What Is a Liquid Compass?

Magnetic compass is an instrument used to indicate bearing based on the principle of compass. It is also called magnetic compass. It is mainly composed of several magnetic needles, scales and magnetic error correction devices arranged in parallel. The magnetic needles are fixed on the back of the scales. Under the influence of geomagnetism, the magnetic needles rotate with the scales to indicate the direction. Often used for navigation on ships and aircraft. In the 13th century, the maritime industry developed, and magnetic compass navigation was gradually adopted, and there was a record of "needle paths", indicating the direction that shipping lines should take. Earlier airplanes had magnetic compasses installed, but because the magnetic field interference caused by steel components and electrical equipment on the aircraft had a large effect, compensation methods must be used to offset the aircraft's magnetic field interference. There are two basic types of aviation magnetic compasses. Direct reading type: The advantage is simple and reliable, but because it is installed in the cockpit with large magnetic interference, the error is large. Remote reading type: The magnetic compass is changed to a magnetic heading sensor, installed outside the cockpit, and the detected magnetic heading information is sent to the dashboard of the cockpit for display at a long distance. The advantage is that the magnetic sensor can be installed on the aircraft. Location with less interference. The magnetic compass will produce a large error when the aircraft is flying at non-uniform speeds or turning, and it is not suitable for use in areas with magnetic anomalies and high latitudes. Therefore, the far-reading magnetic compass on modern aircraft has been replaced by a more complete gyro magnetic compass Or the heading system.

Magnetic compass

The main technical parameters:

Heading measurement range: 0 ~ 360 °.

Tilt measurement range: 0 ~ 180 °.

Heading accuracy: ± 1.5 °.

Inclination accuracy: ± 0.2 °.

Stabilization time: 30 seconds.

Dimensions: 80mm × 60mm × 20mm (L × W × H) or customized.

Power requirements

Working power: DC 12V.

Working current: 100mA.

Output Interface

Communication protocol: RS485, RS232 or customized.

Working environment requirements:

Working temperature: -40 + 50 .

Impact: Triaxial 50g, 11ms, half sine.

Random vibration: 0.04g / Hz, 5 minutes.

The magnetic compass uses the directivity inherent in the geomagnetic field to measure the attitude angle of space. It can measure three-dimensional attitude data of the carrier: horizontal heading, pitch, and roll. It can be widely used in the occasions that need to obtain the attitude angle of the platform (or carrier), such as marine, oil drilling, underwater platform operation, aircraft attitude measurement, robot control and other fields. The digital magnetic compass has the advantages of small size, high heading accuracy, wide tilt range, high frequency response, and low power consumption. It is very suitable for occasions that have high requirements on heading accuracy but also have restrictions on power consumption and volume.

The geomagnetic north and south poles do not coincide with the geographic north and south poles. The direction of the freely hanging magnetic needles has a deviation angle from the geographic north-south (true meridian) direction, which is the magnetic declination (also called magnetic difference). The specially-developed navigation map is marked with the correction amount of magnetic difference of each place. The magnetic heading reading is added to the magnetic difference algebra to get the true heading of the aircraft. The angle between the geomagnetic field strength vector and the horizontal plane is called the magnetic inclination. The magnetic inclination angles are not equal everywhere, and the closer to the geomagnetic pole, the greater the magnetic inclination angle. The geomagnetic horizontal component is weak in high latitudes, and it has little effect on the orientation of horizontally suspended magnetic rods. Therefore, the magnetic compass cannot be applied in the regions of the earth's poles.

In an aviation magnetic compass, a pair of permanent magnetic rods are suspended under a floating ball and have a hem-type structure to keep the magnetic rods horizontal. The floating ball is supported on a gem bowl with a shaft tip and suspended in a floating liquid (ethanol, light kerosene or other compass oil) to reduce friction with the supporting part. An azimuth scale ring is connected with the magnetic rod and the floating ball. The reading of the heading line representing the direction of the nose relative to the azimuth scale ring is the heading of the aircraft. The volume of the floating liquid will expand and contract when the ambient temperature changes, so the expansion chamber is used to compensate. Due to the interference of the magnetic field formed by the steel components and electrical equipment on the aircraft, the magnetic rod will be stable at the orientation of the combined magnetic field (compass meridian) of the geomagnetic horizontal component and the aircraft magnetic field horizontal component. The deviation angle between the compass north and magnetic north is called the compass. The difference in the values of the same aircraft in different directions varies. In order to correctly measure the magnetic heading of the aircraft, a compass correction is installed on the outer shell of the magnetic compass, which can be used to generate an artificially adjustable compensation magnetic field to reduce or cancel the interference effect of the aircraft's magnetic field. The magnetic compass should be installed in a place with weak magnetic interference inside the machine.

There are 2 basic types of aviation magnetic compasses:

Direct reading type: The advantage is simple and reliable, but because it is installed in the cockpit with large magnetic interference, the difference is large.

Remote reading type: The magnetic compass is changed to a magnetic heading sensor and installed outside the cockpit, and the detected magnetic heading information is transmitted to the heading indicator and displayed at a long distance. Its advantage is that the magnetic sensor can be installed on the aircraft where the magnetic interference is small, such as the wing tip, the tail, etc., to reduce the difference. The common disadvantages of these two types of magnetic compasses are: when the aircraft is flying at non-uniform speed, turning, or hovering, because the magnetic sensitive element deviates from the horizontal plane and is affected by the vertical component of geomagnetism, it will produce large acceleration errors and turning errors; the aircraft's course changes Due to the damping effect of the liquid on the magnetic sensitive elements, a significant hysteresis error occurs; the magnetic compass is not suitable for use in areas with magnetic anomalies and high latitudes. The gyro magnetic compass successfully solved these problems.

The direct-reading magnetic compass is simple and reliable. In addition to being used in small aircraft, it is used as an emergency instrument in many aircraft. The far-reading magnetic compass has been replaced by a more complete gyro magnetic compass or heading system.

In addition to magnetic rods, there are also magnetic induction elements commonly used in magnetic compasses. This kind of element has high measurement sensitivity, can directly output electrical signals, and is suitable for long-distance transmission, so it has been widely used in magnetic sensors. The magnetic induction element is composed of a soft magnetic material iron core, an excitation coil and a measurement coil wound thereon. The excitation coil is connected to an AC power source, and the magnetic field formed by the alternating current repeatedly magnetizes the iron core, thereby periodically changing the magnetic permeability of the iron core. At this time, even if a constant geomagnetic field acts on the iron core, the geomagnetic flux generated will be alternating. Under the effect of the geomagnetic alternating magnetic flux, an induced potential will be generated across the measuring coil. When the sensing element is parallel to the geomagnetic field, the induced potential is the largest; when the sensing element is perpendicular to the geomagnetic field, the induced potential is the smallest. Therefore, magnetic induction elements can be used to determine the orientation of the horizontal component of the geomagnetic field.

Normally, the magnetic declination of the compass needs to be corrected before using it. ^[1] The key problem of correction is to judge the direction of the clarity dial correction for two different situations of the magnetic declination. If magnetic north is east of the coordinate north, the coordinate azimuth is greater than the magnetic azimuth. At this time, dial 0 ° of the dial clockwise to increase the count at the north end of the magnetic needle after aiming at the target; if the magnetic north is off to the west of the coordinate north, the coordinate azimuth is smaller than the magnetic azimuth. At this time, you should turn the 0 ° dividing line of the dial counterclockwise to reduce the reading at the north end of the magnetic needle after aiming at the target. The magnetic azimuth of the straight line can be directly measured after the magnetic compass that has been corrected over the circle.