, in one arrangement according to the invention, the valve means comprises two valves in parallel which normally operate as one-way valves, flow in one direction being permitted by one valve and not by the other and vice versa for flow in the opposite direction.If, say on a compression stroke the open valve is forcibly closed before maximum compression has been reached, the remaining part of the compression stroke will act upon the gas in one chamber and not the other. On the return stroke, the one-way valve which was automatically closed during the compression stroke will open as soon as the gas pressure across it is reversed, thus automatically recoupling both chambers back into the resonant system at the required moment on the return stroke. It will be appreciated that an analogous sequence applies, with the roles of the one-way valves reversed, for switching during a gas expansion stroke.

Alternatively the two valves may be in series, in which case means for forcibly opening the valves operate to hold open for a selected part of each stroke the valve which would normally be closed during that stroke.

This is, in effect, a single valve, the one way direction of which is reversible.

The invention also provides a device for extracting energy from waves on water including a mechanical resonant system with controllable resonance as aforesaid.

Specific constructions of resonant systems and devices for extracting energy from waves on water embodying the invention will now be described by way of example and with reference to the drawings filed herewith, in which: Figure 1 is a diagrammatic representation of a resonant system comprising a mechanical spring and mass, Figure 2 illustrates diagrammatically a device for clamping and releasing the spring of Figure 1, Figure 3 shows diagrammatically a modification, Figure 4a illustrates diagrammatically a resonant system comprising a mass supported on a gas spring, Figures 4b and 4c illustrate an alternative valve arrangement from that shown in Figure 4a, Figure 5 illustrates diagrammatically an oscillating liquid column device for extracting energy from waves on the sea, Figure 6 is a graph illustrating an aspect of the operation of the device shown in Figure 5, Figure 6a is a graph illustrating a modified mode of operation of the device shown in Figure 5, Figure 7 illustrates a form of so called "Frog" device in which a mass oscillates on a gas spring within a buoy so tethered as to be free to ride on waves on the sea, and Figure 8 is a block diagram illustrating a form of control equipment.

Referring to Figure 1, the resonant system comprises a mass M suspended upon a spring which is divided into two parts 11, 12 not necessarily equal. At the junction 13 a cylindrical sleeve 14 is attached. A clamp 15 is positioned so as to be able to arrest and release when required the motion of the sleeve 14 and hence of the junction 13 between the two parts 11, 12 of the spring.

If the natural resonant frequency of the system with the clamp released is W1, and with the clamp engaged is W2 where W2 > W1, then it is possible by engaging and releasing the clamp at the appropriate moments in the oscillation cycle to achieve quasi resonance at any frequency W in the range W1W-W2.

Clamping occurs when the mass is moving away from the equilibrium position and release occurs on the return when the forces in the two parts 11, 12 of the spring are equal.

Release at any other moment will cause energy to be dissipated in surging of the parts 11, 12 of the spring The total period during each half cycle of the oscillations for which the clamp is engaged determines the frequency W. Thus, with the clamp engaged all the time W = W2 and, at the other limit with the clamp released all the time W=W1.

Figure 2 shows one of a number of possible mechanisms for arresting motion at a chosen position along the length of the spring and for releasing the spring automatically at the right moment on the return.

Referring to Figure 2, rollers 16 are trapped between inclined faces on Y-shaped members 17 and the sleeve 14. The members 17 are pivoted on fixed pins 18 and are biassed by springs 19 with an over-centre toggle action either into the position shown for arresting upward movement of the sleeve 14 or into the opposite position for arresting downward movement of the sleeve 14. In the position shown the rollers 16, retained from falling out by a cage (not shown) arrest the sleeve 14 by jamming in the convergent passages formed between members 17 and the sleeve 14.

However, when the net force on the sleeve 14 from the two parts of the spring changes to the downward direction, the sleeve 14 is free to- move downwards. At the desired instant in the downward half cycle the members 17 are tripped into the- opposite position, when the rollers stop downward motion. Thus it is only necessary to initiate clamping, release occuring automatically at the right moment.

