GB2252479A - Data transmission by modulation of a power signal - Google Patents

Abstract

Data are transmitted in binary code to a data reception station on a first structure, for example an aircraft, from a data processing station (20) on a second structure, for example a landing gear wheel, which is movable with respect to the first structure. A power signal is transmitted from the first structure to the second structure by means of a rotary transformer (36) and it is picked up by the processing station (20). The processing station comprises an electronic module (29) which supplies processed data in binary-coded form and generates signals with modulation by frequency changing (FSK) representing binary-coded data. The FSK signals are modulated onto the power signal for transmission to the data reception station where they are demodulated and converted into a data form which is suitable for display. That minimises the errors which but for this could result in the alteration in the signals during transmission, and avoids a need for data conversion from the analog form to the digital form after transmission, which would be an additional source of inaccuracy. <IMAGE>

Description

Description of Invention Title: Data Transmission System This invention relates to data transmission systems and is more particularly concerned with a system for transmitting data to data receiving means located on a first structure from data output means located on a second structure which is movable with respect to the first structure.

It is an increasing requirement in vehicles, in particular passenger carrying aircraft, that the pressure of wheel tyres be monitored and made available for display on the vehicle with warning flags being presented in an event such as loss of tyre pressure. This requires that pressure data be transmitted from a sensing device on a wheel which is required to rotate relative to the body of the vehicle on which the data is required to be received. It is important that the accuracy of the data be maintained during transmission.

GB-A-2122757 discloses a device for measuring a parameter, e.g.

pressure, in respect of a pneumatic tyre of a vehicle wheel. The tyre parameter is sensed by a sensor mounted on the wheel rim so as to be internal of the tyre. Electrical power for energising the sensor is provided by a lithium battery mounted with the sensor. Signals output by the sensor are transmitted back to the vehicle by frequency-shift key (FSK) modulation of a capacitively coupled signal. This arrangement gives rise to a weight penalty in that capacitor plates must be provided and, if signals are required to be transmitted when the wheel is stationary, at least one of the plates must extend around the entire circumference of the wheel rim. A further disadvantage of this arrangement is that from time to time the battery must be either recharged or replaced.

It is an object of the present invention to provide a system for transmitting data from a structure that is movable with respect to a structure to which the data is required to be transmitted which substantially overcomes the aforementioned disadvantages.

It is another object of the invention to provide a system for transmitting data from sensing means located on a wheel to receiving means located in a vehicle on which the wheel is rotationally mounted which substantially overcomes the aforementioned disadvantages and that is provided in a package of minimum size and low weight.

Accordingly, one aspect of the present invention provides a method of transmitting data to data receiving means on a first structure from data output means on a second structure which is movable relative to the first structure, comprising the steps of: transmitting a power signal output by the receiving means on the first structure to the second structure for energisation of the data output means; processing data output by the data output means into binary coded data; generating frequency-shift key signals representative of the binary coded data; modulating the frequency-shift key signals onto the power signal; demodulating the modulated power signal at the data receiving means to obtain binary coded data; and converting the binary coded data to a data form suitable for display.

In another aspect the present invention provides a system for transmitting data to data receiving means located on a first structure from data output means located on a second structure which is movable relative to the first structure, comprising means on the first structure for outputting a power signal, means for transmitting the power signal from the first structure to the second structure, data output means on the second structure, data processing means on the second structure connected for receiving data from the data output means, the data processing means including means for converting data to binary coded data, means for generating frequency-shift key signals representative of the binary coded data and means for modulating the frequency-shift key signals onto the power signal for transmission to the data receiving means.

The data processing means may comprise an application specific integrated circuit (ASIC).

The e ASIC may incorporate analogue to digital converter means, means for coding digital signals into binary word data, means for generating frequency-shift key signals representative of logic 0 and logic 1, and means for modulating frequency-shift key signals onto the power signal.

In an embodiment of the invention frequencies of 2025Hz and 2225Hz are generated for logic 0 and logic 1, respectively, and used to modulate a 31.25kHz power signal.

The power signal transmission means preferably comprises a rotary transformer having a primary coil mounted on the first structure and a secondary coil mounted on the second structure.

A data transmission method and system in accordance with the present invention finds particular application in transmitting data from a wheel which is rotatably mounted on a vehicle, such as an aircraft, the data to be transmitted being pressure and/or temperature data. The pressure and temperature data may be derived from any suitable sensing means but, preferably, such sensing means comprises piezoresistive cell means.

