Abstract:
A method assesses the rotational speed of a machine, and more particularly the rotational speed of a rotating equipment prime mover controlled by a governor. Such machines include turbo machinery and relate to a measurement device for measuring speed. The method measures a number of pulses during a measurement interval, determines a portion of a pulse pattern, determines an integration period, and calculates the rotational speed based on the portion of the pulse pattern.

Description:
[0001]    The present invention relates to a measurement device and method for assessing the rotational speed of a machine, and more particular to assessing the rotational speed of a rotating equipment prime mover controlled by a governor. 
       BACKGROUND 
       [0002]    Rotating equipment prime movers are devices that may convert one form of available energy into a particular source of continuous rotational power or motion. The rotational power produced by the prime mover is then put to use by driving another energy conversion device. Examples of rotating equipment prime movers include turbines, such as for wind, gas, steam, or water, reciprocating engines, such as for gasoline, diesel, or natural gas, and electric motors, both ac and dc, that are used to turn drive trains, generators, pumps, or compressors. 
         [0003]    Such rotating equipment prime movers are usually equipped with a governor, a rotational velocity i.e. speed limiting and control device, to manipulate the rotational speed and preferably keep the output of the prime mover at a constant level. The governor is provided with feedback from the prime mover, which may include energy output, rotational speed, angular torque or other parameters. 
         [0004]    One particular way of measuring the rotational speed of the prime mover includes a reference gear having a specific number of teeth and a sensor that detects the teeth passing by and delivers a pulse signal for each detection. The rotational speed may then be determined in terms of frequency by dividing the number of pulses by the measurement interval, the time interval during which a number of pulses is detected: 
         [0000]    
       
         
           
             
               F 
                
               
                   
               
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               Hz 
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             = 
             
               
                 X 
                 pulses 
               
               
                 T 
                 interval 
               
             
           
         
       
