Abstract:
A method for detecting vehicle speed detects existing noise frequency of vehicle power supply and vehicle acceleration value, and compares and processes the detected data to estimate the final vehicle speed. The method includes the procedure of (1) detecting power supply noise frequency and converting to engine speed; (2) detecting acceleration value; (3) comparing the engine speed and the vehicle speed; and (4) selecting the final vehicle speed. By means of the procedure of the invention, the vehicle speed may be provided to the vehicle navigation apparatus to continuously perform navigation during the brief period when the GPS signals are lost.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a method for detecting vehicle speed that measures vehicle power supply noise frequency and vehicle acceleration value and incorporates vehicle driving conditions to derive a final vehicle speed to provide a vehicle navigation apparatus to continuously perform vehicle navigation during a period when GPS signals are absent.  
         [0003]     2. Description of the Prior Art  
         [0004]     Satellite-based vehicle navigation system has become quite popular nowadays. Through signals transmitted by positioning satellites, the location of a vehicle may be positioned on an electronic map and displayed instantly. By entering a destination, a desired driving route may be derived to direct the driver. However, due to geographic conditions or roads and buildings, poor or interrupted signals often occur. For instance, driving in a tunnel, under an overpass, or in the alleys bordered by high rise buildings, the vehicle navigation apparatus could become dysfunction because the satellite signals cannot be properly received. To remedy this problem, some high end navigation apparatus  11  as shown in  FIG. 1  use a gyroscope  12  and a vehicle speed line  13  to derive the location of a driving vehicle to compensate the lost satellite signal  14  for the navigation apparatus  11 . However, the location of vehicle speed line and signal format are different on different vehicles, this results in installation difficulty. Many users purchase high end navigation devices but cannot install by themselves.  
       SUMMARY OF THE INVENTION  
       [0005]     In view of the aforesaid problems such as the navigation device becomes ineffective when GPS signals are lost, and vehicle speed line has different specifications and is difficult to install, the present invention provides a method for detecting vehicle speed that includes the procedure of: (1) detecting power supply noise frequency and converting to engine speed; (2) detecting acceleration value; (3) comparing the engine speed and the vehicle speed; and (4) selecting the final vehicle speed. Through the procedure set forth above, the vehicle speed may be derived to enable the navigation apparatus to continuously perform navigation even the GPS signals are lost in a brief period.  
         [0006]     In one aspect, the method of the invention to detect the vehicle power supply noise frequency mainly employs a vehicle speed detection apparatus which includes at least a vehicle power supply noise detection unit.  
         [0007]     In another aspect, the vehicle speed detection unit includes an accelerometer to detect the acceleration value of the vehicle.  
         [0008]     In yet another aspect, the vehicle speed detection unit gets the vehicle power supply noise frequency by plugging in the vehicle lighter jack which is connected to the power supply so that the vehicle speed detection apparatus and the navigation apparatus can obtain required power supply easily.  
         [0009]     The invention provides a solution to overcome the problems of dysfunction occurred to the conventional vehicle navigation devices when the GPS signals are lost in a brief period, and varying specifications of vehicle speed line and installation difficulty. By providing vehicle speed without the vehicle speed line, the final vehicle speed of a driving vehicle may be derived accurately to meet the requirement of the navigation system, and users can enjoy the plug and use convenience.  
         [0010]     The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a block diagram of a conventional vehicle navigation system.  
         [0012]      FIG. 2  is a circuit block diagram of the invention  
         [0013]      FIG. 3  is a chart showing the characteristic relationship between the engine speed and vehicle speed according to the invention.  
         [0014]      FIG. 4  is a vehicle speed table relating to the gearshift and engine speed according to the invention.  
         [0015]      FIG. 5  is the main flowchart of the invention.  
         [0016]      FIG. 6  is the flowchart of step S 2  of the invention.  
         [0017]      FIG. 7  is the flowchart of step S 3  of the invention.  
         [0018]      FIGS. 8A and 8B  are the flowchart of step S 4  of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     Refer to  FIG. 2  for a circuit block diagram of a vehicle speed detection apparatus  2  for detecting vehicle power supply noise frequency. It includes a vehicle power supply noise detection unit  21  and an accelerometer  22 .  
         [0020]     The vehicle power supply noise detection unit  21  aims to detect the noise frequency (NF) of the existing vehicle power supply and convert to engine speed (ES).  
         [0021]     The accelerometer  22  aims to detect a vehicle acceleration value a.  
         [0022]     The vehicle speed detection apparatus  2  further includes a processing unit  211  to receive satellite signals from a satellite  5 , engine speed signals converted by the vehicle power supply noise detection unit  21  and the signal of the vehicle acceleration value a of the accelerometer  22 , and process and compare the aforesaid signals.  