In a particular embodiment of the invention aircraft undercarriage wheel tyre inflation pressure data is transmitted from each wheel of the undercarriage to an aircraft on-board computer where the data is decoded and made available to an aircraft data bus.

In this particular embodiment pressure data is transmitted as three binary words, each binary word comprising a start bit, 8 data bits, a parity bit and a stop bit. The words have a value which represents the pressure inside the tyre and if the pressure value represented by each word is not identical when decoded by the aircraft on-board computer, the computer votes as to whether the data is acceptable or is corrupt and should be rejected.

If required a fourth binary word may be transmitted to the aircraft on-board computer. This fourth word may comprise temperature data or it may be coded as 'not equipped' if it is not a requirement to transmit temperature data.

By way of example an application of the present invention in a system for measuring aircraft wheel tyre pressures and, if desired, temperatures will now be described with reference to the accompanying drawings of which: Figure 1 is a schematic block diagram of an aircraft tyre pressure measurement embodying the present invention; Figure 2 is a circuit diagram of an electronics module provided as part of the system shown in Figure 1; Figure 3 is a schematic side elevation showing the mechanical construction of the electronics module shown in circuit form in Figure 2; Figure 4 is an exploded schematic view showing the positions of three printed circuit boards of the electronics module shown in Figure 3; Figure 5 is a diagrammatic representation of an application specific integrated circuit provided with one of the printed circuit boards; and Figure 6 is a circuit diagram of a rotary transformer provided as part of the system shown in Figure 1.

A system for an aircraft now to be described, allows the pressure of each tyre on an undercarriage of the aircraft to be monitored continuously whilst the aircraft power system is switched on and, in particular, during taxiing, takeoff, flight and landing, and measured values to be transmitted to a data receiving station on-board the aircraft.

Referring to Figure 1, the system comprises for each wheel of the undercarriage (not shown) a pressure sensor 20 which is attached to the wheel by means of a pressure sensor holder (not shown) which may consist of a stainless steel tubular shell. In this embodiment the pressure sensor 20 comprises a piezoresistive pressure cell 21 connected to an electronics module 22. The pressure cell 21 is a monolithic device in the form of a silicon wafer having a wheatstone bridge circuit 23 implanted thereon, such as is sold by KELLER AG of 119 St. Gallerstrasse, Winterthur, Switzerland. The bridge 23 is energised by a voltage reference signal supplied from the electronics module 22 over lines 25 and 26. The pressure cell 21 is exposed to pressure of nitrogen gas inflating a tyre (not shown) on the wheel in which the sensor 20 is housed.The pressure cell is deformed by the nitrogen gas pressure and the bridge 23, which may be in balance at a pressure of say one atmosphere is unbalanced and outputs a voltage signal to the electronics module 22 over lines 27 and 28. The electronics module 22 comprises an assembly 29 of three printed circuit boards (PCB) 30, 31, 32, and a mechanical interface 33 which will hereinafter be described in more detail. The bridge output voltage signal is converted by the electronics module to digital form before being checked and corrected for linearity and temperature effects. The corrected value is then converted to binary form and passed as frequency-shift key (FSK) signals by way of the mechanical interface 33, a pair of twisted flying leads 34 and a connector 35 to a rotary transformer 36 mounted in the wheel hub (not shown).The rotary transformer 36 comprises a primary coil 37 mounted on a non-rotating component of the wheel hub and secondary coil 38 mounted on a rotating component of the wheel hub. A power signal output by a data receiving station on-board the aircraft, such as an on-board computer (not shown), is transmitted to the electronics module 22 by way of the primary and secondary coils of the rotary transformer and is rectified and regulated by the PCB 33 as will hereinafter be described to produce the voltage reference signal which is used to energise the pressure cell and the PCBs 30 and 31. The FSK signals are modulated onto the power signal and thereby transmitted from the secondary coil to the primary coil and passed from the primary coil to the on-board computer.

The computer sequentially scans the pressure sensor 20 of each wheel of the undercarriage and analyses the FSK signals received back from each sensor to generate pressure and, if transmitted, temperature data and warnings which are made available to the aircraft central maintenance computer via aircraft data bus (not shown).

