| 摘要 |
A linear array of light emitting diodes (CR1-CR7) direct a diffuse beam of radiation across the path of seeds dropping down a seed chute and onto two detectors (46, 48) connected in parallel, so that every seed produces a substantially equal area pulse in the output signal from the photo detectors. The signal is amplified (50) and applied to an analog to digital converter (52) producing digital samples at a high rate for processing by a microprocessor (54). The sensor value is subtracted from an offset value corresponding to the sensor signal when no seed is present and the difference signal is repeatedly added to an area signal. The offset value is adjusted from time to time to prevent the difference signal staying persistently negative or persistently positive. Whenever the area signal reaches a threshold value the passage of a seed is indicated and the area signal is decremented by an estimated, area value corresponding to one seed. This estimated area value is adjusted from time to time so as to avoid long term drift in the area value, thereby maintaining the estimated area value accurately representative. |
