Abstract:
An alarm device is adapted to detect accumulation of fatigue of an automotive vehicle driver and to set an alarm time clock to produce an alarm. The detection of accumulation of fatigue is based on selected driving conditions of the vehicle and the period of time during which the detected driving condition is maintained. Fatigue data obtained based on the driving condition and the time is accumulated to determine a correction or updated value of the set time to be compared with the actual driving time. An alarm device will be activated when the driving time reaches the updated time.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a method and device for detecting vehicle driver fatigue in driving and for generating an alarm for the driver to rest. More particularly, the invention relates to a method and device for measuring a period of driving time in which the driver becomes fatigued and for giving an alarm to the driver to indicate that it is time to take a rest. 
     It has been well known that it is recommendable to take a rest every two or two and a half hours driving for refreshing oneself and for recovering from driving fatigue. It is especially necessary for the driver to take a rest in driving a relatively long time. 
     There have been developed and proposed various alarm devices for generating an alarm for resting. For example, published Japanese Utility Model (Tokko Sho) No. 48-15104 shows an alarm device which is associated with a tachograph to produce an alarm at a given time. On the other hand, unexamined Japanese Utility Model Publication (Jikkai Sho) No. 51-156878 shows a device for displaying a required resting period of time depending on a driving period. 
     Since the foregoing devices are adapted to provide the alarm for the vehicle at certain fixed timing intervals, the timing to generate the alarm does not always correspond to the driver&#39;s fatigue. For example, if the driver takes a rest before the fixed time therefor or if a driving condition is significantly varied, the fixed timing for providing the alarm will not correspond to the driver&#39;s fatigue. 
     To improve the above-mentioned defect, unexamined Japanese Utility Model (Tokkai Sho) No. 52-13232 shows another alarm device which counts a clock signal to detect the timing to give the alarm. In this device, the timing is detected by analog processing of the clock signal. Therefore, if the driving time is relatively long, e.g., 3 hours, the analog processing of the clock signal must be continued for a long time and requires a substantially large capacity of an analog arithmetical element. Further, by accumulation of error in measurement, the accuracy of detecting the timing by calculation will be lowered. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a method for effectively and accurately detecting the driver&#39;s fatigue and giving a suggestive alarm for the driver to take a rest at a suitable time. 
     Another object of the present invention is to provide a device for detecting driving time to generate an alarm taking various driving conditions into account and to produce the alarm based also on the detected driving time. 
     The invention is directed toward an alarm device which is adapted to detect the accumulation of fatigue of an automotive vehicle driver and to preset a time for producing the alarm. The detection of accumulation of fatigue will be made based on a driving condition of the vehicle and on the period of time during which the detected driving condition is maintained. Fatigue data obtained based on the driving condition and the time is accumulated to determine a correction value of the preset time for comparison with a driving time. An alarm device is activated when the driving time reaches the preset time. 
     The invention is also directed to a method for giving a suggestive alarm for a fatigued driver of an automotive vehicle. The method essentially comprises the steps of detecting the variation of a driving condition of the vehicle, measuring the accumulation of fatigue of the driver, updating said measured fatigue data whenever the driving condition is varied, correcting a preset alarm time based on said accumulated fatigue data whenever said variation of the driving condition is detected; and producing an alarm when the driving period reaches said corrected preset time. In this manner, the driver may be alerted that he is becoming drowsy or fatigued and take a rest from further driving. 
     The invention may also be characterized as a device for giving a suggestive alarm for a fatigued driver of an automotive vehicle, comprising, first means for sequentially measuring driving time, second means for presetting a period for producing an alarm, said second means comparing said measured time with said preset period to produce the alarm when said measured period reaches said preset time, third means for detecting a variation of a driving condition of the vehicle and producing a signal indicative of the driving condition being detected, fourth means for processing said signal with time data measured by said first means to obtain fatigue data descriptive of the driver, which fourth means includes a memory for storing the obtained fatigue data which is updated whenever there occurs a variation of the driving condition, and fifth means for correcting said preset time of said second means based on the fatigue data in said fourth means whenever variation of the driving condition occurs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken as limitative of the invention but for elucidation and explanation only. 
