Patent GB-1298654-A (GB)
Title: Automatic calibrating apparatus for electrical instruments
IPC: H01F13/00
Filed: 1971-04-26
Abstract: 1298654 Calibrating electric meters R F L INDUSTRIES Inc 26 April 1971 [21 May 1970] 11381/71 Heading G1U [Also in Divisions H1, H2 and H3] The magnet of an electric meter 11, Fig.2, is magnetized to a level that will give the meter a required sensitivity, thereby causing the meter to become correctly calibrated, by first magnetizing the magnet to saturation and then reducing the level of magnetisation in steps until a given signal applied to the meter produces a given indication on its scale. The whole operation proceeds automatically as soon as the meter has been placed in a holder 10. A lamp 33 illuminates the meter and a lens 31 produces an image 35, 36 of the pointer and scale on a ground glass screen 29. An array of four photo-cells in respective adjustablypositioned heads 38' - 41' carried by a rotatable support plate 37 is rotated until the first photocell (38<SP>1</SP>) is aligned with the image of the extreme scale mark (here, 5V), the third photo-cell (40<SP>1</SP>) then being accurately aligned with the 4V. The magnet is then magnetized to saturation by discharging storage capacitors (202), Fig.8 (not shown) through coils (25) mounted in the holder 10 adjacent the meter, and a 4.0V signal (93) applied to the meter. Demagnetisation is now commenced by applying to the coils (25) bursts of alternating current, each burst being in the form of a decaying sinusoid and the initial magnitudes of successive bursts increasing. The demagnetisation process is stopped when the pointer image becomes aligned with the third photo-cell (40<SP>1</SP>), i.e. when the 4V input signal is producing a meter reading of 4V. Details of the Setting-Up and Magnetising Procedure - When the apparatus is first switched on a motor 55 is energized, but at the same time a relay 81 disengages a clutch 57 and operates a brake 56 so that the photo-cell support plate 37 remains in its extreme left-hand position. When the hinged meter holder 10 is closed the relay 81 is automatically de-energized to the motor to rotate the photo-cells by means of a cam 52, until the image of the extreme scale mark 5V blocks light from the first photo-cell 66 (in head 38<SP>1</SP>). Relay 81 is then re-energized so that the photo-cell array does not rotate any further, storage capacitors (202) are discharged to magnetise the magnet (relay 122), and the calibrated 4V signal applied to the meter (relays 132, 136). Details of the Demagnetizing/Calibrating Procedure - As the magnet becomes magnetized, the pointer image sweeps up the scale, and when it passes the second photo-cell 144 (in head 39<SP>1</SP>, positioned close to the first head 38<SP>1</SP>) the magnetizing circuit is rendered inoperative (relay 126, controlling relays 122, 150) and an inhibit circuit 148 is freed (relay 142) to allow the demagnetization process to commence. As the magnetization level is reduced the pointer image moves down-scale until it blocks light from the third photo-cell 156 (in head 40<SP>1</SP>), when the inhibit circuit 148 is again activated (relay 153) to prevent further demagnetisation. Should the process not be stopped at this time, in due course the pointer image will pass a fourth photo-cell 157 (in head 41<SP>1</SP>) which will also stop further de-magnetization (relay 152) and operate a fault indicator. The demagnetizing process can be temporarily interrupted at any time by closing a switch 170 to activate the inhibit circuit 148. When the holder 10 is opened to remove the treated meter the 4V voltage signal is removed (relays 132, 136) and the motor is again allowed (relay 81) to drive the cam 52 until the photo-cell array is returned to its starting position, when a further cam 53 closes a switch 79 to stop further rotation. A time delay relay 60 automatically disconnects the supply to the apparatus if it has remained unused for a prolonged period of time. A control box 65 carries lamps for indicating the progress of the meter treating operation and for identifying any faults that may have occurred.