GB2186128A - Bimetallic actuator - Google Patents

Abstract

The bimetallic actuator may be used in an electrical toaster as a means for timing the toasting cycle. The actuator includes a bimetallic strip 12 heated by an element 16 in series with the toaster element 24. A thyristor TH1 provides a selective shunt path for diverting a proportion of the power across the bimetallic heating element 16 during a delay period. The length of the delay period can be adjusted by a variable resistor RV1 included in the circuit of an electronic timer IC1. Triggering of the thyristor TH1 is disabled at the end of the delay period whereby sufficient power is dissipated in the bimetallic heating element 16 to cause the bimetallic strip 12 to deflect and move a lever (6), (Fig. 1), resulting in partial release of a bread carrier latch (7), (8), (11), (21) and closure of a switch 22 which shunts the element 16. The strips 12 therefore cools while the toaster element 24 remains energised. After a predetermined cooling period, the strip 12 allows full release of the latch whereby the bread carrier (1) is returned by spring action to its load/unload position. <IMAGE>

Description

SPECIFICATION Bimetallic actuator This invention relates to a bimetallic actuator which may be used, for example in an electrical toaster as a means for timing a toasting cycle.

Conventional electric toasters employ some form of timing device which is usually initiated when a spring-loaded slidable carrier (for supporting slices of bread) is pressed downwardly and latched into an operative toasting position within the toaster body. The timing device releases the latch, after a predetermined interval, to allow the carrier to spring upwardly to prevent toast from burning.

Timing devices generally fall into one of two categories, i.e. electronic or mechanical.

Electronic timing devices vary, but they basically rely on either a counting device which releases the latch after a predetermined amount of time has elapsed, or a temperature comparison device which causes the latch to be released when the respective temperatures sensed by first and second monitoring devices (situated in different positions within the toaster) are equal. However, electronic timing devices and their associated latching mechanism are expensive to manufacture and to install in toasters. Moreover, toasters employing electronic timers are not entirely successful in repeatedly producing consistent "shades" of toast. This is due to the nature of toasting and to the effect of temperature variations within the toaster body.For example, when toasters are used repeatedly over prolonged periods of time, they become hotter and hotter and the "shade" control must be adjusted to maintain the colour or "shade" of the toast.

Most mechanical timing devices rely on either a clockwork mechanism, or on the response of a bimetallic strip to the heat produced by a coil of resistance wire wound on the strip. Clockwork mechanisms have now been largely superceded due to manufacturing expense, but they are also undesirable because they are noisy in operation. Bimetallic actuators provide a cheaper alternative, but they have certain disadvantages.

Many conventional toasters which employ a bimetallic actuator have an element which heats a bimetallic strip as soon as the power is switched on. The heat produced by the element causes the bimetallic strip to deform so that one end of the strip starts to move towards a "target" which is spaced by a gap from the end of the strip. The "target" is usually part of a mechanical system for latching the slidable carrier in its operative toasting position within the toaster body. The width of the gap determines the time taken between the instant at which power is supplied and the instant at which the end of the strip reaches the target.When the target is reached, further deformation of the strip causes movement of the target and this, in turn, causes partial release of the latch which is sufficient to operate electrical contacts to interrupt the supply of power to the element which heats the bimetallic strip. Although this element is then switched off, power continues to be supplied to a radiant heating element (for toasting bread) within the toaster body. As the bimetallic strip cools, it approaches a second target which was previously caused to move into the return path of the strip whilst it deformed under the action of heat. As the strip cools, it eventually contacts this second target, which is thereby displaced, causing the latching mechanism to be fully released to allow the slidable carrier to spring upwardly from the toaster body.With such an arrangement, a variation in the shade of toast is achieved by adjusting a control which moves the first, or second target, or usually both, with respect to the end of the bimetallic strip. However, the further the distance that the bimetallic strip is required to move, the less accurately it moves with respect to time. Moreover, the further that the end of the strip moves, the weaker is the force that can be applied by the strip to overcome friction in the latching system. The net result is that the toaster is not reliable, particularly over longer timing intervals (or darker shade settings) and burning of the toast is quite common due to the inconsistent response of the bimetallic strip. The situation is made worse as the toaster becomes progressively hotter with prolonged use.

One of the problems facing the invention is to provide a bimetallic actuator which is capable of providing variable, but repeatedly consistent, timing intervals and which is relatively inexpensive to manufacture and to install in an appliance, such as a toaster.

