by reference to fig5 and 6 which depict a flow chart and a timing chart respectively referring to fig5 once the sewing machine m is energized the status of the looper cover switch 10 is determined step 1 if the looper cover switch 10 is off the sewing machine m is prevented from driving the motor step 5 this is illustrated at instant t0 t3 of fig6 during this period of time the sewing machine m is not driven even if the controller 13 which serves to initiate the machine m and adjust the speed thereof is operated t2 of fig6 if however the looper cover switch 10 is on the mounting demounting signal generator means s determines whether the second work table 3 is installed step 2 if a demounting signal i e off is indicated by the signal generator means s the controller 13 which controls the sewing speed determines motor operation step 4 if the controller 13 is on the sewing machine m motor is driven step 6 as at instants t4 t11 of fig6 in contrast if the controller 13 is off the sewing machine m motor is not driven step 5 if the looper cover switch 10 is on and the mounting demounting signal generator means s outputs a mounting signal i e on indicating that the.