Abstract:
A tracking unit signal filter  10  for a vehicle, typically tractor/trailer combinations, having at least one signal input  15-17  for receiving a respective signal A, B or C indicative of a respective vehicle parameter eg. signal C for sensing tractor/trailer coupling, and processor means  28  operable to pass the signal C after a predetermined time delay TC, if said signal is stable, and which is also operable to maintain said stable signal C if the signal is interrupted for less than a predetermined time interval tc.

Description:
FIELD 
   This invention relates to a tracking unit signal filter which is used to prevent spurious reports from tracking units which are utilised for locating and tracing vehicles, and vehicle trailers or semi-trailers. 
   BACKGROUND OF THE INVENTION 
   For logistical and security reasons there is an increasing interest for transport operators to equip vehicle trailers and semi-trailers with tracking units. These units typically comprise a GPS (Global Positioning Satellite system) receiver to provide location information and other vehicle parameter sensors and may be passive systems which log data for later retrieval, or may be active systems which include a cellular or satellite transmitter/receiver for transmission of data to a monitoring station for continual monitoring of the vehicle. 
   The transmitters can be programmed to provide location data at desired time intervals or may be programmed to react to particular events such as coupling and decoupling tractor units to a trailer, or opening and closing trailer doors. 
   Trailer units typically have no source of electrical power and when attached to a tractor the trailer electrical system is connected into the tractor electrical system which supplies a 24v dc electricity to the trailer through ISO standard connectors. When tracking units are fitted to trailers they often have their own 12v battery and a battery charger unit which can recharge the battery from the trailer electrical system during the time period that the tractor and trailer are interconnected. 
   It is known to use the voltage from the tractor electrical supply (24v) as a signal to monitor the operation status of the trailer i.e coupled or uncoupled in relation to a tractor. 
   However problems can arise due to the incorrect or incomplete coupling between the tractor and trailer so that for example the trailer electrical system did not receive continuous power but was for example coupled to the power supply only when the brakes are applied or indicators used. Even when correctly connected a continuous power supply may be interrupted due to loose contacts when driving over rough roads. 
   The tracking unit may operate in response to other vehicle sensed parameters and on-board sensors may be triggered by operational conditions, for example door condition sensors may be triggered by driving over bumpy roads, or by shocks caused by fork lift trucks during loading or unloading and the information transmitted may give a false indication of the trailer status. 
   The present inventions seeks to ameliorate the above problems and reduce the number of vehicle condition spurious signals transmitted by a tracking unit. 
   STATEMENT OF INVENTION 
   According to the present invention there is provided a tracking unit signal filter having at least one signal input for receiving a respective signal indicative of a respective vehicle parameter and processor means operable to pass a parameter signal after predetermined time delay if said signal is stable, and which is also operable to maintain said stable signal if the signal is interrupted for less than a predetermined time interval. 
   The term “tracking unit” applies to both active systems and passive logging systems as discussed above. 
   The predetermined time delay and or the time interval may be varied as is required for the particular parameter being sensed by storing numeric constants in a programmable memory device. 
   Preferably, for a tractor/trailer combination the signal input comprises a voltage signal from the vehicle power supply which indicates when the tractor and trailer are coupled. 
   Additionally or alternatively, the filter receives signals from vehicle mounted sensors which each detect the status of particular vehicle conditions, preferably trailer door status sensors. 
   Both the filter and tracking unit are powered by a dedicated battery, which is recharged from the vehicle power supply through a DC-DC converter. The regulated re-charging voltage applied to the battery is controlled by a microprocessor to provide a first higher charging voltage and a second lower charging voltage. Such a device is disclosed in U.S. Pat. Nos. 4,843,299 and 5,049,804. 
   In a preferred arrangement the DC-DC converter may be incorporated into the filter, or vice versa 
   The filter may also include a pulse generator for modifying stable signals into a pulse, and a sequencer for putting pulse signals into a sequence according to a predetermined order of priority as stored in a programmable memory device. 
   A filter of the above type may be integrally incorporated into a tracking unit. 