Figure 3 illustrates a modification in which mass M is suspended on a spring S which operates in tension. Change in spring rate is provided by springs 25, 26, 27, 28 which work in compression and come into operation when engaged by the mass M. The position at which such engagement occurs is adjusted by vertical adjustment of the supports 29, 31 for the springs. Release is automatic when the mass M moves out of engagement with the compression springs. The pairs of springs 25, 26 and 27, 28 may, of course, each be replaced with a single spring encircling the spring S.

Ring or forked prongs like the support 29 in Figure 3 may be adjustably positioned respectively above and below the sleeve 14 in Figure 1. The lower prong would then catch the sleeve 14 and arrest its downward motion, with automatic release on return, whilst the upper prong would arrest the sleeve 14 during its upward motion in the other half cycle, again with automatic release on the return.

Figure 4a shows a resonant system in which a mass M is supported on a gas spring provided by gas trapped in chambers 32, 33 by a piston 34. Partition 35 separating chambers 32 and 33 is provided with gas passages controlled by one-way valves 36, 37.

The valve 36 closes automatically when the gas pressure in chamber 32 is higher than in chamber 33. The valve 37 closes automatically when the gas pressure in chamber 33 is higher than in chamber 32. With no external control, the system will resonate with a natural frequency determined by the mass M and the combined resilience of the gas in both chambers 32 and 33.

If, during the upward stroke, valve 36 is forcibly closed, the mass M thereafter continues upwards and downwards acted upon only by the resilience of the gas in the chamber 32. On the return, valve 37 opens automatically as soon as the pressure in chamber 32 exceeds that in chamber 33. Similarly on the downward stoke after the mass M has passed through the equilibrium position, valve 37 is forcibly closed at the appropriate instant and again the mass M is then acted upon only by the resilience of the gas in chamber 32. Once the gas pressure in chamber 32 falls below that in chamber 33, valve 36 opens automatically and the cycle repeats. It will be appreciated that, when a valve has been forcibly closed it will require to be released at some stage after the mass has next passed through the equilibrium position but before that valve is next required to open automatically.It is convenient to release one valve when the other is forcibly closed.

In an alternative valve arrangement shown in Figures 4b and 4c, two one way valves 36a, 37a are arranged in series, one on each side of a single opening in partition 35a. Light springs 81, 82 bias the valves 36a, 37a into the closed position. A finger 83 is hingedly mounted in the space between the two valves and can be moved by remotely controllable means (not shown) to hold open either one or the other of the valves 36a, 37a. Figure 4b shows the finger 83 in a neutral position in which both valves are closed and no flow is permitted in either direction through the opening. Figure 4c shows the finger 83 holding open valve 37a.

In operation, on the compression stroke, valve 36a would be held open initially permitting flow from chamber 32 to 33 and released to close at a predetermined moment (which, at one extreme, can be right at the start of the stroke-ie. valves 36a, 37a closed all the time during the stroke). For the return, expansion stroke, the position of finger 83 is reversed so as to hold open valve 37a until a predetermined moment at which the valve 37a is released to close.

Again, it is important that, after release of one valve for that valve to close, the other valve is opened before the gas pressure across the opening reverses. Generally, it will be convenient to move the finger 83 into position to hold open one valve as soon as the other valve is permitted to close.

The effect achieved in the examples described above is for parts of the cycle to be at frequency W1, thus achieving a quasi resonance at an intermediate frequency W, the value of which depends upon the relative proportions of the cycle at the respective frequencies W1 and W2. Such a system has a number of advantages over a system in which the mass is clamped at its extreme amplitude, zero velocity, positions. Thus, where the band-width required is not great, the harmonic content of the quasi-resonant motion is much reduced, a feature of particular advantage in extraction of energy from sea-waves. The clamping or switching action is easier to apply to the spring, especially where, for example, the mass is water.By avoiding the discontinuity or infinite "jerk" associated with clamping the mass, where the spring is of gas, transient stress waves in the spring are avoided.

Figure 5 illustrates the system applied to an oscillating water column device for extraction of energy from sea waves.

