WO1998017155A1

WO1998017155A1 - Variable speed rotary vibrator

Info

Publication number: WO1998017155A1
Application number: PCT/GB1997/002888
Authority: WO
Inventors: Gary Alexander Bowles; Donald Anthony Acott Roworth
Original assignee: Locus Technology Limited
Priority date: 1996-10-21
Filing date: 1997-10-20
Publication date: 1998-04-30
Also published as: AU4713997A; GB9621897D0

Abstract

Apparatus and method for producing a varying force by the rotation of an unbalanced mass (1) at a non-constant angular velocity. The system may include an unbalanced mass (1) rotated by way of a uni-directional coupling (3). The apparatus is particularly directed to the rocking of a baby's seat, cot, pram or crib or a hammock for an adult or child.

Description

VARIABLE SPEED ROTARY VIBRATOR

The present invention relates to apparatus for producing a varying force and particularly, although not exclusively, to apparatus for rocking a baby's cradle or seat. The invention also has many other potential applications in diverse fields of engineering and product design including (by way of example) feed systems, vibrators, and therapy systems.

It is well known that a regular rocking motion can be very soothing for a fractious baby, especially if accompanied by pleasant and repetitive sounds which can include the mother's heartbeat. An early method of achieving rocking mechanically was a clockwork escapement mechanism applied to a swinging cradle. More recently, electrically-driven mechanisms using rigidly driven eccentric masses, rotated at a substantially constant angular velocity,

have been available, generally designed to be attached to any item of baby equipment or nursery furniture which can be made to rock or otherwise

oscillate.

It is well known that periodic forces are generated by an unbalanced

rotating mass. If the rotating mass is circular, the forces may be felt by contact

with the periphery, or (whether circular or not) they may be transmitted by an axle to a journal or bearing. For example, railway locomotive wheels impart an undesirable "hammer blow" to the track unless the whole system of the wheel

and any coupled parts is carefully balanced. The direction of the out-of-balance force (the centripetal force) generated by an unbalanced rotating system rotates at the angular velocity of the system (ω). The force resolved in any particular direction shows a sinusoidal variation.

The sinusoidally varying force can be resolved in any direction in the plane of rotation, but the phase alters with the direction chosen. Changing the rate of rotation alters the frequency of variation of the force and also alters its amplitude according to the square of the change in rotary speed.

There are many established applications where this effect is put to practical use to vibrate or oscillate a work-piece or load to which the mechanism is attached, but its capability has severe restrictions when applied to slow speed applications (for example, where the periodicity is of the order of seconds).

The magnitude of the out-of-balance force is dependent upon both the

size of the out-of-balance mass and its rate of rotation. To increase the magnitude of the out-of-balance force for a given system requires either or both the mass or rate of rotation of the system to be increased.

Where an out-of-balance rotating mass is used to drive a periodic system, for example, a baby's cradle, it is preferable for the frequency of rotation of the

mass to be equal to or a factor of the natural frequency of the driven system.

This restricts the rate at which the out-of-balance mass may be rotated. Therefore, in order to generate a driving force of sufficient magnitude at a given frequency it is necessary to increase the mass of the out-of-balance mass. This is inconvenient as the heavier a particular mass the more expensive and difficult to handle it becomes. When a mass is attached to a cradle, for example, it is also important that it is not so heavy as to significantly unbalance the cradle.

As the natural period of oscillation of a typical baby + chair + rocker combination is low, being typically of the order of 2 seconds (i.e. 30 cycles per minute), a rigidly driven eccentric mass rocking system is ineffective for the reasons analysed above, and the amount of energy which it can transfer (and so the amplitude of movement which it can achieve) is severely limited. Attempts to increase this by making the rocker unit larger and heavier tend to be largely self-defeating because the increased weight further reduces the natural frequency of the rocker/chair/baby combination and may unbalance the system.

