the first embodiment of the invention will be described with reference to the drawings . fig1 is a block diagram showing a configuration of a control circuit 100 of a bread maker b , which is the first embodiment of the invention . the bread maker b has the control circuit 100 in place of the control circuit 50 of the conventional bread maker a . in fig1 the same parts and components as those shown in fig9 are designated by the same reference numerals , and the description thereof will be omitted . in fig1 reference numeral 17b designates an internal temperature sensor , whose comparator compares a detected temperature with a heater control reference value that varies according to a condition . reference numeral 52a designates a control program that controls various processes to be carried out by the bread maker b . the program is stored in a rom 52 , and includes a temperature control subroutine program shown in fig3 . the temperature control subroutine program is called when temperature control is requested in the respective processes executed by the control program . reference numeral 53a designates a transition characteristic data table having ambient temperature / bread making process conditions a1 to an , and transition characteristic data s1 to sn of heater control reference values corresponding to the ambient temperature / bread making process conditions a1 to an , each heater control reference value being updated every heater - on / off operation . the transition characteristic data table is stored in a rom 53 . fig2 is a diagram illustrative of the arrangement of the transition characteristic data table 53a . in fig2 reference numeral 55 designates conditions a1 to an such as target temperature ta , ambient temperature , and bread making process ; and 56 , heater control reference values used at the time of controlling the temperature of a bread baking case 13 by turning on and off a heater 12 so as to correspond to the conditions a1 to an , respectively . each heater control reference value corresponds to each of the conditions a1 to an , and is updated every heater - on / off operation . in fig2 the transition characteristic data s1 corresponding to the condition a1 is updated so as to form a table consisting of reference values r0 to rn . the heater control reference value r0 corresponds to the initial - off pulse ; the heater control reference value r1 to the first on - pulse ; the heater control reference value r2 to the first off - pulse ; the heater control reference value r3 to the second on - pulse ; the heater control reference value r4 to the second off - pulse ; and so on . the transition characteristic data s2 corresponding to the condition a2 is updated so as to form a table consisting of reference values q0 to qn . the transition characteristic data s3 corresponding to the condition a3 is updated so as to form a table consisting of reference values p0 to pn . the transition characteristic data sn corresponding to the condition an is updated so as to form a table consisting of reference values m0 to mn . then , the temperature control operation will be described on the basis of the flowchart shown in fig3 . a target temperature ta is specified for each process ( step s1 ). the target temperature ta is , e . g ., the optimal bread baking temperature for the bread baking process and is set through an operation panel 2 in advance . then , judgment of the ambient temperature / bread baking process conditions is made ( step s2 ). this judgment can be made through an ambient temperature detecting sensor for the ambient temperature condition , and by locating in the control program a process ( such as the fermenting process , the bread baking process , and so on ) requiring intervention of the temperature control subroutine , for the bread making process condition . successively , a transition characteristic of the heater control reference value corresponding to the set target temperature ta for each heater - on / off operation is selected by referencing the transition characteristic data table 53a based on the judgment of the condition made in step s2 ( step s3 ). if it is the condition a1 that has been judged in step s2 , the transition characteristic data of the heater control reference value for each heater - on / off operation is s1 ( r0 to rn ). then , the heater control reference value rn is set for each heater - on / off operation based on the transition characteristic data s1 ( step s4 ), and the heater 12 is controlled by each updated reference value rn ( step s5 ). then , 1 is added to n ( step s6 ), and the operation from steps s4 to s6 is repeated after setting the updated reference value r ( n + 1 ). fig4 is a diagram illustrative of an on / off control process of the heater 12 carried out in accordance with the heater control reference values r0 , r1 , r2 , r3 , r4 , . . . rn that are updated on the basis of the transition characteristic data s1 for every heater - on / off operation in steps s4 to s6 . first , the