Abstract:
A device for removing fruit from a plant comprising a vibratory head having means for clamping a fruit plant, and means for controlling the vibratory head, preferably an electronic control means. The vibratory head further comprises at least one reaction mass which is vibratably driveable and connected to the clamping means for relative movement therebetween to provide a unidirectional force transmittable between the reaction mass and the clamping means, and hence transmittable to the plant.

Description:
TECHNICAL FIELD 
     The present invention relates to devices for picking fruit, and in particular for picking olives. 
     BACKGROUND ART 
     The invention has been designed for picking olives, so the discussion of the background art and of the specific embodiments is limited to this particular application of the invention. 
     A known method for picking olives and other fruit is manual labour, whereby fruit is removed from trees by hand picking. This is both time-consuming and costly. 
     Also known are mechanical shaking devices which apply mechanical vibrations directly to the trunk or branches of an olive tree to remove olives, which may be gathered in a net placed on the ground, or by a vertical net having the shape of an upside down cone surrounding the tree trunk. The mechanical shaking method tends to damage the trees and leads to the unwanted removal of leaves and small branches. Some such methods employ a shaker head attachable to an olive tree-trunk or branch. The shaker head can oscillate with a power of up to 100 horse power and is driven by one or more hydraulic motors which impart rotary motion to eccentric masses. The eccentric masses generally comprise rotatable masses with a high inertia which is difficult to control and slow down. Response times for these machines are also slow. The consequences of component failure may also be dangerous in such machines, especially if the eccentric mass is rotating at the top end of its speed range. Some machines provide for adjustability of the speed of the rotating masses. Such oscillating machines generally apply very complicated oscillations of a single dominant frequency or over a very narrow width of dominant frequencies with little regard for the effect on the tree and its roots, and thus the trees are prone to damage often including substantial removal of leaves and/or twigs of the tree. The construction of these oscillating machines generally involves the shaker head being suspended from a carrying frame by means of chains or springs. Another disadvantage of these methods is that only the ripest olives are removed from the tree. The not so ripe olives, which may constitute as much as 40%-50% of the potential crop, remain attached to the plant. To recover as many of these olives as possible the plant is subjected to further mechanical shaking after one or several weeks have elapsed. Notwithstanding this successive treatment, a 10% to 15% residue of olives remains on the plant. Apart from its relative inefficiency, this method of detaching olives from the tree goes counter to one of the main requirements for making high quality extra virgin olive oil which is the use of the younger, less ripe olives. Under ideal conditions the number of trees cropped by this method can be as high as 40 per hour. 
     Devices are also used extensively to detach the fruit from the plant by beating action. A variety of devices, based on this principle, are being employed with varying degrees of success. The most popular consist of pneumatically, or electrically, powered combs made of light thin rods. These devices are mounted on the end of a pole. The other end of the pole is held and pulled by an operator in a comb-like motion through the plant&#39;s thin branches, which bear most of the fruit, while imparting oscillatory beating to the fruit, leaves and branches in the immediate vicinity of the comb&#39;s teeth. The olives are detached from the tree by the device&#39;s combined beating and combing action. Although with this method it is possible to remove almost all the fruit from the plant, a significant amount of small branches and leaves also fall to the ground and the bark of the larger branches is damaged in places by the beating action. One additional problem of all beating methods is that the olives do not fall vertically to the ground. A quantity of olives is thrown outside the gathering net placed on the ground around the tree. The resulting loss of crop and the accidental squashing of olives by the operator(s) walking over the gathering nets can reduce the harvest efficiency by as much as 10%-15%. In addition such methods are prone to bruising the olives, and unless the olives are processed for producing olive oil very soon after harvesting, they develop an increased level of acidity. If the level of acidity increases above 0.5%, the producer may no longer label the oil “extra virgin”. Finally, all hand held methods of cropping olives are notoriously slow (one tree per hour per operator) and hence they are only practical for small groves. 
