Hysteresis removal for positioning systems with variable backlash and stiction

Abstract: A system and method for reducing non-repeatable positioning errors. Non-repeatable positioning errors caused by the effects stiction and backlash in the mechanical system are reduced by "shaking" or vibrating the positioning system in a controlled manner. After the system input is set to the desired value, the positioning system is "shaken" by inputting a series of offsets that oscillate around the desired location and gradually decrease in amplitude, eventually reaching zero. In a specific embodiment, a lead screw (210) is threaded through a follower nut (218) which is connected to a magnetic tape read/write head (216). The lead screw (210) is rotated by a mechanical transmission which provides gear reduction from a computer controlled stepper motor (202). The tape head's (216) non-repeatable positioning error is reduced by alternately stepping the stepper motor (202) in opposite directions a specified number of steps and periodically reducing the specified number of steps until zero is eventually reached.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates generally to systems for the precise positioning of objects. It is contemplated that the invention can be adapted to robotic machines of a general nature including, but not limited to, positioning applications such as X-Y stages, "pick and place" machines, integrated circuit mask aligners, pen plotters, scanners, and machine tools. Statement of the Problem Positioning systems have a variety of applications. For example, X-Y stages may be used to accurately locate a sample under a microscope, or to manipulate a work piece underneath a cutting head. Often, the capabilities of these machines depends on the accuracy of the positioning system. Improved repeatability and accuracy in a "pick and place" machine, as one example, allows a tighter lead pitch and smaller packages to be used for electronic components. This subsequently reduces the cost of the electronic device being manufactured. Likewise, greater positioning accuracy of work pieces allows tighter tolerances at a reduced cost for mechanical manufacturing. One common way of constructing a positioning system is to drive the mechanical elements of the positioning system with a computer controlled stepper motor. The mechanical input to the positioning system is provided by the rotation of the stepper motor's shaft under the computer's control. This arrangement has broad applications which are only limited by the mechanics used to translate the stepper motors rotation to a desired position. One example of such a system is used to position tape and disk read/write heads. Tape and disk recording systems for computers record parallel tracks of data on a magnetic medium. A recording head must be positioned on a track or cylinder such that it is within a predetermined dimensional tolerance of the recorded track or cylinder centerline. To achieve this for a typical tape drive, the head is positioned by driving a lead screw and follower nut connected to the head. The lead screw is driven by a mechanical transmission. The transmission is driven by a stepper motor. The stepper motor may be controlled by a microprocessor. The transmission provides a gear reduction between the stepper motor and the lead screw. This gear reduction makes it possible to reduce system cost by using a stepper motor with a large step increment and still maintain a small system step resolution. In addition to step resolution, variable stiction in the head guiding system and gear backlash affect the system's positioning accuracy. Stiction and backlash create non-repeatable hysteresis which causes the system's response to an input to depend on both the current input, and past values of input and head position. The problem is compounded in high volume production environments where mechanical components suffer a wide variance in component dimensions. Stiction and backlash errors can be remedied by using a closed loop positioning system. A closed loop servo system, however, would cost approximately twice as much as the existing open loop system. Other solutions include tighter tolerances on mechanical components, or a high resolution stepper motor coupled to a simpler transmission. These options would also increase system cost. GB 986216A discloses a closed loop position control system comprising first means for causing a driving action producing relative movement between one member of a mechanical system with respect to another towards the desired relative position and beyond it at a predetermined speed, second means responsive to error combinations indicative of a selected change-speed error position for causing the drive to be reduced in speed and also reversed in direction so that the relative movement towards the desired relative position is produced at a lower speed and third means for terminating the driving action in response to the error combinations when the error combinations are indicative of a known error distance. This system necessarily requires to detect the positioning error and is thus based on the closed loop control approach. US 4703242A relates to a magnetic disk head positioning system using a stepper motor. This prior art system reduces the positioning error