DE7900875U1 - Thermal switch - Google Patents

Description

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Thermal switch

The invention relates to a thermal switch according to t

the preamble of claim 1.

Known thermal switches, such as bimetal strips, fuses or control valves with thermocouples, trigger a switching process when a predeterminable temperature value is exceeded with a relatively large one time delay. Thermal switches are usually used to interrupt an electrical circuit used.

For the operational safety of electrical devices and systems, the incorrect voltage and its duration are particularly important, Amplitude increase and repetition are important. Interference voltages in Devices and systems for malfunctions or destruction of semiconductors in sensitive electronic circuits, to malfunctions in control systems, to the loss of j

or tion ν- information on About beats in poor Isola- \ or too small selected creep or air routes | to lead. Switches with a long time delay are unsuitable for protecting such devices. Bimetallic strips also show signs of fatigue after prolonged use, which lead to a change in the response temperature.

Therrr.oscnalter rcit a relatively small switching delay using a switching process off

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releasing element made of a shape memory alloy are, inter alia, by DE-OS 2 026 629, 2 139 852 and , 2,701,884 known. The relatively large change in shape of this switching element when the temperature increases About the initial temperature of the phase transition from the low to the high temperature phase of the shape memory alloy used to operate a switch. The current to be switched can flow directly through the switching element and be warmed. The heating can also be done by a heater inserted in the circuit. By From DE-OS 2 701 884 it is known to interrupt the current in the circuit when a certain maximum current value is exceeded. A problem with the construction of switches, especially for line and overcurrent protection, is to ensure good selectivity in the Ensure switching conditions, i.e. after reaching a current tolerance value, the switch should only be used when Open when a predeterminable current increase is exceeded and not with all overcurrents.

The object of the invention is to ensure the triggering of a switching process with high selectivity of the switching condition.
y

The object is achieved by the features of claim 1. Further developments of the invention are described in the subclaims.

The invention is based on the new knowledge of the kinetics of the shape memory effect. One advantage of the invention is in that the switching process is selective with respect to the initial value caused by an overcurrent temporal change of the temperature in the switching element

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and is not related to the transmitted energy. The one applied to the switching element by a clamping device The bias voltage can be adjusted so that the switching element cools down after a subcritical overcurrent has passed can be deformed back.

The invention is described below on the basis of exemplary embodiments. Show it:

Fig. 1 shows the principle of the shape memory effect,

a) the one-way effect,
b) the two-way effect,

2 shows the elongation as a function of temperature,

a) for the one-way effect,

b) for the two-way effect,

Fig. 3 shows the scheme of a simple thermal switch in one also suitable for testing purposes,

4 shows the chemical energy G of the two phases martensite and austenite of a shape memory alloy as Function of the temperature T (schematic),

^ Fig. 5 shows the percentage of martensite M as func-

20 tion of the temperature T,

6 shows the electrical resistance R of a shape memory alloy (a) before and (b) after heating as a function of time t,

7 shows the scheme of a mechanical model from a

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Spring K, a shock absorption device with a damping constant oC and a mass m,

8a, 8b, 8c a semi-schematic representation of a Thermal switch ^; in a lhermoelectric switch with additional locking element and mechanical

Switching gain in different positions and

9 is a functional diagram of the thermal switch.

Fig. La) shows the one-way and Fig. Ib) the two-way shape memory effect. These effects occur with shape memory alloys, ie with thermoelastic martensites, which return to their original shape after a so-called pseudo-plastic deformation at low temperature, but when they are heated to a critical temperature. These alloys exist in a martensitic low-temperature phase of low, face-centered orthorhombic or monoclinic symmetry and in an austenitic high-temperature phase of high, cubic, rough, crushed symmetry. In the martensite phase there are no platelets or lamellae with different crystallographic orientations. At a high temperature of, for example, 600 ° C., the alloy is brought into a desired, for example elongated, rod shape and it is annealed. During the cooling, go to the crosser. At a critical temperature the austenitic phase passes over into the arterial phase. If it is now deformed, e.g. bent, at a low temperature T ₁ , stress-induced or reoriented martensite formation results in an a.uasistable shape that carries the original straight structure. At the Aufheiner. At a temperature T_, which is above a critical temperature, the one-way effect spontaneously turns the curved structure back into a straight structure. at

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a subsequent cooling down no longer takes place any spontaneous change in shape. However, the alloy will at the temperature If T is deformed again, the described effect is repeated when it is heated to T.

