galvanometer

Galvanometers (outdated also called rheometers ) are electromechanical current measuring devices that generate a mechanical rotary movement proportional to the electrical current . The principle is used, among other things, in moving-coil measuring mechanisms in combination with a pointer and a scale as a display instrument. Further applications are the galvanometer drive, which is used for quick angle adjustment for light pointers, scanners or in CD players . The galvanometer is named after the Italian doctor and researcher Luigi Galvani .

Schematic construction of a galvanometer according to Weston, as it is used in moving coil measuring mechanisms for current measurement - oblique view of the frame coil and its axis of rotation

history

Early tangential galvanometer with compass ( Bussole ), horizontally adjustable in the black ring coil in which the measuring current flows

Mirror galvanometer according to d'Arsonval. The small circular mirror on the axis can be seen above the black permanent magnet

Galvanometers were the first measuring devices to measure electrical current. The underlying principle was observed by Hans Christian Ørsted in the deflection of magnetic compass needles when an electrical conductor such as B. a piece of wire through which electricity flows. The degree of deflection of the compass needle in the earth's magnetic field corresponded (non-linearly) to the strength of the current through the wire.

The name galvanometer goes back to the work of Johann Salomo Christoph Schweigger at the University of Halle in 1820. This was followed by design improvements, including the fact that the wire was looped around the magnetic needle in several turns in order to multiply the deflection effect with weak currents. Since the windings have the same winding direction, the deflecting force is multiplied by the number of windings for the same current, which is why this type of galvanometer was also called a multiplier or a Schweigger multiplier in the early days .

These early galvanometers with a compass needle were so-called tangential galvanometers , as they had to be aligned in the earth's magnetic field before they could be used and use this as a restoring torque. Later designs with astatic needles avoided this disturbing influence ( André-Marie Ampère , Leopoldo Nobili ). A very sensitive design, the mirror galvanometer, goes back to work by William Thomson from 1858, after preparatory work by Johann Christian Poggendorff from 1826. Instead of a needle as a display, a small mirror is mounted on the axis , and a light beam serves as a display on a projection screen. This enabled even very small deflections of the mirror to be displayed if the projection wall was sufficiently far away.

In 1882 Jacques-Arsène d'Arsonval and Marcel Depréz independently invented a form of galvanometer in which the radially movable coil, suspended on springs , is surrounded by a strong permanent magnet . The magnetic circuit is ensured on the outside by a fastening made of ferromagnetic iron, on the inside of the coil by a rigidly attached cylinder made of iron. The coil moves radially freely in the gap between the inner iron cylinder and the outer permanent magnet, which, as a good approximation, results in a linear relationship between the deflection of the coil and the mirror attached to it for the optical display and the electrical current flowing through the coil. D'Arsonval galvanometers already show a very high sensitivity . With its design in the 1880s, D'Arsonval was able to measure currents in the range of a few microamps .

Edward Weston improved the galvanometer from d'Arsonval and patented these improvements in 1888. Among other things, he attached a fine, spiral-shaped spring to the bobbin, similar in structure to the spiral spring used in clocks on the balance wheel , around the bobbin to move into a defined rest position without current flow and to ensure a defined counterforce for the electromagnetic deflection. Further improvements related to the shape and assembly of the external permanent magnets to ensure the accuracy of the instrument over time. In addition, he replaced the mirror with a pointer, which enabled direct reading of the measured value via a scale and thus avoided the cumbersome adjustment of the mirror, the projection screen and the light source in the construction according to d'Arsonval. This Weston design, also known as a pointer galvanometer, forms the basis of the moving-coil measuring mechanisms that are still used today in electromechanical displays .

Principles

Pointer galvanometer according to Weston, as it is still used today in moving-coil measuring mechanisms

There are different versions of galvanometers, which differ in the type of reading, storage and sensitivity. Like many other analog indicating and electro-mechanically structured instruments are all galvanometer very sensitive to impact and must be protected by closed housing from air currents.

Reading

Historical mirror galvanometer

The following principles for reading the measured value are known:

Dial galvanometer: A pointer is attached to the moving coil; this type is used in moving-coil measuring mechanisms .
Mirror galvanometer: have a small mirror on the moving coil instead of the pointer. A line mark generated by a projection lamp or a laser is reflected by the mirror and projected onto a scale set up separately from the galvanometer. Mirror galvanometers can be designed to be very sensitive, since the massless light pointer can be made of almost any length, provided that the projection lamp or laser is powerful enough. The deflection angle is also doubled due to the reflection. The disadvantage is that there are three separate components that have to be set up and adjusted to one another.
Light mark galvanometer: You avoid these disadvantages by accommodating the three parts of the mirror galvanometer (measuring mechanism, scale and lamp with optics) in a common housing. The length of the light pointer is limited by the size of the housing; however, multiple reflections inside can lengthen the pointer.

storage

Depending on the type of storage, the following types are distinguished:

Tip storage: In galvanometers with a point bearing, the moving coil has an axis with steel points that are stored in spring-loaded precious stones. Power is supplied to the moving coil via two spiral springs, which also provide the reset torque of the pointer in the direction of the zero position.
Tension band storage: Instead of an axis, galvanometers with tension band bearings have two metal bands that are spring-tensioned. The tapes are used to supply power to the moving coil and its torsional moment to reset. A special design combines the functions of the reset and that of the moving coil in that only a single wire loop is used through two parallel tensioned conductor threads with a clamped mirror, which results in a very short response time and oscillographic recordings are possible.
Tape suspension: Galvanometers with tape suspension also have two metal tapes for power supply. The moving coil is suspended vertically from one of the ribbons, the low torsional moment of which is used for resetting. The supporting ribbon can be made so thin that it is just able to withstand the weight of the moving coil. The highest sensitivity is achieved with these devices.

Every mass mounted with torsion springs tends to oscillate like the balance wheel of a watch. A current pulse, i.e. the integral of a current flowing only briefly (typically fractions of a second), can be measured via the amplitude caused, i.e. the maximum deflection of the mirror or pointer. In order to measure continuous currents (alternating current must be rectified), a display that is as calm as possible is required through suitable damping. A mirror rotating in air can be dampened appropriately by the viscosity of the air compared to the very low restoring torque of a twisting thread. Some Weston pointer galvanometers have a paddle in the rear extension of the pointer, which pushes air in a slotted torus segment with a small gap, which enables very strong damping with a time constant of over one second. It is common, however, that the coil is wound on a delicate aluminum frame, the movement of which induces currents in the magnetic field that counteract the cause, i.e. dampen them.

Astatic needles

Galvanometer with astatic needles, protective glass cylinder with a flat window to the mirror

Galvanometers with astatic needles are a form of galvanometer that was invented by the Italian physicist Leopoldo Nobili around 1826.

In this construction, two equally strong hard magnetic needles are attached parallel to each other with opposite polarization and attached to a thread. The opposite polarity eliminates the influence of the earth's magnetic field and the measurement setup becomes astatic. This structure can remain in any position in which it is brought without realigning itself to the earth's magnetic field. In order to enable measurement of external forces, the influence of the earth's magnetic field must not be completely eliminated, otherwise the needle cannot be brought into its starting position. A correction rod is used to control the sensitivity to the earth's magnetic field. This magnetic rod is placed under the needles in the direction of the earth's magnetic field. It acts on the lower needle, and its influence on can be controlled by the distance to it.

Galvanometer constant

Instead of the sensitivity, the galvanometer constant is given as its reciprocal value, preferably the current constant . Depending on the version, the following are marked: ${\ displaystyle G = {\ tfrac {I} {\ alpha}} = {\ tfrac {\ text {current}} {\ text {deflection}}}}$

current sensitive galvanometers: with their current constant; it should be small for high sensitivity. The smaller the constant, the greater the deflection for a given current.
voltage sensitive galvanometers: with its voltage constant U / α (= current constant × (internal + required external resistance)). The requirement for the resistance of the external circuit results from the necessary damping at which the measuring mechanism adjusts itself aperiodically . The circuit must be designed accordingly. High voltage sensitivity also requires the adjustment of the measuring mechanism resistance to the resistance of the measuring circuit, taking into account the correct damping of the measuring mechanism.

dynamic behaviour

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

{\ displaystyle \ Theta \ cdot {\ ddot {\ varphi}} + \ rho \ cdot {\ dot {\ varphi}} + D \ cdot \ varphi = A \; B \; N \ cdot I _ {\ text {tot }}}

It is the time-dependent deflection angle of the galvanometer, its change over time, i.e. the angular velocity of the pointer and the angular acceleration . The constants are the moment of inertia Θ of the rotating member, the mechanical attenuation constant ρ , the spring constant D of the return spring, the coil area A , the magnetic induction B of the permanent magnet and the number of turns N . I _tot is the total current through the coil, consisting of measuring current and induced current. Since the readable deflection α (arc length, often given in millimeters) depends on the pointer length, a geometry factor g is added at the transition from φ to α , and the galvanometer constant results in ${\ displaystyle \ varphi}$ ${\ displaystyle {\ dot {\ varphi}}}$ ${\ displaystyle {\ ddot {\ varphi}}}$

{\ displaystyle G = {\ frac {gD} {ABN}}}

Practical designs

Spherical armor galvanometer

Due to the use as a zero indicator or to detect the smallest currents, the maximum resolution, i.e. the smallest measured value that can still be made visible, is the determining variable for galvanometers.

Pointer galvanometer with tip bearing

Galvanometer drives

Galvanometer mirror from SCANLAB

For the rapid rotation of mirrors and for the rapid movement of read heads in hard disk memories and in CD players, galvanometer drives are still state of the art. Compared to piezo drives or other actuators, galvanometers have the advantage of lower costs and large (angular) strokes. A distinction is made between galvanometer drives with moving coils and those with moving magnets. In both cases, the angle between a coil and a magnet is controlled by adjusting the electrical current in the coil.