by two different control concepts. When the drive member of the system is intended to move from a first stop position to a second stop position, it is moved and stopped temporarily at a third stop position located immediately proceeding the second stop position and, thereafter, moved from the third stop position to the second stop position. In accordance with the alternative concept, when a drive member is intended to move from a fourth stop position to a second stop position, the fourth stop position being located opposite to the first stop position with respect to the second stop position, it is moved from the fourth stop position beyond the second stop position and stopped temporarily at a fifth stop position and, thereafter, moved back to the second stop position. Thus, a single overshooting is performed by this control method. It is the object of the present invention to provide a method and an apparatus for positioning an object that reduce or eliminate the non-repeatable effects of system stiction and backlash, but minimize cost and eliminate the need for redesign by using existing components and assemblies. This object is achieved by a method according to claim 1 and an apparatus according to claim 7. The present invention solves the problem of variable stiction and backlash by "shaking" or vibrating the positioning system in a controlled manner, and with sufficient amplitude to overcome local stiction and gear backlash. The system input is first stepped to the desired location. The system is then "shaken" by inputting a series of offsets that oscillate around the desired location and gradually decrease in amplitude to zero. Since the invention may be implemented through a change in the control software for the stepper motor, it does not require redesign and can use the existing mechanical components and assemblies. The invention has been demonstrated on a test platform. Using conventional techniques to position the head, the test platform has a worst case non-repeatable position error of approximately 600 micro-inches. Use of the controlled shaking of the invention reduced the 3 sigma point in the error distribution to less than 60 micro-inches. The present invention is exemplified by a tape head positioning system for a tape drive backup system. The invention consists of setting the input to the system to a first value corresponding to the desired output location. The input is then perturbed above and below that first value in gradually decreasing amounts until the input is back at the first value. The input to the positioning system may be the angular position of a stepper motor and the input may be perturbed by stepping the motor in one direction, then the opposite direction a number of times, occasionally reducing the number of steps the perturbations move away from the input position that corresponds to the desired output position. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a typical head positioning system. FIG. 2A and 2B are a side, and top view, respectively of the mechanical portion of a typical tape head positioning system. FIG. 3 graphically illustrates an input sequence with a desired location of 8 steps, a shaking amplitude of 5 steps, and a decay of 1 step after every 2 oscillations. FIG. 4 is a flow diagram of control logic to implement the present invention. FIG. 5 is a flow diagram of the control logic used to test the present invention on a tape head positioning system similar to that depicted in FIG. 1, 2A, and 2B. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in terms of a number of different embodiments. The preferred embodiment is a method and apparatus for positioning a magnetic tape read/write head. As will be discussed, the present invention provides improved position control and repeatability for a mechanical system by "shaking" or vibrating the system. A tape head positioning system, shown generally by reference numeral 100, is depicted in FIG 1. The system consists of a controller 104, which controls an actuator 106. The actuator 106 converts the output of the controller 112 to a physical quantity 114 which may be a linear or angular position. The output of the actuator 106 is mechanically coupled to a transmission 108 which converts the output of the actuator to a position of the tape head 110. The transmission 108 may have gear reduction, or mechanical advantage which allows a less expensive actuator to be used. The actuator and mechanical portion of the tape head positioning system enclosed in box 102 of FIG. 1 is shown generally by reference numeral 200 in FIG. 2A and FIG. 2B. These are a side and top view, respectively. In FIG. 2A and 2B, a stepper motor 202 may be connected to a shaft 204 which, in turn, may be connected to a worm gear 206. The stepper motor 202, corresponds to actuator 106 in FIG. 1. The worm gear 206 may engage gear 208 (FIG. 2B) which may be connected to a lead screw 210. The lead screw 210 may be coupled to a head mount 212 through a follower nut 218. The head mount 212 may slidingly engage a guide pin 214 which prevents the head mount 212 from rotating about the lead screw 210. Shaft 204, worm gear 206, gear 208, lead screw 210, follower nut 218, and head mount 212 correspond to transmission 108 of FIG. 1. A tape head 216 may be mounted on the head mount 