Thus, a system in accordance with this embodiment of the present invention uses a pressure cell which will operate over a required temperature range (-550C to +1600C) to measure pressure values over a required pressure range (0 to 254 psi) with a required accuracy and the system transmits measured pressure values to the aircraft on-board computer whether the wheel is rotating or stationary. The system corrects the pressure cell output for both linearity and temperature effect and introduces only one conversion inaccuracy which is that of converting the corrected pressure value to digital form for transmission back to the aircraft on-board computer. This compares with at least two inaccuracies introduced by systems that transmit data in analogue form for conversion to digital form at the on-board computer.These inaccuracies are, for example, converting the pressure value to frequency and converting the frequency to digital form at the computer. An additional advantage of transmitting the data to the computer in digital form is that interference problems are reduced so that the system is more secure, in particular, standard error correction techniques can be used to overcome noisy channel conditions.

A particular advantage of the present invention is that power for energising the data output means, e.g. the pressure cell and electronics module, is provided on a single pair of wires which are also used for the output of data.

The main components of the system will now be described in greater detail commencing with the electronics module 22 and pressure cell 21.

Referring to the circuit diagram of the electronics module 22 shown in Figure 2, the module 22 comprises three PCBs 30, 31, 32, of which PCB 32 is a power supply PCB; PCB 31 is an electrically-erasable programmable-read-only memory (EEPROM) PCB; and PCB 30 is an application specific integrated circuit (ASIC) PCB. The module 22 receives a 31.25 kHz a.c. power signal output by the aircraft on-board computer and transmitted by the rotary transformer 36. The power input to the module 22 is protected by a bidirectional zener diode 39 and reaches the input via a capacitor 40 which offers a high impedance to the FSK signal and a low impedance to the power signal. The power signal is rectified by two diodes 41, 42 and a capacitor 43 and then converted to 5 volts d.c. by a voltage regulator 44.This voltage is used to energise the pressure cell 21 and to power the ASIC and EEPROM PCBs 30, 31, respectively.

The PCB 31 carries an EEPROM 45 which holds a 32 x 16 matrix look-up table that as will hereinafter be described is used to store temperature and pressure data for correcting signals output by the pressure cell 21.

The PCB 30 carries an application specific integrated circuit (ASIC) 46 which receives varying voltage signals output by the pressure cell 21 over lines 27, 28 that are connected with the PCB 30 by pin connections 47, 48. The PCB 30 is provided with pins 49, 50 for connecting with the bridge input lines 25, 26, respectively, whereby the bridge is energised with the 5 volts signal. The PCB 30 further carries resistors 51, 52, 53 and capacitor 54. The output of the cell 21 is non-linear and also varies with temperature change which affects the overall resistance of the bridge 23. The overall resistance of the bridge is monitored by measuring the voltage across the resistor 51 which is a high precision resistor that is substantially unaffected by temperature. The voltage across the resistor 51 is obtained by the ASIC 46 and using the known characteristics of the resistor the current flow through the resistor is calculated. Changes in current flow through the resistor are a measure of variations in the overall resistance of the bridge due to temperature change and, hence a temperature related signal is obtained for correction purposes.

The resistor 52 is a current limiting resistor which uses the power signal to provide a clock for the ASIC 46. The resistor 53 is used to modulate FSK signals carrying data output by the ASIC onto the power signal. In order not to lose most of the FSK signal the value of the resistor 53 is not high enough to protect the output to the rotary transformer over pin 55 of a pair of pins 55, 56, from the high voltages of the power signal, therefore capacitor 54 is necessary to act as a filter to reduce the power signal level on pin 55.

The mechanical configuration of the electronics module 22 is shown in Figures 3 and 4. The PCBs 30, 31, 32 are circular in shape and are connected together by copper wires 57, soldered to each PCB. The wires locate the PCBs relative to one another and allow signals to pass betwen them. As the ASIC 46 is a surface mount ceramic component the PCB 30 is of ceramic construction to prevent problems arising from differential expansion between the ASIC 46 and the PCB 30. PCBs 31 and 32 carry conventional components with through hole pins and are therefore constructed of polyimide. The whole assembly is potted with a suitable potting compound having low weight and low coefficient of thermal expansion.The exterior surface of the ceramic PCB 30 carries no tracks but is provided with pads for connection of the copper wires 57, and is also provided with the pins 47, 48, 49, 50 for connection to the pressure cell 21. This latter connection is made by means of a single layer polyimide circuit 58.

As shown in Figure 5, the ASIC 46 comprises three stages, an analogue to digital conversion stage 60, a serial communications stage 61 and an interpolation logic stage 62. The first stage 60 consists of a switch 63, a programmable gain amplifier 64 and an analogue to digital converter 65. The first stage 60 receives two inputs, the first input being the output of the pressure cell bridge 23 which is proportional to pressure and temperature; and the second input being supplied from the temperature sensing resistor 51 which is a high stability component, this second input being proportional to variation in temperature but relatively insensitive to the variation of pressure.

