Gear mechanism

Abstract

A gear mechanism 1 comprises a drive wheel 3 with a drive axis 9 and first and second drive wheel sections 61, 62 arranged along the drive axis 9, a driven wheel 4 with a driven axis 11 and first and

Description

Field The present invention relates to a gear mechanism for particular, but not exclusive use, in a robotic system. Background Rotating mechanical systems typically need to change speed or torque. Gear systems can be used to achieve this, but they can suffer from backlash whereby the gears lose contact when changing direction. Robotic systems often involve coupling of motors to joints at especially high gear ratios (for a single stage, generally greater than 10). Thus, as a rotating mechanical system, robotic systems also suffer from backlash. Robotic systems, however, can be particularly sensitive to backlash since small changes in position at a pivot can translate into large changes in position at the end of an armature. Another issue is that many robotic mechanisms use wires that pass through a joint and so a hollow shaft is common. Accordingly, drive is transferred across the joint at the periphery. Summary According to a first aspect of the present invention there is provided a gear mechanism. The gear mechanism comprises a drive wheel, a driven wheel and at least one strip wrapped around the drive and driven wheels. The drive wheel has a drive axis and comprises first and second drive wheel sections arranged along the drive axis. The driven wheel has a driven axis and comprises first and second driven wheel sections arranged along the driven axis. The first driven wheel section has a peripheral surface having a profile whose radius angularly varies and includes at least one step and wherein the second driven wheel section has the same profile in reverse. A first length of strip is at least partially wound around the first drive wheel section in a first sense and at least partially wound around the first driven wheel section in a second, opposite sense. A second length of strip is at least partially wound around the second drive wheel section in the second sense and at least partially wound around the second driven wheel section in the first sense. The first length of strip is configured such that, when the drive wheel rotates by more than a rotation in the second sense, the first length of strip is wound on top of itself. The second length of strip is configured such that, when the drive wheel is rotates by more than rotation in the first sense, the second length of strip is wound on top of itself. This can be used to achieve a high gear ratio with few stages, for example, just one stage, and to reduce backlash. The drive wheel may further comprise a third drive wheel section, wherein the second drive wheel section is interposed between the first and third drive wheel sections. The driven wheel may further comprise a third driven wheel section, wherein the second driven wheel section is interposed between the first and third driven wheel sections. The third driven wheel section may have the same profile as the first driven wheel section. A third length of strip maybe at least partially wound around the third drive wheel section in the first sense and at least partially wound around the third driven wheel section in the second, opposite sense. The third length of strip may be configured such that, when the drive wheel is rotates by more than a rotation in the second sense, the third length of strip is wound on top of itself. Using three drive wheel sections and three driven wheel section can help to reduce the net force on the axles of the drive and driven wheels. There may be more than three drive wheel sections and more than three driven wheel sections. According to a second aspect of the present invention there is provided a gear mechanism. The gear mechanism comprises a drive wheel, a driven wheel and at least one strip wrapped around the drive and driven wheels. The drive wheel has a drive axis and comprises first, second and third drive wheel sections arranged along the drive axis. The second drive wheel section is interposed between the first and third drive wheel sections. The driven wheel has an driven axis and comprises first, second and third driven wheel sections arranged along the driven axis. The second driven wheel section is interposed between the first and third driven wheel sections. The first and third driven wheel sections have a peripheral surface having a profile whose radius angularly varies and includes at least one step and wherein the second driven wheel section has the same profile in reverse. A first length of strip is at least partially wound around the first drive wheel section in a first sense and at least partially wound around the first driven wheel section in a second, opposite sense. A second length of strip is at least partially wound around the second drive wheel section in the second sense and at least partially wound around the second driven wheel section in the first sense. A third length of strip is at least partially wound around the third drive wheel section in the first sense and at least partially wound around the third driven wheel section in the second, opposite sense. The first length of strip is configured such that, when the drive wheel is rotates by more than a rotation in the second sense, the first length of strip is wound on top of itself. The second length of strip is configured such that, when the drive wheel is rotates by more than a rotation in the first sense, the second length of strip is wound on top of itself. The third length of strip is configured such that, when the drive wheel