Abstract:
A method for determining the status of a plurality of in a circuit, each switch having first switching arrangement for controlling the supply of a wetting current to the respective switch, and a second switching arrangement, whereby closure of the second switching arrangement enables the status of the respective switch to be determined, the method including the steps of closing the first switching arrangement associated with each of the switches so as to supply a wetting current to each of the switches and, when each one of the first switching arrangements is closed, closing the second switching arrangement associated with the same switch in order to determine the switch status. The invention also relates to an apparatus for determining the status of a plurality of switches.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for determining the status of a plurality of switches. The invention also relates to an apparatus for determining the status of a plurality of switches. 
     In aircraft engine control systems, various switches are used to feed back different aspects of engine and/or airframe condition, the engine control system commonly being controlled by an electronic control unit (ECU). The closing of a switch against a respective switch contact causes a current to pass through the closed switch path to indicate, for example, an overload or limit, or an action by the aircraft pilot. During operation of the engine control system, there is often a need to be able to determine whether the switches are in an open or closed position. 
     Conventionally, switch status is determined by means of a circuit, forming part of the electronic control unit, which periodically supplies a “wetting” current to all of the external switches in the control system by means of an arrangement of primary switches. The wetting current is supplied to each switch via an associated resistor by closing the primary switches. The electrical circuit includes an arrangement of secondary switches, one secondary switch being associated with each of the external switches. By closing the secondary switches, the current flowing through the closed secondary switch path provides an indication of the external switch status. 
     In order to prevent oxidisation and debris build-up on the external switch contacts, it is necessary to ensure a current of at least 4 mA is supplied to the closed external switch contacts. Oxidisation and debris build-up on the switch contacts can occur for smaller currents than this and results in a degradation in performance and reliability of the external switches. 
     However, with larger currents flowing through the closed external switches, a significant power loss occurs across the internal resistors of the ECU. This gives rise to undesirable heating effects which can adversely effect the thermal management of the electronic control unit and can degrade the performance and reliability of overheat protection circuitry contained therein. A problem therefore exists with the conventional method of determining switch status in that a compromise must be made between the most desirable wetting current for reduced heat loss and the most desirable wetting current for reliable switch operation. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus for determining switch status which alleviates the problems of the prior art. 
     According to one aspect of the present invention, there is provided a method for determining the status of a plurality of switches in a circuit, each switch having a first switching means for controlling the supply of a wetting current to the respective switch, and second switching means, closing of the second switching means enabling the status of the respective switch to be determined, the method including the steps of closing the first switching means associated with each of the switches so as to supply a wetting current to each of the switches and, when each one of the first switching means is closed, closing the second switching means associated with the same switch in order to determine switch status. 
     Preferably, the duration for which each pair of the first and second switching means is closed is chosen so as to minimise power dissipation. 
     Conveniently, the method of the present invention may be used to determine whether each of the switches in the circuit is open or closed. The duration is chosen to be sufficient to accurately determine the switch status. Preferably, the duration for which each pair of the first and second switching means is closed is between 50 μs and 1 ms and, typically, may be 500 μs. 
     In a preferred embodiment, the wetting current is supplied to each switch in sequence, the status of the switches being determined sequentially. 
     The method provides the advantage that, as the wetting current is only supplied to the switches in the circuit as a pulse of short duration rather than continuously, heat losses due to resistances within the circuit are reduced. Thus, higher wetting currents can be used than with conventional methods. It has been found that this improves the performance and reliability of the method. 
     A further improvement is obtained by supplying the wetting current to the switches sequentially rather than at the same time. 
     Additionally, the method may include the further steps of reading the status of each switch and storing it in the form of a single bit in a register when the associated second switching means is closed and, prior to closing the first and second switching means associated with the next switch in the sequence, shifting the contents of the register by one register location. 
     Preferably, the method may include the further step of outputting the contents of the register when the register contains the status of all of the switches. Alternatively, the ECU may be arranged to access the data stored in the register at any desired time. 
     The method provides the further advantage that, as the status of each of the switches is determined one after the other, a simplified read-out register arrangement is required. Conventionally, if the status of all the external switches is determined at the same time, a read-out register is required for each switch. 
     The switches may form part of an engine and/or airframe control function. However, the method may be used for determining the status of switches in any circuit. 
     The first and second switching means preferably take the form of first and second multiplexer switch arrangements, the first multiplexer switch arrangement comprising a plurality of first switches and the second multiplexer switch arrangement comprising a plurality of second switches. 