heater 12 is turned on . then , the heater control reference value r0 for the initial off - operation is set for the condition a1 out of the heater control reference transition characteristic data s1 selected from the data 56 in the transition characteristic data table 53a , so that the heater 12 is turned off based on the reference value r0 ( at timing t0 ). unlike in the conventional example in which the sensor curve x depicts such a rise as shown by the broken line ( see fig1 ), the temperature detected by the internal temperature sensor of the invention exceeds the target temperature ta with controlled overshoot , makes a turn , and drops from the target temperature ta as a sensor curve z shown by the solid line . in the meantime , the updated heater control reference value r1 is set , so that the heater is turned on upon coincidence of the detected temperature with r1 ( at timing t1 ) so as to anticipate the turn - on timing . also , in the meantime , the updated heater control reference value r2 is set , so that the heater is turned off upon coincidence of the detected temperature with r2 ( at timing t2 ) so as to anticipate the turn - off timing . the same operation is repeated for r3 ( at timing t3 ), r4 ( at timing t4 ), . . . rn . as is apparent from fig4 the heater 12 is turned off at the timings t0 , t2 , t4 , t6 , . . . that are the timings before the sensor curve z exceeds the target temperature . that is , the heater - off timing is anticipated to reduce overshoot . the times between the heater - off timings and the timings at which the curve rises and reaches the target temperature , i . e ., the anticipating times , are denoted as α0 , α2 , α4 , α6 , . . . . it may be noted that the anticipating time is designed to decrease with increasing number of times of heater - off operations . on the other hand , the heater 12 is turned on at the timings t1 , t3 , t5 , . . . that are the timings before the sensor curve z drops from the target temperature . that is , the heater - on timing is anticipated to correct a heat transfer time difference . the times between the heater - on timings and the timings at which the curve drops and reaches the target temperature , i . e ., the anticipating times , are denoted as β1 , β3 , β5 , . . . . it may be noted that the anticipating time is designed to decrease with increasing number of times of heater - on operations . as shown in fig1 , unlike the sensor curve x of the internal temperature sensor 17 based on the conventional heater - on / off control using a fixed heater control reference value , the sensor curve z of the internal temperature sensor 17b using the heater control reference value being updated in accordance with the heater control reference transition characteristic data of fig4 exhibits improved temperature control accuracy and response with overshoot and undershoot inherent in the on / off control more suppressed than those in the conventional example shown in fig1 and with the fluctuation range d thereof smaller than the fluctuation range d of the curve x at the steady state . while the case where the heater temperature control is effected by a microcomputer on a software basis has been described , the temperature control may be effected by a hardware circuit on a real time basis as well . the second embodiment of the invention will be described next . a bread maker c , which is the second embodiment of the invention , has an internal temperature sensor 17c mounted at such a position as not only to implement temperature control with satisfactory response and high accuracy but also to make high - quality bread . fig5 is a sectional view showing a construction of the bread maker c . in fig5 the same or like parts and components as those shown in fig8 are designated by the same reference numerals , and the description thereof will be omitted . the internal temperature sensor 17c is mounted at a point c that coincides with the first quarter from below when the distance between the uppermost point u ( the lower end of an opening / closing cover 3 ) and the mounting point d of the heater 12 is quartered . it may be noted that points a and b denote the third quarter and the second quarter from below , respectively . fig6 ( a ) to 6 ( c ) show temperature control characteristic curves obtained when the internal temperature sensor 17c is mounted at points a , c , and d . the characteristic curve shown in fig6 ( a ) is obtained when the internal temperature sensor 17c is mounted at point a ; the characteristic curve shown in fig6 ( b ) is obtained when the internal temperature sensor 17c is mounted at point d ; and characteristic curve shown in fig6 ( c ) is obtained when the internal temperature sensor 17c is mounted at point c . the characteristic curve shown in fig6 ( a ) exhibits poor response due to the fact that the internal temperature sensor 17c is remote from the heater 12 . therefore , this characteristic curve exhibits the number of times of heater - on / off operations smaller than that