Abstract:
A method for measuring speed involves calculating and measuring speed of an object based on a distance and a time obtained by a method for measuring distance and a method for measuring time, respectively. The time between distance measuring sessions is obtained using the cycle number of a reference signal based on a clock mask synchronous with the distance measuring sessions. The time is corrected according to a plurality of phase shift signals generated based on the reference signal. An error of the time is minimized by increasing the quantity of the phase shift signals. The method enhances the accuracy of the measured time between distance measuring sessions, speeds up speed measurement, and reduces the required circuit areas. A system for measuring speed is further introduced for use with the method.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 100125213 filed in Taiwan, R.O.C. on Jul. 15, 2011, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The present invention relates to methods and systems for measuring speed, and more particularly, to a precise method and system for measuring speed. 
       BACKGROUND 
       [0003]    A method and system for measuring speed usually start with two stages, that is, distance measurement and time measurement, and then involve calculating the speed or acceleration of an object according to the measured distance and time parameters. Distance measurement usually requires an optical ruler or an encoder. The optical ruler measures the distance traveled by the object during the linear motion thereof. The encoder measures the distance traveled by the object during the circular motion thereof. Time measurement usually entails calculating the time from the commencement of distance measurement of the object to the termination of distance measurement of the object according to a fundamental frequency signal (or defined as known time). 
         [0004]    Time measurement involves calculating a time by a default frequency of the fundamental frequency signal and the number of cycles between the commencement of distance measurement and the termination of distance measurement of an object, wherein the precision of the calculated number of cycles of the fundamental frequency signal affects the accuracy of the calculated time. 
         [0005]    Normally, calculation of the number of cycles of the fundamental frequency signal requires counting, that is, counting the number of cycles of the fundamental frequency signal during a gate time period that starts from a point in time of commencement of the fundamental frequency signal and ends at a point in time of termination of the fundamental frequency signal. 
         [0006]    Nonetheless, the number of cycles of the fundamental frequency signal during the gate time period is seldom an integer, and thus the method is likely to cause an error at the beginning and the end of the gate time period—underestimating or overestimating by a half cycle, for example. In view of this, to measure time, it is usually necessary to maximize the gate time period in order to handle as many cycles as possible and thereby reduce errors. However, the aforesaid solution is performed at the cost of a great increase in the testing time and with the tendency to decrease resolution due to a short gate time period arising from a short period of time measurement and a small number of instances of time measurement. 
       SUMMARY 
       [0007]    It is an objective of the present invention to enhance the speed and accuracy of speed measurement. 
         [0008]    Another objective of the present invention is to reduce circuit-occupied area and reduce power consumption. 
         [0009]    In order to achieve the above and other objectives, the present invention provides a method for measuring speed, by performing computation of speed measurement of an object with a distance and a time obtained by means of distance measurement and time measurement, characterized in that the time measurement comprises the steps of: providing a reference signal; generating a plurality of phase shift signals of a same frequency based on the reference signal, the phase shift signals being spaced apart from each other by a fixed phase; setting a clock mask, the clock mask starting from a start signal of distance measuring sessions and ending at an end signal of the distance measuring sessions; counting a number Nd1 of second triggering states occurring to the phase shift signals during a time period from a point in time of commencement of the clock mask to occurrence of a first triggering state to the reference signal; counting a number Nb of cycles of the reference signal during the time period of the clock mask based on the first triggering state; counting a number Nd2 of second triggering states occurring to the phase shift signals during a time period from a point in time of termination of the clock mask to occurrence of a first triggering state to the reference signal; and obtaining a time t of the object by the equation below: t=(Nb/Fb)+[Nd1/(Fb/M)]−[Nd2/(Fb/M)] where frequency of the reference signal is denoted by Fb and number of the phase shift signals by M, and M≧2. 
