A variable capacitive transducer is employed in a graphical recording device in conjunction with its pen drive mechanism. Two fixed conductive plates are mounted within the recorder parallel to each other and a flexible belt, partly coated with a conductive film, is movably mounted therebetween. The recording pen is attached to the belt which serves as both the feedback element in the pen drive servo system and pen conveyor. Since the pen is attached directly to the belt which, in turn, provides a highly accurate indication of pen position, there is no need to provide any mechanical means for converting the pen's linear motion to rotary motion which avoids the attendant conversion errors.
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a continuation-in-part of my copending application, Ser. No. 688,500, filed on Dec. 6, 1967, now abandoned and filed as copending application Ser. No. 89,514 on Nov. 13, 1970 and both assigned the same assignee as the present invention.
A variable capacitance transducer has a cable loop interlinked with a metallic tube. The cable loop comprises a metallic cable-half joined to an insulative cable-half. The portion of the cable loop passing through the metallic tube is axially positioned in the tube and a capacitive element is formed by the metallic tube and the metallic cable-half. Displacement of the cable loop to vary the amount of the metallic cable-half inside the metallic tube varies the capacitance of the capacitive element in a linear manner.
Three-terminal capacitive servomechanisms are disclosed using single and differential three-terminal capacitors, as the transmitting and receiving elements thereof. Each of the transmitter and receiver capacitors includes a conductive driven element connected to a source of electrical drive signals, a conductive receptor element connected to a high gain amplifier, and shielding circuits connected to ground. The amplifier includes a capacitive feedback loop and clamps the receptor elements to virtual ground. The feed-through capacitances of the transmitter and receiver capacitors are unaffected by stray and cable capacitance thus permitting remote sensing, and the effect of conductance in these capacitors can be cancelled out by use of a synchronous demodulator connected to the output of the amplifier.
Strip chart recorders in which the curve is recorded by means of a rotatable pen (11) which traces the curve in a line of contact (14) being a generatrix of a supporting roller (15), suffer from a plotting error originating from the fact that the ordinate of the curve, which should be proportional to the angle of deflection v of the pen, is proportional to the tangent of said angle. This plotting error, that makes it impossible to carry out measurements on the curve, can be compensated for by incorporating in the position servo, which serves to overcome frictional losses in the recorder, a position sensor of a differential capacitor type (20, 22, 23) the differential capacitance of which, as a function of the angle of deflection v of the pen, is proportional to the tangent of v. According to one embodiment of the invention this is achieved by the rotor plate (20) and the stator plates (22, 23) of the differential capacitor each being bounded by a set of parallel lines (21, and 24, 25), and in that the differential capacitor, when being in its mid-position, displays mirror symmetry about a plane perpendicular to the plates and passing through a zero line (17).
A direct current servo motor operates through a toothed belt linkage to drive the pen carriage of a chart recorder. The position of the drive is monitored by a sectored encoder disc on the motor shaft. A pair of optical sensors is located relative to the encoder disc to provide alternately phased transition signals as corresponding edges of the encoder disc sectors pass the sensors when the motor rotates. A microprocessor interrupted by a transition signal goes to a sub-routine that determines which way the encoder disc is rotating and stores a count signal of the corresponding sign in a register. Further the microprocessor compares an input signal with the stored count signal and directs the motor to rotate in the direction that will reduce the difference or error signal to zero thus causing the pen to follow the magnitude of the input signal. The sub-routine also computes the rate of error change and applies a correction to motor speed or brakes the motor when the error approaches zero to prevent pen overshoot.
A circular array of equally spaced, interleaved, reference elecrodes or pole pieces are arranged about the center of the array for producing an electric or magnetic field in which the resultant vector representing the maximum field strength rotates as a polyphase current or voltage source is successively applied to successive ones of the electrodes or pole pieces. In a preferred embodiment, each of the electrodes is uniquely and generally crescent shaped such that along any radius from the center of the array passing through a given electrode, the radial width of the electrode varies in proportion to the sine of A.multidot..theta. where A is an arbitrary constant and .theta. is the azimuthal angle of the radius as measured from the initial end of the electrode. The radial cross-section ratio may be achieved by varying the width alone, varying the elevation alone, or by a combination of the two. So arranged, the inner and outer edges of each electrode of constant elevation conform substantially to the curves ##EQU1## where r.sub.outer is the radial distance from the center of the array to the outer edge of the electrode at any angle .theta. within the arc subtended by the electrode (O.ltoreq..theta..ltoreq..pi.); r.sub.inner is the radial distance from the center of the array to the inner edge of the electrode at any angle .theta. within the arc subtended by the electrode (O.ltoreq..theta..ltoreq..pi.); R.sub.min is the radial distance from the center of the array to both curves at .theta.=0; r.sub.max is the radial distance from the center of the array to both curves at .theta.=.pi.; .theta. is the azimuthal angle to a given point, measured in radians; d is a parameter which determines the maximum width of the electrode.
