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SOC-based high-precision angle measuring system

In Electronic Infomation Category: S | on April 13,2011

Abstract: In order to solve the automatic level control system and UPC1490HA datasheet and engineering applications, angle measurement cost, low precision, proposed a two-axis tilt sensor using MEMS, signal conditioning circuits, and UPC1490HA price and SOC and UPC1490HA suppliers and other high-precision angle measurement method the stability of the sensor signal from the processing, temperature compensation, signal acquisition and processing, reference design and signal curve fitting method to achieve the inclination angles of low-cost, high-precision measurements. Experiments show that the system is less than the maximum absolute error is 0.005 , the relative error is less than 0.02%.

In geological exploration, equipment installation, road and bridge construction and other engineering applications, and robot control, tanks, artillery platforms and ship control, aircraft attitude control system, automatic level regulation, require high-precision angle measurement. But the high-precision angle measurement devices are usually bulky, high cost and limited for many engineering applications. This departure from the angle of the high precision measurement, focusing on the stability of the tilt sensor output processing, temperature compensation, non-linear processing (sine curve fitting), signal conditioning and measurement circuit of the special treatment.

1 inclination angle measurement system hardware design

Inclination angle measurement system hardware mainly by the MEMS sensor (with dual-axis tilt sensor and temperature sensor), SOC circuits, data processing and transmission, and other auxiliary circuit module. Angle measurement system block diagram shown in Figure 1.


Figure 1 angle measurement system block diagram

1.1 MEMS Inclination Sensor Interface

MEMS tilt sensor by VTI Technologies, Finlands SCA100T series SCA100T-D01, measuring range of 30 . SCA100T series is the use of micro-electromechanical systems (MEMS) technology to produce a high-resolution dual-axis tilt sensor. SCA100T-D01 digital output resolution of 0.035 / LSB, the analog output resolution of 0.002 5 . Analog output resolution is much higher than the resolution of the digital output, so the design uses the analog output. Analog output will involve a more complex analog signal processing, analog signal if not handled properly, the system resolution and accuracy will be greatly reduced, and sometimes not as good as digital output. Using reasonable analog signal processing circuits is one of the ways to ensure system accuracy.

SCA100T-D01 built-in temperature sensor, through its own SPI digital interface to read temperature and processor temperature compensation accordingly. This is another way to ensure system accuracy.

1.2 impedance matching and amplification

SCA100T-D01 output impedance is 10 k, to ensure the MEMS tilt sensor output signal SCA100T-D01 effective communication, which requires the minimum attenuation, designed with high input impedance FET op amp TL081-type design impedance matching circuit, using the same phase input, and increase the input impedance.

Signal amplification circuit ICL7653 chopper-stabilized op amp to complete, as shown in Figure 2. ICL7653 has a very low offset voltage and bias current, high stability and excellent work of high precision zoom function. ICL7653 chopper-stabilized using an internal clock, in the CA, CB and CR with 0.1 F between the end of the low discharge and high stability of the polyester or polypropylene capacitors. Dual power access terminal at the same time filtering and decoupling processing.


Figure 2 signal amplification circuit

1.3 differential conversion and drive

Shown in Figure 3, the differential conversion circuit AD8138AR as the core, the single-ended signal conversion differential signal, not only can improve the common mode rejection ratio, effectively reduce the impact of common mode signals, but also can drive the internal 24-bit differential SOC Sigma-Delta A / D converter. AD8138AR have a wide analog bandwidth (320 MHz, -3 dB. A gain of 1), and surface mount devices AD8138AR, the device small, making the ADC input signal to close the distance, greatly reducing the external noise impact.


Figure 3 differential conversion circuits

1.4 SOC microcontroller resource allocation

Use this design as the Silicon Labs C8051F350 companys processing core. C8051F350 is really able to work independently of the system on a chip (SOC), it comes with 8K bytes of Flash memory can be programmed in the system; integrates a fully differential 24-bit Siva-Delta A / D converter (ADC), the ADC has on-chip calibration, two independent digital decimation filter can be programmed to 1 kHz sampling rate; with 2-way and 1 channel UART SPI interface. And other types of micro-controller to achieve the same combination of features that require multiple chips to complete than, C8051F350 not only reduces system cost and system size, but also improves the reliability of the system. C8051F350

used in the design of the 24-bit Sigma-Delta A / D converter for analog to digital conversion system signals, SPI interface for MEMS tilt sensor the temperature acquisition in order to achieve temperature compensation sensor, UART for serial LED display interface. To ensure the A / D converter is stable, the use of external reference source.

