The present invention is concerned with a strip chart recorder with a recording arm which is rotatable about a shaft, perpendicular to the recording plane and whose angular position is servo-controlled inasmuch as a position sensor in the form of a differential capacitor is incor- porated in a position servo for the recording pen.

In strip chart recorders of the type in which the recorder consists of . a pen motor with a pen, rotatable about the shaft of the pen motor, of length a from the shaft to the tracing point, which is the point of contact between the rotatable pen and a straight line which is a gener¬ ator of a recording roller mounted under the paper strip, the ordinate b of the traced curve, i.e. its distance from a zero line, will not be proportional to the angle of deflection of the pen but will suffer a certain distortion. The length of the ordinate b will be equal to the product of the length a, of the pointer in zero position and the tangent of the angle of deflection v of the pen: b = a tan v

This error distorts the shape of the curve as traced and prevents the measurement thereof.

In combination with such strip chart plotters it is a known practice to use a position servo in order to ensure that the movements of the pen are unaffected by the friction in the recorder, and such a position servo includes a position sensor which may be a potentiometer, an opti¬ cal transducer, an inductive transducer or, most simply, a differential -capacitor consisting of two fixed-plates and a.mαv.ing plate in the form of a sector of a circle, which overlaps the two fixed plates and whose capacitance varies in proportion to the angle of deflection of the rotor.

The aim of the present invention is to provide a strip chart recorder with a differential capacitor which, when incorporated in the'.position servo, is capable of compensating for the above-mentioned tangent error.

OMPI

This is achieved, according to the invention, inasmuch as the moving the fixed plates of the differential capacitor are shaped in such a w that, for a deflection of the moving plate assembly through an angle the overlap area A of the plates changes by an amount ΔA which is pro portional to the tangent of the angle v.

In one embodiment of the invention this is achieved in that the two fixed plates each have two parallel edges, that the moving plate asse bly has two straight edges which each form the same angle with corres ponding parallel edges of the fixed plate assembly, and that when the moving plate assembly is in its mid position, the aforesaid edges of moving and the fixed plate assemblies display mirror symmetry about a plane perpendicular to the plates and passing midway between the tw plate assemblies.

In one embodiment of the invention this is achieved in a particularly simple manner in that the moving plate assembly and the fixed plate assemblies of the differential capacitor are both bounded by two para lel straight lines.

This ensures that the ordinate of the curve on the recording strip is proportional to the input signal.

Other embodiments will be more particularly described in conjunction with the explanation of the drawings.

The invention will now be more particularly described with reference the drawings, whereof Fig. 1 shows a block diagram of a position serv for a strip chart recorder with an embodiment of a differential capac itor in accordance with the invention, and Figs 2 to 4 illustrate var ous modified embodiments of differential capacitors for a strip chart recorder in accordance with the invention.

A pen motor 10 is equipped with a pen 11 with a heated tracing tube 1 which makes contact with a strip 13 of waxed paper along a generator of a supporting roller 15, tracing a curve 16 on the paper strip 43, which is advanced at a constant rate by means not illustrated. When f O PI

plotting pen 11 is deflected through an angle v by an input signal, the curve 16 will assume an ordinate value b in relation to a zero line 17. It can be seen that this ordinate value b is equal to a tan v, where a is the length of the pen from its shaft to the recording point in the 5 zero position of the pen.

A differential capacitor comprises a moving plate 20 with two parallel side edges 21 and a fixed plate assembly 22, 23 bounded by two parallel lines 24, 25. The moving plate 20 is attached along its midline 26 to

10 an extension of the pen 11. For the sake of clarity, the differential capacitor is shown attached to a rearward extension of the pointer. In a practical embodiment, the differential capacitor would be located below the pointer 11. The capacitance ΔC of a differential capacitor is equal to the difference between the capacitances between the moving

15 plate and the two respective fixed plates. A deflection from the centre position into the position illustrated causes the capacitance on the left to increase by the hatched area F1 and the capacitance on the right to decrease by the hatched area F2. Hence ΔC = F1 + F2 = 2BD tan v, where- B is the distance between the parallel edges 24 and 25 of the

20 fixed plates and D is the distance from the shaft of the pen motor to the mid height h of the hatched trapezia. Thus the capacitance of the differential capacitor will be equal to a constant multiplied by the tangent of the angle of deflection v.

25 The signal to be recorded on the strip chart 13 is applied to an input terminal 32, which is connected to a signal amplifier 33, which in turn is connected to one of the input terminals of a differential capacitor 34. The output terminal of the latter is connected to a zero-point amplifier 35, which drives the pen motor 10. The differential capac- -.30 . itor, whose rotor is rigidly.united, .is connected, as illustrated, to the secondary winding of a transformer 36, whose primary winding is con¬ nected to an oscillator 37. The midpoint of the secondary winding of the transformer 36 is connected to an operative amplifier 38 provided with capacitive back-coupling, whose output, together with the signal

35 from the oscillator 37, is applied to a synchronous detector 39, whose output is connected to the other input terminal of the differential - capacitor 34. Apart from the special design of the differential capac-

OMPI

itor, this arrangement forms a position servo which is known per se a which, moreover, serves to overcome losses due to friction in the re¬ corder.

In consequence of the special design of the differential capacitor, a correction is introduced into the servo loop which is proportional the tangent of the angle of deflection v of the pen, whereby the ordi nate of the curve 16 is corrected for the error above-mentioned.

In the differential capacitor shown in Fig. 2 the edges 21 of the movi plate assembly are not parallel, as in the differential capacitor in Fig. 1, but rotate together with the parallel edges 24, 25 of the fix plate assemblies 22, 23 through equal angles in each direction, so th when the moving plate assembly is in zero position, the edge 21 is perpendicular to edge 24 of the fixed plate assembly 22, and similarl for assembly 23. Thus nothing is altered with respect to the capaci¬ tances.

Figs. 3 and 4 illustrate double duplex differential capacitors corres- ponding, respectively, to the embodiments shown in Fig. 1 and Fig. 2.. It will be seen that the double differential capacitors display mirror symmetry about two mutually perpendicular axes. They may be connecte up in the circuit shown in Fig. 1 by cross-connecting the fixed plates or they may be connected up in a balanced oscillator set-up (not illus trated).

Furthermore, if the desired maximum angle of deflection is small, one might use multiple sectors in place of the two sectors shown. This results in an n-fo.ld increase in the deflection sensitivity ΔC/Δ tan v where n is the number of sectors mounted on the rotor.

O