         [0010]    In order to achieve the above and other objectives, the present invention provides a system for measuring speed, comprising: a distance measurement unit for measuring a distance traveled by an object, generating a distance measurement d based on the distance measured, and generating a start signal and an end signal based on distance measuring sessions; a distance measurement signal input end for receiving the start signal, the end signal, and the distance measurement d; a speedometer connected to the distance measurement signal input end for receiving the start signal, the end signal, and the distance measurement d, generating a reference signal of a frequency Fb, generating M phase shift signals based on the reference signal, characterized by a same frequency, and spaced apart from each other by a fixed phase, generating a clock mask starting from the start signal and ending at the end signal, counting a number Nd1 of second triggering states occurring to the phase shift signals during a time period from a point in time of commencement of the clock mask to occurrence of a first triggering state to the reference signal, counting a number Nb of cycles of the reference signal during the time period of the clock mask based on the first triggering state, counting a number Nd2 of second triggering states occurring to the phase shift signals during a time period from a point in time of termination of the clock mask to occurrence of a first triggering state to the reference signal, and outputting values d, Fb, M, Nb, Nd1, and Nd2; and a computing device connected to the speedometer for receiving the values d, Fb, M, Nb, Nd1, and Nd2, performing computation with the equation below to obtain a time t of the object, and performing computation of speed measurement of the object based on the time t and the distance measurement d, t=(Nb/Fb)+[Nd1/(Fb/M)]−[Nd2/(Fb/M)] where M≧2. 
         [0011]    In an embodiment, the speedometer comprises: a fundamental frequency generating unit for generating a fundamental frequency signal; a frequency multiplying unit connected to the fundamental frequency generating unit for turning the fundamental frequency signal into the reference signal by frequency multiplication; and a programmable gate array connected to the distance measurement signal input end for receiving the start signal, the end signal, and the distance measurement d, connected to the frequency multiplying unit for receiving the reference signal, and adapted to generate the values M, Nb, Nd1, and Nd2 and output the values d, Fb, M, Nb, Nd1, and Nd2. 
         [0012]    In an embodiment, the computing device is one of a control unit and a computer. 
         [0013]    In an embodiment, the first triggering state is one of an upper-edge triggering state and a lower-edge triggering state, and the second triggering state is one of an upper-edge triggering state and a lower-edge triggering state. 
         [0014]    In an embodiment, four or eight said phase shift signals are generated. 
         [0015]    In an embodiment, the frequency Fb of the reference signal can be directly replaced with a default value. 
         [0016]    Accordingly, a method and system for measuring speed of the present invention eliminate time measurement errors by quick and precise multiphase processing and multiply the accuracy of measurement in accordance with the quantity of generated phase shift signals, so as to reduce the area occupied by a circuit and reduce power consumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Objectives, features, and advantages of the present invention are hereunder illustrated with specific embodiments in conjunction with the accompanying drawings, in which: 
           [0018]      FIG. 1  is a timing diagram of a method for measuring speed according to an embodiment of the present invention; 
           [0019]      FIG. 2  is a flow chart of the method for measuring speed according to an embodiment of the present invention; and 
           [0020]      FIG. 3  is a function block diagram of a system for measuring speed according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The steps of a method for measuring speed of the present invention are described in specific embodiments thereof and are, unless otherwise specified, interchangeable in terms of sequence. Furthermore, the concept of “connection” used in the description of specific embodiments of a system for measuring speed according to the present invention is not limited to direction connection; instead, connection can also be effectuated by an intervening element. Also, a “first triggering state” and a “second triggering state” used in the description of the method and system for measuring speed of the present invention comprise one of an upper-edge triggering state and a lower-edge triggering state. The first triggering state and the second triggering state are not mutually exclusive; hence, both the first triggering state and the second triggering state may be upper-edge triggering states or lower-edge triggering states. 
         [0022]    In the embodiments of the present invention, a measured time is obtained by making reference to the course of distance measurement of an object, by multiphase processing, and by predetermined equations. 
         [0023]    Referring to  FIG. 1 , there is shown a timing diagram of a method for measuring speed according to an embodiment of the present invention. As shown in  FIG. 1 , this embodiment is exemplified by eight phase shift signals. Persons skilled in the art should be able to understand that, given at least two phase shift signals, the method and system for measuring speed of the present invention is effective in eliminating errors of time measurement and thereby enhancing the accuracy of the time measured. 
