Patent GB-969642-A (GB)
Title: Improvements in or relating to balances
Inventor: CHAPPELL FRANK ARTHUR
IPC: G01G7/04
Filed: 1961-09-14
Abstract: 969, 642. Photo-electric control of weighing. L. OERTLING Ltd. Sept. 11, 1962 [Sept. 14, 1961], No. 33037/61. Headings G1A and G1W. Displacement of the beam 8 of a dispensing balance, Fig. 1, varies the light passing from a source 18 to a photo-electric cell 20 whose output is applied through a high-gain amplifier 24, Fig. 2, to a solenoid coil 15 coacting with a magnet system 10c, 10d attached to the beam, the beam assuming an inclination such that the photo-electric cell is exposed sufficiently to cause the electromagnetic force due to the solenoid to balance the weight causing the original displacement of the beam. The output.from amplifier 24 may thus furnish a signal proportional to the instantaneous outof-balance weight. In accordance with the invention the signal is applied to a feeding device for materials in the form of a hand-held vibratory spatula (not shown in Fig. 1) and is employed to control the feeding, rate-of-feeding and cut-off of the material. For this purpose the output of amplifier 24 is applied through a further amplifier 27 to energize the electric motor or indicative coil of the feeding device and is also applied to a discriminator circuit 28 which detects when the amplifier output falls to zero at the condition of balance and controls a cut-off relay 29 to terminate the supply of material. Preferably the relay is normally energized and is de-energized when zero volts exist across solenoid 15. Alternatively the relay may be arranged to be energized only when zero volts exist across the solenoid. Amplifier 27 is given a characteristic, Fig. 3 (not shown), so that above a predetermined level of current a constant rate of feed of materials is obtained, but below the predetermined level the rate of feed decreases with the decrease in current and levels off at a preset minimum level where it remains until cut-off is produced by relay 29. In operation of the apparatus, a container 5 for the material is first placed on pan and its weight balanced by an electro-magnetic force produced by a solenoid 13 which coacts with a magnet system 10a, 10b attached to the beam 8 and similar to system 10c, 10d. The current in the solenoid is controlled by a potentiometer 31 ("tare controller"), the condition of balance being determined by a meter arranged to indicate the output current of amplifier 24. A weight equivalent to the desired weight of material is then placed in the right-hand pan 4 and the feeding device introduced over container 3 and switched on so as to be controlled as described above by the out-of-balance current produced by amplifier 24. A solenoid 14 connected to receive the time derivative of the output of amplifier 24 is provided to damp movements of the balance so as to prevent oscillation. A further solenoid 16 controlled through a potentiometer 32 ("weight change controller") may be used to counterbalance and indicate change of weight of a dispensed sample, due, e.g. to drying or oxidation. Reference is made (without giving details) to the use of capacitive, inductive or radio-active detectors in place of photo-electric detector 20.