Title of Invention

LINEAR ELECTRO-MECHANICAL ACTUATOR

Abstract

The present invention provides a linear-electromechanical actuator, said actuator comprising a motor with a hollow rotor and a stator housed in a covered body, a bearing housing disposed on both the ends of the covered body, angular contact ball bearings operably disposed in said bearing housing, a rotatable ballscrew-nut lock- integrated with the rotor, said ballscrew-nut having an internal thread profile, a tubular screw-guide with internal key-ways with shock absorbing means axially extending from the covered body, a hollow and non-rotating ballscrew-shaft having a linear motion and with a threaded outer profile axially extending towards the screw-guide and terminating with a plurality of linearly movable ballscrew locking keys, said locking keys disposed in the key-ways of the screw-guide to convert the rotary motion of rotor into linear motion of the ballscrew-shaft, said ballscrew-nut and ballscrew-shaft are Ithread integt'ated to each other, a tubular linear variable differential transformer (L VDT) with a non-contacting probe, said L VDT disposed in said non-rotating ballscrew-shaft, said probe acting as a sensor is directly connected to the ballscrew- shaft to sense the linear movement of the ballscrew-shaft and to provide a corresponding alternate current (At) voltage output to an external electronic control unit, and an attenuation card in functional communication with L VDT is mounted on the screw-guide to adjust the scale factor of the sensor.