of the characteristic curves shown in fig6 ( b ) and 6 ( c ), with large overshoot and undershoot . the characteristic curve shown in fig6 ( b ) exhibits satisfactory response . however , too intense direct heating due to the fact that the internal temperature sensor 17c is located close to the heater 12 makes overshoot and undershoot large . this characteristic curve has the number of times of heater - on / off operations larger than that of the characteristic curves shown in fig6 ( a ) and 6 ( c ), contributing to better response but increasing overshoot as well . in contrast to the characteristic curves shown in fig6 ( a ) and 6 ( b ), the characteristic curve shown in fig6 ( c ) exhibits the smallest overshoot and undershoot with response being in between those of the curves shown in fig6 ( a ) and 6 ( b ) in accordance with the number of heater - on / off operations . the fluctuation range is also minimized at the steady state . therefore , the optimal mounting position for the internal temperature sensor 17c is in the vicinity of point c at which the curve shown in fig6 ( c ) is obtained . fig7 shows a data table for making a general judgment of the mounting point of the internal temperature sensor 17c out of points a , b , c and d so that the optimal temperature control characteristic can be obtained . for example , evaluation points are set for each judgment item in such a manner that two points are given for a temperature fluctuation range of ± 7 ° to ± 6 . 5 ° c . ; one point is given for a temperature fluctuation range of ± 7 ° to ± 7 ° c . ; three points are given for a temperature fluctuation range of ± 7 ° to ± 6 ° c . ; no point is given for a temperature fluctuation range of ± 15 ° to ± 12 . 5 ° c . at a sensor point ; and so on . the mounting position of the internal temperature sensor 17c at which the highest evaluation points are given is judged as the optimal mounting position through general analysis of the respective judgment items . for example , point c is selected as the optimal mounting position . in other words , the optimal mounting position can be found in the area around point c . the temperature control characteristic at point c exhibits , as shown in fig6 ( c ), a narrow temperature fluctuation range . that is , the temperature is constantly controlled within a prescribed temperature range . further , for each point , if a value x 1 minus x 2 ( x 1 - x 2 ) is calculated where x denotes the number of times of turning on the heater for a predetermined period and x 2 denotes the maximum value ( absolute value ) of the temperature fluctuation range , it is determined that a point where this value is substantially the largest is the optimal mounting position for the internal temperature sensor 17c . that is , as shown in fig7 the value is 10 - 7 = 3 when the sensor 17c is mounted at point a ; 14 - 7 = 7 at point b ; 16 - 7 = 9 at point c ; and 19 - 15 = 4 at point d . thus , it is determined that point c is the optimal mounting position for the sensor 17c . heater control ( i . e ., the turning on and off of a heater ) can be effected by an internal temperature sensor in such a manner that a reference value corresponding to a target temperature is updated on a heater - on / off operation basis so as not only to anticipate turn - on and turn - off timings but also to vary the amount of anticipation . therefore , overshoot and undershoot with respect to the target temperature can be controlled . in addition , the temperature fluctuation range can be minimized for the heater - on / off control at the steady state , allowing temperature control with satisfactory response and high accuracy to be obtained . moreover , the internal temperature sensor is mounted at a position where a value obtained by subtracting the maximum value of the temperature fluctuation range from the number of times of turning on the heater for a predetermined period is substantially the largest . this means that the internal temperature sensor is mounted at a position above the heater mounting position , so that not only the sensor is close to the bread making ingredients to be heated by the heater , but also the sensor is not exposed to direct heating by the heater . as a result , overshoot and undershoot are minimized compared with those observed when the internal temperature sensor is arranged at other positions , and temperature fluctuation range is also minimized during the heater - on / off control at the steady state . hence , temperature control with satisfactory response and high accuracy can be implemented .
a bread maker having a reference value setting section for updating a reference value corresponding to a target temperature every heater - on / off control operation , the reference value serving to stabilize a current temperature to the target temperature by comparing the current temperature with the reference value and controlling a heater on the basis of a temperature difference between the current temperature and the reference value .