     In the drawings: 
     FIG. 1 is a block diagram of a first embodiment of an alarm device according to the present invention; 
     FIG. 2 is a block diagram of a driving condition detecting circuit in the alarm device of FIG. 1; 
     FIG. 3 is a timing chart showing an operation of the drive condition detecting circuit of FIG. 2; 
     FIG. 4 is a circuit diagram of the differentiation circuit in the drive condition detecting circuit of FIG. 2; 
     FIG. 5 is a timing chart showing operation of the differentiation circuit of FIG. 4; 
     FIG. 6 is a block diagram of an accumulative calculator in the alarm device in FIG. 1; 
     FIG. 7 is a timing chart showing operation of the accumulative calculator of FIG. 6; 
     FIG. 8 is a block diagram of an alarm timing arithmetic circuit in the alarm device of FIG. 1; 
     FIG. 9 is a timing chart showing operation of the alarm timing arithmetic circuit of FIG. 8; 
     FIG. 10 is a timing chart showing an experimental operation of the alarm device of FIG. 1; 
     FIG. 11 is a block diagram of a second embodiment of the alarm device according to the present invention; and 
     FIG. 12 is a flowchart of an alarm timing calculation program to be processed in the alarm device of FIG. 11. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, particularly to FIG. 1, there is shown the first embodiment of an alarm device according to the present invention. 
     A driving condition detecting circuit 4 is connected to an ignition position switch 1, a brake switch 2 and a speed alarm switch 3. The ignition position switch 1 is associated with an ignition switch (not shown) so that it is turned on to produce an ignition position signal S 1  whenever the ignition switch is turned on. The brake switch 2 is per se well known and produces a braking signal S 2  whenever a foot brake (not shown) is applied. The speed alarm switch 3 is also per se well known and adapted to produce a speed alarm signal S 3  when a driving speed of the vehicle is higher than a preset speed. The driving condition detecting circuit 4 produces various pulse signals S 17 , S 18 , S 19  and S 20  depending on the switch positions of the ignition position switch 1, the brake switch 2 and the speed alarm switch 3, as shown in FIG. 2. The signal S 17  is produced in response to turning on of the ignition switch. The driving condition detector, in turn, produces the signal S 18  in response to turning off of the ignition switch. The signal S 19  is produced in response to either the brake signal S 2  or the speed alarm signal S 3  and the signal 20 is produced in response to turning off of the brake switch 2 or the speed alarm switch 3 with a predetermined delay time. 
     Therefore, the signal S 17  indicates starting of driving of the vehicle, which signal S 17  is thus referred to hereafter as &#34;drive signal&#34;. The signal S 18 , in turn, indicates stopping of driving, which signal S 18  is referred to hereafter as &#34;rest signal&#34;. The signal S 19  represents a driving condition which causes fatigue for the vehicle driver, which signal S 19  is referred to hereafter as &#34;fatigue drive signal&#34; and the signal S 20  indicates ending of the fatigue driving and returning of the driving condition to a normal condition, which signal S 20  is thus referred to hereafter as &#34;normal driving signal&#34;. 
     The driving condition detecting circuit 4 is, in turn, connected to a calculation command generator 5. The calculation command generator 5 produces a calculation command signal S 5  whenever any one of the drive signal S 17 , the resting signal S 18 , the fatigue drive signal S 19  and the normal drive signal S 20  is inputted thereto. The calculation command signal S 5  is fed to a gate circuit 6 which is connected to memories 7 and 8 and a clock 9. The memory 7 stores a time data representative of a time T 0 , which time data T 0  is updated with an absolute time data value T c  fed from the clock 9 whenever the calculation command signal S 5  is inputted to the gate circuit 6. The memory 8 stores an accumulated driving condition data Δt 1  which is updated per one cycle of calculation. The clock 9 may, for example, be an electric vehicle clock adapted to produce a signal S 9  indicative of time data. The gate circuit 6 is responsive to the calculation command S 5   to feed the signals S 7 , S 8  and S 9  of respective memories 7 and 8 and the clock 9 to an accumulative calculator 10. 
     On the other hand, the drive signal S 17 , the fatigue drive signal S 19  and the normal drive signal S 20   are also fed to the accumulative calculator 10. Additionally, the resting signal S 18 , the fatigue drive signal S 19  and the normal drive signal S 20  are also fed to an alarm timing arithmetic circuit 11. The accumulative calculator 10 stores preset coefficients α to be read out in response to respective drive signal S 17 , the fatigue drive signal S 19  and the normal drive signal S 20 . For example, the constants α are -6, 1 and 1.2 respectively corresponding to the rest signal S 18 , the normal drive signal S 20  and the fatigue drive signal S 19 . 
     The accumulative calculator performs a calculation according to an equation: 
     
