Galvanometer Theory of Operation
 

The magnetic circuit that is the basis for the "moving-iron" galvanometer was first disclosed in a United States Patent:

Patented Nov. 22, 1949
2,488,734
Dynamo Transformer
Robert K. Mueller, Newton, Mass.
Application March 7, 1946

The patent teaches how to make a dynamo transformer, otherwise known as a torque motor, with two very special properties. First, torque is linearly proportional to the applied current. Second, the torque produced is independent of the displacement over the working range, which is typically 10 to 20 degrees. The device shown in Fig. 1 comprises a stator 10 and rotor 14, made from magnetically permeable material such as soft iron. Each of the four poles is provided with two coils 16 and 18 (shown in cross-section). Radius 22 is not essential, and can be a flat surface.

Figs. 2 and 3 show the connections of the coil windings 16 and 18 of Fig. 1. For simplicity, the coils are shown outside the frame, but it is understood that they are wound on the pole pieces as shown in Fig. 1. Current flowing in the coils cause the four poles to be magnetized in the direction shown by the arrows. Note that poles b and d show the same direction in Figs. 2 and 3, whereas poles a and c are magnetized in opposite directions.

Fig. 5 combines the coils shown in Figs. 2 and 3, and shows their inputs with series resistors 24 and 26. The resistors, which must have a sufficiently high value, function to convert a voltage input to a corresponding current. In operation, Input 1 is supplied with current i₁ and Input 2 is supplied with current i₂. The currents may be direct current or alternating current of the same frequency. The torque tending to move the rotor from neutral is proportional to i₁i₂. The torque is substantially constant over the range of motion for which fringing effects are negligible.

Note that if Input 1 is supplied with a constant current, and Input 2 is supplied with a variable current, the torque produced will be proportional to the variable current. This is the key operating principle of the "moving-iron" galvanometer, which is a torque motor of the Mueller design. In the galvanometer, Input 1 is replaced with permanent magnets, as shown in the diagram of the G-100PD, below left.

Although at first glance the galvanometer and the Mueller device do not look alike, they are in fact identical magnetic circuits. If the rotor pictured in the Mueller motor is rotated 90 degrees, which may be done without altering its operation, it will look the same as the galvanometer. Note also that the drive coils of the galvanometer may be wound directly on the four poles, as in the Mueller device. The galvanometer coil positions as shown in the diagram at left were chosen for their ease of  manufacturing.

When a current is applied to the Mueller device, the rotor moves to align itself with the stator poles, as shown at right. To make the motor into a useful positioning device, such as a scanner, a torsion spring can be attached to the rotor. The torque produced by a current will then rotate the rotor until the motor torque is exactly balanced by the opposing torque of the spring. This is the operating principle of the so-called "open-loop" galvanometer. In a "closed-loop" device like the G-100PD, the torsion spring is not strictly needed, but is used to center the rotor when power is off, and to provide an electrical ground path for the capacitive sensor.

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