Abstract:
A fan controlling apparatus for controlling a plurality of fans which are tandemly arranged in ventilation direction of a chamber to control a temperature of a hot generating object placed in the chamber, the apparatus includes a memory for storing data of the rotational speed each of fans in relation to the temperature of the heat generating object, and a controller for controlling the rotational speed of each of the fans respectively in dependence on the temperature of the heat generating object in reference to the data stored in the memory.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-123500, filed on May 21, 2009, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are relates to a fan control apparatus and a fan control method. 
       BACKGROUND 
       [0003]    In existing electronic equipment such as a server device, a PC (Personal Computer) and others, in some cases, fans that send air into equipment concerned to radiate heat in the equipment to the outside in order to avoid temperature rising in the equipment with heat generated from a processor or the like are mounted on the equipment. 
         [0004]    In a fan, noise (whirring sounds) is generated owing to formation of air eddies in the vicinity of blades. Noise generated from a fan is increased with increasing an air flow rate at which the fan blows off air. Therefore, if it is intended to increase the number of revolutions (hereinafter, referred to as the revolution number) of the fan to increase the air flow rate, the noise will be increased accordingly. Specifically, it is known that noise from a fan is proportional to the fifth or sixth power of the number of axial rotations of the fan. 
         [0005]    Recently, such a situation has been more and more frequently observed that electronic equipment is installed not only in a specific place such as a computer room but also in a general office and hence consciousness of noise reduction is now being raised. Therefore, how the noise generated from a fan is reduced is one of important problems to be solved. 
         [0006]    As a method of reducing noise generated from a fan, a method of monitoring the temperature of a heating element and the environmental temperature around the heating element and changing the revolution number of the fan concerned in accordance with the temperatures so monitored to control the noise from the fan so as not to increase more than needed is known. The revolution number of the fan is controlled by modulating the pulse width (the PWM value) of a voltage or PWM (Pulse Width Modulation) signal to control energy to be supplied to the motor of the fan. 
         [0007]    On the hand, nowadays, in some cases, a space for an air passage in equipment is reduced with size and thickness reduction of the electronic equipment and such a case in which only a small fan is allowed to be installed is now being more frequently observed. In addition, a heating value of electronic equipment is more and more increased every year as processing speed and performance of electronic equipment get higher. Thus, such a countermeasure is taken that a plurality of fans is superposed on one another in a multi-stage form so as to sufficiently cool the inside of electronic equipment even when installation of only small-sized fans is allowed. 
         [0008]    In this connection, an example in which a plurality of fans is superposed on one another in a multi-stage form is illustrated in  FIG. 26 .  FIG. 26  illustrates an example of electronic equipment in which two fans are disposed in series. 
         [0009]    As illustrated in  FIG. 26 , two fans  321   a  and  321   b  are installed in series in an air passage  310  of electronic equipment  300 . The electronic equipment  300  includes a fan power source section  330  and a control section  340 . The fan power source section  330  is a power source for supplying power to motors not illustrated which are built in the fans  321   a  and  321   b . The control section  340  controls amounts of energy supplied to the fans  321   a  and  321   b  on the basis of temperatures of heating elements  350   a  and  350   b  detected using temperature sensors  341   a  and  341   b  and an environmental temperature detected using a temperature sensor  341   c . Next, a specific configuration of the control section  340  will be illustrated.  FIG. 27  is a block diagram illustrating a configuration of the existing control section  340 . 
         [0010]    As illustrated in  FIG. 27 , the control section  340  includes the temperature sensors  341   a  to  341   c , temperature check sections  342   a  and  342   b , revolution number detecting sections  343   a  and  343   b , a revolution number error check section  344  and a pulse generator  345 . The control section  340  also includes a RAM (Random Access Memory)  346 , a ROM (Read Only Memory)  347  and a processor  348 . 
         [0011]    The temperature sensors  341   a  and  341   b  are attached to the heating elements  350   a  and  350   b  in the electronic equipment  300  to detect the temperatures of the heating elements  350   a  and  350   b . The temperature sensor  341   c  is installed outside of the electronic equipment  300  to detect the environmental temperature around the fan  321   a . The temperature check sections  342   a  and  342   b  check to see to which extent the temperatures of the heating elements  350   a  and  350   b  detected using the temperature sensors  341   a  and  341   b  vary from a target temperature and notify the processor  348  of results of check in the form of predetermined variables. 
         [0012]    The revolution number detecting sections  343   a  and  343   b  detect the revolution numbers of the fans  321   a  and  321   b . The revolution number check section  344  checks to see whether the fans  321   a  and  321   b  normally rotate on the basis of the revolution numbers of the fans  321   a  and  321   b  detected using the revolution number detecting sections  343   a  and  343   b  and notifies the processor  348  of a result of check. The pulse generator  345  inputs pulses for controlling the revolution numbers of the fans  321   a  and  321   b  into the fans  321   a  and  321   b  in pulse widths in accordance with an instruction from the processor  348 . 
         [0013]    The ROM  347  stores therein a table indicating predetermined variables corresponding to temperatures of the heating elements  350   a  and  350   b  and PWM values of pulses to be input into the heating elements  350   a  and  350   b  in one-to-one correspondence and a table indicating environmental temperatures and the PWM values in one-to-one correspondence. The processor  348  determines the pulse widths of pulses to be output to the fans  321   a  and  321   b  on the basis of the predetermined variables sent from the temperature check sections  342   a  and  342   b  and the environmental temperature detected using the temperature sensor  341   c . A specific example thereof is as illustrated in  FIG. 27 .  FIG. 28  is a flowchart illustrating an example of procedures of processing executed using the existing processor  348 . 
         [0014]    As illustrated in  FIG. 28 , first, the processor  348  performs temperature measurement (step S 001 ). That is, the processor  348  acquires predetermined variables corresponding to the temperatures of the heating elements  350   a  and  350   b  detected using the temperature sensors  341   a  and  341   b  from the temperature check sections  342   a  and  342   b . The processor  348  also acquires the environmental temperature detected using the temperature sensor  341   c.    
         [0015]    Next, the processor  348  instructs the pulse generator  345  to modulate the pulse widths on the basis of results of measurement of the environmental temperature and the temperatures of the heating elements  350   a  and  350   b  (step S 002 ). Specifically, first, the processor  348  determines PWM values corresponding to the predetermined variables acquired from the temperature check sections  342   a  and  342   b  or the environmental temperature acquired from the temperature sensor  341   c  on the basis of the tables stored in the ROM  347 . Then, the processor  348  instructs the pulse generator  345  to modulate widths of pulses to be input into the fans  321   a  and  321   b  to have the PWM values so determined (step S 002 ). 
         [0016]    The pulse generator  345  then inputs pulses whose widths have been modulated to have the PWM values as instructed from the processor  348 . As a result, the revolution numbers of the fans  321   a  and  321   b  are changed to the revolution numbers conforming to the environmental temperature and the temperatures of the heating elements  350   a  and  350   b . As described above, the existing processor  348  determines the pulse widths on the basis of the environmental temperature and the temperatures of the heating elements  350   a  and  350   b . Incidentally, the existing control section  340  detects the revolution numbers of the fans  321   a  and  321   b  using the revolution number detecting sections  343   a  and  343   b  to check on errors in the revolution numbers. Specifically, the processor  348  sends an error notification that the fans  321   a  and  321   b  are now in stopped states to the electronic equipment  300  on the basis of results of revolution number error check acquired from the revolution number error check section  344 . 
         [0017]    Incidentally, hitherto, the control section  340  has controlled the fans  321   a  and  321   b  in the same manner. In the following, the fan  321   a  which is situated on the side of taking air into the equipment from the outside will be referred to as a front-stage fan  321   a  and the fan  321   b  which is situated on the side of sending the air into the electronic equipment  300  will be referred to as a rear-stage fan  321   b.    
         [0018]    The processor  348  determines a pulse width of a pulse which is commonly input into the front-stage fan  321   a  and the rear-stage fan  321   b  at step S 002  in  FIG. 28  and notifies the pulse generator  345  of the pulse width. As a result, hitherto, energy of the same amount has been usually supplied from the pulse generator  345  to the front-stage fan  321   a  and the rear-stage fan  321   b  as illustrated in  FIG. 29 . 
         [0019]    However, in the case that the fans  321   a  and  321   b  are superposed on each other in a multi-stage form, the work amount with which the rear-stage fan  321   b  blows off air is reduced influenced by an air current generated from the front-stage fan  321   a . Therefore, if the amount of energy supplied to the front-stage fan  321   a  is the same as that supplied to the rear-stage fan  321   b , the rear-stage fan  321   b  will run idle and hence the revolution number of the rear-stage fan  321   b  will be increased. Noise is generated greatly influenced by the operation of the fan of the largest revolution number and hence an increase in revolution number of the rear-stage fan  321   b  will be a major factor to cause an increase in noise. 
         [0020]    In this connection, a technique for, example, making the revolution number of the front-stage fan  321   a  different from that of the rear-stage fan  321   b  is known. Specifically, in the above mentioned technique, the pulse width of a pulse input into the rear-stage fan  321   b  is controlled to become shorter than that of a pulse input into the front-stage fan  321   a . In the above mentioned case, if the revolution number of the rear-stage fan  321   b  is reduced, an air flow rate (needed air flow rate) needed to cool heating elements may not be possibly obtained. Therefore, it may become needed to increase the revolution number of the front-stage fan  321   a  more than that needed when the energy of the same amount is applied to each of the fans  321   a  and  321   b . An example of the above mentioned technique is disclosed, for example, in Japanese Laid-open Patent Publication No. 2004-179186. 
         [0021]    However, in the above mentioned known technique, the revolution number of the front-stage fan  321   a  is simply increased with no consideration of the system impedance characteristic of the electronic equipment  300 , so that the noise may be rather increased as compared with a case in which the energy of the same amount is applied to each of the fans  321   a  and  321   b . The system impedance characteristic is a pressure loss determined from a density at which components constituting the electronic equipment  300  are packaged and the shape of an air passage therein and is a characteristic intrinsic to the electronic equipment  300  concerned. 