According to the present invention, a bimetallic actuator comprises a bimetallic member; an electrical heating element for heating the bimetallic member, the bimetallic member having a predetermined response time with respect to the power to be supplied to the heating element; and adjustable delay means to delay the supply of sufficient power to the heating element which would otherwise cause the bimetallic member to respond, the delay means being adjustable to vary the overall time taken for the bimetallic actuator to provide a given response.

The above arrangement has the advantages that the delay time can be easily adjusted to vary the overall response time of the actuator and that the overall response time is independent of the movement of the bimetallic member (because the heating element does not receive enough power, during the delay time, to cause any effective response in the bimetallic member). When an actuator in accordance with the invention is used in a toaster, the delay time is controlled to vary the shade of the toast.

Preferably, the delay means comprises circuitry for diverting a proportion of the power supplied to the heating element for a predetermined interval, and an adjustable timer for variably timing the predetermined interval.

According to one preferred arrangement, the circuitry includes a shunt path containing a gate-controlled device, such as a thyristor.

Suitable biasing is provided in the circuitry to trigger the thyristor so as to divert a predetermined proportion of the power supplied to the heating element. A timer is used to switch off the thyristor at the end of a predetermined period so that the full amount of power then flows through the heating element to cause the bimetallic member to respond. An advantage of using a gate-controlled device in a shunt path is that no appreciable power is dissipated in the heating element of the bimetallic member during the delay period.

A mechanical system may be actuated by the response of the bimetallic member, when it is heated, the mechanical system moving into a latched position, against a bias, after the heating period within which the heating element receives sufficient power to cause the bimetallic member to respond. The bimetallic member can be arranged to oppose the bias whilst it is in a heated and deformed state so that, as it cools down, the bias returns the mechanical system to a position where it is released from the latch. This is advantageous, because the bimetallic member does not have to cause any positive displacement of the mechanical system (e.g. by exerting a direct force to cause latch release), since the spring bias provides the return force to cause delatching as the bimetallic member cools down.

Preferably, the mechanical system has a fixed displacement, so that the latch can be released, time after time, after a constant period. When such an arrangement is used in a toaster where the mechanical system includes, for example, a lever which is actuated as the bimetallic member cools down, the cooling period remains substantially constant (whilst the toaster element is energised) to assist in maintaining a repeatedly constant toasting period. Preferably, the response of the bimetallic actuator is employed to operate switch means to terminate the heating of the bimetallic member, i.e. at the end of the period within which the heating element receives sufficient power to cause the bimetallic member to respond. Operation of such a switch means initiates a cooling period within which the bimetallic member returns to a starting position.

The bimetallic member is preferably of a type having an initial and substantially linear response to temperature. The mechanical system and the electrical contacts are preferably actuated within this initial linear response region of the bimetallic member. The bimetallic member is also preferably one which is more sensitive than those used in prior art toasters, e.g. it preferably has a specific deflection greater than 12 x 10-6 C-1, and preferably of the order of 21 x 10-6 C- where the heating element is normally supplied with about 25 watts of rectified AC power during the heating period.

A preferred embodiment of the invention, as applied to an electrical toaster, will now be described with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a bimetallic strip and the essential features of a latching mechanism in a toaster according to an embodiment of the invention, Figure 2 is a schematic circuit diagram of an electronic shade control used in the toaster of the preferred embodiment of the invention, Figure 3 is a schematic timing diagram for explaining certain timing intervals which occur in the toaster, and Figure 4 is a graph showing the characteristic curves of a bimetallic strip used in the toaster, the graph showing deflection (in mm) plotted against temperature ("C) for different power inputs to the bimetallic heating element.

Fig. 1 illustrates the main features of a latching mechanism in a toaster according to a preferred embodiment of the invention. The general construction of a toaster having a spring-loaded bread carrier will be familiar to those skilled in the art and hence no detailed description is necessary.

A spring-loaded, slidable bread carrier 1 (not shown in detail) is normally biased upwardly in the direction of arrow 2. When the slidable carrier 1 is pressed down into the toaster body (not shown), the sloping sides 3, 4 of an arrow-shaped head 5 of a lever 6 are sequentially defiected by abutments 7,8 fixed to the carrier 1. A tension spring 9 has one end fixed to the upper part of lever 6 and its other end anchored to a fixed part 10 of thetoaster body. Spring 9 biases lever 6 in a clockwise direction whereby a camming surface 11 engages abutment 7 when the carrier 1 is in its lowermost operative toasting position. The carrier 1 is thereby latched and is prevented from springing upwardly against the return force of the spring-loading (not shown) which operates in the direction of arrow 2.

