Abstract:
There is provided a state detecting method adopted to an insulation resistance detector including the steps of: calculating a difference between the output of the filter when a pulse signal having a first pulse width is applied to the series circuit, and the output of the filter when a pulse signal having a second pulse width shorter than the first pulse width is applied to the series circuit; and detecting the state of the insulation resistance detector based on the calculated difference.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on Japanese Patent Application No. 2005-355691, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a state detecting method and an insulation resistance detector, in particular to a state detecting method for detecting a degradation of the insulation resistance between a ground and a direct current power supply, and a detector using the same. 
   2. Description of the Related Art 
   Conventionally, an insulation resistance detector using the above described state detecting method is known, for example, Japanese Patent Application Document No. 2005-114496. As shown in  FIG. 1 , the insulation resistance detector  50  includes a detecting resistor Rd connected in series to an insulating resistor Ri between a battery as the direct current power supply and a vehicle body, and a coupling capacitor mounted in between the insulating resistor Ri and the detecting resistor Rd for cutting off the direct current. The insulation resistance detector  50  further includes a pulse oscillation circuit  51  (pulse signal supplying member) for supplying a rectangular wave pulse signal P 1  having a specific peak value with a series circuit consisting of the insulating resistor Ri, the coupling capacitor Co, and the detecting resistor Rd. Here, the peak value means the highest voltage in a pulse signal. 
   The pulse oscillation circuit  51 , for example, includes a constant amplitude pulse generating circuit. When a control circuit  52  inputs a frequency signal S 1  for the rectangular pulse signal P 1  into the constant amplitude pulse generating circuit, a frequency of the rectangular wave pulse outputted from the constant amplitude pulse generating circuit is changed. 
   Further, a connection voltage Vx between the coupling capacitor Co-detecting resistor Rd is expressed by a formula (1) in which the peak voltage of the rectangular pulse signal P 1  is divided by the detecting resistor Rd and the insulating resistor Ri.
 
 Vx=Vp*Ri /( Rd+Ri )  (1)
 
   Where, Vp is a peak voltage of the rectangular pulse signal P 1 . 
   Accordingly, when the insulating resistor Ri is larger than the detecting resistor Rd as normal, the connection voltage Vx is nearly the same peak voltage as the rectangular pulse signal P 1 . On the other hand, when the insulating resistor Ri is reduced and the insulating resistor Ri is smaller than the detecting resistor Rd, the connection voltage Vx is reduced. 
   The insulation resistance detector  50  further includes a low pass filter  53  for outputting the connection voltage Vx after eliminating signals more than specific frequency. This low pass filter  53  is composed of a resistor Rf and a capacitor Cf and aimed for eliminating high frequency noises superimposed on the connection voltage Vx. An output of the low pass filter  53  is shaped at the waveform shaping circuit  54 , then supplied to a control circuit  52 . This control circuit  52  is composed of such as a microcomputer. 
   Next, a detecting principle of insulation resistance detector will be explained with reference to  FIG. 5 . In  FIG. 5 , L 1  is a graph of frequency of the rectangular pulse signal P 1  versus pulse peak value outputted from the low pass filter  53  indicating a normal state where the insulating resistor Ri is not reduced, and the insulation resistance detector  50  is not malfunctioning. 
   As shown in  FIG. 5 , in normal, when the frequency supplied from the pulse oscillation circuit  51  is less than 2.5 Hz, the output peak voltage of the low pass filter  53  is substantially equal to the peak voltage of the rectangular pulse signal P 1  outputted from the pulse oscillation circuit  51 . When the frequency supplied from the pulse oscillation circuit  51  is more than 2.5 Hz, as the frequency increases, the output peak voltage of the low pass filter  53  decreases. 
