DE4237131A1 - Thin film windshield heater for aircraft - Google Patents

Abstract

The electrical windshield heating system includes a windshield structure having an electrical heating film embodied in it, and a circuit for powering the heating film. The circuit comprises a converter (3, 7, 8) which generates a relatively high DC voltage supply from a relatively low DC voltage source. A coupler transfers power from the relatively high DC voltage supply to the heating film and thereby causes a DC voltage to be applied to the heating film.The coupler comprises a commutator (10) which periodically reverses the polarity of the DC voltage at a low rate e.g. every 15 seconds. A transformer has its primary winding connected to the DC supply via a switch.

Description

The invention relates to a windshield heating system and especially affects electrical windshield heating systems systems, especially those for small aircraft where only low span DC electrical supplies voltage (e.g. with a nominal voltage of 24 V =) are available.

When electric heating elements for low voltage and high Electricity was embedded in laminated windshields problems arose due to optical distortion when the windshield is heated by the wire heater ment took place.

With a view to overcoming these problems with opti distortion, it is common for windshield heaters elements in the form of a thin heater designed for high voltage to use filmes in the laminated windshield is included. The even distribution of the heating film or the heating layer and thus the uniform heating the windshield leaves very little optical distortion tion arise. Because of the relatively high electrical resistance However, the thin film heater is relative high operating voltages (i.e. above 100 V) are required, and These have so far been created by converters from Nieder voltage generated higher voltage. The change becomes norma usually performed at the relatively low frequencies, which are common in electronic aircraft AC Supply systems (e.g. 400 Hz to 1 kHz) are used and it is often the generation of rectangle waves lens signals connected at the required frequency. Derar low-frequency converters require the use of un wishes bulky transformers while generating Square wave signals unwanted interference with other off armor (e.g. radio equipment in the aircraft cockpit) let arise.

An object of the present invention is to list them overcome the disadvantages of the prior art.  

According to the invention, an electrical windshield Heating system a windshield structure with one in it embedded electrical heating film and a circuit for Feeding the heating film, the circuit comprising transducers tel to generate a DC power supply with relative high DC voltage from a DC source with relative low voltage and means to couple power from the power supply with relatively high DC voltage in the heating film, the means for coupling power thereby applying a DC voltage to the heating film ver causes.

Eliminating the high frequency components from the at the Windshield electrical supply chip voltage reduces the electricity from this source interference.

The high DC voltage can be used periodically of a commutator circuit can be reversed.

The commutator circuit can be appropriately controlled solid-state Switch means (e.g. thyristors) included to a high voltage power supply with slow rise and To create waste characteristics that match their polarity relatively long intervals (e.g. 15 s) reverses, so perio the polarity of the heating film on the windshield reverse applied DC voltage.

This periodic polarity reversal is provided with regard to this hen that due to a possible film hike in the windbreak optical deterioration could occur.

The system can have means to determine the effective amplitude the voltage applied to the heating film as a function of to change the time. This can change the size of the wind protective power applied at a corresponding rate be changed, so thermal stresses and sudden loading Avoid load of the low-voltage DC power supply  the one from which the system draws its energy.

The commutator means can be from a full bridge circuit stand. Each branch of the bridge can be a controllable electro Include African component, wherein commutation causes is done by placing two components in opposite Branches of the bridge are released, with the output po is determined by which the two components be released. The components can be conventional Thyristors or gate-switchable thyristors (GTOs). Alternatively, conventional transistors such as bipolar transistors interferences or FETs are used that are continuous Tax signals need to be applied in order to to withstand.

The converter can be an inductive component such as a Trans formator, the primary side of the transformer periodically coupled to the DC power source, so an alternating magnetic flux in the transformer core testify. The transformer can be a high voltage secondary have a winding that develops a high AC voltage, whereupon this tension is rectified in order to To generate DC voltage.

Alternatively, a DC converter can be used without an intermediate change voltage are used, for example a flyback converter (flyback converter).

The output voltage developed by the converter can be are tuned by adjusting the amount of time in the stream from the DC voltage supply with the inductive compo nente of the converter is coupled.

The converter can be operated so that it has a number of Output pulses generated at which the voltage of each pulse it increases from substantially zero to a maximum value and then returns to zero. The commutator can are operated essentially synchronously with the pulses, so  that the commutator the polarity of the output pulses during reverses the times at which the output voltage in the we is substantially zero.

