WO2014134688A1

WO2014134688A1 - Anti backlash gearbox and system

Info

Publication number: WO2014134688A1
Application number: PCT/AU2014/000230
Authority: WO
Inventors: Anthony Ross Forsyth
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2013-03-08
Filing date: 2014-03-07
Publication date: 2014-09-12

Abstract

The invention provides an anti-backlash system for reducing backlash between a positional drive and a final drive. In one form, the system is embodied as a gearbox 10 including a pair of differential transmissions 100 & 100', each transmission having first (101 & 101') and second mechanical inputs (105 & 105') and a mechanical output (106 & 106'). The respective first inputs from each transmission are paired together in a sum-input pair and are configured to be driven in a complementary direction by the positional drive (107). The respective second inputs are paired together as a difference-input and are configured to be loaded in an opposing direction by way of a bias means (109), whilst the respective outputs form an output pair for driving a final drive.

Description

ANTI BACKLASH GEARBOX AND SYSTEM

Field of the Invention

[0001] The present invention relates generally to systems for preventing gear backlash and more particularly to gearboxes for reducing and/or eliminating backlash in positional drive systems.

Background of the Invention

[0002] The invention has been developed for use in the positional control and movement of astronomy telescopes and antennas and will generally be described in this application. However, the invention is not intended to be limited to this particular field and may be used in other drives systems and applications where gear backlash presents a significant problem.

[0003] The following discussion of the prior art is intended to facilitate an understanding of the invention and to enable the advantages of it to be more fully understood. It should be appreciated, however, that any reference to prior art throughout the specification should not be construed as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field.

[0004] All moving mechanical devices that involve sliding contact between parts require physical clearances between those parts to account for manufacturing tolerances, misalignment, thermal changes, and commonly, space for a film of lubricant. Such clearances and/or tolerances maybe an intentional part of the design, simply inherent in the design, or both.

[0005] In the case of inter-engaging gears and gear teeth, the clearance may result in a certain amount of disconnect between the gear teeth which becomes particularly noticeable when changing drive direction. This is because the gear engagement transitions between opposing sides of the gear teeth and allows a small degree of free play or gear slop. The slop is commonly referred to as backlash.

[0006] In many applications backlash is of no consequence. For instance in gear drives designed for the transmission of power only. However, in the case of gear drives also designed for positioning, the un-controlled movement between gears during changes of direction introduces inaccuracy into the positional drive.

[0007] Inaccuracy caused by backlash becomes of increasing concern in positioning applications which require precision. One such application which will be described herein relates to telescope and antenna positioning drives where due to simple geometry, backlash can cause positioning errors many orders of magnitude larger than the pointing tolerance required. However backlash can be a serious problem in many other precision applications.

[0008] Several, varied attempts have been made to overcome or limit the problems of backlash. One method of reducing or eliminating backlash is to forcibly engage one set of gears with the other such that the teeth have no clearance. This is a cheap and simple method which provides equivalent anti-backlash in either driving direction. However one notable drawback is that the forced engagement subjects the gears to high frictional loading resulting in power loss and high wear rates, eventually leading to the development of wear- related backlash.

[0009] Another method involves an adjustable, split gear such as seen in US patent 5,409,430. This solution is similar to a forced engagement gears except that two similar gears are used side-by-side on a common axis to drive another gear. The two gears are fixed on the shaft and slightly angularly offset from one another by an adjustment means so as to "fill" any slack between the teeth of the intermeshing gear thereby minimizing backlash. Adjustable, split gears allow equal drive capacity on reversal, and are still cheap and simple. However, they do not provide true anti-backlash operation rather they merely provide backlash minimization. Furthermore, they do not automatically account for gear wear and require constant maintenance and recalibration. They will also only account for backlash in one pair of gears in a gear train.

[0010] Another method involves spring biasing gears to account for back-lash. In one commonly applied system a "split" gear is configured such that one half of the split gear is firmly fixed to the shaft while the other can rotate angularly with respect to the first. This rotation is torque biased, usually with a spring, to a position somewhat angularly offset from the fixed gear.

[001 1] When installed with a mating gear, the teeth of the split gear pair are biased to expand within the tooth space of the mating gear under the influence of the spring, so one part of the split gear contacts one flank of the mating gear teeth, the other gear, the other side. When diving in one direction, torque is transferred through the fixed gear, the other direction through the spring-loaded gear. This split gear solution is relatively simple and cheap. In addition it provides a measure of automatic adjustment for wear. Such an arrangement can be seen in US 1 ,604, 105 for example.