The body 41 floats on the sea and defines two internal chambers 42, 43 which are square or rectangular in plan. Chamber 42 communicates with a passage 44 in which is housed an air turbine 45 driven by bi-directional air flow. The chambers 42 and 43 are separated from one another by a partition 48 in which are located oppositely directed oneway valves 46, 47, the operation of which is analogous to the valves 36, 37 in Figure 4a.

A wave gauge 49 extends downwardly from the partition 48 to provide information about the velocity of the column 50 of water and a rider buoy 51 provides information about the periods of advancing waves.

Restoring force acting upon the mass of the water column 50 is the resultant of the gas pressure in the chamber or chambers above and the water pressure at the bottom of the column. The gas in the chambers 42, 43 thus acts as a gas spring providing part of the resilient restoring force in the oscillating system. Control of the valves 46, 47 thus enables quasi-resonance to be achieved at any frequency between the highest (both valves closed all the time) and the lowest (both valves open all the time).

Referring to Figure 8, the task of the microprocessor is thus to compute from information from the wave gauge 49 and from the rider buoy 51 the required instants for switching the valves 46, 47 and to provide signals accordingly to the valve control gear.

Figure 6 is a graph of velocity against time and shows in chain-line at 53 an assumed velocity profile of an incident wave. The continuous line 54 is the velocity profile of the water column 50 oscillating at its natural resonant frequency in the absence of any forced closure of the valves 46, 47. The dotted line 55 shows the velocity profile of the water column 50 tuned to the incident wave frequency by closing valve 46 at the positions marked 56 on Figure 6, releasing valve 46 and closing valve 47 at position 57. In each case, return to a resonant system in which both chambers 42 and 43 are active occurs with the automatic opening of the "other" valve at the positions marked respectively 56a and 57a on Figure 6.

The lower limit of frequency typical for waves of useful amplitude is of the order of 0.5 s 1. Resonance control in the manner described above can thus be applied provided the device can be constructed with its lowest (natural) frequency of oscillation at or close to this lowest expected wave frequency.

For an oscillating water column device (Figure 5) in particular, this presents difficulties in that, to achieve a low natural frequency of oscillation, the device either has to be very large or has to include means, such as a venturi system, for increasing the apparent inertia.

With the latter, the inherent losses lead to inefficiency which may not be recouped by the ability to achieve quasi resonance with a range of incoming wave frequencies.

A solution is illustrated in Figure 6a. For this the lowest natural frequency (valves 46 and 47 allowed to open) is high (eg 0.7 s-') relative to the lowest expected wave frequency.

To reduce the frequency of quasi resonance, in each half cycle switching is applied to permit one or more complete oscillations at the higher frequency of the device (both valves 46, 47 held closed). Thus, referring to Figure 6a chain line 53a shows an assumed displacement profile of an incident wave. The continuous line 54a is the displacement profile of the water column 50 oscillating at its natural resonant frequency in the absence of any forced closure of the valves 46, 47. The dotted line 55a shows the displacement profile of the water column 50 quasi tuned to the incident wave frequency by closing valve 46 and valve 47 at the positions marked 56p and 56q on Figure 6a, holding both valves closed until one complete cycle of oscillation at the high frequency has taken place and then releasing the valves in appropriate sequence.Thus, on the completion of one cycle of high frequency oscillation after position 56p, the valve 47 is released and thus opens automatically at position 56r, at the moment when pressure in the chamber 43 falls below the pressure in the chamber 42 for the second time following the closing of the valves (at position 56p). Valve 46 has to be held closed until just after valve 47 has opened automatically in this way. In the other half cycle, valve 46 is released one complete cycle of high frequency oscillation after position 56q and opens automatically at position 56s, at the moment when pressure in the chamber 43 rises above the pressure in the chamber 42 for the second time following the closing of the valves (at position 56q).

Valve 47 has to be held closed until just after valve 46 has opened automatically.

It will be seen that the high frequency w2 (both valves closed) must be quite high and that the radiation loss in higher harmonics is likely to be large, more than off-setting the beneficial 'squareness' of the ends, which will give good capture because the amplitude of the fundamental is often greater than the maximum excursion.