is rotates by more than a rotation in the second sense, the third length of strip is wound on top of itself. This can be used to achieve a high gear ratio with few stages, for example, just one stage, and to reduce backlash. The strip may consist of a metal, such as steel or aluminium, or a plastic, such as polyurethane, configured (for example, having a composition and / or a suitable thickness) so as to be inelastic (in other words not stretchable). The profile may be configured, between first and second angles of rotation of the driven wheel, to maintain a gear ratio between the drive and driven wheels as the drive wheel rotates and to maintain a length, L, of one loop of the strip. The first and second angles of rotation of the driven wheel may be limits of rotation of the driven wheel. The profile may be configured, between the second angle of rotation and a third angle of rotation, to maintain another different gear ratio. The profile may vary as a function of a diameter, d, of the drive wheel, a diameter, D, of the driven wheel, the thickness, t, of the at least one strip (5), and an initial number, nl, of turns on the drive wheel when fully wound. The profile may be determined numerically. The profile may be determined analytically. For example, the profile may be approximated using: D = diameter of the larger wheel R = D / 2 d = diameter of the smaller wheel strip_thickness = thickness of the strip used initial_turns = number of turns on the small wheel at full windup small_diameter = d + 2*strip_thickness*initial_turns # working diameter of the small wheel Gear_ratio = D / small_diameter current_curve_number = 0 curve_number_list = a list of curve numbers for that angle last_arc_angle = 0.0 For: angle from 0 to 360 (one full revoultion) { arc_length = n D*((rad - last_arc_rad) / (2 * n )) # length of the segment if arc_length greater than the small wheel + strips diameter, then change the profile { current_curve_number += 1 last_arc_angle = angle small_diameter = small_diameter - 2*strip_thickness D = Gear_ratio *smallDiameter R = D / 2 curve_number[i] = current_curve_number curve_number_list.Append (R) The at least one strip may comprise three strips, wherein the first length of strip is a first strip, the second length of strip is a second strip and the third length of strip is a third strip. The first, second and third lengths of strip may be different parts of one strip. According to a third aspect of the present invention there may be provided a robot 5 comprising the gear mechanism of the first or second aspect. Brief Description of the Drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a gear mechanism comprising a larger gear, a smaller gear and a thin band to couple the gears, in a first position; Figure 2 is a perspective view of the gear mechanism shown in Figure 1 in a second position; Figure 3 is a perspective view of the gear mechanism shown in Figure 1 in a third position; Figure 4 schematically illustrates a first, exaggerated, discontinuous curve profile for a larger gear and a smaller, drive wheel at different stages of unwinding; Figure 5 schematically illustrates strip length; Figure 6 schematically illustrates a second, smoothed, discontinuous curve profile for a larger gear and a smaller, drive wheel at different stages of unwinding; Figure 7 schematically illustrates a third curve profile for a larger gear and a smaller, drive wheel at different stages of unwinding which is similar to the second profile, but with a larger strip thickness of 1.5 mm to accentuate the change in profile size; Figure 8 schematically illustrates a fourth curve profile for a larger gear and a smaller, drive wheel at different stages of unwinding; and Figures 9A to 9E illustrate, in plan view, the larger and smaller gears and a band at different positions. Detailed Description of Certain Embodiments Referring to Figure 1, a gear mechanism 1 and a motor 2 are shown. The gear mechanism 1 comprises a drive wheel 3 (herein also referred to as a "shaft" "capstan" or "gear") which is coupled to the axle of the motor 2, a driven wheel 4 (herein also referred to as a "drum" or "gear") and at least one band 5 (herein also referred to as a "strip", "shim" or "belt") wrapped around the drive and driven wheels 3, 4, generally in a figure-of-eight arrangement. The band 5 may be made of a spring steel, aluminium or another suitably strong, inelastic, but flexible material, such as polyurethane, and which is incompressible. The band 5 may be reinforced, e.g., with wire, glass fibre or carbon, to help avoid stretching. The band 5 may have a strain (in other words, elastic extension) which can lead to an extension of less than or equal to 0.2 mm and, for some materials, less than or equal to 0.05 mm. The drive wheel 3 is generally circular in transverse cross section and had a relatively small diameter. As will be explained in more detail hereinafter, the drive wheel 3 may be divided into first, second and third sections 6, 6i, 62, 63, along its axis 7 between its ends 8, 9 around which corresponding lengths of the band 5 may be wound. There may be only two sections 6 and there may be more than three sections 6. The driven wheel 4 generally has a relatively larger diameter and comprises first, second and third sections 10, 10i, IO2, IO3 (herein also referred to as "layers") disposed along its axis 11 between its ends 12, 13. Each section 10i, IO2, IO3 has a peripheral surface 14, 14i, 142, 14s has a profile 15 whose radius angularly varies and includes at least one step 16. The radius of the surface 14 may vary smoothly (giving the profile the appearance similar to a snail cam) and / or