Another problem associated with the use of an unbalanced rotating mass to drive a periodic system is that the amplitude of the out-of-balance force is the same in all directions in its plane of rotation, so that, unless the load is constrained to move only in a plane it can be vibrated in a circular motion. For example, where a mechanism of this sort is applied to rock a baby seat, it

would tend to impart a side-to-side motion to the seat as well as a vertical one. The sideways motion is undesirable and can be dangerous because it may cause

the seat to "walk" from its original position. Other systems for imparting a periodic force use reciprocating parts, for example, with a spring which is progressively compressed and then, when released, drives a mass in a straight line to an end stop. These are relatively complex and generally less reliable, require more maintenance, and are more noisy than simple out-of-balance rotating systems.

It is an object of the present invention to overcome, or at least mitigate some or all of the performance disadvantages of the simple rotary out-of- balance system, whilst retaining its simplicity and absence of reciprocating parts.

According to a first aspect of the present invention there is provided apparatus for producing a varying force comprising a rotatably mounted unbalanced mass and a means for rotating the unbalanced mass to produce an out-of-balance force wherein said means enables the mass to be rotated at a

non-constant angular velocity.

By rotatably mounted unbalanced mass is to be understood a rotatably

mounted mass the centre of mass of which is displaced from the axis of

rotation.

Preferably the unbalanced mass is rotated so as to produce a

substantially periodic force. Preferably the means for rotating enables the angular velocity of the mass to be varied throughout a single revolution. More preferably the means for

rotating the mass enables the angular velocity of the unbalanced mass to be varied in a predetermined manner, preferably in a repeatable manner. For

example, to accelerate the rate of rotation of the unbalanced mass over a

predetermined part of a revolution and then decelerate the rate of rotation over

the remaining part of the revolution, so as to increase the out-of-balance force

in a given direction.

By varying the rate of rotation of an unbalanced mass it is possible to

increase the out-of-balance force in a given direction whilst maintaining a

desired periodicity.

In one embodiment the means for rotating the mass comprises a motor

which may rotate at constant angular velocity, connected to the mass by means

of a uni-directional rotational coupling. The motor preferably rotates at a

substantially constant angular velocity. The uni-directional coupling is

preferably disposed so that the motor can drive the mass in one direction only

and that the mass can overrun the motor, that is, rotate faster than the motor

in the direction that it is driven. This embodiment is particularly addressed to

applications when the plane of rotation of the unbalanced mass is other than

horizontal.

Where the plane of rotation of the out-of-balance mass is not horizontal the rotation of the mass will tend to be accelerated by gravity during a part of its rotation, thus producing a non-constant angular velocity.

The motor preferably comprises an electric motor. The motor may drive

the mass by way of a gearbox, belt, or any other suitable transmission means.

The uni-directional coupling is preferably effected by a one way roller bearing

although any other suitable means may be employed. It is preferable that the

uni-directional coupling has a minimum of backlash.

Preferably there is also provided means to enable the overall periodicity

of the out-of-balance mass system to be adjusted, to enable the system to be

tuned to the natural frequency of the system to be driven. Where the drive

means comprises a motor and uni-directional coupling the drive means

preferably also comprises a means to vary the rate of rotation of the motor.

The apparatus is preferably self contained. Means for storing a battery

to run the drive means are preferably provided on the apparatus. Rechargeable

batteries may be employed.

There is also preferably provided a control means for controlling the drive

means. The control means may comprise a timer to enable the drive means to

operate for a predetermined period of time and then to stop automatically.

The control means may also include a means to automatically adjust the overall periodicity of the system to the natural frequency of the driven system. To achieve this the control means may include one or more sensing means.

The apparatus is preferably provided with a means to enable it to be

secured to a child's seat, cot, cradle, hammock for use by adults or children or other object, this means preferably comprises a bracket, strap or straps.

Alternatively the apparatus may be integrated with a child's seat, cot, cradle or

other object.

When the apparatus is intended for rocking a baby's cradle, and indeed

for other applications, it is desirable that the apparatus is balanced in order that it can be easily attached to the cot, cradle or other object. For instance, when

the apparatus is suspended or secured at two points it is desirable that the

centre of mass of the device lies substantially at the midpoint between the

suspension points.

According to a second aspect of the present invention there is provided

a method of producing a varying force comprising the steps of rotating an

unbalanced mass to produce an out-of-balance force wherein said mass is

rotated at a non-constant angular velocity.