         [0022]    That is, the noise generated from the fans  321   a  and  321   b  varies depending on the characteristics and the shapes of these fans  321   a  and  321   b  and also depending on the configuration of the electronic equipment  300  concerned on which the fans  321   a  and  321   b  are mounted and locations of the fans  321   a  and  321   b  in the electronic equipment  300  concerned. Therefore, if the revolution number of the front-stage fan  321   a  is simply increased, the noise generated from the front-stage fan  321   a  may be possibly increased more than the noise generated from the rear-stage fan  321   b  when the energy of the same amount is applied to each of the fans  321   a  and  321   b  depending on the locations of the fans. 
       SUMMARY 
       [0023]    According to an aspect of the embodiments, a fan controlling apparatus for controlling a plurality of fans which are tandemly arranged in ventilation direction of a chamber to control a temperature of a hot generating object placed in the chamber, the apparatus includes a memory for storing data of the rotational speed each of fans in relation to the temperature of the heat generating object, and, a controller for controlling the rotational speed of each of the fans respectively in dependence on the temperature of the heat generating object in reference to the data stored in the memory. 
         [0024]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a block diagram illustrating a configuration of a fan control device according to an embodiment 1; 
           [0026]      FIG. 2  is a diagram illustrating a configuration of electronic equipment according to an embodiment 2; 
           [0027]      FIG. 3  is a block diagram illustrating a configuration of a fan control device according to the embodiment 2; 
           [0028]      FIG. 4  is a block diagram illustrating specific configurations of a processor and a ROM according to the embodiment 2; 
           [0029]      FIG. 5  is a diagram illustrating an example of an equipment state management table; 
           [0030]      FIG. 6  is a diagram illustrating information stored in a control data storage section according to the embodiment 2; 
           [0031]      FIG. 7A  is a diagram illustrating an example of a heating-element-temperature-based PWM value determination table; 
           [0032]      FIG. 7B  is a diagram illustrating an example of an environmental-temperature-based PWM value determination table; 
           [0033]      FIG. 8  is a diagram explaining a difference between energy supplied to a front-stage fan and energy supplied to a rear-stage fan; 
           [0034]      FIG. 9  is a diagram illustrating a specific configuration of a database preparing section according to the embodiment 2; 
           [0035]      FIG. 10  is a flowchart illustrating an example of procedures of preparing the heating-element-temperature-based PWM value determination table and the environmental-temperature-based PWM value determination table; 
           [0036]      FIG. 11  is a flowchart illustrating an example of procedures of processing executed using the processor according to the embodiment 2; 
           [0037]      FIG. 12  is a diagram illustrating effects brought about by the embodiment 2; 
           [0038]      FIG. 13  is a diagram illustrating examples of measurement conditions involving experiments for validation; 
           [0039]      FIG. 14A  is a diagram illustrating data indicative of a result of measurement of the front-stage revolution number, the rear-stage revolution number and the device noise performed under a measurement condition A; 
           [0040]      FIG. 14B  is a diagram illustrating data indicative of a result of measurement of the front-stage revolution number, the rear-stage revolution number and the consumption power performed under the measurement condition A; 
           [0041]      FIG. 14C  is a diagram illustrating data indicative of a result of analysis of frequencies of the device noise performed under the measurement condition A; 
           [0042]      FIG. 15A  is a diagram illustrating a result of measurement performed under a condition A- 1 ; 
           [0043]      FIG. 15B  is a diagram illustrating a result of measurement performed under a condition A- 2 ; 
           [0044]      FIG. 15C  is a diagram illustrating a result of measurement performed under a condition A- 3 ; 
           [0045]      FIG. 16A  is a diagram illustrating data indicative of a result of measurement of the front-stage average revolution number, the rear-stage average revolution number and the device noise performed under conditions B- 1  and B- 2 ; 
           [0046]      FIG. 16B  is a diagram illustrating data indicative of a result of measurement of the front-stage average revolution number, the rear-stage average revolution number and the consumption power performed under the conditions B- 1  and B- 2 ; 
           [0047]      FIG. 16C  is a diagram illustrating data indicative of a result of analysis of frequencies of the device noise performed under the conditions B 1  and B 2 ; 
           [0048]      FIG. 17A  is a diagram illustrating a result of measurement performed under the condition B- 1 ; 
           [0049]      FIG. 17B  is a diagram illustrating a result of measurement performed under the condition B- 2 ; 
           [0050]      FIG. 18A  is a diagram illustrating data indicative of a result of measurement of the front-stage average revolution number, the rear-stage average revolution number and the device noise performed under conditions B- 3  and B- 4 ; 
           [0051]      FIG. 18B  is a diagram illustrating data indicative of a result of measurement of the front-stage average revolution number, the rear-stage average revolution number and the consumption power performed under the conditions B- 3  and B- 4 ; 
           [0052]      FIG. 18C  is a diagram illustrating data indicative of a result of analysis of frequencies of the device noise performed under the conditions B 3  and B 4 ; 
           [0053]      FIG. 19A  is a diagram illustrating a result of measurement performed under the condition B- 3 ; 
           [0054]      FIG. 19B  is a diagram illustrating a result of measurement performed under the condition B- 4 ; 
           [0055]      FIG. 20  is a functional block diagram illustrating an example of a computer for executing a fan control program; 
           [0056]      FIG. 21  is a block diagram illustrating a configuration of a fan control device according to an embodiment 3; 
           [0057]      FIG. 22  is a block diagram illustrating specific configurations of a processor and a ROM according to the embodiment 3; 
           [0058]      FIG. 23  is a diagram illustrating information stored in a control data storage section according to the embodiment 3; 
           [0059]      FIG. 24  is a diagram illustrating an example of a rear-stage PWN value change table; 
           [0060]      FIG. 25  is a flowchart illustrating an example of procedures of processing executed using the processor according to the embodiment 3; 
           [0061]      FIG. 26  is a diagram illustrating an example of electronic equipment in which two fans are disposed in series; 
           [0062]      FIG. 27  is a block diagram illustrating a configuration of an existing control section; 
           [0063]      FIG. 28  is a flowchart illustrating an example of procedures of processing executed using an existing processor; and 
           [0064]      FIG. 29  is a diagram illustrating that the amount of energy supplied to a front-stage fan is the same as that supplied to a rear-stage fan. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0065]    Next, preferred embodiments of a fan control device, a fan control method and a fan control program disclosed in the present application will be described in detail with reference to the accompanying drawings. 
       Embodiment 1 
       [0066]    A fan control device according to an embodiment 1 will be described. The fan control device according to the embodiment 1 is a control device that controls operational conditions of a plurality of fans disposed in series relative to an air passage formed in equipment. 
         [0067]    The plurality of fans blow off air to heating elements disposed in the equipment to forcibly air-cool the heating elements. The fan control device controls the revolution number of each of the plurality of fans.  FIG. 1  is a block diagram illustrating a configuration of the fan control device according to the embodiment 1. 
         [0068]    As illustrated in  FIG. 1 , a fan control device  500  according to the embodiment 1 includes a temperature detecting section  510  and a control section  520 . The temperature detecting section  510  detects the temperature of each heating element. The control section  520  then controls the revolution numbers of the fans such that an air flow rate needed for cooling the heating elements is obtained in a state that the revolution numbers of the fans coincide with each other on the basis of the temperatures of the heating elements detected using the temperature detecting section  510 . 
         [0069]    Specifically, the control section  520  according to the embodiment 1 controls the revolution numbers of the fans such that the needed air flow rate (the cooling air flow rate) needed to cool the heating elements obtained from the temperatures of the heating elements, the pipeline resistance in the equipment and the total static pressure-air flow rate characteristic of the fans in a state in which a difference in revolution number between the fans is in a predetermined tolerance is acquired and the difference in revolution number between the fans is set in the predetermined tolerance. By controlling the revolution number of each fan in the above mentioned manner, the respective fans rotate in a state in which the difference in revolution number between the fans is maintained in the predetermined tolerance, that is, rotate in a state in which the revolution numbers of the respective fans coincide with each other in the predetermined tolerance to send air to the heating elements at a needed air flow rate determined in accordance with the current temperature of each heating element. 
         [0070]    As described above, the fan control device  500  according to the embodiment 1 controls so as to maintain the difference in revolution number between the fans in the predetermined tolerance, desirably, so as to control the revolution numbers of the fans to coincide with each other. Therefore, such a situation may be avoided that the total noise generated from the fans is increased owing to surplus noise generated from a fan of the largest revolution number. 
         [0071]    The needed air flow rate attained according to the embodiment 1 is obtained by taking the temperatures of the heating elements, the pipeline resistance in the equipment and the total static pressure-air flow rate characteristic of the fans obtained in a state in which the difference in revolution number between the fans is maintained in the predetermined tolerance into consideration. Therefore, according to the embodiment 1, the air flow rate suited to cool the heating elements may be obtained in a state in which the difference in revolution number between the fans is maintained in the predetermined tolerance regardless of the configuration of the equipment and the locations of the fans in the equipment. 
         [0072]    Therefore, according to the embodiment 1, it may be possible to reduce the noise generated from the plurality of fans while obtaining the air flow rate needed for cooling the heating elements in the equipment using the plurality of fans. 
       Embodiment 2 
       [0073]    Next, a fan control device according to an embodiment 2 will be described. The fan control device according to the embodiment 2 is configured to control the revolution numbers of two fans installed in electronic equipment such as a rack-mounted type server device, a general PC and others. First, a configuration of electronic equipment in which the fan control device according to the embodiment 2 is installed will be described.  FIG. 2  is a diagram illustrating a configuration of the electronic equipment according to the embodiment 2. 
         [0074]    As illustrated in  FIG. 2 , electronic equipment  50  according to the embodiment 2 includes two fans  3   a ,  3   b  and heating elements  52   a  and  52   b  such as processors installed in an air passage  51  formed in the electronic equipment  50 . 
         [0075]    The fans  3   a  and  3   b  are axial fans of the same shape and characteristic. The fans  3   a  and  3   b  are disposed in series relative to the air passage  51  and generate air currents flowing from the fan  3   a  toward the fan  3   b  to forcibly air-cool the heating elements  52   a  and  52   b  disposed downstream of the air currents. In the following, of the fans  3   a  and  3   b , the fan  3   a  for sucking air from the outside of the electronic equipment  50  will be referred to as a front-stage fan and the fan  3   b  for distributing the air which has been sucked using the front-stage fan  3   a  into the electronic equipment  50  will be referred to as a rear-stage fan. 