     SUMMARY OF INVENTION 
     The present invention provides a device for removing fruit from a plant comprising a vibratory head having means for clamping a fruit plant, and means for controlling the vibratory head, the vibratory head further comprising at least one reaction mass which is vibratably driveable and connected to the clamping means for relative movement therebetween to provide a unidirectional force transmittable between the reaction mass and the clamping means, and hence transmittable to the plant. 
     Advantageously, the present invention avoids the application of tangential forces to the bark of the tree. The control means can advantageously allow the frequency, phase and/or the amplitude of the vibrations of the reaction mass and thus the plant to be controlled. Special computer generated complex signal codes, designed to transmit maximum energy for detaching the olives, are transmitted with high fidelity to the tree trunk. 
     The power output from the vibratory head can be adjustable manually or controllable automatically by means of sensors placed on the vibratable body. The vibrations applied to the tree are not forced to be of an uncontrollable amplitude, phase or frequency. Moreover, the vibratory body can be prevented from emitting frequencies that couple substantial vibratory energy to the tree roots. The invention is thus more sympathetic to the tree, and does not damage it. The movement of the reaction mass can be substantially decoupled from the hydraulic arm, advantageously avoiding retro-transmission of the vibrations back to the tractor. By virtue of the various mechanical, electronic and electrical features of embodiments of the invention discussed below, the device is quick and effective, and can be controlled with substantially immediate effect. 
     Preferably, the drive means comprise at least one piston displaceably mounted in a cylinder of the body. The reaction mass may be hydraulically, pneumatically or electromagnetically driven. 
     With embodiments of the present invention, removal of leaves is avoided because the vibrations applied to the tree are not in a range which is likely to cause leaf detachment. Further there is no physical contact between the vibratory components of the present invention and the leaves and no beating being applied thereto, unlike in the known combing methods. Also, as a result, the fruit does not suffer bruising. 
     Advantageously, the vibrations of the reaction mass can be linear in direction and can be applied to the trunk or branch substantially normally to the longitudinal axis of said trunk or branch. This further avoids the application of tangential forces to the tree and consequent damage to the bark. In this connection, the drive means can advantageously be symmetrically arranged with respect to the tree. In one embodiment, two pistons are provided, one on either side of the trunk or branch. The pistons are then to be driven in synchronism to avoid the production of lateral forces or torques about the tree. Alternatively, two or more pistons can be placed radially around the tree trunk and driven sequentially to avoid the production of tangential forces. This drive means has the advantage of coupling the vibratory energy in the optimum direction for maximum transmission through those branches which impart oscillatory motion to the olive-stem. 
     In another advantageous development of the invention the vibrations applied to the tree can have a coded time-varying frequency with constant or time varying phase. For instance, the frequency can be swept from an initial sweep frequency to a lower final sweep frequency, or vice versa. Picking efficiency of 95% of the olives on the tree may thereby be achieved. It is also possible to include a modulation component in the frequency signal, which can vary with time, the phase being optionally time-variable also. The modulation component can have a low frequency (corresponding to olive-stem resonances) and a low amplitude (to avoid damage to tree and roots) and thereby excellent picking efficiency can be achieved. 
     To clarify the invention, the background theory is expanded below. 
     One way to detach efficiently pendulous fruit from the branches of a tree is to apply to the fruit translational and/or bending (shear) forces of a magnitude sufficient to pull or shear off the fruit from the stem. To induce fruit detachment by the application of translational forces, the pendulum-like non linear resonance properties of the fruit-stem can be exploited. 
