Detecting delamination in a laminated structure
We describe a way of detecting delamination of a laminated structure that is heated by DC powered heaters by passing the source and return wires that supply current to the heaters through a toroidal transformer core. Should there be a breakdown in the laminations, current flowing through the heater will flow into the structure, resulting in less current being present in the return wire than in the source wire. The current imbalance between the source and return wires causes the transformer core to saturate. Using the core saturation, caused by the DC current imbalance, the delamination (or imminent delamination) of the laminated structure can be detected.
1. An apparatus for detecting delamination in a laminated structure, the laminated structure comprising a plurality of laminated layers, the apparatus comprising:
one or more heaters mechanically and thermally connected to the laminated structure for heating the laminated structure, the one or more heaters being disposed between first and second ground layers within the laminated structure;
a controller electrically coupled to the one or more heaters via source and return wires, the controller being configured to supply the one or more heaters with a DC current through the source and return wires to heat the one or more heaters;
a magnetically saturable closed loop transformer core, the source and return wires carrying the DC current passing through the transformer core; and
a delamination sensor coupled to the transformer core for detecting a DC current imbalance in the source and return wires,
wherein the delamination sensor detects delamination in the laminated structure upon detection of a DC current imbalance in the source and return wires.
2. An apparatus according to claim 1 , wherein the delamination sensor comprises:
a sensor source winding coupled to the transformer core and receiving an AC sensor voltage;
a detector winding coupled to the transformer core; and
an AC voltage sensor coupled to the detector winding for detecting an AC voltage on the detector winding through transformer coupling with the sensor source winding.
3. An apparatus according to claim 2 , wherein delamination of the laminated structure is detected when the sensor detects a change in the AC voltage.
4. An apparatus according to claim 3 , wherein delamination of the laminated structure is detected when the sensor detects a decrease in the magnitude of the AC voltage detected on the detector winding, and/or when the sensor detects a change in the shape of the AC voltage on the detector winding.
5. An apparatus according to claim 3 , wherein the AC voltage input to the sensor source winding is compared with the AC voltage detected on the detector winding.
6. An apparatus according to claim 1 , wherein the delamination sensor comprises:
a sensor winding coupled to the transformer core, the sensor winding having a first end and a second end, the second end being connected to ground, the sensor winding for sensing a change in the magnetic flux density in the transformer core;
an impedance component having a known impedance and having a first end and a second end, the first end being connected to the first end of the sensor winding;
a signal generator connected to the second end of the impedance component for generating an AC electrical signal for application to the impedance component and sensor winding;
an analogue-to-digital converter for receiving an electrical signal and for generating a digital voltage signal;
a switch having a first end connected to switch alternately between the first end of the impedance component and the second end of the impedance component, and the switch having a second end connected to the analogue-to-digital converter; and
a processor connected to the analogue-to-digital converter for receiving a digital voltage signal and for generating a sensor signal, wherein the processor is adapted to control the switch to connect to the first and second ends of the impedance component sequentially,
wherein delamination of the laminated structure is detected dependent on the sensor signal.
7. An apparatus according to claim 6 , wherein the impedance component comprises a resistor or an inductor.
8. An apparatus according to claim 6 , wherein the signal generator is adapted to generate the AC electrical signal comprising sequential signal blocks for application to the series connected impedance component and sensor winding, and wherein the switch is adapted to switch to connect the analogue-to-digital converter to measure each of the voltages generated at the first and second ends of the impedance component during the same part of the signal block of sequential signal blocks of the signal.
9. An apparatus according to claim 8 , wherein the signal generator includes a digital store storing a signal pattern for at least a part of the signal block, generator means for generating a digital signal by repeatedly using the stored signal pattern, and a digital-to-analogue converter for converting the digital signal to the AC electrical signal.
10. An apparatus according to claim 9 , wherein the signal generator and the processor are adapted to operate synchronously.
11. An apparatus according to claim 8 , comprising a plurality of Analogue to Digital Converters, each for receiving an electrical signal and for generating a digital voltage signal, wherein the switch is adapted to switch each of the plurality of Analogue to Digital Converters sequentially to measure the voltages to allow simultaneous measurements of the voltages, and the processor is adapted to process the sequentially measured voltages for each of the Analogue to Digital Converters.
12. An apparatus according to claim 11 , wherein the processor comprises a digital processing arrangement, and the outputs of the plurality of Analogue to Digital Converters are input into a common multiplexer connected to the processor.
13. An apparatus according to claim 6 , wherein the delamination sensor is adapted to generate an output as a measure of impedance or at least one component of the impedance of the sensor winding.
14. An apparatus according to claim 13 , wherein the delamination sensor is adapted to determine delamination of the laminated structure dependent on the measure of impedance or at least one component of the impedance of the sensor winding is above or below a threshold.
15. An apparatus according to claim 1 , wherein the delamination sensor comprises:
a sensor winding coupled to the transformer core, the sensor winding having a first end and a second end, the sensor winding for sensing a change in the magnetic flux density in the transformer core;
an AC voltage source connected to the first end of the sensor winding for generating a square wave AC electrical signal for application to the sensor winding;
an AC voltage sensor coupled to the first or second end of the sensor winding for detecting an AC voltage on the sensor winding; and
an AC current sensor coupled to the first or second end of the sensor winding for detecting an AC current in the sensor winding.
16. An apparatus according to claim 15 , wherein delamination of the laminated structure is detected when the delamination sensor detects a change in the AC voltage or AC current.
17. An apparatus according to claim 16 , wherein the detected AC voltage on the sensor winding is low-pass filtered, and wherein delamination of the laminated structure is detected when the delamination sensor detects a change in a low-pass filtered AC voltage that is greater than a threshold change.
18. An apparatus according to claim 16 , wherein the AC voltage source is a bipolar excitation source whose polarity is reversed when the AC current in the sensor winding reaches a threshold value, and wherein delamination of the laminated structure is detected when the delamination sensor detects a change in the frequency of the AC voltage on the sensor winding that is greater than a threshold change.
19. An apparatus according to claim 1 , wherein the transformer core is a transformer core having a relative permeability that is greater than 1000, and/or wherein the transformer core is a toroidal transformer core.
20. An apparatus according to claim 1 , wherein for two or more heaters, each of the two or more heaters comprises a return wire, and each of the return wires pass through the transformer core.
21. A method of detecting delamination in a laminated structure, the method comprising:
arranging one or more heaters to be mechanically and thermally connected to the laminated structure between first and second ground layers within the laminated structure,
providing a controller electrically coupled to the one or more heaters via source and return wires, the controller being configured to supply the one or more heaters with a DC current through the source and return wires to heat the one or more heaters;
arranging the source and return wires of the one or more heaters to pass through a magnetically saturable closed loop transformer core;
supplying one or more heaters with a DC current through the source and return wires to heat the one or more heaters;
coupling a delamination sensor to the transformer core for detecting a DC current imbalance in the source and return wires; and
using the delamination sensor to detect delamination in the laminated structure by detecting a DC current imbalance in the source and return wires.