Abstract:
A circuit and method of reducing the occurrence of overheating of a power fold vehicle mirror. The circuit controls the operation of the power fold mirror such that when a portion of the circuit reaches or exceeds a predetermined temperature threshold, the vehicle mirror is prevented from at least folding in. In practice, the vehicle mirror will be allowed to fold out even when the predetermined temperature threshold has been reached or exceeded, to allow the vehicle door to be opened and the car to be driven. The vehicle mirror may be allowed to be folded in when the temperature of the circuit falls to or below a second predetermined temperature threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a National Stage of International Application No. PCT/AU2004/000386 filed Mar. 25, 2004, which claims priority to Australian Patent Application No. 2003901355 filed on Mar. 25, 2003. The disclosures of the above applications are incorporated herein by reference. 
   TECHNICAL FIELD 
   This invention relates to the control of power fold mirrors on vehicles. 
   BACKGROUND OF THE INVENTION 
   A standard feature of many modern vehicles, including private and industrial vehicles, is the ability for one or more side mirrors of the vehicle to fold in and out from the side of the vehicle. The driver is able to manually control the position of the mirror such that, when, for example the car is parked, the driver will switch the mirror into a folded position to lie adjacent to the external surface of the car. When the car is about to be driven, the driver can select the fold out position and the mirror will fold out from the side of the car and assume an extended position in the normal fashion, to allow the mirror to be used while driving. 
   A common factor to be taken into consideration when designing electronic circuits, particularly when associated with mechanical actuators, is the production of heat. In the case of fold out mirrors, small electric motors drive the movement of the mirror and in doing so, generate heat. Traditionally, because the power fold function is used infrequently, the generation of heat is not a major problem. However, there may be instances where the power fold function is used frivolously, for example a child playing with the power fold switch and repeatedly folding the mirror in and out. This may result in overheating of the motor and surrounding electronic components and may damage the components of the mirror system. 
   Traditional ways of dealing with overheating include the provision of heat sinks, which provide a large surface area in contact with the surrounding air to radiate heat therefrom, and in the case of circuit boards, larger PCBs can be used to provide the larger heat radiating area. 
   Other methods of compensating for the generation of heat include using heavier duty components which are more expensive. This is undesirable in a product which is mass manufactured as the overall increase in cost can be dramatic. Similarly, in devices such as power fold control circuits, space is often at a premium and it is necessary to keep component and board sizes as small as possible. 
   It is an object of the present invention to provide an alternative means of addressing the problem of potentially damaging heat. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention, there is provided a control circuit for controlling the operation of a power fold vehicle mirror, the control circuit including:
         a temperature sensor; and   a vehicle mirror actuation control; wherein   upon the temperature sensor sensing that a temperature of at least a portion of the control circuit has reached or exceeded a predetermined temperature threshold, the vehicle mirror actuation control prevents the vehicle mirror from at least folding in.       

   According to a second aspect of the present invention, there is provided a method of controlling the operation of a power fold vehicle mirror, controlled by a control circuit, the method including preventing the vehicle mirror from at least folding in if a temperature of at least a portion of the control circuit reaches or exceeds a predetermined temperature threshold. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will now be described with reference to the following Figures in which: 
       FIG. 1  is a circuit diagram including a control circuit according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a circuit diagram of a control circuit for controlling the operation of a power fold mirror. 
   The vehicle interface has two supply leads (A and B) and a door signal line (D). When door signal line D is open circuit, the motor polarity is determined by the supply polarity. Door signal D equates to the state of the vehicle door. A and B are provided by the state of a “fold in/fold out” control switch, operated by a user. 
   When door signal line D is connected to the negative supply lead, then the polarity of A and B have no effect on the motor polarity. The motor polarity is shown in Table 1 below, where A, D and B are the inputs described above and MA and MB are the terminals of the motor. The “function” column indicates the function of the mirror resulting from the combination of the signals on inputs A, D and B. 
   