         [0023]     The vehicle speed detection apparatus  2  is plugged into a vehicle lighter jack  3  and is connected to a navigation apparatus  4  to get the NF signals of the vehicle power supply, and convert to engine speed (ES). After incorporating with the acceleration value a, and processed by the processing unit  211 , an estimated final vehicle speed V is obtained to be used by the navigation apparatus  4 . The procedure is as follow (referring to  FIG. 5 ): 
        1. Detect the power supply noise frequency and convert to engine speed (step S 1 ): Detect the power supply noise frequency NF by the power supply noise detection unit  21 , and convert to the engine speed ES.     2. Detect the acceleration value (step S 2 ): Process and get a second estimated vehicle speed V 2  based on the acceleration value a detected by the accelerometer  22 .     3. Compare the engine speed and vehicle speed (step S 3 ): Establish a vehicle speed table (referring to  FIG. 4 ) based on comparison of the engine speed ES and a first estimated vehicle speed V 1  of a GPS, and modify according to the gearshift to get a third estimated vehicle speed V 3 .     4. Select a final vehicle speed (step S 4 ): Based on satellite signal receiving conditions and vehicle speed comparison results, estimate and select the final vehicle speed V.        
 
         [0028]     By means of the step S 4  mentioned above, the final vehicle speed V is obtained and provided to the navigation apparatus  4  to perform navigation function.  
         [0029]     Referring to  FIGS. 5 and 6 , to execute the step S 2 , first determine whether the reading of the accelerometer  22  is a constant speed (step  21 ), namely to judge by the acceleration value a. In the event that the reading of the accelerometer  22  is smaller than a set value a 0  (such as the set value is 15 or less), and in a set time period T 0  (such as three seconds), it may be determined that the vehicle is not accelerated, and is at a constant speed condition. The processing unit  211  records the off set value of the accelerometer  22 , and execute step S 22  to be used in the process of compare the engine speed and vehicle speed (Step S 3 ). On the other hand, if the reading exceeds the set value a 0  and not reach the set time period T 0 , it is determined not in a constant speed condition; then a second estimated vehicle speed V 2  is derived based on the acceleration value a of the accelerometer  22  to provide the processing unit  211  to do process and comparison (step S 23 ).  
         [0030]     Referring to  FIG. 7 , at the step S 3 , in the condition of GPS signals are received (step S 31 ), if the acceleration value a is within the range of a set value a 0 , it is considered the constant speed (step S 32 ), the relationship of the engine speed ES and the first GPS estimated vehicle speed V 1  (namely the current speed supplied by the GPS), besides parking and acceleration and deceleration during gearshift, the engine speed ES and the first estimated vehicle speed V 1  form a plurality of non-consecutive constant ratios. Based on this relationship, when the satellite signal condition is desirable, the first estimated vehicle speed V 1  may be measured accurately. With the accelerometer  22  measured that the vehicle is not accelerating (namely at a constant speed condition), the relationship between the first estimated vehicle speed V 1  and the engine speed ES is a non-continuous straight line (as shown in  FIG. 3 ). It means that when the vehicle gearshift is from gearshift  1  to gearshift  4 , the first estimated vehicle speed V 1  and the engine speed ES form respectively a constant ratio relationship of G 1 , G 2 , G 3  and G 4 . Hence the records of the GPS first estimated vehicle speed V 1  and the engine speed ES value may be entered into the statistical data. A plurality of engine speed values (such as two to six times, depending on actual requirements) in the data at the same vehicle speed may be captured for averaging. The resulting average value is included in the vehicle speed table (step S 34 ) to derive a third estimated vehicle speed V 3 . In the event that the acceleration value a exceeds the set value a 0 , it indicates that the vehicle is not at a constant speed condition (step S 31 ), and there is a difference between the actual vehicle speed and the second estimated vehicle speed V 2 , and a modification for a different gearshift is needed (step S 33 ). When the gearshift changes, the engine speed ES rises abruptly then drops, or drops abruptly and rises, then the acceleration and deceleration of accelerometer  22  may be used to determine whether gearshift has actually happened. And the driving gearshift may be determined, and the third estimated vehicle speed V 3  may be derived by referring to the vehicle speed table of the corresponding gearshift (step S 34 ).  
         [0031]     Referring to  FIGS. 8A and 8B , at the step S 4 , the process includes: (1) when the satellite signals are received in a normal condition, the first estimated vehicle speed V 1  obtained from the GPS is selected as the final vehicle speed V (steps S 41 , S 45  and S 46 ); (2) when the satellite signals are lost, if the difference X between the second estimated vehicle speed V 2  derived from the acceleration speed a of the accelerometer  22  and the third estimated vehicle speed V 3  derived by comparing and processing the engine speed and vehicle speed is within a set range (such as 15%, or other value according to requirements), the second estimated vehicle speed V 2  calculated by the accelerometer  22  is selected as the final vehicle speed V (steps S 41 , S 42 , S 44  and S 46 ); (3) if the satellite signals are lost, and the difference X between the second estimated vehicle speed V 2  derived from the acceleration speed a of the accelerometer  22  and the third estimated vehicle speed V 3  derived by comparing and processing the engine speed and vehicle speed exceeds a set range, the three estimated vehicle speed V 3  derived by comparing and processing the engine speed and vehicle speed is selected as the final vehicle speed (step S 41 , S 42 , S 43  and S 46 ). The selected vehicle speed at the steps S 45 , S 44  and S 43  is the final selected vehicle speed V for navigation process (step S 5 ).