GB2357555A - Vibration actuator for toothbrush - Google Patents

Abstract

A toothbrush having a bristle drive shaft 15 rotatably mounted in a toothbrush head includes an actuator for oscillating the shaft. The actuator includes two Shape Memory Alloy (SMA) wires 13 and 14 anchored 22 between the shaft and fixed points 19, 20. The wires are heated by transmitting electrical current causing contraction at their transition temperature (40 {C), hence shaft rotation through a small angle. Alternating wire heating and cooling causes shaft oscillation typically at about 5 Hz. The wires pass around transversely movable arcuate aluminium heatsinks 16, 17 mounted on a slide 33 such that contraction of a first wire 13 causes contact with a heatsink 17, simultaneously moving the other heatsink out of contact with the second wire 14, ensuring rapid cooling of the first and rapid heating of the second wire when the current is switched over.

Description

2357555 VIBRATION ACTUATOR The invention relates to vibration actuators.

In many simple applications vibrations of a work piece is required, such application include toothbrushes, polishers, audible alarms and so forth. Conveniently, because power is readily available from small storage batteries, vibratory motions are generated when required using an electrically responsive device of some sort, such as an electromagnet. With the advent of the readily available cheap and reliable fraction horsepower electric motors, it has been common practice to convert rotary motion by suitable mechanical drives connected to is such motors. Even so, the electric motors themselves may represent a significant relative cost compared to the other components of small handheld appliances so that their overall costs of such appliances are in fact difficult to bring down further.

It is an object of the invention to overcome or at least reduce this problem.

According to the invention there is provided a vibration actuator comprising a work piece constrained to oscillate about a null position, including an elongate SMA component anchored at one end and mechanically coupled at its other end to the work piece, means f or 2 periodically changing the temperature of the component by passing electric current through the component so as to take the component through its transition point to cause the component to reduce its effective length and cause the work piece to oscillate about its null position.

The work piece may be biassed in one direction and the SMA component arranged to overcome the bias as it is heated through its transition point.

The vibration actuator may include two SMA components arranged to move the work piece in or) osing directions IP and the means for periodically changing the temperature is of the SMA component is arranged to pass current through the SMA components in synchronism to cause the oscillation in both respective directions about the null position.

The SMA components may be each pre-tensioned in a direction opposite to the direction of their anticipated direction of reduction in effective length when they pass through the transition point.

The SMA components may be joined to the work piece for opposing operation such that whenever one SMA passes through its transition point its reduction in effective length pre-tensions the other SMA component.

3 The vibration actuator may include a heat sink having an electrically insulated. surface that bears against the SMA component.

The electrically insulated surface may consist of a mica coating. The vibration actuator may include a movable heat sink and means for periodically moving the heat sink into 10 proximity with the SMA component during intervals when no current is being passed through that SMA component. A toothbrush may be provided that incorporates the vibration actuator. is A toothbrush having a vibration actuator according to the invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which:20 Figure 1 is a sectional side view of the toothbrush; Figure 2 is a top plan view of the toothbrush; Figure 3 is an exposed top view of part of the toothbrush; Figure 4 is a part-sectional side view of part of 4 another toothbrush; Figure 5 shows a sectional elevation of theother toothbrush; Figure 6 is a simple schematic arrangement showing electrical components for the toothbrushes; and Figure 7 is an exposed top view of part of a further toothbrush.

Referring to the drawings, in Figures 1 to 3, the toothbrush has a partially hollow handle 10 and an integrally f ormed shank 11 extending to a head 12. A is pair of SMA wires 13 and 14 (only the wire 13 can be seen in Figure 1) extend from the handle to a shaft 15 to a brush base 16 (see Figure 3). The brush base 16 is rotatably supported in the head 12. Corrugated aluminium f ingers 16 and 17 extend along the shank 11 above and below the wires 13 and 14, to act as heat sinks. The inner opposing surfaces of the fingers are coated with mica to electrically insulate the wires 13 and 14 along their lengths from the fingers 17 and 18. The wires 13 and 14 are earthed adjacent their remote ends to the heat sinks which form return current paths for the wires.

The upper surface of the finger 17 is exposed (see Figure 2) to provide an aesthetically attractive design for the top surface of the toothbrush.

Heat sinks (not shown) provide return current paths for currents passing through the wires 13 and 14 in use.

In Figure 3, the SMA wires 13 and 14 are clearly shown.

The wires are both anchored at respective ends 19 and 20 and attached to the shaf t 15 at their other ends by diametrically opposed pins 21 and 22. Thus, if the wires 13 and 14 contract and expand in opposition and in synchronism, the shaft 15 is driven clockwise and anti clockwise in an oscillating manner.

In Figures 4 and 5, a different toothbrush is shown having a head 23 that is rotatably mounted on the end of a toothbrush shank 24. Two SMA wires 25 and 26 extend along the shank 24 and pass over respective pulleys 27 and 28. Remote ends of the wires are attached to the head at points 29 and 30. If the two wires 25 and 26 contract and expand in opposition and in synchronism, the head 23 is driven clockwise and anti-clockwise with respect to the shank 23 in an oscillating manner.

In Figure 6, the electrical components of the toothbrushes consist of a battery 31 (which may be re chargeable), an electrical drive circuit 32 and the SMA wires 13 and 14. (The same drive circuit can be used for the wires 25 and 26). The drive circuit is arranged to supply current pulses to each of the wires 13 and 14 in turn. Current passing through the wires causes the wires to heat up to or beyond their transition temperature so that each of the wires is caused to contract in turn.

