1. TECHNICAL FIELD

The present invention relates to the application of mechanical force to a load and, in particular, to a linear actuator. The linear actuator is advantageous in applications such as in the automotive, aeronautical and robotics sectors where miniaturization, reduced complexity and cost, and symmetry of shape may be desirable.

2. BACKGROUND OF THE INVENTION

Linear actuators are used to apply mechanical force to a load and are typically devices in which the rotary motion of a rotating screw shaft, or lead screw, is converted into linear motion of a traveling nut. In a traditional linear actuator, a rotary motor of some form is attached to the lead screw and causes the lead screw to rotate in one of two rotational directions, which in turn causes the traveling nut, which is typically attached to an actuator rod, to move linearly in one of two linear directions. The rotary motor has a maximum output power capability and will cease causing linear movement in the presence of an excessive load. The presence of such excessive load results in the reduction or loss of efficiency and in excessive power consumption of the linear actuator and may result in damage to the motor and/or linear actuator. Traditional linear actuators typically incorporate limit switches to turn off the rotary motor when the traveling nut has reached its full stroke position in either linear direction. While such conventional limit switches can protect the rotary motor when the traveling nut has reached a full stroke position, no protection is provided in a traditional linear actuator at other positions of the traveling nut. Conventional limit switches are bulky and located adjacent the lead screw and/or adjacent the actuator rod. Such conventional limit switches add to the complexity and/or cost of the linear actuator, and increase size and/or result in an undesirable non-symmetrical shape of the linear actuator. The present invention satisfies the need for a fully protected linear actuator of reduced complexity and cost, and smaller size that may be produced with a symmetrical shape. 3. SUMMARY OF THE INVENTION

The above shortcomings may be addressed by providing, in accordance with one aspect of the invention, a linear actuator which includes an actuator housing, a traveling nut means threadingly engaged with a leadscrew means, a rotary drive means rotatingly engaged with the leadscrew means and a limit switch means, the leadscrew means having an axis of rotation and the traveling nut means being movable along the leadscrew means in a direction parallel to the axis of rotation between first and second limits of motion, wherein the rotary drive means and engaged leadscrew means are suspended within the actuator housing such that they are movable relative to the actuator housing in a direction parallel to the axis of rotation in response to a force transmitted to the rotary drive means and leadscrew means by the traveling nut means, and wherein the limit switch means can detect movement of the rotary drive means and leadscrew means relative to the actuator housing. The rotary drive means and leadscrew means may preferably be suspended within the actuator housing between two spring means, such that the magnitude of a triggering force required to move the rotary drive means and leadscrew means relative to the actuator housing and thereby trip the limit switch means may be controllably set by appropriately selecting the spring forces of the spring means. The spring forces of the two spring means may preferably be selected to provide equal spring force against the motion of the rotary drive means and leadscrew means relative to the actuator housing in either direction. Thereby, when a triggering force greater than the selected spring force of the two spring means is transmitted to the leadscrew means and rotary drive means by the traveling nut means, the leadscrew means and rotary drive means may move relative to the actuator housing and trip the limit switch means. The limit switch means may be used to control power to the rotary drive means, such that when a triggering force greater than the spring force acts on the traveling nut means such as when the traveling nut reaches the first or second limits of motion along the leadscrew means, such force is transmitted to the leadscrew means and rotary drive means, causing them to move relative to the actuator housing and tripping the limit switch means which may desirably cut power to the rotary drive means to stop the motion of the traveling nut means, thus controlling the range of motion of the traveling nut means