Elastic biasing element and encoder arrangement for precise control of force or torque

Abstract

An apparatus, system, and method using an elastic biasing element in combination with an encoder arrangement for precise control of force or torque applied to a moving object, is applied for controlling a feed force applied to an abrasive element of a bore finishing tool, to respond to changes in the feed force such as can arise from contact with a workpiece bore surface and variations therein, such as tapers, hourglass shapes, barrel shapes, and the like. The elastic biasing element can include a single or multiple springs in one or more sets, and the feed force can be selected to have a constant value or vary as a function of time, position, or other variables or conditions.

Claims

1. A feed system for a feeding and applying a feed force to an abrasive element of a bore finishing tool in a lateral direction relative to an axis of rotation thereof, comprising: a drive element supported for movement in a first direction and an opposite second direction and a drive apparatus connected to the drive element and controllably operable to move the drive element in the first direction and the opposite second direction within a predetermined range for generating the feed force; at least one elastic biasing element having a first end and a second end, the first end disposed in predetermined relation to the drive element so as to be displaced by the movement thereof to cause the at least one biasing element to elastically store a quantity of energy proportional to the displacement and representative of the feed force, and the second end being disposed in predetermined relation to an output element disposed to move generally axially in cooperation with a feed element of the bore finishing tool when the output element is positioned to bear thereagainst, to transfer and apply the feed force laterally to the abrasive element and to displace jointly with the output element responsive to changes in the applied force; and a first sensor positioned and operable to measure a value representative of the displacement of the second end of the biasing element and output a signal representative thereof, a second sensor positioned and operable to measure a value representative of the displacement of the first end of the biasing element and output a signal representative thereof, and a processor connected to the first sensor and to the second sensor to receive the signals outputted thereby and configured to determine a responsive value for moving the drive element of the drive apparatus to apply a selected feed force.

2. The feed system of claim 1, wherein the processor is connected in operative control of the drive apparatus.

3. The feed system of claim 1, wherein the drive apparatus comprises a servomotor and apparatus to translate rotary motion thereof to linear motion.

4. The feed system of claim 1 wherein the first sensor and the second sensor comprise encoders, respectively.

5. The feed system of claim 1, wherein the second sensor comprises an encoder incorporated into the drive apparatus.

6. The feed system of claim 1, comprising at least two of the elastic biasing elements.

7. The feed system of claim 6, wherein the biasing elements have different spring constant values.

8. The feed system of claim 1, wherein the at least one elastic biasing element comprises multiple biasing elements arranged in sets.

9. The feed system of claim 1, wherein the at least one biasing element comprises a spring.

10. The feed system of claim 1, wherein the at least one elastic biasing element is selected from a group comprising at least one elastically compressible biasing element, at least one elastically tensionable biasing element, and a combination of at least one elastically compressible biasing element and at least one elastically tensionable biasing element.

11. The feed system of claim 10, wherein the at least one elastic biasing element comprises at least one biasing element that has an elasticity property variable as a function of the displacement of one or both of the ends thereof.

12. The feed system of claim 11, wherein the at least one elastic biasing element comprises at least one variable pitch spring.

13. The feed system of claim 1, further comprising a locking mechanism positioned and operable to lock the feed system such that the displacement of the first end of the biasing element will directly move the output element to apply the feed force.

14. The feed system of claim 1, wherein the drive apparatus comprises a linear drive.

15. The feed system of claim 14, wherein the linear drive comprises a fluid cylinder.

16. The feed system of claim 14, wherein the linear drive comprises a linear motor.

17. The feed system of claim 1, wherein the selected feed force is controlled at least in part using a function based on at least one variable selected from a group consisting of at least time and position.

18. A bore finishing tool comprising: a feed system for a feeding and applying a feed force to an abrasive element of the bore finishing tool in a lateral direction relative to an axis of rotation thereof, the feed system comprising: a drive element supported for movement in a first direction and an opposite second direction and a drive apparatus connected to the drive element and controllably operable to move the drive element in the first direction and the opposite second direction within a predetermined range for generating the feed force; at least one elastic biasing element having a first end and a second end, the first end disposed in predetermined relation to the drive element so as to be displaced by the movement thereof to cause the at least one biasing element to elastically store a quantity of energy proportional to the displacement and representative of the feed force, and the second end being disposed in predetermined relation to an output element disposed to move generally axially in cooperation with a feed element of the bore finishing tool when the output element is positioned to bear thereagainst, to transfer and apply the feed force laterally to the abrasive element and to displace jointly with the output element responsive to changes in the applied force; and a first sensor positioned and operable to measure a value representative of the displacement of the second end of the biasing element and output a signal representative thereof, a second sensor positioned and operable to measure a value representative of the displacement of the first end of the biasing element and output a signal representative thereof, and a processor connected to the first sensor and to the second sensor to receive the signals outputted thereby and configured to determine a responsive value for moving the drive element of the drive apparatus to apply a selected feed force.

