Horizontal motion conveyor

Abstract

A horizontal motion conveyor, for transporting particulate material in a generally horizontal path comprises a longitudinal trough carriage 38 in the transport direction A, and in a asymmetric vibrato

Description

Technical Field The invention relates to an improved horizontal motion conveyor for horizontally transporting particulate material, particularly particulate friable foodstuffs. Background Horizontal motion conveyors are widely used in manufacturing environments where particulate materials are desired to be transported horizontally. Such horizontal conveyors are particularly useful fortransporting friable particles, such as foodstuffs, e.g. snackfoods, cereals etc. Horizontal motion conveyors comprise a length of trough that moves horizontally in the direction of the length of the trough. The motion is arranged so that the particulate material does not move relative to the trough in one direction but does move relative to the trough in the opposite direction. This is typically achieved by use of friction and the speed or acceleration of movement of the trough. In other words, the acceleration in the direction of desired travel of the particulate material does not overcome friction, and in the reverse direction the acceleration does overcome friction. The most common type of device operates in a vibratory manner, wherein the particulate material gradually inches horizontally as the trough vibrates horizontally. Thus, the vibratory movement is not symmetrical, otherwise horizontal movement of the particulate material would not occur, and this leads to complexity in the design of the motors and devices used to induce such asymmetric vibration. For example, so-called out-of-balance rotating weight drives are common in the art. In general, such devices have a slowly accelerating forwards stroke, followed by a fast return stroke, and the peak accelerations and hence peak forces on the trough occur in the return stroke near each extreme of the stroke. Horizontal motion conveyors commonly use out-of-balance rotors to create the asymmetrical forward / reverse motion. They use an ever-present high rotational inertia to smooth out the loads on the drive. These systems use two pairs of imbalanced rotors, each pair generating a sinusoidal acceleration in the transport direction. One pair will rotate at twice the angular velocity of the other. By adjusting the phase shift between the pairs, the summed acceleration profile can be optimized for producing high conveying speeds. However, even when optimized, the profile generated by these systems is far from ideal with respect to the task of conveying quickly. Additionally, optimizing the phase shift for conveying speed in one direction negatively impacts conveying speed in the reverse direction. As such, these systems lack flexibility in the motion profile, they are not well suited for reversing the conveying direction, and due to the large amounts of rotational kinetic energy present in the system, stopping and starting is very slow. The use of position-controlled drives to directly move the trough enables a fully customizable motion profile. The penalty associated with these drives is the size of motor required to be able to deliver the requisite peak torques. In these cases, all the kinetic energy in the system must be transformed into electrical energy and back into kinetic energy twice each cycle. This energy transfer is inefficient and generates significant heat in the electrical circuits. In an alternative approach, the conveyor may not move horizontally, and instead adopts movement in some non-horizontal direction in more than one dimension, in order to transport the particulate materials horizontally. For example, JPS58202209 A discloses a variation of the classic vibratory conveyor, and the flexure setup is no different from that. It achieves a trough trajectory which is slightly arched at about 15° from the transport direction and has a symmetrical vibratory movement. Particulates are moved in the transport direction by this non-horizontal movement. JPS6077015 A is the same arrangement but flipped on its side and aligned in the transport direction, and is otherwise identical. JP2018521927 A is a further example of the same design but with longer springs to minimize the arch in the trajectory. JP2023032507 A: discloses a mechanism with six degrees of freedom, and relies on the properties of the drive to move the platform linearly. Improvements in this area would therefore be desirable. Detailed Description of the Invention In a first aspect, the invention relates to a horizontal motion conveyor, for transporting particulate material in a transport direction that is generally horizontal, the horizontal motion conveyor comprising: a longitudinal trough aligned in the transport direction, having an upper surface for supporting the particulate material thereon, and a lower surface, connected to at least one linear motion compliant mechanism, the linear motion compliant mechanism comprising: at least one rigid beam, a plurality of elastic flexures connected to the at least one rigid beam, and a trough carriage connected to at least two elastic flexures and to the lower surface of the trough; the linear motion compliant mechanism being connected to a fixed support base unit; and comprising at least one motor operable to move a trough carriage in the transport direction in an asymmetrical vibratory manner; wherein asymmetrical vibratory movement of the trough carriage translates to asymmetric vibratory longitudinal movement of the trough. The invention exploits the