         [0076]    The electronic equipment  50  also includes a fan control device  1  and a fan power source section  2  installed outside of the air passage  51 . The fan power source section  2  is a power source for supplying power to motors not shown which are built in the fans  3   a  and  3   b . That is, when the power is supplied from the fan power source section  2 , the motors of the fans  3   a  and  3   b  rotate and blades attached to the motors rotate in cooperation with the rotation of the motors, and hence the fans  3   a  and  3   b  generate air currents flowing toward the heating elements  52   a  and  52   b.    
         [0077]    The fan control device  1  detects the temperatures of the heating elements  52   a  and  52   b  and the temperature (the environmental temperature) outside of the electronic equipment  50  using temperature sensors  11   a  to  11   c . The fan control device  1  then controls the revolution numbers of the fans  3   a  and  3   b  so as to obtain an air flow rate needed for cooling the heating elements  52   a  and  52   b  in a state in which the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance on the basis of the temperatures detected using the temperature sensors  11   a  to  11   c . Next, a specific configuration of the fan control device  1  described above will be illustrated.  FIG. 3  is a block diagram illustrating a configuration of the fan control device according to the embodiment 2. 
         [0078]    As illustrated in  FIG. 3 , the fan control device  1  includes the temperature sensors  11   a  to  11   c , temperature check sections  12   a  and  12   b , revolution number detecting sections  13   a  and  13   b , a revolution number error check section  14  and pulse generators  15   a  and  15   b . The fan control device  1  also includes a RAM  16 , a ROM  17  and a processor  18 . 
         [0079]    The temperature sensors  11   a  and  11   b  are attached to the heating elements  52   a  and  52   b  and detect the temperatures of the heating elements  52   a  and  52   b . The temperature sensor  11 C is disposed outside of the electronic equipment  50  and detects the environmental temperature around the front-stage fan  3   a . The temperature sensors  12   a  and  12   b  check to see to which extent the temperatures of the heating elements  52   a  and  52   b  detected using the temperature sensors  11   a  and  11   b  are varied from a target temperature and notify the processor  18  of results of check in the form of predetermined variables. 
         [0080]    The revolution number detecting sections  13   a  and  13   b  are, for example, pulse counters and respectively detect the revolution numbers of the fans  3   a  and  3   b . The revolution number error check section  14  checks to see whether the fans  3   a  and  3   b  normally rotate on the basis of the revolution numbers of the fans  3   a  and  3   b  detected using the revolution number detecting sections  13   a  and  13   b  and notifies the processor  18  of a result of check. 
         [0081]    The pulse generators  15   a  and  15   b  input pulses used to control the revolution numbers of the fans  3   a  and  3   b  in pulse widths (PWM values) as instructed from the processor  18  into the fans  3   a  and  3   b . Specifically, the pulse generator  15   a  inputs a pulse of a PWM value as instructed from the processor  18  into the front-stage fan  3   a . The pulse generator  15   b  inputs a pulse of a PWM value as instructed from the processor  18  into the rear-stage fan  3   b . As a result, the fans  3   a  and  3   b  rotate with the revolution numbers conforming to the pulse widths of the pulses input from the pulse generators  15   a  and  15   b.    
         [0082]    The ROM  17  stores therein various pieces of data needed for processing executed using the processor  18 . The processor  18  determines PWM values of pulses to be input into the front-stage fan  3   a  and the rear-stage fan  3   b  on the basis of the temperatures detected using the temperature sensors  11   a  to  11   c . Next, specific configurations of the processor  18  and the ROM  17  according to the embodiment 2 will be described.  FIG. 4  is a block diagram illustrating the specific configurations of the processor  18  and the ROM  17  according to the embodiment 2. 
         [0083]    As illustrated in  FIG. 4 , the ROM  17  includes an equipment state storage section  171  and a control data storage section  172 . The equipment state storage section  171  stores an equipment state management table. The equipment state management table is a table used to specify the system impedance (the pipeline resistance) corresponding to the current configuration of the electronic equipment  50 .  FIG. 5  illustrates an example of the equipment state management table  61 . 
         [0084]    As illustrated in  FIG. 5 , in equipment state management table  61 , system impedances and one equipment state specification flag are stored such that the flag corresponds to one of the impedances. In the example illustrated in the drawing, the impedance is a pressure loss determined from a density rate at which the respective components of the electronic equipment  50  are packaged and the shape of an air passage in the equipment. In the equipment state management table  61  according to the embodiment 2, a plurality of impedances measured by changing the configuration of the electronic equipment  50  are stored and the equipment state specification flag is set for a system impedance corresponding to the current configuration of the electronic equipment  50 . 
         [0085]    For example, in the equipment state management table  61  illustrated in  FIG. 5 , system impedances A to C respectively measured by changing the configuration of the electronic equipment  50  are stored and the electronic state specification flag is set for the system impedance A. That is, the equipment state management table  61  illustrated in  FIG. 5  indicates that the system impedance corresponding to the current configuration of the electronic equipment  50  is “A”. In the case that the configuration of the electronic equipment has been changed, the equipment state management table  61  is manually updated by a user of the electronic equipment  50 . Owing to the above mentioned operation, even when the configuration of the electronic equipment  50  has been changed, the system impedance corresponding to a fresh configuration of the electronic equipment  50  may be specified. 
         [0086]    The control data storage section  172  stores a heating-element-temperature-based PWM value determination table and an environmental-temperature-based PWM value determination table. The heating-element-temperature-based PWM value determination table is a table used to specify PWM values of pulses to be input into the fans  3   a  and  3   b  on the basis of the temperatures of the heating elements  52   a  and  52   b . The environmental-temperature-based PWM value determination table is a table used to specify PWM values of pulses to be input into the fans  3   a  and  3   b  on the basis of the environmental temperature. Next, these pieces of information stored in the control data storage section  172  will be specifically described.  FIG. 6  is a diagram for explaining information stored in the control data storage section  172  according to the embodiment 2. 
         [0087]    As illustrated in  FIG. 6 , the control data storage section  172  stores heating-element-temperature-based PWM value determination tables  71   a  to  71   c  and environmental-temperature-based PWM value determination tables  72   a  to  72   c  in one-to-one correspondence with the plurality of system impedances A to C. Specifically, the control data storage section  172  stores the heating-element-temperature-based PWM value determination table  71   a  and the environmental-temperature-based PWM value determination table  72   a  corresponding to the system impedance A. The control data storage section  172  also stores the heating-element-temperature-based PWM value determination table  71   b  and the environmental-temperature-based PWM value determination table  72   b  corresponding to the system impedance B. The control data storage section  172  further stores the heating-element-temperature-based PWM value determination table  71   c  and the environmental-temperature-based PWM value determination table  72   c  corresponding to the system impedance C. 
         [0088]      FIG. 7A  illustrates an example of the heating-element-temperature-based PWM value determination table  71   a . As illustrated in  FIG. 7A , in the heating-element-temperature-based PWM value determination table  71   a , heating element temperatures, common revolution numbers, front-stage PWM values and rear-stage PWM values are stored in one-to-one correspondence. For example, as illustrated in  FIG. 7A , in the heating-element-temperature-based PWM value determination table  71   a , the common revolution number “aaa”, the front-stage PWM value “Nfa” and the rear-stage PWM value “Nra” are stored corresponding to the heating element temperature “A”. The heating element temperatures are stored in the form of predetermined variables output from the temperature check sections  12   a  and  12   b  in accordance with the temperatures of the heating elements  52   a  and  52   b.    
         [0089]    The common revolution number (min −1 ) is a revolution number commonly set for the fans  3   a  and  3   b  in the case that an air flow rate needed for cooling the heating elements  52   a  and  52   b  is obtained in a state in which a difference in revolution number between the fans  3   a  and  3   b  is maintained in a predetermined tolerance. In the example illustrated in  FIG. 7A , the needed air flow rate stored in the heating-element-temperature-based PWM value determination tables  71   a  to  71   c  is an air flow rate determined in accordance with temperatures of the heating elements  52   a  and  52   b , a system impedance in the electronic equipment  50  and a total PQ characteristic (a static pressure-air flow rate characteristic) of the fans  3   a  and  3   b  obtained in a state in which the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance. Incidentally, the system impedance, the PQ characteristic and the common revolution number are obtained from thermal hydraulic simulation and measurement performed on the basis of measurement of temperatures of the heating elements  52   a  and  52   b . Details thereof will be described later. 
         [0090]    In this embodiment, the “predetermined tolerance” is a range in which the difference in revolution number between the front-stage fan  3   a  and the rear-stage fan  3   b  is less than 10%. That is, ideally, the noise generated from these fans  3   a  and  3   b  may be minimized by controlling the revolution number of the front-stage fan  3   a  to coincide with that of the rear-stage fan  3   b . However, even in the case that the revolution number of the front-stage fan  3   a  is slightly different from that of the rear-stage fan  3   b , any problem will not cause as long as an error in noise caused by the slight difference of the revolution number of the front-stage fan from that of the rear-stag fan is so small that it is not recognized with human&#39;s ears. In the case that the difference in revolution number between the front-stage fan  3   a  and the rear-stage fan  3   b  is 10%, the noise generated from these fans  3   a  and  3   b  is larger than the noise generated when the revolution numbers of the front-stage blower  3   a  and the rear-stage blower  3   b  are controlled to coincide with each other by about 3 dB. Basically, it is said that the error of 3 dB is not recognized with human&#39;s ears. Therefore, in this embodiment, the “predetermined tolerance” is defined as a range in which the difference in revolution number between the front-stage fan  3   a  and the rear-stage fan  3   b  is less than 10%. 
         [0091]    However, some persons may recognize the error of 3 dB with their ears, so that, more preferably, the difference in revolution number between the front-stage blower  3   a  and the rear-stage blower  3   b  is set to be less than 5%. With the difference in revolution number between the front-stage fan  3   a  and the rear-stage fan  3   b  of less than 5%, the noise generated from the front-stage fan  3   a  and the rear-stage fan  3   b  becomes larger than the noise generated when the revolution numbers of the front-stage fan  3   a  and the rear-stage fan  3   b  are controlled to coincide with each other by about 1 dB. As described above, a noise reduction effect which is equivalent to that attained when the revolution numbers of the front-stage fan  3   a  and the rear-stage fan  3   b  are controlled to coincide with each other may be more obtained by setting the “predetermined tolerance” to a range in which the difference in revolution number between the front-stage fan  3   a  and the rear-stage fan  3   b  is less than 5%. 
