An air core gauge is a specific type of

rotary actuator A rotary actuator is an actuator that produces a rotation, rotary motion or torque. The simplest actuator is purely mechanical, where linear motion in one direction gives rise to rotation. The most common actuators are electrically powered; other ...

in an analog display gauge that allows an indicator to rotate a full 360 degrees. It is used in gauges and displays, most commonly automotive instrument clusters. A typical automotive application is shown at the right. The air core gauge is a type of "air-core motor". It may be considered a "gauge movement" or "pointer indication device". Simco meter movements
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Background

There are four common types of rotary actuators:Comparing Gauge Movements
/ref> * Physical gauges, in which the needle is attached directly to the value being measured; for example, a mechanical

pressure gauge Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of press ...

* Analog volt meters or d'Arsonval movements, which consist of a coil and a

permanent magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, c ...

Stepper motor A stepper motor, also known as step motor or stepping motor,Clarence W. de Silva. Mechatronics: An Integrated Approach (2005). CRC Press. p. 675. "The terms ''stepper motor'', ''stepping motor'', and ''step motor'' are synonymous and are often u ...

s, which move in one-notch increments or steps * Air-core motors, as described below.

Construction and operation

The air core gauge consists of two independent, perpendicular coils surrounding a hollow chamber. A needle shaft protrudes into the chamber, where a permanent magnet is affixed to the shaft. When

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (hydr ...

flows through the perpendicular coils, their

magnetic fields A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

superimpose and the magnet is free to align with the combined fields. A typical air core gauge has four terminals, two for each coil, as shown. The two coils are identified as the

sine In mathematics, sine and cosine are trigonometric functions of an angle. The sine and cosine of an acute angle are defined in the context of a right triangle: for the specified angle, its sine is the ratio of the length of the side opposite th ...

coil and the

cosine In mathematics, sine and cosine are trigonometric functions of an angle. The sine and cosine of an acute angle are defined in the context of a right triangle: for the specified angle, its sine is the ratio of the length of the side opposite that ...

coil.

Theory

The direction

\theta

of the overall magnetic field is approximately: :

\theta = \arctan \left( \frac \right)

Where

x

and

y

are the coils' sine and cosine currents respectively. The permanent magnet aligns itself with that field, eventually settling near

\theta

. In this way, by proportioning the current through each coil, the needle can reach all 360° of rotation.

Example

If the sin coil current is 29 mA and the cos current is 50 mA: The coil current ratio is 0.58, and arctan 0.58 = 30 degrees.

Drivers

Air core gauges require special electronics to properly drive the coils. Some driver

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

s have a serial input data port and two pair of output lines. One pair of the output lines drives the sin coil and one pair drives the cos coil. The input data defines: * The quadrant to which the actuator will point. This defines the polarity of the

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

to the sin coil and the cos coil. * The desired number of degrees within the quadrant. Some typical driver ICs include: * On Semiconductor CS4172 16 pin dual inline package (DISCONTINUED) * On Semiconductor CS4192 surface mount package (DISCONTINUED) * On Semiconductor CS8190 DIP and SOIC surface mount packages * MLX10407 * Melexis MLX10420

References