spring return force to achieve higher accelerations. The elastic nature of the linear motion compliant mechanism acts to recover the energy in any return force, and thus converting it into useful elastic potential energy to be released for subsequent longitudinal movement. For a given size of conveyor this reduces both the largest torque and power peaks on the motor throughout a full cycle and thereby reducing the size of the motor required. Additionally, the trough carriage is moved in the transport direction, i.e. not at an angle to the transport direction, e.g. if the transport direction is horizontal, then the carriage is also moved in a horizontal orientation. Thus, true linear motion is achieved. Movement of the particulate material is achieved by the asymmetric movement of the trough carriage and therefore the trough. Thus, typically the movement of the trough carriage can be described as one-dimensional. Additionally, due to the efficient structural design of the compliant mechanism using reliable and proven flexural designs, there are no failure prone points in the trough carriage. The bearings in the drive can be isolated from all loads other than those experienced in the transport direction of the trough, which improves their expected lifetime. As used herein, the term “elastic” means that the material deforms under stress whilst storing potential spring energy. This stored energy is released when the stress is removed, and the material returns to its original unstressed conformation. Preferably the at least one rigid beam is parallel to the transport direction, however this need not be the case. The present invention preferably utilises double parallelogram flexures (i,e, the combination of the fixed support base connected to the rigid beam by two elastic flexures and the trough carriage also being connected to the rigid beam by two elastic flexures). Typically, the linear motion compliant mechanism comprises at least one double parallelogram flexure aligned to allow relative motion in the transport direction, connecting to a fixed base unit. The linear motion compliant mechanism can use double parallelogram flexures to eliminate parasitic motion such as any rotation or translation in the lateral and vertical direction and achieves true linear longitudinal motion. This, firstly, eliminates out of plane inertial loads on the trough which leads to potential for a lighter construction and, secondly, ensures that the frictional contact with the conveyed material remains consistent and predictable, which leads to potential for faster and more consistent conveying. In a preferable embodiment of the linear motion compliant mechanism double parallelogram flexures are arranged in pairs that are symmetrical over the centreline of the machine. This greatly improves rigidity of the carriage in the constrained degrees of freedom and greatly reduces the likelihood of the mechanism becoming locked, which occurs when the trough carriage is in the neutral position and the intermediate stage is moved away from the neutral position either by inertial forces or by external forces. Although the most common orientation for the transport direction will be horizontal, it can be inclined, e.g. by up to 20° whilst still being a generally horizontal direction as the term is used herein. In most circumstances, the trough will be of a substantial length, and so preferably the horizontal motion conveyor comprises a plurality of linear motion compliant mechanisms, spaced apart from each other in the transport direction. Thus, the trough is then supported by a plurality of trough carriages along its underside surface, providing stability and consistent motion along its length. For example, the trough may be from 5 to 50m in length. Although each of the linear motion compliant mechanisms may comprise their own motor, preferably at least one of the linear motion compliant mechanisms are not connected to a motor. This is because the elastic nature of the linear motion compliant mechanisms allows for the ones that are not driven directly by the motor to nevertheless support the trough and be driven by their connection to the trough. In one particularly preferred embodiment, the horizontal motion conveyor has only one motor. This avoids any issues with synchronisation of more than one motor, and one large motor is more desirable than a plurality of smaller motors, for reasons of cost and reducing the number of moving parts. Preferably the at least one linear motion compliant mechanism comprises at least two rigid beams parallel to the transport direction. This provides additional structural integrity and stiffness without compromising on the elastic properties of the compliant mechanism. Preferably, the plurality of elastic flexures are connected at both of their ends to a respective rigid beam. The elastic flexures are connected to and are typically substantially perpendicular to the at least one rigid beam. This means that they are perpendicular to the transport direction. In general, the elastic flexures will all be parallel with each other. In general the lengths of the elastic flexures fixed to a common rigid beam will be the same. The cross-section of the elastic flexures must be such that in bending around one principal axis it is substantially rigid, whilst being flexible whilst bending around the other principal axis. Thus preferably the elastic flexures have a cross section in a plane parallel to the transport direction, and wherein the cross section has a vertical dimension that is greater than a horizontal dimension. For example, the cross section could be a rectangular