A bimetallic member or strip 12 has one end anchored to a Z-shaped bracket 14 which is anchored to the toaster body so that its free end 1 5 moves when the bimetallic strip deforms under the action of heat. A coil of resistance wire is wound on the strip 12 and this will be hereinafter referred to as a "bimetallic heating element" 16. When the bimetallic heating element 16 is supplied with sufficient power, it causes the end 15 of the strip 12 to approach the lower end 17 of lever 6.

An adjustment screw 18 threadably engages the anchored end of the strip 12 and bracket 14, the end of the screw bearing against a threaded insert 13 fixed to the toaster body so that the screw may be adjusted to vary the width of a gap 19 between the free end 15 of the strip 12 and a confronting edge 20 of the lower end of the lever. Apart from adjustments for the purpose of manufacturing tolerances, the width of the gap 19 remains constant.

When the bimetallic heating element 16 is supplied with sufficient power to cause the bimetallic strip 12 to deflect, the free end 15 of strip 12 moves towards and eventually engages the edge 20 of the lever 6. Further deflection of the strip 12 causes the lever 6 to rotate in an anticlockwise direction so that the camming surface 11 moves clear of abutment 7. Due to the spring-loaded bias on the carrier 1, the carrier then moves upwardly, by a short distance, until a camming surface 21 on the head 5 engages the abutment 8 fixed to carrier 1. This is represented by a broken line in the drawing. This partial release of the latching mechanism, which enables a small displacement of the carrier 1, is used to operate the contacts of a switch 22 (see Fig. 2) which shunts the bimetallic heating element 16 so that the bimetallic strip 12 is no longer heated.The function of switch 22 will be more readily appreciated from the circuit diagram of Fig. 2, which will be described in more detail below.

As the bimetallic strip 12 is still hot, it has sufficient stored energy to overcome the bias of spring 9 and so the lever 6 is temporarily held in its counterclockwise rotated position.

However, as the bimetallic strip 12 cools, its end 15 starts to move away from the lower end of lever 6 and the spring 9 rotates lever 6 in a clockwise direction until a point is reached where camming surface 21 is drawn clear of abutment 8. At this point, the latching mechanism is fully released, and the carrier 1 springs upwardly due to its spring-loaded bias.

This removes the bread from a position opposite the toasting element to prevent the toast from burning.

It will be noted that the latching mechanism, described with reference to Fig. 1, has few moving parts and that the bimetallic strip 12 does not need to exert any positive force to cause the latching mechanism to be released.

As the strip 12 cools down, it is simply pushed aside by the lower end 17 of lever 6 due to the bias exerted by spring 9.

Referring to Fig. 2, a description will now be given of the electronic shade control circuitry associated with the bimetallic strip 12.

Fig. 3 will also be referred to, since this is a schematic timing diagram which illustrates certain intervals that occur during a toasting cycle.

In Fig. 2, switch 22 and the circuitry 23 (outlined with a broken line) are connected in parallel with the bimetallic heating element 16.

Element 16 is connected in series with a toasting element 24 (e.g. one or more radiant electric heaters) located within the toaster body. A double-pole switch SW1 connects the toaster element and the bimetallic heating element 16 to mains supply terminals T1,T2.

Diode D3 shunts half cycles of one polarity across the bimetallic heating element 16 and thereby reduces the amount of power flowing through the bimetallic heating element 16 due to the half cycles of the opposite polarity. A diode D1 conducts these opposite polarity half cycles to provide a DC power supply having a voltage reduced by resistor R6 and smoothed by capacitor C1 (resistor R1, which is optional, discharges C1 between toasting cycles). This DC supply operates a timing network including an integrated circuit timer IC1 (e.g. of the 555 type).

A gate-controlled device in the form of a thyristor TH1, forms a shunt path for diverting a proportion of the power supplied by the opposite polarity half cycles during a "delay period". During this "delay period" the bimetallic heating element 16 does not receive enough power to cause any appreciable response in the bimetallic strip 12. A resistor network including resistors R5 nd R7, protected by a zener diode Z2, provides a gate voltage for triggering thyristor TH1 during each opposite polarity half cycle of power supplied across the bimetallic heating element 16. This trigger voltage is supplied throughout the "delay period" which is controlled by timer It 1. The thyristor is preferably a sensitive-gate thyristor which can be triggered with a low current signal so that it conducts at an early stage during the supply of opposite polarity half cycles.