         (T.sub.c -T.sub.0)×α+Δt.sub.0 =Δt.sub.1 
    
     where T c  is the absolute time data fed from the clock 9 and T 0  is the time of the previous measurement which was stored in memory 7. The accumulative calculator 10 feeds the obtained Δt 1  to the memory 8 to replace the storage thereof with the same. Thus, in the next calculation, the obtained data Δt 1  serves as Δt 0 . The accumulative calculator 10 produces a condition signal S 24  indicative of the obtained accumulative condition data Δt 1  and feeds the condition signal to the alarm timing arithmetic circuit 11. The alarm timing arithmetic circuit 11 also receives the time data T c  from the clock 9 via the gate circuit 6. 
     The alarm timing arithmetic circuit 11 effects an arithmetic operation according to the following equation: 
     
         (3-Δt.sub.1)/β+T.sub.c =T.sub.a 
    
     where 
     β is a constant preset in the alarm timing arithmetic circuit; and 
     3 is a preset time in hours for producing an alarm. 
     The alarm timing arithmetic circuit 11 produces a timing signal S 25  representative of the calculated alarm time data T a  and feeds the same to an alarm signal generator 12. The alarm signal generator 12 is associated with the clock and is preset to a time to produce the alarm by the alarm time data T a  of the timing signal S 25 . When the time T c   becomes the preset alarm time T a , the alarm signal generator 12 produces an alarm signal S 12  and feeds the same to an alarm device 13. 
     It will be appreciated that the alarm device will be any suitable device such as a visible display device for displaying a visible sign or an audible warning system e.g., buzzer, chime warning voice information etc. 
     Referring to FIGS. 2 to 9, there is illustrated a detailed construction of the alarm device of FIG. 1 for detecting driver fatigue. FIG. 2 shows a detailed circuit construction of the driving condition detecting circuits 4. The ignition switch 1 is connected to a pair of differentiation circuit 17 and 18. The differentiation circuit 17 is responsive to the leading edge of the ignition position signal S 1  to produce an output which serves as the drive signal S 17 . On the other hand, the differentiation circuit 18 is responsive to a trailing edge of the ignition position signal S 1  to produce an output which serves as the resting signal S 18 . The brake switch 2 and the speed alarm switch 3 are connected to an OR gate 14. The OR gate 14 is, in turn connected to another OR gate 15. The OR gate 14 is further connected to the OR gate 15 via a retriggerable monostable multivibrator 16. The monostable multivibrator 16 is responsive to the falling edge of OR signal S 14  of the OR gate 14 to turn on for a given period of time T, as shown in FIG. 3. The differentiation circuit 19 is responsive to the leading edge of the OR signal S 15  of the OR gate 15 and the differentiation circuit 20 is responsive to the trailing edge of the OR signal S 15 . Since the OR gate 14 is connected to the brake switch 2 and the speed alarm switch 3 and is maintained at a high level as long as either of brake switch 2 or the speed alarm switch 3 is maintained high, the OR signal S 15  of the OR gate 15 goes high in response to the braking signal S 2  or the speed alarm signal S 3  and is maintained high for the given period T preset in the monostable multivibrator 16 after the OR gate 14 is turned off. Therefore, the differentiation circuit 20 is turned on in response to the trailing edge of the OR signal S 15  which is produced after a given delay T from the turning off of OR gate signal S 14 . 
     The output of the differentiation circuit 19 serves as the fatigue drive signal S 19  and the output of the differentiation circuit 20 serves as the normal drive signal S 20 . 