     According to a second aspect of the present invention, there is provided an apparatus for determining the status of a plurality of switches in a circuit, comprising; 
     a first switching means associated with each switch for controlling the supply of a wetting current to the respective switch; 
     a second switching means associated with each switch, whereby closing of the second switching means enables the status of the respective switch to be determined; and 
     means for addressing the first and second switching means so as to switch the first and second switching means between open and closed positions such that the first and second switching means associated with a common switch are closed at substantially the same time. 
     Preferably, the first and second switching means associated with a common switch are closed at substantially the same time, for a period of relatively short duration. 
     Preferably, the pairs of first and second switching means associated with each switch are operated sequentially. 
     The apparatus may also include a register into which the status of each of the switches is output. The contents of the register can conveniently be read-out when one sequence has been completed and the status of all of the switches has been determined. 
     Preferably, the apparatus comprises a first multiplexer switch arrangement and a second multiplexer switch arrangement, the first multiplexer switch arrangement comprising a plurality of first switches, each one of the first switches being operable to control the supply of a wetting current to an associated one of the switches, the second multiplexer switch arrangement comprising a plurality of second switches, each of the second switches being operable to enable the status of an associated switch to be determined. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the accompanying FIG. 1 which is a schematic diagram of a circuit in accordance with one aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an electrical circuit, referred to generally as  10 , includes a voltage supply rail  12  for supplying a voltage, +V 1 , across a plurality of resistors  14   a ,  14   b ,  14   c . Each of the resistors  14   a ,  14   b ,  14   c  is associated with an external switch  16   a ,  16   b ,  16   c  respectively within an external control circuit, referred to generally as  18 , one side of each resistor  14   a ,  14   b ,  14   c  being connected to one contact of the respective switch  16   a ,  16   b ,  16   c . The other side of each switch  16   a ,  16   b ,  16   c  is connected to ground. The electrical circuit  10  may be contained within an electronic control unit (not shown) for controlling the operation of an associated engine. Each of the switches  16   a ,  16   b ,  16   c  in the external circuit  18  is operable between open and closed positions to provide status information regarding various aspects of engine and/or airframe operation. The function of the switches  16   a ,  16   b ,  16   c  is beyond the scope of the present invention and will not be described in further detail. 
     The circuit  10  also includes first and second multiplexer switch arrangements  20 ,  24 , the first multiplexer switch arrangement  20  comprising a plurality of first switches  22   a ,  22   b ,  22   c  and the second multiplexer switch arrangement  24  comprising a plurality of second switches  26   a ,  26   b ,  26   c . One contact of each of the first switches  22   a    22   b ,  22   c  is permanently connected to the voltage rail  12 , the other contact of each of the first switches  22   a ,  22   b ,  22   c  being connectable with one side of a respective one of the resistors  14   a ,  14   b ,  14   c . The other side of each of the resistors  14   a ,  14   b ,  14   c  is connected to one of the external switches  16   a ,  16   b ,  16   c  respectively and, in addition, is in connection with one contact of a respective one of the second switches  26   a    26   b ,  26   c.    
     Each of the second switches  26   a ,  26   b ,  26   c  shares a common address line  28   a ,  28   b ,  28   c  with an associated one of the first switches  22   a ,  22   b ,  22   c . In use, the address lines  28   a ,  28   b ,  28   c  for addressing associated ones of the first and second switches,  22   a  and  26   a ,  22   b  and  26   b ,  22   c  and  26   c , are provided with address pulses by means of an application specific integrated circuit (ASIC)  30  which forms part of circuit  10 , the ASIC  30  being connected to a voltage supply, +V 2 . In use, the common address lines  28   a ,  28   b ,  28   c  are addressed in turn by address pulses supplied by the ASIC  30 , the ASIC controlling both the duration of the address pulse applied to the common address lines  28   a ,  28   b    28   c , and the time period between sequential address pulses. Alternatively, in place of the ASIC, a Field Programmable Gate Array (FPGA), may be used. 
     The ASIC  30  also includes a register  32 , one contact of each of the second switches  26   a ,  26   b ,  26   c  being connectable with the register  32  such that, when any one of the second switches  26   a ,  26   b ,  26   c  is in the closed position, the voltage across the closed switch is input to the register  32 . The analogue value of the voltage across each of the second switches  26   a ,  26   b ,  26   c  is converted to digital form by suitable analogue to digital conversion means (not shown) prior to input to the register  32 . In addition, the ASIC  30  includes a latch  34  into which the contents of the register  32  are latched for access by a computer processor data bus  36 . 
