Assume that the slotted encoder shown in Figure P14.5 has a length of 1 m and a total of 1,000 slots (i.e., there is one slot per millimeter). If a counter is incremented by 1 each time a slot goes past a sensor, design a digital counting system that determines the speed of the moving encoder (in meters per second).

Question

Answer 1

Analysis:

Assuming a maximum speed of 10 m/s and a minimum speed of 1 mm/s, we can calculate the instantaneous speed of the slotted encoder by counting the number of clock pulses between slots using a fixed frequency clock. This resolution should be sufficient to measure the speed of the encoder over the range of interest. The figure depicts the arrangement: A 10 kHz clock increments a 16-bit binary counter. The choice for a 16-bit counter is due to the maximum speed requirement: 216 = 65,536, will be the maximum count between slots. At a speed of 10 m/s, the time for one slot to go by is 10-4 s, thus the number of counts would be 1 count; at the minimum speed of 1 mm/s the number of counts would be 104. A 14-bit counter would be sufficient, but in practice it is easier to cascade two 8-bit counters; thus the choice of a 16-bit counter. The count is held by a latch, and then converted to BCD for use with seven-segment displays. The details of the seven-segment display encoders are not shown (see Focus on Measurements: Seven-Segment Display). If a decimal point is placed to the right of the second seven-segment display (starting from the left in the figure), the display will read the speed in m/s, up to a maximum of 10 m/s, with a resolution of 1 mm/s.