     FIG. 3 shows the function of the driving condition detecting circuit 4 of FIG. 2. As is apparent herefrom, the differentiation circuit outputs S 17 , S 18 , S 19  and S 20  are pulse signals having substantially short pulse widths. Assuming the ignition switch is turned on at a time T 0 , the ignition position switch 1 turns on to produce the ignition position signal S 1 . In response to the ignition position signal S 1 , the differentiation circuit 17 produces the drive signal S 17  at the time T 0 . In response to the drive signal S 17 , an OR gate of the calculation command generator 5 produces the calculation command S 5 . Then, at a time T 1 , the foot brake is applied to turn the brake switch 2 on. In response to the braking signal S 2 , the differentiation circuit 19 produces the fatigue drive signal S 19 . When the brake is released at the time T 2  the differentiation circuit 20 turns on at the time T 3  with the given delay T from the time T 2  to produce the normal drive signal S 20 . Likewise, in response to the speed alarm signal S 3  produced from the time T 4 , the differentiation circuit 19 becomes operative to produce the fatigue drive signal S 19  and the differentiation circuit 20 becomes operative at the time period T 6  with the delay time T after the period T 5  at which the speed alarm switch 3 is turned off. 
     As appreciated from FIG. 2, the calculation command generator 5 is responsive to any one of the drive signal S 17 , the resting signal S 18 , the fatigue drive signal S 19  and the normal drive signal S 20  to produce the calculation command S 5 . Further, the command signal may be generated periodically at fixed intervals of time, as for example, every 5 seconds, 30 seconds, 60 seconds etc. Such timing signals T c1  may be taken from the clock g using a counter (not shown). 
     As shown in FIG. 4, each of the differentiation circuits 17 and 19 is constituted of a capacitor C 1 , a resistor R 1  and diode D 1 . The resistor R 1  is grounded, and the cathode of the diode D 1  is connected to the capacitor C 1  and the anode thereof is grounded. With this construction, the differentiation circuits 17 or 19 are responsive to the rising edge of the ignition position signal S 1  or the OR signal S 15 , as shown in FIG. 5. On the other hand, each of the differentiation circuits 18 and 20 is constituted by a capacitor C 2 , a resistor R 2  and a diode D 2 . The resistor R 2  is connected to a power source +Vcc and the cathode of the diode D 2  is also connected to the power source +Vcc. The anode of the diode D 2  is connected to the capacitor C 2 . By this construction, the differentiation circuit 18 or 20 is responsive to the falling edge of the drive signal S 1  or the OR signal S 15 , as shown in FIG. 5. 
     However the differentiation circuits 17, 18, 19 and 20 are constructed as set forth and as illustrated in FIG. 4, may each be replaced with a circuit including a pair of monostable multivibrators. 
     FIG. 6 shows the accumulative calculator 10 in detail. To the accumulative calculator 10 are inputted the initial time data T 0  stored in the memory 7, the time data T 1  fed from the clock 9 and the accumulated driving condition data Δt 0 . In the specific construction, a subtractor 21 is connected to the gate circuit 6 to receive therefrom the initial time data T 0  and the time data T 1 . The subtractor 21 performs a subtraction to obtain the time interval (T 1  -T 0 ). The difference obtained by the subtraction is fed to a multiplier 23. To the multiplier 23, one of three constants, i.e., =-6, =1 and =1.2 are inputted from a constant generator 22. A specific one of the preset constants is taken depending upon which one of the three condition signals, i.e., rest signal S 18 , the normal drive signal S 20  and the fatigue drive signal S 19  are inputted thereto. In the multiplier, the obtained time interval (T 1  -T 0 ) is multiplied by the selected constant. The multiplier 23 outputs a signal representative of the product of the multiplying operation to be fed to an adder 24. To the adder 24, the accumulated drive condition data Δt 0   is inputted through the gate circuit 6. In the adder, the result (T 1  -T 0 )×α of the multiplying operation is added to the condition data Δt 0  read from the memory 8. By this, the foregoing equation 
     
