Abstract:
The invention concerns an apparatus for monitoring the operation of electrical circuits on a vehicle trailer  10,  and has a monitoring unit  18  including a monitor circuit  32  for outputting a signal in response to failure of a component such as a lamp  12   a   -12   d  of a trailer electrical circuit. The monitoring unit also has a transmitter  34  for passing a failure indication signal to a receiver  90  of an indicator unit  26  which is mounted in a towing vehicle  14  in a position where it can give an indication of failure of the component to the vehicle driver.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is the U.S. national stage application of International Application PCT/GB99/02896, filed Sep. 2, 1999, which international application was published on Mar. 16, 2000 as International Publication WO 00/14554 A1 in the English language. The International Application claims the priority of British Patent Application 9818963.2, filed Sep. 2, 1998. 
     BACKGROUND OF THE INVENTION 
     This invention relates to an arrangement for monitoring the operation of electrical circuits on a vehicle and, in particular, lamps or other electrical components on a vehicle trailer. 
     When a trailer (which may be for example a box trailer, a boat trailer or a caravan) is being towed by a vehicle, it is a legal requirement to equip the trailer with a set of rear warning lamps which duplicate the brake lamps, tail lamps and rear turn indicators of the towing vehicle. From time to time, a lamp on the trailer may fail, requiring, for example, the replacement of the lamp bulb. However, because trailer lamps are normally not visible to the driver, the driver may continue to drive unaware that a lamp has failed, which can be dangerous. 
     Although it is known to have lamp failure monitors for motor vehicle lamps, these are built into the circuitry of the motor vehicle, and therefore cannot be removed for use elsewhere. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided apparatus for monitoring operation of electrical circuits on a vehicle trailer supplied with electrical power from a towing vehicle via a plug and socket wiring connector, the apparatus comprising a monitoring unit including means for outputting a signal in response to the failure of a component of a trailer electrical circuit; an indicator unit to be mounted in the towing vehicle in a position where it can give an indication to the vehicle driver, the indicator unit including means for indicating to the driver the failure of the component; and means for passing a component failure signal from the monitoring unit to the indicator unit, characterised in that the monitoring unit comprises a plug and socket adapted to be connected in series between the plug and socket otherwise used to connect the vehicle wiring to the trailer wiring, and the means for passing a component failure signal from the monitoring unit to the indicator unit is a radio transmitter and receiver. 
     The component may be a trailer or other trailer electrical component. The failure of a lamp or other component may be due, for example, to the failure of a bulb within a lamp, a fault with the connections to the socket holding the bulb the failure of a fuse, or any other discontinuity in the trailer circuitry associated with that component, and it will be understood that the term “component failure” includes any such fault. 
     If one of the trailer components fails, the driver of the towing vehicle will be alerted and will be able to take the appropriate action to replace the component or otherwise correct the fault. 
     It may be convenient for the monitoring unit and indicator units to be separate units. 
     The transmitting means contained in the monitoring unit may have a range limited, depending on the system&#39;s particular intended use, e.g. say to approximately five metres, sufficient to reach the indicator unit whilst avoiding undue interference with other equipment not on board the vehicle or trailer. 
     So that the monitoring unit can be easily connected into the electrical circuit powering the trailer electrical components, the monitoring unit may comprise a plug and socket adapted to be connected in series between the plug and socket otherwise used to connect the vehicle wiring to the trailer wiring. The plug and socket may be a standard 7-pin connection as is well known in respect of trailers being towed behind cars and vans. The invention can equally be applied to trailer systems which use different couplings; e.g. the coupling conventionally used on articulated lorries. 
     Such an arrangement allows the monitoring unit to be removed for use with different trailers or towing vehicles, since the plugs and sockets in existing use are normally standardised. 
     Means for releasably attaching the monitoring unit to either the towing vehicle or the trailer may be provided. 
     However, in a preferred embodiment, the monitoring unit comprises a clamp allowing it to be releasably attached to the tow arm of the trailer. 