19. A method of controlling a feed force applied to an abrasive element of a bore finishing tool laterally relative to an axis of rotation of the tool, comprising steps of: providing a drive apparatus controllably operable to move a drive element in a first direction and an opposite second direction within a predetermined range; providing an elastic biasing element having a first end and a second end, the first end disposed in predetermined relation to the drive element so as to be displaced by the movement thereof to cause the biasing element to elastically store a quantity of energy proportional to the displacement and representative of the feed force, and the second end being disposed in predetermined relation to an output element disposed to move generally axially in cooperation with an element of the bore finishing tool to transfer and apply the feed force laterally to the abrasive element and to transfer changes in the applied force from the abrasive element to the second end of the biasing element so as to cause displacement thereof representative of the changes; and during rotation of the tool in a bore with the abrasive element in contact with a surface bounding the bore, measuring positions of the output element or the second end of the biasing element and responsively determining displacements of the second end of the biasing element, and controlling the drive apparatus to move the drive element as required to variably displace the first end of the biasing element responsive to changes in the applied feed force as represented by the displacements of the second end thereof, to apply a predetermined feed force to the abrasive element.

20. The method of claim 19, wherein during the rotation of the tool in the bore, positions of the drive element or the first end of the biasing element are measured.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a simplified side view of a representative embodiment of a motion control system of the invention, used for controlling feed of a bore finishing tool;

(2) FIG. 2 is a simplified side view of a another embodiment of a motion control system of the invention, used for controlling feed of a bore finishing tool;

(3) FIG. 3 is a simplified side view of a another embodiment of a motion control system of the invention, used for controlling feed of a bore finishing tool;

(4) FIG. 4 is another simplified side view of the motion control system of FIG. 3, shown in another operating mode;

(5) FIG. 5 is a graphical representation showing a relationship between encoder difference and output force for the motion control system of FIGS. 3 and 4;

(6) FIG. 6 is a perspective view of a representative bore finish machine incorporating the invention, showing a machine controller operable to perform control aspects of the invention;

(7) FIG. 7 is a perspective view of aspects of the machine of FIG. 6, including a rotary spindle carrying a tool holder on which is mounted a representative bore finishing tool which is a honing tool, along with associated workpiece holding apparatus of the machine; and

(8) FIG. 8 is another perspective view of the machine, showing the spindle and tool holder.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(9) Referring now to the drawings, a bore finishing feed system 18 incorporating an apparatus, system, and method of motion control adapted for imparting a precise feed force to an abrasive element of a tool of a bore finishing tool, is shown. The feed system 18 is operable to control the applied feed force as a constant force, or a force controlled to any required function, either based on time, position or other variables that are monitored by the system, as desired or required for a particular application.

(10) Input motion is accomplished with a drive apparatus 20 such as a servomotor 30 or such, having a drive element 22, and capable of responding to externally measured or computed digital signals. When a rotary servomotor is used, some apparatus to convert the motor rotation to linear motion 24 is required. In this embodiment this is accomplished using a ball screw 26 and a ball nut 28, but it could be a rack and pinion or any other device capable of converting rotary motion to linear motion. The ball nut 28, moving linearly, pushes against a first end 34 of an elastic biasing element 32 which in this embodiment is a spring, and which has a second end 36 that in turn bears against an output element 38, which can comprise for instance, an output rod of the bore finishing or honing machine.

(11) In the bore finishing or honing machine, the output element 38 will drive a wedge 42 of a honing tool 40. The wedge 42 imparts force and motion to an abrasive element or elements 44, such as a honing stone or stones. This honing tool assembly is rotated and stroked by the honing machine (spindle not shown) relative to a workpiece 48 that has a bore surface 46 within a bore to be finished.

