Electromagnet weighing balance

• US 3,968,850 A
• Filed: 10/31/1974
• Issued: 07/13/1976
• Est. Priority Date: 10/31/1974
• Status: Expired due to Term

• Current Assignee: International Electronics Incorporated (Melrose Industries Plc)
• Original Assignee: International Electronics Incorporated (Melrose Industries Plc)
• Inventors: Gaskill, Charles David Sinclair
• Primary Examiner(s): Miller, Jr., George H.
• Application Number:
  US05/519,773
• Time in Patent Office: 621 Days
• Field of Search: 177/210, 177/212, 177/185
• US Class Current: 177/210.00R
• CPC Class Codes: G01G 7/04 with means for regulating the current to solenoids

Abstract

An electromagnetic weighing balance includes a load support system for supporting the mass to be weighed and including an electromagnetic balance coil disposed in a magnetic field and fed with a balance current producing on the support system a force balancing the weight. The magnitude of the current is taken as a measure of the weight. In order to reduce the effect of vibration the balance coil is fed from a high-impedance source and the balance circuit controlling the balance current in response to displacement of the support system from a datum position includes threshold means increasing the high-frequency gain in response to displacements in excess of threshold levels. Sensor means developing a control signal representative of support system displacement includes a balanced inductive displacement transducer, the output signal from which is combined with components derived by rectifying the energizing alternating voltage to devise a control signal representing the magnitude and direction of the support system.

Claims

1. In an electromagnetic weighing balance including a movable support system for a mass to be weighed, said support system including an electromagnetic balance coil disposed in the field of a permanent magnet, sensor means responsive to displacement of said support system from a datum position to develop a control signal representative of said displacement, a signal-controlled current source feeding to said balance coil a balancing current producing on said support system a force balancing said mass, balance circuit means fed with said control signal and applying a balancing signal derived therefrom to control said current source, and indicator means responsive to said balancing current to provide a measure of said mass, the improvement which comprises constructing said current source as a high impedance source comprising a transistor (45) having emitter, base and collector electrodes, with said balance coil connected between the collector electrode of said transistor and one terminal of a direct voltage source and the emitter electrode of the transistor returned to the other terminal of said voltage source, and means applying said balancing signal to said transistor base electrode, thereby to control the balancing current supplied to said balance coil.

2. The invention claimed in claim 1 wherein said sensor means comprises a support member (12) secured to said support system and supporting a ferromagnetic core (14) movable within a coil system (15) from a median position therein in response to displacement of said support system from said datum position, said coil system comprising a central energizing coil (15a) flanked by mutually similar sensing coils (15b, 15c) symmetrically disposed thereabout and electrically connected in series opposition, an alternating voltage source (21) connected by way of a transformer (22) having a centre-tapped secondary winding (22b) coupled to feed said energizing coil (15a), the series combination of oppositely poled diodes (28, 29) with intervening similar capacitors (26, 27) connected in shunt with said secondary winding (22b), with one end of said series connected sensing coils being connected to said secondary winding centre-tap (22c) and the other end of said coils being connected to the junction of said capacitors (26, 27) and to a first output terminal (25), and with like resistors (31, 32) each connected between a respective junction of a said diode with a said capacitor and a second output terminal (30).

3. The invention claimed in claim 1 wherein said balance coil (8) is connected electrically in series opposition with a similar compensating coil (8c) fixedly placed upon said permanent magnet.

4. The invention claimed in claim 3 wherein said permanent magnet means includes a central cylindrical pole member (3b) concentrically surrounded by a hollow cylindrical permanent magnet member (3e) magnetized in the direction of its length, with said balancing coil surrounding said cylindrical pole member and wherein said compensating coil also surrounds said pole member.

5. The invention claimed in claim 1 wherein said balance circuit means comprises:

first amplifier means (33) fed with said control signal and providing said balance signal to said current source;

switchable gain-control means (62-66) operable to increase the high-frequency gain of said first amplifier means from a low value to a higher value in response to a switching signal; and

threshold means (35-39 &

50-61) responsive to excursions of said control signal in excess of a predetermined magnitude, of either polarity, to apply signal to said gain-control means.

6. The invention claimed in claim 5 wherein said threshold means includes second amplifier means (35) fed with high-frequency components of said control signal, said second amplifier means feeding amplified high-frequency components to a respective input of a difference amplifier (59) by way of a respective series combination of a Zener diode (53,55) an oppositely poled diode (54,56) and a resistor (60,61), said Zener diodes being oppositely poled, whereby said difference amplifier yields an output signal to increase the high-frequency gain of said first amplifier means in response to the high-frequency components of said control signal exceeding the Zener voltage of said diodes.