     The plug of the monitoring unit connecting to the towing vehicle may be joined to the monitoring unit body via a short cable to allow the monitoring unit body to be positioned conveniently in or on the towing vehicle or trailer. 
     Since the monitoring unit can be exposed to rain and mechanical shock while the trailer is being towed, the electrical circuitry of the monitoring unit may be housed within a hard casing that is sealed. The casing may be fabricated from a suitable metal, with the inclusion of a non metallic window to allow for the transmission of radio signals from within the casing. 
     The indicator unit which alerts the driver to a component failure may have a front panel comprising a plurality of lights, each light being designated to turn on in response to a fault in a given trailer component or set of components, e.g., to allow the driver to identify the faulty component of components, the indicator panel may comprise markings or differently coloured lights such as light emitting diodes, for example. The indicator unit could alternatively or additionally emit a distinctive sound to warn of component failure, or provide some other indication to the driver. 
     The indicator unit may be placed in the driver&#39;s compartment and may be powered from an on-board power socket, such as a cigarette lighter socket, for example. However, it is within the scope of the invention for the indicator unit to be powered by a battery housed within the unit. 
     The monitoring unit may comprise means for monitoring each of a number of trailer components continuously and, upon failure, passing a signal to a multichannel transmitter. However, if a multichannel transmitter is not available, the monitoring unit may comprises means for checking trailer components sequentially in a continuous cycle and, upon failure, passing a signal to a single channel transmitter. 
     Further examples of trailer electrical circuits that may require monitoring are the battery charging circuit on a caravan, which is powered from the electrical supply of the towing vehicle and refrigeration circuits on articulated lorries. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 shows a vehicle towing a trailer and trailer bulb failure monitor, in accordance with the invention; 
     FIG. 2 shows a block diagram of a monitoring unit circuit; 
     FIG. 3 shows a detailed representation of a possible brake monitor board circuit; 
     FIG. 4 shows a detailed representation of a possible lamp monitor board circuit 
     FIG. 5 shows a detailed representation of a possible transmitter circuit arrangement and sequential decoder arrangement; 
     FIG. 6 shows a block diagram of an alternative signal transmitter unit; and 
     FIG. 7 shows a block diagram of a signal receiver unit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The figures show a monitoring circuit suitable for use with a 12 volt car electrical system. The circuit could easily be adapted for use with a 24 volt electrical system such as used on a lorry. 
     When a trailer  10  is towed by a vehicle  14  as shown in FIG. 1, a set of trailer lamps  12   a - 12   d , powered by the electrical supply of the vehicle  14 , are usually mounted at the rear of the trailer  10  in order to duplicate the rear lights of the vehicle  14 . 
     Without a trailer lamp monitor in place, power to the trailer lamps  12   a - 12   d  is delivered from the vehicle  14  via a power cable  20  directly connected to the vehicle  14  by joining a plug  24   b  and a socket  22   a , where the socket  22   a  is mounted on the vehicle  14  and the plug  24   b  is joined to the end of the cable  20 . When the trailer  10  is unhitched, it is disconnected from the vehicle electrical supply by releasing the plug  24   b  from the socket  22   a.    
     The monitoring unit  18 , shown in place in FIG. 1, includes a plug  22   b  and a socket  24   a  which are adapted to connect with the existing socket  22   a  and plug  24   b  respectively. Hence, the monitoring unit can easily be inserted into the trailer lamp circuit by disconnecting the socket  22   a  and the plug  24   b  from each other and the connecting monitoring unit  18  between them, without any modification being required of the existing trailer lamp circuit components. 
     In the present example, the socket  24   a  is mounted on the body of the monitoring unit  18  directly, whereas the plug  22   b  is linked to the monitoring unit body via a cable  19 . This allows the monitoring unit body to be carried in or on the trailer, but it is within of the scope of the invention for the monitoring unit body to be carried in or on the vehicle  14 . 