   This is because normally, when a time constant of the low pass filter  53  is large, when the frequency of the rectangular pulse signal P 1  increases over the 2.5 Hz, the time between the rising time of the rectangular pulse signal P 1  and the time for the output of the low pass filter  53  to reach 5 V as the peak value of the rectangular pulse signal P 1  is shorter than the pulse width of the rectangular pulse signal P 1 . Namely, when the pulse width decreases corresponding to the increase of the frequency, before the output of the low pass filter  53  reaches 5 V as the peak voltage of the rectangular pulse signal P 1 , the supply of the rectangular pulse signal P 1  is cut off, and the peak value is less than 5 V. When supplying the higher frequency of the rectangular pulse signal P 1 , the rectangular pulse signal P 1  having the shorter pulse width is supplied, and the peak value of the low pass filter  53  is further reduced. 
   L 2  in  FIG. 5  shows a graph of the frequency of the rectangular pulse signal P 1  versus the output peak voltage of the low pass filter  53 , when the insulation resistance detector  50  is malfunctioning, for example, coupling capacitor Co or capacitor Cf is open. 
   As shown in  FIG. 5 , at the malfunction, even when the frequency increases, namely, when reducing the pulse width, the output peak voltage of the low pass filter  53  is substantially constant. This is because when the coupling capacitor or the capacitor Cf is open, the time constant of the low pass filter  53  decreases, and the rising time of the output of the low pass filter  53  is shorter than the normal state. 
   According to the above, when the coupling capacitor Co is open, namely, the connection between the insulation resistance detector  50  and the insulating resistor Ri is broken, the output peak voltage of the low pass filter  53  is constantly equal to the peak voltage of the rectangular pulse signal P 1 . Therefore, even when the insulating resistor Ri is reduced, the output of the low pass filter  53  is not reduced, and the reduction of the insulating resistor Ri cannot be detected. 
   Further, when the capacitor Cf of the low pass filter  53  is open, the noise elimination at the low pass filter  53  cannot be achieved, and the noise superimposed signal is inputted into the control circuit  52 . In this case also, the reduction of the insulating resistor Ri cannot be detected correctly. 
   L 3  in  FIG. 5  shows a graph of the frequency of the rectangular pulse signal P 1  versus the output peak voltage of the low pass filter  53 , when the insulation resistance detector  50  is malfunctioning, for example, coupling capacitor Co or capacitor Cf is short. 
   As described above, when the coupling capacitor Co or capacitor Cf is short, then even applying the rectangular pulse signal P 1 , the output of the low pass filter  53  does not rise. Therefore, even when the frequency increases, namely, the pulse width decreases, the output voltage is constantly about 0.2 V. 
   As described above, when the coupling capacitor Co or capacitor Cf is short, the output of the low pass filter  53  is constantly low. Therefore, even when the insulating resistor Ri is not reduced, the output of the low pass filter  53  is reduced so that the reduction of the insulating resistor Ri cannot be detected. 
   Therefore, in a conventional detecting method, as shown in  FIG. 5 , when applying the rectangular pulse signal P 1  of 2.5 Hz, and the output peak value of the low pass filter  53  is more than a threshold value X 1 , and when applying the rectangular pulse signal P 1  of 5.5 Hz, and the output peak value of the low pass filter  53  is less than a threshold value X 2 , the insulation resistance detector  50  is judged as a normal state. 
   On the other hand, when the rectangular pulse signal P 1  of 2.5 Hz and 5.5 Hz are applied, and the outputs of the low pass filter  53  are more than the threshold voltage X 1 , the insulation resistance detector  50  is judged as an open state. 
   Further, when the rectangular pulse signal P 1  of 2.5 Hz and 5.5 Hz are applied, and the outputs of the low pass filter  53  are less than the threshold voltage X 3 , the insulation resistance detector  50  is judged as a short state. 
   Further, in Japanese Patent Application Document 2005-114496, the pulse width of the rectangular pulse signal P 1  is changed by changing the frequency of the rectangular pulse signal P 1 . In Japanese Patent Application Document 2005-114497, a duty ratio of the rectangular pulse signal P 1  is changed for changing the pulse width of the rectangular pulse signal P 1 . 
   Incidentally, in the insulation resistance detector  50 , the output of the low pass filter  53  in response to the same frequency of the rectangular pulse signal P 1  is different in each product. This is because a circuit constant of the insulation resistance detector  50 , the voltage source, and circuit characteristics of the low pass filter  53  are varied in each product. 