within the first and second limits of motion along the leadscrew means, and protecting the drive means and other components of the linear actuator from damage. In another embodiment, the inventive linear actuator may comprise an actuator housing, a traveling nut means threadingly engaged with a leadscrew means, a rotary drive means rotatingly engaged with the leadscrew means and a linear potentiometer means, the leadscrew means having an axis of rotation and the traveling nut means being movable along the leadscrew means in a direction parallel to the axis of rotation between first and second limits of motion, wherein the rotary drive means and engaged leadscrew means are suspended within the actuator housing such that they are movable relative to the actuator housing in a direction parallel to the axis of rotation in response to a force transmitted to the rotary drive means and leadscrew means by the traveling nut means, and wherein the linear potentiometer means can detect movement of the rotary drive means and leadscrew means relative to the actuator housing. The rotary drive means and leadscrew means may preferably be suspended within the actuator housing between two spring means having known spring rates, such that the magnitude of the force applied to the traveling nut by an operational load or the first or second limits of motion and transmitted to the rotary drive means and leadscrew means may be determined by the corresponding magnitude of movement of the rotary drive means and leadscrew means relative to the actuator housing detected by the linear potentiometer means, according to the relation between force and movement for the spring rates of the two spring means. In another further embodiment according to the present invention, a method of limiting the force applied by a linear actuator is provided, the method comprising the steps of: - providing a linear actuator comprising an actuator housing, a traveling nut means for applying a force, the traveling nut means threadingly engaged with a leadscrew means, a rotary drive means rotatingly engaged with the leadscrew means and a displacement detection means, the leadscrew means having an axis of rotation and the traveling nut means being movable along the leadscrew means in a direction parallel to the axis of rotation between first and second limits of motion, wherein the rotary drive means and engaged leadscrew means are suspended within the actuator housing such that they are movable relative to the actuator housing in a direction parallel to the axis of rotation in response to the force applied by the traveling nut means, and transmitted to the rotary drive means and leadscrew means by the traveling nut means, wherein the displacement detection means can detect movement of the rotary drive means and leadscrew means relative to the actuator housing; - powering the rotary drive means to apply a force through the motion of the traveling nut means along the leadscrew; - detecting the degree of movement of the rotary drive means and leadscrew means relative to the actuator housing corresponding to the magnitude of force applied through the traveling nut means; - cutting power to the rotary drive means to stop the application of force through the traveling nut means when the degree of movement of the rotary drive means and leadscrew means exceeds a level corresponding to the application of a force of a magnitude exceeding a limit. Other aspects and features of the present invention will become apparent to one of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 4. BRIEF DESCRIPTION OF THE DRAWINGS

In drawing figures which illustrate by way of example only specific embodiments of the invention: FIG. 1 is a sectional view of a linear actuator according to a first embodiment of the invention; FIG. 2 is a diagram of an electrical circuit for use with the first embodiment shown in Figure 1; FIG. 3 is a perspective view of the first embodiment shown in Figure 1, showing an implementation of the electrical circuit shown in Figure 2; FIG. 4 is a sectional view of a linear actuator, showing a switch which does not include a printed circuit board mounted directly on a side of the motor; FIG. 5 is a sectional view of a linear actuator, showing a potentiometer; FIG. 6 is sectional view of a linear actuator according to a second embodiment of the invention, showing enlarged inset views of electrically conductive resilient components; FIG. 7 is a linear actuator according to a third embodiment of the invention; and FIG. 8 is a schematic showing a single-pole triple-throw switch having three states.