comprise two or more arcs having respective radii and includes a plurality of steps. The second driven wheel section IO2 is interposed between the first and third driven wheel sections 101, IO3. The profile of the second driven wheel section IO2 is reversed (or "inverted") with respect to those of the first and third driven wheel sections 10i, IO3, in other words, the profile changes in the same way but in the opposite sense. The profiles of the first and third sections 10i, IO3 are aligned. The strip(s) 5 is (are) wrapped around a relatively large drum 4 either in a first sense 17i (for instance, anti-clockwise) or in a second, opposite sense 172 (for example, clockwise), and affixed at one end to it. The strip(s) 5 is (are) wrapped around the smaller driving wheel three multiple times. If a single piece of strip material is used, it can be 'Y'-shaped to stop its ends interfering, and it passes through the middle of the driving wheel 3. If multiple strips are used they are affixed to the driving wheel at the other end. The strips 5 run along the surface of the driven wheel and transfer torque through both friction and by pulling the wheel. Figures 1 to 3 show rotation of the driving wheel 3 and driven wheel 4 during the course of one revolution of one layer 6 of the larger, driven wheel 4. The smaller, driving gear 3 rotates many times. The strip 5 is wrapped, in this example, eight turns around a top section 63 of the driving gear 3 In Figure 2, the large wheel 4 is rotated by 180 degrees and the small gear 3 and strips 5 are equally wound. The top and bottom strips 5i, 5s wound around the small gear 3 have reduced in diameter. Thus, the large gear 4 needs to decrease in diameter to maintain the gear ratio. As will also be explained in more detail, the length of the strip 5 around one loop of the small and large gear 3, 4 is constant. Figure 3 shows the top and bottom shim 5 has been fully unwound and the device is near its clockwise extent. Figure 4 shows a simplified curve profile of the peripheral surface 12 obtained by analytical means. The curve can be generated from an idealised profile, and a number of simplifications. In deriving the profile, there are two, two-dimensional wheels, namely a smaller, driving wheel and a larger, driven wheel which are connected by a strip. The strip follows the profile of either the large or small wheel at all times. In reality, there is another wheel pulling in the reverse direction. This means that the gear ratio remains constant (or at least, the same as the wheel below), at least over a given range of rotation. Thus, in some cases, the profile may be configured such that the gear ratio is constant over a full rotation of the large gear (or its extent of rotation) and, in other cases, the profile may be configured that the gear ratio is a first constant for a first given range of rotation but changes to a second constant for a second given range of rotation. This case be used where different gear ratio are needed at different positions of travel. The derivation considers both, but assumes that they are the same (but mirror images) and so most of the consideration is of one layer of wheels at a time. Referring to Figure 4, it is easier to consider the large wheel as stationary and the small wheel as moving around some fixed radius: D : Instantaneous Diameter of the large wheel d : diameter of the small wheel The strip is incompressible and transfers the force along its length to the driven larger wheel. Referring to Figure 5, a strip 5 has a a strip length L between the small and large wheels 3, 4 remains constant. The length is calculated by considering three sections: the Archimedes spiral section 5a around the small wheel, the tangent section spanning the inner tangent between the wheels 5b, and the arcuate large wheel section 5c. The large wheel is made up of a sum of lengths of a series of circular profiles for simplicity. In Figure 5, two strips 5i, 52 having respective strip lengths Li, L2 are shown. The first strip length Li is for the upper layer 10i (or the bottom layer IO3), and the second strip length L2 is for the middle layer IO2. Each strip 5 has the same length, in other words, Li = L2. Pseudocode for the analytical derivation is given below: D = diameter of the larger wheel R = D / 2 d = diameter of the smaller wheel strip_thickness = thickness of the strip used initial_turns = number of turns on the small wheel at full windup small_diameter = d + 2*strip_thickness*initial_turns # working diameter of the small wheel Gear_ratio = D / small_diameter current_curve_number = 0 curve_number_list = a list of curve numbers for that angle last_arc_angle = 0.0 For: angle from 0 to 360 (one full revoultion) { arc_length = n D*((rad - last_arc_rad) / (2 * n )) # length of the segment if arc_length greater than the small wheel + strips diameter, then change the profile { current_curve_number += 1 last_arc_angle = angle small_diameter = small_diameter - 2*strip_thickness D = Gear_ratio *smallDiameter R = D / 2 curve_number[i] = current_curve_number curve_number_list.Append (R) } } In this pseudocode, the program generates a radius that is appropriate for the number of layers of strip that is around the gears. The mechanism is able to make multiple rotations of the drive (smaller) gear without using rope or wire. This is achieved by allowing the strips to wind multiple turns and modifying the profile of the larger wheel. Specific curves are used in the larger wheel to compensate for the changes in radius of the smaller wheel as the strips wind around it. A family of curves can be found numerically to minimise the change of gear ratio or