   Namely, as shown by an alternate long and short dash line in  FIG. 6A , there is a product of which output peak voltage of the low pass filter  53  is shifted up in response to the frequency of the rectangular pulse signal P 1  against the other product shown in a solid line. As shown by an alternate long and short dash line in  FIG. 6B , there is a product of which output peak voltage of the low pass filter  53  is shifted down in response to the frequency of the rectangular pulse signal P 1  against the other product shown in a solid line. 
   However, according to the conventional state detecting method, on an assumption that the output peak voltage of the low pass filter  53  is constant in each product, and by comparing the output of the low pass filter  53  with the threshold voltages X 1 , X 2 , X 3 , normal, open, short states are detected. 
   Therefore, as shown by the alternate long and short dash line in  FIG. 6A , when the output peak voltage of the low pass filter  53  is shifted up, even when the insulation resistance detector  50  is normal, the output peak voltage of the low pass filter  53  may be more than X 2  in response to the 5.5 Hz rectangular pulse signal P 1 . Therefore, even when the insulation resistance detector  50  is normal, the output peak voltage of the low pass filter  53  may be more than X 1  in response to 2.5 Hz rectangular pulse signal P 1 , and the output peak voltage of the low pass filter  53  may be less than X 2  in response to 5.5 Hz rectangular pulse signal P 1 . Thus, the normal state of the insulation resistance detector  50  cannot be detected. 
   Further, as shown by the alternate long and short dash line in  FIG. 6B , when the output peak voltage of the low pass filter  53  is shifted down, even when the insulation resistance detector  50  is open, the output peak voltage of the low pass filter  53  may be less than X 1  in response to the 5.5 Hz rectangular pulse signal P 1 . Therefore, even when the insulation resistance detector  50  is open, the output peak voltage of the low pass filter  53  may be less than X 1  in response to 2.5 Hz and 5.5 Hz rectangular pulse signal P 1 , and the open state of the insulation resistance detector  50  cannot be detected. 
   It is difficult to correctly detect the state of the insulation resistance detector  50  according to the comparison of the output peak voltage of the low pass filter  53  and the threshold voltages, because there is a shift up or shift down of the output peak voltage of the low pass filter  53 . Namely, using the comparison of the output peak voltage of the low pass filter  53  with the threshold voltages X 1 , X 2 , X 3  cannot correctly detect the state of the insulation resistance detector  50 . 
   Further, according to the above, if the insulation resistance detector  50  is normal in response to the 2.5 Hz rectangular pulse signal P 1 , and then a short is occurred before the rectangular pulse signal P 1  outputs 5.5 Hz pulse, the short state of the insulation resistance detector  50  cannot be detected. Namely, in a rare short that the short is intermittently occurred, a possibility of detecting the short state decreases. 
   Accordingly, an object of the present invention is to provide a state detecting method to detect a reduction of an insulating resistor correctly and easily, and to provide an insulation resistance detector using the state detecting method. 
   SUMMARY OF THE INVENTION 
   In order to attain the object, according to the present invention, there is provided a state detecting method adopted to an insulation resistance detector including: 
   a detecting resistor connected in series to an insulating resistor interposed between a ground and a direct current power supply; 
   a coupling capacitor interposed between the insulating resistor and the detecting resistor; 
   a pulse signal applying member for applying a pulse signal to a series circuit composed of the insulating resistor, the coupling capacitor, and the detecting resistor; 
   a filter for filtering a specific frequency and outputting a node voltage between the coupling capacitor and the detecting resistor; and 
   a reduction detecting member for detecting a reduction of the insulating resistor based on the output of the filter, 
   said method including the steps of: 
   calculating a difference between the output of the filter when a pulse signal having a first pulse width is applied to the series circuit, and the output of the filter when a pulse signal having a second pulse width shorter than the first pulse width is applied to the series circuit; and 
   detecting the state of the insulation resistance detector based on the calculated difference. 