   
     DESCRIPTION OF DRAWINGS 
     The invention will be described by way of example and with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic layout of the signal filter according to the present invention, 
       FIG. 2  is a circuit diagram of a filter according to  FIG. 1 , and 
       FIG. 3  shows signals before and after passing through the filter. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1  there is shown schematically a signal filter  10  according to the present invention for use with a vehicle trailer tracking unit  21 . A typical tracking unit is MH Fleet Plus manufactured by proTime GmbH of Prien am Chiemsee, Germany. The electrical system of the vehicle trailer is typically powered from the tractor unit through electrical couplings. 
   The filter  10  has outputs  11 ,  12 ,  13 A &amp;  13 B, connected to a tracking unit  21  and has signal inputs  15 - 17 . The filter  10  is also connected to a power source  18  such as a 12v dc battery  18 , via input  14 , for example a Yuasa NP7-12 available from Yuasa Batteries Ltd. The battery  18  may be connected to DC-DC converter  19  which receives a 24v DC current from the vehicle tractor unit. The converter  19  produces an approximate voltage of 14.5 volt DC for charging for the battery  18 . An example of such a converter is an Exegon “Vepax” (tradename) 24v-12v manufactured by Exegon of Melksham, England and described in U.S. Pat. Nos. 4,843,299 and 5,049,804. The tracking unit  21  receives its operational power from the battery  18  or DC-DC converter  19 . 
   The input  17  is connected to a relay or transistor switch type sensor  22  allowing the filter unit to sense the 24v signal from the tractor unit. The input  15  is connected to a “door open” output  25  of at least one door condition sensor and the input  16  is connected to a “door closed” output  26  of the door condition sensor. The door sensor is for example heavy duty reed proximity switches or magnetic proximity switches with changeover contacts. 
   The filter  10  includes a microprocessor  28 , preferably a programmable processor, which monitors the signals C, A, &amp; B respectively, received from the sensors  22 ,  25 ,  26  and processes the signals according to set instructions. 
   With also to  FIG. 3 , in  FIG. 3A , there is shown a typical raw signal from a sensor. The signal may include an initial unstable signal and once stabilised may include interruptions due to operational problems. 
   The processing of the signals from each sensor Is similar in each case. The 24v signal C indicating a trailer coupled/uncoupled status is processed as follows:
     a) The 24v signal C from sensor  22  is detected and switched off by a time delay  41 . After a pre-set time period TC the delay  41  switches on and the stable signal C passes through to an interrupt filter  42 . The time period T may be varied as is required by storing numeric constants in the processor  28 . This is represented by  FIG. 3B     b) The interrupt filter  42  maintains the signal C if there is an interruption of less than a pre-set period tc in the signal C detected by the sensor  22 . This pre-set period of interruption may be varied as is required by storing numeric constants in the processor  28 .   c) The treated signal from the interrupt-filter  42  shown in  FIG. 3C  is then passed either
       i) directly into a sequencer  44  before passing through to the tracking unit  21  via a transistor switching assembly  29  and output  13 B, and will keep the tracking unit  21  in active mode as long as signal C remains on, or   ii) is passed into a pulse generator  43  which modifies an established signal into a pulse, the time period Pc of which may be varied, which is passed into the sequencer  44 . The pulsed signal is shown in FIG.  3 D. The pulse may be used to put the tracking unit  21  to “sleep mode”. The pulsed signal Pc is passed to the unit  21  via output  13 A. The sequencer  44  prevents simultaneous occurrence of inputs to the tracking unit  21  as this could result in malfunctioning of the unit, for example the generation of false status or confusing messages, or loss of status information.   
       

   The signals A and B from the doors sensor outputs  25   26  respectively are similarly processed through respective delays  51 , 61 , interrupt filters  52 , 62 , and pulse generators  53 , 63 . The door status signals from the interrupt filters  52 , 62  may be passed into a pulse generator  53  or  63  to modify the signal as previously discussed to reset the tracking device to “sleep mode” after an initial “wake-up” when the doors are either open or closed for a prolonged time period. The door status signals are then passed into the sequencer  44  and through to the tracking device via outputs  11  &amp;  12 . 
   It has been found for this typical example using a semi-trailer tractor combination, that suitable a time delay for sensing vehicle coupling or uncoupling is 100 secs with an interruption period of 10 second, whereas the door status signals should have a delay period of 3 secs with an interruption period also of 3 seconds. Typical tolerances are ±0.5 secs to ±10% of the nominal value. 
   These periods can be varied to suit the operation conditions and sensor type. 
   The microprocessor may also include an inhibit instruction that prevents a transmission of a signal to the tracking unit within a given time period of an earlier transmission. 
   A circuit diagram for the filter is shown in FIG.  2  and those parts corresponding with the schematic have been assigned the same reference numerals. 
   It will be apparent that other sensors for other vehicle parameters can be added alternatively or additionally to the filter  10 , and that the microprocessor  28  can incorporate any number of “delay:interrupt filter:pulse generator” chains as is required for the number of vehicle parameters to be sensed and when necessary more than one microprocessor  28  can be used.