     In order to determine the status of the switches  16   a ,  16   b ,  16   c  in the external circuit  18 , the circuit  10  shown in FIG. 1 is operated in the following way. During the first stage of the operating cycle, the ASIC  30  addresses the common address line  28   a  causing the first switch  22   a  and second switch  26   a  to be closed at substantially the same time. The voltage V 1  is therefore applied across the resistor  14   a  and a wetting current, determined by the value of the resistor  14   a  and the voltage V 1 , is supplied to the external switch  16   a . If the external switch  16   a  is open when the first and second switches  22   a ,  26   a  are closed, the contact of the external switch  16   a  connected to the resistor  14   a  will be at a relatively high voltage, this voltage being fed to the second switch  26   a and the register  32 , and stored, for example, in the form of a logic “1”. However, if the switch  16   a  is closed, then the voltage at the contact of the switch  16   a  connected to the resistor  14   a  will be relatively low, and this low voltage will be fed through the second switch  26   a  to the register  32  where it is stored, for example in the form of a logic “0”. The voltage level input to the register  32  therefore provides an indication of whether the external switch  16   a  is open or closed. 
     After a predetermined time period determined by the ASIC  30 , typically between 50 μs and 1 ms, the first bit of the register  32  is shifted by one place so that the register  32  is ready for a subsequent input. In addition, the address pulse applied to the common address line  28   a  is removed causing the first and second switches  22   a ,  26   a  to open. The wetting current is therefore no longer applied to the external switch  16   a.    
     During the next stage of the operating cycle, the ASIC  30  applies an address pulse to the common address line  28   b  causing the first and second switches  22   b ,  26   b  to close. As the first switch  22   b  is closed, a current flows through the resistor  14   b  and a wetting current is supplied to the external switch  16   b . As described previously, a signal indicative of the voltage value is input to the register  32  which provides an indication of the status of the switch  16   b . As in the first stage of the operating cycle, after the predetermined time period, the ASIC  30  removes the address pulse from the second common address line  28   b  and the contents of the register  32  are again shifted by one place. 
     Finally, during the third stage of the operating cycle, the ASIC  30  applies an address pulse to the common address line  28   c  causing the first and second switches  22   c ,  26   c  to be closed. As described previously for the external switch  16   a , the signal indicative of the voltage value is input to the register  32  and provides an indication of the status of the external switch  16   c . After the predetermined time period, the contents of the register  32  are shifted to the latch  34  such that the latch  34  contains information regarding the status of all three of the external switches  16   a ,  16   b ,  16   c.    
     The process may be repeated for any number of switches. 
     Once the operating cycle has been completed, and the latch  34  contains the status of each of the external switches  16   a ,  16   b ,  16   c , the switch status information held in the latch  34  is read by the computer via bus  36 . The reading of the latch contents provides a signal to the ASIC  30  to repeat the operating cycle, and an address pulse is again applied to the common address line  28   a  and the procedure is repeated. As the status of the external switches  16   a ,  16   b ,  16   c  is determined by supplying a wetting current pulse to each of the switches, rather than supplying a continuous wetting current, power loss is reduced. The loss is further improved by using the multiplexed, sequential method described hereinbefore in which power loss only occurs across one of the resistors  14   a ,  14   b ,  14   c , at any one time. This reduces the amount of heat generated within the electronic control unit, thereby improving thermal management of the unit. 
     The values of the resistors  14   a ,  14   b ,  14   c  are selected such that the wetting current supplied to the external switches  16   a ,  16   b ,  16   c  is greater than that which will cause oxidisation effects and debris build up on the switch contacts. A wetting current of 4 mA is adequate, but it has been found that the effects of oxidisation and debris build-up on the switch contacts can be improved further if the wetting current is at least 10 mA. As the wetting current is supplied to the external switches as a pulse, the present invention permits an increased wetting current to be used without creating an excessive undesirable heat loss across the resistors  14   a ,  14   b ,  14   c.    
     It will be appreciated that the invention may be used with an external circuit having any number of switches for which the status is to be determined and is not limited to the number described hereinbefore. In each case, a corresponding number of first and second switches to the number of external switches is provided in the circuit  10  so that each has an associated first and second switch controlled by a common address line. It will be further appreciated that the switches of the circuit  18  may form part of the circuit  10  and need not be external thereto. Additionally, the electrical circuit  10  need not be incorporated within the electronic control unit but may be a separate circuit. 
     The invention may also be applied to relays in which secondary relay contacts are provided to give feed back regarding the status of the relay output. The secondary relay contacts have to carry at least a certain proportion of the primary relay current and hence there will be significant power dissipation in the secondary contact limit resistor. The use of the pulsed method described hereinbefore gives a reduced power dissipation, and the use of the sequential method further reduces the power dissipation.