5. DETAILED DESCRIPTION OF THE INVENTION

A linear actuator having a housing and a driving mechanism for causing linear displacement of an object includes resilient means for resisting linear movement of the driving mechanism, the resilient means having a first end abutting at least one portion of the driving mechanism and a second end abutting the housing of the linear actuator; and control means for controlling the driving mechanism in response to movement of the at least one portion of the driving mechanism relative to the housing. Referring to Figure 1, a linear actuator according to a first embodiment of the invention is shown generally at 100. The linear actuator 100 includes an actuator housing 2, which may be made of any suitable material, including plastic and metal. The actuator housing 2 may be constructed of assembled housing parts (not all of which are shown in Figure 1). The actuator housing 2 preferably encloses a rotary drive means or other driving mechanism, which preferably includes an electrically powered rotary motor 4 as shown in Figure 1. The motor 4 is coupled to leadscrew means, such as the lead screw 6 shown in Figure 1 , and functions to rotationally drive the lead screw 6. Other components of the linear actuator 100 include a traveling nut means, including an internally threaded end cap or other similar component such as the traveling nut 7, an actuator rod 27, spring means or other resilient component or components such as springs 3 and 5, and limit switch means or other control mechanism such as switch 1, as shown in Figure 1. The lead screw 6 preferably has a longitudinally threaded outwardly facing surface as shown in Figure 1 and the traveling nut 7 has a threaded inwardly facing surface which is threadedly engageable to the lead screw 6. Rotation of the traveling nut 7 is typically constrained by the actuator housing 2 such that rotation of the lead screw 6 causes longitudinal linear motion of the traveling nut 7. The traveling nut 7 and actuator rod 27 attached thereto travel within bore 10 in the actuator housing 2 by means of threading engagement with lead screw 6 such that when lead screw 6 is rotated by motor 4, traveling nut 7 and attached actuator rod 27 move along lead screw 6 within the useful range of motion of traveling nut 7 between travel limit flanges 8 and 9 positioned at either end of bore 10 within the actuator housing 2. The actuator rod 27 may longitudinally extend beyond the actuator housing 2 for applying mechanical force to a load (not shown in Figure 1). Springs 3 and 5 abut opposing longitudinal ends of the motor 4 such that motor 4 and lead screw 6 may move relative to the actuator housing 2 in a direction parallel to the longitudinal axis of lead screw 6 in response to a force transmitted to the motor 4 and lead screw 6 by traveling nut 7 and attached actuator rod 27. Additionally, the springs 3 and 5 may in some applications act to suspend the motor 4 and lead screw 6 within the actuator housing 2. Switch 1 is coupled to motor 4, which is coupled to lead screw 6, such that switch 1 can detect movement of the motor 4 and lead screw 6 relative to the actuator housing 2 in response to such a force, and trip the switch 1 accordingly. Tripping the switch 1 may involve the breaking or opening of an electrical contact (not shown) within the switch 1. hi this way a single switch 1 coupled to the motor 4 and/or lead screw 6 may be used to detect when the traveling nut 7 reaches either end of its range of motion and bears against limit flange 8 or 9, transmitting a force to motor 4 and lead screw 6 and causing movement of the motor 4 and lead screw 6 relative to the actuator housing 2, and to trip the switch 1 at that point. Preferably, the switch 1 may be connected to the power source for motor 4 such that when the switch 1 is tripped in response to the force transmitted to the motor 4 and lead screw 6 by the traveling nut 7 bearing against limit flange 8 or 9, the power to the motor 4 may be interrupted and/or reversed such that the traveling nut 7 will not be further forced against limit flange 8 or 9. Such control of the power to motor 4 by means of switch 1 may be used to prevent damage to the linear actuator 100, including in particular the motor 4 and lead screw 6 of the linear actuator 100. Still referring to Figure 1, the switch 1 may be located along the axis of rotation of the lead screw 6 on the opposite end of the motor 4 from the lead screw 6. Such a location for the switch 1 may be chosen to maintain a symmetrical actuator form factor, and simplified actuator design, which may be desirable. Switch 1, motor 4 and springs 3 and 5 may be housed within actuator housing 2 such as within a substantially cylindrical bore or other opening within the actuator housing 2 suited to the size and shape of the housed components of the linear actuator 100. In order to optimize packaging of components within actuator housing 2, motor 4 may preferably be of substantially cylindrical or semi-cylindrical shape, including having at least one flat