Patent Publication Number: US-11043345-B2

Title: Load driving device and load driving method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-020593 filed on Feb. 8, 2018, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a load driving device having a relay and a load driving method. 
     Description of the Related Art 
     Japanese Laid-Open Patent Publication No. 2012-070484 discloses an on/off control device that obtains, as data for diagnosing lifetime of an opening/closing device that controls on and off of a load, an accumulated operating time, an accumulated conduction time, and an accumulated number of opening/closing operations of the opening/closing device. 
     SUMMARY OF THE INVENTION 
     The degree of degradation of relays (opening/closing devices) varies depending on the magnitude of the electric current conducted when the contact is in a closed state. Accordingly, the technique of Japanese Laid-Open Patent Publication No. 2012-070484 is unable to estimate the degree of degradation of a contact of a device whose conduction current differs in each operation, such as a dynamic brake circuit etc., and the accuracy of estimation of the time for which the relay will remain usable is low. 
     The present invention has been made to solve the problem above, and an object of the present invention is to provide a load driving device and a load driving method that can accurately estimate the degree of degradation of a relay at the time of measurement and the time during which the relay will remain usable after that time. 
     According to an aspect of the present invention, a load driving device having a relay includes a driving portion configured to drive the relay so that a contact of the relay enters a closed state or an open state; a driving control portion configured to output to the driving portion a closing command for bringing the contact to the closed state, and also to output to the driving portion alternately the closing command and an opening command for bringing the contact to the open state until the current runs through the contact; a number-of-retries obtaining portion configured to obtain, as a number of retries, a number of times that the driving control portion repeated the opening command and the closing command until the current runs through the contact after the driving control portion has outputted the closing command to the driving portion; and a usable period estimating portion configured to estimate a usable period of the relay in accordance with the number of retries. 
     According to another aspect of the present invention, in a load driving method by a load driving device having a relay, the load driving device includes a driving portion configured to drive the relay so that a contact of the relay enters a closed state or an open state, and the load driving method includes a driving control step of outputting to the driving portion a closing command for bringing the contact to the closed state, and also outputting to the driving portion alternately the closing command and an opening command for bringing the contact to the open state until the current runs through the contact; a number-of-retries obtaining step of obtaining, as a number of retries, a number of times that the opening command and the closing command were repeated until the current runs through the contact after the closing command has been outputted to the driving portion at the driving control step; and a usable period estimating step of estimating a usable period of the relay in accordance with the number of retries. 
     According to the present invention, it is possible to accurately estimate a time for which the relay will remain usable after the present time. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings, in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the configuration of a motor driving device; 
         FIG. 2A  is a block diagram showing a functional configuration of a first control section, and  FIG. 2B  is a block diagram showing a functional configuration of a second control section; 
         FIG. 3  is a flowchart showing a process of estimating a time for which a first relay and a second relay will remain usable, which is performed in the first control section and the second control section; 
         FIG. 4  is a flowchart showing the process of estimating the time for which the first relay and the second relay will remain usable, which is performed in the first control section and the second control section; 
         FIG. 5  is graph schematically showing the degree of degradation of a contact of the first relay and the second relay with respect to the number of times that the contact is opened and closed; and 
         FIG. 6  is a time chart schematically showing the degree of degradation of the contact at the present time and conditions of progress of degradation of the contact depending on the magnitude of conduction current. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 
     [Configuration of Motor Driving Device] 
       FIG. 1  is a block diagram showing the configuration of a motor driving device  10 . The motor driving device  10  controls electric power supplied from a power supply  12  to a motor  14 . 
     The motor driving device  10  includes a switching circuit  16 , a main power-supply circuit  18 , and a dynamic brake circuit  20 . The switching circuit  16  is PWM-controlled by a motor control device (not shown) for controlling the motor  14 , so as to control the power supplied to the motor  14 . The motor driving device  10  constitutes a load driving device  80 , and the motor  14  constitutes a load  82 . 
