An air core gauge is a specific type of rotary actuator 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 pre ...

* 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 ...

Stepper motor A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can be commanded to move and hold at one of these steps without any posi ...

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 (stre ...

flows through the perpendicular coils, their magnetic fields 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 that is opp ...

coil and the cosine 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 or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

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 electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

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 * Melexis MLX10407 * Melexis MLX10420

References