face. Similarly, lead screw 6 and traveling nut 7 may be housed inside bore 10 which may preferably be of substantially cylindrical shape. The exterior shape of the actuator housing 2 may be chosen to suit the installation or operational requirements of a given application of the linear actuator 100, and may preferably be substantially cylindrical in shape. The motor 4 may be selected from any rotary motor suitable to rotate the lead screw 6 to drive the traveling nut 7 of the linear actuator 100 for a chosen linear actuation application. The motor 4 may also comprise gearing means such as a gearbox, or other power transmission means for use in combination with the rotary motor in order to rotate the lead screw 6. The motor 4 may preferably be a rotary electric motor suitable for direct connection to the lead screw 6, but alternatively may connect through a gearing means to the lead screw 6 for rotating the lead screw 6 to drive the traveling nut 7. Preferably, the rotary motor and gearing means if any may be selected so as to cooperate with the threading characteristics (e.g. pitch and depth) of the lead screw 6 to provide suitable rotary torque, and associated linear force characteristics to the traveling nut 7 for a desired application of the linear actuator 100. Similarly, the lead screw 6 and traveling nut 7 may preferably be selected from suitable materials such as metal, plastic, composite or combinations thereof to cooperate in threading engagement to drive the traveling nut 7 by the rotation of the lead screw 6. Springs 3 and 5 may be selected from any elastically deformable object, including conventional coil springs, elastomeric or other compressible bulk material springs, Belleville washer springs, wave springs, solid pieces of resilient foam, plastic or other resilient material, or combinations thereof which may be suitable to be positioned between the actuator housing 2 and the motor 4, including abutting the motor 4 and/or the actuator housing 2, thereby allowing movement, of the motor 4 and the lead screw 6 along the axis parallel to the rotational axis of the lead screw 6, in response to a force transmitted to the motor 4 and lead screw 6 by the traveling nut 7, as described above. The compressive characteristics of springs 3 and 5, such as the compressive force of the springs 3 and 5, may be selected to determine a minimum force required to be exerted to move the motor 4 and lead screw 6 relative to the actuator housing 2 as described above. In such a manner the magnitude of the force required to be transmitted to the motor 4 and lead screw 6 by the traveling nut 7 in order to trip a particular switch 1 may be varied by the selection of the compressive characteristics of springs 3 and 5. In particular, the compressive characteristics of springs 3 and 5 may preferably be selected such that the force required to be exerted on motor 4 and lead screw 6 in order to trip the switch 1 is less than the maximum force that may be exerted by the linear actuator 100, and less than the force that may cause damage to the linear actuator 100, or any part thereof, when the traveling nut 7 bears against a limit flange 8 or 9 at the extent of its range of motion. Switch 1 may be selected from any known switch technology suitable to detect the relative movement of the motor 4 and lead screw 6 relative to the actuator housing 2 and upon detection of such relative motion, to trip to provide a signal that such movement has been detected. The switch 1 is preferably capable of providing an electrical, mechanical, or electromechanical signal when tripped in a manner such as the interruption of an electrical circuit. As described above, such interruption of an electrical circuit by the switch 1 upon tripping may be used to interrupt power to the motor 4 to stop the further motion of traveling nut 7 that resulted in the force that tripped the switch 1 , such as to prevent damage to the linear actuator 100 upon reaching the extent of the range of motion of the traveling nut 7. In addition to the prevention of damage to the linear actuator 100 at the limits of the range of motion of the traveling nut 7, the compressive characteristics of springs 3 and 5 may also be selected to define a maximum desired exertion force of the traveling nut 7, such that if the traveling nut 7 or attached actuator rod 27 (or any external load in contact with the actuator rod 27) comes into contact with an object (animate or not) or is otherwise loaded beyond such maximum desired exertion force, that the motor 4 and lead screw 6 will move relative to the actuator housing 2 against the spring forces of springs 3 and 5 to trip the switch 1 and prevent further movement of the traveling nut 7 and attached actuator rod 27 in that direction. This additional force-limiting feature of the linear actuator 100 may be particularly desirable in applications such as automation and/or robotics where the normal path of motion of the traveling nut 7 or attached actuator rod 27 may be blocked by a person or other object, and the exertion of