     The main power-supply circuit  18  includes the power supply  12 , a first relay  22 , a first driving portion  24 , and a first control section  26 . The first relay  22  is provided between the power supply  12  and the switching circuit  16 , and switches the state of the circuitry connecting the power supply  12  and the switching circuit  16  between a conducting state and a non-conducting state. The first relay  22  includes a contact  28  and a coil  30 . The contact  28  establishes the conducting state between the power supply  12  and the switching circuit  16  when the contact  28  closes, and establishes the non-conducting state between the power supply  12  and the switching circuit  16  when the contact  28  opens. When a current is passed to the coil  30 , the coil  30  goes in an excited state, and the magnetic force of the coil  30  closes the contact  28 . On the other hand, when the current passed to the coil  30  is stopped, the coil  30  goes in a non-excited state and the contact  28  is opened by an urging force of an urging member (not shown). 
     The first driving portion  24  passes a current to the coil  30  so as to place the coil  30  in the excited state, and stops the current passed to the coil  30  so as to place the coil  30  in the non-excited state. The first driving portion  24  is controlled by the first control section  26  to control the passage of current to the coil  30 . 
     When the coil  30  goes in the excited state and the contact  28  closes, then the power supply  12  and the switching circuit  16  are connected. The switching circuit  16  controls the power supplied to the motor  14 . The motor  14  is thus driven. 
     The dynamic brake circuit  20  includes a resistance  32 , a second relay  34 , a second driving portion  36 , and a second control section  38 . The second relay  34  is provided between the motor  14  and the resistance  32  and switches the state of the circuitry connecting the motor  14  and the resistance  32  between a conducting state and a non-conducting state. The second relay  34  includes a contact  40  and a coil  42 . The contact  40  establishes the conducting state between the motor  14  and the resistance  32  when the contact  40  closes, and establishes the non-conducting state between the motor  14  and the resistance  32  when the contact  40  opens. When a current is passed to the coil  42 , the coil  42  goes in an excited state, and the magnetic force of the coil  42  closes the contact  40 . On the other hand, when the current passed to the coil  42  is stopped, the coil  42  goes in a non-excited state and the contact  40  is opened by an urging force of an urging member (not shown). 
     The second driving portion  36  passes a current to the coil  42  so as to place the coil  42  in the excited state, and stops the current passed to the coil  42  so as to place the coil  42  in the non-excited state. The second driving portion  36  is controlled by the second control section  38  to control the passage of current to the coil  42 . 
     When the coil  42  goes in the excited state and the contact  40  closes, then the motor  14  and the resistance  32  are connected. The switching circuit  16  controls the power supplied to the motor  14 . The motor  14  is thus driven. 
     When the brakes are applied to the motor  14 , the supply of power from the switching circuit  16  to the motor  14  is stopped, and the coil  42  is placed in the excited state so as to close the contact  40 , thereby connecting the motor  14  and the resistance  32 . Rotational resistance is thus caused to the motor  14  and a braking force is applied to the motor  14 . 
     A notice portion  44  gives notice to an operator by displaying an image, letters, etc. on a screen. The notice portion  44  may be configured to give notice to the operator by sound or voice. The notice portion  44  gives notice to the operator on the basis of a control by the first control section  26  and the second control section  38 . 
     An ammeter  46  is provided between the switching circuit  16  and the motor  14 . The ammeter  46  detects a conduction current flowing through the contact  28  of the first relay  22  and a conduction current flowing through the contact  40  of the second relay  34 . 
     [Configuration of First Control Section] 
       FIG. 2A  is a block diagram showing a functional configuration of the first control section  26 . The first control section  26  includes a driving control portion  48 , a number-of-retries obtaining portion  50 , a response time obtaining portion  52 , a conduction current obtaining portion  54 , a conduction current integrating portion  56 , a usable period estimating portion  58 , and a notice control portion  60 . 
     When supplying power to the motor  14 , the driving control portion  48  outputs a closing command to the first driving portion  24 . Based on the closing command, the first driving portion  24  passes a current to the coil  30 , so as to close the contact  28 . When not supplying power to the motor  14 , the driving control portion  48  outputs an opening command to the first driving portion  24 . Based on the opening command, the first driving portion  24  stops the current to the coil  30 , so as to open the contact  28 . When the ammeter  46  does not detect conduction current (when the current does not run through the contact  28 ) even after a certain time period has elapsed after the output of the closing command to the first driving portion  24 , the driving control portion  48  repeats alternate output of the closing command and opening command to the first driving portion  24  until conduction current is detected (until the current runs through the contact  28 ). 