Mirrors moved by galvanometer drive are mainly used to move laser beams in space. Laser scanners with galvanometer drive are used in laser show devices, in material processing machines - e.g. B. laser sintering machines , stereolithography machines and laser markers - but also in dermatological and ophthalmological devices. The mirror rotation is usually measured with the help of a built-in angular position encoder and electronically controlled with its output signal. Almost all of these galvanometer drives are of the moving magnet type (see 'From the galvanometer to the galvanometer'). A major problem with modern mirror galvanometers is the very high angular acceleration in the axial direction. The very thin mirrors that are placed on top can deform at high torques and begin to swing open during movement. Various imaging errors can result from this.

Resonant galvanometers are also manufactured for special applications; these oscillate with a fixed angular amplitude around the axis of rotation. Resonant mirror galvanometers are used in certain printing applications or in space travel. In a vacuum, fat-free storage is a decisive advantage.

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

From the galvanometer to the galvanometer

1- rotor (permanent magnet) 2- stator (coils) 3- iron core 4- outer wall

Mirrors that move with a galvanometer drive are called galvanometer scanners, or galvo scanners for short. A galvo scanner should achieve the highest possible speeds and accelerations. To do this, friction and moments of inertia must be as small as possible. For this reason, aluminum coils are preferably used instead of copper coils in galvanic scanners with moving coils.

For high dynamics, the cooling of the coil must also be as good as possible. However, a rotor coil in air is not thermally well connected to its surroundings. In addition, a coil through which air flows is deformed by the centrifugal forces that occur. These disadvantages can be avoided with systems in which the magnet moves and the coil is at rest. Most systems are produced in this design today. This has several advantages: no electrical contacts are required to the rotor, the coil no longer deforms due to the high speeds and the coil windings can be cooled over a larger area and better thermal connection. An appropriate construction of the magnet guarantees constant properties up to about 135 ° C.

In the picture on the right, the rotor (1) as a permanent magnet, a small air gap and the wound coils (2) on the iron core (3) can be seen. The outer wall (4) is made of metal and is used to cool the coils. The best properties of such a galvo scanner can be achieved with permanent magnets made of FeNdB .

Web links

Commons : Galvanometer - collection of images, videos and audio files

Walter Luhs: EXPERIMENT 28 - LASER scanner. Meos GmbH, 2003. (PDF; 567 kB)

Individual evidence

↑ Experimenta circa effectum conflictus electrici in acum magneticam , self-published 1820.
^ Meyers Großes Konversations-Lexikon , 6th edition, 1905–1909.
↑ Wolfgang Schreier (Ed.), Biographies of important physicists, People and Knowledge 1984, p. 133
^ Joseph F. Keithley: The story of electrical and magnetic measurements: from 500 BC to the 1940s . John Wiley and Sons, 1999, ISBN 0-7803-1193-0 , pp. 196-198 .
↑ Patent US381304 : Electrical coil and conductor. Published April 17, 1888 , inventor: Edward Weston. ‌
^ Klaus Beneke: Biographies and scientific résumés of colloid scientists . Knof, 1999, ISBN 3-934413-01-3 , pp. 97-99 .
^ Siemens & Halske AG: Pocket book for electrical measurement technology . 1959, p. 85 .
↑ Melchior Stöckl, Karl Heinz Winterling: Electrical measurement technology . Teubner, 1987, ISBN 3-519-46405-5 , pp. 29 .
^ Karl Strecker: Auxiliary book of electrical engineering . 10th edition. Julius Springer, Berlin 1925, p. 124 - 125 (high voltage output).

[1] Experimenta circa effectum conflictus electrici in acum magneticam , self-published 1820.

[2] Meyers Großes Konversations-Lexikon , 6th edition, 1905–1909.

[3] Wolfgang Schreier (Ed.), Biographies of important physicists, People and Knowledge 1984, p. 133

[4] Joseph F. Keithley: The story of electrical and magnetic measurements: from 500 BC to the 1940s . John Wiley and Sons, 1999, ISBN 0-7803-1193-0 , pp. 196-198 .

[Weston1-5] Patent US381304 : Electrical coil and conductor. Published April 17, 1888 , inventor: Edward Weston. ‌

[6] Klaus Beneke: Biographies and scientific résumés of colloid scientists . Knof, 1999, ISBN 3-934413-01-3 , pp. 97-99 .

[7] Siemens & Halske AG: Pocket book for electrical measurement technology . 1959, p. 85 .

[8] Melchior Stöckl, Karl Heinz Winterling: Electrical measurement technology . Teubner, 1987, ISBN 3-519-46405-5 , pp. 29 .

[kugal1-9] Karl Strecker: Auxiliary book of electrical engineering . 10th edition. Julius Springer, Berlin 1925, p. 124 - 125 (high voltage output).