Patent GB-900609-A (GB)
Title: Improvements in or relating to weighing apparatus
Inventor: FLETCHER LAWRENCE ALFRED
IPC: B07C5/32
Filed: 1958-06-25
Abstract: 900,609. Weighing apparatus. AUTOMA ENGINEERING GROUP Ltd. Sept. 21, 1959 [June 25, 1958], No. 20375/58. Class 40 (1). [Also in Groups XVIII and XXXVI] Weighing apparatus comprises a platform with a cross-spring suspension system and transducers to give an output signal depending on the magnitude of the unbalance caused by an article on the platform differing from a nominal value and also depending on the velocity at which the system is deflected from balance, this output signal being arranged to restore the system to balance. In Fig. 1, members 3 and 4 are attached by cross-springs to horizontal beams 5 and 6 and these are attached similarly to rigid members 1 and 2. The weight of an article on platform 8 attached to member 4 is balanced by a counterweight 7 on member 3. Any difference causes tilting of the beam 5 and consequent movement of a member 9 attached thereto. The latter carries a coil 10 which co-operates with a permanent magnet 11 to restore the beam to its normal position by means described below. The member 9 also carries a position transducer and a velocity transducer (not shown in Fig. 1). The position transducer (Fig. 2) consists of a differential capacitor having a movable section 16 attached to member 9 and a stationary section 12 fixed to the base of the machine. The stationary section consists of two identical pieces 13 each having the shape shown, in which the parts A, B, C are conductive areas on an insulating body. Each of these five parts is joined to the corresponding part of the other piece by its own conductive spacer 14, and each piece is backed by another conductively-coated piece 15. The four areas C and the two pieces 15 are all earthed. The moving portion 16 is conductively-coated on each side and differentially varies the capacitance between part B and each part A. The parts A are energized from a 50 kc/s. push-pull oscillator (Fig. 6, not shown) so that the output from B is zero when the moving portion is in its central position and varies proportionally to movement, in appropriate phase-sense, as the member 9 is deflected. This transducer is insensitive to transverse movement. The velocity transducer (Fig. 3, hot shown) consists of a cylindrical permanent magnet moving in a coil coaxial therewith, to give an output proportional to velocity. As shown in Fig. 4, the position transducer 23 is fed with a push-pull signal from the oscillator 21 (which is of the thermistor-stabilized R.C. type), and its output is amplified, rectified by a phasedetector 26 and applied to a mixer 28 in conjunction with the signal from the velocity transducer 29. This latter signal is applied with the correct phase and amplitude to give critical damping. The mixer is a cathode-coupled balanced amplifier with the signals applied to its two grids. The mixer output goes to a pushpull cascode amplifier 30 which delivers current to the restoring coil 10 and a resistor 32 in series, so that the magnitude of the current through the latter is a measure of the difference between the weight of the object and the counterweight. A double-pole double-throw relay in the sampler unit 33 repetitively charges a capacitor to the voltage across the resistor and then transfers its voltage to a discriminator unit 34 comprising a pair of Schmidt trigger circuits. The relay is synchronized with mechanical handling apparatus for introducing the object on to the weighing platform (e.g. as in Specification 900,610). The discriminator compares the capacitor voltage with two preset standards and operates relays to divert the object into " low," " correct " and " over " weight categories. By increasing the number of trigger circuits more than three categories can be provided. The detailed circuitry of the various units appears in Figs. 5 to 11 (not shown).