Design of high-precision tilt measurement system based on SOC

High-precision inclination measurement is required in engineering applications such as geological and petroleum exploration, equipment installation, road and bridge construction, as well as in the automatic leveling of systems such as robot control, tank and ship artillery platform control, and aircraft attitude control. However, high-precision inclination measurement equipment is usually large in size and high in cost, which limits many engineering applications. Starting from the high-precision measurement of inclination, this article focuses on the inclination sensor output stability processing, temperature compensation, nonlinear processing (sinusoidal curve fitting), signal conditioning, and special processing of its measurement circuit.

1 Hardware Design of Inclination Measurement System

The hardware part of the inclination measurement system is mainly composed of MEMS sensors (including dual-axis inclination sensors and temperature sensors ), SOC circuits, data processing and transmission, and other auxiliary circuits. The block diagram of the inclination measurement system is shown in Figure 1.

1.1 MEMS tilt sensor interface

The MEMS inclination sensor uses the SCA100T-D01 in the SCA100T series of VTI Technologies of Finland, with a measurement range of ±30°. The SCA100T series is a high-resolution dual-axis inclination sensor manufactured using micro-electromechanical system (MEMS) technology. The digital output resolution of SCA100T-D01 is 0.035°/LSB, and the analog output resolution is 0.002 5°. The resolution of analog output is much higher than that of digital output, so this design uses its analog output. Analog output will involve more complex analog signal processing. If the analog signal is not processed properly, the resolution and accuracy of the system will be greatly reduced, and sometimes even worse than digital output. Using a reasonable analog signal processing circuit is one of the ways to ensure system accuracy.

SCA100T-D01 has a built-in temperature sensor, which can read the temperature value through its own SPI digital interface and perform corresponding temperature compensation in the processor. This is another way to ensure system accuracy.

1.2 Impedance matching and amplification

The output impedance of SCA100T-D01 is 10 kΩ. In order to ensure that the signal output by the MEMS inclination sensor SCA100T-D01 is effectively transmitted, that is, the attenuation is required to be minimal, the impedance matching circuit is designed using the field effect tube type operational amplifier TL081 with high input impedance, and the same-phase input is used to increase the input impedance.

The signal amplification circuit is completed by using the ICL7653 chopper-stabilized zero operational amplifier , as shown in Figure 2. The ICL7653 has extremely low offset voltage and bias current, high working stability and excellent high-precision amplification function. When the ICL7653 chopper-stabilized zero uses the internal clock, a 0.1 μF low-discharge, high-stability polyester or polypropylene capacitor is added between the CA, CB and CR terminals. At the same time, filtering and decoupling are performed at the dual power supply access terminal.

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1.3 Differential conversion and drive

As shown in Figure 3, the differential conversion circuit uses AD8138AR as the core, converting single-ended signals into differential signals, which can not only improve the common-mode rejection ratio and effectively reduce the impact of common-mode signals, but also drive the 24-bit differential Sigma-Delta analog/digital converter inside the SOC . AD8138AR has a wide analog bandwidth (320 MHz, -3 dB. When the gain is 1), and AD8138AR is a surface-mount device with a small size, so that the distance between the ADC and the signal input point can be very close, greatly reducing the impact of external noise.

1.4 SOC microcontroller resource allocation

This design uses Silicon Labs' C8051F350 as the processing core. C8051F350 is a truly independent system-on-chip (SOC). It has an 8K-byte Flash memory and can be programmed in the system. It integrates a fully differential 24-bit Siva-Delta analog-to-digital converter (ADC) with on-chip calibration function. Two independent digital decimation filters can be programmed to a sampling rate of 1 kHz. It has two UARTs and one SPI interface. Compared with other types of microcontrollers that require a combination of multiple chips to achieve the same function, C8051F350 not only reduces system cost and system size, but also greatly improves system reliability.

The design uses C8051F350's 24-bit Sigma-Delta analog/digital converter for analog-to-digital conversion of system signals, SPI interface for temperature acquisition of MEMS tilt sensor to achieve temperature compensation of sensor, and UART as serial LED display interface. To ensure the stability of analog/digital converter, an external reference source is used.

1.5 ADC reference source and sensor power supply

When the inclination angle of the MEMS inclination sensor SCA100T is 0°, the analog output is 1/2 times of its power supply voltage. If the power supply voltage of the inclination sensor fluctuates, its output will fluctuate accordingly. Therefore, when designing, the output of the reference source is provided to the analog-to-digital conversion circuit (as shown in Figure 4), and after improving the driving capability, it is provided to the MEMS inclination sensor SCA100T as a power supply (as shown in Figure 5). On the one hand, the output ripple of the reference source is extremely small and the performance is stable; on the other hand, the reference source of the analog-to-digital converter and the power supply of the MEMS inclination sensor SCA100T change in the same direction at the same time, offsetting the influence of the zero drift of the MEMS inclination sensor caused by the power supply.