Full Text

Linear Electro-mechanical Actuator Technical Field The invention generally relates to a linear electromechanical actuator. The present invention particularly relates to a linear electromechanical actuator with key-ways and a gear-free assembly to convert rotary motion of rotor in to a linear motion and for adjusting the scale factor of the linear motion. Background and Prior art Linear electromechanical actuators are used in industries such as turf care, specialty vehicles, agriculture, service vehicles, construction vehicles, and material handling, which typically use a motor, a gear box of a specified ratio, and a screw and nut combination to extend and retract a load. The actuator is used usually in applications where a reciprocating linear motion is needed for some intended purpose. The US Publication No. 2004/0061382 titled "Electromechanical Screw Drive Actuator" depicts an electromechanical actuator assembly having inline axial load support of its screw drive shaft. The bearing support structure provides a single in-line ball bearing accommodated within a hardened end fitting and a screw pivot recess in the axial screw drive shaft. For axial loading in an opposite direction, a number of smaller ball bearings are provided around the outer periphery of the screw shaft in a groove, and are retained within the hardened end fitting by a bearing retainer. Further, integrated electronics may also be provided for position sensing and power efficiency control. Linear electromechanical actuators widely used are screw driven, with or without gears. Compact linear electromechanical actuator published in Pub. No. US 2003/0111924 Al titled "Compact Electromechanical Linear Actuator", contains a rotor of the brushless permanent magnet servo motor, which is connected to the ball screw shaft through a hub and the ball screw nut is adapted to move linearly along the axis of the ball screw shaft as the shaft turns with the rotor and the hub. This invention discloses a linear electromechanical actuator that contains an electric servo motor mounted within a compact housing. A thermally conductive path is provided to efficiently transfer heat out of the housing into the surrounding ambient. A further mechanism has been provided for holding the stator windings of the motor in undisturbed contact with the inner wall of the housing when the motor is subjected to thermal stress. The motor is arranged to linearly position a push rod through means of a ball screw unit. The ball screw nut and the push rod ride on bearings within guide ways to insure that the push rod tracks along a linear path. The US Patent No. 4,500,805 "Electromechanical Linear Actuator" depicts an electromechanical linear actuator comprising an electric motor driving a cog belt and pulley device in turn rotating a lead screw of the ball type. The lead screw is restrained axially along the screw in one and an opposite direction. The drive nut in turn moves an actuator assembly comprising a drive ring, four circumferentially spaced rods and an actuating head connected at a remote end of the rods. Many actuators have been devised. Attempts have been made to make it compact to a certain extent. However, none of the actuators has fully attained the objective of simplicity, compactness and adaptability. In general, while some of the actuators have certain features or the other, it may lack on certain other counts. The actuator working in closed loop, has an associated control electronics for driving the actuator and the maximum travel/stroke depends on the scale factor of the feedback position sensor used in the actuator. There is no provision in the actuator to change the scale factor for the fine adjustments in actuator stroke. For applications which require high degree of position accuracy, it is necessary to use a linear position feedback sensor directly connected to the output shaft. The resolver, if used in the position feedback loop will not consider the ball screw lead errors. It is highly desirable to apply a preload to the ball bearings and the ball screw nut to reduce the backlash to a minimum. In general it is seen that, it is not easy to vary the preload of either the ball bearing or the ball screw nut without doing a careful selective assembly of the critical components. Further, a separate mechanism is provided on either the ball screw nut or the shaft to convert the rotary to linear motion. All the above drawbacks have been overcome using the present invention. Objects of the invention The primary object of the present invention is to provide a compact and adaptive linear electromechanical actuator with an adjustable scale factor. An object of the present invention is to provide a linear electromechanical actuator that utilizes an integral key in the ballscrew-shaft to arrest rotational motion and for converting rotary motion into linear motion. Another object of the present invention is to provide an adaptive linear electromechanical actuator by virtue of fine adjustments in the scale factor. Yet another object of the present invention is to provide an linear electromechanical actuator wherein preloads are applied to the ball bearings and ballscrew-nut so as to reduce backlash in linear motion. Summary of the invention The present invention provides a linear-electromechanical actuator, said actuator comprising a motor with a hollow rotor and a stator housed in a covered body, a bearing housing disposed on both the ends of the covered body, angular contact ball bearings operably disposed in said bearing housing, a rotatable ballscrew-nut lock-integrated with the rotor, said ballscrew-nut having an internal thread profile, a tubular screw-guide with internal key-ways with shock absorbing means axially extending from the covered body, a hollow and non -rotating ballscrew-shaft having a linear motion and with a threaded outer profile axially extending towards the screw-guide and terminating with a plurality of linearly movable ballscrew locking keys, said locking keys disposed in the key-ways of the screw-guide to convert the rotary motion of rotor into linear motion of the ballscrew-shaft, said ballscrew-nut and ballscrew-shaft are thread integrated to each other, a tubular linear variable differential transformer (LVDT) with a non-contacting probe, said LVDT disposed in said non-rotating ballscrew-shaft, said probe acting as a sensor is directly connected to the ballscrew-shaft to sense the linear movement of the ballscrew-shaft and to provide a corresponding alternate current (AC) voltage output to an external electronic control unit, and an attenuation card in functional communication with LVDT is mounted on the screw-guide to adjust the scale factor of the sensor. Brief Description of the drawings Fig. 1 depicts a longitudinal cross-section of the linear electromechanical actuator device. Fig. 2 gives a view of the rotor assembly depicting the pre-loading mechanism. Fig. 3 