Galvanometer

Galvanometer are electromechanical electricity meters, which is proportional generate rotary mechanical motion to electricity. The principle is used among other things in moving coil instruments in combination with a pointer and a scale as indicating instrument. Other applications are in the galvanometer, which is used for fast angle adjustment for light pointer, scanner or CD players. The galvanometer is named after the Italian doctor and scientist Luigi Galvani.

History

Galvanometer were the first instruments for measuring the electric current. The underlying principle is observed by Hans Christian Oersted in the deflection magnetic compass needle, when in the vicinity of an electrical conductor, such as a piece of wire, was traversed by electricity. The degree of deflection of the compass needle in the earth's magnetic field corresponded to ( non-linear) of the current through the wire. The term galvanometer goes back to works by Johann Salomo Christoph Schweigger at the University of Halle in 1820. This was followed by design improvements, among other things, by the wire was performed in several turns around the magnetic needle, in order to multiply the effect of the displacement at low currents.

These early galvanometer with a compass needle represented so-called tangential galvanometer, as they had to be aligned prior to their use in the Earth's magnetic field and use this as a restoring moment. Later designs with astatic needles to avoid this disturbing influence. A very delicate design, the mirror galvanometer draws on the work of William Thomson to 1858, according to preliminary work by Johann Christian Poggendorff from 1826, back. Instead of a needle as a display, a small mirror is mounted on the axis, and a light beam is used as a display on a projection screen. This could, at sufficiently far distance of the projection screen, even very small deflections of the mirror are displayed.

1882 invented independently Jacques- Arsène d' Arsonval and Marcel Deprez a form of galvanometer in which the radially movable, suspended on springs coil is surrounded by a carrier of a strong permanent magnet. The magnetic circuit is ensured by a fastener made ​​of ferromagnetic iron in the outer area, the indoor area of ​​the coil by a rigidly mounted cylinder of iron. The spool is moved radially freely in the gap between the internal iron cylinder and the outside lying permanent magnet, resulting in a linear relationship between the displacement of the coil and the attached mirror for the optical display and the current flowing through the coil electric current is given to a good approximation. Galvanometer d' Arsonval already have a very high sensitivity. D' Arsonval could compete with its construction in the 1880s currents of a few microamps.

Edward Weston improved the galvanometer of d' Arsonval and patented these improvements in 1888. Among other things, he brought a fine spiral spring on the bobbin in, similar in structure to the spiral spring, as it is used in watches in the balance, so the bobbin no current flow to move into a defined rest position, and to ensure a defined counter-force for the electromagnetic deflection. Other improvements included the shape and installation of exterior mounted permanent magnets to ensure the accuracy of the instrument over time safely. He also replaced the mirror with a pointer, which allowed for the direct reading of the measured value on a scale and so avoided the cumbersome adjustment of the mirror, the projection screen and the light source in the structure according to d' Arsonval. This design of Weston, also referred to as Zeigergalvanometer, constitutes the basis of the Drehspulmesswerke today used in electro- mechanical displays

Principle

There are different types of galvanometers, which differ according to the type of reading, storage and sensitivity. Like many other analog indicating instruments and electro -mechanical based galvanometer are all very sensitive to shock and must be protected from air currents by closed housing.

Reading

The following principles of the reading of the measured value is known:

Storage

By type of storage, the following types are distinguished:

Each rotationally resilient mounted mass tends as the balance of a clock to vibrate. Current pulse measurements, ie the integral only for a short time (typically a fraction of a second) flowing streams can caused the amplitude, ie the mirror or pointing maximum deflection can be measured. To steady-state currents (AC must be rectified ) to measure, it needs a quiet possible display by suitable damping. A rotating mirror in air can already be suitably damped by the viscosity of air compared to the very low restoring torque of a rotating thread. Some pointer galvanometer Weston have in the rearward extension of the pointer a paddle, the air moves in a slit and extending with a small gap spacing torus segment, which can be a very strong attenuation with a time constant of about a second implement. The art, however, that the coil is wound on a soft aluminum frame, the movement in the magnetic field induces currents which counteract the cause, so vapors.