212. Tape head 216 in FIG. 2 corresponds to tape head 110 of FIG. 1. As the stepper motor 202 rotates shaft 204, shown by arrow 220, the head mount 212 and the head 216 move in an upward or downward direction as shown by arrow 222 in FIG. 2A. The stepper motor 202 can turn either clockwise or counter-clockwise, therefore, the head 216 may be positioned either upward or downward. FIG. 3 is one example of a plot of the stepper motor position verses time implementing the present invention for the system of FIGS. 2A and 2B. In FIG. 3, the stepper motor 202 starts at zero steps clockwise and the target head position corresponds to eight steps clockwise. The controller waits approximately two milliseconds between each step to allow time for the physical actuation to occur. FIG. 4 illustrates a flow diagram of an embodiment of the invention. First, the input to the mechanical system is set to correspond to the desired output location (see boxes 400 and 402). Then the input to the mechanical system is perturbed around that location until it is time to adjust either the frequency or amplitude of the perturbations (see boxes 404 and 406). Thereafter, the frequency and/or the amplitude of perturbations is adjusted, if necessary (see boxes 408 and 410). If the shaking is complete, (for example, when the amplitude reaches zero) the system stops (see boxes 412 and 414). If the shaking is not complete, then the perturbations continue using the current frequency and amplitude until it is time for another adjustment. FIG. 5 illustrates a flow diagram of an embodiment of the invention using a stepper motor and a mechanical transmission such as that shown in FIGS. 2A and 2B. As shown, the parameters for the move are acquired and the input to the mechanical system is set. For the system of FIGS. 2A and 2B, the stepper motor 202 is stepped to the position which corresponds to the desired head location (see boxes 500, 502, and 504). Thereafter, stepper motor 202 is stepped, in the direction that brings the head down, by one-half the desired amplitude of perturbation, rounded to the nearest integer number of steps (box 506). Thereafter, stepper motor 202 is stepped up, then down the desired amplitude number of steps (box 508). This last step is repeated until it is time to reduce the amplitude of the perturbations (box 510). Thereafter, if the amplitude is odd, one step up is taken (boxes 512 and 514). The magnitude of the amplitude is then reduced by one step (box 516). If the magnitude of the amplitude is now zero, the perturbations are stopped and the system is finished (box 518 and 520). If the magnitude of the amplitude is not zero, the stepper motor is repeatedly stepped up, then down by the new amplitude until it is time to change the amplitude again (boxes 508 and 510). The present invention has been tested with a T-1000 tape drive manufactured by Colorado Memory Systems, a division of Hewlett-Packard Co., Loveland, Colorado, part # C4320A. The T-1000 has a controller, stepper motor, transmission, and head configuration similar to that shown in FIGS. 1, 2A, and 2B. For testing purposes, the existing positioning controller was disconnected and the stepper motor was hooked up to the printer port of a personal computer through an adapter. Such adapters are well known and can be easily obtained through many electronic supply stores. The final head position was measured by a non-contact laser triangulation gauge. First, a series of tests were performed to see what the worst case non-repeatable position error was using the conventional positioning technique of stepping to the desired location and stopping. On the test platform this technique resulted in a worst-case non-repeatable position error of approximately 600 micro-inches. Each step nominally translates to 140 micro-inches of head movement. Therefore, the error without the invention was on the order of 3-4 steps. The present invention was then employed using an initial amplitude of 5 steps, and reducing the amplitude by one step every two complete (up, then down) cycles. Hence, the stepping pattern consisted, in order, of stepping to the desired location, then stepping down 3 steps, up 5, down 5, up 5, down 5, up 5, down 4, up 4, down 4, up 3, down 3, up 3, down 3, up 3, down 2, up 2, down 2, up 1, down 1, up 1, down 1, up 1. This produced a three-sigma non-repeatable position error of less than 60 micro-inches. Since one step is nominally 140 micro-inches, the repeatability has been improved to less than one step. The amplitude and decay of the stepping sequence is the same as the one illustrated in FIG. 3. The code used to program the personal computer is included as an appendix to this application. The net offset of one step low was chosen so that the head motion for the last move is up. This was chosen because the surfaces and spring force for the test platform are designed for movement in the up direction. Should the characteristics of the mechanical system be different, a net offset of zero, or one step high could be chosen. The present invention improves the performance of positioning systems by minimizing the non-repeatable hysteresis effects. The system is also economical. As demonstrated, the repeatability of existing, inexpensive, positioning systems may be improved by the present invention. The