METER AND METHOD OF CALIBRATION

• US 3,757,217 A
• Filed: 08/20/1971
• Issued: 09/04/1973
• Est. Priority Date: 08/20/1971
• Status: Expired due to Term

• Current Assignee: David B. Pearson
• Original Assignee: David B. Pearson
• Inventors: Pearson, David B.
• Primary Examiner(s): Smith, Alfred E.
• Application Number:
  US05/173,424
• Time in Patent Office: 746 Days
• Field of Search: 324/151R,151A,74
• US Class Current: 324/151.00R
• CPC Class Codes: G01R 1/28 Provision in measuring instruments for reference values, e.g. standard voltage, standard waveform
• CPC Class Codes: G01R 11/02 Constructional details
• CPC Class Codes: G01R 5/04 with magnet external to the coil

Abstract

A D'"'"''"'"'Arsonval type moving coil meter with linear scale deflection characteristics. The magnetic circuit of the meter is such that the sum total of flux which cuts the coil is the same for any operative position of the coil. Correspondingly, angular deflection of the coil is in direct proportion to the current in the coil. The magnetic circuit includes a magnet having fixed locating surfaces thereon which cooperate with fixed locating surfaces on a yoke. This locating arrangement assures that the sum total of flux which cuts the coil is the same for any operative position of the coil and further assures repeatable accuracy in successively assembled meters. The meter is calibrated by magnetizing the magnet to obtain full scale deflection when rated current is applied to the meter terminals. Where a calibrating resistor is used, the resistor is connected to the meter prior to calibration with the result that accuracy of the meter is assured without the need for using a precision resistor.

Claims

1. A moving coil measuring instrument wherein deflection of the coil is directly proportional to the current in the coil, and wherein the instrument is adapted to be calibrated after assembly by magnetizing a magnet of the instrument to a required extent;

comprising in combination, an adjustment free magnetic circuit comprising a pair of unitary yoke pieces abutting each other to form a ring shaped two piece yoke, and a core within and magnetically connected to said yoke, one of said yoke and core being of magnetic material and the other of said yoke and core including a fixed unitary permanent magnet;

said yoke and core having fixed, abutting, cooperating locating surfaces for mounting said yoke and core in only one predetermined aligned position with respect to each other;

opposed surface means of said yoke and core defining a coil receiving gap therebetween;

a coil assembly;

means mounting said coil assembly with a portion thereof extending through said gap and for movement through a predetermined angle of rotation at least as great as the angular deflection of the coil from a zero position to a full scale position;

said opposed surface means cooperating with said magnet to provide flux which cuts the coil, the sum total of said flux which cuts the coil being substantially the same for any position of said coil within said predetermined angle of rotation;

whereby deflection of said coil in said gap is directly proportional to the current in said coil.

2. A measuring instrument according to claim 1 wherein said opposed surface means of said yoke and core cooperate to provide a crescent shaped gap;

and the flux density in said gap is substantially uniform throughout said predetermined angle of rotation of said coil.

3. A measuring instrument according to claim 1 wherein said cooperating locating surfaces include first and second fixed loCating surfaces on said core, said yoke includes first and second fixed locating surfaces in engagement respectively with said locating surfaces of said core;

whereby, said core is in a predetermined fixed position relative to said yoke.

4. A measuring instrument according to claim 3 wherein said first and second locating surfaces of said core are generally rectangular;

and said first and second locating surfaces of said yoke include rectangular slots to receive said rectangular portions of said core whereby, said core is maintained in a fixed position within said yoke.

5. A measuring instrument according to claim 1 wherein said yoke is of uniform cross-sectional configuration;

said core is of uniform cross-sectional configuraton;

said opposed surface means defining said gap include a smooth arcuately curved inner surface of said yoke and a smooth arcuately curved exterior surface of said core.

6. A moving coil measuring instrument wherein deflection of the coil is directly proportional to the current in the coil, comprising in combination, a magnetic circuit comprising an adjustment free hollow yoke of magnetic material, and a unitary permanent magnet in fixed abutting relation with respect to, and clamped within said yoke;

said yoke having an inner surface in opposed relation to an outer surface of said magnet, said inner surface and said outer surface defining a coil receiving gap therebetween;

a coil assembly;

means mounting said coil assembly with a portion thereof extending through said gap and for movement through a predetermined angle of rotation at least as great as the angular deflection of the coil from a zero position to a full scale position;

said mountng means including means exerting an opposing force on said coil proportional to the angular deflection of said coil from said zero position;

said opposed surfaces of said magnet and yoke providing a flux gap in which the sum total of the flux which cuts the coil is substantially the same for any position of said coil within said predetermined angle of rotation of the coil assembly so that the magnetic circuit requires no adjustment but can be calibrated by permanently magnetizing the permanent magnet after assembly;

whereby, deflection of said coil assembly in said gap is directly proportional to the current in the coil.

7. A measuring instrument according to claim 6 wherein said means mounting said coil assembly for rotation includes taut band suspension means extending between said coil assembly and a supporting base.

8. A measuring instrument according to claim 6 wherein said means mounting said coil assembly for rotation includes pivot bearing means extending between said coil assembly and a supporting base.

9. An instrument according to claim 6 wherein said magnet includes means to permit installation thereof in only one predetermined oriented position.

10. An instrument according to claim 9 wherein said instrument includes a base having a positioning surface, and said means on said magnet includes a notch cooperating with said surface to permit mounting said magnet in only one predetermined oriented position.

11. A measuring instrument according to claim 6 wherein said yoke is comprised of two semi-circular yoke elements, and said magnet is clamped between said yoke elements.

12. A method of making a linear scale meter of predetermined sensitivity comprising in combination, the steps of providing a movable coil;

providing an adjustment free magnetic circuit having a permanent magnet and a flux gap and with a portion of the coil in said gap, said magnetic circuit including an adjustment free two piece yoke, said gap and coil cooperating so the sum total of the flux in the gap which cuts the coil is the same for any operative position of the coil in the gap;

changing the magnetization of said permanent magnet with a magnetizing source separate from the meter while applying a predetermined current to said coil until said coil deflects to a predetermined calibration position;

aNd removing the meter from the magnetizing source.

13. A method according to claim 12 wherein said step of providing a coil includes connecting a resistor between said coil and a terminal of the meter before changing the magnetization of said magnet;

and said step of applying a predetermined current to said coil includes applying said current to the meter terminals.

14. A method according to claim 12 wherein said steps of providing a magnetic circuit and providing a coil include providing an assembled meter movement, and wherein the method further includes placing said movement in a casing;

mounting a linear scale adjacent said pointer; and

adjusting the zero position of said pointer prior to changing the magnetization of said magnet.

15. A method of calibrating an electric meter of predetermined sensitivity comprising in combination, the steps of, providing a movable coil;

providing a magnetic circuit having a permanent magnet and a flux gap, and with a portion of the coil in said gap;

connecting a resistor between the coil and a terminal of the meter;

subsequently changing the magnetization of said permanent magnet with a magnetizing source separate from the meter while applying a predetermined current to said coil through said resistor until said coil deflects to a predetermined calibrated position; and

removing the meter from the magnetizing source.

16. A method according to claim 15 wherein said gap and coil cooperate with each other so the sum total of flux in the gap which cuts the coil is the same for any operative position of the coil in said gap.

17. A method according to claim 15 wherein said step of connecting said resistor between said coil and a terminal of the meter includes mechanically supporting said resistor on a supporting base of said magnetic circuit.