Astatic needles

Astatic galvanometer needles represent a form of galvanometer, which were invented by the Italian physicist Leopoldo Nobili 1826.

In this structure, two equally strong hard magnetic needles are mounted parallel to each other with opposite polarization and by a thread. By the opposite polarity of the influence of the terrestrial magnetic field is eliminated, the measuring structure is astatic. This structure can be in any position to which it is put, remain without realign the Earth's magnetic field. To enable measurement of external forces, the influence of the earth's magnetic field may not be completely eliminated, because the needle is otherwise not be brought into its initial position. In order to control the sensitivity of the Earth's magnetic field, a correction rod is used. This magnetic bar is placed in the effective direction of the geomagnetic field under the needles. It acts on the lower needle, and its influence can be controlled by the distance to the latter.

Galvanometerkonstante

Instead of the sensitivity of the Galvanometerkonstante is given as the reciprocal of which, preferably the current constant. Depending on the model are identified:

Dynamic behavior

The dynamic behavior of a galvanometer is described by the following differential equation:

Here, the time-dependent deflection angle of the galvanometer, which change over time, so the angular velocity of the pointer and the angular acceleration. The constants are the moment of inertia Θ of the rotating member, the ρ mechanical damping constant, the spring constant D of the return spring, the coil area A, the magnetic induction B of the permanent magnets and the number of turns N. Itot is the total current through the coil consists of measuring current and induced current. Since the read rash α ( arc length, often expressed in millimeters ) depends on the vector length, comes in the transition from φ after α nor a geometry factor g added, and the Galvanometerkonstante is given by

Practical realizations

Due to the application as a null indicator or for detecting small currents is galvanometers at the maximum resolution, ie the smallest measurement that can still be made visible, the determining variable.

Galvanometer

For rapid rotation of mirrors and rapid movement of read heads in hard disk drives and CD players are still galvanometer prior art. Have galvanometer compared to piezo actuators or other actuators the advantage of low cost and large (angular) strokes. A distinction galvanometer with moving coils and those with moving magnet. In both cases, the angle between a coil and a magnet, by adjusting the electric current is controlled in the coil.

By galvanometer mirror moving are used above all to move laser beams in space. Laser scanner with galvanometer come in laser show equipment, in material processing machinery - such as laser sintering machines, stereo equipment and Laser Engraving - but also in dermatological and ophthalmological appliances. The mirror rotation is typically measured using a built- angle position sensor and electronically controlled with the output signal. Almost all of these are of type galvanometer moving magnet (see ' From galvanometer to galvoscanner ').

For special applications, resonant galvanometer be manufactured; this swing with a fixed angular amplitude about the axis of rotation. Resonant mirror galvanometer are applied to specific pressure applications or space. In vacuum, grease-free storage is a crucial advantage.

Galvanometer in CD reading heads are mainly used in CD players for cars, in which it is important to achieve shock resistance. Galvanometer in hard disk drives move the magnetic read head over the disc; they are consistently in order to keep the type of moving coil, inertia, and thus the access times as low as possible.

From galvanometer to galvoscanner

With galvanometer mirror moving is called the galvanometer or short galvo scanner. A galvo scanner to achieve the highest possible speeds and accelerations. For this purpose, friction and moments of inertia must be as small as possible. Therefore, moving coil preferably aluminum coils instead of copper coils used in Galvanoscannern.

For a high dynamics and the cooling of the coil must be as good as possible. A rotor coil in air, however, is connected thermally bad for your environment. In addition, an air- carrying coil deformed by the Zentrifugalkäfte occurring. These disadvantages can be avoided by systems in which moves the magnet and the coil is at rest. Most systems today are produced in this embodiment. This brings several advantages: the rotor does not require any electrical contacts, the coil no longer deformed by the high speeds and the cooling of the coil windings can be realized over a larger area and better thermal connection. An appropriate design of the magnet guarantees constant properties to about 135 ° C.

In the right image, the rotor (1) as a permanent magnet, a small air gap, and then the wound coils (2) on the iron core (3) are visible. The outer wall (4) is made of metal and serves to cool the coils. The best features of such Galvoscanners is achieved with permanent magnets FeNdB.