The 2.5 V voltage output by the reference source LM236 in Figure 4 is processed by the follower circuit composed of the rail-to-rail operational amplifier OPA340, which increases the driving capability. It serves as the reference source of the analog-to-digital conversion circuit, and also provides the center voltage for the differential conversion circuit and the power input of the MEMS inclination sensor SCA100T.

The input in Figure 5 is the reference voltage (VREF) output in Figure 4. The op amp circuit composed of the low drift, high stability op amp OPA340 provides power to the inclination sensor SCA100T, which can ensure small power ripple and stable operation. [page]

2 Mathematical processing of signals

2.1 ADC Accuracy Control

The C8051F350 has two independent decimation filters (SINC3 filter and fast filter) and a programmable gain amplifier . According to the reference, the SINC3 filter has low RMS noise and high precision, but the disadvantage is that the output rate is low, while the fast filter is the opposite. This design has low speed requirements and high precision requirements, so the SINC3 filter is selected. The typical RMS noise of the SINC3 filter is shown in Table 1.

As can be seen from Table 1, a higher decimation ratio requires a longer conversion period, ie, a lower output word rate, but with lower noise.

According to the reference, when using the SINC3 filter, the actual resolution of the analog/digital converter is:

According to the actual resolution formula (1), when the decimation ratio is 1920 and the output word rate is 10 Hz, the actual resolution is about 20.00 bits.

The sensitivity of SCA100T sensor is 70mV/(°), the resolution is 0.0025°, and the ADC reference voltage VREF is 2.5V. The minimum detectable signal is 0.0025°x70mV/°= 0.175 mV. According to 0.175 mV/2.5 V="1"/14 286, the ADC bit number should be at least 14 bits, that is, 214=16 384>14 286. According to the derating design requirements, 20 bits are used, so this design fully meets the design requirements.

2.2 Temperature compensation

According to the reference, the temperature error curve of SCA100T-D01 is shown in Figure 6.

Through curve fitting, the curve equation is:

After the signal is collected by the analog-to-digital converter and converted into an angle output, the corresponding angle value can be compensated according to the temperature compensation curve based on the real-time collected temperature value of the inclination sensor SCA100T, thus minimizing the impact of temperature on inclination measurement. [page]

2.3 Curve Fitting

Since there is a nonlinear relationship between the output of the SCA100T series sensor and the tilt angle (the nonlinear error is 0.11° within the measurement range), it is not conducive to analyzing and processing the measurement results. Therefore, corresponding linearization measures must be taken to compensate for the nonlinearity introduced by the sensor. Most of the traditional methods use hardware methods, which are relatively complex to implement and difficult to control stability and reliability.

Since the nonlinear characteristics of the SCA100T series sensors are known, the corresponding correction function can be used for compensation. Since the microprocessor has strong function calculation and data processing capabilities, the required correction function can be easily implemented by programming. This design uses SOC to correct nonlinearity through software programming.

During the design, the measurement range of the SCA100T-D01 sensor was further subdivided, such as dividing the curve with an inclination angle of 3° into 2.5°~3.5°, and modifying the fitting curve to the following formula:

Where XIN is the sampled value obtained after the output value of the analog-to-digital converter is filtered by the internal SINC3 filter, TER is the real-time temperature compensation value of the SCA100T sensor, and PI is the pi.

The above formula not only corrects the nonlinearity of the sensor output, but also corrects the effect of temperature on the sensor.

3 Measured data

The inclination measurement system was indexed and tested on the MC019-JJ2 digital 2" optical indexing head standard instrument. The MC019-JJ2 digital 2" optical indexing head is a precision optical metrology instrument for angular indexing or angle inspection of workpieces clamped on its spindle, and its display equivalent is 1". The test data is shown in Table 2.

From the test data in Table 2, we can see that the deviation of each test point is positive and negative. The main reason is that the curve fitting of these test points is independent and does not affect each other. In addition, the absolute error is the largest at 30°, with the maximum absolute error being 0.0044°, and the relative error is the largest at 1°, which is 0.001 8/1≈0.018%.

4 Conclusion

This paper adopts the analog interface of MEMS inclination sensor SCA100T as output, and uses its digital interface to realize temperature compensation. At the same time, the reference source and op amp driver are used as the power supply of the sensor to improve the accuracy and stability of the sensor output; when processing the signal, the low drift op amp processing circuit and the differential analog-to-digital conversion circuit are used to effectively improve the signal-to-noise ratio and common mode rejection ratio of the signal; the sine curve fitting is used to effectively improve the linearity of the signal output. After the above multi-faceted signal processing and optimization, the maximum absolute error of the system within the measurement range is 0.004 4°. In addition, the system has a high degree of integration, a small size, and a low cost. It can meet the engineering applications such as geological and petroleum exploration, equipment installation, road and bridge construction, as well as the automatic level adjustment applications of robot control, tank and ship artillery platform control, and aircraft attitude control systems.