DE19933956B4 - Electrical circuit arrangement with temperature compensation for a shape memory metal - Google Patents

Abstract

Electrical circuit arrangement (1) comprising
- A shape memory metal (3), via which a current is feasible for heating and thus the change in shape at a transition temperature, wherein the required to achieve the transformation temperature and thus to initiate the change in shape nominal current (I _soll ) decreases with increasing ambient temperature,
- A parallel to the shape memory metal (3) connected thermistor (5), and
- A current source (2), which outputs a constant total current (I _ges ), which corresponds to the sum of the desired current (I _soll ) of the shape memory metal (3) and the current (I _thermistor ) of the thermistor (5),
wherein the resistance characteristic of the thermistor (5) is selected to compensate for the dependence of the desired current for triggering the change in shape of the ambient temperature so that it can absorb with increasing ambient temperature substantially the excess due to the temperature dependence of the desired current excess current.

Description

• – ein Formgedächtnis-Metall, über welches zur Erwärmung und damit zur Formänderung bei einer Umwandlungstemperatur ein Strom führbar ist,
• – und eine Stromquelle zur Bereitstellung des dem Formgedächtnis-Metall zuführbaren Stroms.

The invention relates to an electrical circuit arrangement comprising.

• A shape-memory metal over which a current can be conducted for heating and thus for changing the shape at a transformation temperature,
• - And a current source for providing the shape memory metal feedable stream.

In such circuits can as special components switching elements or actuators are used wherein the actual switching operation, by means of which, for example a circuit opened or closed, by means of by preferably pulse-shaped application a current achievable shape change of the shape memory metal is reached. Such metals, mostly alloys based on a CuAlNi systems or a NiTi system are also referred to as memory alloys. They are characterized by the fact that they are due to appropriate treatment due a martensitic transformation of their shape in dependence change from the temperature. requirement for the Occurrence of the shape memory effect is a transformation from a crystal structure into a martensitic one Structure with a specific stacking sequence. This conversion is reversible, which is due to that in the transformation of martensite to austenite only slight elasticity Tensions arise, the practical no irreversible deformation effect due to dislocation movement. In these shape memory metals As a rule, a distinction is made between one-way and two-way effect. The one-way effect is the original Form after a strengthening, apparently plastic deformation assumed again when the metal heated becomes. As a two-way effect is called the deformation in which the metal hysteresis as it heats up and cools down in its respective temperature-related shape changes, that is, the metal increases with temperature change in one for each alloy characteristic temperature range in each "trained" form.

As can be described as common Shape memory materials especially CuAlNi alloys and NiTi alloys are called. The size of the memory effect and the stability The memory properties of NiTi are superior to those of CuAlNi. However, the transformation temperatures, ie the temperatures, where the deformation starts and which is also called "austenite start temperature", of NiTi alloys to a maximum of 100 ° C limited. In contrast, Cu-AlNi alloys Conversion temperatures up to about 200 ° C, sometimes over it, so that these alloys even at high ambient temperatures (up to about 180 ° C; for comparison: NiTi alloys up to 80 ° C) can be used. Such inserts are For example, in the automotive sector or in the field of electronic Circuit technology conceivable. The stability of these alloys against Aging at elevated Temperatures are low. As a result of the temperature load degrade these alloys, resulting in a continuous Change the transformation temperature and a decreasing deformation can express. To ensure stable memory characteristics even at high switching speeds must therefore be overheated materials are avoided during operation. Such overheating occur in devices with an electrically driven shape memory metal at elevated Ambient temperature, as at the room temperature compared to room temperature significantly increased a constant heating current pulse is passed through the shape memory metal, which leads to a certain increase in temperature, the quasi "additive" to the already elevated temperature added, causing destruction lead of the device can. Although the alloy has to be switched to a certain temperature for switching are heated above the transition temperature, e.g. 20 ° -40 ° higher. This "overheating" is in the short switching or heating cycles not harmful even in the long term. The injury occurs more frequently significant warming over the Conversion temperature, to which it is used at elevated ambient temperature but can come easily. Such elevated ambient temperatures from, for example, to about 80 ° C enter e.g. in an automobile under the bonnet or else in control cabinets without further ado. An insert of the described components under Such is highly variable with respect to the temperature conditions not possible.