(12) The force being delivered by the above described apparatus will cause the elastic biasing element 32 (spring) to compress by an amount that is a function of the force being applied. If x and y represent the linear positions of rigid components connected to ends 34 and 36 of biasing element 32, and if x.sub.0 and y.sub.0 are some pair of positions where the biasing element 32 is uncompressed, then the force in the element 32 is a function of the differences in these positions:
F=f[(yy.sub.0)(xx.sub.0)] where F is the delivered force
For conventional springs, this function is a simple proportional relationship such that
F=k[(yy.sub.0)(xx.sub.0)] where k is the spring constant

(13) However, nonlinear springs or other elastic components may be used as biasing element 32 as long as their force vs. compression function is known and can be digitally computed.

(14) The position x is measured continually by a primary encoder 50, which here is a linear encoder connected to a process controller 76 of the system for outputting positional signals thereto. This encoder 50 is considered primary because it measures the position of the output element 38 and other rigid components connected to it relative to a fixed location such as a frame 52 of the machine on which the system is used. It is often useful or necessary to display this position or use it for other control functions. In the example of the honing machine, the honing cycle can be stopped when the position x reaches a value known to correspond to a desired bore size.

(15) A secondary encoder 54 also connected to controller 76 for outputting positional signals thereto can be placed directly on the ball nut 28 (or in that vicinity) to measure the position y. However servomotors will commonly be equipped with internal rotary encoders so it becomes simpler to instead use that encoder as the secondary encoder 54. A mathematical conversion will be necessary:
y=r
where r is the ratio of an increment of linear motion to the corresponding increment of rotation as determined by the specific parameter of the ball screw or other rotary to linear conversion device. represents the number of increments of rotary motion (counts).

(16) To control the output force, the system 18 will continually sample both the primary and secondary encoders 50 and 54 and compute their differences. This difference is compared to a desired difference which, by the functions shown above, corresponds to a desired level of feed force. This computed difference then becomes the feedback variable controlling the motion of the drive apparatus 20. If the difference is too large then the drive apparatus 20 is operated in the direction that will reduce that difference; if the difference is too small then the apparatus 20 is operated in the direction that will increase the difference.

(17) In this manner the output element 38 can be moved through a range of motion continually imparting the desired (programmed) level of feed force.

(18) The use of an elastic element (such as a spring) as opposed to more rigid connections serves to maintain the constancy of the delivered feed force as follows.

(19) If the output element 38 encounters sudden resistance (in the honing example, when the abrasive element or honing stone contacts the workpiece bore surface 46) then there will be a force spike at impact. The system 18 always requires some amount of time to react to sudden changes and in that amount of time the force will continue to increase. If the elastic biasing element 32 is fairly soft (low value of k), then the force will increase only minimally in the brief time that the system can respond to the change in position x. Therefore this system can be designed to have a spring with a spring constant that keeps those sudden force changes within a very small tolerance band as required by the particular application.

(20) In operation, during rotation of the tool 40 in a bore with the abrasive element 44 in contact with a surface 46 bounding the bore, drive apparatus 20 can be automatically controlled by controller 76 to move the drive element 22 as required to variably displace the first end 34 of the biasing element 32 responsive to changes in the applied feed force as represented by displacements of the second end 36, to apply a predetermined feed force to the abrasive element 44. Thus it should be apparent that the system can efficiently and quickly respond to variations in the applied feed force resulting from bore diameter variations and the like, including during concurrent rapid stroking actions of the tool.

(21) It is known that due to the nature of motion control systems, changes to motor or other driver position cannot be instantaneous, and although properly tuned, there can still be overshoot and undershoot which in a purely rigid system would result in brief fluctuations in the force delivered by the output element. Having an elastic member in the drive train as afforded by the present invention allows errors in servomotor and other driver position to cause only very minimal errors in delivered force. Again, the degree of force precision can be designed into the system by proper selection of the spring constant.

(22) Variations

(23) FIGS. 1 and 2 show one simple embodiment of the system and apparatus of the invention, but many other variations are envisioned:

(24) As described above, a rotary encoder is not required if the secondary encoder is a linear encoder located near the input to the spring. In such a case, the driver could be a linear driver 56 such as a linear motor or a fluid cylinder such as a hydraulic cylinder controlled by a servo valve, which often can be supplied with embedded linear encoders. This variation is shown in FIG. 2.

(25) The same principle of operation can used to control an output torque. In that case there is no ball screw to convert rotary motion to linear motion and the spring is a torsion spring. The primary encoder then is a rotary encoder. Again the difference between the two rotary encoders represents some level of torque as determined by the spring constant of the torsion spring.