         [0092]    The front-stage PWM value (s) is a PWM value of a pulse to be input into the front-stage fan  3   a  in order to rotate the front-stage fan  3   a  with a common revolution number. The rear-stage PWM value (s) is a PWM value of a pulse to be input into the rear-stage fan  3   b  in order to rotate the rear-stage fan  3   b  with the common revolution number. That is, for example, when the heating element temperature is “A”, the pulse of the front-stage PWM value “Nfa” is input into the front-stage fan  3   a  and the pulse of the rear-stage PWM value “Nra” corresponding to the front-stage PWM value “Nfa” is input into the rear-stage fan  3   b , thereby rotating the fans  3   a  and  3   b  with the common revolution number “aaa”. These front-stage and rear-stage PWM values may be obtained from measurement performed in advance. 
         [0093]      FIG. 7B  illustrates an example of the environmental-temperature-based PWM value determination table  72   a . As illustrated in  FIG. 7B , environmental temperatures, common revolution numbers, front-stage PWM values and rear-stage PWM values are stored in the environmental-temperature-based PWM value determination table  72   a  in one-to-one correspondence. For example, as illustrated in  FIG. 7B , the common revolution number “fff”, the front-stage PWM value “Nff” and the rear-stage PWM value “Nrf” are stored in the environmental-temperature-based PWM value determination table  72   a  corresponding to the heating element temperature “F”. In the example illustrated in the drawing, the environmental temperature is a temperature detected using the temperature sensor  11   c.    
         [0094]    The common revolution number is a revolution number which is commonly set for the fans  3   a  and  3   b  in order to obtain an air flow rate needed to cool the heating elements  52   a  and  52   b  in a state in which the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance as in the case in the heating-element-temperature-based PWM value determination tables  71   a  to  71   c . In the example illustrated in the drawing, the needed air flow rate stored in the environmental-temperature-based PWM value determination tables  72   a  to  72   c  is an air flow rate which is obtained in accordance with an environmental temperature, a system impedance in the electronic equipment  50  and a total PQ characteristic of the fans  3   a  and  3   b  obtained when a difference in revolution number between the fans  3   a  and  3   b  is in the predetermined tolerance. 
         [0095]    The front-stage PWM value and the rear-stage PWM value are PWM values of pulses to be input into the front-stage fan  3   a  and the rear-stage fan  3   b  in order to rotate the front-stage fan  3   a  and the rear-stage fan  3   b  with a common revolution number as in the case in the heating-element-based PWM value determination tables  71   a  to  71   c . That is, for example, with the heating element temperature “G”, the pulse of the front-stage PWM value “Nfg” is input into the front-stage fan  3   a  and the pulse of the rear-stage PWM value “Nrg” corresponding to the front-stage PWM value “Nfg” is input into the rear-stage fan  3   b  to rotate the fans  3   a  and  3   b  with the common revolution number “ggg”. 
         [0096]    As described above, the control data storage section  172  corresponds to control data storage means and stores input values of pulses to be input into the fans  3   a  and  3   b  per temperature in order to obtain the air flow rate needed to cool the heating elements in a state in which the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance. In addition, the control data storage section  172  corresponds to control data storage means and stores input values of pulses to be input into the front-stage fan  3   a  and the rear-stage fan  3   b  per system impedance. 
         [0097]    As illustrated in  FIG. 4 , the processor  18  includes an error processing section  181 , a temperature information acquiring section  182 , an input value determining section  183  and database preparing section  184 . 
         [0098]    The error processing section  181  executes an error notifying process on the basis of information acquired from the revolution number error check section  14 . The error processing section  181  acquires a result of check indicating that the front-stage fan  3   a  or the rear-stage fan  3   b  is now in a stopped state from the revolution number error check section  14 . The error processing section  181  then sends an error notification that the fan  3   a  or  3   b  is in the stopped state to the electronic equipment  50  on the basis of the result of check acquired. As a result, an error message that the fan  3   a  or  3   b  is now in the stopped state is displayed on a display not illustrated of the electronic equipment  50 . 
         [0099]    The temperature information acquiring section  182  acquires predetermined variables corresponding to temperatures of the heating elements  52   a  and  52   b  from the temperature check sections  12   a  and  12   b  as data indicative of the heating element temperatures. The temperature information acquiring section  182  also acquires the environmental temperature from the temperature sensor  12   c . As described above, the temperature sensors  11   a  and  11   b , the temperature check sections  12   a  and  12   b  and the temperature information acquiring section  182  functions as an example of temperature detecting means for detecting the temperatures of the heating elements  52   a  and  52   b  installed in the electronic equipment  50 . The temperature sensor  11   c  and the temperature information acquiring section  182  also function as an example of temperature detecting means for detecting the temperature outside of the electronic equipment  50 . 
         [0100]    The input value determining section  183  specifies a system impedance corresponding to the current configuration of the electronic equipment  50  with reference to data stored in the equipment state storage section  171 . In addition, the input value determining section  183  acquires the respective PWM values of pulses to be input into the fans  3   a  and  3   b  from the control data storage section  172  on the basis of the specified system impedance and the heating element temperatures or the environmental temperature acquired from the temperature information acquiring section  182 . 
         [0101]    Specifically, the input value determining section  183  acquires a front-stage PWM value and a rear-stage PWM value corresponding to a combination of the specified system impedance with temperature information of the heating elements  52   a  and  52   b  acquired from the temperature information acquiring section  182  from the heating-element-temperature-based PWM value determination tables  71   a  to  71   c . In addition, the input value determining section  183  acquires a front-stage PWM value and a rear-stage PWM value corresponding to a combination of the specified system impedance with environmental temperature information acquired from the temperature information acquiring section  182  from the environmental-temperature-based PWM value determination tables  72   a  to  72   c.    
         [0102]    For example, in the case that the specified system impedance is “A” and the heating element temperature acquired from the temperature information acquiring section  182  is “C”, the input value determining section  183  acquires the front-stage PWM value “Nfc” and the rear-stage PWM value “Nrc” from the heating-element-temperature-based PWM value determination table  71   a  as illustrated in  FIG. 7A . 
         [0103]    The input value determining section  183  then instructs the pulse generator  15   a  to input the pulse of the acquired front-stage PWM value into the front-stage blower  3   a  and instructs the pulse generator  15   b  to input the pulse of the acquired rear-stage PWM value into the rear-stage blower  3   b . In response to the instructions, the pulse generators  15   a  and  15   b  respectively input the pulses of the front-stage PWM value and rear-stage PWM value as instructed from the input value determining section  183  into the front-stage fan  3   a  and the rear-stage fan  3   b . As a result, the front-stage fan  3   a  rotates with the revolution number conforming to the front-stage PWM value of the pulse input from the pulse generator  15   a  and the rear-stage fan  3   b  rotates with the revolution number conforming to the rear-stage PWM value of the pulse input from the pulse generator  15   b.    
         [0104]    Incidentally, as described above, the front-stage PWM value and the rear-stage PWM value are PWM values of pulses to be input into the fans  3   a  and  3   b  in order to obtain the needed air flow rate in a state in which the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance. Therefore, the difference in revolution number between the fans  3   a  and  3   b  is set in the predetermined tolerance by inputting pulses of the above mentioned front-stage and rear-stage PWM values into the respective fans  3   a  and  3   b  and air is fed into the equipment at the needed air flow rate conforming to the temperatures of the heating elements  52   a  and  52   b.    
         [0105]    As described above, in the fan control device  1  according to this embodiment, since the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance, that is, the revolution numbers of the fans  3   a  and  3   b  are controlled to coincide with each other in the predetermined tolerance, such a situation may be avoided that surplus noise is generated from a fan of the largest revolution number to increase the total noise from the fans  3   a  and  3   b.    
         [0106]    In addition, the needed air flow rate according to this embodiment is determined by taking the temperatures of the heating elements  52   a  and  52   b , the system impedance within the electronic equipment  50  and the total PQ characteristic of the fans  3   a  and  3   b  obtained when the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance into consideration. Therefore, according to this embodiment, the air flow rate suitable for cooling the heating elements  52   a  and  52   b  may be obtained in a state in which the revolution numbers of the fans  3   a  and  3   b  are controlled to coincide with each other in the predetermined tolerance regardless of the configuration of the electronic equipment  50  concerned and the locations of the fans  3   a  and  3   b  in the electronic equipment  50 . 
         [0107]    Incidentally, the input value determining section  183  determines different PWM values for the front-stage fan  3   a  and the rear-stage fan  3   b  and inputs pulses of the determined PWM values into the fans  3   a  and  3   b  via the pulse generators  15   a  and  15   b . That is, as illustrated the graph  1001  in  FIG. 8 , different amounts of energy  1010  are supplied to the front-stage fan  3   a  and the rear-stage fan  3   b . Specifically, the work amount of the rear-stage fan  3   b  is reduced influenced by the air currents generated from the front-stage fan  3   a , so that the amount of energy supplied to the rear-stage fan  3   b  becomes smaller than that supplied to the front-stage fan  3   a  in the case that the revolution numbers of the fans  3   a  and  3   b  are controlled to coincide with each other. 
         [0108]    In addition, in the case that the common revolution number obtained on the basis of the environmental temperature is larger than the common revolution number obtained on the basis of the heating element temperatures, the input value determining section  183  controls the revolutions numbers of the fans  3   a  and  3   b  by using the environmental-temperature-based PWM values. Specifically, the input value determining section  183  acquires common revolution numbers corresponding to the environmental temperature and the heating element temperatures acquired from the temperature information acquiring section respectively from the environmental-temperature-based PWM determination tables  72   a  to  72   c  and the heating-element-temperature-based PWM value determination tables  71   a  to  71   c . The input value determining section  183  then compares a common revolution number obtained from the environmental temperature with a common revolution number obtained from the heating element temperatures, and in the case that the common revolution number obtained from the environmental temperature is larger than that obtained from the heating element temperatures, acquires the front-stage PWM value and the rear-stage PWM value corresponding to the environmental temperature concerned from the environmental-temperature-based PWM value determination tables  72   a  to  72   c . As described above, the revolution numbers of the fans  3   a  and  3   b  are controlled on the basis of one of the environmental temperature and the heating element temperatures which will influence the heating elements  52   a  and  52   b  more greatly than another, so that the heating elements  52   a  and  52   b  may be maintained at more appropriate temperatures. 