shape, with a long vertical dimension greater than the short horizontal dimension. The vertical dimension may be as much as 10 times greater, or even 100 times greater than the horizontal dimension. This provides for greater stiffness in response to any shear forces in the vertical direction, whilst retaining high flexibility in the transport direction. Additionally, this arrangement provides for very few surfaces where dust and debris can accumulate, which reduces dirt build-up. For example, the horizontal dimension of the cross-section may be less than 5mm, whereas the vertical dimension of the cross-section may be from 20 to 200mm. The elastic flexures may be horizontally aligned, although they need not be. The elastic flexures may be made from any suitable elastic material. However composite materials, such as glass fibre composites may be conveniently used as these have been shown to have excellent resilience and resistance against fatigue failure. In one preferred embodiment, the at least one linear motion compliant mechanism is made from a single unitary elastomeric structure. This permits simpler manufacturing methods to be employed such as 3-D printing, and reduces the need for assembly. Preferably the at least one linear motion compliant mechanism is connected to the fixed support base unit via at least two elastic flexures, e.g. in a double parallelogram flexure. This allows the linear motion compliant mechanism to have some degree of movement in the transport direction, so that it can deploy and recover elastic energy to reduce the peak torque and peak power requirements of the motor. In a preferred embodiment, the rigid beam comprises at least four elastic flexures, two carriage flexures connected to the trough carriage, and two base flexures connected to the fixed support base i.e. a double parallelogram flexure. In this embodiment, each pair consists of flexures of the same type, and the spring stiffness of the carriage flexures should ideally be equal to the spring stiffness of the base flexures. In a preferred embodiment, any double parallelogram flexure consists of flexures of equal length and preferably equal spring stiffness. Thus, preferably the rigid beams are not directly connected to the fixed support base unit. Ideally the rigid beams are therefore floating but nevertheless provide overall structural rigidity to the compliant mechanism. The rigid beams may be made from any suitably rigid material, such as plastic or metal, e.g. stainless steel. The rigid beams therefore perform a variety of essential functions, i.e. providing a rigid connection between the elastic flexures to provide a load-bearing path for the trough, provides rigidity to the carriage so that it is constrained to move only in the transport direction, and it can move laterally, cancelling out any lateral deflection of the elastic flexures. The compliant mechanism thus supports the trough carriage so that it is rigidly supported in the three principal axes of rotation, as well as being rigidly supported in the vertical direction and in the horizontal direction perpendicular to the transport direction. However, it provides a convenient elastic spring support of the trough carriage in the transport direction. These properties ensure that any unexpected external loads that are not in the transport direction are only supported by the compliant mechanism and not the drive. The motor can be any suitable electric motor that is capable of driving the trough carriage. However, preferably the motor is a stepper motor or a servo motor, as these provide a high degree of control. Preferably the at least one motor is rigidly attached to the fixed support base unit. With this arrangement the motor is fixed in space and not subject to any vibration, as it is flexibly connected to the compliant mechanism. In a preferred alternative arrangement, the motor is mounted on an independent linear motion compliant mechanism. In this arrangement, the motor moves in the opposite direction to the trough when the motor exerts a force on the trough. This arrangement can therefore be dynamically balanced. Thus, preferably the horizontal motion conveyor comprises a linear motion compliant mechanism comprising: at least one rigid beam parallel to the transport direction, a plurality of elastic flexures connected to and being substantially perpendicular to the at least one rigid beam, and a motor carriage connected to at least two elastic flexures and to a motor; the linear motion compliant mechanism being connected to the fixed support base unit. In this arrangement, the weight of the motor and the motor carriage provide a counterbalance to the weight of the trough and its trough carriage. For ideal balance, the ratio of the weight of the motor and the motor carriage to the weight of the trough and trough carriage is the same as the ratio of the spring stiffness of the trough compliant mechanism to the spring stiffness of the motor compliant mechanism, as this ensures a perfect balance of static and dynamic forces in the transport direction. Furthermore, it can be shown that the heavier the motor and motor carriage, the more energy efficient the mechanism becomes. Preferably therefore, the ratio of the weight of the motor and the motor carriage to the weight of the trough and trough carriage should be as high as practically possible. For example, a ratio of at least 2:1, preferably at least 4:1 or more preferably at least 6:1, e.g. around 8:1 provide excellent energy efficiencies. This is particularly useful for large conveyors, e.g. with a trough in excess of 15kg, as the dynamic forces