         (T.sub.1 -T.sub.0)×α+Δt.sub.0 =Δt.sub.1 
    
     is completed to calculate the condition data value. the adder 24 produces the condition signal S 24  indicative of the result of the foregoing calculation and feeds the condition signal to the alarm timing arithmetic circuit 11 and the memory 8 to update the content therein. FIG. 7 shows experimental values illustrating the variation of the condition data as a function of the foregoing accumulative output of calculator 10. Assuming the condition data accumulated in the memory 8 at the time period T 0  is Δt 0  as illustrated, and the normal drive signals S 20  are produced at the time periods T 0  and T 2 , the fatigue drive signal S 19  is produced at the time period T 1  and the drive signal is produced at the time period T 3 , the condition data value Δt n  is varied as illustrated. The condition data values Δt 1 , Δt 2  and Δt 3  at respective time periods T c  =T 1 , T 2  and T 3  can be obtained from the following equations: 
     
         Δt.sub.1 =(T.sub.1 -T.sub.0)×1+Δt.sub.0  T.sub.1 
    
     
         Δt.sub.2 =(T.sub.2 -T.sub.1)×1.2+Δt.sub.1T.sub.2 
    
     
         Δt.sub.3 =(T.sub.3 -T.sub.2)×(-6)+Δt.sub.2 T.sub.3 
    
     Here, the constants α are selected depending on the driving condition. However, the above specific values have been selected empirically with respect to driving physiology. That is, normally, 30 minutes of rest time is necessary for 3 hours of driving under normal driving condition and for 2 and half hours driving under fatigue driving condition. Therefore, the foregoing specific constant values, i.e., α=-6, 1 and 1.2 will be reasonable. 
     FIG. 8 shows the alarm timing arithmetic circuit 11 in detail. The alarm timing arithmetic circuit 11 comprises a time preset circuit 25, a subtractor 26, a divider 28, a constant generator 27 and an adder 29. The subtractor 26 is connected to the adder 24 of the accumulative calculator 10 to receive therefrom the condition signal S 24 . To the subtractor 26, the time preset circuit 25 is also connected. The time preset circuit 25 produces a preset time signal S 25  representative of the driving time interval requiring the driver to a rest. As set forth, it has been considered that it is recommendable to have a rest after every 3 hours driving. Therefore, the preset time in the time preset circuit 25 is 3 hours in the shown embodiment. The subtractor 26 effects a subtracting operation of (3-Δt 1 ). The subtractor 26 then produces a signal S 26  having a value corresponding to the difference obtained as a result of the subtracting operation. The signal S 26  is fed to the divider 28. The divider 28 also receives from the constant generator 27 a signal S 27 . The signal value of the signal S 27  is valuable depending on the condition signal fed from the driving condition detecting circuit 4. As shown in FIG. 8, since the constant generator 27 is connected to the differentiation circuits 19 and 20, the fatigue drive signal S 19  and the normal drive signal S 20  is inputted to the constant generator 27. The signal S 27  of the constant generator 27 is representative of the constant β for dividing operation in the divider 28. The constant β is presetted in the constant generator 27 and has values 1.2 and 1 respectively corresponding to the fatigue drive signal S 19  and the normal drive signal S 20 . The divider 28 divides the signal value (3-Δt 1 ) of the signal S 26  by the constant of the signal S 27  to produce a signal S 28  having value (3-Δt 1 )/β. The signal S 28  is fed to an adder 29. To the adder, the clock signal S 9  representative of the time T c  is also inputted. The adder 29 adds the time data T c  and the signal value of the signal S 28  to complete the equation for obtaining the set time T a  for the alarm signal generator 12. 
     The alarm signal generator 12 sets the set time data T a . The set time data T a  represents a time to produce the alarm. In the alarm signal generator 12, the set time data T a  is compared with the time data T c . When the time data T c  reaches the value of the set time data T a , the alarm signal generator 12 produces an alarm signal S 12  to activate the alarm device 13. On the other hand, when a rest has been taken, the set time data T a  is reset and in the calculation for obtaining the set time data in response to the next drive signal S 17 , the set time becomes equal to (3-Δt 2 )/β. 
     FIG. 9 shows the function of the alarm timing arithmetic circuit 11 as set forth in relation to the condition signals. Assuming the fatigue drive signal S 19  is produced at the time period T 1  &#39;, the accumulated condition data is Δt 1  &#39;, and the normal drive signal S 20  is produced at the time period T 2  &#39;, the set time data T a  obtained from the foregoing arithmetic operation in the alarm timing arithmetic circuit 11 is in a relation with respect to the predetermined alarm level, i.e., 3 hours, as shown in FIG. 9. If the driver take a rest at the time period T 2  &#39; and the accumulated condition data Δt 3  &#39; when the next drive signal S 17  is inputted, the set time T a  will be obtained from (3-Δt 3  &#39;). 
     FIG. 10 shows the function of the alarm device of the shown embodiment in relation to the condition signals produced by the drive condition detecting circuit 4. At the time period T 1  &#34;, the fatigue drive signal S 19  is produced by application of the brake or vehicle speed exceeding the predetermined speed. In response to the fatigue drive signal S 19 , the calculation command S 5  is produced. In response to the calculation command, the accumulative calculator 10 effects calculation to obtain the condition data Δt 0  &#34;. Based on the condition signal S 10   indicative of the condition data Δt 0  &#34;, the alarm timing arithmetic circuit 11 obtains the set time T a0  for the alarm signal generator 12 at the time period T 1  &#34;. Then, at the time period T 2  &#34;, the resting signal S 18  is produced by turning off of the ignition switch. In response to the calculation command S 5 , the accumulative calculator obtains Δt 2  &#34; and the alarm timing arithmetic circuit 11 obtains T a1 . 
     During these operations, the content of the memories 7 and 8 and the alarm signal generator 12 are varied as illustrated in FIG. 10. 
     In view of the examples shown in FIGS. 7, 9 and 10 one may write general equations for the n the calculation as follows: 
     