Unfortunately, the bandwidth obtainable is rather small. This could be remedied by allowing another full 'vibration' in the high frequency mode, thus giving two ranges of bandwidth, one in which, after closing the valves, release occurs to permit the appropriate valve to open on the second reversal of pressure difference between the chambers 42 and 43, and the other in which, after closing the valves, release occurs to permit the appropriate valve to open on the third reversal of pressure difference between the chambers 42 and 43.

In Figure 7 a so called "Frog" device comprises a large buoy 61 moored so as to be free to follow the up and down movement of the surface waves.

Within the buoy 61 a cylindrical mass M is free to move up and down in a cylinder 62 fixed to the main body of the buoy 61. Two partitions 63, 64 divide the interior of the buoy into three gas spaces 65, 66, 67.

In the partition 64 between gas spaces 66 and 67 are oppositely directed one-way valves 68, 69.

As with the example of Figure 5, a rider buoy 71 provides information about the period of incident waves.

The resonant system comprises the mass M supported by the gas spring of the gas contained in chambers 66 and 67. There is also a contribution from gas in chamber 65. Control via the valves 68 and 69 is analogous to that described for Figure 4.

Valves may be provided in the partition 63 instead of the partition 64, or possibly as well as the valves in partition 64. In the latter case more complex effects can be introduced with corresponding added complication to the control system.

The invention is not restricted to the details of the foregoing examples.

Claims (13)

1. A mechanical resonant system with controllable resonance comprising a moveable mass and resilient means for providing restoring force which acts upon the mass, the resilient means being switchable between two or more states having differing rates of the dependence of restoring force upon displacement from equilibrium, and means for effecting switching between the said states for periods of controlled duration and such that on switching there is no, or little substantially instantaneous change in force acting upon the mass.

2. A mechanical resonant system as claimed in claim 1, wherein the resilient means is a mechanical spring means.

3. A mechanical resonant system as claimed in claim 2, wherein the mechanical spring means is switched between states by means for arresting and releasing the spring means at a point between and spaced from its ends.

4. A mechanical resonant system as claimed in claim 3, wherein the means for arresting and releasing the spring means comprise a releasable clamp.

5. A mechanical resonant system as claimed in claim 3, wherein the means for arresting and releasing the spring means comprise a one way latching mechanism which, when triggered clamps a portion of the spring means against movement in one direction but automatically releases when that portion of the spring means moves in the opposite direction.

6. A mechanical resonant system as claimed in claim 2, wherein the mechanical spring means comprises one spring connected permanently to the mass and another spring or springs engaged by the mass at an adjustable position at which the mass is displaced from equilibrium.

7. A mechanical resonant system as claimed in claim 6, wherein the said other spring or springs is a compression spring or are compression springs.

8. A mechanical resonant system as claimed in claim 1, wherein the resilient means is a gas "spring", the oscillating mass acting to vary the volume of the gas and the gas pressure providing the restoring force, the gas is contained in two or more chambers intercommunicating via valve means, and switching between different states of the resilient means is effected by opening and closing the said valve means.

9. A mechanical resonant system as claimed in claim 8, wherein the valve means comprises two valves in parallel which normally operate as one-way valves, flow in one direction being permitted by one valve and not by the other and vice versa for flow in the opposite direction.

10. A mechanical resonant system as claimed in claim 8, wherein the valve means comprises two valves in series, and valve opening means are provided for forcibly opening the valves, the valve opening means being operative to hold open for a selected part of each stroke the value that would normally be closed during that stroke.

11. A mechanical resonant system as claimed in any of the preceding claims incorporated in a device for extracting energy from waves on water.

12. A mechanical resonant system substantially as herein described with reference to, and illustrated in, Figure 1, or Figure 2, or Figure 3, or Figure 4a, or Figure 4b, or Figure 4c, or Figures 5 and 6 and 8, or Figure 7 of the drawings filed herewith.

13. A mechanical resonant system substantially as herein described with reference to, and illustrated in, Figures 5 and 6a of the drawings filed herewith.