     The non-linear oscillatory motion of the olive with the stem attached to a moving support (the tree branch) has been investigated using the Duffing oscillator mathematical model. According to this non-linear model the resonant frequency is a function of the driving force magnitude and its frequency, stem mechanical properties such as elasticity damping and physical dimensions of both stem and olive. Also under certain driving conditions the phase of the moving olive-stem locks to the phase of the applied drive; i.e. the resonant frequency of the olive-stem follows the frequency of the applied drive. This property is very useful because non linear resonance can be maintained over a wide range of magnitudes and frequencies. This is achieved with a modest amount of drive energy and without the need to synchronize the phase of the drive to that of the olive-stem oscillator 
     Since it is likely that the physical dimensions and mechanical properties of stems and olives in a tree are all different, their resonant frequencies (both linear and non-linear) cover different ranges. Under these conditions it is more appropriate to employ time variant frequency signals such as a coded frequency sweep to cover a set or sets of resonant frequency ranges, rather than mono or narrow frequency band drives, as used in the prior art. When the trunk or branch is driven with a vibratory force, the frequency range for olive detachment should be limited to the set of resonances (linear and non-linear resonances) of branches with the attached stems and olives. In fact it may not be advisable to use frequencies as low as a few hertz to only cover the olive-stem set of resonances. At these low frequencies the resulting high amplitude motion applied to the tree trunk can displace and damage the tree&#39;s micro root system. To overcome this problem, the modulation components mentioned above are coded into the drive signal to produce low frequency sidebands of lower magnitude compared to the higher frequency components occupying the sweep frequency band. The resulting complex signal is not deleterious to the root system but still supplies energy to the olive-stem set of resonances. 
    
    
     
       By way of example, there now follows a detailed description of an embodiment of the invention with reference to the accompanying drawings. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary construction of the device according to a first embodiment of the invention; 
         FIG. 2  is a schematic view of the first embodiment; 
         FIG. 3  is a schematic view of a hydraulic system forming part of the first embodiment invention; 
         FIG. 4  is a schematic view of a second embodiment of the invention; 
         FIG. 5  is a schematic view of a hydraulic system according to the second embodiment; and 
         FIG. 6  is a schematic cross sectional view of a part of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1  can be seen a vibratory head comprising a vibratable body  1  engaged with the trunk  8  of an olive tree. The body  1  is resiliently attached to carrying means  2  in the form of a Y-shaped bracket  2  by means of springs  3 . Thus, the body is decoupled from the hydraulic arm and hence the tractor. The body  1  more specifically is mounted on top of the bracket  2  and thus the springs  3  are in compression. The springs  3  need to be of sufficient strength to withhold the weight of the body  1 , which can have a mass of 450 kg for example. The bracket  2  is disposed at the end of a robotic hydraulic arm as may be provided on a tractor for instance. Along the sides of the body  1  are provided a hydraulic cylinder  4  and a corresponding subsidiary piston  5  (not visible on the rear side) which serve to open and close adjustable arms  6   a  and  6   b  in order that the body may be securely held to the trunk  8  for effective transmission of vibrations therebetween. As viewed in  FIG. 1 , a first arm  6   a , which is nearest the viewer, is provided with cushioning means in the form of a black rubber pad for abutment with the bark of the tree. A second arm  6   b  which is distal from the viewer is not provided with cushioning means. Alternatively, a pad could be located on the second arm  6   b . In operation, the first arm  6   a  is rotated by means of the subsidiary piston  5  until the pad comes into contact with the bark. The second arm  6   b  lags behind the first arm, and after the first arm has come into engagement with the tree, the second arm comes into abutment with the first arm  6   a  and applies pressure thereto. It is thereby possible to adjust the pressure applied by arms  6   a ,  6   b  very finely, thereby allowing an optimum coupling of the vibratory energy between the body  1  and the tree  8  and avoiding undue stress on the bark. A corresponding pad is fixed to the vibratable body  1  for abutment with the opposite side of the branch. In this way it is provided that there are only two lines of contact between the pads and the tree, and the direction of the applied vibrational forces is parallel to the displacement between the two lines of contact. Thus there are no tangential forces applied to the bark avoiding damage thereto. There is also no need for lateral pads to be provided. 