         [0024]    A method for measuring speed according to an embodiment of the present invention comprises the steps of: 
         [0025]    As shown in  FIG. 1 , in the course of the distance measurement of an object, a distance measurement unit generates a start signal SS and an end signal ES upon commencement of measurement and termination of measurement, respectively. In an embodiment of the present invention, time measurement starts with the step of providing a reference signal Fb and the step of generating multilevel phase shift signals Fb-p 1 ˜Fb-p 8  of the same frequency based on the reference signal, wherein the phase shift signals Fb-p 1 ˜Fb-p 8  are spaced apart from each other by a fixed phase. 
         [0026]    The reference signal Fb functions as a fundamental frequency. The phase shift signals are generated from the reference signal Fb. Normally, the phase shift of a signal is effectuated by a digital clock manager (DCM) of a programmable gate array (FPGA). In this embodiment, eight phase shift signals Fb-p 1 ˜Fb-p 8  are processed by two digital clock managers, and the reference signal Fb is decomposed by a digital clock manager to form four phase shift signals. However, persons skilled in the art should be able to understand that a user can still selectively shut down four of the phase shift decomposition processes even with just one digital clock manager. Hence, with only one digital clock manager, it is still possible to decompose the reference signal Fb into two or three phase shift signals. Hence, users can select the quantity of required phase shift signals as needed and as appropriate for operation of a digital clock manager. Regarding the spacing of phase shift signals, a digital clock manager divides 360° into equal phase portions and distributes the equal phase portions among the phase shift signals. For example, the phase equals 360°/(M−1), where M denotes the number of phase shift signals. 
         [0027]    Afterward, a clock mask mk is set. The clock mask mk thus set starts from the start signal SS of distance measuring sessions and ends at the end signal ES of the distance measuring sessions. Hence, the clock mask mk can be triggered synchronously with the start signal SS and the end signal ES, such that the time period of time measurement equals the time period of distance measurement. Regarding the upper-edge triggering signal SS and signal ES shown in  FIG. 1 , persons skilled in the art understand that the signal SS and the signal ES can be replaced by lower-edge triggering states for denoting the commencement and termination of the distance measuring sessions, respectively. 
         [0028]    Upon initialization of the clock mask mk, time measurement kicks off. Referring to  FIG. 1 , the reference signal Fb does not synchronize with the clock mask mk; hence, the time actually taken to effect the number Nb of cycles of the reference signal Fb measured does not fall within the range of the clock mask mk, thereby resulting in front-end errors and back-end errors. 
         [0029]    Hence, in an embodiment of the present invention, the front-end and back-end errors which occur in the course of time measurement are eliminated by means of the phase shift signals. 
         [0030]    Regarding the front-end errors, the number Nd1 of second triggering states (upper or lower triggering states) that occur to the phase shift signals Fb-p 1 ˜Fb-p 8  during the period from the point in time of commencement of the clock mask mk to the point in time when a first triggering state occurs to the reference signal Fb is counted. 
         [0031]    Regarding the back-end errors, the number Nd2 of second triggering states (upper or lower triggering states) that occur to the phase shift signals Fb-p 1 ˜Fb-p 8  during the period from the point in time of termination of the clock mask mk to the point in time when a first triggering state occurs to the reference signal Fb is counted. 
         [0032]    Counting the second triggering states that occur to the phase shift signals Fb-p 1 ˜Fb-p 8  means that elimination of back-end errors requires selecting the upper triggering state as the second triggering state when elimination of front-end errors requires selecting the upper triggering state as the second triggering state, or means that elimination of back-end errors requires selecting the lower triggering state as the second triggering state when elimination of front-end errors requires selecting the lower triggering state as the second triggering state. As shown in  FIG. 1 , the upper triggering state is selected to be the second triggering state, thereby setting Nd1 to 3 and Nd2 to 5. 
         [0033]    As shown in  FIG. 1 , the time actually taken to measure the distance traveled by an object is denoted by t as expressed by equation (1): 
         [0000]        t=tb+td 1 −td 2  (1)
 
         [0034]    Hence, in a subsequent calculation process, the number Nd1 and the number Nd2 are used in calculating front-end error time td1 and back-end error time td2 to therefore eliminate front-end and back-end errors. 