[0012] Split gear systems have some drawbacks particularly when driving in through the spring loaded gear. If drive loads exceed the resilient biasing force, backlash is reintroduced as the spring takes up. Therefore the system provides unequal drive characteristics in each direction. Accordingly, split gear backlash systems are usually only used in applications where power transmission is relatively light and the biasing spring will not be overloaded. Another disadvantage, as with many other existing anti-backlash systems is that split gears only account for backlash in the one pair of gears in a train where they are installed.

[0013] Split gear systems also suffer from a loss of positioning accuracy when the drive torque in the direction of the spring-biased gear approaches and exceeds resilient biasing force. At that drive load, further rotation of the diving motor is absorbed into deflection of the bias spring without a corresponding rotation of the drive output, leading to a loss of output position control and potential system instability.

[0014] In more complex forms the fundamental principle of a split gear bias system may be embodied by utilising two drive paths within a differential gear box to provide an anti- backlash bias within the drive train. For instance, US 3,665,482 provides a bias to a bevel drive differential gearbox via a bevel drive gear, while WO 02/09998 provides a torsion axel connecting paired planet gears in an epicyclic gearbox. These configurations may provide bias on one or both drive paths however still provide many of the disadvantages of the split gear bias system. Particularly because if the bias limit is exceeded, the positional accuracy and anti-backlash capability can be compromised.

[0015] As such, in many heavy load applications, none of the above solutions adequately cope with larger loads in either drive direction. One common solution is the provision of two complete gear drives and respective servo systems for each moving axis. Each drive has an output pinion that engages the final drive gear on the moving axis. The servo drives are programmed to be biased against the other drive by a predetermined amount. [0016] The sum of the input torques matches the output drive torque, thus when the input torques are equal and opposite, i.e., the total output torque applied to the final drive gear is zero. However backlash is reduced or eliminated because the output drive pinions are driven by the respective servo motor to each contact opposite sides of a respective tooth flank on the final drive gear. Of course it will be appreciated that in order to make the final drive gear move there must be a non-zero torque applied, requiring the torque applied by one drive to be larger . This is achieved by programming a set "difference" between the torque outputs of the two drives.

[00 7] A torque plot for a two-servo drive is shown as Figure 1. Input torque for each of the two drives are shown as lines T and T'. Discounting losses and inefficiencies, the sum of the input torque T + T matches the output torque shown as line T₀.

[0018] Referring to Figure 1 , if the difference between T and T is constant (K), then the output torque is:

To = 2T - K

[0019] As there is a difference between T and T at all points between CCW-C (Counter Clockwise) and CW-C (Clockwise) crossover, the gear teeth are loaded in opposite directions and are therefore in an anti-backlash mode. Beyond the CCW-C and CW-C crossover points either torque T or torque T crosses zero, then changes its direction of loading and the two drives work together instead of opposing. The gears are no longer in anti-backlash mode.

[0020] In two-servo telescope/antenna drives this is usually called high-torque mode, and might occur for instance, under high wind loads of a telescope. Loss of anti-backlash in this mode is of less importance as the output load keeps the gears positively loaded in one direction.

[0021] There is no loss of position control at the crossover as the load is taken by the drive that is not crossing zero torque.

[0022] A significant advantage of this method is that it accounts for backlash in every stage of a multi-gear train, as the anti-backlash loading comes from the drive system input shaft connected to the servo motor. The drawback is that this solution is expensive, complicated, and usually takes a great deal of work to install and calibrate. Furthermore, the system consumes a more power than would otherwise be needed, as the two drives constantly operate against each other, even when stationary.

[0023] It is an object of the present invention to overcome or substantially ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative.

Summary of the Invention

[0024] Accordingly, in a first aspect the invention provides an anti-backlash gearbox for reducing backlash between a positional input drive and a final drive, the system including: a pair of differential transmissions each having first and second inputs and an output;

the outputs of the transmissions being paired together as an output pair to drive the final drive;

the first inputs being paired together as a sum input and configured to be driven in a common direction by the positional input drive under a drive load thereby driving the respective outputs in a common direction; and

the second inputs being paired together as a difference input and configured to be biased in opposing directions under an input bias load thereby loading the respective outputs in opposing directions.