(26) The spring need not be a conventional coil spring. It can be any component that is significantly less rigid than the rest of the drive train.

(27) The spring can be one capable of tension so that a pulling force can be controlled, or both compression and tensions springs may be used in combination to have a system capable of pushing and pulling. Likewise this elastic component may be multiple springs arranged in an assembly so that push and pull forces can be controlled with the same device.

(28) The elastic component may be an assembly of multiple springs with various spring constants, to provide finer force control for lower levels of force than for higher levels of force, thereby keeping the overall length of the device to a minimum. Likewise non-linear springs may be used to achieve the same effect.

(29) These last two variations are employed in one particular embodiment for a honing machine as shown in FIGS. 3 and 4.

(30) In this embodiment, a set of several springs are used as the elastic biasing element 32. Each set is comprised of a subset of springs of various strengths, each subset acting in series, but nested together to optimize the utilization of space. Each subset is comprised of multiple springs in parallel, which allows for achieving the low spring rate (for better control) also in a relatively small space.

(31) Two such spring sets can be arranged in a housing that is pushed or pulled by a ball screw 26 and ball nut 28 and servomotor 30 as previously described. FIG. 3 shows an assembly that is being pushed by the ball screw and motor. By this motion a push-side spring Set 64 is partially compressed to deliver a push force F through a central feed rod that comprises the output element 38 (in connection with the feed element of the tool such as wedge 42 (FIGS. 1 and 2). In this mode the other set of springs is a pull-side set 62 which is relaxed and not used.

(32) FIG. 4 shows an assembly that is being pulled by the ball screw and motor arrangement. By this motion the pull-side spring set 62 is partially compressed, delivering a force to the opposite side of a flange on the feed rod so that it will provide a pulling force F on the other end of the feed rod acting as output element 38 in connection with the wedge or other feed element of the associated tool (see FIGS. 1 and 2), the force F comprising the feed force in both instances.

(33) FIG. 4 also illustrates a locking cylinder or mechanism 72 (pneumatic or hydraulic) which can engage a locking notch 74 in the feed rod. If the system is placed in a neutral position (both spring sets relaxed) then this mechanism 72 can lock in the notch 74 which will lock the entire feed system. When locked, neither set of springs are used and the motion of the servomotor 30 will result in direct motion of the feed rod with no control of feed force. This feature mimics the behavior of older honing machine feed systems and is still useful for some honing applications.

(34) The spring sets shown in FIGS. 3 and 4 employ springs of various strengths in series to create a non-linear (or piece-wise linear) relationship between the measured difference in encoder positions and the force being applied. FIG. 5 is a graph showing this relationship between encoder difference and output force.

(35) This relationship between the encoder difference and the feed force allows the honing or other bore finishing feed system to produce a very wide range of feed forces and yet have very precise control of the lower levels of feed force. This is necessary for small tool applications where very light feed forces must be applied with precision.

(36) As an alternative to using sets of various-sized springs, a non-linear spring could be designed and employed as elastic biasing element 32. A well-known way to achieve that is with a coil spring wound to have a continuously varying pitch. A special spring of that type could be produced to give nearly the same curve as shown as shown in FIG. 5. and hence the same benefit.

(37) For the particular embodiment of a honing machine feed system, this above described apparatus and system can maintain the feed force very closely to the desired feed force set by the system or input by the operator. In many applications, closer control of feed force improves the bore size control of the honing operation and optimizes the life and performance of the abrasive element or honing stone.

(38) In light of all the foregoing, it should thus be apparent to those skilled in the art that there has been shown and described an apparatus, system, and method using an elastic biasing element in combination with an encoder arrangement for precise control of force or torque applied to a moving object, namely, for controlling feed force of a bore finishing tool such as a honing tool. However, it should also be apparent that, within the principles and scope of the invention, many changes are possible and contemplated, including in the details, materials, and arrangements of parts which have been described and illustrated to explain the nature of the invention. Thus, while the foregoing description and discussion addresses certain preferred embodiments or elements of the invention, it should further be understood that concepts of the invention, as based upon the foregoing description and discussion, may be readily incorporated into or employed in other embodiments and constructions without departing from the scope of the invention. Accordingly, the following claims are intended to protect the invention broadly as well as in the specific form shown, and all changes, modifications, variations, and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention, which is limited only by the claims which follow.