         Δt.sub.n =(T.sub.c -T.sub.n-1)×α+Δt.sub.n-1 
    
     
         (3-Δt.sub.n)/β+T.sub.c =T.sub.a 
    
     FIGS. 11 and 12 show the second embodiment of the alarm device according to the present invention. In this embodiment, a microcomputer 30 is applied for precessing the calculations effected by the accumulative calculator and the alarm timing arithmetic circuit of the foregoing first embodiment. 
     Likewise to the foregoing first embodiment, the ignition position switch 1, the brake switch 2 and the speed alarm switch 3 are connected to the driving condition detecting circuit 35. The driving condition detecting circuit 35 produces the drive signal S 17 , the resting signal S 18 , the fatigue drive signal S 19  and the normal drive signal S 20  depending on the signals inputted from the switches 1, 2 and 3. The condition signals, i.e., the drive signal S 17 , the resting signal S 18 , the fatigue drive signal S 19  and the normal drive signal S 20 , are fed to an interface 31 of the microcomputer 30. At the same time, the condition signals, together with a fixed rate clock signal representative of the desired calculation rate are fed to the calculation command generator 5. The calculation command generator 5 is responsive to the condition signals and fixed rate clock signal to produce the calculation command S 5  every time one of the condition signals or clock signals is inputted. The calculation command is fed to the interface to make CPU 32 execute a calculation program to determine the alarm timing. The interface 32 is also connected to an alarm signal generator 12 which functions in the same manner as that of the foregoing first embodiment. The alarm signal generator 12 produces the alarm signal S 12  when the time reaches the set time, and activates the alarm device 13. 
     The microcomputer 30 therefore includes RAM 34 and ROM 33. RAM stores data obtained in each cycle of execution of the program, and the ROM stores the program as illustrated in FIG. 12. As apparent from FIG. 11, the interface 31 of the microcomputer 30 also connected to the clock 9 to receive therefrom the signal S 9  representative of time T. 
     The operation of the microcomputer 30 will be described with reference to FIG. 12. The microcomputer 30 executes in normal condition a background job as indicated by the term &#34;other routine&#34; in a block 301. In the background job, there is provided a step 302 for checking the presence of the calculation command S 5 . When the calculation command S 5  is detected at the step 302, the routine of steps 303 to 316 is executed as an interrupt program. At the step 303, accumulated condition data Δt 0  which is stored in RAM 34 and updated from time to time, a time data T 0  of the time when the previous calculation command S 5  and the time signal S 9  representative of the time data T 1  fed from the clock 9 are read out to CPU 32. Thereafter, the condition signal is checked. If the condition signal is the drive signal S 17 , the constant=-6 is read out from ROM. If the condition signal is fatigue drive signal S 19 , the constant=1 is read out. If the condition signal is the normal drive signal S 20 , the constant=1.2 is read out. Based on the read out constant, the calculation of the equation of: 
     
         Δt.sub.n =(T.sub.c -T.sub.n-1)×α+Δt.sub.n-1 
    
     is effected at a step 308. 
     After the step 308, the presence of the resting signal S 18  is checked at a step 309. If the resting signal S 18  is detected at the step 309, the set time of the alarm signal generator 12 is reset at a step 310. Otherwise, the condition signal is checked to determine whether the signal is either the fatigue driven signal  19  or the normal drive signal S 20  at a step 311. If the condition signal inputted is the fatigue drive signal S 19 , the constant=1.2 is read out from ROM at a step 312. On the other hand, if the signal is the normal drive signal S 20 , the constant=1 is read out from ROM at a step 313. Based on the read out constant and the condition data Δt n  read from RAM 34, the calculation according to the equation: 
     
         T.sub.a =(3-Δt.sub.n)/β+T.sub.c 
    
     is effected at a step 314. The result T A  of the calculation at the step 314 represents the set time data to be set in the alarm signal generator 12. The set time data T a  is fed to the alarm signal generator 12 via the interface 31 at the step 315. 
     Thereafter, at a step 316, the time data T 1 , and the condition data Δt n  are written in RAM. The time data T c  and the condition data Δt n  written in RAM 34 respectively serve as the time data T 0  and the condition data Δt n-1  in the next cycle of the execution of the foregoing program. Then the interrupt routine ends to return to the background job. 
     Thus, the invention fulfills all of the object and advantages sought therefor. 
     While the invention has been described in detail with reference to the drawings of the preferred embodiments, the invention should be understood as including all of the possible modifications embodied without departing from the principle of the invention.