     
       
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Input 
               Output 
                 
             
           
        
         
             
               A 
               D 
               B 
               MA 
               MB 
               Function 
             
             
                 
             
             
               + 
               Open 
               − 
               − 
               + 
               Fold In 
             
             
               − 
               Open 
               + 
               + 
               − 
               Fold Out 
             
             
               − 
               − 
               + 
               + 
               − 
               Fold Out 
             
             
               + 
               − 
               − 
               + 
               − 
               Fold Out 
             
             
                 
             
           
        
       
     
   
   Door signal line D input is supplied with a positive bias current from R 19  via D 6  from whichever of A or B is positive. From Table 1, it can be seen that the function is always “fold out” unless A is positive, D is open and B is negative. In order to select “fold in”, digital transistors Q 13  must be biased on. This occurs only when Q 12  is biased on which can only occur if B is negative and D is open. In this case, a current flows from D 6  to R 19  to R 21 , to bias Q 12 B on and draw current through R 22  which biases digital transistors Q 13  on. If D is negative, then Q 12 B cannot be biased on and if B is positive, Q 12 B cannot draw current through R 22  to bias Q 13  on. Thus, Q 13  is biased on only when the “fold in” function is required. 
   It will be appreciated that the mirror should not be able to fold in if the vehicle door is open, as this will interfere with the operation of the mirror. 
   Transistor Q 12 A is used to protect the junction of Q 12 B from excessive reverse bias voltage. Capacitor C 7  provides immunity from high frequency interference and R 20  provides immunity from RF and small leakage currents on the D input line. Capacitor C 8  is provided to prevent changes to the input states that only last a short time from changing the function of the device. Diodes D 2  to D 5  form a conventional full wave rectifier circuit. Capacitor C 2  is used to bias the circuit during supply interruptions, reducing interfering signals from the motor being transmitted to the vehicle interface leads and improve the input impulse transient immunity. While the vehicle interface is energised, the cathode of diode D 2  is positive and anode of diode D 3  is negative, regardless of the polarity of the incoming supply signal. 
   Capacitors C 3  and C 4  connect the input to the output of the rectifying circuits or bridge, for high frequencies even when none of the diodes is positively biased. This reduces EMC emissions and susceptibility. 
   Transistors Q 10  and Q 7  form a conventional H bridge allowing polarity control. When transistor Q 13  is on, transistor Q 7 A is on making the motor input terminal MA negative. Q 13 , being on, also biases Q 10 A off and Q 14  off (via Q 9  and R 23 ). Transistor Q 14  is biased off, allowing R 16  to bias Q 10 B on and Q 7 B off, making motor terminal MB positive. When Q 13  is off, all of the above states are reversed and motor terminal MA is positive and motor terminal MB is negative. 
   Capacitor C 9  reduces the interference produced by the motor passing through the circuit to the vehicle interface. Diode D 7  prevents transients on the supply from damaging transistors Q 7 A and Q 10 A via R 15 . 
   Diode D 1 , biased by R 2 , provides a voltage reference, which may be shunted off by either of the SCR-configured transistor pairs Q 11 A with Q 11 B and Q 8 A with Q 8 B (the function of which will be discussed further below). Transistor Q 3  and resistor R 9  buffer the voltage regulator output. 
   Transistors Q 2 A with Q 2 B form a voltage comparator of the “long tail pair” variety, biased by R 3 . The current provided by R 3  is steered in varying proportions between the collector of transistor Q 2 B to supply return or to the collector of transistor Q 2 A, generating a voltage on R 4 . 
   The input of the comparator is fed with the voltage reference on one side and with a percentage of the voltage supplied to the output polarity control circuit (incorporating transistors Q 10  and Q 7  as described above) by the ratiometric potential divider made up by resistors R 7 , R 10  and R 6 . 
   The output of the comparator (being the voltage developed across resistor R 4 ) controls the bias of transistor Q 6 , which forms the voltage regulator pass element. 
   Capacitor C 1  and resistor R 30  control the gain roll-off and phase of the loop to prevent oscillation of the circuit. 
   The voltage of the output polarity control circuit and hence the output to the motor is thus voltage regulated to a multiple of the referenced voltage. 