     
         [0005]    The accuracy of this approach mainly depends on the accuracy of determining the time duration of the measurement interval. The length of the measurement interval influences the accuracy of the measurement, but also the delay time of feedback to the governor. A longer interval increases accuracy, but increases the delay time for feedback. For an engine running at low speed a long measurement interval is required to get a valid measurement, whereas for engines running at high speed a short delay time for feedback is required to control the engine at a constant speed. 
         [0006]    A further problem is that for certain engines, such as reciprocating engines, the angular velocity is not constant, e.g. with a stroke of the engine the angular velocity will first accelerate and decelerate thereafter. 
         [0007]    Some prior art solutions address this by adapting the manner of calculation to the speed of the engine. For example, by introducing a threshold to select between calculating the speed based on one pulse-to-pulse delay or calculating the speed by averaging over multiple pulse-to-pulse delays. Though this allows to increase accuracy at high speed, it may show variations between consecutive calculations due to the variations in angular velocity. Other prior art solutions therefore measure the speed with an increased measurement interval, despite that this delays the feedback to the governor. 
       SUMMARY OF INVENTION 
       [0008]    It is an object of the invention to overcome the disadvantages of the prior art. This is achieved by providing a method that measures a number of pulses during a measurement interval, determines a portion of a pulse pattern, determines an integration period, and calculates the rotational speed based on the portion of the pulse pattern. 
         [0009]    According to another aspect, there is provided a measurement device carrying out the above method. 
         [0010]    Determining which portion of a rotational movement pattern of a rotation of a reference gear is represented by a measurement, allows to compensate for variations in angular velocity of the engine. As prior art solutions measure the rotational speed based only on measurements representing a fraction of the rotational movement of the reference gear, consecutive measurement samples may show variations that reflect the variation in angular velocity but not the speed of the engine when actually running at a constant speed of rotations per minute (RPM). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    By way of example only, the embodiments of the present disclosure will be described with reference to the accompanying drawing, wherein: 
           [0012]      FIG. 1  illustrates schematically an example of an engine with a speed governor; 
           [0013]      FIG. 2  is a flow diagram illustrating an example of a method in accordance with the invention; 
           [0014]      FIG. 3  is a flowchart illustrating a further example of a method in accordance with the invention; and 
           [0015]      FIG. 4  is a block diagram schematically illustrating an example of a measurement device in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Shown in  FIG. 1 , is an example of an engine  1  with a prime mover  2  driving a piece of rotating equipment  3  via a drive axis  4 . The engine  1  further has a reference gear  5 , a sensor  6  and a governor  7 . The reference gear  5  is equipped with teeth and mounted on the drive axis  4 . The sensor  6  is arranged for detecting teeth of the rotating reference gear  5  passing by and delivering a pulse to the governor  7  for each instance of detection. The governor  7  is in turn connected to a valve  8 . The valve  8  is arranged for controlling the supply of an energy source  9  to the prime mover  2 . In this example, the prime mover  2  may be a diesel engine to be supplied with diesel as energy source. The supply of diesel  9  will run the prime mover  2  which in turn will drive the rotary equipment  3  via the drive axis  4 . The reference gear  5  will rotate in the same direction, as indicated by arrow  12  and at the same speed as the prime mover  2  and allows to feedback the rotational speed to the governor  7 . 
         [0017]    The output  10  delivered by the rotating equipment  3  may be measured by a meter  11  to be fed back to the governor  7  and taken also in account for controlling the energy supply  9  via the valve  8 . 
         [0018]    The governor  7  is provided with a measurement device  13  configured for performing a method for assessing a rotational speed of an engine. Turning to  FIG. 2 , an example of such method for assessing the rotational speed of the engine  2  of  FIG. 1  is shown. The method requires measuring  101  a number of pulses X during a measurement interval, determining  102  a portion of a pulse pattern, determining  103  an integration period, and calculating  104  the rotational speed based on the portion of the pulse pattern. 
         [0019]    The sensor  6  detects the teeth of the gear passing by and provides an input signal  14  consisting of pulses for each detection. The pulses delivered by the sensor  6  may be measured  101  by a pulse counter which counts the number of pulses. A time counter may measure the time delay between consecutive pulses. And the pulse index of each pulse is also monitored. 
         [0020]    When an engine starts running the measurements may start immediately. As the reference gear  5  has a fixed number of teeth which is known to the governor  7 , each incoming pulse can be assigned a pulse index number, running from one to the number of teeth M. On completing one full rotation of the reference gear  5 , the next pulse corresponds to the first pulse index again. This allows to acquire a pulse pattern of the reference gear  5  by storing the time proportional delay for each pulse index. Hence, for pulse index number two, the time proportional delay between the first pulse and the second pulse is stored. In this embodiment the acquired pulse pattern is stored in a pattern type table, which initial measurement may in general be referred to as a learning phase. In other embodiments the pattern type table may be provided in advance by the manufacturer of the engine. A full rotation of the reference gear  5  may take several measurement intervals i.e. sampling periods of the pulse counter. Depending on the structure of engine, e.g. the number of cylinders, a pulse pattern may correspond to one revolution or several revolutions of the reference gear  5 . The pattern table will thus contain time delays for pulse indices for N pulses, with N equal to the number of teeth M on the reference gear, or a multiple thereof, like 2*M, 3*M, or 4*M. In order to determine  102  the portion of the pulse pattern the incoming measured pulses are compared against the stored pulses of the pattern type table. In one embodiment, this may be done by comparing the pulse indices and selecting the portion that matches the index of the measured pulses. In another embodiment, this may be done by comparing the consecutive time delays against the proportional time delays of the pattern type table and then selecting the portion matching that matches the measured pulses. 
         [0021]    Selection of the portion based on the indices of the pulses requires the pulse counter to be synchronised with the pulse pattern table. Which is readily obtained when the learning phase has preceded the phase in which the measurements are used by the governor. If the pulse pattern table is supplied by the manufacturer, synchronization of the pulse counter with the start index of the pulse pattern table may be required. 
         [0022]    Selection of the portion based on comparison of consecutive pulses does not require synchronization of the pulse counter and the pulse pattern table, but instead results in the portion being selected by identifying the consecutive pulse indices of which the proportional time delays match the measured pulses. 
         [0023]    Either way, based on the pulses X and time delays of the corresponding pulse indices a corrective value Portion is calculated as the ratio of N pulses of the full pattern divided by the sum of the total time delay of the full pattern and the received pulses X divided by the sum of the time delays of the corresponding pulse indices. As shown in the Formula below: 
         [0000]    
       
         
           
             Portion 
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                     N 
                     Pulses 
                   
                   
                     T 
                     
                       N 
                       pulses 
                     
                   
                 
                 
                   
                     X 
                     Pulses 
                   
                   
                     T 
                     
                       X 
                       pulses 
                     
                   
                 
               
               = 
               
                 
                   
                     N 
                     pulses 
                   
                   
                     X 
                     pulses 
                   
                 
                  
                 
                   
                     T 
                     
                       X 
                       pulses 
                     
                   
                   
                     T 
                     
                       N 
                       pulses 
                     
                   
                 
               
             
           
         
       
     
         [0024]    In other words, the corrective value Portion is the ratio of the average speed of pulses N of a full pattern and the relative speed of pulses X. 
         [0025]    The integration period may be determined  103  from the sampling period, i.e. the period during which pulses were measured, and the time delay from the last measured pulse till the end of the sampling period. This method is known from European Patent EP 2172785 B1, granted to Schneider Electric on Mar. 16, 2016. The method disclosed therein allows to determine more accurately the period of the measurement interval during which pulses were received. In particular, it deducts the time from the last detected pulse till the end of the sampling period and adds the corresponding time interval of the previous sampling period. 
         [0026]    With the number of pulses measured in the measurement interval, the portion of the pulse pattern and the integration period available, the rotational speed of the engine may be calculated  104  as a frequency based on the portion of the pulse pattern. By taking the formula: 
         [0000]    
       
         
           
             
               F 
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               [ 
               Hz 
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             = 
             
               
                 
                   X 
                   pulses 
                 
                 * 
                 Portion 
               
               
                 T 
                 integration 
               
             
           
         
       