[0025] The terms "input/s" and "output/s" as used herein generally refer to the mechanical means for the transmission of mechanical loads and/or torque to and from the transmissions and/or gearbox. Inputs and outputs maybe in the form of shafts, levers, racks, belts, gears or any type of mechanical device for transmitting a mechanical force/torque to or from the system.

[0026] The term "differential transmission" is used herein to define a transmission type having at least three input/output components, whereas the term "epicyclic transmission" is used to refer to a planetary gearing system consisting of one or more outer gears, or planet gears carried on an planet carrier, revolving about a central, or sun gear within an annulus.

[0027] Preferably, the difference input is configured to load each second input in equal but opposing directions thereby loading respective transmissions and corresponding outputs in equal but opposing directions. [0028] Preferably, the input bias load is generally constant. [0029] Preferably, the input bias load is variable.

[0030] Preferably, the input bias load is variable in response to predetermined parameters.

[0031] Preferably, the parameters include the drive load.

[0032] Preferably, the input bias load is adjusted to remain greater than the drive load.

[0033] Preferably, the parameters are monitored by a control unit.

[0034] Preferably, each transmission is an epicyclic transmission including respective sun, annulus and planet carrier components.

[0035] More preferably, each transmission is a like epicyclic transmission including respective sun, annulus and planet carrier components and wherein each component is paired with a like component to form sun, annulus and planet carrier component pairs and wherein each component pair is configured as first input, second input and output pairs respectively.

[0036] Preferably, the planet carrier component pair is configured as a first input.

[0037] Preferably, the sun component is configured as a first input.

[0038] Alternatively, each transmission is a differential bevel gear transmission.

[0039] In a second aspect, the invention provides an anti-backlash system for reducing backlash between an input drive and a final drive, the system including:

an anti backlash gearbox in accordance with any one of the preceding claims; positional drive means for providing rotational drive to the positional input drive; and

biasing means for providing a bias load to the difference input. [0040] Preferably, the positional drive means is an electronic servo motor. [0041] Preferably, the biasing means is passive torque generating device.

[0042] Preferably, the biasing means includes a spring and damper.

[0043] Preferably, the biasing means includes an active torque generating device for providing the bias load to the difference input.

[0044] Preferably, the active torque generating device is controllable to provide a bias load of selectable magnitude.

[0045] Preferably, the biasing means includes an electronic servo motor.

[0046] Preferably, the system includes a control unit for controlling the biasing means.

[0047] Preferably, the control unit includes a microprocessor.

[0048] Preferably, the control unit includes an input sensor for monitoring positional drive means.

[0049] Preferably, the control unit actively varies the bias load output from the biasing means in response to the input sensor.

[0050] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are intended to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Brief Description of the Drawings

[0051] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0052] Figure 1 is a graphical representation of the torque output of an anti-backlash gearing system;

[0053] Figure 2 is a schematic representation of an anti-backlash gear system in accordance with the invention; [0054] Figure 3 is a schematic view of an anti-backlash gearbox in accordance with the invention; and

[0055] Figure 4 is a schematic representation of another anti-backlash gear system in accordance with the invention.

Preferred Embodiments of the Invention

[0056] The invention provides an anti-backlash system for reducing backlash between a positional drive and a final drive. In one form, the system is embodied as a gearbox including a pair of differential transmissions, each transmission having first and second mechanical inputs and a mechanical output. Each of the inputs and the output of one transmission are paired with the corresponding inputs and output of the other transmission for simultaneous operation.

[0057] Specifically, the respective first inputs from each transmission are paired together in a sum-input pair and are configured to be driven in a complementary direction by the positional drive. The respective second inputs are paired together as a difference-input and are configured to be loaded in an opposing direction by way of a bias means, whilst the respective outputs form an output pair for driving a final drive.

[0058] In operation, when the positional drive applies a rotational torque to the sum-input pair, a portion of that rotational torque is transmitted through each transmission to drive their respective output in a common direction thereby effecting rotational drive of the final drive, at the output pair.