Alternatively, the high DC voltage supply can be constant be, the commutator causes a polarity reversal. If the commutator is made up of components, their Conductivity can be controlled continuously Commutator itself can be made to be the size the output voltage changes between zero and maximum so that commutation takes place at times when the output voltage is essentially zero. Because the out output voltage needs to change only slowly, right relatively cheap slow-switching power components the commutator can be used.

Alternatively, you can change the output voltage in assignments to a switching commutator a discrete in series or Controller connected in parallel is used will.

In another embodiment of the invention, the Heater applied voltage can be changed in that a Variable impedance control between the DC power supply and the heater is used, in which In case a switching commutator can be used.

If it is ensured that the one applied to the heater Voltage with a relatively low voltage change rate In terms of time, d. H. essentially no high contains frequency components, a disturbance is significantly re induced and the power supply source with low voltage is not exposed to sudden changes in electricity needs.

The invention is hereinafter based on non-limiting embodiment Rungsbeispiele with reference to which the drawing explains tert; in this shows:  

Fig. 1 is a block diagram of an electric windshield heating system according to the invention;

FIG. 2 is a circuit diagram of part of the heating system from FIG. 1;

Fig. 3 is a waveform diagram illustrating the operation of the embodiment of Figs. 1 and 2 in use;

. Figure 4 shows a portion of Figure 3 on an enlarged scale; FIG. and

Fig. 5 is a waveform diagram illustrating the operation of the embodiment of FIGS. 1 and 2 when first switched on.

In Fig. 1 and 2, the same reference numerals are used to load drawing the same units used. The solid-boxed boxes of FIG. 1 correspond to the dashed boxes with the same number in FIG. 2.

In Fig. 1 provides a DC power source 1 with low voltage of z. B. 28 V power to a low voltage bus 101 . An input filter 2 blocks the coupling of faults back into the busbar rods 101 . Schaltele elements 3 control the current flow in the primary windings of a transformer 7th A rectifier 8 rectifies the AC voltage output signal of the transformer 7 and applies it to an output filter 9 , which removes high-frequency hum from the rectified AC output signal. A DC supply current with a higher voltage of approximately 100 V appears at its output terminals 90 . A commutator 14 couples the supply direct current with high voltage to its output 100 and optionally applies the polarity of this direct voltage to its output 100 . An output filter 12 after the commutator 14 blocks the coupling of faults to the windshield heater 13 . Feedback signals on line 6 from transformer 7 and external control signals on line 5 are applied to a control circuit 4 which controls switch elements 3 and commutator 14 . In certain applications that do not require polarity reversal, commutator 14 may be left as will be described later.

A detailed description of the circuit will now be given with reference to Figs. 2-4.

Transistors TR1, TR2, the transformer 7 and the control circuit 4 comprise allowing a push-pull inverter circuit is generated in the control circuit 4 wel chem signals on lines 16 conduct alternately so as TR1 and TR2. This allows current from the low-voltage DC power source to alternately flow into the windings P1 and P2, whereby an AC voltage is developed across the secondary winding SA. A feedback winding SB develops a feedback voltage proportional to the secondary winding voltage. This feedback voltage is rectified and fed back via line 6 to the control circuit 4 , whereby both transistors TR1 and TR2 are switched off when the instantaneous output voltage has reached the required value. Input signals on line 5 can be used to further control the switching times of transistors TR1 and TR2 and thereby to control the secondary AC output voltage. The secondary winding SA generates the required amount of voltage increase. The transistors TR1, TR2 are at a very high frequency of z. B. switched 50 kHz. The AC output signal high voltage is at a correspondingly high frequency and is rectified in the bridge rectifier 8 . The rectified AC voltage is smoothed in an LC filter circuit 9 . C1 is a high-frequency capacitor for creating a shunt path for the high-frequency hum components, the capacitor C2 is an electrolytic capacitor. The smoothed direct current is applied to a commutator circuit 10 which contains four thyristors TH1-TH4 in a bridge configuration. The switching of the thyristors TH1-TH4 is effected by the thyristor trigger circuit 11 , which is controlled by signals from the control circuit 4 via lines 15 .

The thyristor bridge allows the output polarity of the voltage occurring at its output terminals 100 to be reversed. For example, when TH1 and TH3 are turned on, an output voltage of one polarity is generated, while when TH2 and TH4 are turned on, an output voltage of the opposite polarity is generated. The voltage occurring at the terminals 100 is further filtered in the filter 12 and applied via output terminals 120 to the windshield heater, not shown.

