Patent GB-686599-A (GB)
Title: Improvements in or relating to a follow-up system
IPC: G08C21/00
Filed: 1950-03-31
Abstract: 686,599. Electric follow-up systems. CONSOLIDATED ELECTRIC CO. March 31, 1950 [April 4, 1949], No. 8104/50. Class 40 (i) [Also in Group XXXVI] In a direct transmission or follow-up control system, primarily for tele-autographic apparatus wherein a stylus movable in two co-ordinated directions deflects corresponding controlling elements through a parallelogram linkage to transmit e.g., frequency modulated signals over a common channel to a remote station where the signals are separated to operate controlled elements deflecting a pen through a similar parallelogram linkage, the controlled elements producing the follow-up motion comprise virtually frictionless electromagnetic deflection movements linearly responsive to their energizing currents energized primarily in accordance with the second time derivative of the appropriate control signal, and secondarily in accordance with the instantaneous value, and/or. the first time derivative of the control signal, or of the signal representing the positional difference between corresponding and controlled elements. Fig. 3 shows a servo operated tele-autographic system wherein a stylus 11 at the transmitter is supported by a parallelogram linkage for displacement along the X and Y co-ordinates of a writing surface 10 and is mechanically connected to vary the tuning of two oscillators which transmit distinctive frequencies, modulated in accordance with the respective co-ordinate displacements, over a line or radio transmission channel 15 to a remote receiver. The oscillator 13 corresponding to the Y motion is tuned by a variable inductance 26 and a fixed condenser 27 and the oscillator 14 corresponding to the X motion is variably tuned by a similar arrangement (not shown). At the receiver, the incoming signals are separated by filters 32, 33 and the Y - signal from filter 33 is amplified and passed to a limiter pentode 53, whose anode is coupled through a condenser 66 to a tuned circuit 67, 68, resonant at the frequency corresponding to the neutral stylus position, and to grid 64 of a pentagrid discriminator tube 54. The limiter also energizes the primary 74 (tuned to the neutral frequency) of a transformer whose secondary 73 supplies the grid 65 of the pentagrid. The anode of the latter supplies a low pass filter, 55 whose output is a unidirectional voltage, deviating from a datum level in proportion to the frequency deviations from neutral and the Y displacement of the stylus, which is differentiated twice in the input and output circuits of the triode 59, and the resulting signal proportional to the Y - acceleration of the stylus is applied to the input of a balanced D.C. amplifier 45 (Fig. 4) in which the phasesplitting triodes 105, 107 drive pushpull cathode loaded triodes 101, 102 between whose cathodes is connected the moving coil 41 of a D'Arsonval galvanometer movement 42, which is coupled through a parallelogram linkage similar to that at the transmitter to displace the receiving stylus in the Y direction. While the system described is free from dynamic or transient errors, static positional or frictional errors are removed by applying to the grid of the amplifier 45 a unidirectional voltage proportional to the Y - coordinate deflection of the stylus, obtained from a double-diode discriminator 91 in which the centre tapped transformer secondary 92 supplies antiphase signals to the anodes, which are interconnected through resistors 94. 95, to the corresponding cathodes, of which one is earthed while the other is connected to the input of amplifier 45 earthed through condenser 93. The transformer secondary 73. the condenser 97 and the inductance 96, variable through a linkage by the Y displacement of the receiver stylus, constitutes a series tuned circuit normally resonant at the neutral frequency of the Y - signal and the junction of condenser 97 and inductance 96 is coupled to the centre tap of secondary 92 through condenser 98, so that the discriminator output represents the frequency deviation between the incoming Y - signal and the tuning of circuit 73, 96, 97, and thus the positional error between the transmitting and receiving styli in the Y - direction. A similar circuit and galvanometer movement is responsive to the X - signal derived from oscillator 14 and filter 32, and is coupled to the parallelogram linkage to displace the receiving stylus in the X - direction. The effect of the back E.M.F. induced in the galvanometer coils due to their movements is reduced by the high output impedance of the energizing D.C. amplifiers, and may be compensated by tapping off a portion of the unidirectional velocity voltage from the grid resistor 58 of the differentiating amplifier 59 and injecting it to the grid of the phase splitting triode 107 of the D.C. amplifier 45 in the appropriate phase. The system may be damped by inserting a parallel resistor-condenser differentiating circuit or a differentiating amplifier in series with the cathode output circuit of the discriminator double diode 91. Figure 7 shows a directtransmission tele-autographic system which is similar to that shown in Fig. 3 up to the receiver limiter stage. The Y signal at the limiter anode energizes the transformer primary 123 of a known double diode discriminator, which produces a unidirectional voltage at point 114 deviating from zero level in magnitude and sense in proportion to the deviations of the signal frequency corresponding to the displacements of the transmitter stylus in the Y direction. This voltage is converted to an acceleration voltage by the differentiating circuits 116, 117 which together with the displacement voltage supplied from point 114 through a series resistance, energizes one input of a D.C. amplifier 119 of the kind shown in Fig. 4, the other input being supplied with a damping velocity voltage from a tap on resistor 138. The galvanometer coil 41 energized from the D.C. amplifier cathodes is normally maintained in its mid or neutral position when unenergized, by a restoring spring 52, so that the displacement voltages position the coil until the spring restoring couple balances that due to the coil current. A similar circuit and galvanometer is responsive to the voltages derived from X - direction displacements of the transmitting stylus, and the galvanometer movements are coupled to a parallelogram linkage similar to that at the transmitter to displace the receiving stylus in accordance with the displacements of the transmitting stylus. In Fig. 1 (not shown) an instrument pointer varies a unidirectional line voltage in accordance with a quantity to be telemetered. and at the receiver the voltage is amplified, twice differentiated. and applied together with a portion of the displacement voltage to deflect a direct transmission or servo type galvanometric recording instrument. The invention is applicable to instruments employing linear displacements and accelerations.