     The number-of-retries obtaining portion  50  counts the number of times that the opening command and the closing command are repeated (hereinafter referred to as the number of retries) after the driving control portion  48  first outputted the closing command in order for the current to run through the contact  28 , and obtains the number. The number of retries is counted by regarding a set of opening and closing commands as one time. 
     The response time obtaining portion  52  measures the time length from the instant the driving control portion  48  outputs the closing command to the first driving portion  24  to the instant the ammeter  46  detects conduction current (until the current runs through the contact  28 ) (hereinafter referred to as a response time). Since the response time is the time between the instant the driving control portion  48  outputs the closing command and the instant the current runs through the contact  28 , the response time is not measured when the current does not run through the contact  28  even after the driving control portion  48  has outputted the closing command. 
     The conduction current obtaining portion  54  obtains the conduction current detected by the ammeter  46 . The conduction current integrating portion  56  integrates the conduction currents that have flowed through the contact  28  from the start of use of the first relay  22  until the present time, and stores the integrated value (which will be hereinafter referred to as a conduction current integrated value). 
     The usable period estimating portion  58  estimates a period for which the first relay  22  will stay usable after the present time (hereinafter referred to as a usable period). The usable period is estimated based on the number of retries, the conduction current, the response time, and the conduction current integrated value. The process of estimating the usable period will be described in detail later. 
     The notice control portion  60  controls the notice portion  44  so as to display an image, letters, or generate sound to give notice of the estimated usable period to an operator when the conduction current integrated value has become equal to or more than a set current value, when the response time has become equal to or more than a set time, or when the number of retries has become equal to or more than a set number of times. 
     [Configuration of Second Control Section] 
       FIG. 2B  is a block diagram showing a functional configuration of the second control section  38 . The second control section  38  includes a driving control portion  62 , a number-of-retries obtaining portion  64 , a response time obtaining portion  66 , a conduction current obtaining portion  68 , a conduction current integrating portion  70 , a usable period estimating portion  72 , and a notice control portion  74 . 
     When connecting the resistance  32  to the motor  14 , the driving control portion  62  outputs a closing command to the second driving portion  36 . Based on the closing command, the second driving portion  36  passes a current to the coil  42 , so as to close the contact  40 . When not connecting the resistance  32  to the motor  14 , the driving control portion  62  outputs an opening command to the second driving portion  36 . Based on the opening command, the second driving portion  36  stops the current to the coil  42 , so as to open the contact  40 . When the ammeter  46  does not detect conduction current (when the current does not run through the contact  40 ) even after a certain time period has elapsed after the output of the closing command to the second driving portion  36 , the driving control portion  62  repeats alternate output of the closing command and opening command to the second driving portion  36  until conduction current is detected (until the current runs through the contact  40 ). 
     The number-of-retries obtaining portion  64  counts the number of times that the opening command and the closing command are repeated (hereinafter referred to as the number of retries) after the driving control portion  62  first outputted the closing command in order for the current to run through the contact  40 , and obtains the number. The number of retries is counted by regarding a set of opening and closing commands as one time. 
     The response time obtaining portion  66  measures the response time from the instant the driving control portion  62  outputs the closing command to the second driving portion  36  to the instant the ammeter  46  detects conduction current (until the current runs through the contact  40 ). Since the response time is the time between the instant the driving control portion  62  outputs the closing command and the instant the current runs through the contact  40 , the response time is not measured when the current does not run through the contact  40  even after the driving control portion  62  has outputted the closing command. 
     The conduction current obtaining portion  68  obtains the conduction current detected by the ammeter  46 . The conduction current integrating portion  70  integrates the conduction currents that have flowed through the contact  40  from the start of use of the second relay  34  until the present time, and stores the integrated value (which will be hereinafter referred to as a conduction current integrated value). 
     The usable period estimating portion  72  estimates a period for which the second relay  34  will stay usable from the present time (hereinafter referred to as a usable period). The usable period is estimated based on the number of retries, the conduction current, the response time, and the conduction current integrated value. The process of estimating the usable period will be described in detail later. 
