The function of the liquid level sensor is to pick up the liquid level height information in the confined space and transform it into a quantity that has a definite function relation with the measured and is convenient for transmission and processing.In construction machinery, the level sensor is usually used to detect the amount of fuel in the tank and display it to the user through the level gauge.The commonly used liquid level sensors are generally divided into the following four categories.

I. Variable resistance type liquid level sensor (i.e. oil float type liquid level sensor)

Figure 1 shows the appearance of this type of sensor.It makes use of float and connecting rod to determine the level of liquid in a mechanical way to make the instrument move.Variable resistor type liquid level sensor is float, the built-in sliding resistance of ontology, and connection of the float arm that float level with the upper and lower movement, the arm is then in a sliding resistance, thus changing the resistance value between iron and float, take advantage of this resistance changes to the size of the current control loop, and displayed on the meter.

1, float 2, float arm 3, sliding resistance 4, terminal

FIG. 1 Variable resistance level sensor

Two, dry spring pipe type liquid level sensor

As shown in Figure 2, the sensor is composed of a shaft rod formed by a circular control and a buoy that can be moved up and down along the shaft rod.The shaft contains a circuit board consisting of a reed pipe and a resistor, and the buoy contains a permanent magnet.The inside of the reed pipe is a pair of thin metal contacts. When a permanent magnet approaches the reed pipe, many magnetic lines of force pass through the pipe, so the contacts close.The shaft goes from bottom to top, with many reed pipes, so that the contacts close or break depending on the buoy position.As can be seen from the internal circuit diagram, with different position changes, different spring closure, so that the resistance value in the circuit is different.The variation of resistance value in the circuit is used to control the different display of meter readings.

Connection panel 3, shaft rod 4, buoy 5, resistance

6. Reed pipe

FIG. 2 Reed pipe level sensor

Iii. Capacitive level sensor

According to the principle of capacitance, parallel to each other, each other at very close but two parallel plates of the insulation, constitute a capacitor, the capacitor of capacitance changes over liquid level, with level changes corresponding to the change of capacitance JingBian send circuit of current signal is converted into a standard, can be output to the instrument shows the corresponding level.

Δ = C (0.885).Epsilon, Δ S/d

(In the above formula: 0.885 is the calculation constant)

Is the dielectric constant of the measured medium

Δ S surface area for further media

D is the distance between the poles

In the above formula, if the invariants are ignored, only the distance into the medium causes the change.So the capacity varies with the height of the material.

Ultrasonic level sensor

The sensor is based on the theory of ultrasonic propagation.When ultrasonic waves travel through the air, they are severely attenuated.Conversely, the sound wave is greatly enhanced when it travels through a liquid.The electronic control unit emits a series of electrical signals, which are converted by the sensor into ultrasonic energy pulses and propagated in the detected area.When the sound wave hits the liquid level, it returns.By calculating the round trip time, the height of the liquid level can be obtained.The sensor has a wide range of applications and can be used in liquids with high viscosity or low specific gravity, including some that are difficult to handle.

Variable resistance level sensors are generally used in construction machinery vehicles, but due to the high vibration frequency of construction machinery vehicles, the damage rate of variable resistance level sensors is relatively high.At present, the spring pipe type liquid level sensor is used in the vehicle. It has strong impact resistance, which reduces the failure rate and achieves good results.