Air pulse fruit harvester

Universidad Nacional del LitoralCONICETAngers SRL

Air pulse fruit harvester

The advent of intensive farming methods for olive requires of new mechanical methods to replace manual crop harvest, due to its inefficiency and inherent risks.

The vast majority of current designs for automatic harvesting are mechanical. The fruit drop occurs shaking the ground by rotating counterweights applied to the trunk or hitting the branches with rods.

This type of harvester produces damage to the leaves, fruit buds and in general the whole structure of the plant.

They also tend to be heavy, damaging the roots especially in areas of low soil compaction.

Harvesting methods that do not have physical contact with the ground are sought long ago.

One possibility is the use of strong air streams. In fact, it is the natural way the fruits usually fall during storms and high winds.

However, the wind speed required to produce fruit drop is too high, requiring very powerful turbines with the disadvantage of high weight and plant damage (as with mechanical harvesters).

It is estimated that the force required to remove an olive is 300 grf (grams force), while the drag force produced by a wind of 360kmh is less than 50grf.

The force required to pick the olive can be dramatically reduced cyclic loads are applied, causing material fatigue.

Steel spring broken by cyclic fatigue

All mechanical systems have "resonance frequencies". By applying cyclic loading at these frequencies it is more likely that the system breaks.

The Tacoma Narrows bridge was destroyed by the resonance produced by a wind of 67kmh whose aerodynamic loads excited resonant mode at 0.2Hz.

The Angers bridge was destroyed in 1850 when an army of 478 soldiers marching on it excited resonance and caused its collapse.

Which is the best frequency for olives?

Here we can see the most important resonance modes

First mode (lowest frequency)

Second nd mode (highest frequency). Note the compression stresses (red) and traction (blue) are higher.

It has a low frequency and produces low stress on the stalk.

The base of the peduncle and the fruit itself move in phase.

There is a large displacement of the center of mass of the the fruit (hence low frequencies) (video elastld-w1.asf)

First resonance mode

Typically has higher frequency and produce larger stress on the peduncle.

The base of the peduncle and the fruit move in opposite phase.

Small displacements of the center of gravity of the olive. Practically the fruit rotates around its center of gravity (vide elastld-w2.asf)

Second resonance mode

How can we produce cyclic loads? There are

several conceptual designs of pulsed air harvesters.

Most of them generate pulses by cyclically opening and closing an air passage.

But accelerating and decelerating the air flow is inefficient!!

This strategy is very inefficient because the flow accelerates and decelerates, which requires large amounts of energy due to the inertia of the fluid . While air is a lightweight fluid the high speeds required (150kmh) and pulse rates required (approx 20Hz) imply an important effect of fluid inertia. To get an idea, the thrust produced by the air jet in this conditions is 240 Kgf.

How can we produce pulsating air without stopping the flow? The solution is to DIVERT the flow without

STOPPING it!

Analogy: Aikido is a martial art that is based on divert and redirect the momentum of the opponent instead of opposing it directly.

Alternative vs. rotative movementFinally, there is a further improvement. Instead of moving alternatively the device or part of it, diverting the flow is produced by a rotor.

This has many advantages: Simplifies the mechanical design Avoids cyclic loading on the structure

(fatigue) Further improves aerodynamic efficiency

(steady flow in a rotating system)

How can we divert the air stream?

Let's first see what happens when the rotor is stationary. The rotor blades divert the the main stream in three powerful jets.

One way to visualize the flow is think as if the air were composed of tennis balls. To begin, we see here how the air behaves when the rotor spins at low speeds. (video shaker42.asf)

How can we divert the air stream?

Spinning rotor

As the rotor starts spinning the points where the jets are emitted perform a circular motion and the air jets are bent into helices. (videos shaker41.asf)

But the air stream follows a straight line!!

In the figures and animations presented tennis balls (representing the air blown by the fan) form a helix-shaped jet. This would seem to suggest that the air jet is bent but it's really not! Each ball follows a straight path. It is the change in position of the point of emission that generates the helical pattern. (see video shaker40.asf). This is in line with the strategy of disturbing the flow as little as possible so that the aerodynamic efficiency is improved.

How the cyclic load on the fruit is produced?

As the air jets spin, the fruit passes alternately through the jet zone and outside the jet zone producing the desired cyclic load (see video shaker39.asf)

How the pulse generator is applied to the whole tree?

By moving the generator sweeps a fringe in the tree line. Depending on the configuration this area may have a height of 1.5m and a depth of 2m.(see videos shaker36.asf-shaker37.asf-

shaker38.asf) Área barrida por un generador

How the pulse generator is applied to the whole tree?

Using 4 pulse generators, two on each side of the row and covering the top and bottom part of the tree. (see videos shaker33.asf-shaker34.asf-shaker35.asf)

Area barrida por 4 generadores

Novelty of the design A harvesting technology that does not make physical

contact with the tree. The picking is produced by pulses of air, producing fatigue in the peduncle.

High aerodynamic efficiency: The whole design is based on diverting the airflow, never stopping it.

The mechanical design is based on a rotor and has no reciprocating components.

The air stream follows a straight path.

Efficiency of the air pulse The ideal pulse should have an

intensity as high as possible and a frequency as close as possible to the frequency of resonance.

The intensity is measured as the difference between peak and valley pressure.

The generator must produce a pulse at the resonance frequency, but due to variations in the weight and stiffness, a frequency range is covered. The generator must be able to change the frequency, keeping an appropriate intensity throughout this range.

Controls parameters

Fan power: Directly affects the pulse intensity. Speed and number of openings of the rotor: directly influence the

frequency.

However, increasing too much the rotational speed or the number of openings may

indirectly affect adversely the amplitude, as the air jets begin to interfere with each

other.

The jets interfere with each other because the frequency is very high.

The jets interfere with each other because there are many openings.

When the jets interfere with each other?

That can be determined either by

Laboratory experimentation

Computational Fluid Dynamics simulation

Pulse generator prototype

A full scale pulse generator prototype was built. The fan rotates up to 2200rpm, driven by a hydraulic motor which

in turn is driven by a 40 HP electric motor. The rotor is controlled by an electromagnetic brake. Pulse intensity measurements based on different parameters

measuring equipment based on Pitot tubes and a computer data acquisition were performed.