present invention may also have various other applications besides positioning magnetic tape read/write heads. For example, it may be used with an x-y stage, "pick and place" machines, or lithography machines. Additionally, although a stepper motor controlled by a computer has been described, it may be possible to use other types of actuators to apply the input to the mechanical system. For example, hydraulic or pneumatic actuators, if appropriately controlled, may be used. It is to be understood that the claimed invention is not to be limited by the preferred embodiments but encompasses other modifications and alterations within the scope and spirit of the inventive concept. For example, the mechanical transmission described includes a worm gear 206, gear 208, lead screw 210, and nut 218. Other mechanical transmissions which translate the actuators input to a position could be used. One example would have the lead screw driven directly by the stepper motor. Other shaking patterns may also be employed. For example, the shaking pattern used on the test platform had an offset of one step low so that the head motion for the last step was in the up direction. It is understood that the pattern could have an offset of one step up so that the last step was in the down direction. It is also understood that the first offset from the desired location could be in the up direction. It is also understood that the minimum amplitude of one step is determined by the resolution of the actuator. Should an actuator be chosen that is capable of smaller vibrations, the decay of the shaking could continue until the limit of that actuator is reached. Also it is understood that separate actuators may be used to position the mechanical system and to implement the shaking.

Claims: An open loop control method for reducing non-repeatable positioning errors when positioning an object (110), said object (110) having a position, comprising the steps of: inputting a first system input value which corresponds to a desired location of the object (110) to a transmission system (108) having a system input (114), a system input value, and a system output, said output being said position of said object (110); exciting said transmission system (108) with a plurality of excitations that displace said system input value in periodically decreasing displacements around said first system input value. The method of claim 1 wherein said plurality of excitations comprise at least one excitation that displaces said system input value to a value that is less than said first system input value, and at least one excitation that displaces said system input value to a value that is more than said first system input value. The method of claim 1 wherein said system input (114) is connected to a shaft (204) of a stepper motor (202) having steps, said shaft (204) having a rotational position, said stepper motor (202) having steps with each step corresponding to a specified rotational offset of said shaft (204) of the stepper motor (202) and said system input value being an angular quantity describing the rotational position of said shaft (204) of the stepper motor (202). The method of claim 3 wherein the step of inputting a first system input value comprises stepping the shaft (204) of the stepper motor (202) to a rotational position which corresponds to the desired location of the object (110). The method of claim 4 wherein the step of exciting said system input (114) comprises displacing said rotational position of said shaft (204) of said stepper motor (202). The method of claim 5 wherein said excitations comprise repeatedly displacing the rotational position of the shaft (204) of the stepper motor (202) in both rotational directions a specified number of steps, and periodically reducing the specified number of steps until zero is reached. An open loop control apparatus for reducing non-repeatable positioning errors when positioning an object (110), said object (110) having a position, comprising: transmission system means (108) having a system input (114), a system input value which corresponds to a desired location of the object (110), and an output, said output being said position of said object (110); means for exciting said system input with a plurality of excitations that displace said system input value in periodically decreasing displacements around a first system input value. An apparatus as in claim 7 wherein said plurality of excitations comprise at least one excitation that displaces said system input value to a value that is less than said first system input value, and at least one excitation that displaces said system input value to a value that is more than said first system input value. An apparatus as in claim 8 wherein said system input value is the angular position of the shaft (204) of a stepper motor (202). An apparatus as in claim 9 wherein said transmission system means (108) comprises at least a lead screw (210) disposed through a follower nut (218), said follower nut (218) connected to said object (110), and said object (110) is a magnetic tape read/write head (216).

Inventors: LIEPE STEVEN F, RICHARDSON KENNETH G, LIEPE, STEVEN F., RICHARDSON, KENNETH G.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1996
Main classification: G05B19
IPC codes: G05D3, G05B19