Out JP 01-143301 A (abstract) discloses an actuator, the one helically comprises a shaped memory alloy element. This Actuator element is by means of a heating device to a transition temperature to heat the alloy. To a temperature-controlled heating to ensure the actuator element For example, as a tubular heater, the heater is the actuator element enclosing Heating element made of a PTC material.

Also in the JP 09-231891 A (Abstract) removable current limiting interrupter is a temperature-controlled heating of an actuator element made of a shape memory alloy. For this purpose, the actuator element is also of a heating device surrounded by PTC material.

From the DE 35 38 964 C2 and the DE 30 22 168 A1 In principle, it is known to eliminate temperature-induced fluctuations in the electrical measured quantities of ultrasonic transducers or of the holding current of relay by means of integration of temperature-dependent resistors in the circuits of these devices.

Of the The invention is therefore based on the problem, a circuit arrangement specify the type mentioned, the shape memory metal at least within a certain temperature fluctuation range can be used without the use of a heating current This temperature range operational overheating are to be feared.

to solution This problem should be the measures specified in claim 1 be provided.

at the circuit arrangement is therefore on the electrically controllable Shape-memory metal for warming and thus to the change in shape a current, preferably pulse-shaped, feasible wherein a current limit occurs, with an unintentional temperature change of the metal, in a change of the required to reach a predetermined heating temperature Current results within one below the transformation temperature lying of the metal temperature change range at least partially so compensated that when concerns one independently of the temperature constant current substantially only the still required to reach the predetermined heating temperature Electricity over that Metal guided becomes.

With the inventively provided Device is advantageously achieved that the excess power component is dissipated and not over the shape memory metal flows, but only the proportion of electricity to the "Resterwärmung" on the above the transformation temperature lying "operating temperature" starting from the increased Temperature of the metal is still required. As "unintentional temperature change" is not one actively or deliberately scored warming of the metal, for example, from an increase in the ambient temperature, as they are z. B. occurs in the engine compartment of a car while operating the same, occurs. It is thus achieved that on the metal only the Electricity share led which is at the given actual temperature of the metal is required to bring it to the deformation-inducing transformation temperature to warm up. Overheating will thereby imposed, so that with particular advantage shape memory metals can also be used where with considerable temperature fluctuations is to be expected, whereby this fluctuation range can be maximally so large how the device is able to compensate.

To a current limit according to the invention to a shape memory metal parallel connected thermistor be provided. The parallel connected thermistor is characterized by it out that its electrical resistance with increasing temperature decreases and thus the proportion of the current flowing through the thermistor, increases. This leader becomes dependent selected from the thermal current characteristic of the shape memory metal, i. it will be first determines which current at a given outlet temperature the metal is still needed to assist in the heating of the rest each to achieve a constant current pulse time. Starting from the This behavior can then be determined by the appropriate leader chosen whose resistance behavior is a compensation of the excess current component allows the constantly applied at each temperature current. It should expediently a temperature change up to at least 60 ° C, in particular up to at least 80 ° C - starting from room temperature - compensatable this being ultimately dependent of the shape memory metal used in each case and the specific transformation temperatures. It is on it to point out that, of course Also embodiments possible are that when using a CuAlNi alloy with a transformation temperature from about 180 ° C and an intended use temperature range of, for. B. 100-150 ° C compensation only in this temperature range is sufficient, since others, lower Temperatures do not occur during operation and therefore no compensation is to be provided.

Further Advantages, features and details of the invention will become apparent the embodiments described below and with reference to the Drawings. Showing:

1 a schematic diagram of a circuit arrangement of a device according to the invention using a thermistor,

2 a diagram showing the applied to the shape memory metal target and actual current when using a thermistor at a current pulse time of 2 seconds, and

3 a diagram showing the applied to the shape memory metal target and actual current when using a thermistor at a current pulse time of 1 second.

1 shows a basic circuit arrangement 1 according to the invention with a component, such as a switching element or other Actuator. The circuit arrangement comprises a current source 2 , by means of which a constant total current I _{ges is} pulsed gebbar. The circuit arrangement further includes a shape memory metal 3 in the form of a wire, which may for example be 10 mm long and 0.6 mm in diameter. About this is to be heated to the transition temperature at which the deformation occurs, a temperature-dependent target current I _want to lead, the size of which depends on the actual temperature of the shape memory metal, which is usually specified on the ambient temperature dependent. Parallel to the shape memory metal 3 is a facility 4 in the form of a thermistor 5 over which the excess current component (I _thermistor ) which is not required to heat to the "operating temperature" above the transition temperature and which, in the event that it flows over the shape memory metal, would result in overheating thereof, to be led. The higher the actual temperature of the metal, the lower I _soll and the higher I _thermistor . This ensures that regardless of the actual temperature, the metal is just heated to the desired temperature. The electricity balance is thus as follows: I ges = constant = I should + I thermistor

In this case, the thermistor is chosen such that its resistance changes so that with increasing temperature essentially the current is "branched off", which according to I _thermistor = I _ges - I _soll (temperature-dependent) is excess.