     The notice control portion  74  controls the notice portion  44  so as to display an image, letters, or to generate sound to give notice of the estimated usable period to the operator when the conduction current integrated value has become equal to or more than a set current value, when the response time has become equal to or more than a set time, or when the number of retries has become equal to or more than a set number of times. 
     [Usable Period Estimating Process] 
       FIGS. 3 and 4  constitute a flowchart illustrating the flow of a usable period estimating process concerning the first relay  22  and the second relay  34  that is performed in the first control section  26  and the second control section  38 . The usable period estimating process concerning the first relay  22  performed in the first control section  26  is substantially the same as the usable period estimating process concerning the second relay  34  performed in the second control section  38  except that the target of the usable period estimation differs. 
     At step S 1 , the driving control portion  48  (driving control portion  62 ) outputs the closing command to the first driving portion  24  (second driving portion  36 ) and the process proceeds to step S 2 . At step S 2 , the response time obtaining portion  52  (response time obtaining portion  66 ) starts measurement of the response time and the process proceeds to step S 3 . 
     At step S 3 , the driving control portion  48  (driving control portion  62 ) determines whether the current runs through the contact  28  (contact  40 ) before a certain time period passes after the driving control portion  48  (driving control portion  62 ) outputted the closing command to the first driving portion  24  (second driving portion  36 ). When the current runs through the contact  28  (contact  40 ), the process proceeds to step S 7 . When the current does not run through the contact  28  (contact  40 ), the process proceeds to step S 4 . 
     At step S 4 , the response time obtaining portion  52  (response time obtaining portion  66 ) resets the response time and the process proceeds to step S 5 . By resetting the response time at step S 4 , the response time is not measured if the current does not run through the contact  28  (contact  40 ) even after the certain time period has passed after the driving control portion  48  (driving control portion  62 ) outputted the closing command. 
     At step S 5 , the driving control portion  48  (driving control portion  62 ) outputs the opening command to the first driving portion  24  (second driving portion  36 ) and the process proceeds to step S 6 . At step S 6 , the number-of-retries obtaining portion  50  increments the previous value of the number of retries and the process returns to step S 1 . 
     When it is determined at step S 3  that the current is running through the contact  28  (contact  40 ) before the certain time has elapsed after the closing command was outputted to the first driving portion  24  (second driving portion  36 ), the response time obtaining portion  52  (response time obtaining portion  66 ) terminates, at step S 7 , the measurement of the response time and the process proceeds to step S 8 . 
     At step S 8 , the conduction current integrating portion  56  (conduction current integrating portion  70 ) integrates the conduction current flowing through the contact  28  (contact  40 ) and the process proceeds to step S 9 . The conduction current integrated value is obtained by adding the time integral of the conduction current while the current runs through the contact  28  (contact  40 ) to the conduction current integrated value up to the time when the current runs through the contact  28  (contact  40 ) last time. 
     At step S 9 , the usable period estimating portion  58  (usable period estimating portion  72 ) estimates the usable period of the first relay  22  (second relay  34 ) and the process proceeds to step S 10 . The usable period is estimated to be shorter when the number of retries is larger, to be shorter when the conduction current is larger, to be shorter when the response time is longer, and to be shorter when the conduction current integrated value is larger. 
     At step S 10 , the notice control portion  60  (notice control portion  74 ) sets a set current, a set time, and a set number of times that are used in the following steps, and the process proceeds to step S 11 . As the conduction current becomes larger, the set current is set to be smaller, the set time is set to be shorter, and the set number of times is set to be smaller. The set current, the set time, and the set number of times that are set in the notice control portion  60 , and the set current, the set time, and the set number of times that are set in the notice control portion  74 , may be equal to each other, or may be different from each other. 
     At step S 11 , the usable period estimating portion  58  (usable period estimating portion  72 ) determines whether or not the conduction current integrated value is equal to or more than the set current. When the conduction current integrated value is equal to or more than the set current, the process proceeds to step S 15 . When the conduction current integrated value is less than the set current, the process proceeds to step S 12 . 
     At step S 12 , the usable period estimating portion  58  (usable period estimating portion  72 ) determines whether or not the response time is equal to or more than the set time. When the response time is equal to or more than the set time, the process proceeds to step S 15 . When the response time is less than the set time, the process proceeds to step S 13 . 