depicts a cross-section and a longitudinal section view of the screw guide showing the ballscrew-shaft with locking keys in the keyway. Detailed description of the invention The embodiments of the present invention are now explained by referring to figures of the accompanied diagrams. Referring to Fig. 1, the longitudinal cross-section of the linear electromechanical actuator depicts three primary mechanical components which are Brushless DC Torque motor comprising a hollow rotor (15) and a stator (14), Ballscrews comprising a Ballscrew-nut (17) and a ballscrew-shaft (16) and Linear Variable Differential Transformer (LVDT) (6) with a non-contacting probe (7). The Brushless DC Torque motor is a three-phase, permanent magnet (PM), brushless, direct current (DC) motor having ten poles. The brushless motor comprises a hollow rotor (15) and a stator (14). Brushless motors have high torque to weight ratio and high torque capability at low speeds. They are able to operate at high speeds with moderately linear outputs (torque versus speed) curves. The no load speed of the motor is in the range of 1000 to 1500 RPM. One of the most significant features of the brushless PM motor used here is its hollow rotor (15). Placement of the brushless windings in the outer stationary member and field magnets on the inner rotating member allow significant reductions in the rotor (15) inertia and the winding heat can be transferred directly to the housing which acts as the heat sink. The stator (14) windings carry current and heats up the stator body (14). The heat gets transferred from the stator (14) to body (1) by conduction and then from body (1) to the surrounding atmosphere by radiation. The stator (14) is fixed to the body (1) by means of a stator key (4). The motor is housed in a body (1) and enclosed at both ends by bearing housing (2) and a cover (3). The body (1), Cover (3) and bearing housing (2) are preferably made of aluminium alloy. The body (1) which houses the motor stator (14) also houses a contact ball bearing-L (8) on the left side. The bearing housing (2) houses one angular contact ball bearing-R (9) on the right side. The hollow rotor (15) of the motor is keyed over the ballscrew-nut (17) thereby lock-integrating directly to the ballscrew-nut (17). Thus the ballscrew-nut (17) holds the motor rotor (15), the inner races of the two angular contact ball bearings (8 & 9), the spacers (13) and the preload spacer (12). This provides two distinct advantages. The rotor (15) inertia gets significantly reduced as well as it avoids the usage of gears and other couplings which will introduce additional inertia and backlash type of non linearity. A tubular screw-guide (5) axially extending outwards from the cover (3) of the body (1) or covered body comprises a plurality of internal key-ways (19) equipped with shock absorbing means. A hollow and non-rotating ballscrew-shaft (16) with a threaded outer profile axially extending towards the screw guide (5). Said ballscrew-shaft having a linear motion. At one end of the ballscrew-shaft (16) there is a set of locking keys (18) which is an integral part of the ballscrew-shaft (16) and moves inside the key-ways (19) of screw-guide (5) thereby converting the rotary motion of the rotor (15) and the ballscrew-nut (17) to linear motion. Thus the rotational motion of the motor rotor (15) is converted to a linear motion of the ballscrew-shaft (16). Referring to Fig. 1 & 3, a plurality of locking keys (18), preferably about 4 in number are disposed in the key-ways (19). In the present invention, as an exemplary embodiment a pair of locking keys (18) is disposed at an angle of about 180 degrees apart, said locking keys (18) are integral to the ballscrew-shaft (16). However, said angular position may vary according to the number of locking keys (18). The locking keys (18) travel along the key-ways (19) that are provided inside said screw-guide (5). Grease lubrication is provided between the key (18) and keyway (19) for reducing the wear of the keys (18) and keyways (19). A suitable shock absorbing mechanism (28) is provided at both the ends of the keys (18). The shock absorbing mechanism, which is disposed in said key ways (19) in close proximity to locking keys (18) comprises wave springs stacked in parallel to each other and a cushioning arrangement, preferably in the form of Teflon bush disposed in said key-ways (19) to act as shock absorbers for the linear movement generated by the ballscrew-shaft (16). Ballscrews viz, the ballscrew-shaft and the ballscrew-nut (16 & 17) are high efficiency linear devices which provide a robust means of transmitting very high loads with considerable accuracy. They consist of a threaded screw and an internally threaded nut having recirculating balls in between. The rotary force transmitted is spread over a number of balls so that the contact stress is relatively low. The rolling friction between the ballscrew-shaft (16) and nut (17) results in extremely low coefficient of friction for the ballscrews. Due to this, the efficiency of the ball screws range above 90%. Fig. 2 depicts the mechanism for achieving pre-loading. When the screw set-I (23 & 24) get tightened, it closes the gap-I (27), thereby pulling the left half of the ballscrew-nut (17) to the left side. The retainer ring (11) pushes the inner race of angular contact ball bearing-L (8), spacers (12 & 13), motor rotor (15), inner race of angular contact ball bearing-R (9) and bearing nut (21) to the right side. The bearing nut (21) pulls the right half of the ballscrew-nut (17) to the right side thereby preloading the split nut (23). When the screw set-II (25) gets tightened, it closes the gap-II (26), thereby pulling the body (1), the bearing housing (2), pre-load spacer (12) and the outer race of the angular contact ball bearing-R (9) to the left side. The left cover (3) pushes the outer race of the angular contact ball bearing-L (8) to the right side thereby preloading the bearings. The directions of preloading are shown in the form of arrows in Fig. 2. The ballscrew used in the actuator comprises a split nut (17) thereby facilitating the pre-loading to avoid backlash. The pre-loading is done by using a retainer ring (11) which is tightened to one end of the ballscrew-nut (17) by using fasteners. The ballscrew-nut (17) which rotates is supported by two angular contact ball bearings (8 & 9) stacked face to face to carry the axial load in both the directions and radial load. The outer race of the angular contact ball bearing-R (9) on the right side which is housed inside the bearing housing (2) butts against the pre-load spacer (12), thereby preloading both the ball bearings (8 & 9). By using a spacer (13) of appropriate thickness, it is possible to restrict the ballscrew preload to a desirable value. By changing the thickness of the preload spacer (12), the preload force on the ball bearings can be varied. By this method it is possible