Abstract:
A method for measuring a capacitance using a capacitance meter. The capacitance meter includes an AC power source with a controllable frequency which is fed to a capacitor to measure its capacitance. A first measurement of the capacitance is performed by the capacitance meter using a first frequency. When the first measurement of the capacitance indicates the capacitance is below a threshold capacitance a lower capacitance measurement is performed in the capacitance meter, using a second measurement of the capacitance using a second frequency. When the first measurement of the capacitance indicates the capacitance is above a threshold capacitance, a higher capacitance measurement is performed in the capacitance meter, using a second measurement of the capacitance using a third frequency, the third frequency being lower than the second frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European patent application 09170649.9 filed 18 Sep. 2009. 
     BACKGROUND 
     Since the advent of capacitors, there has been a need to measure the main magnitude of capacitors, i.e. the capacitance. 
     One known way to measure the capacitance of a capacitor is to connect an alternating current to the capacitor and observe the voltage. It is also known to use two frequencies to increase the accuracy of the measurement. 
     However, there is always a need to further improve the accuracy of capacitance measurements. 
     SUMMARY 
     The present invention overcomes or at least reduces the problems of the prior art. 
     It is presented a method for measuring a capacitance using a capacitance meter, the capacitance meter comprising an AC power source with a controllable frequency which is fed to a capacitor to measure its capacitance. The method comprises the steps of: performing, in the capacitance meter, a first measurement of the capacitance using a first frequency; when the first measurement of the capacitance indicates the capacitance being below a threshold capacitance, performing a lower capacitance measurement in the capacitance meter, using a second measurement of the capacitance using a second frequency; and when the first measurement of the capacitance indicates the capacitance being above a threshold capacitance, performing a higher capacitance measurement in the capacitance meter, using a second measurement of the capacitance using a third frequency, the third frequency being lower than the second frequency. 
     This method will result in more accurate measurements due to the following effects. Higher frequencies provide a higher current or measurement signal at low capacitances. Furthermore, lower frequencies reduce the current and voltage drop in the cable for higher capacitances. Consequently, it is beneficial to use higher frequencies for lower capacitances and lower frequencies for higher capacitances. 
     The performing the lower capacitance measurement may involve using the second frequency and a fourth frequency, wherein the second and fourth frequencies differ, and the performing the higher capacitance measurement may involve using the third frequency and a fifth frequency, wherein the third and fifth frequencies differ. Using two frequencies improve the accuracy of the measurement even further. Furthermore, using two frequencies makes it possible to calculate a parasitic resistance, e.g. in cables, whereby this can be compensated for in calculations. 
     The fourth frequency and the fifth frequency may be equal. This simplifies the construction of the capacitance meter by reducing the number of frequencies that are required to be generated. 
     The first frequency, the fourth frequency and the fifth frequency may all be equal. This simplifies the construction of the capacitance meter even further by reducing the number of frequencies that are required to be generated. 
     The first frequency may be higher than the third frequency and the first frequency may be lower than the second frequency. In other words, the first frequency is the middle frequency. 
     The second measurement being performed may be more accurate than the first measurement. This is due to the active selection of frequencies depending on the first measurement of the capacitance. 
     The method may further comprise the step of presenting a measured capacitance as the capacitance measured in the performed second measurement. 
     A second aspect of the invention is a capacitance meter for measuring a capacitance. The capacitance meter comprises: an AC power source with a controllable frequency which is fed to the capacitor to measure its capacitance, and a controller. The controller is arranged to: perform a first measurement of the capacitance using a first frequency; when the first measurement of the capacitance indicates the capacitance being below a threshold capacitance, perform a lower capacitance measurement in the capacitance meter, using a second measurement of the capacitance using a second frequency; and when the first measurement of the capacitance indicates the capacitance being above a threshold capacitance, perform a higher capacitance measurement in the capacitance meter, using a second measurement of the capacitance using a third frequency, the third frequency being lower than the second frequency. 