[0059] In contrast, a bias torque applied to the difference-input will act through each transmission in a direction opposite to the direction of the other transmission. Since the output pair are commonly attached to the same final drive gear, the bias torque causes the output pairs work against each other. Moreover, the bias torque is equally split between each transmission, and thus the transmitted bias through each transmission is effectively cancelled out by the other transmission from a positional drive perspective. Furthermore, the bias torque is applied not only to the engagement between each drive pinion and the final drive gear, but also every gear engagement throughout each transmission thereby removing backlash. [0060] A schematic view of the concept of the invention is displayed in Figure 2. This schematic representation is provided for the purposes of explaining the inventive concept and is not intended to limit or define the scope of invention in any way. In the figure, a first transmission may be thought of as comprising first and second input rods 1 & 2 and output pin 3 while second transmission comprises first and second input rods 1 ^' & 2' and output pin 3'. The first input rods 1 and V are paired as sum input pair and connected to sum input shaft 5. The second input rods 2 and 2' are paired as difference input pair and connected to difference input shaft 6. The output pins 3 and 3' are paired as the output pair and configured to engage teeth of a final drive represented by toothed rack 4.

[0061] In this figure it can clearly be seen that clock-wise rotation of sum input shaft 5 will cause first input pair rods 1 & 1 ' to move to the left of the page. On the other-hand, a clockwise rotation of difference input shaft 6 will cause rod 2 & 2' to move in opposite directions, rod 2 to the left of the page and rod 2' to the right of the page.

[0062] The movement of the each rods 1 , 1 ', 2, and 2' are respectively translated to each drive pin 3 & 3' by means of connecting arms 7 & 7'. As such it will be appreciated that the clockwise rotation of sum input shaft 5 will cause each of the pins 3 & 3' to move to the left thereby moving rack 4 to the left.

[0063] On the other hand, a clock-wise bias torque applied to difference input shaft 6 will cause pin 3 to be driven towards the left of the page and pin 3' to be driven in the opposite direction toward the right of the page.

[0064] It will be appreciated that this bias torque loads each pin 3 & 3' in contact with a respective tooth but such that each pin is engaged with a corresponding, opposite side of the tooth. Thus, because application of a torque through difference input 6 sees each side of the system work against the other, torque applied through the difference input cannot cause movement of the rack 4. Rather it merely biases each side of the system into engagement with the rack such that output drive in one direction is loaded through one pin whilst output drive in the other direction is loaded through the other pin. In the figure shown, where each input shaft is loaded in a clockwise direction, drive is affected through pin 3. On change of drive direction, drive load is automatically passed to the pin preloaded in that direction. [0065] It will also be appreciated that the direction that the torque load is applied through difference input shaft 6 is unimportant in this configuration, provided each input rod 2 & 2' are biased in opposing directions. Swapping the direction of bias torque through shaft 6 will merely cause each pin 3 and 3' to move across the respective gear tooth gap to abut the adjacent gear tooth.

[0066] As displayed in Figure 2, the drive torque applied to the sum input shaft is less than the bias torque applied to the difference input shaft. In this situation the net loading on the drive pins makes them act in opposite directions and they will rest on opposite sides of the respective gear teeth gap as shown. However, when the drive torque is increased beyond the level of the bias torque, the net output of each pin will pins 3 and 3' will begin to operate in the same direction. This effectively mimics the torque operation of the two-servo drive system as depicted in the graph of Figure 1 , beyond the CCW-C and CW-C crossover points. For instance referring to Figure 2, as the positional torque applied to shaft 5 increases, and the crossover point is reached, the net load on pin 3' will swap from acting to the right, to acting to the left. Accordingly at this point it will cross-over the gear teeth gap, and begin to drive to the left, in the same direction as pin 3.

[0067] It will also be appreciated that the arrangement shown in Figure 2 will only provide for very limited movement of rack 4. Accordingly as can be seen with reference to Figure 3, in a preferred embodiment of the gearbox 10, each differential transmission is in the form of an epicydic differential 100 & 100' having respective, annulus (101 & 101 '), sun (102 & 102') and planet carrier (103 & 103') components. Other configurations of differential type transmissions and/or epicydic gearboxes may also be considered to provide similar or equivalent results. Otherwise, returning to Figure 3, the components interact in the usual manner as is known to provide a predetermined gear ratio from reduction to overdrive as is required. Like many differential transmissions, the epicydic transmission may be configured to have one input component, one output component and one stationary component, one input component and two output components or, as configured in the invention, two input components and one output component.