     A pair of main pistons  13  are mounted in cylinders  12  and form part of the drive means responsible for vibrating the body  1 . These are discussed in more depth below. An electronic control unit and a hydraulic pressure supply in the embodiment shown are mounted on the tractor (not shown) thereby facilitating user control. The hydraulic equipment placed on the rear of the tractor for connection of the hydraulic pump to the tractor&#39;s power take-off. 
     Some of the hardware required to implement the invention is shown schematically in  FIG. 2 . The vibratable body  1  contains drive means in the form of two hydraulic cylinders  12 , each of which contains one of the corresponding main pistons  13 . A hydraulic unit  7  discussed in more depth below provides pressurised fluid to a valve  14 , more particularly a servo valve, and takes away low pressure fluid. Upper and lower holding plates  15  hold the vibratable body  1  together. The trunk or branch  8  is held against the body by the adjustable arms or clamps, which are not shown here. Vibrations of the body  1  are transmitted to the appropriate trunk or branch, and because the body can move with respect to the carrying means, transmission of the vibrations to the carrying means is inhibited. An electronic control unit  18  sends an electric signal s(t), represented by equation 1 below, to the servo valve  14  mounted on the central part of a manifold  11 . This valve alternates the flow of hydraulic fluid, supplied by high and low pressure lines  16  and  17 , to either the right side or the left side of piston/cylinder sealing rings via the ducts A and B respectively. Corresponding ducts are provided but not shown in connection with the second main piston  13 , shown in  FIG. 2  adjacent to the first main piston  13 . The resulting translational motion imparted to the main pistons, relative to the cylinders, is the analog of the servo valve drive signal s(t). Reaction forces from the vibration of the main pistons, as internal masses, cause vibration of the body. In addition the two pistons provide the inertial mass (e.g. 350 kg) required to deliver a force of 30,000 N, which is sufficient to vibrate the tree strongly enough to remove olives. 
     In order to avoid damage to the tree bark, the vibrations are applied to the tree without tangential forces. This is achieved with the present structure of the device because the forces caused by the vibrations are linear in direction and are applied substantially normally to the longitudinal axis of the trunk or branch. By virtue of the two pistons  13  being controlled by the same valve  14 , they remain in phase with each other, and thus do not produce tangential components of vibrational motion. Any form of drive means fulfilling such criteria is contemplated within the invention. 
     A detailed hydraulic fluid flow schematic diagram is shown in  FIG. 3 . A hydraulic pump  21 , driven from a tractor power supply increases the pressure of the fluid pumped from a tank  31  to 3,000 psi (20.7 MPa). The maximum pressure of the fluid is regulated by check valve  22  and after the fluid has gone through a filter  23  it is fed to the servo valve  14 . This valve is controlled by the electronic control unit and as pointed out earlier, its function is to switch the direction of the fluid fed into the cylinders  12 . Two nitrogen accumulators  24  are located along the high pressure line to dampen high energy impulses which could irreparably damage the pump or other components in the hydraulic circuit. A hand operated valve  28  B in the open position, bypasses the fluid from the high pressure line to the tank. Valve  28  A leads to a pressure gauge. The valves  28  A and  28  B are only operated during warm up and shut down operations. Due to the different pressures occurring throughout the hydraulic system, different hoses are used to cope with the stresses caused. The hydraulic pump  21  is connected to a hydraulic fluid reservoir  31  by suction hoses, whereas the hoses connecting the hydraulic pump  21  to the valve  14 , via the intermediate components, are high pressure hoses. A high pressure pilot hose connects the hydraulic pump to a solenoid valve  29  which is operable to direct the hydraulic fluid e.g. oil, through a cooler  30  should such be necessary. After it has served its purpose of driving the hydraulic cylinders  26 , the hydraulic fluid passes back to the reservoir  31  via a return hose  33 . 