In the two-way effect, a curved body made of a shape memory alloy, for example, deformed at temperature T, "spontaneously only partially reverts to its original shape" when it is subsequently cooled to T, it deforms again spontaneously, but not up to the degree of deformation applied from the outside. The two temperature-induced, spontaneous changes in shape can be _{repeated with successive temperature cycles between T 1} and T ".

Fig. 2 shows another representation of the memory effects.

Depending on the alloy composition, for the one-way effect shown in FIG. 2a, expansions or pseudo-plastic deformations of up to about 8 % can be recovered during heating, while for the two-way effect shown in FIG. 2b, expansions or deformations up to 1.5 % are known. While the deformation with the one-way effect goes back to zero through heating and this re-formed state is maintained when cooling down, with the two-way effect a deformation of about 2 % remains. When cooling below a certain temperature, a spontaneous deformation of about 1.5 % occurs, which spontaneously regresses when the alloy is heated up.

The two-way effect can be achieved by creating dislocations and lattice defects e.g.

a) by an irreversible plastic deformation over the pseudoplastic Range of the alloy beyond, i.e. from more

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than about 8%, or

b) be produced by increasing the temperature by about 50 K above the final temperature of the austenite transformation.

The portion of the deformation leading to dislocations is not recovered during heating. When cooling down again The internal stress field of the lattice disturbance favors the regression of martensitic orientations created by the originally applied tension, and leads to a smaller one as it cools down Shape change takes place in the direction of the original deformation. For subsequent thermal cycles | a small two-way effect is created.

If a voltage is applied to a memory alloy exercises, the temperature range of the phase transition shifts to higher temperatures, as dashed in Fig. 2a indicated. The transition from the low to the high temperature phase between about -180 ° C can take place unloaded and +250 C, depending on the alloy composition.

In Fig. 4, the chemical energy G is the martensite and Austenite phase shown as a function of temperature T.

At low temperatures, martensite has a lower *

Energy as austenite. Above a temperature T _n , at \

both phases are in thermodynamic equilibrium and have the same chemical energy G, austenite has a lower energy than martensite.

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-S-

If austenite is to be converted into martensite by cooling, so the shape memory alloy has to be cooled below T. are, as shown in Fig. 5, since elastic or nucleation energy is included because of the volume differences must become.

At a martensite transformation start temperature T begins the transformation, at a final martensite transformation temperature T --_ it is completed. i.e. no austenite remains return. Even at temperatures above T, however, an externally applied stress can cause a

chemical energy contribution to the nucleation or / for the martensite supply, ie j it came ei ··, stress induced he · martensite are generated. If a body made of a shape memory alloy is first _{cooled below T MO} or T and then

IHo 1 "Ir

is formed, among other things, by a twin vision processB a reorientation of the martensite platelets is produced which, when heated, leads to a reversal of the deformation leads.

The transformation from martensite to austenite begins at a so-called critical Te
at a temperature T._.

so-called critical temperature 1. " above T _n and ends

To create a large shape memory effect ^ u. the volume change caused by the phase transition must be small, otherwise many dislocations will be generated in order to adapt the volume differences. As a result of an increased dislocation density, the martensite is solidified or hardened and reorientation due to an applied stress is no longer possible, as is the case with steel, for example. The most important property of a shape memory alloy for use as a switching element is that the

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Reconversion of the deformation during heating work done or a voltage can be generated. For Ni-Ti based alloys an energy density of 2 MJ / m can be achieved, which is much higher than that of bimetal. There are tensions due to a hindrance to the change in shape during heating of more than 650 MPa can be generated. The achievable force and the path depend on the sample geometry. In the case of a round rod, the length of which is 10 times as large as its diameter, the bending force is 20 times smaller than the tensile force, but the displacement of the free end in the case of bending is 10 times as large as the displacement with tensile stress.