Resistors R2,R3,R4, capacitors C2,C3 and variable resistor RV1 are connected to the integrated circuit timer Inc1, as shown, to provide suitable biasing for operating the timer and for controlling the "delay period". The adjustable resistor RV1 can be adjusted (by the user) to vary the length of the "delay period" to control the shade of the toast. Resistor R4 (which is optional) has a value which adjusts the limits of the range of adjustment of variable resistor RV1. Various other connections of the timer to the positive and negative supply rails are as shown in Fig. 2.

The operation of the circuitry will now be described.

When switch SW1 is closed, AC power is supplied to toaster lement 24 and to the bimetallic heating element 16. Both positive and negative half cycles of power are dissipated in the toaster element 24, whereby the toaster element radiates heat for toasting bread when the carrier is in its operative position. As explained above, half cycles of one polarity have no effect on the bimetallic heating element 16.

Half cycles of the opposite polarity would normally be dissipated in the bimetallic element 16, but a proportion of this power is diverted through the shunt path containing thyristor TH1 during the "delay period". The trigger voltage is set at a low value so that, for example, only about a quarter of the power in the positive half cycles flows through the bimetallic heating element 16 in the delay period. In a typical arrangement, the bimetallic heating element 16 normally dissipates about 25 watts of power, but only 6-8 watts when thyristor TH1 provides a shunt path.

Thyristor TH 1 is triggered in each opposite polarity half cycle during delay period T1 as shown in Fig. 3. During this delay period, the bimetallic heating element 16 is on "low power", since most of the power is diverted through thyristor TH1. The period T1 can be adjusted by means of variable resistor RV1 to control the shade of the toast. The timer IC 1 controls the length of the delay period T 1. At the end of this period, the network which provides a gate trigger voltage is disabled, so that thyristor TH1 no longer conducts during any part of each opposite polarity half cycle.

The bimetallic heating element 16 is then supplied with "full power" during a heating interval T2 as shown in Fig. 3. During interval T2, the bimetallic strip 12 deflects until it operates the latching mechanism as described above, with reference to Fig. 1. Switch 22 closes after interval T2 (as a result of the small displacement of the bread carrier, i.e. when camming surface 11 is released from abutment 7 so that the camming surface 21 subsequenly engages abutment 8). When switch 22 closes, positive and negative half cycles of power are shunted across the bimetallic heating element 16 so that the bimetallic strip 12 is no longer heated. The strip 12 therefore cools during an interval T3 as shown in Fig.

3. At the end of interval T3, the strip 12 has cooled sufficiently to release abutment from the camming surface 21, whereupon the carrier 1 springs upwardly from the body of the toaster.

The toasting cycle may be repeated by once more depressing the bread carrier 1 into its lowermost latched position.

The intervals T1,T2 and T3 can each be maintained substantially constant despite variations in temperature within the body of the toaster. Therefore, depending on the setting of variable resistor RV1 (for obtaining a desired shade of toast), accurate and fixed toasting intervals can be repeatedly maintained. Generally, the heating interval (T1 and T2) may be roughly equai to the cooling interval (T3), but a longer cooling interval is preferable. Typically, the periods T1,T2 and T3 are respectively 35 seconds, 15 seconds and 100 seconds thereby giving a toasting cycle of 150 seconds. The toasting cycle is preferably kept as short as possible so that the toast is crisp on the outside and soft on the inside.

Preferably, the bimetallic member 1 is more sensitive than those used in prior art toasters.

For example, in a toaster embodying the invention, the specific deflection of the bimetallic member is about 20.8x10-6 "C-' Such a bimetallic member is available from Bulten Kanthal under their Serial No: Kanthal 200. Bimetallic members used in prior art toasters are usually of the Kanthal type 115 TB 1170 having a specific deflection of about 11.7x10-6 "C-'. The reason for using a more sensitive bimetallic member will be appreciated from the following description of the characteristic curves shown in Fig. 3.