     In alternative embodiments of the invention not shown in the figures, it is envisaged that two or more pistons and cylinders are provided, the pistons being arranged orthogonally to one another. The pistons can further be arranged and driven so as to avoid the production of tangential forces. Such a construction advantageously allows a greater olive picking efficiency, by allowing excitation of the tree in more than one plane. 
     As shown in  FIG. 2 , the electronic control unit  18  is connected to the hydraulic equipment  7  and the valve  14 . The control unit  18  emits a signal represented by equation (1) below which controls the valve  14  and hence the vibrations of the pistons  13 . 
     Equation (1) defining the signal properties can be written as:
 
 s ( t )={cos [({acute over (ω)}−α t ) t+m ( t )]}* g ( t )  (1)
 
     Where 
     s(t)=signal driving the servo valve 
     {acute over (ω)}=2πfh 
     fh=initial sweep frequency 
     α=frequency sweep rate=2π(fh−fl)/sl, fh&gt;fl and (fh−fl)≦Δfd 
     fl=final sweep frequency 
     sl=sweep length 
     Δfd=bandwidth suitable for olive detachment 
     m(t)=modulation function having a frequency value&lt;&lt;fl 
     *=convolution operation 
     g(t)=band pass filter function with a bandwidth˜Δfd 
     Equation 1 shows that the signal essentially consists of a linear sweep parameter {acute over (ω)}−αt, and a modulating function m(t). This modulation function is a low frequency signal, which is optionally sinusoidal, and which causes the production of side band energy at the low end of the bandwidth Δfd. This side band energy drives the olive-stem system into resonance but due to its low amplitude does not inflict damage on the tree&#39;s micro root system. Lastly, the purpose of the band pass filtering function g(t) is to ensure that the frequency components emitted by the device are within the range of frequencies Δfd which are suitable for detaching olives. As already pointed out above the emitted signal is band limited at the low frequency end to prevent root damage. The signal is also band limited at the high frequency end to reduce energy loss caused by the tree&#39;s spurious resonant modes and to concentrate the available energy within the frequency range for olive detachment. Also, the production of high frequencies is limited to prevent the detachment of leaves from the tree. It should be emphasized that the abovementioned signal type is not the only one that can be used with the embodiments of the invention. New signals can be programmed into the electronic control unit, the invention being flexible in this regard. 
     In operation, the user controls the robotic arm to guide the device into engagement with the tree. Then, the adjustable arms  6  are closed around the trunk or branch to secure the arrangement. Alternatively, it is also envisaged that the device be equipped with a position sensor which can take the form of an ultrasonic or a laser positioning device for instance, and means for automatically controlling the position of the body  1 , whereby the user simply brings the device into proximity with a trunk or branch and the automatic control means then guide the body into engagement with the tree in a very short time, avoiding any potentially damaging contact with the bark which might occur before engagement. As with previous techniques, the olives can be gathered by placing a large net on the ground beneath the tree or by a vertical net shaped like an upside down cone with its narrower end secured to the base of the tree trunk. Vibrations as described above are then applied to the tree and the fallen olives collected. 
