speed this can be overcome by periodically incrementing the initial ratio dp 1 fig5 a and 5b are flowcharts for incremental setting of the initial ratio dp 1 in fig5 a block 70 initiates a tachogenerator interrupt routine in response to the receipt of an output pulse from tachogenerator 23 at interrupt terminal irq2 this tachogenerator interrupt calls for block 71 which directs the resetting of a timer tm and block 72 which terminates the tachogenerator interrupt in fig5 b block 80 initiates a timer interrupt routine which calls for decision block 81 which tests for the presence of an end 1 flag indicating the end of the needle position interrupt routine mentioned previously if the end flag is one control exits to block 86 to terminate the timer interrupt routine if the end flag is not one the exit is to operations block 82 which causes the timer to increment by one exit then is to decision block 83 which checks to see if the timer count value tm exceeds a predetermined value t if the answer is negative the exit is to operations block 85 and if it is affirmative control proceeds to operations block 84 which directs the addition of an incremental value u03b4dp to the initial value dp 1 which was obtained at time t 0 see fig6 by the power on routine 40 fig3 a.