jumps to a step s 216 at the step s 216 an increment of the second timer tm 2 takes place following this the count value of the timer tm 2 is compared with the second threshold time period tth 2 to check whether or not the timer tm 2 reaches the termination s 217 if it does not reach the termination tm 2 gt tth 2 the operational flow returns to the step s 210 on the other hand if in the step s 213 the load w is equal to or more than wth 1 the timer tm 2 is reset and the flag f 2 is reset to u201c 0 u201d s 219 then the operational flow returns to the step s 210 if the decision at the step s 210 shows that the flag k is u201c 0 u201d u201c child u201d the load w is read s 221 and the load w read is compared with the smaller load threshold wth 2 s 222 if the load w is equal to or more than wth 2 the timer tm 4 is reset and the flag f 4 is reset to u201c 0 u201d s 2220 then followed by a step s 223 on the other hand if the load w is smaller than wth 2 the timer tm 3 is reset and the flag f 3 is reset to u201c 0 u201d s 2221 then followed by a step s 233 when in the step s 223 the load w is equal to or more.