In FIG. 6, curve a shows the resistance R of a wire made of a mechanically loaded or prestressed shape memory alloy before heating as a function of time t. Curve b was generated by sending a short-term current pulse from a capacitor discharge (875 V, 25 μF ) through this wire, thereby causing heating. The resistance characterizes the structure or the phase change of the alloy. The movement of the wire, which corresponds to the measured resistance oscillation en ^{v, is calculated using an in Fip.} 7 explains that K is the spring constant of a spring, m is a mass, γ is the damping constant, x is the path or a deflection, χ or x 'is the first and second derivative of the path according to time and g is the acceleration due to gravity mean. The differential equation applies to this model:

mx '+ ^ χ - Kx ■ = -mg (1)

with the initial condition:

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χ = χ + 2jS and χ = O for t = O, (2)

max K

where χ means the maximum elongation that can be achieved by Aufmax

heating a spring body as a result of a shape memory effect is attainable. Under these assumptions, the differential equation (1) has the solution:

χ ₌ JHax. _exp (-a to t). cos («* t + y, ) + ψ- (3)

COSJ; K

with

ω- =
/ m (1 - λ ² )

tan If = -2 λ and (5)

a = s

^U = \ ^K

represents the elastic energy of the spring, which, when heated, changes its shape within a characteristic static movement time

The spring constant K depends on the type of movement, such as tension, bending, etc., on the type of shape memory alloy and on the magnitude of a mechanical preload. Without pre-tensioning, K for tension or compression results in:

K = - ^. ^ ^s ' ^Ah · ^d (9)

At = 4 f ^ '(8) 1

m 2 'K I

is implemented with an effective force F.

advice

q Cross-sectional area of the shape memory body 1 length

d relative density

C specific heat capacity

; .. s specific entropy

/ lh specific enthalpy

C specific heat capacity

(■ relative change in shape caused by deformation and d

£ maximum relative change in shape due to phase change.

If a body or wire made of a shape memory alloy is heated up by one flowing through it electric current I, then applies to temperature changes over time dT / dt in a temperature range in where there is no change in shape caused by a shape memory effect, the relationship:

dT _ RI (t) ² (10)

dt "C V d

V ^! where V is the volume of the body.

Above the critical temperature T for the recovery, a mechanical tension (Γ proportional to T - T _flC,:

constant
where (T _{0 is} a material · ''. This voltage 6 "is in

those shown in FIGS. 3 and 8a, reference numeral 38 Thermal switches used to trigger a switching process.

The principle of the thermal switch is illustrated in FIG. 9.

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purifies. Any time-dependent physical variable f (t) j which is intended to trigger a switching process or to provide a switching signal y in a predeterminable condition, e.g. when a certain limit value is exceeded, is converted into one of these input variables f (t) in a converter 41 assigned time-dependent heat or thermal energy Q (t) transformed, if it is not already specified as heat. This heat Q (t) is transformed in a further converter 42 into an acceleration χ, which is compared in a comparator 43 with an acceleration limit value x'q. For χ> x _Q , the switching signal y i is present at the output y of the comparator 43, otherwise the signal y. _{This signal y 1} or y, which is dependent on the output of the comparison, has the same meaning as "switching on" or "switching off" or "switching over" a switch.