     At step S 13 , the usable period estimating portion  58  (usable period estimating portion  72 ) determines whether or not the number of retries is equal to or more than the set number of times. When the number of retries is equal to or more than the set number of times, the process proceeds to step S 15 . When the number of retries is less than the set number of times, the process proceeds to step S 14 . 
     At step S 14 , the usable period estimating portion  58  (usable period estimating portion  72 ) determines whether the usable period is less than a predetermined set period. When the usable period is less than the set period, the process proceeds to step S 15 . When the usable period is equal to or more than the set period, the process is terminated. The set period used to make the determination in the usable period estimating portion  58  and the set period used to make the determination in the usable period estimating portion  72  may be equal to each other or may be different from each other. 
     At step S 15 , the usable period estimating portion  58  (usable period estimating portion  72 ) estimates the usable period of the first relay  22  (second relay  34 ), and the notice control portion  60  (notice control portion  74 ) controls the notice portion  44  so as to give notice about the estimated usable period to the operator, and the process is terminated. The process of estimating the usable period of the first relay  22  (second relay  34 ) performed by the usable period estimating portion  58  (usable period estimating portion  72 ) will next be described in detail. 
     [Estimation of Usable Period] 
       FIG. 5  is a graph schematically showing the degree of degradation of the contact  28  (contact  40 ) with respect to the number of opening and closing operations of the contact  28  (contact  40 ) of the first relay  22  (second relay  34 ). Every time the contact  28  (contact  40 ) is opened/closed, the contact surface of the contact  28  (contact  40 ) wears and the contact  28  (contact  40 ) suffers corrosion etc., and thus the degradation of the contact  28  (contact  40 ) progresses. The degradation of the contact  28  (contact  40 ) progresses faster as the conduction current flowing through the contact  28  (contact  40 ) is larger. Accordingly, as shown in  FIG. 5 , even when the number of opening and closing operations of the contact  28  (contact  40 ) is the same, the degree of degradation of the contact  28  (contact  40 ) is higher as the conduction current is larger. That is to say, the degree of degradation of the contact  28  (contact  40 ) cannot be estimated correctly only from the number of opening and closing operations of the contact  28  (contact  40 ). 
     Accordingly, the degree of degradation of the contact  28  (contact  40 ) is estimated on the basis of the number of retries. The contact  28  (contact  40 ) is more likely to cause contact failure as the degree of degradation of the contact  28  (contact  40 ) becomes higher, and then the number of retries tends to become larger. In this way, the degree of degradation of the contact  28  (contact  40 ) can be correctly estimated based on the number of retries. 
     Further, the degree of degradation of the contact  28  (contact  40 ) may be estimated in accordance with the response time in addition to the number of retries. As the degradation of the contact  28  (contact  40 ) progresses, opening/closing of the contact  28  (contact  40 ) takes more time due to corrosion of the contact  28  (contact  40 ). Using the response time in addition to the number of retries improves the accuracy of estimation of the degree of degradation of the contact  28  (contact  40 ). 
     Further, the degree of degradation of the contact  28  (contact  40 ) may be estimated in accordance with the conduction current integrated value in addition to the number of retries. As described above, the degree of degradation of the contact  28  (contact  40 ) becomes higher as the conduction current through the contact  28  (contact  40 ) becomes larger. Using the conduction current integrated value in addition to the number of retries improves the accuracy of estimation of the degree of degradation of the contact  28  (contact  40 ). 
     Further, the usable period of the first relay  22  (second relay  34 ) is estimated based on the estimated degree of degradation and the magnitude of the conduction current that is flowing at present through the contact  28  (contact  40 ).  FIG. 6  is a time chart schematically showing a condition of the degree of degradation of the contact  28  (contact  40 ) at the present time and the progress of degradation of the contact  28  (contact  40 ) which depends on the magnitude of the conduction current. As mentioned above, the progress of degradation of the contact  28  (contact  40 ) is faster if the conduction current flowing through the contact  28  (contact  40 ) is larger. Therefore, as shown in  FIG. 6 , even if the present degradation degree is the same, the degree of degradation of the contact  28  (contact  40 ) reaches a reference value for replacement of the first relay  22  (second relay  34 ) in a shorter period as the conduction current is larger; that is, the usable period of the first relay  22  (second relay  34 ) becomes shorter. The speed of degradation of the contact  28  (contact  40 ) in the future can be estimated from the magnitude of the conduction current flowing through the contact  28  (contact  40 ) at present. The accuracy of estimation of the usable period of the first relay  22  (second relay  34 ) can thus be improved. 