maintain the same tension in the strips. A geometrical derivation with some simplifications can be used. First example Referring to Figure 1, the system consists of a axle 11, and a larger, driven wheel 4. An electric motor 2 is used to drive the larger wheel via a smaller drive wheel 3 and three metal drive belts 5i, 5?, 5s are employed to push and pull the larger wheel 4 at a high gear ratio. In order to achieve constant push and pull a particular curve 15 is selected for the larger wheel. In Figure 1, the motor is shown wound to one extent. The top and bottom metal belts 5i, 5s are shown wound around the motor drive wheel many times the profile of the larger wheel at the corresponding positions reflects the change of diameter of the drive wheel 3 caused by the wound belts 5. Referring to Figure 2, as the motor 2 rotates, the top and bottom belts 5i, 5s have unwound and the middle belt 52 has wound around the drive wheel 3. At this stage of rotation, the diameter of the driven wheel 4 diameter has changed. Referring to Figure 3, the motor 2 has wound up most of the single, middle belt 52 and unwound the top and bottom belts 5i, 5s. Second example The pseudocode above describes the creation of a suitable profile for one layer. Typically three or more layers are stacked in alternating winding directions. This allows the device to work in both directions. In order to avoid locking, the device maintains the same gear ratio throughout the angle of operation. For some profiles this can be nearly 360 degrees, for other arrangements the profile is expected to operate in a smaller range, for example in Figure 7, where operation is limited to around 270 degrees. The code works out the gear ratio when the larger, driven wheel is at its largest diameter and the driving wheel is fully wound up (this is used to set the gear ratio). Then it calculates the angle of an arc-length equal to the length of the spiral formed by the shim around the small wheel. For derivation the profile of the large wheel is kept stationary and the small wheel is moved in an orbital path around it. At this point the spiral has unwound one rotation, so the new radius of the larger wheel profile is calculated. This process repeats until the whole profile is built up. The profile is then smoothed to allow for the bend radius of the shim and tangential shim from the small wheel to the surface of the profile when the small wheel has a reduced diameter. Third example Referring to Figures 9A to 9E, an example of a drive wheel 3, driven wheel 4 and strip 5 at different angular positions gear ratio are shown. The drive wheel 3 has a diameter of 3 units, the large wheel 5 has a diameter of 150 units and he strip 5 has a thickness of 2 units. The gear mechanism maintains an approximately constant across the illustrated range of rotation, which is about 270°. Advantages The gear mechanism can exhibit one or more advantages over existing gear mechanisms, such as Capstan drives. First, it can be more compact than, for example, existing Capstan drives. Secondly, the mechanism exerts force in the plane of rotation whereas a Capstan drive exerts force at some angle to the plane of rotation. Thus, the mechanism can be more efficient. Thirdly, the thickness of the shim can be much smaller compared to a wire of equivalent strength. Thus, the expected fatigue can be lower for a given gear ratio. Furthermore, the shim can be made wider without increasing thickness to increase strength. Fourthly, the shim can have exceptionally low creep. Furthermore, backlash can be reduced and allow control to less than or equal to 0.1 mm tolerance and joint position tolerance to less than or equal to 0.01°. Modifications It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of robotic systems and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. The driven wheel may have only two sections or may have more than three sections 9. There may be N sections 9, were N is a positive, number equal to or greater than 2. Plastic bands, such as bands made from polyurethane, may be reinforced with carbon fibre or glass fibre to reduce the elasticity of the band. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. A gear mechanism (1) comprises:■ a drive wheel (3);■ a driven wheel (4);■ at least one strip (5) wrapped around the drive and driven wheels,wherein the drive wheel has a drive axis and comprises first and second drive wheel sections (6i, 62) arranged along the drive axis (9);wherein the driven wheel has a driven axis (11) and comprises first and second driven wheel sections (10i, IO2) arranged along the driven axis, wherein the first driven wheel section has a peripheral surface (14) having a profile (15) whose radius angularly varies and includes at least one step (16) and wherein the second driven wheel section has the same profile in reverse,wherein a first length of strip (5i) is at least partially wound around the first drive wheel section in a first sense (17i) and at least partially wound around the first driven wheel section in a second, opposite sense (17?); andwherein a second length of strip (5?) is at least partially wound around the second drive wheel section in the second sense and at least partially wound around the second driven wheel section in the first sense;wherein the first length of strip is configured such that, when the drive wheel is rotates by more than a rotation in the second sense, the first length of strip is wound on top of itself; andwherein the second length of strip is configured such that, when the drive wheel is rotates by more than a rotation in the first sense, the second length of strip is wound on top of itself.