   Preferably, when the calculated difference is more than or equal to a short judging value, the method judges that the insulation resistance detector has a short circuit malfunction. 
   Preferably, when the filter output at the pulse signal having the first pulse width is over a specific value, and the calculated difference is less than a open judging value which is smaller than the short judging value, the method judges that the insulation resistance detector has an open circuit malfunction. 
   Preferably, when the calculated difference is less than the short judging value, and more than or equal to a normal judging value which is less than the short judging value, the method judges that the insulation resistance detector is normal. 
   Preferably, when the calculated difference is more than or equal to the normal judging value, the method judges that the insulation resistance detector is normal. 
   Preferably, when the filter output at the pulse signal having the first pulse width is over a specific value, and the calculated difference is less than a open judging value which is equal to or smaller than the normal judging value, the method judges that the insulation resistance detector has an open circuit malfunction. 
   Preferably, when the filter output at the pulse signal having the first pulse width is less than an initial short judging value, the method judges that the insulation resistance detector has an initial short state. 
   According to another aspect of the present invention, there is provided an insulation resistance detector including: 
   a detecting resistor connected in series to an insulating resistor interposed between a ground and a direct current power supply; 
   a coupling capacitor interposed between the insulating resistor and the detecting resistor; 
   a pulse signal applying member for applying a pulse signal to a series circuit composed of the insulating resistor, the coupling capacitor, and the detecting resistor; 
   a filter for filtering a specific frequency and outputting a node voltage between the coupling capacitor and the detecting resistor; and 
   a reduction detecting member for detecting a reduction of the insulating resistor based on the output of the filter, 
   said detector further including: 
   a pulse width varying member to make the pulse signal applying member apply pulse signals having a first pulse width and a second pulse width shorter than the first pulse width; 
   a differential operation member for calculating a difference between the filter output at the pulse signal having the first pulse width and the filter output at the pulse signal having the second pulse width; and 
   a detecting member for detecting a state of the insulation resistance detector based on the difference calculated by the differential operation member. 
   These and other objects, features, and advantages of the present invention will become more apparent upon reading of the following detailed description along with the accompanied drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing an embodiment of an insulation resistance detector implementing a state detecting method according to the present invention; 
       FIG. 2  is a graph showing relations between frequency of a rectangular pulse signal P 1  at normal, open, and short states, and output peak voltage of a low pass filter  53 ; 
       FIGS. 3A and 3B  are graphs for explaining the state detecting method according to the present invention; 
       FIG. 4  is a flow chart showing a fault detection process of a control circuit  52  composing the insulation resistance detector; 
       FIG. 5  is a graph for explaining a conventional state detecting method; and 
       FIGS. 6A and 6B  are graphs for explaining a problem of the conventional state detecting method. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A state detecting method and an insulation resistance detector will be explained with reference to figures. The insulation resistance detector includes a detecting resistor Rd connected in series to an insulating resistor Ri between a battery B as the direct current power supply and a vehicle body E, a coupling capacitor Co, a pulse oscillation circuit  51  (pulse signal supplying member), a low pass filter  53 , a waveform shaping circuit  54 , and a control circuit  52 . 
   Next, a principle for the state detecting method of the insulation resistance detector  50  shown in  FIG. 1  will be explained with reference to  FIG. 2 . A curve L 1  of  FIG. 2  shows a relationship between the frequency of the rectangular pulse signal P 1  and a peak voltage outputted from the low pass filter  53  at a normal mode where the insulating resistor Ri is not reduced, and the insulation resistance detector  50  is not malfunctioning. 
   As shown in  FIG. 2 , in the normal state, when the frequency supplied from the pulse oscillation circuit  51  is less than 2.5 Hz, the output peak voltage of the low pass filter  53  is substantially equal to that of the rectangular pulse signal P 1  outputted from the pulse oscillation circuit  51 . On the other hand, as the frequency supplied from the pulse oscillation circuit  51  increases from 2.5 Hz, the output peak voltage of the low pass filter  53  decreases. 