     The usable period of the first relay  22  (second relay  34 ) can be estimated also by using a predetermined speed as the speed of degradation. Further, the usable period of the first relay  22  (second relay  34 ) can be estimated also by using a speed of degradation in the past. Still further, the usable period of the first relay  22  (second relay  34 ) can also be estimated by using an estimated degradation speed that has been obtained by estimating a future degradation speed of the contact  28  (contact  40 ) by using, for example, an average value of the magnitude of conduction current that has flowed through the contact  28  (contact  40 ) in the past. 
     [Functions and Effects] 
     The degree of degradation of the first relay  22  and the second relay  34  varies also depending on the magnitude of conduction current flowing when the contact  28  or the contact  40  is in the closed state. Accordingly, the accuracy of the degree of degradation of the first relay  22  or the second relay  34  was low when it is estimated from, for example, the number of opening and closing operations of the contact  28  or the contact  40 . Especially, for the dynamic brake circuit  20 , the degree of degradation of the contact  40  could not be estimated because the conduction current through the contact  40  of the second relay  34  differs every time the second relay  34  operates. 
     In this embodiment, the usable period estimating portion  58  (usable period estimating portion  72 ) estimates the usable period of the first relay  22  (second relay  34 ) in accordance with the number of retries. The degree of degradation of the contact  28  (contact  40 ) can be highly accurately estimated on the basis of the number of retries, and using the estimated degree of degradation improves the accuracy of estimation of the usable period of the first relay  22  (second relay  34 ). 
     Further, in this embodiment, the usable period estimating portion  58  (usable period estimating portion  72 ) estimates the usable period of the first relay  22  (second relay  34 ) in accordance with the number of retries and the magnitude of the conduction current flowing through the contact  28  (contact  40 ) at present. A future degradation speed of the contact  28  (contact  40 ) can be highly accurately estimated based on the magnitude of the conduction current flowing through the contact  28  (contact  40 ) at present, and the accuracy of estimation of the usable period of the first relay  22  (second relay  34 ) can be improved. 
     Furthermore, in this embodiment, the usable period estimating portion  58  (usable period estimating portion  72 ) estimates the usable period of the first relay  22  (second relay  34 ) in accordance with at least one of the response time and the conduction current integrated value in addition to the number of retries. The degree of degradation of the contact  28  (contact  40 ) can be estimated also from the response time and the conduction current integrated value, and the degree of degradation of the contact  28  (contact  40 ) can be highly accurately estimated by using the response time and the conduction current integrated value in addition to the number of retries. Thus, by using the estimated degree of degradation, the accuracy of estimation of the usable period of the first relay  22  (second relay  34 ) can be improved. 
     Still further, in the embodiment, the notice portion  44  notifies the operator of the estimated usable period of the first relay  22  (second relay  34 ). It is thus possible to give notice such that the operator can recognize the usable period of the first relay  22  (second relay  34 ). 
     Further, in this embodiment, the notice portion  44  notifies the operator of the usable period of the first relay  22  (second relay  34 ) when the number of retries is equal to or more than a set number of times. When the number of retries is equal to or more than the set number of times, or when the response time is equal to or more than the set time, or when the conduction current integrated value is equal to or more than the set current, the degree of degradation of the first relay  22  (second relay  34 ) is high and the usable period of the first relay  22  (second relay  34 ) is short. Therefore, when the usable period of the first relay  22  (second relay  34 ) is long, no notice is given to the operator and the operator is not bothered by frequent notices. When the usable period of the first relay  22  (second relay  34 ) is short, notice is given to the operator so that the operator can recognize the usable period of the first relay  22  (second relay  34 ). 