   The current through the output terminals, the output polarity control circuit and the voltage regulator, flows through current sense resistors R 17  and R 18 , generating a voltage feeding resistor R 14 , proportional to the current. This voltage is summed with the voltage produced by a current from R 1  acting on R 14 . This current is introduced to modify the voltage produced by the current according to the temperature of the circuit. Applying the reference voltage as described above to the series combination of resistors R 5  and R 8  creates the current. Resistor R 8  has a high and non-linear defined negative temperature coefficient of resistance to temperature (otherwise known as a thermistor). The current this produces increases with temperature. Given that the base current of transistor Q 1 A is small, the emitter current and collector current are approximately equal. This additional current flows into resistor R 14  via resistor R 1 . Once the current in R 1  is sufficient to produce a voltage across it such that Q 1 B starts to bias on, then any additional current flows in the negative supply and not through R 14 . 
   Once the summing of R 14  is such that the transistor Q 5 B starts to be biased, drive from transistor Q 6  is pulled away via the base-emitter junction of transistor Q 5 A. In this way, the current is limited through transistor Q 6  and hence the motor current is limited. 
   When transistor Q 5 B draws current away from the gate of transistor Q 6  to control the current, transistor Q 5 A is turned on and diode D 8  is biased via resistors R 13  and R 26  to provide a voltage of approximately 1.2 volts below the positive supply rail. This charges capacitor C 5  and C 6  through resistors R 25  and R 24  respectively. 
   In the “fold in” function, transistor Q 13  is biased on and as such, capacitor C 6  is prevented from charging but capacitor C 5  can charge. In the “fold out” function, the reverse is true and transistor Q 14  is on which prevents capacitor C 5  from charging but allows capacitor C 6  to charge. 
   When capacitor C 5  charges enough, transistor Q 8 A will start to bias on, which will bias transistor Q 8 B on, which will further bias transistor Q 8 A on. The effect is that transistors Q 8 A and B snap on (as in an SCR). When this happens, the voltage reference across diode D 1  is shunted and the regulated voltage falls close to zero causing the motor to be off. This state only ends when transistor Q 14  is biased on to unlatch the arrangement, which happens when the function changes from “fold in” to “fold out”. Resistor R 27  prevents leakage currents causing false activation of the circuit. 
   When capacitor C 6  charges enough, transistor Q 11 A will start to bias on which will bias Q 11 B on, which will further bias transistor Q 11 A on. The effect is that Q 11 A and B snap on (again, as in an SCR). When this happens, the motor is off. This state only ends when transistor Q 13  is biased on to unlatch the arrangement, which happens when the function changes from “fold out” to “fold in”. Resistor R 28  prevents leakage currents causing false activation of the circuit. 
   The portion of the circuit which controls the function of the mirror in accordance with the invention includes resistors R 13  and R 12  which form a potential divider from the referenced voltage. Resistor R 13  has a high and non-linear defined negative temperature coefficient of resistance to temperature as does resistor R 8  as previously described. As the temperature increases, the voltage feeding resistor R 11  increases (provided that the reference voltage is not shunted). 
   When this voltage is high enough to start biasing resistor Q 4 A on, transistor Q 4 B is biased on which in turn biases Q 4 A harder on. The result is that the circuit latches on, pulling transistor Q 13  off. As previously described, when transistor Q 13  is off, the control circuit assumes the “fold out” function and cannot fold the mirror in. 
   Resistor R 29  prevents leakage current from falsely activating the circuit. Accordingly, if the unit were asked to operate (with voltage reference being unshunted) but at too high a temperature, the control circuit will be forced to an “fold out” state and will remain in that “fold out” state until the temperature reduces. 
   As will be obvious to the person skilled in the art, the value of resistor R 13  and associated components will be chosen such as to cause transistor Q 13  to turn off, and hence assume the “fold out” position, when the temperature of resistor R 13  reaches a preset threshold. Accordingly, once the temperature of the circuit, and more particularly resistor R 13 , reaches an undesirable level, the control circuit will prevent the mirror from folding in, until the temperature has dropped to a safer level, at which time transistor Q 13  will be allowed to turn on, and the control circuit will be allowed to assume the “fold in” state. The sense resistor R 13  is placed adjacent to the components most needing temperature protection, typically the output transistors Q 7  and Q 10 . 
   Because when transistor Q 13  is on, and the circuit is in the “fold in” state, the mirror is still able to be operated, but only to cause it to be folded out. In this way, even though the temperature of the circuit and surrounding components may be too high, the mirror may still be folded out to allow the door to be opened, and the car to be driven, however, the mirror will not be allowed to be folded in until the temperature has reduced to safer levels. 
   This is a preferred arrangement, however, it is within the scope of the present invention to prevent any activation of the mirror fold function. 
   The above has been described with reference to a particular embodiment and it will be understood by the person skilled in the art that many variations and modifications may be made within the scope of the present invention. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.