Patent GB-1436858-A (GB)
Title: System for digital weighing
IPC: G01G23/37
Filed: 1973-11-19
Abstract: 1436858 Measuring weight electrically: measuring voltage MANGOOD CORP 19 Nov 1973 [11 Dec 1972] 53524/73 Headings G1N and G1U Weighing apparatus comprises: a first switch to connect an analogue transducer to a power supply, and means to turn the switch on and off; and A/D converter, and coupling means operative with the first switch on to couple the converter to the transducer output; generating and storage means operative with the first switch off to generate and store a signal representing the transducer output with the power supply disconnected; means to use the stored signal in converter so as to produce a converter output devoid of any component representing the transducer output with the transducer power supply disconnected; and control logic means to inhibit each one of the coupling means and the generating and storage means when the respective other one is operative. With a power supply 11 Fig. 1 switched off at 12, any thermal voltage generated in a load cell  circuit 16, together with any offset voltage in a preamplifier 19, is stored in an integrator 30 while a thermal zero switch 26 is on, the integrator 30 reaching a steady state when its input signal is neutralized by an inverter 33 which feeds back signal to the preamplifier input 22. Similarly, with a preamplifier switch 42 off, a reference switch 61 off, and an A-to-D zero switch 63 closed, any offset in an A-to-D integrator 44 and a D.C. amplifier/comparator 48 is stored in a sample-and-hold 55 while an A-to-D offset switch 54 is on, the sample-and-hold 55 reaching a steady state when the offset is neutralized by a feedback path 56. To measure a weight a logic circuit, Fig. 2, turns off the thermal zero switch 26 and the A-to-D offset switch 54, leaving the integrator/inverter 30, 33, and the sample-and-hold 55 to maintain their corrective outputs. The power switch 12 is turned on, and after a short interval for stabilization of power-voltage-sensitive circuits, a clock pulse, initiated by an oscillator 101, Fig. 2 and master counter control logic 79, turns on the preamplifier switch 42, Fig. 1 and opens the A-to-D zero switch 63, causing the A-to-D integrator 44, via the D.C. amplifier comparator 48, to impose on a line 50 a potential representing the corrected analogue output of the load cells 16. The preamplifier switch 42 is then turned off and the reference switch 61 is turned on, giving the integrator 44 an input reference voltage via a line 62, proportional to the power supply voltage and of opposite polarity. to the load cell analogue voltage. The time taken for the reference voltage, integrated by the integrator 44, to neutralize the previously held corrected analogue voltage is a measure of that analogue voltage and is signalled by a pulse from an A.C.-coupled D.C. amplifier 51 in response to a zero crossing of the potential on a line 50. The time thus signalled is measured by reversible decade counters 81, Fig. 2 and stored in in output latches 83 for display on printout at 109'. If the dead load of a weighing platform, or the tare of a loaded container placed on it, is to be discounted, these quantities are stored in respective zero latches 82, Fig. 2 and tare latches 84. Immediately before the weighing phase the control logic 79 resets the reversible decade counters 81 to the contents of the latches 82 and/or 84 and commands the counters 81 to begin counting down until reaching zero, then to count up, so that the output latches 83 finally contain the net weight  after deduction of the discountable quantity from  the gross weight derived from the corrected load cell signal. If both dead load and tare are to be discounted, two count-down phases are arranged to run consecutively. In an alternative embodiment Fig. 3 (not shown), thermal and offset voltages are not neutralized but are measured immediately before power-supply switch-on and stored in a thermal-zero latch (131), being subsequently discounted from the gross output similarly to, and consecutively upon, the dead load and tare. In this embodiment the A-to-D conversion occurs in a voltage-controlled oscillator (124) coupled directly to the load cells (16) and sampled by a standard-duration pulse from a clock (127).