1.5 ADC reference and sensor power supply SCA100T

MEMS tilt sensor in the inclination of 0 , the power supply voltage for analog output 1 / 2 times, if the angle sensor power supply voltage fluctuations, its output will produce a corresponding fluctuations. Therefore the design, ADC will provide reference circuit output (see Figure 4), after increased drive capability, the provision of a MEMS tilt sensor SCA100T for power supply (see Figure 5). On the one hand, reference output ripple is extremely small, and stable performance; the other hand, the ADC reference and power MEMS tilt sensor SCA100T changes in the same direction at the same time, MEMS tilt sensor offset caused by zero drift due to power impact.


reference circuit in Figure 4

Figure 4 LM236 reference voltage of 2.5 V output through the rail to rail op amp OPA340 circuit consisting of treatment to follow, increasing the drive capability, both as the ADC reference circuit, but also for the center differential to provide the voltage conversion circuit and MEMS tilt sensor SCA100T power input.

Figure 5 to Figure 4, the input reference voltage (VREF) output. Low drift, high stability of the composition of the op amp OPA340 op amp circuit to provide power to the tilt sensor SCA100T, to ensure the power supply ripple, stable.


Figure 5 SCA100T power circuit

2 signal mathematical treatment

2.1 ADC precision control

C8051F350 has two internal independent decimation filter (SINC3 filters and fast filters), and a programmable gain amplifier. Reference SINC3 filter according to RMS noise, high accuracy, low output rate disadvantage, and quickly filter on the contrary. The design requirements of rates low, while high precision, so we choose SINC3 filter. SINC3 typical RMS noise filter shown in Table 1. Table 1 SINC3 filter

typical RMS noise


From Table 1, the higher the conversion taking longer than the cycle, the output word rate is low, but with lower noise.

Under reference, using SINC3 filter, the A / D converter, the actual resolution:


where full-scale input range is:


Formula based on the actual resolution (1) shows that, when the extraction ratio is 1 920, the output word rate of 10 Hz, according to the actual resolution of the formula (1) get the actual resolution of about 20.00.

SCA100T sensitivity of the sensor is 70mV /(), resolution of 0.0025 , ADC reference voltage VREF is 2.5V, you need to detect the smallest signal is 0.0025 x70mV / = 0.175 mV, 0.175 mV under / 2.5 V = 1 / 14 286 known, ADC number of bits should be at least 14, that is, 214 = 16384> 14 286, according to reduce the amount of design requirements, with 20 bits, so the design meets the design requirements.

2.2 Temperature Compensation

Under reference, SCA100T-D01 temperature error curve shown in Figure 6.


Figure 6 SCA100T-D01 temperature error curve

By curve fitting, the curve equation is:


After collection in the signal through the ADC, is converted to angle output, according to real-time acquisition of the tilt sensor SCA100T the temperature value, temperature compensation curve can be compensated according to the corresponding point values, the impact of temperature on the angle measurement to a minimum.

2.3 Curve Fitting

The SCA100T series tilt sensor output and the nonlinear relationship (non-linear error in the measurement range is 0.11 ), this is not conducive to analysis and processing measurement results. Therefore, the linearization must be taken measures to compensate for the introduction of the nonlinear sensor. Most of the traditional method using the hardware method, more complex implementations, and the stability and reliability difficult to control.

The nonlinear characteristics SCA100T series of sensors is known, you can use the appropriate calibration function to compensate. As a function of the microprocessor has a strong computing and data processing capabilities, with the programming method can easily achieve the desired correction function. This design uses software programming method SOC modified nonlinear.

In the design, the SCA100T-D01 sensor measurement range is further broken down, such as the inclination of 3 of this curve is divided into 2.5 ~ 3.5 , and fitting curve for the next change type:


Where, XIN ADC output value for the filter is filtered by the internal SINC3 obtained samples, TER SCA100T sensors for the real-time temperature compensation value, PI is pi.

Both on the type of nonlinear correction of the sensor output, and correction of temperature on the sensor.

3 measured data

The angle measurement system in MC019-JJ2 Digital 2 "optical dividing head on a sub-standard equipment and performance testing. MC019-JJ2 Digital 2" optical dividing head is a kind of clamping in the spindle angle of the workpiece on the sub-degree angle or testing of precision optical measuring instruments, the display equivalent to 1. "test data shown in Table 2.

Table 2 Test Data


Test data from Table 2 shows, the deviation of each test point has both positive and negative, mainly due to the curve fitting of these test points are independent, independently of each other. In addition, the maximum absolute error at 30 , the largest absolute error is 0.0044 , 1 while the maximum relative error for 0.001 8 / 1 0.018%.

4 Conclusion

MEMS tilt sensor SCA100T this paper, the analog interface output, the use of its digital interface temperature compensation at the same time reference and the op amp used as a sensor-driven power, improved accuracy and stability of the sensor output; signal processing, Treatment with low drift op amp circuit and differential analog-digital conversion circuitry, to effectively improve the signal to noise ratio and common mode rejection ratio; sine curve fitting, effectively improve the output linearity. After the above various signal processing and optimization, within the measurement system has a maximum absolute error is 0.004 4 . And a higher degree of system integration, small size, low cost, to meet the geology of oil exploration, equipment installation, road and bridge construction and other engineering applications, and robot control, * and ship artillery platform control, aircraft attitude control system, automatic level adjustment application.

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