By rotatably mounted unbalanced mass is to be understood a rotatably

mounted mass the centre of mass of which is displaced from the axis of

rotation. Preferably the angular velocity of the mass is varied throughout a single rotation of the mass. More preferably the rate of rotation is varied in a

predetermined and repeatable way throughout each revolution.

In order that the invention may be more clearly understood there are now

described embodiments thereof, by way of example and with reference to the

accompanying drawings in which:-

Figure 1 a shows diagrammatically the manner in which a rotating out-of-

balance mass generates a force;

Figure 1 b shows a graph of the force generated by the mass illustrated

in Fig.1 a, resolved in a particular direction;

Figure 2a shows a perspective view of a first embodiment of the

invention;

Figure 2b shows an end elevation of the embodiment illustrated in Fig.2a;

Figure 3 shows an embodiment of the invention mounted on a baby

carriage;

Figure 4 shows an embodiment of the invention mounted on a baby's

chair; Figure 5 shows an embodiment of the invention mounted on a baby's rocking chair, the chair is resting on a carpeted surface;

Figure 6a shows a part perspective view of a further embodiment of the present invention;

Figure 6b shows a side elevational view of the embodiment illustrated in Figure 6a, hidden detail is shown by way of dashed lines;

Figure 7a shows a part cross-section of a third embodiment of the invention;

Figure 7b shows an end view of the embodiment illustrated in Figure 7a, secured to a part of a baby chair;

Figure 8 shows a part cross-sectional view of a still further embodiment

of the invention;

Figure 9a shows a side elevation of a rocking chair mounted on runners;

and

Figure 9b shows a part cross-sectional view of a runner illustrated in

Figure 9a. Referring to Figure 1 a, when mass M is constrained to travel in a circular path force f is generated as follows:-

f = mω²r Newtons where f is the force acting radially ω is the angular velocity in radians/sec m the effective unbalanced mass in kg and r the effective radius of rotation of the mass in m.

Figure 1 b shows a graph of the force produced by the mass m of Figure 1 a resolved in a particular direction, plotted with respect to the angular position θ of the mass m. The force varies sinusoidally with maximum and minimum values of mα/²r and -mω^zτ.

From the above it can be seen that the amplitude of the radial force generated by a rotating out-of-balance mass is dependent upon the frequency

of rotation and that to generate a significant force requires a large unbalanced mass and/or a large radius, especially at slow rotational speed. For very slow rotation the force becomes minimal, even with a substantial rotating mass,

because of the square law relationship between force and angular velocity

Referring to Figures 2a and 2b the apparatus comprises an out-of-balance mass rotor 1 mounted on a shaft 2 by way of a one way roller bearing 3. The

shaft 2 is connected to motor 4 by way of reduction gearbox 5. The one way roller bearing allows the rotor 1 to rotate in the direction of arrow 1 a, relative to the shaft 2. The intended direction of rotation of shaft 2 is shown by the

arrow 2a.

The motor 4 and gearbox 5 enable the shaft 2 to be driven at a low, say

between 10 and 100 revolutions per minute, but adjustable and substantially

constant rate of rotation.

The mode of operation of the device is as follows: Consider the rotor 1

initially to have its centre of mass M rising, being raised slowly but steadily by

the rotation of the output shaft 2 via the locked one-way roller bearing 3. As

the centre of mass M passes its highest point 6, it will fall forward as it over¬

runs the shaft and is no longer restrained by the one-way roller bearing 3. The

rotor will accelerate under gravity, and will reach its maximum angular velocity

when M passes the lowest point 7, imparting a significant impulse vertically

downwards as it does so. As it continues to rotate further, the rotor 1 will slow

down as its centre of mass M rises, reducing angular velocity until it is again

locked onto the shaft by the one-way roller bearing 3 at about point 8 when the

rates of rotation of the shaft 2 and rotor 1 coincide.

In effect the rotor is coupled to the shaft only for a portion of the rising

part of the cycle. For the rest of each cycle it is free to rotate, spending

approximately the first 1 80 ° falling under gravity and a part of the next 180 °

decelerating under gravity until the constant rotation of the shaft "catches" it. The exact proportion of the cycle for which the rotor is driven by the shaft will depend upon the rate of rotation of the shaft and the amount of energy which

is imparted to the load during the impulse around its lowest point. The smaller the part of the cycle for which the bearing is locked, the higher the impulse rate

for a given shaft speed.

