| 摘要 |
<p>A sheet counter has an elongate element (18) in which is formed a suction port (19), connected to a vacuum source by a duct (26). A pressure sensor is arranged to sense the pressure in the duct, in the vicinity of the port (19), and produces an electrical signal indicative thereof. The counter is set up by sensing the pressure in the duct with the port (19) open, either with the vacuum source operating or before operation thereof, and then by sensing the pressure in the duct with the port closed and the vacuum source operating. A pressure count-value part-way between the two sensed pressures is computed and during subsequent operation of the counter, a valid count signal is generated only if the instantaneous sensed pressure in the duct falls below the computed count-value.</p> |