At the moschaltern according to Pig. 3 and Pig. 8a, reference number 38, is the time-dependent physical quantity f (t) an electric current I (t). The transducers 4l and 42 are made of a shape memory alloy by the switching element 18 educated. The electric current I (t) is in this switching element according to

t
Q (t) = ' ^d Rl ^ dt (12)

in heat, this in accordance with equation (11) in mechanical tension and this in accordance with equations (7) and (1) in force F or acceleration ^ transformed. The comparator 43 is represented by an adjustable cash register l6, the Inertia of this mass is used to realize the acceleration limit value x ". Only when one through the

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If the force effect or acceleration x 'generated when the switching element is heated up exceeds this predeterminable limit value, the switching signal or a switching process is triggered. Slowly running current or temperature changes in the switching element 18 do not trigger a switching process. Likewise, releasing current changes, which correspond to temperature changes outside the phase transitions between T. " and T." or between T _M "and T, _p J , do not emit any switching signals. It is thus a selectively acting thermal switch which, among other things, is suitable for quickly switching off overcurrents. The phase transition from martensite to austenite corresponding to the temperature range from T to Tp is used for this. This switching range can be set by dimensioning the switching element, the composition of the alloy and mechanical preload.

The thermal switch shown schematically in Fig. 3 consists of the switching element 18 in the form of a round rod

2 or wire of 6.5 mm length and 0.33 mm cross section made of an alloy of 3 M % Ni, 13 M % Al and 84 M % Cu, compare wire number 9 in Tables 1 and 2. Another example for a switching element is designated there with wire number 3. The upper end of the formwork element is held in an upper holder 19 which is immovably attached to a fastening 40. The ur.tere end of the switching element is coupled via a bracket 15 with an adjustable mass 16 of 14.8 g and a tension spring 14, which can also be omitted and which is set in the present example with the spring constant K = O. The train spring is on one side in a fixture 13 keep immobile ge. Mass 16 and tension spring 14 work together as a tensioning device and generate mechanical tension

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that in the low-temperature phase by stretching more pseudo-plastic deformed switching elements 18, which are below its rupture stress lies. A current I is sent through the switching element at a room temperature of 20 C. the occurrence of an overcurrent is to be monitored. Exceeds the current-time integral

SZ = l ~ ² I ² dt, ^ t = t "- t ₁ (13)

within a time interval with Δί = 2 ms the value of

SZ = 1000 A s, the switching element 18 breaks and interrupts the circuit, as shown in FIG. 3b. The switching element acts as an electrical fuse with adjustable sensitivity. If the increase in current is faster than a predeterminable Wej / c, the mechanical stress T exceeds the rupture voltage of the switching element before the mass 16 begins to move. The initial shape of the switching element is chosen so that the distance between the fragments after tearing is large enough to ensure separation of the arc. In the event of a slowly increasing overcurrent v /, the mass 16 is moved and C will never exceed the rupture voltage of the switching element. The values of m and K are expediently selected using a two-way memory alloy for the switching element in such a way that the switching element is deformed back by cooling, compare FIG. 2b, after an overcurrent with a subcritical increase has passed. This thermal switch can also be used with a switching element with a one-way memory effect, with changes in shape of up to about 8 % being usable. Instead of a direct interruption of the circuit, the change in position when tearing

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Les switching element in a known manner .ösen off of another switching process, if necessary lit intermediaries gain of the switching signal are erwendet ^r as a means to. The thermal switch acting as an electrical fuse according to FIG. 3 can be used instead of a fuse in a conventional thermoelectric switch 50 according to FIG. 8, which is known for protecting electrical circuits against slowly or suddenly increasing overcurrents.

'ig. 8a shows such a thermoelectric switch OK in a semi-schematic representation, but with a thermal switch 38 which has a switching element 18 with a two-circuit memory effect which cannot be reached during heating and thus cannot directly affect the circuit. In contrast to the thermal switch according to ¹ Ig. 3, the upper holder 9 of the switching element 18 is not mounted immovably in the thermal switch 38, but is connected to a linkage 12 in the catch arm 23 via a mechanical coupling 20 and is movable against a tension spring 11.

er) electrical circuit passes from one connecting line 'a fixed contact 6, a contact end 8 of a contact Lrir.es 4, which is pivotable about an articulation 1, a ^r Getting Connected line 28, an additional securing element 9, a ^r Getting Connected line 22, the upper bracket 19 through the switching element 18 to the lower bracket 15 to a connection line 17-

'reads through the switching element an overcurrent, the one change in shape or here for contraction due to the shape ; memory effect leads, so will

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1) the linkage 12 is offset against the spring force of the tension spring 11 downwards and

2) the mass l6 and the tension spring 14 coupled to it moved upwards.