     A third aspect of the invention is a computer program for measuring capacitance, the computer program comprising computer program code which, when run on in a capacitance meter, causes the capacitance meter to perform the method according to the first aspect. 
     A fourth aspect of the invention is a computer program product comprising a computer program according to the third aspect and a computer readable means on which the computer program is stored. 
     Whenever the term equals is used herein, it is to be interpreted to be the same within a margin of error. The margin of error is set appropriately, such as to a few percent or similar. 
     Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention is now described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram showing a capacitance meter according to an embodiment of the present invention, where the focus is on elements used to measure a capacitance of a component to be tested, 
         FIG. 2  is a schematic diagram showing the capacitance meter of  FIG. 1 , where the focus is on interfaces of the capacitance meter, 
         FIG. 3  is a graph illustrating the use of frequencies in capacitance measurements in the capacitance meter of  FIGS. 1 and 2 , 
         FIG. 4  is a flow chart showing a method which can be executed in the capacitance meter of  FIGS. 1 and 2 , and 
         FIG. 5  shows one example of a computer program product comprising computer readable means. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description. 
       FIG. 1  is a schematic diagram showing a capacitance meter  1  according to an embodiment of the present invention, where the focus is on elements used to measure a capacitance of a capacitor  3 . It is to be noted that while the capacitive effects of the capacitor  3  are dominant, there may be resistive and inductive components to the capacitor  3  and/or cabling connecting the capacitor  3  and the capacitance meter  1 . It will be shown below with reference to  FIG. 4  how the effect of resistive components can be reduced or even eliminated. The capacitor  3  can for example be a capacitor in a capacitor bank. 
     The capacitance meter  1  connects to the capacitor  3  by means of connectors  9   a - b . The capacitance meter  1  comprises an AC voltage source  2  which generates a measurement voltage. The AC voltage source  2  typically comprises resistors and other components (not shown) as well as the actual power source. The AC source can comprise a battery and an inverter to generate the AC power. The capacitance meter  1  is adapted to measure capacitances from 1 to 1000 μF. The AC voltage source can for example be controlled using pulse width modulation from a microprocessor (e.g. controller  8 ). The AC voltage source further comprises an amplifier and a low pass filter, e.g. an inductor. While the AC source can have any suitable output voltage, it can be restricted to 1-1.4 volts in order to stay below the trigger voltage of any connected diodes. 
     The AC measurement voltage will result in a current going to and from the capacitor  3 . A voltage sensor  5 , such as a voltmeter, measures the voltage across the capacitor  3  and a current sensor  6 , such as a ammeter, measures the current going to and from the capacitor  3 . 
     Optionally, there are other capacitors  7   a - b  connected in parallel to the capacitor  3  to be measured. In this situation, a second current sensor  4  is used to measure the current going to or from the capacitor  3  to be measured. The second current sensor  4  is also connected to the controller  8  for calculation of the capacitance of the capacitor  3  to be measured. 
     A controller  8  oversees the whole measurement process and calculates a measured capacitance using the measured voltage and current. The controller can be a CPU (Central Processing Unit), a FPGA (Field-programmable Gate Array), a DSP (Digital Signal Processor) or any suitable programmable electronic logic unit. 
     In order to reduce the risk of singularities affecting the measurement, the capacitance measurement can be performed three times, whereby the median value is used as the measured capacitance. Optionally, the measurement is performed at two different frequencies, where three measurements are taken at the two frequencies. The measurement used can then be taken as the average of the two median values of the six measurements. 
       FIG. 2  is a schematic diagram showing the capacitance meter of  FIG. 1 , where the focus is on interfaces of the capacitance meter. The capacitance meter  1  comprises a display  11  and a keypad  10 . This allows a user to interface with the capacitance meter  1 , e.g. to measure capacitance of a specific capacitor  3  and store the measured capacitance along with an identity of the capacitor, allowing history to be kept. 