     Furthermore, in the embodiment, the set number of times, the set time, and the set current, described above, are set in accordance with the magnitude of the conduction current flowing through the contact  28  (contact  40 ) at present. The usable period of the first relay  22  (second relay  34 ) becomes shorter as the magnitude of the conduction current flowing through the contact  28  (contact  40 ) at present becomes larger. Thus, the set number of times, the set time, and the set current can be set in accordance with the usable period of the first relay  22  (second relay  34 ), and hence it is possible to give notice so that the operator can recognize the usable period of the first relay  22  (second relay  34 ) when the usable period of the first relay  22  (second relay  34 ) has become short. 
     [Technical Ideas Obtained from Embodiment] 
     Technical ideas that can be grasped from the above-described embodiment will be described below. 
     A load driving device ( 80 ) having a relay ( 22 ,  34 ) includes: a driving portion ( 24 ,  36 ) configured to drive the relay ( 22 ,  34 ) so that a contact ( 28 ,  40 ) of the relay ( 22 ,  34 ) enters a closed state or an open state; a driving control portion ( 48 ,  62 ) configured to output to the driving portion ( 24 ,  36 ) a closing command for bringing the contact ( 28 ,  40 ) to the closed state, and also to output to the driving portion ( 24 ,  36 ) alternately the closing command and an opening command for bringing the contact ( 28 ,  40 ) to the open state until the current runs through the contact ( 28 ,  40 ); a number-of-retries obtaining portion ( 50 ,  64 ) configured to obtain, as a number of retries, the number of times that the driving control portion ( 48 ,  62 ) repeated the opening command and the closing command until the current runs through the contact ( 28 ,  40 ) after the driving control portion ( 48 ,  62 ) outputted the closing command to the driving portion ( 24 ,  36 ); and a usable period estimating portion ( 58 ,  72 ) configured to estimate a usable period of the relay ( 22 ,  34 ) in accordance with the number of retries. Thus, it is possible to estimate the degree of degradation of the contact ( 28 ,  40 ) of the relay ( 22 ,  34 ) and improve the accuracy of estimation of the usable period. 
     The above-described load driving device ( 80 ) may further include a conduction current obtaining portion ( 54 ,  68 ) configured to obtain a conduction current flowing through the contact ( 28 ,  40 ), and the usable period estimating portion ( 58 ,  72 ) may be configured to estimate the usable period in accordance with the number of retries and the conduction current. It is thus possible to improve the accuracy of estimation of the usable period of the relay ( 22 ,  34 ). 
     The above-described load driving device ( 80 ) may further include a notice portion ( 44 ) configured to notify an operator of the usable period. It is thus possible to give notice such that the operator can recognize the usable period of the relay ( 22 ,  34 ). 
     In the above-described load driving device ( 80 ), the notice portion ( 44 ) may be configured to notify the operator of the usable period when the number of retries is equal to or more than a set number of times. In this way, when the usable period of the relay ( 22 ,  34 ) is long, no notice is given to the operator so that the operator is not bothered by frequent notices. When the usable period of the relay ( 22 ,  34 ) is short, notice is given to the operator so that the operator can recognize the usable period of the relay ( 22 ,  34 ). 
     The above-described load driving device ( 80 ) may further include a conduction current obtaining portion ( 54 ,  68 ) configured to obtain a conduction current at the contact ( 28 ,  40 ), and the notice portion ( 44 ) may be configured to set the set number of times in accordance with the conduction current. It is thus possible to give notice to the operator so that the operator can recognize the usable period of the relay ( 22 ,  34 ) when the usable period of the relay ( 22 ,  34 ) has become short. 
     The above-described load driving device ( 80 ) may further include a response time obtaining portion ( 52 ,  66 ) configured to obtain a response time from when the driving control portion ( 48 ,  62 ) outputs the closing command to the driving portion ( 24 ,  36 ) to when the current runs through the contact ( 28 ,  40 ); a conduction current obtaining portion ( 54 ,  68 ) configured to obtain a conduction current flowing through the contact ( 28 ,  40 ); and a conduction current integrating portion ( 56 ,  70 ) configured to integrate the conduction current to calculate a conduction current integrated value, and the usable period estimating portion ( 58 ,  72 ) may be configured to estimate the usable period in accordance with at least one of the response time, the conduction current, and the conduction current integrated value in addition to the number of retries. It is thus possible to improve the accuracy of estimation of the usable period of the relay ( 22 ,  34 ). 