In this embodiment it will be seen that (i) the effective rate of rotation of

the rotor is greater than that of the shaft (ii) the force exerted on the load is

impulsive, not sinusoidal (iii) the impulse is always downwards (iv) for slow

rates of rotation, the amplitude and shape of the impulse are fixed and

independent of the rate of rotation.

It can be shown that the maximum impulse force F_A exerted by the freely-

rotating unbalanced rotor (i.e. at the point where its centre of mass is vertically

below the axle) is given by

F_A = mω²r + 4mg

where g is the acceleration due to gravity (i.e. 9.81 m/s²) and the other

variables are as defined before, whereas for a fixed drive rotor the force F_B at

the same position is given by

Fo = mα/ r The ratio of the "free-fall" force to the "fixed " force for a given set of parameters is thus

F_A/F_B = 1 + (4g/ω²r)

Taking a typical rotational speed of the shaft as 60 revolutions per minute and the radius of rotation of an unbalanced mass of 0.1 kg as 25 mm, it can be calculated from the above that the downwards impulse imparted to the load as the freely rotating centre of mass passes its lowest point is approximately 40 times greater than that obtained from the same weight and size of rotor if the rotor were permanently fixed to the shaft. The downwards momentum imparted to the load is (as required) much greater than the upwards or transverse momentum.

At low rates of revolution, changing the shaft speed has very little effect on the amplitude of the impulse, but its frequency will change and can be

simply adjusted to be close to resonance with the natural frequency of the load. Because of the non-sinusoidal force waveform, the impulsive force has a wider spectrum and will continue to impart significant energy to the load even if

somewhat away from synchronism.

Compared with the established method of rotating an out-of-balance rotor at a constant angular velocity by a rigid drive, it can be seen that this invention enables the same effect to be obtained with a much reduced weight, especially when the periodicity is low. This gives a considerable improvement in safety, the reduced weight being much less likely to destabilise the load or to cause

damage should it become detached.

It is known that to obtain the maximum transfer of energy between two coupled oscillating systems they need to be operating at the same frequency

but out of phase. Thus to obtain the maximum effect when an out-of-balance

rotating mass is applied to a load (for example, to rock a chair), the periodicity

of rotation must be adjusted to match the natural period of oscillation of the

chair. This may simply be achieved by a manual adjustment to the voltage

applied to the drive motor (for example, by means of a potentiometer), but if

there is no feedback means to maintain synchronicity under changing load or

drive characteristics coupling efficiency will be lost and the effect on the load

will diminish. The present invention shows a significant advantage over a

simple rigid drive system in this respect, because it has an inherent degree of

self-adjustment to the natural period of oscillation of the load. This arises from

the free-wheeling effect of the one-way coupling: the angle through which the

rotor turns freely before being "caught" by the coupling is greater if the drive

is lagging the load and less if the drive is tending to lead the load, due to the

relative movement of the free rotor and the load. Thus the effective periodicity

of the drive will reduce in the former case and increase in the latter case,

tending to correct for unequal periodicities of drive and load. Because this

process is effective in providing such stabilising negative feedback over only a

limited range either side of the "correct" periodicity, it requires the user initially to set the drive to approximately the correct speed. However, once set it is effective in maintaining a closer synchronism than the conventional fixed-rotor approach where there is no such feedback mechanism.

Referring to Figures 3 to 5 there are illustrated a number of possible mounting positions for embodiments of the present invention, indicated as 9 in each Figure. In Figure 3 the rocker 9 is attached by straps 1 1 to the handle of a baby carriage 10 which it causes to oscillate on its springs. A second example is shown in Figure 4, where a baby chair 12 having a springy frame 13 has the rocker 9 attached to the top of the chair by adaptor 14.

In general to obtain the maximum effect, it is necessary to place the rocker as far as possible from the pivot point to obtain the maximum leverage. This is achieved with the baby carriage 10 by attaching the rocker to the handle, and with the chair 12 by attachment to the top of the frame.