7. The invention claimed in claim 1 wherein said balance coil (8) is connected between said transistor collector electrode and said voltage source pole in series with a resistor (43) and said indicator means comprises voltmeter means (44) connected to provide an indication corresponding to the voltage appearing across said resistor (43).

8. The invention claimed in claim 7 wherein said voltmeter means is a digital voltmeter.

9. An electromagnetic weighing balance including a movable support system for a mass to be weighed;

permanent magnet means having a magnet gap in which a magnetic flux is developed;

said support system including an electromagnetic balancing coil disposed in said magnet gap;

sensor means responsive to displacement of said support system from a datum position to develop a control signal representative of said displacement, said sensor means comprising a support member secured to said support system and supporting a ferromagnetic core movable within a coil system from a median position therein in response to displacement of said support system from said datum position, said coil system comprising a central energizing coil flanked by mutually similar sensing coils symmetrically disposed thereabout and electrically connected in series opposition, an alternating voltage source connected by way of a transformer having a centre-tapped secondary winding coupled to feed said energizing coil, the series combination of oppositely poled diodes with intervening similar capacitors connected in shunt with said secondary winding, with one end of said series-connected sensing coils being connected to said secondary winding centre-tap and the other end of said coils being connected to the junction of said capacitors and to a first output terminal, and with like resistors each connected between a respective junction of a said diode with a said capacitor and a second input terminal,

a signal controlled current source comprising a transistor having emitter, base and collection electrodes, with said balance coil connected between the collector electrode of said transistor and one terminal of a direct-voltage source;

balance circuit means fed with said control signal and applying a balancing signal derived therefrom to control said current source;

and indicator means responsive to said balancing current to provide a measure of said mass.

10. An electromagnetic weighing balance including a movable support system for a mass to be weighed;

permanent magnet means having a magnet gap in which a magnetic flux is developed;

said support system including an electromagnetic balancing coil disposed in said magnet gap;

sensor means responsive to displacement of said support system from a datum position to develop a control signal related to said displacement, said sensor means comprising a support member secured to said support system and supporting a ferromagnetic core movable within a coil system from a median position therein in response to displacement of said support system from said datum position, said coil system comprising a central energizing coil flanked by mutually similar sensing coils symmetrically disposed thereabout and electrically connected in series opposition, an alternating voltage source connected by way of a transformer having a centre-tapped secondary winding coupled to feed said energizing coil, the series combination of oppositely poled diodes with intervening similar capacitors connected in shunt with said secondary winding, with one end of said series-connected sensing coils being connected to said secondary winding centre-tap and the other end of said coils being connected to the junction of said capacitors and to a first output terminal, and with like resistors each connected between a respective junction of a said diode with a said capacitor and a second output terminal;

a signal controlled current source feeding to said balance coil a balance current producing on said support system a force balancing said mass;

balance circuit means fed with said control signal and applying a balancing signal derived therefrom to control said current source, said balance circuit means comprising first amplifier means fed with said control signal and providing said balance signal to said current source;

switchable gain-control means operable to increase the high-frequency gain of said first amplifier means from a low value to a higher value in response to a switching signal; and

threshold means responsive to excursions of said control signal in excess of a predetermined magnitude, of either polarity, to apply signal to said gain-control means, said threshold means comprising second amplifier means fed with high-frequency components of said control signal, said second amplifier means feeding amplified high-frequency components to a respective input of a difference amplifier by way of a respective series combination of a Zener diode an oppositely poled diode and a resistor, said Zener diodes being oppositely poled, whereby said difference amplifier yields an output signal to increase the high-frequency gain of said first amplifier means in response to the high-frequency components of said control signal exceeding the Zener voltage of said diodes.

11. An electromagnetic weighing balance including a movable support system for a mass to be weighed;

permanent magnet means having a gap in which a magnetic flux is developed;

said support system including an electromagnetic balancing coil disposed in said magnet gap;

sensor means responsive to displacement of said support system to develop a control signal related to said displacement;

a signal controlled direct current source coupled to supply to said balance coil a balance current of a magnitude to produce on said support system a force balancing said mass;

and indicator means responsive to said current to provide an indication denotive of said mass;

wherein said balance circuit means comprises;

first amplifier means fed with said control signal and providing said balance signal to said current source;

switchable gain-control means (62-66) operable to increase the high-frequency gain of said first amplifier means from a low value to a higher value in response to a switching signal; and

threshold means (35-39 &

50-61) responsive to excursions of said control signal in excess of a predetermined magnitude, of either polarity, to apply said switching signal to said gain-control means.