     The monitoring unit  19  can be detachably mounted on a trailer or vehicle, for example with the use of clamps, thereby allowing it to be switched from one trailer to another. 
     When an attempt is made by the vehicle driver to activate a non functional trailer lamp or set of lamps, circuitry within the monitoring unit  18  emits a radio frequency fault signal corresponding to that lamp or set of lamps, which is received by an indicator unit  26  located in the driver compartment  16  of the vehicle  14 . 
     The indicator unit  26  is equipped with a front panel, on which a plurality of light emitting diodes are mounted to alert the driver of a trailer lamp fault. Circuitry included within the indicator unit enables each diode to become lit in response to a given fault signal which corresponds to a given trailer lamp or set of lamps, thereby enabling the driver to identify the faulty lamp or set of lamps. 
     In the present example, the indicator unit  26  is powered by the vehicle electrical supply via a plug  28  adapted to connect with an on board power socket such as the cigarette lighter socket of the vehicle, although it is within the scope of the invention for the indicator to be powered by a battery housed within the indicator unit. 
     A block diagram illustrating the electrical circuit of the monitoring unit  18  is shown in FIG.  2 . The circuit comprises a brake lamp monitor board  30  and a lamp monitor board  32 , the brake lamp monitor board  30  monitoring the operation of the brake lamp  12   a , and the lamp monitor board  32  monitoring the remaining trailer lamps  12   b - 12   d , each board being capable of producing a fault signal in response to a fault in a given trailer lamp, and passing the fault signal to a transmitter circuit  34 . 
     The monitoring unit circuit is wired to the plug  22   b  and the socket  24   a , which are schematically shown connected to socket  22   a  and plug  24   b  of the vehicle and trailer respectively. For clarity, a simplified version of the vehicle circuit  23  is included, showing the vehicle lamp power supply at voltage Vs and supply rails S 1 -S 4  wired to the socket  22   a  with their corresponding lamp switches  25  in series with the power supply. A typical trailer lamp circuit wired to the plug  24   b  is also shown, with each of the trailer lamps  12   a - 12   d  being connected to a respective supply rail S 1 -S 4  at one terminal, and to the ground rail S 0  at the other terminal. 
     In the present example the supply voltage Vs is chosen to be positive and is provided by the vehicle battery, which is normally at a voltage of 12 V. 
     The lamp monitoring board contains three independent channels with respective channel input and output L 1  and L 7  for the first channel, L 2  and L 8  for the second channel, and L 3  and L 9  for the third channel, the channels being placed in series with the supply rails S 1 , S 2  and S 3  respectively. In a similar fashion, the brake lamp monitor board  30  contains a single channel in series with a supply rail S 4 , with channel input and output B 1  and B 3  respectively. 
     Channel outputs L 7 , L 8 , L 9  and B 3  are low impedance outputs in order to ensure that the intensity of the lamps is not significantly attenuated by the presence of the monitoring boards. 
     When a supply rail S 1 -S 4  rises to voltage Vs due to the closure of a corresponding switch  25 , a current flows through the lamp and that supply rail if the lamp is functional. If the lamp becomes non functional, no current flows through the supply rail for that lamp but the rail remains at voltage Vs provided the corresponding switch remains closed. 
     Current measurement circuitry present in each channel of the lamp monitor board produces a fault signal at a signal output L 4 -L 6  when a respective supply rail S 1 -S 3  is not passing current and the switch to that supply rail is closed, these two conditions together being indicative of lamp failure, the closure of the switch being determined from the voltage of the supply rail, which is close or equal to Vs independent of the lamp when the switch is closed. In a similar fashion, brake monitor board  30  produces a fault signal at signal output B 2  in response to a null current reading through supply rail S 4 . 
     A fault signal at a signal output L 4 , L 5 , L 6  or B 2  of a monitor board is represented by a low state of that output, the high state of that output being indicative of correct lamp operation. 