[0068] In addition, each input/output component includes drive means enabling drive and/or a torque load to be transmitted to and/or from the component. For instance, with reference to Figure 3, each annulus component (101 & 101 ') includes a respective annulus drive gear (104 & 104') on an outer periphery surface. Each sun component (102 & 102') is connected by means of a shaft to a respective sun drive gear (105 & 105') and each planet carrier (103 & 103') is connected by means of a shaft to a respective planet carrier drive pinion (106 & 106').

[0069] In this embodiment the annulus components (101 & 101 ') are paired together as the sum input by connecting respective annulus drive gears (104 & 104') to sum input drive shaft 107 and sum input gear 108. It is noted that sum input gear 108 is directly mated to each annulus drive gear (104 & 104') such that rotation of the sum input drive shaft 107 and gear 108 result in the annulus components being rotated in a complementary direction.

[0070] In contrast, the sun components (102 & 102') are mated together as the difference input by connecting respective sun drive gear (105 & 105') to difference input shaft 109 and a pair of difference input gears 110 & 1 11. It is noted that the difference input shaft 109 and gears 110 & 111 , are configured to rotate each sun drive gear 105 & 105' and respective sun component in opposing directions.

[0071] In addition, the planet carrier components (103 & 103') are configured as the output pair. Each planet carrier drive pinion 106 & 106', is engaged with a common final bull drive gear or drive rack (not shown).

[0072] In operation, applying rotational motion to the sum input shaft 107 rotates the components of each transmission in a similar direction such that each planet carrier drive pinion 106 & 106', will drive the final bull drive gear in the same direction. Referring to Figure 3, clockwise rotation of shaft 107 (shown as solid arrow T will cause a corresponding anti-clockwise rotation in each of the annulus components (101 & 101 '). This in-turn causes a corresponding anti-clockwise rotation of the planets and a corresponding clockwise rotation of each plant carrier component (103 & 103') and output drive pinions 106 & 106. As such the final bull drive gear will be driven cooperatively by the drive pinions 106 & 106', in an anticlockwise direction as shown by solid arrows I^s _m and T We can refer to the output torque at each shaft 106 and 106' due to the sum drive as T and T^s ₀₆'. The torques should be generally equal such that:

[0073] T^s ₁₀₆ = T^s ₁₀₆<

[0074] On the other hand, applying a clock-wise torque to the difference input shaft 109 (shown as broken line arrow T generates an equal bias in each transmission in opposite directions such that each planet carrier drive pinion 106 & 106', will be biased to engage the final drive gear in opposing directions. Specifically, referring again to Figure 3, applying a clock-wise torque to the difference shaft 109 generates an anti-clockwise loading in sun component 102, but a clockwise loading in sun component 102'. These conflicting loadings will result in planet carrier 103 being biased in an anti-clockwise direction and planet carrier 103' being biased in a clockwise direction. As such, because the difference input is effectively equally split by the difference input gear 1 10 & 110', the drive pinions 106 & 106' will be equally but oppositely biased as shown by broken line arrows T and T This effectively cancels out any movement due to the difference input but at the same time biases the drive pinions against one another. Referring to the output torque at each shaft 106 and 106' due to the difference drive as T^D ₀₆ and T^D _W. The torques should be generally equal but in opposite direction such that:

[0075] -T^D ₁₀₆ = T^D™

[0076] Of course the total output torque at each shaft 106 and 106' denoted as Τι_0β and Τιο6' will be equivalent to the sum of the torques due to the sum drive and difference drive. Thus:

[0077] (T_10e = T - T ) and [0078] (T_1fl6. = T^s ₁₀₆. + T^D _10f5 )

[0079] From these equations it will be appreciated that no matter which direction of either input 107 or 109, the net torque at one drive pinion will be the addition of the sum and difference outputs while at the other drive pinion will be the difference of the sum and difference outputs on the other drive pinion.

[0080] Furthermore, in the case where the net input value of T^s is less than the net input value of T^D for each shaft, the net output drive of each pinion 106 and 106' will be working in opposite directions. On the other hand if the net input value of T^s is greater than the net input value of T^D, both pinions 106 and 106' will drive in the same direction albeit with one shaft driving with greater torque than the other.

[0081] In this regard, a plot of torque from this device is generally identical to that shown for the servo anti-backlash in Figure 1 provided the bias torque applied to the difference input is held constant. [0082] Referring again to our equations, the CCW-C and CW-C crossover points will occur when the output drive torque from the sum drive exceeds the output bias torque from the difference drive. Or put another way, when the crossover will occur when T^s = T^D and will be exceeded when T^s > T^D.

