Controlled linear motor

Details Controlled linear motor Controlled linear motor

1990-02-07

1991-07-02

Shoop, Jr., William M.

Electricity: motive power systems

Positional servo systems

With particular motor control system responsive to the...

346141, 346140R, 318135, 335258, 335229, G05B 1100

Patent

active

050288563

DESCRIPTION:

BRIEF SUMMARY
This invention relates to linear drive motors, and more particularly to the control of such motors.
Conventional linear drive motors employ a standard motor/gearbox with a rotary to linear conversion device, typically lead screws or lever mechanisms. Problems incurred with this technique are backlash, mechanical vibration, unwanted mechanical resonances, and low bandwidth. It is known to control such motors by providing a velocity feedback from a tachometer, but this introduces unwanted AC components, e.g. caused by the commutating action of the tachometer. When the AC components are filtered out, the system bandwidth is reduced further. For precision mechanical drive systems, some or all of these characteristics are undesirable.
The present invention provides a linear drive motor, comprising:
a proportional solenoid, having an electrical input, and a mechanical output member which is driven with a linear motion in proportion to the electrical input; a velocity transducer connected to the mechanical output member of the solenoid and producing an electrical output signal dependent on the velocity thereof; and
a feedback circuit which receives the electrical output signal of the velocity transducer and controls the electrical input to the solenoid in accordance therewith, thereby controlling the velocity of the mechanical output member.
Examples of the present invention will be described with reference to the accompanying drawings, in which:
FIG. 1 is a schematic diagram of the basic elements of a first embodiment of a controlled linear motor;
FIG. 2 is a schematic section through part of a proportional solenoid; and
FIGS. 3, 4 and 5 are schematic circuit diagrams of three further embodiments of controlled linear motor.
In FIG. 1, L1 is a proportional solenoid such as used in hydraulic systems. Suitable devices are available from Ledex Electromechanical Products (Ledex Inc, 801 Scholz Drive, PO Box 427, Vandalia, Ohio, USA) or Elektroteile GmbH (Germany). The solenoid L1 produces a linear output motion of an output member 18, which can be used as desired. It is also connected to drive a velocity transducer F1, which produces a DC output proportional to the velocity. Appropriate devices could be piezo ceramic elements (flexible or rigid), or an electromagnetic linear velocity transducer (LVT) such as available from Schaevitz Corp., USA.
A proportional solenoid, as used for the solenoid L1, is distinguishable from a conventional solenoid as follows. A conventional solenoid basically consists of a coil, to carry current and establish a magnetic flux; an iron shell, to contain and direct the flux in a manner commensurate with the desired operation of the solenoid; and a movable armature, to act as the working element. The magnetic flux lines are transmitted through a path consisting of air and iron; the iron, of course being the more efficient of the two, and the air gap being necessary for physical movement. The force of attraction between the stationary shell and the movable armature is inversely proportional to the square of the distance between them, across the air gap. This results in the familiar snap action as the armature completes its stroke. It is this type of magnetic action that makes a constant velocity difficult to achieve with servo electronics.
The proprietary proportional solenoids mentioned above look very much like conventional solenoids; both have coils, armatures, housings; major differences are in the pole pieces and bearing systems. As previously noted, the air gap diminishes in a standard solenoid. However, as shown in FIG. 2, in a proportional solenoid the working air gap 10, between the movable armature 12 and the pole piece 14, is perpendicular to solenoid motion (indicated by arrow 16). The lines of magnetic flux passing across the working air gap 10 are indicated by arrows 15. Thus the air gap remains constant through the solenoid's linear stroke. In this configuration, the positioning of the pole pieces, and consequently the magnetics, can be controlled by design to achieve the desi

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