Evaluation of the Possibility of Shaking off Raspberry Fruits with a Pulsating Air Stream

Abstract

The aim of the study was to determine the possibility of harvesting raspberries with a pulsating air stream that causes the shoots of plants to vibrate. Two tractor-driven test devices were developed under this specific objective. The first one was a high capacity compressor, and the stream of air produced by it was directed onto one side of a row of plants. The second one was a device producing two counter-flowing air streams colliding in the middle of a row of plants. The frequency of the air pulses was adjusted steplessly by varying the rotational velocity of the shutters closing and opening the outlets of the fans. In the field trials with the device based on a compressor-generated air stream, two air pulse frequencies were used: 500 and 540 pulses per minute (8.3 and 9 Hz). The mean detachment force was 0.727 N. About 50% of ripe raspberries were removed from the bushes. The results achieved during harvesting with the device operating on the principle of colliding two pulsating air streams were markedly better and put the harvesting effectiveness within the range achieved in the trials with combine harvesters with mechanical finger shakers conducted by other researchers. For two pulse frequencies (8 and 9 Hz), almost 62% of fruits were harvested. Increasing the pulse frequency of the counter-flowing air streams above 9 Hz did not improve the effectiveness of detaching raspberry fruits from shoots.