     An alternative construction embodying the invention is shown schematically in  FIG. 4 . In this, the preferred embodiment, a C-shaped clamp  41  is provided at the distal end of the hydraulic arm (not shown), and is adapted to receive the trunk or branches of an olive tree. The C clamp  41  comprises a first arm  42  and a second arm  50 , the first arm  42  having a first pad (not shown) for non-damaging contact with the bark of the tree. The second arm  50  comprises connection means for securing the C-clamp  41  to a main piston  44 . A second pad (also not shown) is provided on a movable clamping element  43 , which pad in use is brought into contact with the bark of the tree to firmly clamp the tree. As with the first embodiment, this arrangement allows applied vibrational forces which are parallel to the displacement between the respective lines of contact between the first and second pads and the bark, ie. substantially no tangential forces are applied to the bark. The clamping element  43  is provided on the end of a hydraulically driveable rod (not visible in  FIG. 4 ) which is provided within the main piston  44 . Therefore, the main piston  44  acts as a hydraulic cylinder itself within which the rod is hydraulically driveable. (See  FIG. 6 ) 
     In contrast to the first embodiment, the drive means comprise one vibratory reaction mass  47  rather than two, which mass is arranged to vibrate along an axis which passes directly through the tree, although as before, more than one vibratory mass can be used, so long as the arrangement does not result in any significant tangential forces being applied to the bark. The vibrating mass comprises a cylinder  47  guided for vibrating along the axis by the cage  45 . The cylinder  47  is slidably held in the cage  45  by four bars  51 , although a smaller or larger number of bars could be utilised. Where the cylinder  47  rests against each bar  51 , the latter has friction reducing means  46  to allow the cylinder  47  to slide with ease. In the preferred form of the second embodiment, the friction reducing means comprises Teflon® strips  46 , extending substantially the length of the bar  51 . The use of Teflon® strips  46  advantageously provides a simple slide construction with very few working parts. 
     The cage  45  is firmly attached to the hydraulic arm, with the cylinder  47  being freely slidable within the cage  45 . The C-clamp  41  is firmly attached to the piston  44 , which piston extends longitudinally through the entire length of the cylinder  47 . The piston  44  is also movable relative to the hydraulic arm and the cage  45 . A tube  49  can be provided for receiving the proximal end of the main piston  44 , to protect the piston  44  from engaging with and being damaged by other components of the hydraulic arm. In practice, the tube  49  is not always necessary. Indeed, the length of the piston  44  need not be so great as to extend further back than the cage  45 . 
     In operation, the user guides the C-clamp  41  into co-operation with the tree. The user brings the pad on the first arm  42  into engagement with the tree either by adjusting the position of the hydraulic arm or by moving the tractor itself. Then hydraulic pressure is applied through a solenoid valve (not shown) to extend the rod to bring the clamping element  43  and its pad into engagement with the tree. As with the first embodiment, the hydraulic system including its valves is controlled by the electronic control unit. With the tree firmly clamped, the hydraulic rod is held in its position relative to the main piston  44  by shutting the solenoid valve. It is then preferable to ensure that the piston  44  is centralised with respect to the cylinder  47  and that the cylinder  47  is centralised with respect to the cage  45 . To centralise the piston  44 , hydraulic fluid is fed as appropriate to cylinder chambers  47   a ,  47   b , a process which can be done automatically under the control of the electronic control means. The position information of the main piston  44  with respect to the cylinder, required by the electronic control means, is continuously provided by a displacement transducer placed between the cylinder and the main piston. A variety of displacement transducers, including a linear variable differential transformer (LDTV), can be used to provide position information. The cylinder  47  is then to be centralised within the cage  45 , and in this regard there is provided at each end of the cage a proximity sensor (not shown) which measures the distance to the cylinder  47 . Using this information the hydraulic arm is adjusted to centralise the cylinder relative to the cage. Again, this process can be done automatically by the electronic control means. The hydraulic arm can include a telescopically extendable region, to allow such movement of the cage. Alternatively, the tractor can be moved to directly adjust the cage position. This relative centralisation avoids any banging of the cylinder  47  against the cage  45  at the limits of its motion. 
     Then, pressurized hydraulic fluid is applied to the cylinder  47  via the proportional valve (not shown). The proportional valve is mounted on a adapter plate  52  in  FIG. 4 . This valve is controlled by the electronic control unit and is operable to provide a switching action with a high fidelity reproduction of the electrical signal applied thereto. Thus, the pressurized hydraulic fluid is directed alternately by the valve to first and second hydraulic chambers of the cylinder  47 , causing oscillatory relative motion between the cylinder  47  and the main piston  44 . 