become increasingly difficult for the supporting steelwork to handle. Although ideally, the counterbalance weight of the motor and motor carriage would lie on the axis of the trough, as this is not possible the motor and motor carriage should be located as close to the base of the trough as is practically possible. In order to induce the horizontal movement of particulate material supported on the trough, preferably the motor is operable to accelerate the trough carriage in the transport direction, at an acceleration that does not exceed the limit of static friction between the trough and the particulate material, and is lower in magnitude than the acceleration of the trough carriage in the opposite direction to the transport direction, during which the acceleration does overcome the limit of static friction between the trough and the particulate material. During the period of lower acceleration, the particulate material does not move relative to the trough, whereas during the return movement with high acceleration, the particulate material does move relative to the trough. This asymmetric vibration thus causes the transport of the particulate material. The trough carriage may be vibrated in the transport direction with a frequency of from 5 to 20Hz. The amplitude of displacement may be from 2 to 5cm for example. The accelerations achieved may be for instance of the order of 10ms'2 in the transport direction, and of the order of 50ms'2 in the return stroke direction. In one preferred arrangement the motor is operable such that the transport direction is reversible, so that particulate material may be transported in the opposite direction along the trough. This provides for a bi-directional function, allowing the direction of movement of the particulate material to be reversed, if desired. One advantage of the present invention is that the trough may be removably attached to the at least one trough carriage, such that the trough can be removed and replaced by a different trough or be temporarily removed for cleaning. Such replaceable troughs may be attachable by a quick-coupling mechanism. This simplifies the construction, and allows for a modular construction with a range of different types of trough available. In use, the upper surface of the trough preferably comprises friable particulate foodstuff. However, a wider range of particulate material may be conveyed by the apparatus according to the present invention. Thus, in a second aspect, the invention relates to a method of operating a horizontal motion conveyor as described herein, wherein the motor accelerates the trough carriage in the transport direction at an acceleration that is lower than the acceleration of the trough carriage in the opposite direction to the transport direction. The invention will now be illustrated, by way of example only, and with reference to the following figures, in which: Figure 1 is a perspective view of a linear motion compliant mechanism and motor, for use in the present invention. Figure 2 is a plan view of the linear motion compliant mechanism and motor shown in figure 1. Figure 3 is a is a side sectional view through the trough carriage of the linear compliant mechanism shown in figures 1 and 2. Figure 4 is a plan view of a prototype linear motion compliant mechanism for use in the present invention, made from a single unitary elastomer. Figure 5 is a plan view of another linear motion compliant mechanism and motor, for use in the present invention, with the trough carriage in a central neutral position. Figure 6 is a plan view of the linear motion compliant mechanism and motor shown in figure 5, with the trough carriage in an extreme position. Figures 7a to 7c are views of a horizontal motion conveyor according to the present invention. Figures 8a and 8b show views of another horizontal motion conveyor according to the present invention. Turning to the figures, figures 1 and 2 show a linear motion compliant mechanism 10 and motor 12 for use in the present invention. The linear motion compliant mechanism 10 comprises four rigid beams 14, 16, 18, 20 constituting two intermediate stagesmade from stainless steel, the ends of which are mounted within rigid plastic supports 22, 24, 26, 28 and which are parallel to a transport direction A. Extending between rigid plastic supports 22, 24 are two elastic flexures 30, 32, and extending between rigid plastic supports 26, 28 are two more elastic flexures 34, 36. A rigid trough carriage 38 is attached to elastic flexures 32, 34. Additionally, elastic flexures 30, 36 are attached to fixed supports 40, 42 which form part of a fixed support base unit (not shown). The motor 12 is mounted on the fixed support base unit (not shown) and is connected to the trough carriage 38 via a crank mechanism similar to that shown in figure 3. In use a trough would be placed to rest on top of the trough carriage 38 such that it is aligned with the transport direction A. The motor 12 is then activated to move the trough carriage 38 in the transport direction A in an asymmetric vibratory manner. This has the result that the elastic flexures 32, 34 begin to deform from the neutral position shown in the figures. This causes a deformation in the elastic flexures 30, 36 attached to fixed supports 40, 42. As they deform, elastic energy is stored in the elastic flexures. On the return stroke, when the trough carriage is moving in the opposite direction, this stored elastic energy is released, which is used to move the trough carriage in the reverse direction, relieving the motor 12 from some of the burden of driving the return motion. Figure 3 is a