Electrical generator or motor structure – Dynamoelectric – Linear
Reexamination Certificate
1999-05-03
2001-01-16
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Linear
C318S135000, C414S935000, C074S4710XY
Reexamination Certificate
active
06175169
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates generally to sensing and control of electric motors, and more particularly, to precise sensing and control of the position and orientation of a planar linear motor incorporating a monolithic planar alternating current (AC) magnetic sensor.
2. Description of Prior Art
Motors that can move in a straight line (linear motors) are well known in the art. Motors that can move freely in the plane are less well known, but several examples exist. For example, the planar linear motor (henceforth referred to simply as a planar motor) due to Sawyer (U.S. Pat. No. 3,376,578) can provide linear motion in two mutually orthogonal directions in the plane as well as a small rotation in the plane.
Such a planar motor generally combines four linear-motor sections into one forcer assembly that is capable of producing forces and torques in the plane. The forcer is magnetically attracted to a patterned iron platen surface while being forced away from the surface by an air bearing film; the equilibrium separation being typically 10 to 15 &mgr;m. The motor sections have fine teeth [typically 0.5 mm (0.020 in.) wide on a 1.0 mm (0.040 in.) pitch] and the platen has a two-dimensional array of square teeth of corresponding width and pitch. After chemical or physical machining, the platen surface is planarized using epoxy to form the air-bearing surface. The combined motor sections making up the forcer ride above (or hang below) the platen (stator) surface, and typically operate on a flux-steering principle in open-loop microstepping mode. That is, a string of pulses from the control computer serves to increment counters which set proportional currents in the drive coils which, in turn, move the stable magnetic equilibrium point which, in turn, provides a force which moves the motor forward. These developments are chiefly due to Sawyer, and date from the late 1960s.
Planar motors have many desirable attributes. Commercial systems such as RobotWorld (V. Scheinman, “RobotWorld: a multiple robot vision guided assembly system,” in
Robotics Research, the Fourth International Symposium
, Santa Cruz Calif., 1987, pp. 23-27, and “RobotWorld—unrolled motors turn assembly on its head,”
Industrial Robot
, Vol. 20, No. 1, 1993, pp. 28-31) use forcers carrying vertical and rotational axes and vision cameras suspended from a platen ceiling for automated assembly. Similar systems have been developed by AT&T (P. F. Lilienthal, et al., “A flexible manufacturing workstation,”
AT&T Technical Journal
, 1998, pp. 5-14) and Megamation (Anon., “Speed and precision from novel assembly robot,”
Assembly Automation
, Vol. 9, No. 2, 1989, pp. 85-87) for a wide variety of automation applications such as the placement of surface-mount components on circuit boards (B. D. Hoffman, “The use of 2-D linear motors in surface mount technology,”
Proc.
5
th Int'l SAMPE Electronics Conference,
1991, pp. 141-151).
While offering many benefits, current planar motion systems are severely limited because of their open-loop stepping operation which prevents the achievement of maximum potential performance. To help ensure against loss of synchrony (missing steps), only two-thirds to three-fourths of the available force margin is used, reducing the forcer's potential maximum acceleration and velocity. Even so, the forcer motors remain susceptible to loss of synchrony if large enough unanticipated external forces are acting. Additionally, settling times after moves are longer than desirable and there is no way to reject low-frequency external disturbances. The forcer has only moderate stiffness requiring high power dissipation when holding a position.
Many have recognized that these problems can be solved or considerably reduced in severity by incorporating a suitable position sensor that can accurately measure the relative displacements of forcer and platen at high enough bandwidth to be used for servo control for greatly improved performance. Among the possible sensing strategies are laser interferometry, tracking from light sources attached to the forcer, optical sensing of teeth in the platen, capacitive sensing of teeth, and magnetic sensing of teeth.
Interferometric or other optical tracking techniques are expensive and run into trouble when multiple forcers are used in a cluttered environment. On the other hand, sensors which are self contained and can be mounted on or incorporated into the forcer would appear to be the most desirable. Such sensors could use either magnetic, capacitive or optical principles to generate electrical output when the forcer is driven over the platen array. The output signals, either pulses or continuous waveforms, would correspond to the platen array dimensions. These could be used for closed-loop coarse distance control by pulse counting and/or intra-tooth fine control by interpolating the analog waveform.
Sawyer himself recognized the desirability of a platen tooth sensor and patented a method based on magnetic induction (U.S. Pat. No. 3,735,231). One embodiment of the sensor in U.S. Pat. No. 3,735,231 includes a four pole magnetic member having a pair of sense windings which can be in the form of a printed circuit board disposed on non-adjacent poles at the exposed end of the poles. The pair of windings provide outputs which are a periodic function of the head relative to the platen along a single axis. The patent does not teach control of the motor from the sensor signals.
U.S. Pat. No. 3,857,078 to Sawyer discloses a closed loop planar motor using magnetic sensing. For detection along each axis, two pickoff assemblies are utilized. Each pickoff includes two magnetic cores joined by a magnetic cross piece having a drive coil wrapped around it. Each magnetic core has two poles, each with three teeth. The two poles of one core are spaced in a phase quadrature relationship with the two poles of the other core. The flux in each core varies with the linear positioning of the pickoff relative to the platen and the fluxes in the two cores are in a quadrature relationship with each other. A sense coil is wound around an upper horizontal portion of each core member. The two sense coils provide quadrature related output signals having periodic relationships in accordance with the actual displacement of the head along the platen. This sensor, however, suffers from the disadvantage that since the magnetic path is not symmetrical on both sides of the drive coil the outputs have a large common mode (bias field) component which is not cancelled.
A magnetic sensing technique is disclosed by Brennemann, et al., (“Magnetic sensor for 2-D linear stepper motor,”
IBM Technical Disclosure Bulletin
, Vol. 35, No. 1B, June 1992). This sensor is an AC magnetic sensor based on self inductance of coils integrated with a planar motor. The sensor includes four linearly shaped poles, each having a plurality of teeth. Two poles on the left are separated from the two poles on the right by a magnetic spacer. A sense coil (L1-L4) is wound around each of the poles. The sensor for each axis consists of eight coils wound on eight poles. Four poles are positioned in one quadrant of the forcer and four are positioned in the diagonally opposite quadrant. The inductance of a first sense coil L1 is at a maximum when the inductance of a second coil L2 is at a minimum and vice-versa. The sense coils L3 and L4 are in phase quadrature with the coils L1 and L2. Each four-pole sensor produces quadrature related output voltages which vary sinusoidally with forcer displacement along a single axis of the platen. The sensors can be used to measure displacement along one of two axes and rotation about the z axis. The above sensors are, however, relatively complex to make and use, expensive to manufacture, difficult to shield from unwanted external fields and have a relatively small signal. There is no discussion of motor control based on such signals.
U.S. Pat. No. 5,434,504 to Hollis, et al. discloses a position sensor for planar motors using inductive coupling to the pl
Butler Zachary J.
Hollis, Jr. Ralph L.
Quard, III Arthur E.
Rizzi Alfred A.
Dougherty Thomas M.
Hollis, Jr. Ralph L.
Jones Judson H.
LandOfFree
Closed-loop planar linear motor with integral monolithic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Closed-loop planar linear motor with integral monolithic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Closed-loop planar linear motor with integral monolithic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2486425