     Fault signals from the signal outputs L 4 , L 5 , L 6  and B 2  of the monitor boards are passed to respective inputs T 1 , T 2 , T 3  and T 4  of the transmitter circuit  34 , and are subsequently radio transmitted to the indicator unit  26 . 
     The transmitter circuit  34  is powered by supply rails S 1 -S 4  via supply input T 5  when at least one of the corresponding switches is closed, this arrangement conveniently ensuring that the transmitter circuit is only powered when the lamps are required to be illuminated. Similarly, the monitor boards are powered by the supply rails feeding into inputs L 1 -L 3  and B 1 . 
     A diode  36  is provided between each supply rail and the transmitter circuit supply input T 5 , the diodes being orientated so that each diode faces input T 5  in order to isolate the supply rails from each other. 
     The brake lamp monitor board  30 , the other lamp monitor board  32  and transmitter circuit  34  are each provided with a respective ground terminal B 0 , L 0 , and T 0 , each terminal being connected to the ground rail S 0 . 
     One embodiment of the brake lamp monitor board  30  is shown in FIG. 3, where connections B 0 , B 1 , B 2  and B 3  correspond to those in FIG.  2 . Brake lamps  12   a  connected in parallel between B 3  and ground are also shown. When the supply rail S 4  is at Vs, a first potential divider between the supply rail S 4  and the ground rail S 0  formed by the brake lamps in series with a resistor R 1  defines a voltage V+ at the non inverting input (+) of an operational amplifier  40  acting as a comparator. A variable resistor R 2  and a resistor R 3  in series form a second potential divider in parallel with the first, that defines a voltage V−, adjustable by means of a variable resistor R 2  at the inverting input (−) of the operational amplifier  40  whose output is connected via an adjustable potential divider R 4  and R 5  to the base of a npn transistor  42  which acts as a switch to the resistor R 6  and light emitting diode  44  in series between the collector of the transistor  42  and the supply rail S 4 , the emitter of the transistor being connected directly to ground. 
     The variable resistor R 2  is adjusted and the resistor values R 1  and R 3  are chosen so that when both brake lamps are operational, V+ is less than V− and therefore the output of the operational amplifier  40  is close to ground, with the result that the npn transistor  42  is cut off and signal output B 2  at the emitter terminal is close to Vs and is therefore high. 
     Failure of one or both lamps causes the voltage V+ at the non inverting input (+) to rise to a value greater than the voltage V− at the inverting input (−), with the result that the operational amplifier output is close to the positive supply voltage Vs and the npn transistor is biased into conduction mode, allowing current to flow through the resistor R 6  and light emitting diode  44  in series between the collector and the supply rail, which causes output B 2  to be close to ground and therefore low, the low state of output B 2  constituting a fault signal. 
     Appropriate biasing of the npn transistor  42  when the operational amplifier output is close to the positive supply voltage is achieved via the adjustable potential divider at the operational amplifier output formed by a variable resistor R 4  and resistor R 5  in series. 
     FIG. 4 shows a possible logic circuit for the lamp monitor board, where each channel includes a differential amplifier  50  whose inputs are placed across a resistor R 10 . The monitor board inputs and outputs are labelled as in FIG.  2 . 
     With reference to the channel with channel input L 1  as an example, it can be seen that when L 1  is at the supply rail voltage Vs and when the corresponding lamp  12   b  in the circuit is on, the current through the resistor R 10  causes a voltage to develop across its terminals, which is amplified by the differential amplifier  50 , causing the voltage at the inverted input of an AND gate  52  connected to the output of the differential amplifier to be high. Consequently, the output of the AND gate  52  is low, and therefore the npn transistor  54  is cut off, since it is connected at the base to the AND gate output and grounded at the emitter, resulting in a negligibly small current flow though the resistor R 12  and the light emitting diode  56  connected in series between the collector of the npn transistor  54  and the channel input L 1 . Hence the signal output L 4  at the collector transistor  54  is close to the supply rail voltage Vs and is therefore high. 