         [0109]    As described above, the input value determining section  183  and the equipment state storage section  171  function as an example of equipment state acquiring means for acquiring information on the system impedance within the electronic equipment  50 . In addition, the input value determining section  183  and the pulse generators  15   a  and  15   b  function as an example of control means for controlling the revolution number of each fan on the basis of the temperature information acquired using the temperature information acquiring section  182  such that the air flow rate of air for cooling the heating elements  52   a  and  52   b  which is determined in accordance with the temperatures of the heating elements  52   a  and  52   b , the system impedance within the electronic equipment  50  and the total static pressure-air flow rate characteristic of the fans  3   a  and  3   b  attained when the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance may be obtained and the difference in revolution number between the fans  3   a  and  3   b  may be set in the predetermined tolerance. 
         [0110]    The database preparing section  184  executes various calculating processes in order to prepare the heating-element-temperature-based PWM value determination tables  71   a  to  781   c  and the environmental-temperature-based PWM value determination tables  72   a  to  72   c  before the fan control device is incorporated into the electronic equipment  50 . Next, a specific configuration of the database preparing section  184  will be described.  FIG. 9  is a block diagram illustrating a specific configuration of the database preparing section according to the embodiment 2. 
         [0111]    As illustrated in  FIG. 9 , the database preparing section  184  includes a measured (already measured) system impedance information storage section  81 , an equipment configuration information storage section  82 , a system impedance information storage section  83 , a temperature information storage section  84  and a temperature standard value information storage section  85 . The database preparing section  184  also includes a needed air flow rate information storage section  86 , a measured (already measured) PQ characteristic information storage section  87 , a PQ characteristic information and revolution number information storage section  88  and a PWM value information storage section  89 . The database preparing section  184  further includes a system impedance calculating section  91 , a needed air flow rate calculating section  92 , a fan PQ performance calculating section  93  and a fan PWM value calculating section  94 . 
         [0112]    The measured system impedance information storage section  81  stores measured system impedances of the electronic equipment  50 . For example, the measured system impedance information storage section  81  stores system impedances corresponding to full configurations and popular configurations of the electronic equipment  50 . The equipment configuration information storage section  82  stores a plurality of patterns of configurations of the electronic equipment  50 . For example, the equipment configuration information storage section  82  stores information on the number of CPUs and the number of power sources which are built into the electronic equipment  50  per configuration of the electronic equipment  50 . The system impedance information storage section  83  stores system impedances which are calculated per configuration of the electronic equipment  50  using the system impedance calculating section  91 . 
         [0113]    The temperature information storage section  84  stores the environmental temperature and the temperatures of the heating elements  5   a  and  52   b . For example, the temperature information storage section  84  stores the temperatures of the CPUs and memories built into the electronic equipment as the temperatures of the heating element  52   a  and  52   b . The temperature standard value information storage section  85  stores standard values of temperatures of the heating elements  52   a  and  52   b . The needed air flow rate information storage section  86  stores the needed air flow rates calculated using the needed air flow rate calculating section  92 . 
         [0114]    The measured PQ characteristic information storage section  87  stores the information on the maximum and minimum revolution numbers of the fans  3   a  and  3   b . The PQ characteristic information and revolution number information storage section  88  stores the total PQ characteristic of the fans  3   a  and  3   b  and the common revolution numbers of the fans  3   a  and  3   b  calculated using the fan PQ performance calculating section  93 . The PWM value information storage section  89  stores the front-stage PWM values and rear-stage PWM values calculated using the fan PWM value calculating section  94 . 
         [0115]    The system impedance calculating section  91  calculates each system impedance of each configuration of the electronic equipment  50  on the basis of the information on the measured system impedances stored in the measured system impedance information storage section  81  and the information on the configurations of the electronic equipment  50  stored in the equipment configuration information storage section  82 . The needed air flow rate calculating section  92  calculates the needed air flow rates on the basis of the temperatures stored in the temperature information storage section  84  and the temperature standard values stored in the temperature standard value information storage section  85 . 
         [0116]    The fan PQ performance calculating section  93  calculates the total PQ characteristic of the fans  3   a  and  3   b  and the common revolution numbers of the fans  3   a  and  3   b . The calculation is performed on the basis of information on the system impedances stored in the system impedance information storage section  83 , the needed air flow rates stored in the needed air flow rate information storage section  86  and the maximum and minimum revolution numbers of the fans  3   a  and  3   b  stored in the measured PQ characteristic information storage section  87 . The fan PWM value calculating section  94  calculates the front-stage PWM values and rear-stage PWM values. The calculation is performed on the basis of information on the maximum and minimum revolution numbers of the fans  3   a  and  3   b  stored in the measured PQ characteristic information storage section  87  and the total PQ characteristic of the fans  3   a  and  3   b  and the common revolution numbers of the fans  3   a  and  3   b  stored in the PQ characteristic information and revolution number information storage section  88 . 
         [0117]    Next, procedures of preparing the heating-element-temperature-based PWM value determination tables  71   a  to  71   c  and the environmental-temperature-based PWM value determination tables  72   a  to  72   c  will be described.  FIG. 10  is a flowchart illustrating an example of procedures of preparing the heating-element-temperature-based PWM value determination tables  71   a  to  71   c  and the environmental-temperature-based PWM value determination tables  72   a  to  72   c.    
         [0118]    As illustrated in  FIG. 10 , as work to be previously performed in preparation of the heating-element-temperature-based PWM value determination tables  71   a  to  71   c  and the environmental-temperature-based PWM value determination tables  72   a  to  72   c , first, measurement and simulation are performed (step S 101 ). Specifically, system impedances corresponding to the full and popular configurations of the electronic equipment, and maximum and minimum revolution numbers, air flow rates, static pressures and noise levels of the fans  3   a  and  3   b  are obtained by the measurement and simulation. 
         [0119]    Next, various calculations are performed on the basis of the information obtained at step S 101  using the database preparing section  184  (step S 102 ). Specifically, each system impedance of each equipment configuration and each needed air flow rate, each common revolution number, each front-stage PWM value and each rear-stage PWM value at each temperature are calculated (step  102 ). 
         [0120]    Then, each system impedance calculated for each equipment configuration and each needed air flow rate, each common revolution number, each front-stage PWM value and each rear-stage PWM value which are calculated at each of the environmental temperature and the heating element temperatures are arranged in the form of databases and stored in the ROM  17  (step S 103 ). Owing to the above mentioned operations, the heating-element-temperature-based PWM value determination tables  71   a  to  71   c  and the environmental-temperature-based PWM value determination tables  72   a  to  72   c  are stored in the ROM  17 . The fan control device  1  is then incorporated into the electronic equipment  50  in a state in which the heating-element-temperature-based PWM value determination tables  71   a  to  71   c  and the environmental-temperature-based PWM value determination tables  72   a  to  72   c  are stored in the ROM  17 . 
         [0121]    Next, specific operations of the processor  18  according to this embodiment will be described.  FIG. 11  is a flowchart illustrating an example of procedures of processing executed using the processor according to the embodiment 2. Incidentally, only a procedure involving revolution number control of the front-stage fan  3   a  and the rear-stage fan  3   b  of the procedures of processing executed using the processor  18  is illustrated in  FIG. 11 . 
         [0122]    As illustrated in  FIG. 11 , first, the input value determining section  183  acquires an equipment state (step S 201 ). Specifically, the input value determining section  183  specifies a system impedance to which an equipment state specification flag is set as the system impedance corresponding to the current configuration of the electronic equipment  50  with reference to data stored in the equipment state storage section  171 . 
         [0123]    Next, the input value determining section  183  selects a database corresponding to the specified system impedance (step S 202 ). Specifically, the input value determining section  183  selects one heating-element-temperature-based PWM value determination table from within the heating-element-temperature-based PWM value determination tables  71   a  to  71   c  and one environmental-temperature-based PWM value determination table from within the environmental-temperature-based PWM value determination tables  72   a  to  72   c.    
         [0124]    Next, the processor  18  executes temperature measurement (step S 203 ). Specifically, the temperature information acquiring section  182  acquires predetermined variables corresponding to temperatures of the heating elements  52   a  and  52   b  from the temperature check sections  12   a  and  12   b  as heating element temperatures and acquires an environmental temperature from the temperature sensor  12   c . The temperature information acquiring section  182  then notifies the input value determining section  183  of the acquired heating element and environmental temperatures. 
         [0125]    Next, the input value determining section  183  selects a common revolution number corresponding to the heating element temperatures acquired at step S 203  with reference to the heating-element-temperature-based PWM determination table (step S 204 ). The input value determining section  183  also selects a common revolution number corresponding to the environmental temperature acquired at step S 203  with reference to the environmental-temperature-based PWM value determination table (step S 205 ). 
         [0126]    Next, the input value determining section  183  judges whether the heating-element-temperature-based common revolution number is larger than the environmental-temperature-based common revolution number (step S 206 ). In the above mentioned process, in the case that it has been judged that the common revolution number obtained from the heating element temperature is larger than the common revolution number obtained from the environmental temperature (Yes at step S 206 ), the input value determining section  183  determines the PWM values of pulses to be input into the fans  3   a  and  3   b  using the heating-element-temperature-based PWM value determination table  71 . 
         [0127]    Specifically, the input value determining section  183  determines the front-stage PWM value corresponding to the common revolution number selected at step S 204  with reference to the heating-element-temperature-based PWM value determination table  71  (step S 207 ). The input value determining section  183  then determines the rear-stage PWM value corresponding to the front-stage PWM value with reference to the heating-element-temperature-based PWM value determination table  71  (step S 208 ). For example, in the case that the current system impedance of the electronic equipment  50  is “A” and the heating element temperature acquired using the temperature information acquiring section  182  is “C”, the input value determining section  183  selects the common revolution number “ccc” with reference to the heating-element-temperature-based PWM value determination table  71   a  illustrated in  FIG. 7A . Then, the input value determining section  183  determines the front-stage PWM value “Nfc” and the rear-stage PWM value “Nrc” corresponding to the common revolution number “ccc” as the PWM values of the pulses to be input into the fans  3   a  and  3   b.    
