Apparatus for detecting relative movement wherein a detecting means is positioned in the region of natural interference

An apparatus capable of detecting relative movement of a member which includes a diffraction grating having a period P and a characteristic which diffracts light at a preselected wavelength .lambda. into positive and negative first orders that interfere with one another in a region of natural interference adjacent the diffraction grating, the apparatus comprising

• a head structure;
• a carrier structure supported by the head structure;
• a light source which provides collimated, coherent light of the pre-selected wavelength .lambda. and supported by the head structure;
• a periodic detector positioned on the carrier structure, having a sensing plane and a having a period which is a function of the preselected wavelength .lambda. and the period P of the diffraction grating, wherein the periodic detector provides an output signal in response to light incident thereon;
• electronic circuitry positioned on the carrier structure, for conditioning the output signal from the periodic detector;
• a mirror structure supported by the head structure;
• wherein the mirror structure has a position on the head structure with respect to the light source so as to be capable of directing onto the periodic detector natural interference between positive and negative first orders which are diffracted by the diffraction grating in response to light incident on the diffraction grating from the light source when the head structure is positioned adjacent the diffraction grating.

A method for controlling the position of a component which is located in a closed space within an enclosure, wherein the component is positionable along a path within the closed space by a positioning mechanism in response to positioning signals, the positioning mechanism being located within the closed space, and the enclosure having a port through which at least a portion of the component is visible from the outside of the enclosure over the entire path of the component, the method comprising the steps of

• providing an interference structure on the portion of the component which is visible from the outside of the enclosure through the port, wherein the interference structure is capable of providing interference fringes at points outside of the enclosure when light of a wavelength .lambda. is incident upon the interference structure;
• illuminating the interference structure with light of wavelength .lambda.;
• positioning an interference detector within the interference fringes so that the interference detector provides a reference signal as an output;
• controlling the positioning mechanism so that the reference signal is maintained as the output of the interference detector for any position of the interference detector;
• and
• moving the interference detector to a designated position, whereby the positioning mechanism is controlled to cause the component to move to the designated position in like fashion.

A method for controlling the position of a component which is located in a closed space within an enclosure, wherein the component is positionable along a path within the closed space by a positioning mechanism in response to positioning signals from a servo control circuit, the positioning mechanism being located within the closed space, and the enclosure having a port through which at least a portion of the component is visible from the outside of the enclosure over the entire path of the component, and the component having an axis of rotation, the method comprising the steps of

• providing a grating structure on the portion of the
• component which is visible from the outside of the enclosure, wherein the grating structure is capable of providing interference fringes at points outside of the enclosure when light of a wavelength .lambda. is incident upon the interference structure;
• supporting an interference detector and a source of light of wavelength .lambda. on a rotatable structure which has an axis of rotation which is substantially in common with the axis of rotation of the component;
• positioning the rotatable structure so that the light from the source illuminates the grating structure and the interference detector is positioned within the resulting interference fringes;
• designating an output of the interference detector corresponding to a selected segment of the interference fringes detected by the interference detector as a grating home location;
• providing the interference detector output corresponding to the grating home location to the servo control circuitry so that the positioner assembly is controlled to maintain the grating home location as the output of the interference detector throughout any movement by the component;
• and
• rotating the rotatable structure under servo control to a designated position so that the positioning mechanism moves the component to a current position.

An enclosure capable of use with a positioning system which includes an interference detector supported for movement on a positioning structure, wherein the positioning structure is capable of being moved into selected positions, and processing circuitry which is capable of analyzing signals from the interference detector and supplying control signals indicative of the difference between the signals from the interference detector and a desired signal, the enclosure comprising

• a housing which defines a closed space;
• a component positioned within the housing and which is moveable along a path within the housing;
• a positioning mechanism capable of responding to control signals from the control circuitry and which is capable of positioning the component along the path;
• a grating structure positioned on the component within the housing, wherein the housing includes a window through which the grating structure is optically visible at a selected wavelength from the outside of the sealed housing and over the path of the component, and further wherein the grating structure is capable of generating interference fringes outside of the sealed housing when illuminated with light of the selected wavelength so that the interference detector is positionable with respect to the window of the housing to detect the interference fringes, and the control circuitry is capable of supplying control signals to the positioning mechanism so that the component is positioned along the path in response to the control signals.

An apparatus for detecting relative movement comprising

• a light source providing a beam of collimated light of wavelength .lambda.;
• a wave front correction structure positioned to receive the beam of collimated light of wavelength .lambda., and which provides first and second beams of wavelength .lambda. at an angle, .theta..sub.wfc, from the axis of the beam of collimated light of wavelength .lambda.;
• a diffraction grating relatively movable with respect to the light source and wave front correction structure and positioned to have a region illuminated by the first and second beams from the wave front correction structure, wherein the diffraction grating has a period P.sub.G and a characteristic which diffracts light at a wavelength .lambda. into positive and negative first orders which interfere with one another in a region of natural interference adjacent the diffraction grating;
• a periodic detector having a sensing plane positioned within the region of natural interference of the diffraction grating, wherein the periodic detector is suitable for detection of interference fringes having a period P.sub.D, and wherein the periodic detector provides an output signal in response to light incident thereon;
• wherein the wave front correction structure is formed so that the angle at which the first and second beams are produced causes the interference between the positive and negative first orders in the region of natural interference to have a period of substantially P.sub.D, whereby the periodic detector responds principally to interference at said sensing plane between the positive and negative first orders diffracted from said diffraction grating.

An apparatus for detecting relative movement comprising

• a diffraction grating relatively movable with respect to a source and having a period P and a characteristic which diffracts light at a preselected wavelength X into positive and negative first orders which interfere with one another in a region of natural interference adjacent the diffraction grating;
• a light source providing a beam of light of wavelength .lambda.;
• a polarizing beam splitter;
• a quarter-wavelength phase retarder, wherein the polarizing beam splitter is positioned in a path of the beam to reflect substantially all of the beam through the phase retarder and onto the diffraction grating, and further wherein the polarizing beam splitter and the phase retarder are positioned so that positive and negative first orders diffracted from the diffraction grating pass through the phase retarder and the polarizing beam splitter;
• and
• a detector having a sensing plane positioned within a region of natural interference between positive and negative first order diffractive beams from the diffraction grating which have passed through the phase retarder and the polarizing beam splitter.