force on the object by the linear actuator 100 is desired to be limited as a safety feature. Similarly, in some applications, it may be desired to define the range of motion of the actuator rod 27 and attached traveling nut 7 by the exertion of a selected maximum force, and the compressive characteristics of springs 3 and 5 may be selected to define such selected maximum force limit (which is less than the maximum force the linear actuator 100 is capable of exerting of course) beyond which the switch 1 will trip to stop the movement of the traveling nut 7, as described above. In an alternative exemplary version of the first embodiment, switch 1 may be selected to be sensitive to rotational movement of the motor 4 relative to the actuator housing 2 instead of translational movement relative to the actuator housing 2, in which case springs 3 and 5 may be selected to permit the motor 4 and lead screw 6 to rotate in the rotational direction around the axis of lead screw 6, by means of a torsional spring force whereby exceeding a chosen maximum torque applied to the traveling nut 7 will cause relative rotational movement between the motor 4 and actuator housing 2 and trip the switch 1. In such a way, the above described force limitation at end of range, safety, and maximum force limiting features may alternatively be enabled through the detection of such relative rotational motion of the motor 4 in response to the application of torque on the traveling nut 7 greater than a limiting magnitude defined by the torsional characteristics of springs 3 and 5. In both the translational motion and alternative rotational motion sensitive versions of the first embodiment, springs 3 and 5 may be preloaded within the actuator housing 2, when the motor 4 and lead screw 6 are located in their neutral position relative to the actuator housing 2, as depicted in Figure 1. Such preloading may improve the application of normal working loads by the motor 4 through the lead screw 6 to the traveling nut 7 and actuator rod 27, when the force applied in such working loads is less than the chosen magnitude of force required to trip the switch 1 by moving the motor 4 and lead screw 6 relative to the actuator housing 2 against the spring force of springs 3 and 5. Thus, there is provided a linear actuator having a housing, the linear actuator including a driving mechanism having at least one portion thereof resiliently coupled to the housing; and a control mechanism adapted to control said driving mechanism in response to movement of said at least one portion of said driving mechanism relative to said housing. Referring now to Figures 2A, 2B and 2C an electrical circuit in accordance with the first embodiment of the present invention is illustrated in 3 switching modes and shown generally at 200. The electrical circuit 200 may be used in cooperation with, or integrated into the switch 1 of the linear actuator 100 to control the power to the motor 4 upon the tripping of the switch 1 resulting from the movement of the motor 4 relative to the actuator housing 2 (Figure 1). Integrated in such a manner, the switch 1 is operable as a single-pole triple-throw switch containing switching elements. In Figure 2A, motor 4 is connected via electrical leads 44 and 45 to switching circuits 41 and 42 respectively. Either of switching circuits 41 and 42 may be switched by default to connect to power source 43, or by tripping each switch, either of switching circuits 41 or 42 may individually be disconnected from power source 43 as shown in Figures 2B and 2C respectively. The motor 4 may preferably be bi-directional, or reversible, and be an electrical motor, such that connection of power source 43 to electrical leads 44 or 45 will cause the motor 4 to rotate in opposite directions, corresponding to opposite rotational directions of lead screw 6 (Figure 1), and associated longitudinal movement of traveling nut 7 (Figure 1) in opposite directions along lead screw 6 between travel limit flanges 8 and 9 (Figure 1). Switching circuits 41 and 42 may be sensitive to movement between motor 4 and actuator housing 2, so that upon such relative movement of motor 4 in either direction against the spring force of at least one of spring 3 and 5 (Figure 1), the appropriate switching circuit 41 or 42 of switching circuits 41 and 42 will trip, disrupting power to one of the electrical leads to motor 4, and thereby stopping the driving action of the motor 4 in the direction that caused the appropriate switching circuit 41 or 42 to trip, while continuing to allow power to connect to the motor 4 through the other electrical lead to cause the motor 4 to drive in the opposite direction to retract the traveling nut 7 from whatever caused the appropriate switching circuit 41 or 42 to trip (may be contact with a travel limit flange 8 or 9, or other obstruction to path of traveling nut 7). The use of diodes or other suitable analog or digital electrical current limiting devices may be made to control the direction of current permitted to pass through the switching