The input to the amplifier 64 is toggled by the switch 63 between the input from the bridge 23 and the input from the resistor 51 at the clock frequency and the gain of the amplifier 64 is adjusted accordingly. Each time a pair of readings are taken the polarity of the amplifier 64 and the analogue to digital converter 65 are reversed in order to cancel any errors caused by offset voltages. The analogue to digital converter provides two digital words, one describing the temperature and the other the pressure to which the cell 21 is subjected. These words are passed to a serial communications circuit 61a of the serial communications stage 61 which passes the most significant bits of the two words to the EEPROM 45 as addresses to enable stored correction data to be retrieved.Each individual correction data word is stored in the EEPROM 45 as an eight bit byte and its inverse to allow data integrity checks to be performed on the data obtained from the EEPROM. The data obtained from the EEPROM consists of four eight bit bytes and their inverse. These are checked and are passed to an interpolation logic circuit 62a of the interpolation logic stage 62, together with the least significant bits of the two digital words derived from the analogue to digital converter 65. These latter are used together with the data from the EEPROM to calculate the true corrected pressure in the tyre. The corrected pressure data is modulated onto the 31.25 kHz power signal using frequency-shift key techniques and a digital frequency synthesiser 70 incorporated as part of the stage 62 of the ASIC 46.

Referring now to Figure 6, the rotary transformer 36 comprises two air coupled screened windings of 0.132mm diameter copper wire insulated with polyimide. The primary (stator) winding 38 has 190 turns and the secondary (rotor) winding 37 has 115 turns. The transformer allows the power signal output by the aircraft on-board computer to be transferred from the on-board computer to the pressure sensor 20 on the wheel (not shown) and the FSK signal to be transmitted back from the pressure sensor to the on-board computer. The mechanical function of the transformer is to locate the primary and secondary windings relative to one another to ensure the effective transfer of power at 31.2SkHz and FSK signals between the rotating and fixed parts of the wheel.

After assembly of the pressure cell 21 and the electronic module 22, pressure sensor 20 is calibrated over the operating temperature range -550C to +1600C and the operating pressure range 0 to 254 psi. Two voltage output signals are derived, one V being proportional to p pressure but being influenced by temperature, and the other Vt being proportional to temperature but not being substantially influenced by pressure. In order to avoid storing a large number of values which would normally be necessary to achieve the required accuracy a curve fitting method is used to define the values to be written into the matrix look-up table in the BEPROM 45 using a portable computer 67 (Figure 5) which is connected by a serial data line 68 and a serial clock line 69 to the EEPROM 45.

In operation, with aircraft power switched on, the aircraft on-board computer outputs a 31.25 kHz power signal sequentially to the primary coil 38 of the rotary transformer 36 associated with each undercarriage wheel. The power signal is applied to each wheel for 300 milliseconds (ms) of which the first 150 ms is a data acquisition and processing period and the second 150 ms is a data transmission period. The power signal is transmitted to the secondary coil 37 and enters the power supply PCB 32 which processes the power signal and outputs a 5 volts d.c. signal. This voltage is used to energise the pressure cell 21 and power the EEPROM and ASIC PCBs 31 and 32, respectively.

The ASIC 46 reads four memory locations in the EEPROM 45 to bring down pressure and temperature gain values for the input amplifier. The gain values plus their inverse values are read to validate EEPROM operation.

(Prior to calibration default values are loaded into the EEPROM). The gain is then adjusted to give full range of the analogue to digital converter and the gain values are reloaded into the EEPROM.

The ASIC 46 next reads the differential pressure and single-ended input temperature values for a minimum of 100 samples. The polarity of inputs are reversed on alternate clock cycles in order to average out sensor and amplifier offsets. The analogue to digital converter feeds the averaging logic which outputs 1 byte of data each for temperature and pressure. These are then truncated into a 9 bit address (5 bits for temperature and 4 bits for pressure).

The 9 bit word is used to address eight locations in the EEPROM. The 8 locations are derived by incrementing the temperature portion by one bit and then the pressure portion to give a sequence of four points.

This is repeated for the four inverse data locations. The data and inverse data is checked for corruption and four map points are derived.

If corruption is detected an error word is passed to the FSK output and no other processing is performed. If the data is good, the ASIC then uses the least significant bit information for pressure and temperature to carry out a linear interpolation to calculate a true pressure value.