The operation of the commutator 14 will now be described. It is assumed that all thyristors TH1-TH4 are non-conductive in the initial state. A positive voltage pulse on line 15 makes TR3 conductive. This results in a current flow in the primary winding of the pulse transformer PT1 and induces respective voltage pulses in the secondary windings S1, S2. These voltages are passed through their respective coupling networks to the gate electrodes of TH3 and TH1. The operation of the coupling networks is easily understood by the person skilled in the art and need not be described further. Once turned on, TH1 and TH3 remain on until turned off when the current through them drops to zero. After TH1 and TH3 have switched off, it is necessary to generate an output voltage of the opposite polarity, to apply a pulse to line 15 ', which then allows TH2 and TH4 to be switched on.

The operation of the circuit will now be described with reference to FIGS. 3 and 4.

It is assumed that at time t ₀ both TR1 and TR2 are off and there is no voltage in the circuit somewhere between the transformer 7 and the windshield heater. The control circuit 4 now allows TR1 and TR2 to begin their changing control state and simultaneously applies a signal to line 15 to switch on TH1 and TH3. The control circuit 4 controls TR1 and TR2 so that the proportion of time during which they are on increases progressively. This causes the AC output voltage and thus the voltage of the high voltage supply voltage on line 90 and the voltage on output terminal 120 to grow progressively with time from t ₀ until a maximum voltage value is reached at time t ₁ . TR1 and TR2 are then controlled so that the voltage values at 90 and 120 remain essentially constant between times t ₁ and t ₂ . At time t ₂ , control circuit 4 controls the leading state of TR1 and TR2 so that voltages 90 and 120 decrease progressively until voltage t ₃ is reached at time t ₃ . If the voltage drops to zero at time t ₃ , the current flow through TH1 and TH3 ceases and these thyristors switch off. Now the control circuit 4 TR1 and TR2 alternately come into the leading state and essentially applies a control signal to line 15 '. This causes the thyristors TH2 and TH4 to turn on at time t ₄ , and since the polarity of the voltage at terminal 100 reverses from the previously existing polarity. TR1 and TR2 are controlled between times t ₄ and t ₅ in the same manner as they were between times t ₀ and t ₁ , and thereby a progressively increasing voltage on line 90 is generated. The voltage on line 90 reaches a maximum value at time t ₅ , and this value is maintained until time t ₆ . The voltage is then progressively reduced from time t ₆ until it reaches a value of essentially zero again at time t ₇ . The thyristors TH2 and TH4 turn off when the current passing through them drops to zero at time t ₇ . At time t ₈ , TR1 and TR2 are again caused to alternate conductance, and a Steueri signal is applied to line 15 to turn on the thyristors TH1, Th3 again. The cycle then repeats itself as before, the voltage again reaching a maximum at time t ₉ .

FIG. 4 shows part of a cycle of the voltage waveform at 120 of FIG. 3 on an enlarged scale. It can be seen that the voltage up to time t _{2 is} + 100 V. From time t ₂ to t ₃ , it progressively goes to zero. It remains zero between times t ₃ and t _{4 in} order to give the thyristors TH1 and TH3 time to switch off. The length of this period can be of the order of 30 ms. At time t ₄ , thyristors TH2 and TH4 are switched on and the voltage reaches a value -V at time t ₅ . From time t ₆ to t ₇ , it progressively drops to zero. Between times t ₇ and t ₈ , it remains zero in order to have the thyristors TH2 and TH4 switched off. In the example shown, the time length of a half cycle of the output voltage on line 120 is 15 s.

Although the waveforms are represented as piece-wise linear curves with sharp corners to facilitate the display, in practice the corners will be rounded due to the smoothing effects of the filter circles. This is desirable to remove the high frequency components in the output waveform. Furthermore, the inclined portions of the waveforms between t ₂ and t ₃ , t ₄ and t ₅ , etc. are shown as consisting of straight lines, but they may contain curves of arbitrary shape, e.g. B. exponential or sine curves. To keep the interference small, it is only necessary that the waveforms are essentially free of high-frequency components. The shape of the inclined portions of the waveforms can be determined by applying appropriate driving signals to the high frequency power switch 3 . For example, the output voltage applied to the heater contains essentially no high-frequency components that could otherwise radiate and cause interference. It will be appreciated that this output voltage is generated without the use of physically large and heavy inductive components which would be necessary to generate sine wave current waveforms at the relatively low frequencies used in aircraft electrical supply systems.

