Application of Electrolyte Tilt Sensor in Antenna Control

1. Introduction

Sensors are devices or equipment that can sense the specified measured quantity and convert it into a usable output signal according to certain rules. As a key basic component of information systems, they have received extensive attention at home and abroad in recent years. As one of the classic sensors, the tilt sensor is also being replaced by new materials, new principles, multi-functions, and new structures. It is increasingly integrated with digital technology and communication technology, and is developing towards integration, intelligence, and miniaturization.

Figure 1

2. Tilt sensor principle

In order to measure the inclination angle of the measured object and the standard horizontal plane, an electrolyte sensor is often used. The figure shows a schematic diagram of a dual-axis sensor in a single axis when it is slightly tilted. The sensor is composed of a sealed cylinder, and the cylinder is filled with a fluid medium of about half of the entire capacity. The electrolyte is a viscous liquid. Electrodes are installed in the cylinder and immersed in the electrolyte. Each electrode has a pin lead. When the sensor is tilted, the liquid surface remains horizontal due to gravity, and the conductivity between the two electrodes is proportional to the length of the electrode immersed in the liquid. For example, at the inclination angle shown in the figure, the conductivity between electrodes a and b is greater than the conductivity between electrodes b and c. It can be seen that in terms of electrical characteristics, the sensor is similar to a voltage divider, and the change in impedance is proportional to the angle of inclination. The relationship between the sensor output signal and the change in the inclination angle is shown in Figure 2. Note that when the inclination angle is greater than 20°, the output signal becomes nonlinear. It can be proved that the inclination range that the sensor can measure is a function of the electrolyte capacity, the electrode spacing and the electrode length. The sensor is similar to a lead-acid battery in some ways. The current can cause a chemical reaction in the electrolyte, which eventually causes the electrolyte to lose its conductivity. Therefore, in order to prevent the occurrence of electrolytic reactions, the sensor excitation must be an alternating current with a sufficiently high frequency. For some electrolytes, this frequency can be 25Hz, while some electrolytes need to reach 1000Hz to 4000Hz.

Figure 2 Sensor output characteristics

3. Application of Tilt Sensors in Shipborne Antenna Control

3.1 The ship's forward movement and the turbulence of the waves will cause the shipborne antenna to tilt randomly, so in order to ensure that the antenna can continuously and accurately track the satellite, the antenna axis frame must be adjusted in real time. Since the rotation control of the antenna includes not only azimuth and pitch, but also a vertical plane of pitch (cross level), three angular velocity sensors for detecting motor speed and one tilt sensor for detecting horizontality are required. As shown in the figure:

Figure 3 Schematic diagram of shipborne satellite antenna

3.2 Sensor parameters and applications

Measuring range ±45°; input voltage +5v; output +1~4vDC or 4~20mA; resolution 0.01°; nonlinearity ±2°; operating temperature -40°C~+80°C; impact resistance 1000g, 1msec.

The dual-axis sensor has similar properties to the single-axis sensor, but also has its own complexity. Since the dual-axis shares the central electrode, the four peripheral electrodes are ideally distributed at the four corners of the square, so two methods are used to measure each axis independently: one is to excite only one axis at the same time, and the other is to load the dual-axis with different frequencies of excitation at the same time. As shown in the figure, the excitation signal frequency between electrodes a and c is twice that between electrodes d and e. Note that the two orthogonal axes in method one are the diagonal ac and de directions, while the two orthogonal axes in method two are the edges ae and ad directions of the peripheral electrode square.

Figure 4 Peripheral electrode waveform

3.3 Sensor interface circuit

Figure 5 Input circuit diagram

As can be seen from the figure, since the sensor output is a weak analog signal, the analog output of the sensor must be pre-processed, also known as signal conditioning, and converted into digital quantity through A/D conversion before the processor can analyze and process it. Specifically for the electrolyte type tilt sensor, taking a certain type of shipborne antenna as an example, the actual application circuit is shown in the figure below:

Figure 6 Sensor application circuit

In the figure, U5 constitutes the inverting amplifier circuit of the sensor output CTR terminal signal. F1 and F2 come from the processor output port control signal, which is a square wave with a frequency of 50HZ and a phase difference of 180°. After the inverter, it is used as the LV and CL electrode drive of the sensor, which can not only realize the alternating change of the signal polarity on each pair of electrodes, but also provide the option of horizontal and vertical two-dimensional tilt measurement. F1 and F2 also act on the control terminals A and B of the multi-channel input selector U6, corresponding to the change of the signal polarity on each pair of electrodes, and select the signal of the corresponding polarity as the output.

4. Conclusion

Electrolyte type inclination sensor has good reproducibility, reliability and high precision. In application, special attention should be paid to the following: ⑴ The driving signals F1 and F2 must be AC ​​voltage signals with zero DC component, because DC will cause the electrolyte to produce electrolysis reaction and lose conductivity, causing irreversible damage to the sensor. ⑵ Avoid using wave soldering and chemical organic solvent washing to prevent changes in sensor output characteristics and electrolyte leakage.

The author's innovation: To ensure reliable operation, a CMOS inverter should be connected between the processor port pin and the sensor. The microprocessor can be set to wake up once or several times per second to make a new measurement, while sampling the midpoint voltage of the drive signal as a reference. In this way, each measurement is completed in two steps: first, the value of the sensor signal minus the reference signal is calculated, then the inverted drive signal is added and the value of the reference signal minus the sensor signal is calculated. The two measurements are subtracted to obtain twice the required tilt value and the deviation generated by the system is offset.