     The cylinder  47  acts as a reaction mass whose vibrations are transmitted via the piston  44  and the clamping element  43  and the C-clamp  41  to the tree. The cylinder  47  preferably has a substantial inertial mass e.g. 100 to 500 kg. Correspondingly, it is preferable that the main piston  44  and C-clamp  41  are as light as possible, since in use they are fast with the tree. In practice, a mass ratio of the components fast with the tree to the cylinder  47  of 1:3 has been achieved and has produced effective results. 
     Accelerometers or similar such sensors can be fitted on the cylinder  47  and on the C-clamp  41  to measure the acceleration, the amplitude and the relative phase of their motions. The cylinder motion and the C-clamp motion are primarily out of phase with each other, since when the hydraulic fluid is pushing the cylinder  47  in one direction, the corresponding force on the main piston  44 , and hence the C-clamp is acting in the opposite direction. However, due to the compliance of the tree, this phase difference may vary. Using the sensors it is possible for the system to recognise the onset of any tree resonance, which might damage the tree, and to take automatic evasive action, eg. by reducing the amplitude and/or momentarily increasing rate of change of the frequency of the driving motion. 
     First and second pressure accumulators  48  are also shown in  FIG. 4  and are provided to absorb and dampen unwanted high pressure impulses which could damage the other components of the assembly. 
       FIG. 5  shows schematically the hydraulic system of the second embodiment. The assembly comprises three basic sub-assemblies: a three point hook-up for attaching the assembly to the rear of a tractor; the hydraulic arm which allows manipulation of the vibratory head, and the vibratory head itself situated at the end of the arm. In the left hand box of  FIG. 5  are illustrated the components of the assembly which are mounted on the tractor three point hook-up subassembly. These components comprise firstly a hydraulic pump  56  connected via a speed increaser  57  to a tractor power take off shaft  58 . The pressurised hydraulic fluid exits the pump  56  and goes through a high pressure relief valve  59 , which in one embodiment allows through a range of pressures from 10 to 210 bar. This is followed by a high pressure filter  60  to remove impurities which in turn is followed by a check valve  61 . Next is an accumulator  62  for dampening excessive pressure impulses in the system. From here, the high-pressure fluid is transmitted to the vibratory head via a first quick release connection means  64 , allowing easy removal and replacement of the vibratory head sub assembly. The tractor mounted components further include a low-pressure return section where the hydraulic fluid returns, the section including a second quick release connection means  64 , a low pressure filter  65 , an oil cooler  63 , and a reservoir  66  from which the hydraulic pump  56  sources hydraulic fluid. 
     In the vibratory head subassembly there is provided firstly the accumulator  48   a  corresponding to that shown in  FIG. 4 , followed by the solenoid operated valve  68 , which in particular may be a four-way valve, which valve is operable to allow hydraulic fluid to enter a first cavity  44   a  inside the main piston  44  via a pressure control valve  69  to push the rod  67  outwards so as to clamp the tree. The pressure control valve  69  reduces the clamping pressure to an appropriate level for firmly gripping the tree without damaging the bark. By virtue of this pressure regulation, it is possible to use the same source of pressurized fluid for both the clamping mechanism and the vibratory mechanism. The pressure control valve  69  includes a flow partition line  70 , extending from the valve  69  to the return conduit, which line allows the hydraulic fluid pressure applied to the cavity  44   a  to be reduced. 
     To withdraw the rod  67  from its clamping position after the olives have been removed from the tree, the valve  68  is operated, via the solenoid, to apply fluid to a second cavity  44   b  in the main piston  44 . One of the objectives of the invention is to improve the harvesting rate, so it is desirable to open the clamp quickly. Therefore, hydraulic fluid is transmitted from the solenoid valve  68  to the cavity  44   b  at full pressure. This is achieved by providing opening conduits, separate to those illustrated in  FIG. 5  (the closing conduits), connecting the solenoid valve  68  and the two cavities  44   a,b  which conduits transmit fluid at full pressure. Another possibility is for the pressure regulating valve  69  to be a two-way valve which restricts the fluid pressure in one flow direction (corresponding to the closing of the clamp) and allows full pressure in the opposite direction (corresponding to the opening of the clamp). The solenoid valve  68  itself could alternatively incorporate the necessary means to allow directionally dependent pressure control. 