         [0035]    Nb denotes the number of cycles of the reference signal Fb measured within the time period of the clock mask mk. Fb denotes the frequency of the reference signal Fb. M denotes the number of the phase shift signals. The feature “being based on the first triggering state” means that the counting of the number of cycles of the reference signal Fb is in line with the point in time of termination of the front-end error time period td1. Hence, as shown in  FIG. 1 , the point in time of commencement of the counting of the number of cycles of the reference signal Fb starts from the upper-edge triggering state rather than the lower-edge triggering state. Conversely, if the time period td1 is changed to a time period starting from the point in time of commencement of the clock mask mk and ending at the point in time of occurrence of a lower-edge triggering state (first triggering state) to the reference signal Fb, the front-end error time period td1 will end at the lower-edge triggering state, and the point in time of commencement of the counting of the number of cycles of the reference signal Fb will start from the lower-edge triggering state. 
         [0036]    Hence, as indicated by the relationship between time, frequency, and number of cycles, clock mask time tb can be calculated by equation (2), 
         [0000]        tb =( Nb/Fb )  (2)
 
         [0037]    the front-end error time td1 is calculated by equation (3), 
         [0000]        td 1 =[Nd 1/( Fb/M )]  (3)
 
         [0038]    the back-end error time td2 is calculated by equation (4), 
         [0000]        td 2 =[Nd 2/( Fb/M )]  (4)
 
         [0039]    Accordingly, the time t is calculated by equation (5), 
         [0000]        t =( Nb/Fb )+[ Nd 1/( Fb/M )]−[ Nd 2/( Fb/M )]  (5)
 
         [0040]    wherein M denotes the number of the phase shift signals, with M≧2, indicating that there are at least two said phase shift signals. 
         [0041]    Furthermore, as indicated by equation (5), the more the phase shift signals are, the more the multiplication of enhancement of accuracy of measurement is. Hence, when compared with a conventional method which is not based on calibration of front-end errors and back-end errors, the method shown in  FIG. 1  and disclosed in an embodiment of the present invention increases accuracy eightfold. The more the phase shift signals are, the shorter the time period is, thereby eliminating increasingly small errors. 
         [0042]    Referring to  FIG. 2 , there is shown a flow chart of the method for measuring speed according to an embodiment of the present invention. Referring to  FIG. 1 , time measurement depends on distance measurement, except for provision of the reference signal Fb and the phase shift signals Fb-p 1 ˜Fb-p 8  thereof in advance. Time measurement comprises the steps of: providing the reference signal Fb, and a plurality of phase shift signals Fb-p 1 ˜Fb-p 8  (S 101 ); setting the clock mask mk based on signals SS and ES (S 102 ); obtaining number Nd1 of front-end errors (S 103 ); shutting down the clock mask mk and obtaining number Nb (S 104 ); obtaining number Nd2 of back-end errors (S 105 ); and performing computation by equation (5) (S 106 ). Distance measurement comprises the step of generating signals SS and ES and providing distance measurement d (S 201 ). Eventually, in step S 301 , computation of speed measurement is performed, using time t and distance measurement d to determine speed (d/t) and even determine acceleration [d(d)/d(t)]. 
         [0043]    Referring to  FIG. 3 , there is shown a function block diagram of a system for measuring speed according to an embodiment of the present invention. As shown in  FIG. 3 , a system  100  for measuring speed comprises a distance measurement unit  105 , a distance measurement signal input end  110 , a speedometer  120 , and a computing device  130 . 
         [0044]    The distance measurement unit  105  measures the distance traveled by an object, generates a distance measurement d based on the distance thus measured, and generates a start signal SS and an end signal ES based on distance measuring sessions. 
         [0045]    The distance measurement signal input end  110  receives the start signal SS, the end signal ES, and the distance measurement d. 