2 shows a diagram in which the experimentally determined target current I _soll and the current actually flowing through the shape memory metal when using a thermistor actual current I _is applied. Along the abscissa, the temperature is plotted along the ordinate of the current applied to the shape memory metal. A shape memory metal made of a CuAlNi alloy with an austenite start temperature of A _s = 160 ° C. was used. At this temperature, the transformation of the martensite responsible for the change in shape starts an austenite, which is completed at the austenite finish temperature. The temperature difference is about 5 K.

In general, the austenite start temperature is referred to as the transition temperature. The current pulse time was 2 seconds, the applied, constant total current I _ges was 17.26 A. As a thermistor, a thermistor from Siemens-Matsushita with the type designation 5464-10M was used. The "operating temperature" required for switching in this example is about 200 ° C at room temperature.

As can be seen from the graph, the power requirement for heating the metal wire decreases with a pulse duration of 2 seconds from 15 A at 25 ° C to 12, 6 A at 80 ° C (I _soll curve). Such ambient temperature variations readily occur, for example, in automotive applications. If no compensation were made in this case, the temperature of the wire would rise during the current pulse at constant 15 A of about 200 ° C at 25 ° C up to 300 ° C at 80 ° C. The wire would be so much overheated, which can lead to degradation to destruction. When using the thermistor, however, it turns out that the actual current I flowing over the wire _is approximately equal to the desired current I _soll . As described, a constant total current of 17.26 A is present at each measuring point. As a result of the change in temperature, the resistance of the thermistor changes, so that the current flowing through this thermistor current I _thermistor increases, the actual current share decreases, however, as the figure is clear. The example shows that in this case a sufficient compensation in the temperature range 25 ° C-80 ° C was achieved. For use, this means that such a device can be used readily in the temperature range 25 ° C-80 ° C, as is achieved due to the compensation effect of the thermistor that at least in this temperature range on the memory metal only to heat to the predetermined Temperature actually required power component is applied.

3 shows a further embodiment with the same experimental components. In contrast to the example described above, however, the pulse duration was only 1 second, the constant total current was 21 A, was thus somewhat higher than in the above-described embodiment, because due to the much shorter pulse time more current must flow to heat. When looking at the nominal current curve, it shows that it drops from approx. 20.7 A at 25 ° C to 16 A at 80 ° C. As the actual current curve can be seen, was also here by using the thermistor, which comes also from the company. Siemens Matsushita, a sufficient current limit is reached, the curve of the actual current I _is equal to a good approximation, the curve of the target Electricity I _should .

Claims (4)

Electrical circuit arrangement ( 1 ), comprising - a shape memory metal ( 3 ), over which a current can be conducted for heating and thus for a change in shape at a transformation temperature, the desired current (I _soll ) required to reach the transformation temperature and thus triggering the change in shape decrease with increasing ambient temperature, - a shape memory metal ( 3 ) connected thermistor ( 5 ), and - a power source ( 2 ), which gives a constant total current (I _ges ), which at room temperature the sum of the desired current (I _soll ) of the shape memory metal ( 3 ) and the current (I _thermistor ) of the thermistor ( 5 ), wherein to compensate for the dependence of the desired current for triggering the change in shape from the ambient temperature, the resistance characteristic of the thermistor ( 5 ) is selected so that it can absorb with increasing ambient temperature substantially the excess due to the temperature dependence of the desired current excess current. Circuit arrangement according to claim 1, characterized in that the total current (I _ges ) can be fed in a pulse shape. Circuit arrangement according to claim 1 or 2, characterized characterized in that a temperature changes up to at least 60 ° C, in particular until at least 80 ° C is compensable. Circuit arrangement according to one of the preceding claims, characterized in that the shape memory metal ( 3 ) is a CuAlNi alloy or a NiTi alloy.