[0083] As noted, for the linear configuration illustrated in Figure 2, the crossover points marks the point at which the both pinions begin driving the bull gear in the same direction. At this point the previously retarding pinion transitions from engagement with one side of the bull gear teeth to engagement with the other side.

[0084] The invention also includes an anti-backlash system including the gearbox. In this embodiment the gearbox is configured such that the sum input shaft would be driven by a positional drive. This may take a variety of forms however, most commonly a rotational drive would be employed such as an electric single servo motor able to provide rotational drive in both directions. However, when not moving the system, in contrast to the two motor system of the prior art, the motor would not be drawing power.

[0085] On the other hand, the difference input shaft 109 may be driven by a passive or active torque generating device. For instance passive torque generating devices might include a simple bias means such as a spring, mass bias or electronic, hydraulic or gas pressure bias means.

[0086] In preferred embodiments, the bias means includes a damping means. Damping means are particularly advantageous in managing the gearbox during unexpected (or expected) CCW-C and CW-C crossover when the system transfers from opposite drive to co-operative drive. This may occur in gusting wind loads or the like. Damping means can also help maintain positional control and prevent damage to the system.

[0087] In some embodiments, the passive or constant bias means can be replaced or complimented by an active or variable bias means, or active torque generator such as a small low-cost servo motor. This allows the system to adjust the anti-backlash bias force as required. For instance, such active torque generating devices enables the system to have multiple operational modes. In one operational mode, the bias torque applied to the difference input may be maintained just above the torque applied to the sum input. This would maintain the paired transmissions in an unconditional anti-backlash mode where the respective drives always oppose, rather than swapping from an opposed drive at low torques to co-operational drive or shared load at high torque.

[0088] Alternatively, the system can switch between the two modes to achieve load sharing under some conditions of high load or unconditional backlash control under others.

[0089] In further preferred embodiments, a control unit is incorporated to monitor system parameters indicative of the system operation to determine which mode to operate in and adjust the bias torque accordingly. The control unit may utilise a microprocessor and inputs from torque and other sensors to control the torque generating device.

[0090] These control units, microprocessors, sensors, motors and torque generating devices may be incorporated into the gearbox or added as separate components. As such the inventive concept extends beyond a gearbox as previously described, to a anti-backlash drive system including an anti-backlash gearbox, and a control system

[0091] Another embodiment of the invention is shown in Figure 4. In this embodiment, of the gearbox 20 the differential transmissions are bevel gear differential transmissions 200 and 200'. As with the epicyclic transmissions, each of the bevel differentials have been configured to operate with two inputs and a single outputs. The respective inputs are paired as are the outputs.

[0092] Referring to the Figure 4, each transmission 200 and 200' respectively includes first and second sun gears (201 , 201 ') and (202, 202') each connected to a carrier (203, 203') by means of a pair of planet gears (204, 204') and (205, 205'). Each carrier (203, 203') includes and output drive pinion (206, 206'). The first pair of sun gears (20 , 201 ') are configured as the sum input being connected by shafts to a sum input drive shaft 207 and gear 208. The second pair of sun gears (202, 202') are configured as the difference input being connected to difference input shaft 209 and a pair of difference input gears 210 & 211.

[0093] Notwithstanding the differences in configuration, the bevel differential transmissions operate in an analogous manner to the epicyclic differential transmissions shown in Figure 3. [0094] Each planet carrier drive pinion 206 & 206', is engaged with a common final bull drive gear or drive rack (not shown).

[0095] Similarly, the bevel drive transmission can also be provided with either passive and/or active torque generating devices.

[0096] It will be appreciated that the present invention provides many differentiators from existing technology. The gearbox provides anti-backlash to the entire gear train, not simply the final drive. It provides an equal drive capacity in either direction compared to split-gear designs, and does not apply high wear gear tooth loads compared to forced engagement designs. Moreover, in comparison to these two systems, the bias can be adjusted externally and on the fly, to account for increase torque requirements without moving to a crossover drive situation. In this regard the system provides superior positional control. Furthermore, it provides for equal bias in each direction and automatically compensates for gear wear.