2. The gear mechanism of claim 1:wherein the drive wheel comprises a third drive wheel section (63), wherein the second drive wheel section is interposed between the first and third drive wheel sections (61,62);wherein the driven wheel comprises a third driven wheel sections (IO3), wherein the second driven wheel section is interposed between the first and third driven wheel sections,wherein the third driven wheel section has the same profile as the first driven wheel section;wherein a third length of strip (5s) is at least partially wound around the third drive wheel section in the first sense and at least partially wound around the third driven wheel section in the second, opposite sense; andwherein the third length of strip is configured such that, when the drive wheel is rotates by more than a rotation in the second sense, the third length of strip is wound on top of itself.

3. The gear mechanism of claim 1 or 2, wherein the profile (15) is configured, between first and second angles of rotation of the driven wheel, to maintain a gear ratio between the drive and driven wheels (4, 5) as the drive wheel rotates and to maintain a length, L, of one loop of the strip.

4. The gear mechanism of claim 3, wherein the first and second angles of rotation of the driven wheel are limits of rotation of the driven wheel.

5. The gear mechanism of claim 3, wherein the profile (15) is configured, between the second angle of rotation and a third angle of rotation, to maintain another different gear ratio.

6. The gear mechanism of claim 1 or any one of claims 2 to 5, wherein the profile varies as a function of a diameter, d, of the drive wheel (3), a diameter, D, of the driven wheel (4), the thickness, t, of the at least one strip (5), and an initial number, nl, of turns on the drive wheel when fully wound.

7. The gear mechanism of claim 1 or any one of claims 2 to 6, wherein the profile is determined numerically.

8. The gear mechanism of claim 1 or any one of claims 2 to 7, wherein the profile is determined analytically.

9. The gear mechanism of claim 8, wherein the profile is determined using:.D = diameter of the larger wheelR = D / 2d = diameter of the smaller wheelstrip_thickness = thickness of the strip used initial_turns = number of turns on the small wheel at full windupsmall_diameter = d + 2*strip_thickness*initial_turns # working diameter of the small wheelGear_ratio = D / small_diametercurrent curve number = 0curve_number_list = a list of curve numbers for that angle last_arc_angle = 0.0For: angle from 0 to 360 (one full revoultion) { arc_length = n D*((rad - last_arc_rad) / (2 * n )) # length of the segment if arc_length greater than the small wheel + strips diameter, then change the profile { current_curve_number += 1 last_arc_angle = angle small_diameter = small_diameter - 2*strip_thickness D = Gear_ratio *smallDiameter R = D / 2 curve_number[i] = current_curve_number curve_number_list.Append (R)10. The gear mechanism of claim 1 or any one of claims 1 to 9, wherein the at least one strip comprises two strips, wherein the first length of strip is a first strip, and the second length of strip is a second strip.

11. The gear mechanism of claim 1 or any one of claims 1 to 9, wherein the first and second lengths of strip are different parts of one strip.

12. A robot comprising the gear mechanism of claim 1 or any one of claims 2 to 11.

Images