     The load driving device ( 80 ) may further include a notice portion ( 44 ) configured to notify an operator of the usable period. It is thus possible to give the operator notice such that the operator can recognize the usable period of the relay ( 22 ,  34 ). 
     In the above-described load driving device ( 80 ), the notice portion ( 44 ) may be configured to notify the operator of the usable period when the number of retries is equal to or more than a set number of times, or when the response time is equal to or more than a set time, or when the conduction current integrated value is equal to or more than a set current. In this way, when the usable period of the relay ( 22 ,  34 ) is long, no notice is given to the operator so that the operator is not bothered by frequent notices. When the usable period of the relay ( 22 ,  34 ) is short, notice is given to the operator so that the operator can recognize the usable period of the relay ( 22 ,  34 ). 
     In the above-described load driving device ( 80 ), the notice portion ( 44 ) may be configured to set the set number of times, the set time, or the set current in accordance with the conduction current. It is thus possible to give the operator notice such that the operator can recognize the usable period of the relay ( 22 ,  34 ) when the usable period of the relay ( 22 ,  34 ) has become short. 
     In the above-described load driving device ( 80 ), the notice portion ( 44 ) may be configured to notify the operator of the usable period when the usable period is less than a set period. It is thus possible to give the operator notice such that the operator can recognize the usable period of the relay ( 22 ,  34 ) when the usable period of the relay ( 22 ,  34 ) has become short. 
     In a load driving method for a load driving device ( 80 ) having a relay ( 22 ,  34 ), the load driving device ( 80 ) includes a driving portion ( 24 ,  36 ) configured to drive the relay ( 22 ,  34 ) so that a contact ( 28 ,  40 ) of the relay ( 22 ,  34 ) enters a closed state or an open state, and the load driving method includes: a driving control step of outputting a closing command for bringing the contact ( 28 ,  40 ) to the closed state to the driving portion ( 24 ,  36 ), and also outputting to the driving portion ( 24 ,  36 ) an opening command for bringing the contact ( 28 ,  40 ) to the open state and the closing command in an alternate and repeated manner until the current runs through the contact ( 28 ,  40 ); a number-of-retries obtaining step of obtaining, as a number of retries, the number of times that the opening command and the closing command were repeated until the current runs through the contact ( 28 ,  40 ) after the closing command has been outputted to the driving portion ( 24 ,  36 ) at the driving control step; and a usable period estimating step of estimating a usable period of the relay ( 22 ,  34 ) in accordance with the number of retries. Thus, it is possible to improve the accuracy of estimation of the usable period of the relay ( 22 ,  34 ). 
     The above-described load driving method may further include a conduction current obtaining step of obtaining a conduction current flowing through the contact ( 28 ,  40 ), and the usable period estimating step may estimate the usable period in accordance with the number of retries and the conduction current. It is thus possible to improve the accuracy of estimation of the usable period of the relay ( 22 ,  34 ). 
     In the above-described load driving method, the load driving device ( 80 ) may further include a notice portion ( 44 ) configured to give an operator notice, and the load driving method may further include a notice control step of controlling the notice portion ( 44 ) to notify the operator of the usable period. It is thus possible to give notice such that the operator can recognize the usable period of the relay ( 22 ,  34 ). 
     In the above-described load driving method, the notice control step may control the notice portion ( 44 ) to notify the operator of the usable period when the number of retries is equal to or more than a set number of times. In this way, when the usable period of the relay ( 22 ,  34 ) is long, no notice is given to the operator so that the operator is not bothered by frequent notices. When the usable period of the relay ( 22 ,  34 ) is short, notice is given to the operator so that the operator can recognize the usable period of the relay ( 22 ,  34 ). 
     The above-described load driving method may further include a conduction current obtaining step of obtaining a conduction current of the contact ( 28 ,  40 ), and the notice control step may set the set number of times in accordance with the conduction current. It is thus possible to give notice to the operator so that the operator can recognize the usable period of the relay ( 22 ,  34 ) when the usable period of the relay ( 22 ,  34 ) has become short. 
     The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the gist of the present invention.