circuits 41 and 42 when they are tripped to determine the appropriate direction of rotation of the motor 4 after tripping switching circuit 41 or switching circuit 42. Figure 2A shows the switching circuits 41 and 42 in closed states that permit electrical current to flow in either direction. Figure 2B shows the switching circuit 41 in an open state such that diode 202 permits a clockwise (as seen in Figure 2B) flow of positive electrical current while preventing a counter-clockwise flow of positive electrical current, and diode 204 is bypassed by the switching circuit 42 which is in a closed state. Figure 2C shows the switching circuit 41 in a closed state so as to bypass diode 202 and shows the switching circuit 42 in an open state such that diode 204 permits a counter-clockwise (as seen in Figure 2C) flow of positive electrical current while preventing a clockwise flow of positive electrical current. The arrangement of switching circuits 41 and 42 in cooperation with motor 4 thereby provides the desired force-limiting at end of range, safety, and maximum force- limiting functionality of the linear actuator 100 (Figure 1), as described above. Switching circuits 41 and 42 may both be incorporated into switch 1 (Figure 1), including being incorporated in accordance with the single-pole, triple-throw switch shown in Figure 8. Switch 1 is operable to trip and interrupt power to the motor 4 in response to relative movement between motor 4 and actuator housing 2 in either direction, and by tripping thereby interrupt power to the motor 4 in the direction of rotation that caused the trip. Figure 3 shows the linear actuator 100 having the electrical circuit 200 preferably implemented on a printed circuit board 300, including being implemented in accordance with the switch shown in Figure 8. The printed circuit board 300 is preferably mounted on a flattened surface of the motor 4. The printed circuit board 300 includes contacts 302 between electrically conductive strips 304 and electrically conductive fingers 306 of an arm 308 mounted on the printed circuit board 300. The arm 308 is resiliently mounted on the printed circuit board 300 to urge contact between the fingers 306 and the surface of the printed circuit board 300. The integrated circuit 310 is mounted on the printed circuit board 300 and includes diodes as described in connection with the electrical circuit 200 (Figure 2). In the preferred embodiment shown in Figure 3, the printed circuit board 300 extends beyond the actuator housing 2 to facilitate connection to external circuitry, which typically includes a direct current (DC) power supply (not shown in Figure 3) and a reversing 3-position power switch (not shown), and may include external monitoring circuitry (not shown) for monitoring an output from the printed circuit board 300. The placement of the printed circuit board 300 along the side of motor 4 as shown in Figure 3 advantageously retains the force-limiting at end of range, safety, and maximum force- limiting functionality of the linear actuator 100 (Figure 1), and may allow for shortening of the overall length of the actuator housing 2, which may be desirable in some applications where space for the linear actuator 100 may be constrained. Referring now to Figure 4, the advantage of a shorter overall length of the actuator housing 2 of the linear actuator 100 (Figure 1) may be retained where a switch 22 not including a printed circuit board mounted directly on a side of the motor 4 is desirable. Figure 4 shows switch 22, which may be sensitive to relative axial motion (parallel to the longitudinal axis of lead screw 6) of motor 4 such that switch 22 may trip from a central untripped position 25 shown in the inset of Figure 2, to a displaced tripped position 26 in either direction. Such tripping of switch 22 may produce a signal, such as an electrical, mechanical or electromechanical signal, which may be used to control a power supply (not shown) for motor 4 and stop the further motion of lead screw 6 and traveling nut 7 in the direction that had resulted in the tripping of switch 22, as described above and depicted in Figure 1. Referring now to Figure 5, the advantage of a shorter overall length of the actuator housing 2 of the linear actuator 100 (Figure 1) may be retained where the use of a potentiometer 50 is desirable. The linear potentiometer 50 is typically sensitive to the movement of motor 4 relative to actuator housing 2, such movement being measurable by the linear potentiometer 50 by the relative movement of reference point 52 on motor 4. In accordance with the above description, motor 4 and lead screw 6 rotatingly coupled thereto are positioned between springs 51 and 53 such that a force exerted on traveling nut 7 which is threadedly engaged with lead screw 6 is transmitted to motor 4 through lead screw 6 and moves motor 4 relative to actuator housing 2. Additionally, springs 51 and 53 may in some applications suspend motor 4 and lead screw 6 within actuator housing 2. Typically, the springs 51 and 53 have a defined spring