to fix a desirable value of bearing preload by selecting an appropriate value of pre load spacer (12) thickness. A Linear Variable Differential Transformer (LVDT) (6) disposed within the non-rotating ballscrew-shaft (16). The Linear Variable Differential Transformer (LVDT) (6) provides an ac voltage output proportional to the displacement of a core passing through the windings. The LVDT (6) has an advantage of having a longer life due to the non contacting probe (7). The LVDT body (6) is connected to the stationary member, screw-guide (5) and its probe is connected to the linearly moving member which is the ballscrew-shaft (16). The electrical signal which varies according to the position of the probe (7) is fed back to the control electronics. The electrical connectors for the motor are mounted on the body (1) and for the LVDT (6) on the screw-guide (5). The LVDT body (6) is assembled to the Sensor nut (20) using the flange provided at the end of the LVDT body (6). The sensor nut which has thread on its outer diameter is screwed to the end of the screw-guide (5) and locked using a Sensor lock nut (22). This is for adjusting the position of the LVDT body (6) with respect to the LVDT probe (7) during the LVDT assembly; to make the LVDT output zero corresponding to the nominal length of the actuator, i.e. the nulling operation. Wires are provided in the LVDT body (6) which is connected to the primary and secondary windings situated inside the LVDT body (6). Sinusoidal excitation voltage is given to the primary windings and the output of the secondary windings which gives the position information of the LVDT probe (7) is fed back to the control electronics. The overshooting of the ballscrew-shaft (16) in the keyway (19) is controlled by the shock absorber, said shock absorber acting as mechanical dead stop to the ballscrew-shaft (16). The attenuation card (10) is mounted on the screw-guide (5) in functional communication with the LVDT (6) connector for adjusting the scale factor of the feedback sensor. The LVDTs which are being procured from various sources comes in a wide range of scale factors. To bring the actuator scale factor values within a narrow band, usually the attenuation is done in the output lines of LVDT signal conditioner card located inside the control electronics to make it compatible with a particular actuator. But this practice has a disadvantage in it that this particular actuator need not be compatible with other control electronics boxes and vice versa. For realising the actuators with scale factors falling in a tighter tolerance of specification, there need to have an arrangement for effecting fine adjustments of LVDT scale factor. By introducing the attenuation card (10) which is a printed circuit board with resistors, we can contain the LVDT scale factor with in a tight specification tolerance thereby making the actuator compatible with other Control Electronic boxes. More over, the realisation of Control electronics can be made completely independent which otherwise need the LVDT scale factor value for the trimming to be made in the control electronics to achieve the required actuator scale factor. Advantages 1. The scale factor of the system can be controlled within a tight specification tolerance due to the attenuation card in the device. 2. The actuator is compatible with any other control electronic box due to an attenuation card present in the system. 3. The rotor inertia is reduced significantly due to keying of the hollow rotor to the ball screw nut. 4. The usage of gears and couplings is avoided due to the keying of the hollow rotor to the ball screw nut. 5. High degree of position accuracy is achieved by using a linear position feedback sensor and directly connecting it to the output shaft. 6. The backlash is reduced to a minimum by applying preload to the ball bearings and the ballscrew-nut. 7. In the present invention, by way of adopting the selective assembly of preload spacer with the required thickness, the bearing and the ballscrew preloading force is controlled to a desirable value. We claim: 1. An improved linear electromechanical actuator for adjusting the scale factor of linear movement in which a motor with a hollow rotor and a stator housed in a covered body, wherein the improvement comprises: a bearing housing with angular contact bearings disposed on both the ends of the covered body, a rotatable ballscrew-nut lock-integrated with the rotor, said ballscrew-nut having an internal thread profile, a tubular screw-guide axially extending from the covered body, said screw-guide further comprising a plurality of internal key-ways with shock absorbing means, a hollow and non-rotating ballscrew-shaft with a threaded outer profile axially extending towards the screw-guide, said ballscrew shaft having a linear motion, a plurality of linearly movable ballscrew locking keys which is integral to the ballscrew-shaft, said locking keys disposed in the key-ways of the screw-guide, said keys disposed to convert the rotary motion of rotor into linear motion of the ballscrew-shaft, said ballscrew-nut and ballscrew-shaft are thread integrated to each other, a tubular linear variable differential transformer (LVDT) disposed in said non-rotating ballscrew-shaft, a non-contacting probe disposed in said LVDT, said probe acting as a sensor is directly connected to the ballscrew-shaft to sense the linear movement of the ballscrew-shaft and to provide a corresponding alternate current (AC) voltage output to an external electronic control unit, and an attenuation card mounted on the screw-guide, said attenuation card is in functional communication with said LVDT to adjust the scale factor of the sensor. 2. The linear electromechanical actuator as claimed in claim 1, wherein the locking keys are disposed at about 180 deg apart. 3. The linear electromechanical actuator as claimed in claim 1, wherein the total number of locking keys can be variable, preferably up to 4 in number. 4. The linear electromechanical actuator as claimed in claim 1, wherein the lock integration between the rotor and ballscrew-nut is gear-free. 5. The linear electromechanical actuator as claimed in claim 1, wherein spacers are provided on the ballscrew nut, whose thickness controls the preloading force on the ballscrew nut. 6. The linear electromechanical actuator as claimed in claim 1, wherein preload spacers are provided in the bearing housing, whose thickness controls the preloading force on the angular contact ball bearings.

Documents:

768-che-2004-abstract.pdf

768-che-2004-claims duplicate.pdf

768-che-2004-claims original.pdf

768-che-2004-correspondnece-others.pdf

768-che-2004-correspondnece-po.pdf

768-che-2004-description(complete) duplicate.pdf

768-che-2004-description(complete) original.pdf

768-che-2004-drawings.pdf

768-che-2004-form 1.pdf

768-che-2004-form 19.pdf

768-che-2004-form 26.pdf

768-che-2004-form 3.pdf

768-che-2004-form 5.pdf