When the load to be driven is lossy, as shown in Figure 5, where a baby chair with integral rockers 15 is placed on a soft carpet 16 which absorbs

energy as the chair rocks, there is no opportunity for the amplitude of oscillation

to build up from a small sinusoidal driving input, and a fixed-drive rocker with

its low energy output can be ineffective.

Referring to Figures 6a and 6b, a further embodiment of the invention is illustrated comprising an eccentric rotor which is formed from a shaped disc 17 of material, rotated on its axis by a motor/gearbox combination 1 8 to which it is coupled. The drive between gearbox and disc is turned through 90 ° by

means of a bevel gear or spur gear set 1 9, resulting in a compact unit.

The disc is coupled to the drive by means of uni-directional coupling 21 .

Alternatively the gearbox may be so constructed that one of the gears

disengages under reverse torque, a technique well established in clockwork

mechanisms to allow winding without disconnection from the load.

Referring to Figures 7a and 7b there is illustrated a further embodiment,

contained in a substantially cylindrical housing 31 , which may be attached to

the handle 51 of a baby chair by (for example) straps 22.

This embodiment employs an electric motor/gearbox combination 23

controlled by a switch 25 and speed control potentiometer 24 to enable the rate

of rotation of the motor to be adjusted.

The motor/gearbox 23 drives out-of-balance mass 26 via shaft 27 and

one way roller bearing coupling 28. The out-of-balance mass 26 is cast or

machined from a substance of high density such as steel or cast iron, and is

linked to shaft 27 by one-way coupling 28.

Bearings 29 support the motor shaft and the rotor, and must be of

sufficient strength to withstand the out-of-balance forces as the rotor rotates. The cast or moulded housing 21 may be designed so that it is made from two identical halves (to minimise tooling cost), each of which may incorporate ears 30 for the attachment of straps or clips by which the unit is secured to the load. The housing is preferably produced from a plastics material.

The embodiment of Figure 7 gives a long slim unit ideally suited to attachment to the handle of a pram or push-chair.

The motor 23 is preferably a dc, permanent magnet electric motor. The power supply may be external although preferably there is made provision for

the storage of batteries on the body 31 of the apparatus (not shown).

Figure 8 shows a still further embodiment, in which the rotor is in effect folded back to rotate around the motor. The cylindrical case 32 is again cast

or moulded from two identical halves and incorporates ears 33 for attachment purposes. The motor/gearbox combination 34 is supported at the rear end of

the motor by a moulding (or pair of split mouldings) 35 which have a key into the motor casing to prevent it rotating under torque. Moulding 35 is itself supported firmly by ribs 36 in the case into which it is fitted when the case is

assembled. An axial hole 37 in moulding 35 permits the wiring from the

potentiometer 38 and power socket 39 to reach the motor.

Moulding 35 is so formed that a portion of its outside face forms a cylindrical bearing surface for an extension 40 to the rotor 41 . The extension may be a moulding or a casting, and is designed to be clipped or screwed to rotor 41 during assembly, one end of moulding 35 being not greater in diameter than the bearing area to permit the circular bearing aperture in extension 33 to pass along it to its correct position during assembly. Although the bearing thus formed has a relatively large diameter, friction levels may be kept low by the use of self-lubricating plastic for one or both parts. The other end of rotor 41 is supported by a one-way roller bearing 42 carried on the gearbox output shaft 43, which is extended to a plain bearing 44 supported by ribs 45. This bearing provides the support for the other end of motor/gearbox 34 and must withstand and convey the eccentric impulse from one end of rotor 41 as it rotates. This approach approximately halves the overall length of the unit for a small increase in diameter, and achieves a centrally-balanced impulse to the load.

A cavity 46 formed between the case halves provides space for electronics, if required, for versions of the product with added features, which may include a soothing sound generator and/or automatic activation. The sound generator may be designed to produce soothing music or an approximate simulation of maternal heartbeat. The automatic activation may be designed to turn the rocking on and off at predetermined or random intervals, or by the use

of sensors may respond to movement or sound from the baby.

