Geometrical instruments – Gauge – With support for gauged article
Patent
1995-05-12
1998-04-21
Will, Thomas B.
Geometrical instruments
Gauge
With support for gauged article
33504, 33551, G01B 520, G01B 728
Patent
active
057406162
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a metrological instrument for measuring surface finish parameters, particularly the shape (form) and/or roughness or smoothness (texture) of a surface of a workpiece, and to methods of using such an instrument.
Metrological instruments are known in which a stylus is mounted on an arm, and the arm is slowly retracted horizontally by a drive mechanism while the tip of the stylus rests on the surface of a workpiece. A transducer outputs a signal in accordance with movement of the stylus relative to the arm on which it is mounted, and the transducer output as the arm is retracted is used to obtain information about the shape or the roughness of the surface of the workpiece. The drive mechanism which supports and retracts the arm is itself mounted on a column and is drivable up and down the column. The entire device is supported by a stand and will normally be placed on a high precision optical table which also supports the workpiece. The precise position of the arm up and down the column and the precise degree to which the arm is extended or retracted is monitored, typically using an optical grating displacement measuring system. Accordingly, the precise position of the tip of the stylus is known during a measurement operation.
Such instruments can provide very precise measurements of the form or texture of the surface of a workpiece. However, they are large and expensive. They are difficult to transport and will normally be set up on an optical table on which they will remain more or less permanently.
In one aspect, the present invention provides a portable metrological instrument for measuring surface form and/or texture, comprising an arm for supporting a stylus to be traversed over a workpiece and a drive means for driving the arm so as to traverse a stylus supported by it, arranged for the drive means to sit on a work bench next to the workpiece. The stylus and its transducer will normally be provided as a separate detachable unit mountable on the arm.
In another aspect of the present invention there is provided a portable bench-mountable metrological instrument for measuring surface roughness and/or shape, comprising an arm for supporting a stylus, a motor for driving the arm relative to an arm mounting, and an arm position detecting means mounted on the arm for detecting the position of the arm relative to the arm mounting, and control means for controlling the speed of the motor, the control means using the output from the arm position detecting means as an indication of the current speed or position of the motor and controlling the motor (e.g. controlling the motor speed) in response thereto. Preferably the motor drives the arm through a linkage. The-linkage may comprise a reduction gearing so that one rotation of the motor drives the arm through less distance than it would if a reduction gearing was replaced by a direct connection. Typically, a reduction ratio in the range of 100:1 to 1000:1 is used. The linkage may comprise a flexible connection, for example a drive band, between the motor and the arm, which will tend to isolate the arm from motor vibrations.
In the past, it has been normal for the position of the motor to be detected using a shaft encoder on a motor shaft and to detect the position of the arm separately using an arm position detecting means mounted on the arm. Any flexibility or looseness in the linkage through which the motor drives the arm means that a position detector on the motor cannot indicate precisely the position of the arm and a position indicator on the arm cannot indicate precisely the position of the motor. However, it has now been found that a satisfactory device can be made in which a position detector mounted on the arm is used both for arm position data and motor position or speed data. Because the position detector is mounted on the arm, the position of the arm can still be detected very precisely, to enable high accuracy measurement using the instrument. Uncertainty over the precise position of the motor does not prevent effectiv
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Coleman Alan John
Dixey Trevor
Onyon Dean
Seddon Peter
Taylor Hobson Limited
Will Thomas B.
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