The corrected or true pressure value and, if required, the temperature value, are coded in four binary words for transmission to the aircraft on-board computer. The four binary words W1, W2, W3 and W4 have the same structure which is as follows: 1 start bit (logic 0) 8 data bits, with most significant bit first 1 parity bit, (data bits + parity - even) 1 stop bit (logic 1) The words W1, W2 and W3 are identical and have a value which represents the pressure inside the tyre.

If the system is required to transmit temperature data, word W4 has a value which represents the temperature. Otherwise, word W4 is coded to give a 'not equipped' code.

The e ASIC incorporates built in tests (BITE) which check the pressure cell inputs are not short circuited to 5V, low impedance to 0V, and open circuit. The BITE also tests the EEPROM and the ASIC itself. If an error is detected this is reported by an error code in the words W1, W2 and W3. The ASIC also checks that the tyre pressure is within a required range, in this embodiment 0 to 254 psi. The ASIC does not report an error for any pressure above 254 psi, instead any higher pressure is limited to a 254 reading.

Examples of the data bits for words W1, W2 and W3 in the pressure range are as follows: Pressure psi Decimal Binary 0 0 0000 0000 1 1 0000 0001 2 2 0000 0010 253 253 1111 1101 254 and above 254 1111 1110 The error code 1111 1111 is sent when the ASIC BITE detects: a. The failure of the EEPROM test b. A failure of the pressure cell c. The failure of the ASIC self test.

The word 4 'not equipped' code is 1111 1111.

The word data is passed to the digital frequency synthesiser 70 which generates one of two frequencies. A frequency of 2225Hz is generated for logic 1 and a frequency of 2025Hz is generated for logic 0. These frequencies are modulated onto the 31.25kHz power signal and transmitted to the aircraft on-board computer where the data is decoded and made continuously available to the aircraft data bus together with any warning flags.

Whilst the particular embodiment of the invention hereinbefore described with reference to the accompanying drawing is concerned with transmission of data from an aircraft undercarriage wheel to a data receiving station on the aircraft, it is to be appreciated that a system in accordance with the invention is not limited to such application and may be used to meet other requirements for transmission of data from a data output station located on a structure which is movable with respect to a structure on which the data receiving station is located.

Claims (11)

1. A method of transmitting data to data receiving means on a first structure from data output means on a second structure which is movable relative to the first structure, comprising the steps of: transmitting a power signal output by the receiving means on the first structure to the second structure for energisation of the data output means; processing data output by the data output means into binary coded data; generating frequency-shift key signals representative of the binary coded data; modulating the frequency-shift key signals onto the power signal; demodulating the modulated power signal at the data receiving means to obtain binary coded data; and converting the binary coded data to a data form suitable for display.

2. A system for transmitting data to data receiving means located on a first structure from data output means located on a second structure which is movable relative to the first structure, comprising means on the first structure for outputting a power signal, means for transmitting the power signal from the first structure to the second structure, data output means on the second structure, data processing means on the second structure connected for receiving data from the data output means, the data processing means including means for converting data to binary coded data, means for generating frequency-shift key signals representative of the binary coded data and means for modulating the frequency-shift key signals onto the power signal for transmission to the data receiving means.

3. A system as claimed in Claim 2, wherein the data processing means comprises an application specific integrated circuit (ASIC).

4. A system as claimed in Claim 3, wherein the ASIC includes analogue to digital converter means, means for coding digital signals into binary word data, means for generating frequency-shift key signals representative of logic 0 and logic 1, and means for modulating frequency-shift key signals onto the power signal.

5. A system as claimed in claim 4, wherein frequencies of 2025Hz and 2225Hz are generated for logic 0 and logic 1, respectively, and used to modulate a 31.25kHz power signal.

6. A system as claimed in any one of Claims 2 to 5, wherein the power signal transmission means comprises a rotary transformer having a primary coil mounted on the first structure and a secondary coil mounted on the second structure.

7. A system as claimed in any one of Claims 2 to 6, wherein the first structure comprises a wheel rotatably mounted with respect to the vehicle body structure.

8. A system as claimed in Claim 7, wherein inflation pressure data for a wheel tyre is transmitted as three binary words, each word comprising a start bit, a number of data bits, a parity bit and a stop bit, each word having a value representative of the pressure inflating the tyre.

9. A system as claimed in Claim 8, wherein a fourth binary word comprising temperature data is transmitted.

10. A system for transmitting data substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

11. Any new or improved features7 combinations and arrangements described, shown and mentioned, or any of them together or separately.