Fig. 3 illustrates the deflection (in mm) plotted against temperature T ("C) for a bimetallic strip having a specific resistivity of about 20.8 x 10-6 "C-'. The strip was wound with a coil of resistance wire through which various currents were passed to dissipate the power values shown on the graph. The characteristic curves have an initial short linear portion followed by a curved portion where the bimetallic strip deflects less and less with increasing temperature. In a toaster embodying the invention, the bimetallic strip and its heating eie- ment are arranged to operate the latching mechanism on a steep and linear portion of the characteristic curve, e.g. with a heating element dissipating about 25 watts.Typically, the bimetallic strip would be deflected by about 3-6mm when heated at this power value. Bimetallic strips having a lower specific deflection would have characteristic curves similar to those shown at the lower power values (e.g. 5 watts) when supplied with about 25 watts of power. Therefore, in conventional toasters, the bimetallic strips would tend to be operating more on the curved portion of the characteristics, especially in order to achieve sufficient deflection to operate the latching mechanism when the shade control is on the darker settings.

Generally, bimetallic strips are preferred having a specific deflection of more than about 12 x 10 6"C 1. The use of more responsive bimetallic strips means that the release point of the latch and the switching of electrical contacts occur at a point on the steep linear portion of the characteristic curve (see Fig. 4).

The faster the response of the strip, the less likely is the chance of a timing error in successive toasting cycles. Bimetallic strips used in most conventional toasters are subject to such timing errors because they have a slower response and because the performance of the strip, under severe deflection, is not consistent in successive toasting cycles.

In toasters embodying the invention, apart from adjusting manufacturing tolerances, it is not necessary to adjust the gap between the end of the bimetallic strip and any targets in order to adjust the timing of the toasting cycle. The timing of the toasting cycle is carried out electronically by simply adjusting the variable resistance RV1 to vary the delay period.

Hence, the gap between the end of the bimetallic strip 12 and the end 17 of lever 6 can be maintained constant, as can the displacement of the camming surface 21 with respect to the abutment 8, in order to promote repeatedly constant response times within the actuator system. This is particularly advantageous compared with prior art toasters where such gaps and displacements must be varied to adjust the shade of the toast and where longer response times mean extending the displacements or gaps by an amount which contributes to timing errors in successive toasting cycles.

Whilst a preferred embodiment of the invention has been described by way of an example, variations and modifications are possible within the scope of the invention.

Claims (12)

1. A bimetallic actuator comprising a bimetallic member; an electrical heating element for heating the bimetallic member, the bimetallic member having a predetermined response time with respect to the power to be supplied to the heating element; and adjustable delay means to delay the supply of sufficient power to the heating element which would otherwise cause the bimetallic member to respond, the delay means being adjustable to vary the overall time taken for the bimetallic actuator to provide a given response.

2. A bimetallic actuator according to claim 1, wherein the delay means comprises circuitry for diverting a proportion of the power supplied to the heating element for a predetermined interval, and an adjustable timer for variably timing the predetermined interval.

3. A bimetallic actuator according to claim 2, further including a mechanical system which is actuated by the bimetallic member so as to move into a latched position, against a bias, after a heating period within which the heating element has been supplied with sufficient power to cause a predetermined deflection of the bimetallic member, the bimetallic member opposing the bias whilst it is in a heated and deformed state so that, as it cools, the bias releases the mechanical system from its latched position.

4. A bimetallic actuator according to claim 3, further including switch means for terminating the supply of power to the heating element after said heating period, said switch means being operated by a displacement in the mechanical system.

5. A bimetallic actuator according to claim 3 or 4, wherein the mechanical system includes a lever which is actuated by the bimetallic member, a fixed gap being provided between the bimetallic member and the lever apart from any adjustment for manufacturing tolerances.

6. A bimetallic actuator according to any one of claims 3-5, wherein the bimetallic member has an initial and substantially linear response to temperature, the mechanical system being actuated within the linear response region.

7. A bimetallic actuator according to claim 6, wherein the bimetallic member has a specific deflection greater than about 12x 10-6 0C-1.

8. A bimetallic actuator according to claim 7, wherein the bimetallic member has a specific deflection of about 21 X 10-6 "C-' and the heating element is supplied with about 25 watts of electrical power during the heating period.

9. A bimetallic actuator according to any one of claims 2-8, wherein the means for diverting a proportion of the power supplied to the heating element comprises a gate-controlled device.

10. An electrical appliance comprising the bimetallic actuator according to any one of the preceding claims and further including an additional heating element which is connected in series with the heating element of the bimetallic actuator.

11. An electrical appliance according to claim 10, where the appliance is a toaster and the additional heating element is provided for toasting bread.

12. A toaster substantially as herein described with reference to the accompanying drawings.