An air core gauge is a specific type of rotary actuator

Rotary actuator

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

 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".

Background

There are four common types of rotary actuators :

• Physical gauges, in which the needle is attached directly to the value being measured; for example, a mechanical pressure gauge
• Analog volt meters or d'Arsonval movements, which consist of a coil and a permanent magnet
• Stepper motor
  Stepper motor
  A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
  
  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

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 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

Sine

In mathematics, the sine function is a function of an angle. In a right triangle, sine gives the ratio of the length of the side opposite to an angle to the length of the hypotenuse.Sine is usually listed first amongst the trigonometric functions....

 coil and the cosine coil.

Theory

The direction of the overall magnetic field is approximately:

Where and are the coils' respective currents. The permanent magnet aligns itself with that field, eventually settling near . 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 50 mA and the cos current is 29 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. Driver integrated circuit

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s typically 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
  Electrical polarity
  Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...
  
   of the voltage
  Voltage
  Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
  
  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
• On Semiconductor CS4192 surface mount package
• Melexis MLX10407
• Melexis MLX10420