     An data interface  12  allows the capacitance meter  1  to send and/or receive data with a computer  13 , such as a general purpose stationary or portable computer. The data interface  12  can for example be an interface of type USB (Universal Serial Bus), a Centronics parallel interface, an RS-232 serial interface, or an Ethernet interface. The data interface  12  can also be a wireless interface such as Bluetooth, wireless LAN or wireless USB interface. For example, the data interface  12  can be used to allow central collection of capacitance measurements. 
       FIG. 3  is a graph illustrating the use of frequencies in capacitance measurements in the capacitance meter of  FIGS. 1 and 2 . The idea is to take a first rough measurement at a middle frequency  20  of the capacitor. If the rough measurement indicates that the capacitor is below a threshold capacitance, a second measurement  24  is performed using a higher frequency  21  and a middle frequency  20 ′, which may be equal to the middle frequency  20  used for the first rough measurement. If the rough measurement indicates that the capacitor is above a threshold capacitance, a second measurement  25  is performed using a lower frequency  22  and the middle frequency  20 ′, which may be equal to the middle frequency  20  used for the first rough measurement. The threshold capacitance can be selected as any suitable capacitance; in this embodiment, the threshold capacitance is selected as 200 μF. 
     The reason for this is to compensate for the following effects. Higher frequencies provide a higher current or measurement signal at low capacitances. Furthermore, lower frequencies reduce the current and voltage drop in the cable for higher capacitances. Consequently, it is beneficial to use higher frequencies for lower capacitances and lower frequencies for higher capacitances. The frequencies can be selected as any suitable frequencies. In this embodiment, the lower frequency  22  is selected to be 40 Hz, the middle frequency  20 ,  20 ′ is selected to be 80 Hz and the higher frequency  21  is selected to be 160 Hz. 
       FIG. 4  is a flow chart showing a method which can be executed in the capacitance meter  1  of  FIGS. 1 and 2 . The method starts when a measurement is triggered, e.g. by a user of the capacitance meter  1  activating capacitance measurement of a capacitor. 
     In an initial perform first measurement step  30 , a first rough measurement of the capacitance is performed. 
     In a subsequent conditional compare C (capacitance) to threshold step, the rough measurement of the capacitance from the previous step is compared to a threshold capacitance. If the first rough measurement of the capacitance is below the threshold, the method continues to a perform lower C (capacitance) measurement step  34 . If the first rough measurement is above the threshold, the method continues to a perform higher C (capacitance) measurement step  36 . It is not important what happens if the first rough measurement of the capacitance is exactly on the threshold capacitance. This would be a very rare singularity and the method could be configured to in this case either go to the perform lower C (capacitance) measurement step  34  or the perform higher C (capacitance) measurement step  36 . 
     In the perform lower C (capacitance) measurement step, a second, more accurate, measurement of the capacitance is performed. This is done using a higher frequency. Optionally, two frequencies are used, such as a higher frequency and the frequency used in the first rough measurement in step  30 . 
     In the perform higher C (capacitance) measurement step, a second, more accurate, measurement of the capacitance is performed. This is done using a lower frequency. Optionally, two frequencies are used, such as a lower frequency and the frequency used in the first rough measurement in step  30 . 
     After the second measurement has been performed in either of steps  34  and  36  above, the measured capacitance is presented in a present measured capacitance step  38 . In this step, the second, more accurate, measurement of the capacitance is presented, as obtained from either of the prior steps  34  or  36 . The measurement can for example be presented in the display of the capacitance meter or sent over the data interface. 
       FIG. 5  shows one example of a computer program product comprising computer readable means  50 . On this computer readable means  50  a computer program can be stored, which computer program can cause a controller to execute the method according to embodiments described herein. In this example, the computer program product is an optical disc, such as a CD (compact disc), a DVD (digital versatile disc) or a blu-ray disc. The computer readable means can also be solid state memory, such as flash memory or a software package distributed over a network, such as the Internet. 
     The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention.