GB2139315A - Friction drive mechanism - Google Patents

Abstract

A high-speed, high-accuracy closed loop servo-system uses a friction drive mechanism to move an output element between a plurality of positions without the back-lash, noise and resonance associated with gear excitation in gear drive mechanisms. The friction drive mechanism in a drive train having an input and output shaft (42, 20) has a drive wheel 43 connected to a drive motor 19, a driven wheel (38) connected to the output shaft (20) and the drive and driven wheels mounted in tangential relationship for movement relative to one another. A pressure wheel (49) is mounted for tangential engagement with one of the drive and driven wheels and is located generally on the side of one of the wheels opposite from the other wheel to produce frictional engagement between the drive and driven wheels at the point of tangency. A spring loaded plunger 54 regulates the pressure imparted by the pressure wheel. <IMAGE>

Description

SPECIFICATION Friction drive mechanism BACKGROUND OF THE INVENTION The present invention relates to a drive mechanism and deals more particularly with a friction drive mechanism.

Drive mechanisms of the type with which this invention is concerned, for example those that are used in high-speed and high-accuracy applications requiring start and stop stepping action and direction reversals are well known in the art. A typical application is a positioning and measuring system controlling a servomotor to automatically position a tool and workpiece relative to one another in accordance with a desired work program. Such a measuring and positioning apparatus utilizing laser interferometry to provide position feedback signals is described and illustrated in Patent No. 3,884,580 assigned to the assignee of the present invention. Such applications require the output driving movement to accurately respond to an input command signal without introduction of error from the drive mechanism.Although the present invention has utility in any application requiring a drive mechanism, it is disclosed in particular in a high resolution plotting system in which the tool is an optical head used to plot graphic information on a workpiece.

Drive mechanisms having rigid mechanical connections utilizing gears, ball lead screws and the like have been devised in an attempt to provide the faster response times required to yield the speed and accuracy necessary in high-speed and highaccuracy servosystem applications. However, these servosystems because of the high stiffness of the drive mechanisms and generally low damping are susceptible to apparatus resonance that occurs, for example, within the range of gear teeth excitation frequencies. This resonance in turn contaminates feedback control information which causes instability as the system hunts to reduce the feedback error signal to zero.

Sources of instability are well known to those skilled in the art and elaborate solutions have been proposed to minimize their effects but it is best to eliminate the problems at their source.

Gear or other toothed drive mechanisms also suffer from backlash as the drive mechanism moves a lead back and forth from position to position. Backlash introduces error directly into the system since the output gear position relative to the input driving gear is not consistent at all times and load positions.

If the output gear is being driven in one direction and the direction of the input gear is reversed, the input gear must rotate some small amount before its teeth contact the opposite sides of the gear teeth on the output gear. Also, the teeth of the input gear and the output gear act like springs as torque is applied and induce backlash chatter at constant drive speeds.

Consequently, backlash becomes the major source of excitation causing apparatus resonance. Methods to minimize or eliminate backlash are generally very expensive, requiring precision manufacturing to cut perfect gears or to use spring loaded split gears or preloaded ball lead screws to maintain pressure on both sides of the driving elements so any reversal in direction is followed without position lag.

Additionally, gear chatter generates generally annoying and undesirable noise. In order to minimize the noise generated and also to prolong the useful life of the gearing mechanism, lubrication is required at periodic intervals.

Accordingly, a general object of the present invention is to provide a friction drive mechanism wherein the sources of resonance are minimized or eliminated and the dynamic performance of both the drive mechanism and the apparatus being driven is improved.

A further object of the present invention is to provide a friction drive mechanism particularly suited for high-speed, high-accuracy applications.

Another object of the invention is to provide a drive mechanism wherein reliability is improved and operating noise is minimized.

Other objects and advantages of the invention will be apparent from the drawings and from the following written description.

The invention relates to a friction drive mechanism comprising a drive wheel connected to a drive motor, a driven wheel connected to an output shaft and the drive and driven wheels being mounted for movement relative to one another and in tangential relationship with the centers of the wheels and the point of tangency disposed along a straight line extending through the centers. A pressure wheel is mounted for tangential engagement with one of the drive and driven wheels and is located generally on the side of one of the wheels opposite from the other of the wheels to apply pressure between the drive and driven wheels at the point of tangency. Pressure producing means regulates the amount of pressure imparted by the pressure wheel to one of the drive and driven wheels.

The invention further relates to a closed loop servosystem having a servomotor and the friction drive mechanism for moving an output element between a plurality of positions in accordance with an input position command signal. Sensing means are included for producing a feed-back signal representing the output element movement by the friction drive mechanism, and summing means receives the feedback signal for producing a position error signal that drives the servomotor.

Fig. 1 is a perspective view of a photoplotter employing the mechanism of the present invention.

Fig. 2 is a frontal view of a preferred form of the mechanism as employed in the photoplotter.

Fig. 3 is a top plan view of the mechanism sectioned on line 3-3 of Fig. 2.

Fig.4 is a side elevational view of the mechanism sectioned along the line 4-4 of Fig. 3.

Prior to describing the invention, a typical high-speed, high-accuracy servosystem will be described to help familiarize the reader with the type of system in which the present invention is employed.

Fig. 1 illustrates a photoplotter, generally designated by the numeral 10, having frictional drive mechanisms, generally designated 21 and 29, embodying the present invention.

The photoplotter automatically positions an optical photohead 11 and a film plate F relative to one another in accordance with a desired work program to expose the film with lines and symbols that are located with an accuracy of + 0.002 inch. The invention is not, however, limited to the photoplotter application and may be used with other apparatus requiring a drive mechanism.

The photoplotter shown in Fig. 1 includes a stationary support frame 12 having legs 13, a support platform 14 and an overlying bridge 15. The bridge is rigidly connected to the frame and spans the midsection of the platfform at an elevated height.

A work table 16 has a work surface on which the film plate F is held stationary. The table is mounted on pillow blocks fixedly connected to the underside of the table and is guided in the illustrated X-coordinate direction by a pair of round ways 17,18 mounted on the platform 14.

Movement of the table 16 in the X-coordinate direction is produced by means of an Xdrive motor 19, a single lead screw 20 rotated by the friction drive mechanism 21 of the present invention and a ball nut (not shown) connected to the underside of the central portion of the table. The drive mechanism 21 is contained generally in a frame 40 attached to the stationary support frame 12.

The drive motor 19 is connected to a plot controller 22 through a command signal cable 23. The controller 22 derives plotting commands from a program tape 24 and transmits the commands to the drive motor 19 in order to position the work table 16 and the film F thereon at various locations along the X-coordinate axis during a plotting operation.

The optical photohead 11 is supported and moved over the film plate F by means of a tool carriage 25. The photohead has an optical axis 26 extending generally perpendicular to the photosensitive surface of the film plate F and the X and Y coordinate axes, and by projecting a beam of light along the axis, the photohead exposes the film surface. Exposure can occur while the film plate and photohead are moving relative to one another so that a continuous line is exposed.

Still referring to Fig. 1, the tool carriage 25 is mounted on the bridge 15 for movement along the bridge in the illustrated Y-coordinate direction by means of a Y-drive motor 27, a single lead screw 28 rotated by the friction drive mechanism 29 of the present invention and a ball nut 30 attached to the tool carriage. The drive mechanism 29 is contained generally in a frame 57 mounted on bridge 15. Command signals also derived from the program tape 24 are supplied by the controller 22 to the Y-drive motor 27 to position the photohead transversely along the bridge 15 relative to the film plate F. It will be understood that composite movements of the film plate F and the photohead by means of the X and Y drive motors 19,27 allow the photohead to expose any area of the photosensitive surfface of the film F.

Movements of the photohead 11 and the work table 16 are measured by means of a laser interferometer system in order to accurately expose a plot on the film plate F.

Briefly, the interferometer system is a dual axis system for measuring motion in both the X and Y coordinate directions and includes a heliumneon laser 31 mounted on the platform 14 for generating a laser beam that is used to measure along both coordinate axes, an x-axis photodetector in the laser and Y-axis photodetector in the laser for measuring movements along the X and Y axis respectively. The generated laser beam is transmitted to a beam splitter (not shown) mounted on the bridge 15. The beam splitter directs one portion of the beam to the x-axis interferometer 32 mounted on the tool carriage 25 and another portion of the beam to the Y-axis interferometer 33 mounted on the bridge 15.The Xaxis interferometer 32 operates with a remote elongated mirror 34 mounted on the work table 16 at one edge and has a measuring axis 35 extending between the interferometer 32 and the elongated mirror 34 in the illustrated X-direction. The reflective surface of the mirror 34 is perpendicular to the measuring axis 35 and is elongated in the Y-coordinate direction so that the measuring axis intersects the reflective surface at each position of the tool carriage 25 along the lead screw 28.

The Y-axis interferometer 33 has a measuring axis 36 extending between the interferometer and a retroreflector 37 mounted on the tool carriage 25. Since the tool carriage moves in the Y-coordinate direction parallel with the axis 36, the retroreflector 37 is not elongated and remains in intersecting relationship with the axis 36 at each position of the tool carriage and the work table 16.

The term "measuring axis" refers to the axis along which an interferometer projects a beam or beams of coherent light to a remote reflector for the purpose of measuring relative movement between the interferometer and the reflector. The axis may in fact be the median line between two or more parallel beams of light and the beams may be deflected at points between the interferometer and reflector so that the measuring axis is bent with two or more portions extending in different directions. The term "intersect" is intended to refer to the geometric condition in which a line or extension of that line passes through a specific point, line or plane.

Turning now to Figs. 2-4, the X-axis friction drive mechanism 21 contained within the frame 40 as described in Fig. 1 is shown in accordance with one embodiment of the present invention. It will be understood that the Y-axis friction drive mechanism 29 contained generally in frame 57 mounted on bridge 15 is similar in appearance and operation to the X-axis friction drive mechanism. Consequently, the illustration and description of the Y-axis friction drive mechanism is omitted.

The X drive motor 19 is mounted on a pendular support frame 41 as best illustrated in Figs.2 and 4. The pendular frame is connected to one end of a flex hinge 44. The term "flex hinge" is used here to describe a resilient integral hinge formed by weakening directly opposite sides of a member by forming grooves or channels. A hinge fabricated in this manner permits the member to pivot at a pivot axis and is sufficiently rigid to function as a support. The opposite end of the flex hinge is connected to a shank 45 which extends through the frame 40. The shank is held in place by a retaining nut or collar 46.

The frame 41 is rotatable about center line 47 of shank 45 to facilitate alignment within the friction drive mechanism 21 as explained in greater detail below. The pivoting action of flex hinge 44 permits limited arcuate lateral movement of the pendular frame 41 and the attached X drive motor.

The friction drive mechanism 21 has a friction drive wheel 43 mounted on the drive shaft 42 of the motor 19 and a friction driven wheel 38 connected to an output shaft or lead screw 20 shown in phantom in Fig. 2. The lead screw is mounted in a conventional manner such as through a ball bearing 39 in the support frame 12. Such an arrangement prevents lateral movement by the lead screw 20 while allowing the lead screw to rotate.

The drive wheel 43 is supported by the motor 19 at one side of the driven wheel 38 in tangential relationship and can be moved toward and away from the driven wheel through the lateral movement of the pendular frame 41.

A pressure wheel 49 is shown rotatably mounted in a yoke 50 at the side of the drive wheel 43 opposite from the driven wheel 38.

Yoke 50 is rigidly connected to the upper end of a movable arm 58 having a flex hinge 51 near the opposite lower end. The hinged end of the arm is fastened to the frame 40 by screws 52 to permit axial alignment of the pressure wheel and the drive wheel to maximize surface contact area and promote uniform wear. The pivoting action of the flex hinge 51 permits limited arcuate lateral movement of the arm 58, the yoke 50 and the pressure wheel 49 toward and away from the drive wheel 43.

As best shown in Figs. 3 and 4, the axial ends of pressure wheel 49 have flanges having rims 56 which provide surface contact on drive wheel 43 at locations on both sides of the surface area that contacts the driven wheel 38. The wheels are made of stainless steel having a Rockwell C hardness of 55 to 60 for minimum peripheral surface wear and to obtain preferred frictional surface contact for driving torque transmission between the drive and driven wheels.

A spring-loaded plunger 54 mounted in frame 40 applies force to the movable arm 58 in a direction which urges the pressure wheel 49 into contact with the peripheral surface of drive wheel 43. The resilent support of the motor 19 by the pendular frame 41 allows the drive wheel in turn to be urged into contact with the peripheral surface of driven wheel 38 for frictional torque transmission. A pressure adjusting screw 55 provides means for increasing or decreasing the compression on the springloaded plunger 54 to regulate the force applied to arm 58 and between the wheels 38 and 43. Increasing the force on the arm 58 causes the pressure wheel 49 to impart a greater force at the point of tangency between the two wheels, 38 and 43.The arrangement of the pressure wheel and the spring loaded plunger in the same plane with the drive and driven wheels also eliminates bending stresses in the motor shaft 42 of the motor 19.

The drive wheel 43 transmits maximum torque to the driven wheel 38 when the axes of the drive and driven wheels are aligned in perfect parallelism because maximum friction occurs when the surface contact area between the two wheels is the largest. Alignment is accomplished by rotating the pendular frame 41 about the center line 47 of shank 45 to bring the drive wheel axis into parallel alignment with the driven wheel axis. The alignment is held by a set screw 48 in collar 46 tightened against shank 45 and is held permanent by fastening shank 45 to frame 40 with a set screw 53. Although some slippage may occur between the drive and driven wheels the amount is minimal and of no consequence in positional feedback system applications.

In summary, a friction drive mechanism has been described above that avoids prior prob lems associated with gear excitation in gear drive mechanisms used in high speed, high accuracy servosystem applications. Sources of resonance and noise are minimized or eliminated and the dynamic performance of both the drive mechanism and the driven apparatus is improved.

It should be understood that the foregoing description illustrates one preferred embodiment of the present invention and that numerous substitutions and modifications can be had without departing from the spirit of the invention. For example, pressure wheel 49 could be arranged to urge a moveably supported driven wheel 38 toward and away from a fixedly mounted drive wheel 43. Similarly, the hinge function of flex hinges 44 and 51 might be carried out by pinned pivot hinges. Accordingly, the present invention has been described merely by way of illustration rather than limitation.

Claims (8)

1. A friction drive mechanism in a drive train having an input and an output shaft comprising: a driven wheel connected to the output shaft for transmitting rotational driving movement to said output shaft; a drive wheel connected to the input shaft for transmitting the rotational driving movement from said input shaft; said drive wheel and said driven wheel being mounted in tangential relationship with the centers of said wheels and the point of tangency disposed along a straight line extending through said centers of said wheels; a pressure wheel mounted for tangential engagement with one of the drive and driven wheels and located generally on the side of said one of the wheels opposite from the other of the wheels; a first support means holding said pressure wheel for at least limited movement.toward and away from said one of the drive and driven wheels;; a second support means holding said one of the drive and driven wheels for at least limited movement toward and away from the other of the drive and driven wheels to allow said pressure wheel to apply pressure between said drive and driven wheels at said point of tangency; and pressure producing means for urging said pressure wheel toward said one of the drive and driven wheels and thereby producing pressure between the drive and driven wheels at the point of tangency and permitting the drive wheel to transmit driving torque to the driven wheel by friction.

2. A friction drive mechanism in a drive train as defined in claim 1 further comprising: a third support means for holding stationary relative to the second support means, the other of the drive and driven wheels and permitting the other of the wheels to rotate.

3. A friction drive mechanism in a drive train as defined in claim 2 wherein said first support means comprises a movable arm mounted on said third support means for holding said pressure wheel.

4. A friction drive mechanism in a drive train as defined in claim 3 wherein said movable arm comprises a flex hinge located near one end of said arm.

5. A friction drive mechanism in a drive train as defined in claim 1 wherein said pressure wheel includes a flange at each axial end, said flange having a rim for peripheral surface contact engagement with said one of the drive and driven wheels.

6. In combination with a closed loop servosystem for moving an output element between a plurality of positions and having a servomotor for moving the output element in accordance with an input command signal, sensing means for producing an output element movement representing feed-back signal, and summing means receiving the feedback signal for producing an error signal to drive said servomotor, the improvement comprising:: a driven wheel connected to the output element; a drive wheel connected to the servomotor; said drive and driven wheels being mounted in tangential relationship for at least limited movement relative to one another with the centers of said wheels and the point of tangency disposed along a straight line extending through said centers of said wheels; a pressure wheel mounted for tangential engagement with one of the drive and driven wheels and located generally on the side of said one of the wheels opposite from the other of the wheels with the point of tangency disposed in the vicinity of said straight line extending through said centers of said wheels; pressure producing means for urging said pressure wheel toward said one of the drive and driven wheels to produce frictional engagement of said drive and driven wheel to move said output element by means of the servomotor.

7. A friction drive mechanism substantially as herein described and showin in the accompanying drawings.

8. A servosystem substantially as herein described and shown in the accompanying drawings.