         [0128]    On the other hand, at step S 204 , it has been judged that the heating-element-temperature-based common revolution number is not larger than the environmental-temperature-based common revolution number (No at step S 206 ); the input value determining section  183  determines the PWM values of pulses to be input into the fans  3   a  and  3   b  by using the heating-element-temperature-based PWM value determination table  71 . 
         [0129]    Specifically, the input value determining section  183  determines a front-stage PWM value corresponding to the common revolution number selected at step S 205  with reference to the environmental-temperature-based PWM value determination table  72  (step S 209 ). The input value determining section  183  determines a rear-stage PWM value corresponding to the front-stage PWM value with reference to the heating-element-temperature-based PWM value determination table  71  (step S 210 ). For example, in the case that the current system impedance of the electronic equipment  50  is “A” and the environmental temperature acquired using the temperature information acquiring section  182  is “G”, the input value determining section  183  selects the common revolution number “ggg” with reference to the environmental-temperature-based PWM value determination table  72   a  as illustrated in  FIG. 7B . The input value determining section  183  then determines the front-stage PWM value “Nfg” and the rear-stage PWM value “Nrg” corresponding to the common revolution number “ggg” as the PWM values of pulses to be input into the fans  3   a  and  3   b.    
         [0130]    At the completion of execution of the process at step S 208  or step S 210 , the input value determining section  183  instructs to rotate the fans (step S 211 ). That is, the input value determining section  183  instructs the pulse generators  15   a  and  15   b  to input pulses of the front-stage and rear-stage PWM values determined at steps S 207  and S 208  or at steps S 209  and S 210  respectively into the front-stage fan  3   a  and the rear-stage fan  3   b . In response to the instruction, the pulse generator  15   a  inputs the pulse of the above front-stage PWM value into the front-stage fan  3   a  as instructed from the input value determining section  183 . Likewise, the pulse generator  15   b  inputs the pulse of the above rear-stage PWM value into the rear-stage fan  3   b  as instructed from the input value determining section  183 . As a result, the front-stage fan  3   a  and the rear-stage fan  3   b  rotate with the revolution numbers corresponding to the front-stage PWM value and the rear-state PWM value which have been input from the pulse generators  15   a  and  15   b.    
         [0131]    Incidentally, as described above, the front-stage PWM value and the rear-stage PWM value are PWM values of pulses to be input into the fans  3   a  and  3   b  in order to obtain the needed air flow rate in a state in which the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance. Therefore, the fans  3   a  and  3   b  rotate in a state in which the revolution numbers of the fans coincide with each other in the predetermined tolerance by inputting the pulses of the above front-stage and rear-stage PWM values respectively into the fans  3   a  and  3   b  and air is fed into the heating elements  52   a  and  52   b  at the needed air flow rates conforming to the temperatures of the heating elements  52   a  and  52   b.    
         [0132]      FIG. 12  is a diagram illustrating an effect of the embodiment. In  FIG. 12 , a left part is a graph indicative of changes in static pressure, revolution number and noise relative to a change in air flow rate obtained when the PWM values of the pulses applied to the front-stage fan  3   a  and the rear-stage fan  3   b  are made the same as each other at a predetermined temperature. 
         [0133]    As illustrated in the graph, in the case that pulses of the same PWM value have been applied to the fans  3   a  and  3   b , if it is intended to obtain a needed air flow rate  3003  at which the total aerodynamic performance  3001  of the fans  3   a  and  3   b  intersects the system impedance  3002 , the revolution number of the rear-stage fan  3   b  (the rear-stage revolution number  3004 B) will become larger than the revolution number of the front-stage fan  3   a  (the front-stage revolution number  3004 A). The reason therefore lies in the fact that the work amount with which the rear-stage fan  3   b  blows off air is reduced under the influence of the air currents generated from the front-stage fan  3   a  to increase the revolution number of the rear-stage fan  3   b . The noise  3005  is also increased with increasing the revolution number of the rear-stage fan  3   b.    
         [0134]    On the other hand, in  FIG. 12 , a right part is a graph indicative of changes in static pressure, revolution number and noise relative to a change in air flow rate obtained in the case that the fan control device  1  according to this embodiment has controlled such that the difference in revolution number ( 4004 A,  4004 B) between the front-stage fan  3   a  and the rear-stage fan  3   b  is maintained in the predetermined tolerance at a predetermined temperature. As illustrated in this graph, according to this embodiment, it may become possible to obtain the needed air flow rate  4003  at which the system impedance  4002  within the electronic equipment  50  intersects the total aerodynamic characteristic  4001  (the PQ characteristic) of the fans  3   a  and  3   b  when the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance in a state in which the difference in revolution number between the fans is set in the predetermined tolerance at the predetermined temperature. 
         [0135]    Therefore, according to this embodiment, such a situation may be avoided that the revolution number of any one of the fans becomes larger than that of another fan to generate surplus noise. In addition, according to this embodiment, the air flow rate suited to cool the heating elements  52   a  and  52   b  may be obtained in a state in which the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance regardless of the configuration of the electronic equipment  50  and the locations of the fans  3   a  and  3   b  within the electronic equipment  3   a  and  3   b.    
         [0136]    Next, results of experiments performed to validate that the noise generated from the plurality of fans  3   a  and  3   b  is reduced by using the fan control device  1  according to this embodiment will be described. Respective measurement conditions  3010  are illustrated in  FIG. 13 .  FIG. 13  illustrates measurement conditions of the experiments for validation. As illustrated in  FIG. 13 , a case in which two fans of the same type are disposed in series is assumed to be a measurement condition A and a case in which two fans of the same type are disposed in series and four fans of the same type are disposed in parallel is assumed to be a measurement condition B. 
         [0137]    Under the measurement condition A, experiments were performed under three conditions of conditions A- 1  to A- 3 . Specifically, under the condition A- 1 , voltages of the same amount were supplied to the front-stage and rear-stage fans. Under the condition A- 2 , the same air flow rate as that under the condition A- 1  was set and voltages were controlled such that the ratio of the front-stage fan revolution number to the rear-stage fan revolution number is 1:1. The condition A- 2  corresponds to a condition under which the same control as that performed using the fan control device  1  according to this embodiment is performed. Under the condition A- 3 , the same air flow rate at that under the condition A- 1  was set and voltages were controlled such that the ratio of the front-stage fan revolution number to the rear-stage fan revolution number is 1:0.9. 
         [0138]    In addition, as illustrated in  FIG. 13 , under the measurement condition B, experiments were performed under four conditions of conditions B- 1 , B- 2 , B- 3  and B- 4 . Specifically, under the condition B- 1 , voltages of the same amount were supplied to the front-stage and rear-stage fans. Under the condition B- 2 , the same air flow rate as that under the condition B- 1  was set and voltages were controlled such that the ratio of the front-stage fan evolution number to the rear-stage fan revolution number is 1:1. The condition B- 2  corresponds to a condition under which the same control as that performed using the fan control device  1  according to this embodiment is performed. 
         [0139]    Under the condition B- 3 , voltages of the same amount which is different from that of the voltages supplied under the condition B- 1  were supplied to the front-stage and rear-stage fans. Under the condition B- 4 , the same air flow rate as that under the condition B- 3  was set and voltages were controlled such that the ratio of the front-stage fan revolution number to the rear-stage fan revolution number is 1:1. The condition B- 4  corresponds to a condition under which the same control as that performed using the fan control device  1  according to this embodiment is performed. 
         [0140]    First, results of measurements performed under the measurement condition A will be described.  FIG. 14A  illustrates data indicative of the result of measurement of the front-stage fan revolution number  5001 A, the rear-stage fan revolution number  5001 B and the device noise  5002  performed under the measurement condition A.  FIG. 14B  illustrates data indicative of the result of measurement of the front-stage fan revolution number  6001 A, the rear-stage fan revolution number  6001 B and the consumption power  6002  performed under the measurement condition A.  FIG. 14C  illustrates data indicative of the result of frequency analysis of the device noise performed under the measurement condition A.  FIG. 15A  is a diagram  3020 A illustrating a result of measurement performed under the condition A- 1 ,  FIG. 15B  is a diagram  3020 B illustrating a result of measurement performed under the condition A- 2  and  FIG. 15C  is a diagram  3020 C illustrating a result of measurement performed under the condition A- 3 . 
         [0141]    As illustrated in  FIGS. 14A and 15A , under the condition A- 1 , the front-stage revolution number  5001 A was 12257 min −1  and the rear-stage revolution number  5001 B was 13523 min −1 , indicating that in the case that voltages of the same amount have been supplied to the front-stage and rear-stage fans, the rear-stage fan revolution number  5001 B is increased influenced by air currents generated from the front-stage fan. As illustrated in  FIGS. 14A and 15C , under the condition A- 3 , the front-stage revolution number was 13643 min −1  and the rear-stage revolution number was 12277 min −1 , indicating that the front-stage fan revolution number measured under the condition A- 3  is larger than the rear-stage fan revolution number measured under the condition A- 1 . On the other hand, as illustrated in  FIGS. 14B and 15B , under the condition A- 2 , the front-stage revolution number was 12615 min −1  and the rear-stage revolution number was 12555 min −1 , indicating that these revolution numbers are smaller than the rear-stage revolution number measured under the condition A- 1  and the front-stage revolution number measured under the condition A- 3 . 
         [0142]    In addition, as illustrated in  FIG. 14A  and  FIGS. 15A to 15C , the device noise  5002  measured under the condition A- 1  was 55.5 dB (A), the device noise  5002  measured under the condition A- 2  was 53.4 dB(A) and the device noise  5002  measured under the condition A- 3  was 56 dB(A). That is, the device noise was the lowest when measured under the condition A- 2  corresponding to a control system of the fan control device  1  according to this embodiment and was reduced from the value measured under the condition A- 1  by about 2 dB(A). On the other hand, the highest device noise  5002  was measured under the condition A- 3 . It is thought that the reason therefore lies in the fact that the front-stage revolution number  5001 A measured under the condition A- 3  exhibited the highest value of the values measured under the respective conditions A- 1  to A- 3 . 