rate (relation between force applied and spring extension/compression) that is known. With known spring rate values for springs 51 and 53, any force exerted on traveling nut 7 at any point of its travel along lead screw 6 will translate into a degree of movement of motor 4 relative to the actuator housing 2, and such movement may be measured by linear potentiometer 50. The magnitude of the force exerted on traveling nut 7 may then be calculated from the measured movement of motor 4 using the known spring rates of springs 51 and 53. Preferably, the force exerted on traveling nut 7 may be calculated in such a manner by a control device capable of monitoring the output of the linear potentiometer 50, and capable to control power to motor 4 based on the calculated force on traveling nut 7. In such a manner, the desirable force-limiting at end of range (limit detection and switching), safety, and maximum force- limiting functionality of the linear actuator 100 (Figure 1), as described above, may be implemented by the control device, which may cut power to the motor 4 when the force exerted on traveling nut 7 exceeds a desired maximum force, while still allowing the motor 4 to be powered in the opposite direction to retract the traveling nut 7 from the limit flange 8, 9 or other obstruction that may have resulted in the over-force condition. Alternatively or additionally, a magnetic sensor (not shown) may be used to detect the position of the motor 4 or other component of the driving mechanism, thereby directly or indirectly measuring the degree of relative movement between the motor 4 and actuator housing 2 resulting from a force exerted on traveling nut 7. Such magnetic sensor may be monitored by a control device as described above in the case using linear potentiometer 50, in order to calculate the force exerted on traveling nut 7, and to control the power to motor 4 according to the desired functionality of the linear actuator 100 (Figure 1). Referring now to Figure 6, a linear actuator according to a second embodiment of the invention is shown generally at 600. The linear actuator 600 includes the motor 4, which is positioned between the electrically conductive springs 12 and 15 and in some applications may be suspended between the electrically conductive springs 12 and 15. In the second embodiment, electrically conductive spring 12 and associated electrically conductive endplates 11 and 13 form an electrical circuit breaker such that when the end of the motor 4 distal from lead screw 6 is in engaging contact against conductive endplate 13, endplates 11 and 13 and conductive spring 12 are in electrically conductive contact with each other, and an electrical current can flow from endplate 11 to endplate 13 through conductive spring 12 to complete an electric circuit. However, when motor 4 moves away from endplate 11, the extension of conductive spring 12 results in the separation of at least two of endplates 11 and 13 and conductive spring 12, thus breaking the electric circuit between endplates 11 and 13, as shown in one inset depiction of Figure 6. Similarly at the opposite end of motor 4 which is proximate to lead screw 6, electrically conductive endplates 14 and 16 may form an electric circuit breaker in combination with electrically conductive spring 15, wherein when conductive endplate 14 is in engaging contact with the end of motor 4, an electrical circuit is formed between endplates 14 and 16 through conductive spring 15, and when motor 4 moves away from endplate 14, the extension of spring 15 results in the separation of at least two of endplates 14 and 16 and conductive spring 15, thus breaking the electric circuit, as also shown in one inset depiction of Figure 6. Conductive springs 12 and 15 may preferably be preloaded between conductive endplates 11 and 13 and conductive endplates 14 and 16 such that motor 4 may move longitudinally relative to actuator housing 2 in response to the exertion of a force on traveling nut 7 greater than the spring forces of conductive springs 12 and 15 thereby compressing one of the springs 12 and 15 and extending the other of the springs 12 and 15. Additionally, the conductive springs 12 and 15 in some applications may suspend the motor 4 within the actuator housing 2. Such longitudinal movement of motor 4 relative to actuator housing 2 may then cause the electrical circuit formed by the combination of endplate 11, spring 12, and endplate 13 or the electrical circuit formed by the combination of endplate 14, spring 15 and endplate 16 to break. In such a manner, each conductive endplate-spring-endplate circuit may act as a control mechanism sensitive to relative longitudinal movement between motor 4 and actuator housing 2 resulting from a force greater than the spring force acting on traveling nut 7 and transmitted to the motor 4 through lead screw 6. Similar to the first embodiment, the electrical circuits formed by the endplate-spring-endplate contact may be used to control the power to motor 4, such that when either of such circuits is broken due