   This is because in the normal state, the time constant of the low pass filter  53  is large. When the frequency of the rectangular pulse signal P 1  increases over 2.5 Hz, the rising time to peak voltage 4.7 V is longer than the pulse width of the rectangular pulse signal P 1 . Namely, when the pulse width is short according to the increase of the frequency, before the output of the low pass filter  53  reaches the peak voltage 4.7 V, the source of the rectangular pulse signal P 1  is cut off, and the peak voltage is less than 4.7 V. As the frequency of the rectangular pulse signal P 1  increases, the pulse width decreases and the peak voltage of the low pass filter  53  decreases. 
   A curve L 2  of  FIG. 2  shows a relationship between the frequency of the rectangular pulse signal P 1  and the output peak voltage of the low pass filter  53  when the insulation resistance detector  50  is malfunctioning, for example, the coupling capacitor Co or the capacitor Cf is open. 
   As shown in  FIG. 2 , when the insulation resistance detector  50  is open, the output peak voltage of the low pass filter  53  is constant even when the frequency of the rectangular pulse signal P 1  increases. This is because the time constant of the low pass filter  53  is short, and the rising time of the output of the low pass filter  53  in the short state is shorter than that in the normal state. 
   A curve L 3  of  FIG. 2  shows a relationship between the frequency of the rectangular pulse signal P 1  and the output peak voltage of the low pass filter  53  when the insulation resistance detector  50  is malfunctioning, for example, the coupling capacitor Co or the capacitor Cf is short. 
   When the capacitor Co or Cf is short, the output of the low pass filter  53  does not rise even when the rectangular pulse signal P 1  outputs. Even when the frequency of the rectangular pulse signal P 1  increases, the output of the low pass filter  53  is constantly about 0.2 V. 
   According to the state detecting method of the present invention, when the peak voltage at 2.5 Hz (V 2.5 Hz ) is in a specific range (more than or equal to 0.5V, and less than or equal to 2V), the method judges that the insulating resistor Ri is reduced, and a short circuit is detected. Further, when the V 2.5 Hz  is less than the initial shot judging value 0.5V, the method judges that the insulation resistance detector  50  is in the initial short state. Further, as described above, the specific range is more than the initial short judging range 0.5 V. Therefore, the method judges both the reduction of the insulating resistor Ri, in particular, several meg ohm to several kilo ohm, and the initial short state. 
   Next, a case when the short circuit is occurred between acquiring V 2.5 Hz  and V 4.5 Hz  will be explained. As shown in  FIG. 2 , the insulation resistance detector  50  is normal when acquiring V 2.5 Hz  and short when acquiring V 4.5 Hz , the peak voltage V 2.5 Hz  is about 5 V, and V 4.5 Hz  is about 0.2 V. The difference (V 2.5 Hz −V 4.5 Hz ) is large, and about 4.8 V. Therefore, according to the present invention, when the difference (V 2.5 Hz −V 4.5 Hz ) is more than the short judging value 3 V, the method judges that the insulation resistance detector  50  is in the short malfunction. Accordingly, the method can detect the short malfunction after acquiring V 2.5 Hz  in the normal state. 
   Next, a case that the insulation resistance detector  50  is normal will be explained. As shown in  FIG. 2 , when the insulation resistance detector  50  is normal, the peak voltage V 2.5 Hz  is about 5 V, and V 4.5 Hz  is about 2.5 V. This 2.5 V is less than the short judging value 3 V. Accordingly, when the difference (V 2.5 Hz −V 4.5 Hz ) is less than 3V, and more than or equal to 2V, the method judges that the insulation resistance detector  50  is in the normal state. 
   Next, a case that the insulation resistance detector  50  is in the open state will be explained. As shown in  FIG. 2 , if the insulation resistance detector  50  is in the open mode, the peak voltage V 2.5 Hz  and V 4.5 Hz  are about 5 V, and the difference (V 2.5 Hz −V 4.5 Hz ) is almost 0 V. However, when the insulation resistance detector  50  is in the initial short malfunction, the difference (V 2.5 Hz −V 4.5 Hz ) is also almost 0 V. So, for distinguishing the open malfunction from the initial short malfunction, the method judges that the insulation resistance detector  50  is in the open malfunction when the peak voltage V 2.5 Hz  is over a specific value 2 V, and the difference (V 2.5 Hz −V 4.5 Hz ) is less than open judging value 2 V. 