         [0143]    As described above, the noise generated from each fan may be reduced by controlling so as to attain a needed air flow rate in a state in which the revolution numbers of the front-stage and rear-stage fans ( 5001 A,  5001 B) are controlled to coincide with each other in the predetermined tolerance as in the case with the fan control device  1  according to this embodiment. Incidentally, the device noise  5002  under each of the conditions A- 1  to A- 3  is of a value calculated on the basis of the result of analysis of the frequency of the device noise illustrated in  FIG. 14C . 
         [0144]    In addition, as illustrated in  FIG. 14B  and  FIGS. 15A to 15C , the total consumption power  5003  of the front-stage and rear-stage fans was 7.20 W under the condition A- 1 , was 6.78 W under the condition A- 2  and was 7.65 W under the condition A- 3 . That is, as in the case of the device noise  5002  the total consumption power  5003  was also the lowest when measured under the condition A- 2  corresponding to that of the control system of the fan control device  1  according this embodiment and was reduced from the value measured under the condition A- 1  by about 6%. As described above, the consumption power  5003  of each fan may be reduced by controlling so as to attain the needed air flow rate in a state in which the revolution numbers of the front-stage and rear-stage fans ( 5001 A,  5001 B) are controlled to coincide with each other in the predetermined tolerance as in the case with the fan control device  1  according to this embodiment. 
         [0145]    Next, results of measurements performed under the measurement condition B will be described. First, the results of measurements performed under the conditions B- 1  and B- 2  will be described.  FIG. 16A  illustrates data indicative of the result of measurement of the front-stage average revolution number  7001 A, the rear-stage average revolution number  7002 A and the device noise  7003  performed under the conditions B- 1  and B- 2 .  FIG. 16B  illustrates data indicative of the result of measurement of the front-stage average revolution number  7001 A, the rear-stage average revolution number  7001 B and the consumption power  7003  performed under the conditions B- 1  and B- 2 .  FIG. 16C  illustrates data indicative of the result of frequency analysis of the device noise  7002  performed under the conditions B- 1  and B- 2 .  FIG. 17A  is a diagram  3030  illustrating the result of measurement performed under the condition B- 1  and  FIG. 17B  is a diagram  3040  illustrating the result of measurement performed under the condition B- 2 . In the examples illustrated in the drawings, the front-stage average revolution number  7001 A is the average value of revolution numbers of four front-stage fans and the rear-stage average revolution number  7001 B is the average value of revolution numbers of four rear-stage fans. 
         [0146]    As illustrated in  FIGS. 16A and 17A , the front-stage average revolution number  7001 A was 14488 min −1  and the rear-stage average revolution number  7001 B was 15548 min −1  under the condition B- 1 . These values indicate that in the case that the voltages of the same amount have been supplied to the front-stage fan and the rear-stage fan as in the case under the condition A- 1 , the revolution number of the rear-stage fan  7001 B is increased influenced by the air currents generated from the front-stage fan. On the other hand, as illustrated in  FIGS. 16A and 17B , the front-stage average revolution number  7001 A was 14825 min −1  and the rear-stage average revolution number  7001 B was 14859 min −1  under the condition B- 2 . These revolution numbers  7001 A,  7001 B are smaller than the rear-stage average revolution number measured under the condition B- 1 . 
         [0147]    As illustrated in  FIG. 16A  and  FIGS. 17A and 17B , the device noise  7002  under the condition B- 1  was 62.6 dB(A) and the device noise  7002  under the condition B- 2  was 61.6 dB(A). That is, as in the case under the measurement condition A, the device noise  7002  measured under the condition B- 2  which is the same as that of the fan control device  1  according to this embodiment was lower than that measured under the condition B- 1  and was reduced from the value measured under the condition B- 1  by about 1 db(A). 
         [0148]    As described above, even when the front-stage fans and the rear-stage fans are disposed in series and in parallel, an effect equivalent to that attained when the front-stage fan and the rear-stage fan are disposed only in series is obtained. Incidentally, the values of the device noise illustrated in  FIG. 16A  were calculated on the basis of the result of analysis of the frequency of the device noise  7002  illustrated in  FIG. 16C . 
         [0149]    In addition, as illustrated in  FIG. 16B  and  FIGS. 17A and 17B , the total consumption power  7003  of the front-stage and rear-stage fans was 43.08 W under the condition B- 1  and was 41.83 W under the condition B- 2 . That is, the total consumption power  7003  was the lowest when measured under the condition B- 2  corresponding to that of the fan control device  1  according to this embodiment and was reduced from the value measured under the condition B- 1  by about 3%. 
         [0150]    Next, results of measurements performed under the conditions B- 3  and B- 4  will be described.  FIG. 18A  illustrates data indicative of the result of measurement of the front-stage average revolution number  8001 A, the rear-stage average revolution number  8001 B and the device noise  8002  performed under the conditions B- 3  and B- 4 ,  FIG. 18B  illustrates data indicative of the result of measurement of the front-stage average revolution number  8001 A, the rear-stage average revolution number  8001 B and the consumption power  8003  performed under the conditions  18 - 3  and  18 - 4  and  FIG. 18C  illustrates data indicative of the result of analysis of the frequency of the device noise  8002  performed under the conditions B- 3  and B- 4 .  FIG. 19A  is a diagram  3050  illustrating the result of measurement performed under the condition B- 3  and  FIG. 19B  is a diagram  3060  illustrating the result of measurement performed under the condition B- 3 . 
         [0151]    As illustrated in  FIGS. 18A and 19A , under the condition B- 3 , the front-stage average revolution number  8001 A was 12576 min −1  and the rear-stage average revolution number was 13354 min −1 . This result is the same as those attained under the conditions A- 1  and B- 1 . On the other hand, as illustrated in  FIGS. 18A and 19B , under the condition B- 4 , the front-stage average revolution number was 12826 min −1  and the rear-stage average revolution number  8001 B was 12809 min −1 . That is, these values are smaller than the value of the rear-stage average revolution number  8001 B measured under the condition B- 3 . 
         [0152]    As illustrated in  FIG. 18A  and  FIGS. 19A and 19B , the device noise  8002  under the condition B- 3  was 58.7 db(A) and the device noise  8002  under the condition B- 4  was 57.7 dB(A). That is, the device noise  8002  measured under the condition B- 4  which is the same as that of the fan control device  1  according to this embodiment was lower than that measured under the condition B- 3  and was reduced from the value measured under the condition B- 3  by about 1 db(A) as in the case in the relation between the conditions B- 1  and B- 2 . 
         [0153]    As described above, it has been found that even when the air flow rates of air generated from each front-stage fan and each rear-stage fan are varied, the same result may be obtained by changing the amount of voltages applied in the case that the front-stage fans and the rear-stage fans are disposed in series and in parallel. Incidentally, the values of the device noise illustrated in  FIG. 18A  are calculated on the basis of the result of analysis of the frequency of the device noise illustrated in  FIG. 18C . 
         [0154]    As illustrated in  FIG. 18B  and  FIGS. 19A and 19B , the total consumption power  8003  of the front-stage and rear-stage fans was 27.80 W under the condition B- 3  and was 26.87 W under the condition B- 4 . That is, the total consumption power  8003  was also the lowest when measured under the condition B- 4  which is the same as that of the fan control device  1  according to this embodiment as in the case in the relation between the conditions B- 1  and B- 2  and was reduced from the value measured under the condition B- 3  by about 3%. 
         [0155]    As described above, according to the embodiment 2, the air flow rate of air cooling the heating elements  52   a  and  52   b  which is determined from the temperatures of the heating elements  52   a  and  52   b , the system impedance within the electronic equipment  50  and the total PQ characteristic of the fans  3   a  and  3   b  attained in a state in which the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance is obtained. In addition, the revolution number of each of the fans  3   a  and  3   b  is controlled on the basis of the temperatures of the heating elements and the environmental temperature such that the difference in revolution number between the fans  3   a  and  3   b  is set in the predetermined tolerance. That is, in the embodiment 2, the revolution number of each of the fans  3   a  and  3   b  is controlled on the basis of the heating element temperatures and the environmental temperature such that the needed air flow rate which is needed for cooling the heating elements  52   a  and  52   b  and is determined from the temperatures of the heating elements  52   a  and  52   b , the system impedance within the electronic equipment  50  and the total PQ characteristic of the fans  3   a  and  3   b  obtained in a state in which the revolution numbers of the fans  3   a  and  3   b  are controlled to coincide with each other in the predetermined tolerance may be obtained in a state in which the revolution numbers of the fans  3   a  and  3   b  coincide with each other in the predetermined tolerance. 
         [0156]    That is, according to this embodiment, such a situation that the total noise generated from the fans  3   a  and  3   b  owing to the surplus noise generated from the fan of the largest revolution number may be avoided by maintaining the difference in revolution number between the fans  3   a  and  3   b  in the predetermined tolerance. In addition, in this embodiment, the needed air flow rate may be obtained by taking the temperatures of the heating elements  52   a  and  52   b , the system impedance within the electronic equipment  50  and the total PQ characteristic of the fans  3   a  and  3   b  obtained in a state in which the difference in evolution number between the fans  3   a  and  3   b  is in the predetermined tolerance into consideration. Therefore, according to this embodiment, the air flow rate suited to cool the heating elements  52   a  and  52   b  may be obtained in a state in which the difference in revolution number between the fans  3   a  and  3   b  is maintained in the predetermined tolerance regardless of the configuration of the electronic equipment  50  concerned and the locations of the respective fans  3   a  and  3   b  within the electronic equipment  50 . 
         [0157]    As described above, according to this embodiment, the noise generated from the plurality of fans  3   a  and  3   b  may be reduced while attaining the air flow rate needed for cooling the heating elements  52 A and  52 B within the electronic equipment  50  using the plurality of fans  3   a  and  3   b.    
         [0158]    In addition, according to this embodiment, the revolution number of the rear-stage fan  3   b  is reduced more remarkably than that attained when the energy of the same amount is supplied to the both fans  3   a  and  3   b  and hence the service life of the rear-stage fan  3   b  may be increased. Further, according to this embodiment, addition of components is not needed and hence the above mentioned effects may be realized at a cost equivalent to an ever attained cost. 