side sectional view through the carriage 38, showing how the motor 12 is connected to the trough carriage 38 via a slider-crank mechanism. The motor 12 comprises a crank arm comprising two hingedly connected links 50, 52 hinged together at point A. Link 50 is also connected to the motor acting as a crank arm, whereas link 52 is also hingedly connected to point B on the trough carriage 38 acting as a connecting rod. In use, link 50 is driven by the motor 12 to rotate, causing point A to prescribe a circular motion. This acts to move link 52 which has the effect of causing movement of point B, and therefore the trough carriage 38, in the transport direction A. Figure 4 is an image of a prototype linear motion compliant mechanism 60 for use in the present invention, made from a single unitary elastomer. The compliant mechanism 60 comprises two rigid beams (i.e. intermediate stages) 62, 63, made rigid by their increased thickness, and four elastic flexures 65, 66, 67, 68. A trough carriage 69 is also provided that is connected to elastic flexures 66, 67. Also shown is two rigid vertical struts 70, 71. In use the vertical struts 70, 71 would be held in place so that they are fixed during operation of the linear motion compliant mechanism 60. Figures 5 and 6 show plan views of another linear motion compliant mechanism 100 and motor 150, for use in the present invention. The linear motion compliant mechanism 100 comprises four rigid beams 104, 106, 108, 110 constituting two intermediate stages made from rigid plastic, the ends of which are mounted within rigid plastic supports 112,114,116, 118 and which are parallel to a transport direction A. Extending between rigid plastic supports 112, 114 are two elastic flexures 120, 122, which in this case are glass-fibre composite materials. Extending between rigid plastic supports 116, 118 are two more elastic flexures 124, 126 which are also glass-fibre composite materials. A rigid trough carriage 128 is attached to elastic flexures 122, 124. Additionally, elastic flexures 120,126 are attached to fixed supports 130, 132 which form part of a fixed support base unit 134. The motor 150 is mounted on the fixed support base unit 134 and is connected to the trough carriage 128 via a crank mechanism similar to that shown in figure 3. In use a trough would be fixed on top of the trough carriage such that it is aligned with the transport direction A, and the compliant mechanism would be in the neutral position shown in figure 5. The motor 150 is then activated to move the trough carriage 128 in the transport direction A in an asymmetric vibratory manner. This has the result that the elastic flexures 122, 124 begin to deform from the neutral position shown in the figure 5 to the extreme position shown in figure 6. This causes a deformation in the elastic flexures 120, 126 attached to fixed supports 130, 132. As they deform, elastic energy begins to be stored in the elastic flexures. On the return stroke, when the trough carriage is moving in the opposite direction, this stored elastic energy is released, which is used to move the trough carriage in the reverse direction, relieving the motor 150 from the burden of driving the return motion. Figures 7a to 7c show a front view, a side view and a plan view respectively, of a horizontal motion conveyor 200 according to the present invention for horizontally transporting particulate material in a transport direction, comprising a longitudinal trough 202 aligned in the transport direction A, a linear motion compliant mechanism 204 and a motor 206. As can be seen, the trough 202 is connected to a trough carriage 208 via connecting struts 210, 212. The linear compliant mechanism 204 is fixed to a fixed support base unit 206. The linear motion compliant mechanism 204 comprises four rigid beams 212 (i.e. two intermediate stages) parallel to the transport direction A, and four elastic flexures 214 connected to the rigid beams 212. In use, the motor causes the trough carriage 208 to vibrate asymmetrically in the transport direction, horizontally transporting particulate material in a transport direction. Figures 8a and 8b show a plan view and a side view respectively, of a dynamically balanced horizontal motion conveyor according to the present invention, but wherein the trough is not shown. The conveyor comprises a linear motion compliant mechanism 240 comprising two rigid beams 242, 243 parallel to the transport direction A. Elastic flexures 246, 249 are connected to and being perpendicular to the rigid beams 242, 243, and connected to two trough carriages 250, 251. Elastic flexures 247, 248 are connected to and being perpendicular to the rigid beams 242, 243, and connected to fixed supports 254, 255. The conveyor also comprises a linear motion compliant mechanism 260 comprising two rigid beams 262, 263 parallel to the transport direction A. Elastic flexures 267, 268 are connected to and are substantially perpendicular to the rigid beams 262, 263 and a motor carriage 270 upon which is mounted a motor 280. Elastic flexures 266, 269 are connected to and are perpendicular to the rigid beams 262, 263, and connected to fixed supports 254, 255. The motor 280 is therefore mounted on an independent linear motion compliant mechanism 260. In this arrangement, the motor 280 moves in the opposite direction to the trough when the motor 280 exerts a force on the trough. This arrangement can therefore be dynamically balanced. The weight of the motor and the motor carriage provide a counterbalance to the weight of the trough and its trough carriage.

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