For the second movement, the movement time is 4t according to Equation (8) characteristic. As soon as the movement is completed, the tension T disappears, compare Equation (11).

The first movement will only trigger switch 50 if 5 "rises above a prescribable critical value, which is necessary for the translation of the articulation 12 downwards by e.g. 1 mm is sufficient. Otherwise, the switching element 18 due to the setting of the values of mass 16 and spring force K of the tension spring 14 after the passage of a deformed back by cooling under ^ critical overcurrent

However, if there is a downward deflection of the linkage 12, the catch arm 23 is moved around a linkage 21 in the Pivoted clockwise, as indicated by dashed lines, and gives an angle arm 37 for pivoting against the Clockwise free. This pivoting is blocked as long as a hook end 25 of the tentacle arm 23 is at one end of the catch 24 of the angle arm 37 is applied. Under the action of a compression spring 2, which is supported on an attachment 3, and seeks to move the contact arm 4 to its articulation 1 clockwise, the angle arm 37 is by means of a push arm 29 pivoted about a linkage 26 counterclockwise, as shown in Fig. 8b. The contact arm 4 lifts from the fixed contact 6 and interrupts the electrical connection between the

t · · t · ι ι ι ι ι

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Connecting line 7 and the connecting line 28. The Push arm 29 is connected to contact arm 4 by a linkage 5. At its lower end it has a linkage 27, in an opening 36 of the angle arm 37 can slide when the angle arm is pivoted. When pivoting, the articulation 27 slides in the opening 36 to the left, whereby the also around the articulation 27 swiveling Ruckstellhel. el 32 by means of a push arm is moved into an off position shown in Fig. 8c.

^öer reset lever 32 is under tensile stress of a tension spring 34 which is anchored in a fastening 37. It is connected to one end of the push arm 30 via a movable articulation 31. The other end of the push arm 30 is connected to the push arm 29 by the movable articulation 27. The switch 50 can also be switched on again with the reset lever 32 by pivoting it clockwise.

The thermal switches according to FIG. 3 and FIG. 8a, reference number 38, are expediently in addition to an additional switching element ment 9 applied, which can be connected to the tentacle arm 23 by means of a mechanical coupling 10 and to others responds to parameters relevant to protection.

Rated values for switching elements 18, which are suitable for use in the thermal switch 38, are in the Tables 1 and 2 under the. Wire numbers 1 through 7 specified. The values in Table 1 apply to a room temperature of 20 C, a deflection due to one of the shape memory effects of 1 mm and a spring constant K = O of the tension spring 14. The tension spring 14 can be described in both cases Thermal switches are omitted, but it is suitable for Lagestkbilisierung the thermal switch and for application

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a bias on the switching element 18.

Among other things, the material properties are suitable for the shape memory alloy of the switching element 18 and in terms of price, especially alloys based on Ni Ti, Ni Ti Cu according to DE-OS 2 6Ί4 OHl ,. Cu Zn, Cu Al, Ni Zn also with ternary and other components such as Cu Al Ni, Cu Zn Al Ni etc.

It goes without saying that the invention is not restricted to what has been presented above. For example, shaping of the switching element 18 strip-shaped, tubular, spiral-shaped, curved, etc. be designed. The switching element can be deformed by bending, twisting, upsetting or stretching. The tripping of a thermoelectric switch 50 according to FIG. 8 can be done by pulling or pushing. The change in position caused by the switching element can be carried out in a known manner, e.g. by means of optical, electrical or hydraulic detectors are recorded, amplified and transmitted to another switching element.

