Patent GB-935173-A (GB)
Title: Improvements relating to stylus control systems
IPC: G08C21/00
Filed: 1961-06-14
Abstract: 935,173. Correspondence control. P. G. S. MERO. June 14, 1961 [June 27, 1960], No. 21522/61. Class 40 (1). in a graphic communication system in which movements of a transmitter stylus cause frequency modulation of a carrier signal, further modulation means are provided to change the frequency of the carrier signal at a fixed rate, substantially greater than the maximum rate of change of frequency caused by movements of the transmitter stylus, in response to movements of the stylus normal to the recording medium. Transmitter (or transceiver) 10 includes a stylus 14 supported by pantograph linkage 17 for movement over a writing surface 12. The pantograph arms are connected to variable inductors 25, 28 (or to variable capacitors), each forming part of a signal generator, so that movements of the stylus modulate the generator output frequencies. Receiver (or transceiver) 11 includes filters 32, 33 to separate the X- and Y-coordinate signals, amplifiers 34, 35 and discriminators 36, 37, the outputs of which are fed through control circuits 38, 39 to driving motors 40, 41 coupled to discriminators to complete the servocontrol systems. The transmitter also incorporates a low-frequency generator 50 which is operative whenever the stylus is placed on the recording medium to frequency-modulate the X-co-ordinate carrier signal at a fixed rate (e.g. 100 c.p.s.) substantially greater than the maximum rate of change of frequency caused by movements along the X-co-ordinate (less than 10 c.p.s.). This signal is recovered in the receiver by a high-pass filter 52 the output of which actuates a motor, solenoid or other device to move the receiver stylus on to the recording medium. Pen-lift signal generator. In Fig. 2, X- co-ordinate movements vary capacitor 68 in the frequency-determining circuit of oscillator 69, the principle inductance 73 of the circuit forming part of a field-sensitive variable reactor 72. Field coil 74 is coupled to lowfrequency oscillator 70 controlled by a switch 75 connected to the stylus. Oscillator 70 operates on closure of switch 75 and causes modulation of the carrier oscillator signal at the frequency of oscillator 70. In Fig. 3 (not shown), a voltage-sensitive capacitor is connected across the low-frequency oscillator in parallel with the frequency-determining circuit of the carriersignal oscillator. The low-frequency oscillator may be controlled without a switch by utilizing changes of capacity between the stylus and recording medium. In a preferred modulator system (Fig. 4), the carrier-signal oscillator comprises a transistor 87 with a frequencydetermining circuit 98 and output winding 93, forming the primary winding of transformer 94. A second transistor 103 constitutes a limiter amplifier, its base being connected to secondary winding 95 of transformer 94, and its collecter to the base of transistor 87. A position transformer 109 is coupled to the stylus so that a bucking or boosting signal is fed back from coil 113 to vary the effective capacitance of capacitor 99 and thus vary the oscillator frequency as a function of movements along the X-co-ordinate. A bridge-type modulator 122 coupled to a low-frequency signal source 130 and controlled by switch 129 provides the pen-lift signal. A preferred receiver circuit is also described. Specification 901,972 is referred to.