         [0046]    The speedometer  120  is connected to the distance measurement signal input end  110  for receiving the start signal SS, the end signal ES, and the distance measurement d. The speedometer  120  generates the reference signal Fb, M phase shift signals spaced apart from each other by a fixed phase, the clock mask mk starting from the start signal SS and ending at the end signal ES, the number Nd1 of second triggering states occurring to the phase shift signals during the time period of front-end errors, the number Nb of cycles of the reference signal Fb during the time period of the clock mask mk, and the number Nd2 of second triggering states occurring to the phase shift signals during a time period of back-end errors, and outputs values d, Fb, M, Nb, Nd1, and Nd2. 
         [0047]    In an embodiment, the speedometer  120  comprises a fundamental frequency generating unit  121 , a frequency multiplying unit  123 , and a programmable gate array  125 . The fundamental frequency generating unit  121  generates a fundamental frequency signal. Normally, a low fundamental frequency is generated by a crystal oscillator to cut costs, and then the fundamental frequency is boosted by the frequency multiplying unit  123  connected to the fundamental frequency generating unit  121  for functioning as the reference signal Fb. 
         [0048]    The programmable gate array  125  comprises a digital clock manager for functioning as a phase shift generating circuit, a differential circuit for performing upper or lower differentiation (upper-edge triggering or lower-edge triggering) to count Nd1 and Nd2, and a mask circuit for generating the clock mask mk and counting the reference signal Fb. Accordingly, the programmable gate array  125  generates the values M, Nb, Nd1, and Nd2 and outputs the count values d, Fb, M, Nb, Nd1, and Nd2. 
         [0049]    The programmable gate array  125  is a conventional element. The system for measuring speed according to an embodiment of the present invention achieves the objectives of the present invention by means of logical elements of the system for measuring speed. The system for measuring speed according to an embodiment of the present invention reduces the required number of the logical elements, dispenses with a large-sized programmable gate array chip, and thus reduces the circuit-occupied area and downsizes the product. For example, if the computing function of a computing device is also incorporated into the programmable gate array, the required number of the logical elements will be greatly increased, thereby increasing the circuit-occupied area. Due to its structural design, it is necessary for the programmable gate array to perform computation by logic, and thus the computation is rapid; however, the required logical elements are bulky. Although a special programmable gate array having a computation structure circuit disposed therein has low logical element spatial requirements and can perform high-speed computation, it incurs an excessively high cost. 
         [0050]    The computing device  130  is connected to the speedometer  120  for receiving the values and performing computation by equation (5) to obtain the measured time t, and performing computation of speed measurement by the distance measurement d. The computing device  130  is a control unit (MCU) or a computer. If the computing device  130  is a control unit, then the control unit is usually disposed on the same circuit board as the speedometer  120  is, such that the speed measuring system  100  in its entirety is integrated onto a module; however, the computing device  130  can also be an external computer device for processing a computation procedure in whole with data provided by a measuring module. 
         [0051]    To reduce errors further, it is feasible to perform a high-precision measurement process on the generated reference signal Fb beforehand. To preclude any error which might otherwise be produced as a result of a discrepancy between a frequency actually generated by a fundamental frequency generating unit and a frequency multiplier and a given frequency level, it is feasible to measure the reference signal Fb in advance by means of a high-precision frequency counter having a higher resolution than the frequency of the reference signal Fb, and then use the measured reference signal Fb as a default value to be directly stored in the computing device  130 . In doing so, in every instance of measurement, the default value always applies to the frequency of the reference signal Fb, thereby dispensing with the need to use a parameter set forth in the specifications of a fundamental frequency generating unit and a frequency multiplier. 
         [0052]    In conclusion, a method and system for measuring speed of the present invention eliminate time measurement errors by quick and precise multiphase processing and multiply the accuracy of measurement in accordance with the quantity of generated phase shift signals. An embodiment of the present invention achieves eightfold reduction (corresponding to eight phase shift signals) in errors, effectuates high-precision measurement of speed, and reduces the area occupied by a circuit. 
         [0053]    The present invention is disclosed above by preferred embodiments. However, persons skilled in the art should understand that the preferred embodiments are illustrative of the present invention only, but should not be interpreted as restrictive of the scope of the present invention. Hence, all equivalent modifications and replacements made to the aforesaid embodiments should fall within the scope of the present invention. Accordingly, the legal protection for the present invention should be defined by the appended claims.