This provides a "vibration actuator" that causes toothbrush bristles to be rotated in Figures 1 to 3 (or the head 23 to be rotated in Figures 4 and 5), as explained above. In this way, the vibration actuator is used to provide simple oscillations to aid brushing of teeth, in a fashion previously provided by using a conventional fractional horsepower direct current motor.

In Figure 7, an embodiment of the invention having a is movable heat sink is shown. The heat sink consists of two arcuate fingers 16 and 17 inside surface-coated as before with mica mounted on a slider 33. A headed pin 34 holds the sliderto the toothbrush and allows the slider to move up and down (as depicted in Figure 7) as required. The movement of the heat sink takes place automatically and in synchronism with the heating up and cooling down of the wires 13 and 14. As shown, the wire 13 has just heated up and contracted. The resulted tightening of the wire 13 causes it to press against the finger 17 and urge the slider 13 upwards. An upper end of the slider urges the wire 14 out-of-contact with the finger 16.

In a next step in a cycle of the oscillator actuator, current flowing through the wire 13 is turned OFF and instead, current a pulse is passed through the wire 14.

The wire 14 is initially out of contact with the finger 16, so allowing the wire to heat up rapidly. At this time the wire 13, being in contact with the f inger 17 can cool more rapidly. As soon as the wire 14 reaches its transition temperature, the wire contracts and tightens so that a lower end of the slider 13 is urged against the wire 13 to move it downwards, away. from and out-of-contact with the surface of the finger 17.

Thus, the heat sink is moved automatically, in effect, by the wires 13 and 14 in synchronism with oscillations is of the shaft 15. Importantly, in this way, the heat sink is rendered ineffective for each wire during periods of heating up of the wires, and effective to aid cooling of the wires at other times.

The overall general arrangement of the toothbrushes according to the invention is otherwise well-understood and not described further in this specification. Certain particular aspects however of the embodiments of the invention are now mentioned.

The SMA wires are made of Nickel-Titanium or Nickel Titanium-Copper alloys that have a transition temperature of around 400C. The wires are typically 200 mm long with a diameter of 0.002 to 0.005 mm. In normal operation, the wires contract by between 3% and 4% of their length and it has been found advantageous to pre tension each wire respectively by 1.5% to 2% of its length at say 150C. When a suitable current pulse is applied to each wire, the wire rapidly heat up to 400C and when the current ceases to flow the wire looses heat to the heat sink fingers 16 and 17 so as to cool relatively quickly as required. Electrical pulses are applied by the drive circuit 32 alternatively for 0.1 seconds to each of the pairs of wires 13 and 14. An oscillating frequency of 5 cycles per second is thus provided by the vibration actuator.

various mechanical changes can be made while making use of the SMA wire characteristics to create different forms of oscillation. For example, one of the wires 13 (or 14) can be replaced by an elongate spring which biasses the shaft 15 against the contraction of the single remaining wire. The spring returns the shaft 15 to a null position when the wire cools down and returns to its uncontracted length.

The rate at which the wires can be heated up and cooled down through the transition temperature cannot normally be increased significantly without using much more 9 electrical power. The oscillation rate can be increased however by using more than two wires. For example, if three pairs of wires are used, the pairs can be pulsed with current in tandem, and in sequence with the other pairs of wires. As such, the pulse rate is readily increased threefold to 15 cycles per second.

In a further arrangement, the amount of oscillation is increased by having a ",gear coupler". A gear wheel is located between the remote ends of the SMA wires and the toothbrush head. The SMA wires are attached to diametrically opposite sides of the wheel and two respective normal (nonSMA) wires or rigid links are attached between the wheel and the shaft 15. If the is normal wires are attached to points further from the rotational axis of the wheel than the SMA wires, the amount of oscillation of the shaft 15 is respectively increased. An opposite effect can be achieved, where required, by attaching the normal wires closer to the rotational axis of the wheel than the SMA wires.

Claims (10)

1. A vibration actuator comprising a work piece constrained to oscillate about a null position, including an elongate SMA component anchored at one end and mechanically coupled at its other end to the work piece, means for periodically changing the temperature of the component by passing electric current through the component so as to take the component through its transition point to cause the component to reduce its effective length and cause the work piece to oscillate about its null position.

2. A vibration actuator according to claim 1, in which is the work piece is biassed in one direction and the SMA component is arranged to overcome the bias as it is heated through its transition point.

3. A vibration actuator according to claim 1, including two SMA components arranged to move the work piece in opposing directions and the means for periodically changing the temperature of the SMA component is arranged to pass current through the SMA components in synchronism to cause the oscillation in both respective directions about the null position.

4. A vibration actuator according to claim 3, in which the SMA components are each pre-tensioned in a direction opposite to the direction of their anticipated direction of reduction in effective length when they pass through the transition point.

5. A vibration actuator according to claim 3, in which the SMA components are joined to the work piece for opposing operation such that whenever one SMA passes through its transition point its reduction in effective length pre-tensions the other SMA component. 10

6. A vibration actuator according to any of claims 1 to 5, including a heat sink having an electrically insulated surface that bears aaainst the SMA component.

7. A vibration actuator according to claim 6, in which the electrically insulated surface consists of a mica coating.

8. A vibration actuator according to any of claims 1 20 to 7, including a movable heat sink and including means for periodically moving the heat sink into proximity with the SMA component during intervals when no current is being passed through that SMA component. 25

9. A toothbrush incorporating a vibration actuator according to any one of claims 1 to 8.

10. A vibration actuator substantially as herein described with reference to the accompanying drawings.