[0097] The invention replicates performance of electronic anti-backlash, by a passive mechanism. The parts used in the invention are comparatively standard, commercially available and well developed. Moreover, it requires fewer complex and expensive parts because it only requires one servo motor and servo amplifier. However the largest cost saving is derived from the fact that the design does not rely on a pair of servo motors constantly driving against each other continuously. This drastically reduces power consumption and further increase service intervals. It thus will be appreciated that in these and other respects, the invention represents a practical and commercially significant improvement over the prior art.

[0098] As noted, one particularly preferred application for the drive system is for moving and aligning large terrestrial telescopes whether they be optical telescopes, radio telescopes or other types of telescopes. In such applications precision movement and control is of course critical. The ability of the system to monitor and adjust bias to suit variable conditions such as wind loads is also a significant advantage. Moreover in such applications where the system must remain active for long durations during tracking operations, power savings over traditional paired drive systems can be considerable. In the case of arrays of telescopes, saving will be multiplied. [0099] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0100] Similarly it should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, FIG., or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.

[0101] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0102] Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

[0103] In the description provided herein, numerous specific details are set forth.

However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0104] Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms "coupled" and "connected," along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled" may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

[0105] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as falling within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.

Claims

1 . An anti-backlash gearbox for reducing backlash between a positional input drive and a final drive, said system including:

a pair of differential transmissions each having first and second inputs and an output;

the outputs of the transmissions being paired together as an output pair to drive said final drive;

the first inputs being paired together as a sum input and configured to be driven in a common direction by said positional input drive under a drive load thereby driving the respective outputs in a common direction; and

2. A gearbox according to claim 1 wherein said difference input is configured to load each second input in equal but opposing directions thereby loading respective transmissions and corresponding outputs in equal but opposing directions.

3. A gearbox according to claim 1 or 2 wherein said input bias load is generally constant.

4. A gearbox according to claim 1 or 2 wherein said input bias load is variable.

5. A gearbox according to claim 1 or 2 wherein said input bias load is variable in response to predetermined parameters.

6. A gearbox according to claim 5 wherein said parameters include the drive load.

7. A gearbox according to claim 6 wherein said input bias load is adjusted to remain greater than the drive load.

8. A gearbox according to any one of claims 5 to 7 wherein said parameters are monitored by a control unit.

9. A gearbox according to any one of the preceding wherein each transmission is an epicyclic transmission including respective sun, annulus and planet carrier components.

10. A gearbox according to claim 9 wherein each transmission is a like epicyclic transmission including respective sun, annulus and planet carrier components and wherein each component is paired with a like component to form sun, annulus and planet carrier component pairs and wherein each component pair is configured as first input, second input and output pairs respectively.

1 1. A system according to claim 9 or 10 wherein said planet carrier component pair is configured as a first input.

12. A system according to any one of claims 9 to 11 wherein said sun component is configured as a first input.

13. A gearbox according to any one of claims 1 to 8 wherein each transmission is a differential bevel gear transmission.

14. An anti-backlash system for reducing backlash between an input drive and a final drive, said system including:

an anti backlash gearbox in accordance with any one of the preceding claims; positional drive means for providing rotational drive to said positional input drive; and

biasing means for providing a bias load to said difference input.

15. An anti-backlash system according to claim 14 wherein said positional drive means is an electronic servo motor.

16. An anti-backlash system according to claim 14 or 15 wherein said biasing means is passive torque generating device.

17. An anti-backlash system according to claim 16 wherein said biasing means includes a spring.

18. An anti-backlash system according to claim 14 or 15 wherein said biasing means includes an active torque generating device for providing said bias load to said difference input.

19. An anti-backlash system according to claim 18 wherein said active torque generating device is controllable to provide a bias load of selectable magnitude.

20. An anti-backlash system according to claim 18 wherein said biasing means includes an electronic servo motor.

21. An anti-backlash system according to any one of claims 14 to 20 wherein the system includes damping means.

22. An anti-backlash system according to claim 18 wherein said system includes a control unit for controlling said biasing means.

23. An anti-backlash system according to claim 21 wherein said control unit includes a microprocessor.

24. An anti-backlash system according to claim 21 or 22 wherein said control unit includes an input sensor for monitoring positional drive means.

25. An anti-backlash system according to claim 23 wherein said control unit actively varies the bias load output from the biasing means in response to the input sensor.

26. A drive system for a telescope including an anti-backlash system according to any one of claims 14 to 24.