   For distinguishing the open malfunction from the initial short malfunction, the specific value 2 V is larger than the initial short judging value 0.5 V. Further, for distinguishing the reduction of the insulating resistor from the open malfunction, the specific value 2 V is the maximum value of the specific range (0.5 V to 2 V). The open judging value 2 V is equal to the normal judging value 2 V. The open judging value 2 V is smaller than the short judging value 3 V. 
   Incidentally, the pulse width of the rectangular pulse signal P 1  at the frequency 2.5 Hz is the first pulse width in claims. The first pulse width is longer than a minimum pulse width for keeping the peak voltage 4.7 V. In detail, the first pulse width is longer than the pulse width when the rectangular pulse signal P 1  is applied and the output of the low pass filter  53  reaches the peak voltage 4.7 V of the rectangular pulse signal P 1 . 
   The pulse width of the rectangular pulse signal P 1  at the frequency 4.5 Hz is the second pulse width in claims. The second pulse width is shorter than a minimum pulse width for keeping the peak voltage 4.7 V. In detail, the second pulse width is shorter than the pulse width when the rectangular pulse signal P 1  is applied and the output of the low pass filter  53  reaches the peak voltage 4.7 V of the rectangular pulse signal P 1 . As shown in  FIG. 2 , when the insulation resistance detector  50  is normal, the output peak voltage of the low pass filter  53  is 2.5 V at the frequency 4.5 Hz. 
   An effect of the insulation resistance detector  50  will be explained. As shown by an alternate long and short dash line in  FIGS. 3A and 3B , the output peak voltage of the low pass filter  53  is shifted down or shifted up in a range ±ΔV relative to a standard product as shown by a solid line. Therefore, in the conventional comparison of the output peak voltage of the low pass filter  53  and the threshold voltage, owing to the variation of the output peak voltage of the low pass filter  53 , the open or the short cannot correctly detected. Namely, according to the conventional method which judges the insulation resistance detector  50  is normal when the peak voltage V 4.5 Hz  is less than or equal to 3 V and more than or equal to 2 V, as shown in  FIG. 3A , when the output peak voltage of the low pass filter  53  is shifted up, the peak voltage V 4.5 Hz  is more than 3 V even when the insulation resistance detector  50  is normal, and the method cannot detect the normality of the insulation resistance detector  50 . As shown in  FIG. 3B , when the output peak voltage of the low pass filter  53  is shifted down, the peak voltage V 4.5 Hz  is less than 2 V even when the insulation resistance detector  50  is normal, and the method cannot detect the normality of the insulation resistance detector  50 . 
   However, the difference (V 2.5 Hz −V 4.5 Hz ) of the low pass filter  53  cancel the variety of the output peak voltage of the low pass filter  53 . Namely, the difference (V 2.5 Hz −V 4.5 Hz ) is not varied even when the output peak voltage of the low pass filter  53  is varied. Namely, there is no variation among the products. 
   Accordingly, the method calculates the difference (V 2.5 Hz −V 4.5 Hz ), and judges normal, short malfunction, open malfunction based on the difference (V 2.5 Hz −V 4.5 Hz ). Therefore, the method can correctly detect the state of the insulation resistance detector  50 . 
   An operation of the insulation resistance detector  50  will be explained with reference to a flow chart of the control circuit shown in  FIG. 4 . The control circuit  52  starts the operation with a specific trigger such as ignition switch on. First, the control circuit  52  works as a reduction detecting member, and outputs a signal S 1  at the frequency 2.5 Hz (step S 1 ). Then, the pulse oscillation circuit  51  outputs the rectangular pulse signal P 1  at the frequency 2.5 Hz. The control circuit  52  reads the output peak voltage V 2.5 Hz  outputted from the waveform shaping circuit  54  and stores the output peak voltage V 2.5 Hz  in a memory member (step S 2 ). 