     As an extra protection against overpressure in the clamping system, there can be provided along the high pressure line between the solenoid valve  68  and the cavity  44   a , a check valve (not shown) which in the event of an overpressure diverts fluid through a run-off line, bypassing the piston cavities  44   a,b  and thereby avoiding damage to the piston/rod system. 
     When the tree is clamped, and harvesting is to begin the proportional valve  71  is operated, as mentioned above to centralise the main piston with respect to the cylinder located within the cage. In  FIG. 5 , the main piston  44 , as shown inside the cylinder  47 , is represented schematically by a thin line, which is of course not to scale. A piston ring  44   c  which is fixedly connected to the surface of the main piston  44  is also represented in  FIG. 5 . This ring is designed for withstanding the high forces resulting from the vibratory pressures in the cylinder  47 . The proportional valve  71  is then operated to allow the pressurized hydraulic fluid to act on the respective cylinder chambers  47   a  and  47   b  alternately to cause the cylinder  47  and the main piston  44  (fast with its internal rod) to undergo vibratory motion relative to one another. Thereby, vibrations are applied to the tree, with virtually no retro transmission to the hydraulic arm and tractor. In other words, the vibrations of the cylinder, piston and C-clamp are substantially decoupled from the rest of the system. 
     The valve  71  can be electronically driven and the signal s(t) as specified above can be used, although other signals deemed suitable can be programmed into the control unit. A high fidelity reproduction of the signal is possible with the valve  71 , despite the high fluid pressures which are involved e.g. 200 bar. A return conduit feeds hydraulic fluid leaving the cylinder  47  or the piston cavities  44   a,b  to the tractor-mounted components via a low-pressure accumulator  48   b . The conduits which transmit fluid from the pump  56  to the cylinder  47  and the piston cavities  44   a,b  all need to withstand the high pressure of the fluid, so appropriate high strength hoses are used. 
       FIG. 6 , again not to scale, is a cross-sectional view of part of the cylinder  47 , showing the ends of the main piston  44  located inside the cylinder  47  and the rod  67  located inside piston  44  all concentric with one another. The cylinder chambers  47   a,b  and the piston cavities  44   a,b  cannot be seen in this view. The cylinder  47  includes a number of notches  81  corresponding to the number of Teflon® strips  46 , the notches and strips having matching profiles for low-friction sliding contact therebetween. 
     Compared to known shaker heads, especially those of the rotating mass type, the vibratory head can be engaged with the tree at a relatively low height, since the amplitude of the vibrations of the vibratory head is low. The tree can be considered to effectively pivot about the ground level, such that oscillations applied near the base of the tree are amplified higher up the tree. Not only is this sympathetic to the tree but also it means the user should easily be able to find an appropriate point on the tree to engage the clamp. It is also possible to engage the vibratory head higher up in the tree, and in particular, it can be engaged with individual branches. The vibrations are transmitted almost entirely to the tree, and not back to the tractor via the hydraulic arm. A camera (not shown) can be provided on the vibratory head or hydraulic arm to relay pictures of the C-clamp and tree to the user of the assembly, who may be sitting in the cabin of the tractor. Thereby, the user is able to accurately guide the clamp into engagement with the tree. The user controls the assembly using a joystick which can be provided in the tractor cabin. Alternatively, a portable remote control can be provided. This can include an rf transmitter for transmitting signals to a corresponding receiver of the electronic control unit or can simply be wired to the electronic control unit. It is envisaged that a much smaller and lighter machine can be built, using the same principles, which applies coded signals to the smaller branches near to or where the olive stems are attached.