         [0159]    Incidentally, a fan control program is stored in the ROM  17  of the fan control device  1  and functions as mentioned above are implemented by executing this fan control program using the processor  18 . Next, an example of a computer for executing the fan control program will be illustrated.  FIG. 20  is a functional block diagram illustrating a computer for executing the fan control program. 
         [0160]    As illustrated in  FIG. 20 , a computer  600  serving as the fan control device  1  includes an HDD  610 , a RAM  620 , a ROM  630 , a CPU  60  and a bus  650  for connecting these elements with one another. The ROM  630  corresponds to the ROM  17  illustrated in  FIG. 2  and stores in advance the fan control program. The fan control program includes a temperature detection program  631  and a control program  632 . 
         [0161]    Then, the CPU  640  reads the temperature detection program  631  and the control program  632  out of the ROM  630  and executes these programs. As a result of execution of these programs, the programs  631  and  632  come to function respectively as a temperature detection process  641  and a control process  642 . As described above, the CPU  640  corresponds to the processor  18  in  FIG. 2 . 
         [0162]    The ROM  630  includes the equipment state storage section  171  and the control data storage section  172  and stores the equipment state management table  61 , the heating-element-temperature-based PWM value determination tables  71   a  to  71   c  and the environmental-temperature-based PWM value determination tables  72   a  to  72   c  in these storage sections. The CPU  640  reads these tables stored in the ROM  630  and stores these tables in the RAM  620  such that the processes  641  and  642  execute various processes utilizing the data stored in the RAM  620 . 
       Embodiment 3 
       [0163]    Although, in the above mentioned embodiment 2, the evolution numbers of the fans  3   a  and  3   b  detected using the revolution number detecting sections  13   a  and  13   b  are used only for revolution number error check, the revolution numbers of the fans  3   a  and  3   b  may be utilized as feedback information used to set the difference in revolution number between the fans  3   a  and  3   b  in the predetermined tolerance. Next, an embodiment involving a case as mentioned above will be described.  FIG. 21  is a block diagram illustrating a configuration of a fan control device according to an embodiment 3. Incidentally, the same numerals are assigned to parts having the same configurations as those previously described and description thereof will be omitted. 
         [0164]    As illustrated in  FIG. 21 , in a fan control device  1 ′ according to this embodiment, a processor  18 ′ acquires information on the revolution numbers of the fans  3   a  and  3   b  from the revolution number detecting sections  13   a  and  13   b . Next, specific configurations of the processor  18 ′ and a ROM  17 ′ according to this embodiment will be described herein below.  FIG. 22  is a block diagram illustrating specific configurations of the processor  18 ′ and the ROM  17 ′ according to the embodiment 3. 
         [0165]    As illustrated in  FIG. 22 , the processor  18 ′ according to this embodiment includes the error processing section  181 , the temperature information acquiring section  182 , an input value determining section  183 ′, the database preparing section  184  and a revolution number information acquiring section  185 . The revolution number information acquiring section  185  acquires information on the revolution numbers of the front-stage fan  3   a  and the rear-stage fan  3   b  respectively from the revolution number detecting sections  13   a  and  13   b . As described above, the revolution number information acquiring section  185  and the revolution number detecting section  13   a  function as an example of front-stage revolution number detecting means for detecting the revolution number of the front-stage fan  3   a  and the revolution number information acquiring section  185  and the revolution number detecting section  13   a  functions as an example of rear-stage revolution number detecting means for detecting the revolution number of the rear-stage fan  3   b.    
         [0166]    The input value detecting section  183 ′ then performs feedback control on the revolution number of the rear-stage fan  3   b  on the basis of information acquired from the revolution number information acquiring section  185 . Specifically, in the case that a difference in revolution number between the front-stage fan  3   a  and the rear-stage fan  3   b  is out of a predetermined tolerance, the input value determining section  183 ′ changes the revolution number of the rear-stage fan such that the difference in revolution number between the front-stage fan  3   a  and the rear-stage fan  3   b  is set in the predetermined tolerance. Processing as mentioned above is executed with reference to a rear-stage PWM value change table stored in a control data storage section  172 ′. 
         [0167]    The control data storage section  172 ′ according to this embodiment stores a PWM value for the rear-stage fan  3   b  obtained when a difference in evolution number between the front-stage fan  3   a  and the rear-stage fan  3  is reduced to have a value in the predetermined tolerance per revolution number. Next, information stored in the control data storage section  172 ′ will be described.  FIG. 23  is a diagram illustrating information stored in the control data storage section according to the embodiment 3. 
         [0168]    As illustrated in  FIG. 23 , rear-stage PWM value change tables  73   a  to  73   c  corresponding to the system impedances A to C are stored in the control data storage section  172 ′ according to this embodiment in addition to the heating-element-temperature-based PWM value determination tables  71   a  to  71   c  and the environmental-temperature-based PWM value determination tables  72   a  to  72   c .  FIG. 24  illustrates an example of the rear-stage PWM change table  73   a.    
         [0169]    As illustrated in  FIG. 24 , in the rear-stage OWM value change table  73   a , each front-stage revolution number indicative of the revolution number of each front-stag fan  3   a  is stored corresponding to each rear-stage PWM value. For example, in the rear-stage PWM value change table  73   a , the front-stage revolution number “jjj” is stored corresponding to the rear-stage PWM value “Nrj”. 
         [0170]    The rear-stage PWM value stored in the rear-stage PWM value change table  3   a  is a PWM value needed to control the revolution number of the rear-stage fan  3   b  to have a difference relative to the evolution number of the front-stage fan  3   a  in a predetermined tolerance. That is, for example, in the case that the revolution number “kkk” of the front-stage fan  3   a  has been acquired from the revolution number information acquiring section  185 , the input value determining section  183 ′ determines the rear-stage PWM value “Nrk” corresponding to the acquired revolution number with reference to the rear-stage PWM value change table  73   a . The input value determining section  183 ′ then inputs the pulse of the determined rear-stage PWM value “Nrk” into the rear-stage fan  3   b  via the pulse generator  15   b . Owing to the above mentioned operations, even when the revolution number of the front-stage fan  3   a  differs from the revolution number of the rear-stage fan  3   b , the fan control device  1 ′ will be capable of performing correction such that the difference in revolution number between these fans is set in the predetermined tolerance. 
         [0171]    Next, specific operations of the processor  18 ′ according to this embodiment will be described.  FIG. 25  is a flowchart illustrating an example of processing executed using the processor  18 ′ according to the embodiment 3. Incidentally, in  FIG. 25 , in procedures of processing executed using the processor  18 ′, only a procedure of processing involving control of the revolution numbers of the front-stage and rear-stage fans  3   a  and  3   b  will be described. Processes executed at steps S 301  to S 311  in  FIG. 25  are the same as the processes executed at steps S 201  to S 211  in  FIG. 11  and hence description thereof will be omitted. 
         [0172]    As illustrated in  FIG. 25 , after an instruction to rotate the fans  3   a  and  3   b  has been given at step S 311 , the input value determining section  183 ′ measures the revolution numbers of the fans (step S 312 ). Specifically, the input value determining section  183 ′ acquires information on the revolution numbers of the front-stage and rear-stage fans  3   a  and  3   b  from the revolution number information acquiring section  185 . 
         [0173]    Next, the input value determining section  183 ′ judges whether a difference in revolution number between the front-stage and rear-stage fans is in a predetermined tolerance (step S 313 ). In the case that it has been judged that the difference in revolution number between the fans is in the predetermined tolerance in execution of the above mentioned process (Yes at step S 313 ), the input value determining section  183 ′ shifts the process to step S 312 . 
         [0174]    On the other hand, in the case that the difference in revolution number between the fans is not in the predetermined tolerance (No at step S 313 ). The input value determining section  183 ′ changes the rear-stage PWM value (step S 314 ). Specifically, the input value determining section  183 ′ determines, on the basis of information on the revolution number of the front-stage fan  3   a  acquired from the revolution number acquiring section  185 , the rear-state PWM value corresponding to the acquired revolution number of the front-stage fan  3   a  with reference to the rear-stage PWM value change table. The input value determining section  183 ′ then gives an instruction to rotate the fans (step S 315 ). Specifically, the input value determining section  183 ′ instructs the pulse generator  15   b  to input the pulse of the rear-stage PWM value which has been determined at step S 312  into the rear-stage fan  3   b . Owing to the above mentioned operation, the pulse of the rear-stag PWM value conforming to the revolution number of the front-stage fan  3   a  is input into the rear-stage fan  3   b  and the revolution numbers of the front-stage fan  3   a  and the rear-stage fan  3   b  come into coincidence with each other in the predetermined tolerance. 
         [0175]    As described above, in the embodiment 3, in the case that the difference in revolution number between the front-stage fan  3   a  and the rear-stage fan  3   b  is out of the predetermined tolerance, the revolution number of the rear-stage fan is changed such that the difference in revolution numbers between the fans is set in the predetermined tolerance. Owing to the above mentioned operation, it may become possible to more set the difference in revolution number between the front-stage fan  3   a  and the rear-stage fan  3   b  in the predetermined tolerance. 
         [0176]    Although several embodiments haven been described in detail with reference to the accompanying drawings, these embodiments are merely examples and the embodiments may be modified in other forms which are altered and improved in a variety of ways on the basis of skills of persons in the art including the embodying aspects as described above. 
         [0177]    For example, although in the above mentioned embodiments, the database preparing section  184  is used only in the case that heating-element-temperature-based PWM value determination tables  71   a  to  71   b  and the environmental-temperature-based PWM value determination tables  72   a  to  72   c  are prepared, the revolution numbers of the fans  3   a  and  3   b  may be controlled using the database preparing section  184 . 
         [0178]    Specifically, first, the database preparing section  184  acquires the temperatures of the heating elements or the environmental temperature from the temperature information acquiring section  182 . Then, the database preparing section  184  calculates a front-stage PWM value and a rear-stage PWM value of pulses to be input into the respective fans  3   a  and  3   b  in the case that an air flow rate needed for cooling the heating elements  52   a  and  52   b  is obtained in a state in which a difference in revolution number between the fans  3   a  and  3   b  is in a predetermined tolerance on the basis of the acquired temperature(s). Then, the database preparing section  184  notifies the input value determining section  183  (or the input value determining section  183 ′) of the calculated front-stage PWM value and rear-stage PWM value. 
         [0179]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.