   Next, when the peak voltage V 2.5 Hz  is lower than or equal to the specific voltage 2 V and larger than or equal to the initial short judging value 0.5 V (“Y” in step S 3  and “Y” in step S 4 ), the control circuit  52  judges that the insulating resistor Ri is reduced and the insulation resistance detector  50  is short-circuited (step S 5 ) and the process ends. On the other hand, when the peak voltage V 2.5 Hz  is less than 0.5 V (“N” in step S 4 ), the control circuit  52  detects the short state of the insulation resistance detector  50  (step S 9 ), and the process ends. 
   When the peak voltage V 2.5 Hz  is more than or equal to 2 V (“N” in step S 3 ), the control circuit  52  outputs a frequency signal S 1  4.5 Hz (step S 6 ). In response to this signal, the pulse oscillation circuit  51  outputs the rectangular pulse signal P 1  of 4.5 Hz. Namely, the control circuit  52  works as a pulse width changing member. Of course, the second pulse width is shorted than the first pulse width. Then, the control circuit  52  reads out the output peak voltage V 4.5 Hz  outputted from the waveform shaping circuit  54 , and stores the output peak voltage V 4.5 Hz  in the memory member (step S 7 ). 
   Then, the control circuit  52  calculates the difference (V 2.5 Hz −V 4.5 Hz ). When the difference (V 2.5 Hz −V 4.5 Hz ) is more than or equal to the short judging value 3 V (“Y” in step S 8 ), the control circuit  52  detects the short malfunction (step S 9 ) and the process ends. On the other hand, when the difference (V 2.5 Hz −V 4.5 Hz ) is less than the short judging value 3 V, and more than or equal to the normal judging value 2 V, (“N” in step S 8  and “Y” in step S 10 ), the control circuit  52  detects that the insulation resistance detector  50  is normal (step S 11 ) and the process ends. 
   When the difference (V 2.5 Hz −V 4.5 Hz ) is less than the open judging value 2 V (“N” in step S 10 ), the control circuit  52  detects the insulation resistance detector  50  is in the open malfunction (step S 12 ), and the process ends. According to the above, the control circuit  52  works as the differential operation member, and the detecting member. 
   According to the above, the control circuit  52  is composed of the microcomputer. However, the control circuit  52  may be composed of a comparator or the like. 
   According to the above, the first pulse width is a pulse width of the frequency 2.5 Hz. However, this invention is not limited to this. The first pulse width may be a pulse width more than the time from when the rectangular pulse signal P 1  applies to when the output of the low pass filter  53  reaches the peak voltage 4.7 V of the rectangular pulse signal P 1 . 
   Further, the second pulse width is a pulse width of the frequency 4.5 Hz. However, this invention is not limited to this. The second pulse width may be a pulse width less than the time from when the rectangular pulse signal P 1  applies to when the output of the low pass filter  53  reaches the peak voltage 4.7 V of the rectangular pulse signal P 1 . 
   According to the above, the pulse width of the rectangular pulse signal P 1  is changed by changing the frequency of the rectangular pulse signal P 1 . However, the pulse width of the rectangular pulse signal P 1  may be changed by changing a duty ratio of the rectangular pulse signal P 1 . 
   According to the above, when the difference (V 2.5 Hz −V 4.5 Hz ) is more than or equal to the short judging value 3 V, the control circuit  52  detects the short malfunction. When the difference (V 2.5 Hz −V 4.5 Hz ) is less than the short judging value 3 V and more than or equal to the normal judging value 2 V, the control circuit  52  detects the normal state. However, the present invention is not limited to this. For example, if it is unnecessary to detect the short malfunction after the V 2.5 Hz  is measured, the short judging value 3 V is not used, and only when the difference (V 2.5 Hz −V 4.5 Hz ) is more than or equal to the normal judging value 2 V, the control circuit  52  may detect the normal state. 
   Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein. 
   The threshold voltages may be changed in the scope of the invention.