933,785. Electric measuring systems. RANK PRECISION INDUSTRIES Ltd., [trading as TAYLOR, TAYLOR & HOBSON]. Nov. 13, 1961 [Nov. 16, 1960],- No. 39341/60. Class 40 (1). In apparatus for determining the duration of a test piece with respect to a basic closed curve, the test piece is rotated relatively to a detector which generates a combined A.C./ D.C. signal representing the shape of the surface under test and from this signal is extracted a further signal which includes the A.C. harmonics above a chosen one and the Fournier D.C. component of the signal and which includes a D.C. signal equal in magnitude to the mean level of the signal over one or more complete relative rotations of the detector and test piece the two signals then being depicted in superimposition to indicate the shape of the surface under test and the closed curve from which deviations can be measured. The detecting device may be as described in Specification 706,673 and consists of a worktable on which the piece under test is mounted and a rotating test spindle carrying an inductive device the output of which varies in accordance with the profile of the workpiece (V.I.). This output modulates a carrier at D (Fig. 4) is amplified (D1) and fed to a modulator D2 to provide a signal output between D3, D4. The signal is fed direct to an electrically marked polar recorder which may be as described in Specification 613,102, when a switch F is in its upper position. To draw the profile of the curve, when switch F is in its lower position the reference profile is drawn on the indicator and this is achieved by filter circuits G (V.I.) which extract only the A.C. signal at the rotation frequency of the test piece relative to the detector. A further switch 11 allows a condenser, charged through a resistor and amplifier K to produce a signal directly indicative of the mean D.C. signal level. These signals are combined and indicated on the same record as the workpiece and produce a circle indicative of the eccentricity of the workpiece relative to the axis of rotation of the detector. Ovality may also be determined by an additional circuit which is in parallel with the circuit deriving the fundamental and which derives the second harmonic. The combined reference signals then give an ellipse in the display. The sequence of events is controlled by a timing disc mounted on the spindle driving the detector and a cam device (Fig. 7) which may be on the same spindle as a slightly faster one. Initially, cam C is held as shown and the integrating condenser is earthed. Operation of a push-button allows a timing device to rotate supplied current to the inkless pen of the recorder and energizes a solenoid L1. This connects condenser J1 through a resistor to the output of D2 and allows cam 7 to rotate. Condenser J1 starts to charge. After a revolution the timing disc and cam L are stopped by their detents but switch M is held in its mid-position by track L3. The push-button is again pressed to release the timing disc and cam for one revolution. Switch M is now held in its mid-position and the reference curve is drawn by means of switch P which transfers the connection to recorder E from member D to the condenser J1 and filter G. Network G for extracting fundamental (Fig. 5) consists of a cathode follower input stage G feeding the filtering stages G2, each comprising an anode load constituted by a parallel T network which itself constitutes part of the feed back network through a cathode follower G6. The output of the third filter stage is fed to an output stage consisting of an untuned amplifier G8 and cathode follower G9 so adjusted that the amplitude of the extracted fundamental is the same as that of the component in the whole signal. Resistance G11 allows the D.C. level of the extracted component to be zero. Integrating device (Fig. 6). Capacitor J1, charged through resistor J2 is arranged to operate well down the straight line portion and the A.C. part of the signal is shorted to earth. The potential across the condenser is applied to a D.C. amplifier with stages J4, J5, J6. Variable resistance network J7 enables the gain of the amplifier to be so adjusted that the size of the steady D.C. signal is exactly equal to the mean D.C. level of amplifier D2. Detector (Fig. 2). The detector is brought near the profile under test by a bar C adjustable at right angles to the rotation spindle. The detector comprises a member C9 pivoted at C14 and carrying at its opposite end a common armature for differential windings C12, C13. A fine adjustment is produced by pivoting the whole assembly about a strip hinge C2 by means of a screw C7 driven by rotor C5. Modifications (Fig. 10). To indicate the average deviation of the profile from the reference the whole signal and reference signal are fed through cathode followers to opposite corners of a bridge rectifier with an averaging meter p in its diagonal. The maximum crest or trough of the profile may be indicated by peaking meters with oppositely connected rectifiers Q, Q1. The peaking meters may be of the kind where a pointer is latched to its highest reading by a pawl mechanism.
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