     
         [0027]    Herein, X pulses  is the number of pulses X measured. T integration  is the time of the measurement interval. And Portion is the corrective value as described above. 
         [0028]    Turning to  FIG. 3 , a flowchart is shown illustrating an example of the method in more detail. The method includes measuring  201  a number of pulses X during a measurement interval, determining  202  a portion of a pulse pattern, determining  203  an integration period, and calculating  204  the rotational speed based on the portion of the pulse pattern. And the method further includes providing a pattern type table  205 , triggering  206  a measurement interval through a clock signal from a time base generator, and verifying  207  the size of the integration period. 
         [0029]    As described above, a pattern type table may be provided  205  in advance by the manufacturer of the engine. Or it may be provided by storing the measurements obtained when the engine is started, i.e. during the learning phase. The learning phase is preferably performed when the engine is running at normal operational speed. Preferably, the load of the engine is kept stable during the learning phase, so the engine may be running more or less at constant speed to allow capturing a pulse pattern representative of the engine. The learning phase will only take a few revolutions to capture and store the pulse pattern. 
         [0030]    A time base generator may be used to provide a clock signal for triggering  206  the measurement interval. This time base determines the length of the measurement interval, which due to its&#39; periodic nature may also be referred to as a sampling period. 
         [0031]    When the engine is running at low frequencies, e.g. less than 60 revolutions per minute, it could be that no pulse is delivered by the sensor in the measurement interval as the reference gear is rotating equally slow. It may also be that the integration period does not meet the expected accuracy, which may e.g. happen when the integration period becomes less than half of the sampling period. Or, for example, when the frequency of the engine lies between two and three times the sampling rate, the integration period may fluctuate between half and less than once the sampling rate. The higher the ratio between frequency and sampling rate, the closer the integration period will be in size to the sampling period. Which optimises the potentially achievable accuracy. 
         [0032]    Hence, to guarantee the accuracy of the calculation, the size of the integration period needs to be verified  207 . And the integration period needs to be extended  208  if no pulse was measured or if the integration period is less than a predetermined threshold. For example, to obtain an accuracy of 0.005% of error or more, the integration period must be greater than 20000 times the system clock period. 
         [0033]    Referring to  FIG. 4 , a measurement device  20  for measuring a rotational speed of an engine is shown. The measurement device includes an input port  21  for an input signal. The measurement device further including: A system clock  22  providing a system clock signal. A time base generator  23  determining a sampling period derived from the system clock signal. A counter clock  24  providing a counter clock signal derived from the system clock signal. An input pulse counter  25  counting pulses of the input signal during a sampling period and keeping track of an index of the input pulses. A time counter  26  counting pulses of the counter clock  24  between two consecutive pulses of the input signal. And a latching memory  27  for capturing and storing the index of a value of the input pulse counter and a value of the time counter at an end of the sampling period of both a current sample period and a previous sample period. 
         [0034]    The latching memory  27  may comprise two pairs of latching registries  28  and  29 . Each pair  28 ,  29  consists of two latching registries  28   a ,  28   b  &amp;  29   a ,  29   b  for storing the values for the pulse index and the pulse delay. One pair  29  will store the values of the previous measurement interval, which may be referred to as “sample begin”. The other pair  28  will store the values of the current measurement interval, which may be referred to as “sample end”. 
         [0035]    In the embodiment of  FIG. 4  the time base generator  23  and the counter clock  24  derive their signal frequency from the system clock  22 . The signal frequency of the time base generator  23  determines when measurements are updated. The signal frequency of the counter clock  24  relative to the signal frequency of the time base generator  23  determines the accuracy of the measurement. For example, for an accuracy of 0.005% the frequency of the counter clock  24  must be 20000 times higher than the frequency of the time base generator  23 . 
         [0036]    The measurement device  20  may further include a pattern type table  30 . The pattern type table  30  holding at least one pulse pattern of a series of pulse indices and a proportional delay for each of the pulse indices. The measurement device may further include a portion calculator  31  for comparing the stored pulse indices of the latching memory  27  against the pulse pattern table and calculating the portion of the pulse pattern from the proportional delays associated with the stored pulse indices of the latching memory  27 . 
         [0037]    The measurement device  20  may further include a processing unit  32 . The processing unit  32  being arranged for calculating the number of pulses by subtracting the pulse index stored of the previous sample period from the pulse index stored of the current sample period. Further arranged for calculating an integration period from the sampling period, the proportional delay of the pulse index stored for the previous sample period and the proportional delay of the pulse index stored for the current sample period. And arranged for calculating the frequency of the rotational speed from the number of pulses, the portion of the pulse pattern and the integration period. 
         [0038]    In another embodiment, the processing unit  32  of the measurement device  20  may also be arranged for performing the function of the portion calculator  31 . Hence, no separate hardware for portion calculator  31  is present. 
         [0039]    Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims. 
         [0040]    Furthermore, although exemplary embodiments have been described above in some exemplary combination of components and/or functions, it should be appreciated that, alternative embodiments may be provided by different combinations of members and/or functions without departing from the scope of the present disclosure. In addition, it is specifically contemplated that a particular feature described, either individually or as part of an embodiment, can be combined with other individually described features, or parts of other embodiments.