Patent GB-743413-A (GB)
Abstract: 743,413. Balanceable networks. LEEDS & NORTHRUP CO. Jan. 15, 1953 [April 24, 1952], No. 1232/53. Class 37. [Also in Groups XXXVII, XXXVIII and XL (b)] In a measuring system comprising an automatically balanceable network with a balancing element adjusted at a rate dependent on a voltage related to network unbalance, which is applied to a detector, an error current integrator connected across the detector is removed when the balancing rate exceeds a predetermined level and connected when the rate falls below a certain level. A thermocouple voltage ET (Fig. 1) is compared with voltage ES between tap 13a of potentiometer 13 and earth and the difference voltage fed to a detector comprising a converter 17 and A.C. amplifier 9 which feeds a bridge A, one arm of which is formed by one winding 25 of two-phase motor 23, which, in accordance with the phase sense and magnitude of the amplifier output drives the tapping point 13a in a sense to balance the network and to drive a recording pen 40 across a continuously driven chart 42. Hunting of the system is reduced by the connection of resistor 11 and capacitor 12 in shunt with the thermocouple, which adds a negative voltage proportional to motor speed to the voltage ET-ES, so that the detector input first becomes zero at a point short of true balance thereby reducing overshoot. The sensitivity of this system which is low, due to a large fraction of the voltage ET-ES being developed across resistor 11, is increased by connection of capacitor 33 and resistor 34 across the input. This integrates the error current so that full error voltage may be fed to the detector, while a speed-responsive switch which short-circuits the capacitor at high motor speeds prevents development of a large error voltage across it with a sudden change in ET which, due to the long time constant of the circuit, would persist and cause overshoot. The switch 44 comprises a contact 45 vibrated with varying amplitude between spaced contacts by the output of speed responsive bridge A, which is a minimum at zero motor speed. The lagging of voltage ES behind ET, occurring whenever ET changes at a uniform rate with which ES can keep pace, which is due to voltage developed across resistor 11 by the current charging capacitor 12, is eliminated by the circuit of Fig. 7. Capacitor 12 is connected to the junction of capacitor 105 and resistor 106, instead of to one side of the detector so that at low speeds when capacitor 105 is in circuit i.e. when maximum accuracy in following is desirable, no current flows in resistor 11 and the full error voltage is developed across the detector. Speed of response is improved by connection of a series CR circuit 120, 121 in shunt with detector 9A (Fig. 9). The circuit associated with capacitor 120 produces a positive velocity dependent voltage, if the joint resistance of 11 and 121 is greater than the detector resistance, which is arranged to be smaller than the negative voltage introduced by capacitor 12 but with a longer time constant so as to produce a resultant voltage dependent on acceleration. Capacitor 105 and resistor 106 form the error integrator. A further circuit, basically similar to that of Fig. 1 has in addition the shunt circuit 120, 121, while a further arrangement combines all the advantageous features already described. An arrangement for measuring a current variable in accordance with some condition differs from the Fig. 1 arrangement in having the capacitor and resistor corresponding to 12 and 11 in parallel with one another across the current source, while the speed-responsive switch is of the centrifugal flyball type driven by the motor. Another form of speed responsive switch (Fig. 5, not shown) comprises a resilient contact-making arm carrying permanent magnets, which is deflected by interaction with a field produced by eddy currents in a conductive disc rotated adjacent the magnets by the balancing motor. In a similar circuit for comparison of spectrum line intensities the current source is a photo-multiplier cell exposed to a spectrum line and is connected to the network through a filter, while the reference voltage is tapped off a potentiometer fed via a filter network with a voltage proportional to the current in a photo-multiplier tube exposed to a reference line of known intensity.