     When the lamp  12   b  is non functional, a voltage Vs applied at L 1  results in the same voltage appearing at L 7 , since no current is passed through the resistor R 10 . Hence there is no voltage difference between the inputs of the differential amplifier  50  and the amplifier output appearing at the inverted input of the AND gate  52  is low. Since the other AND gate input is connected to channel input L 1  at voltage Vs and is therefore high, the AND gate output biasing the base of transistor  54  is also high, with the result that the transistor  54  is in conduction mode and signal output L 4  goes low, the low state of signal output L 4  constituting a fault signal. 
     The other channels of the lamp monitor board operate in the same way, producing a low signal output when the corresponding lamp in the circuit is non functional and the supply voltage Vs is applied to the channel input. 
     Additional channels may be incorporated into the lamp monitor if required, not necessarily according to the circuit of the existing channels. 
     The light emitting diodes  44  and  56  in or on the monitoring unit are optional, being provided to indicate lamp failure without a separate indicator unit, for example during tests. 
     A possible configuration of the transmitter circuit  34  is shown in FIG.  5 . Single channel transmitters  60  transmit a radio frequency fault signal when a regulated voltage from a regulator  62  is applied to one of the transmitter inputs. Each transmitter input is connected to the regulator output via a respective pair of switches in series formed by a by a first relay switch  64  and a second relay switch  66 . 
     Each of the first relay switches  64  is controlled by a respective signal input T 1 -T 4 , each becoming closed in response to a fault signal at that input. Each of the second relay switches  66  is controlled by a counter  68  through a corresponding set of npn transistors  70 , the counter being operated by a timer  72 . The counter  68  sequentially causes each switch  66  to be closed for a time in a cyclic manner, such that only one of the second set of switches are closed at any given moment. 
     Only when both relays of a given pair are closed is the voltage from the regulator passed to the transmitter, resulting in a fault signal being transmitted. This arrangement prevents two or more fault signal being passed to a transmitter simultaneously, which is necessary since the transmitters  60  operate on a single channel. 
     The relays  66  are driven by the output from the regulator  62  connected to one terminal of the relay coil, with a return to ground at the other terminal provided by npn transistors  70  acting as a switch capable of being biased at the base into conduction by a high signal from the counter  68 . Thus a high signal from the counter  68  to a given transistor allows a current to flow through the corresponding relay coil which closes the corresponding switch in the relay. A protector diode  67  in parallel with the relay coil of each relay  66  is included to prevent damage to the transistors  70 . 
     The relays  64  are driven directly by a supply rail via supply input T 5 . A path to ground though each relay coil is provided by a signal input T 1 -T 4  when that input goes low, thereby closing the corresponding relay switch. 
     The regulator  62 , counter  68 , timer  72  and transmitters  60  are powered by a supply rail through input T 5 , with electrolytic smoothing capacitors  73  and  75  provided at the regulator output and at the supply to the counter  68  respectively. This relay type circuit may, of course, be replaced by one using solid state components. 
     An alternative configuration of the transmitter circuit is shown in FIG.  6 . In this example, each of the signal inputs T 1 -T 4  is passed to an input ip 1 -ip 4  of a signal transmitter  80  via a set of pnp transistors  82  in the emitter follower configuration. Each input of the signal transmitter operates on a single channel, the signal transmitter transforming the single channel inputs to form a multi channel single output passed to a radio transmitter  84  which transmits a radio frequency fault signal. The transmitted fault signals are received by the indicator unit  26 . 
     A block diagram for the indicator unit circuitry is shown in FIG. 7. A radio frequency receiver  90  picks up a radio frequency fault signal sent by the radio transmitter  60 , 84  and passes the corresponding data to a signal receiver  92 , the signal receiver  92  being capable of decoding that data in order to identify the signal input in the monitoring unit that is registering the fault signal, and as a result of the identification, illuminating one or more designated light emitting diodes  94  on the front panel of the indicator unit. Illumination of the light emitting diodes  94  will indicate to a driver that a lamp in the trailer has failed.