Vibration Resistant Measuring Device

The invention relates to a measuring device, particularly an encapsulated longitudinal measuring device, comprising a scale, a plurality of guide surfaces, and a scanning unit guided at the scale and urged resiliently against the guide surfaces. Such measuring devices are utilized primarily to measure the relative movement of mobile machine parts.

In the operation of machine tools equipped with such measuring devices, or measuring devices in general, oscillations and other accelerations are often produced which may influence the measurement. The elimination or at least the reduction of the effect of such oscillations and accelerations on an optical instrument is the problem on which the invention according to the German Offlegungschrift DT-OS No. 1,900,475 is based. An arrangement is described there for stabilizing a pair of binoculars held by an observer. To maintain the binoculars in the desired orientation about their pitching axis, a counterweight and a resilient guide are used, preferably with aperiodic attenuation.

Counterweight and attenuation members arranged to reduce the effect of oscillations and thrusts have also been used in other areas as well. However, such known methods for reducing the repurcussions of oscillations and thrusts cannot be simply applied to high precision longitudinal measuring devices, which are often capable of a precision of a few micrometers.

Due to the frequently rough operation of machine tools such measuring devices require special protection against external mechanical influences and special provisions for guiding mobile components. Generally such high precision measurement devices include a scanning unit which travels along the measuring scale. Both the scale and the scanning unit are surrounded by a tubular housing member which defines guide surfaces, and the scanning unit is positioned by rollers which travel along the guide surfaces. A powerful spring is used to bias the rollers against the guide surfaces and thereby prevent the scanning unit from lifting off the guide surfaces in the event of oscillations or accelerations of the tubular member.

For reasons of precision the scanning unit is guided in at least one plane directly at the measuring scale itself in many measuring devices. However, the scale must be protected against distorting compressive forces and friction against the scanning unit and, therefore, biasing forces acting to hold the scanning unit on the scale must be limited. Thus the danger exists that at peak acceleration of the tubular member the sensing unit may lift off the scale, thereby producing the possibility of both measuring errors and mechanical damage to the measuring device.

The present invention is directed to an improved measuring device in which the effects of oscillations and thrusts on the scale and the sensing unit are substantially reduced. The scanning unit of the device is effectively connected with a compensating mass which is so dimensioned and arranged that even at high accelerations the scanning unit is kept in contact with the guiding surface at the scale. The adverse effect of acceleration on the scanning unit may be further reduced by resiliently mounting any incandescent lamps and condensers included in the scanning unit. Special advantages result if the holding means for lamp and condenser has not too small a mass and if the lamp mounting means is formed from an elastomer having a high internal attenuation.

The invention, together with further objects and attendant advantages, will be best understood by reference to the following detailed description taken in connection with the accompanying drawings.

FIG. 1 is a cross-sectional view of a simplified presentation of a preferred embodiment of a longitudinal measuring device according to the present invention.

FIG. 2 is a longitudinal sectional view of the longitudinal measuring device of FIG. 1.

Referring now to the drawings, FIG. 1 shows a tubular housing 2 which is fastened with screws 3 to a machine component 1. A scale 4 is mounted within the housing 2 and a scanning unit 5 with mass M is seated inside the housing 2 at the guide surfaces 2' and 2" and at the guide surfaces 5" defined on the scale 4. Roller 5' is mounted on the scanning unit 5 and is biased by the powerful spring 6 against the guide surface 2'. A clutch 7 connects the scanning unit 5 with an entrainment means 7' fastened in a manner (not shown) to a machine component (likewise not shown) which carries out a relative movement in relation to the machine component 1 to be measured. The inner area of the tubular member 2 is sealed toward the outside by sealing lips 8 which embrace the sword-shaped entrainment means 7'.

FIG. 2 shows the same measuring installation in longitudinal section. The scanning unit 5 is seated via guide surfaces 5" on the scale 4. However, scale 4 is a highly precise measuring scale which must be protected from excessive distorting forces or excessive friction between the scale 4 and scanning unit 5. A contact lever 10 is pivotably connected to the scanning unit 5, and a roller 11 is rotatably mounted on the lever 10. The lever 10 is biased away from the scanning unit 5 by the spring 9, and the force supplied by spring 9 operates to press the roller 11 into contact with the guide surface 2" and the scanning unit 5 into contact with the guide surfaces 5" on the scale 4.

A compensating mass 12 having mass m is mounted on the free end of the lever 10. The length of the lever arms a and b of the lever 10 are so dimensioned that the product of the mass M and the distance a is equal to the product of the mass m and the distance b. If a positive acceleration X acts on the scanning unit 5 a clockwise torque is produced about the axis of the roller 11 equal to the product of the acceleration X, the mass of the scanning unit M and the distance a. However, the same acceleration X also acts on the compensating mass 12 and thereby produces a counterclockwise torque equal to the product of the acceleration X, the mass of the compensating mass m and the distance b. Because of the relationship between the distances a and b, the counterclockwise torque substantially cancels the clockwise torque, thereby reducing the tendency of the scanning unit 5 to lift off the scale 4.

In operation, the scanning unit 5 is normally held in sliding contact with the scale 4 by the spring 9. The stiffness of spring 9 is chosen so that the scale 4 is not distorted excessively by either the contact force or the resulting friction between the scanning unit 5 and the scale 4. The compensating mass 12 and lever arm 10 operate to maintain this contact force more nearly constant during periods of acceleration. Accelerations act on both the compensating mass 12 and the scanning unit 5 and the torques generated thereby about the axis of the roller 11 tend to cancel. Thus, the tendency of the scanning unit 5 to rotate about the axis of the roller 11 under lateral acceleration is reduced, and the contact face between the scanning unit 5 and the scale 4 is maintained more nearly constant.

To increase the insensitivity of the measuring device to acceleration, it is possible to mount the illuminating unit 13 comprising a lamp 14 and a condenser 15, resiliently on the scanning unit 5. In this case it is preferable not to keep the mass of the illuminating unit 13 too small and to use an elastomer with high internal attenuation as resilient mount 16.

Of course, it should be understood that various changes and modifications to the preferred embodiment described above will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is, therefore, intended that such changes and modifications be covered by the following claims.