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Tabel

1 mm

5.2 5.2 53 53 78 78 780 .3.6 6.5

mm

20 20

40 1.5 0.33

m g

38 950 950 3 14.8 3 14

38 14.8

K (II) MN / m

0.36

0.36

3.6

3.6

0., 3

0.3

SZ
A ² S

1. ΙΟΙ, ιοί. 10-1.10 ^ί
1.10 ^ί
1.10 ^ί
1.1O ¹

ms

1 5 5

10 2

10

20th

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OJ

Q) rH rH

CD

.α cd

κη O σ • 10 O < O in CO CO Eh r-i r-i co O bO • Η · Η rs 3 -H rH 3 C.
Ph rH See O O S <U Φ · Η in in VO • Η Φ S. O o m. =} · bO-P ^ r in η rH m rH CO Φ C ■ = r - = r O J cd C-
CO in S. t- O . α-, C! I. O Ln c— vo σ ι
«O
r-i r-i c—
rH r-i C— O ^ j- Λ O O _! _ S. O OJ rt O CO W. VO r-i ,--1 - CO OJ O . = r O hfl I. m r-i ■ = r Q- c— Μ VO O * ^, 1-3 O ε O "ω r-i C- CU U Q I. in

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Designation list

1 Linkage of 4 2 Compression spring 3 Attachment of 2 4th Contact poor 5 Articulation 6th Fixed contact 7th Connecting cable 8th End of contact from 4 9 Assurance Element 10 mechanical coupling 11th Tension spring 12th Articulation 13th Fortification of 14 14th Tension spring 15th lower bracket from 18 16 Dimensions 17th Connecting cable 18th triggering switching element 19th upper bracket of 18 20th mechanical coupling 21 Linkage of 23 22nd Connecting line 23 Tentacle 24 Catcher from 37 25th Hook end from 23 26th Linkage of 37 27 Articulation 28 Connecting line 29 Push arm 30th Push arm 31
> * ■ Articulation

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»■ ft t · * * · ■ ·

32 Reset lever 33 Articulation 34 Tension spring 35 Fortification of 34 36 Breakthrough from 37 37 Angle arm 38 Thermal switch 40 Attachment 41 Converter 42 Viand ler 43 Comparator 50 counter

A austenite ■ ·

C specific heat capacity

d relative density ^;

f (t) time-dependent physical quantity?

P force {

g acceleration due to gravity

G chemical energy |

h specific enthalpy |

I electric current?.

K spring constant I

1 large j

m mass

M martensite

q cross-sectional area

Q (t) time-dependent energy

s specific entropy

t time c

t mechanical movement time?

T temperature

U elastic energy of the spring

I · ·! It t «·« ■

• t ti · (ι ·· tat

t ■ t t it it

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χ y Path, deflection X *1 speed X ^y 2 acceleration *G SZ Acceleration limit Max © <: maximum elongation if Switch output, switching signal ^ d Switching signal "on" Signal "off" Current-time integral Damping constant λ elastic deformation applied by deformation relative change in shape (Γ maximum relative change in shape through phase transformation > Damping constant UJ specific electrical resistance was standing mechanical tension Material constant Phase angle Angular frequency

Claims (1)

Protection claims t ο 1. thermal switch, - With a switching element (l8) of a temperature-dependent variable shape made of a memory alloy - That at a first point in a first holder (15) and - Is attached to a second point in a second holder (19), characterized in that - This first holder (15) is movably arranged and - is firmly connected to a body (l6) of predeterminable mass - 2. Thermal switch according to claim 1, characterized in that - This second holder (19) is immovable. 3. Thermal switch according to claim 1, characterized in that - this second holder (19) is attached to a movable part of an elastic body (11), - which is immovable in at least one place. . Thermal switch according to claim 3, characterized in that
- This elastic body (11) is a spring. * ··· «■ · ■■>» I! ■ - 2 - 175/78 5. Thermal switch according to at least one of the preceding claims, characterized in that - This first holder (15) is attached to an elastic body (14) which is at least one point (13) is arranged immovably. 6. Thermal switch according to claim 5, characterized in that that - the elastic body (14) is a spring. 7- Thermal switch according to at least one of the preceding Claims, characterized in that - The switching element (18) consists of a shape memory alloy with a two-way memory effect. 8. Thermal switch according to at least one of the preceding claims, characterized in that - The switching element (18) is rod-shaped.