A modification of the invention will now be described with reference to FIG. 5. There may be advantages to progressively increasing the power applied to the windshield heater, both in terms of minimizing the thermal shock to the heater and windshield, and in order to lower the sudden power requirement on the DC power source Reduce tension. For this purpose, the control circuit 4 can be operated so that the effective voltage value applied to the heater increases progressively over time, e.g. B. during a variety of polarity reversal cycles of the output voltage. Thus, while in Fig. 3 the voltage at 90 was allowed to reach its maximum value of + V at time t ₁ , the maximum voltage was limited to a value in the course of the initial warm-up time, which increases progressively with time , and it may only reach voltage V at time t _m .

There are a number of variations within the protection area realm of the invention possible.

While in the embodiment described above, the com mutator thyristors, other types of ak tive components are used by creating a corresponding control signal can be switched off can. This enables commutation at other times take place than when the current is through the commutator Is zero.

In a further modification, the high DC voltage be kept constant while the one applied to the heater Voltage is progressively varied by varying the Wi the current value of one in series between the high dc control and the windshield heater connected element. If the commutator transistors or others active components used with an essentially continuous  nuously variable conductivity between the ON and OFF states, these transistors can be the control element form.

As mentioned before, the commutator can also be completely gone be left. This is possible if the windbreak disc heating by applying a unipolar voltage remains impaired.

During the execution of a push-pull DC converter uses an intermediate alternating voltage, can also be other switching mode DC / DC converters be used, e.g. B. flyback converters, in which no intermediate AC voltage as such is present. It is only necessary that the converter from a lower input DC voltage without using heavy and bulky in ductive components have a relatively high output DC voltage can generate voltage.

Claims (13)

1. Electrical windshield heating system with a windshield structure with an embedded electrical heating film and a circuit for feeding the heating film, characterized in that the circuit comprises converter means ( 3 , 4 , 7 , 8 , 9 , 10 , 11 , 12 ) to produce a direct current supply with a relatively high direct voltage from a direct current source with a relatively low voltage and means ( 120 , 13 ) for coupling power from the direct current supply with a relatively high direct voltage into the heating film, the means for coupling the power thereby can apply a DC voltage to the heating film. 2. System according to claim 1, characterized in that the means for coupling power comprises a commutator means ( 14 ) for periodically reversing the polarity of the DC voltage applied to the heating film. 3. System according to claim 2, characterized in that the Commutator means the polarity of the heating film periodically applied DC voltage at a very low rate reversed. 4. System according to any one of the preceding claims, characterized in that the converter means contains inductive Schaltmit tel ( 7 ), means ( 3 ) to periodically to the DC voltage source with the inductive switching means ( 7 ), and means ( 14 ), to draw energy from the inductive switching means for generating the DC supply voltage. 5. System according to claim 4, characterized in that the inductive switching means comprises a transformer ( 7 ) with primary winding means (P1, P2), which is coupled to the direct current source via switch means (TR1, TR2), and secondary winding means (SA), in which the DC supply voltage is generated by rectifying the AC voltage generated by the secondary winding means. 6. System according to any one of the preceding claims, characterized ge indicates that it contains means for the effective Am plitude of the voltage applied to the heating film as radio tion of time to vary. 7. System according to claim 6 depending on claim 4 or 5, characterized in that the means for changing which includes the means to change the amount of time during which the DC voltage source with the indukti ven circuit means is coupled. 8. System according to any one of claims 6 to 7, characterized indicates that the means for changing the effective Am plitude between the DC supply voltage and the heating film-coupled agents with controllable conductivity holds. 9. System according to claim 8, characterized in that the Commutator means the means (TH1, TH2, TH3, TH4) with tax contains conductive conductivity. 10. System according to any one of claims 6 to 9, characterized records that it warms up rise signal generator means includes to change the effective amplitude so that the power applied to the windshield heater increases progressively during the initial heating time. 11. System according to any one of claims 2 to 10, characterized ge indicates that the commutator means a full bridge network (TH1-TH4).   12. System according to any one of claims 2 to 11, characterized records that it contains means to repeat the chip substantially zero DC power supply up to a maximum value and back to essentially Change zero to a sequence of unipolar voltage im generate pulses, and means for controlling the commuta means in such a way that any polarity reversal takes place during a period in which the chip The value of the DC supply voltage is essentially zero is. 13. System according to any one of the preceding claims, characterized characterized in that the equal applied to the heating film voltage essentially free of high-frequency changes is voltage components.