An alternative version of the compact design of Figure 8 may be achieved by eliminating the eccentric mass and using the motor itself as the eccentric

mass. Most reduction gearboxes applied to small motors have the output shaft offset from the motor axis. By clamping the shaft so that it cannot rotate, and supporting the remote end of the motor in a suitable eccentric bearing, the motor will rotate in such a way that its centre of mass rises and falls. If the gearbox incorporates a gear which disengages under reverse torque, the motor will describe an impulsive eccentric path producing an effect similar to that from an out-of-balance rotor. Special steps must be taken to convey power to the rotating motor, and this may be achieved (for example) by slip rings, by inverting the motor construction so that the coils surround the permanent magnet and are stationary while the magnets rotate, or by introducing electrical isolation between the motor and the gearbox output shaft so that an electrical circuit may be obtained via the two bearings.

The embodiment of Figure 8 is so designed that its centre of mass lies approximately half way between ears 33 to enable it to be conveniently attached to a chair or pram or other rockable item without the rocking action

causing the chair to walk.

Referring to Figures 5 and 9, even with the greater impulsive drive to the

load obtained by the application of a one-way drive to the rotor, effective rocking results cannot be obtained when there is much energy loss due to a soft carpet as exemplified in Figure 5. Such a problem may be simply overcome by interposing a hard surface, such as a sheet of hardboard, between the chair

rockers and the carpet. A more compact and convenient solution, which has the added advantages of lower cost and inhibiting "creeping " of the load across the floor as it rocks, may be achieved by the use of short lengths of appropriately shaped aluminium or plastic extrusion 47 as shown in Figure 9. The cross-section of the extrusion is such that if the rocker 48 is tending to run off line there is a self-centring action due to the "valley" shape 49. Ridges 50 on the underside lock the extrusion into the carpet and prevent it from moving, while those on the topside act as a safety barrier to prevent the rocker falling off the extrusion under extreme misalignment. The extrusion cross-section is reversible so that the user need not be concerned with placing it the right way

up.

The above embodiments are described by way of example only and many variations are possible without departing from the invention.

Claims

1 . Apparatus for producing a varying force comprising a rotatably mounted unbalanced mass and a means for rotating the unbalanced mass to produce an out-of-balance force wherein said means enables the mass to be rotated at a non-constant angular velocity.

2. Apparatus as claimed in claim 1 , wherein the means for rotating enables the angular velocity of the unbalanced mass to be varied throughout a single revolution.

3. Apparatus as claimed in any preceding claim, wherein the means for rotating the mass enables the angular velocity of the unbalanced mass to be varied in a predetermined manner.

4. Apparatus as claimed in any preceding claim, wherein the apparatus is

arranged to produce a substantially periodic force.

5. Apparatus as claimed in any preceding claim, wherein the means for

rotating the mass comprises a uni-directional coupling.

6. Apparatus as claimed in any preceding claim, wherein the means for

rotating the mass comprises a motor.

7. Apparatus as claimed in any preceding claim, wherein there is also provided means to enable the overall periodicity of the rotation of unbalanced mass to be adjusted.

8. Apparatus as claimed in claim 7, when appendant to claim 6, wherein said means to enable the overall periodicity of the unbalanced mass system to be adjusted comprises a means for controlling the rate of rotation of the motor.

9. Apparatus as claimed in any preceding claim, wherein there is also provided a control means for controlling the drive means operative to stop the drive means automatically after a predetermined period of time.

10. Apparatus as claimed in any preceding claim, wherein there is also provided a control means adapted to automatically adjust the periodicity of the unbalanced mass system to the natural frequency of any article driven by the apparatus.

1 1 . Apparatus as claimed in any preceding claim, wherein there is provided

on the apparatus a means for storing an electric battery.

12. Apparatus as claimed in any preceding claim, wherein means are provided to enable the apparatus to be mounted onto an object to which it is desired to

apply a force.

13. Apparatus as claimed in any preceding claim, wherein the apparatus is adapted to be integrated into an object to which it is desired to apply a force.

14. A method of producing a varying force comprising the steps of rotating an unbalanced mass to produce an out-of-balance force, wherein said mass is rotated at a non-constant angular velocity.

15. A method as claimed in claim 14, wherein the angular velocity of the mass is varied throughout a single revolution.

16. A method as claimed in either claim 14 or 15, wherein the angular velocity of the mass is varied in a predetermined manner.

17. A method as claimed in any of claims 14 to 16, wherein the mass is so rotated to produce a substantially periodic force.