to relative movement between the motor 4 and actuator housing 2, the motion of traveling nut 7 may be halted, thus providing force-limiting at end of range, safety, and maximum force-limiting functionality of the linear actuator 600. Conductive springs 12 and 15 may preferably be provided as metal wave springs such as metal Bellville washers, however any conductive spring suitable for completing and breaking an electrical circuit as described above may be used. Additionally, electrically insulative and elastically compressible annular spacers 19 and 20 may preferably be included between endplate 11 and spring 12, and between endplate 16 and spring 15, respectively. Such insulative spacers may assist in breaking contact between endplates 11 and 16 and associated springs 12 and 15 as soon as springs 12 and 15 begin to be extended due to relative movement between motor 4 and actuator housing 2, by means of expansion of the spacer, in order to provide a clean break of electrical contact. Any suitable annular spacer comprising electrically insulative and elastically compressible material may be used as a spacer, such as an elastomeric or rubber o-ring. Alternatively or additionally, a strain gauge or other contact pressure sensor (not shown) may be arranged to measure the force exerted on springs 12 and 15. Such strain gauge or other contact pressure sensor may be monitored by a control device as described above in the case using linear potentiometer 50, in order to calculate the force exerted on traveling nut 7, and to control the power to motor 4 according to the desired functionality of the linear actuator 100 (Figure 1) and/or linear actuator 600. Referring now to Figure 7, a linear actuator according to a third embodiment of the invention is shown generally at 700. The linear actuator 700 includes a motor 4 coupled to a lead screw 6. The motor 4 is coupled to the actuator housing 2 by abutting a single spring 31 within actuator housing 2. The motor 4 and lead screw 6 are capable of movement relative to the actuator housing 2 in the direction parallel to the longitudinal axis of the lead screw 6 in response to a force greater than the compressive or expansive spring force of spring 31 acting on traveling nut 7 and/or attached actuator rod 27 and transmitted to the motor 4 through the lead screw 6. As in the first and second embodiments described above, traveling nut 7 is capable of movement along the axis of lead screw 6 between travel limit flanges 8 and 9 within preferably cylindrically shaped bore 10 in actuator housing 2. In order for single spring 31 to exert a spring force restricting longitudinal movement of motor 4 relative to actuator housing 2 during unloaded and unimpeded travel of traveling nut 7 in both directions of movement, the ends 32 and 33 of spring 31 may be fixedly attached to spring endplate 35 and 34 respectively. Spring endplate 35 may then be fixed to motor 4 and spring endplate 34 may be fixed in place relative to actuator housing 2 between flanges 36 and 37 in the actuator housing 2. In this way, upon a force exceeding the spring force of spring 31 being exerted on traveling nut 7 (such as by traveling nut 7 bearing against travel limit flanges 8 or 9 at the extent of its range of motion or by encountering an excessive load or external force applied to the actuator rod 27, for example) and transmitted to motor 4 through lead screw 6, motor 4 may move relative to the actuator housing 2 by either compressing or extending spring 31. As described similarly with reference to the above illustrated embodiments, a control mechanism (not shown in Figure 7) may be incorporated in the present embodiment, preferably located so as to be sensitive to relative movement of motor 4 relative to the actuator housing 2, such that such relative movement of motor 4 may trip the control mechanism, including tripping a switching element of the control mechanism, upon the exertion of a force on traveling nut 7 greater than the spring force of spring 31. Also, similar to as described above, the signal produced by the control mechanism upon tripping may preferably be used to control power to the motor 4 to stop the motion of traveling nut 7 to provide force-limiting at end of range, safety, and maximum force- limiting functionality of the linear actuator 700. Also similar to as described above, the compressive and extensive characteristics of spring 31 may be selected so as to determine the magnitude of force required to be exerted against traveling nut 7 in order to cause movement of the motor 4 relative to the actuator housing 2, and to trip a control mechanism. The use of a single spring 31 in the present embodiment may allow for the overall length of the actuator housing 2 to be advantageously shortened relative to otherwise similar embodiments incorporating two springs abutting motor 4, which may be particularly beneficial in some applications requiring compact linear actuators. As will be apparent to those skilled in the art, in light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.