1、附录 A Transducer and sensor excitation and measurement techniques Many of todays industrial and instrumentation applications involving sensor. The function of the sensor system is to monitor changes, and then this data back to the main controller. For a simple voltage or current measurement sensors m
2、ay be resistance in nature. However, some sensor system may be inductive or capacitive in nature, that is to say, the frequency range of the sensor resistance change is nonlinear. Impedance sensors such typical example is the proximity sensor - a campaign for the detection of the relative distance o
3、f objects； In addition, capacitive sensors or sensor sensibility - in the medical devices used to measure blood flow or blood pressure or blood qualitative analysis. In order to use these complex impedance sensors, the realization of measurement, to provide an exchange of (AC) excitation source freq
4、uency in the frequency range of the sensor for scanning. This article attempts to explain how the use of single-chip digital waveform generator to easily achieve this up to 10 MHz frequency scanning. Also introduced a kind of integrated incentive, response and digital signal processor (DSP) features
5、 a complete single-chip sensor solution that suited the requirements of up to nearly 50 kHz frequency applications. Sensors: working principle. Inspired by the frequency of sensor signals based on sensor values of L or C to show the corresponding instantaneous magnitude, frequency or phase changes.
6、For example, the ultrasound will show a flow of phase offset, while the proximity sensor will cause the rate to change. Tracking changes in impedance that is the most commonly used to monitor the resonant frequency circuit. Capacitance value of the resonant frequency is equal to the frequency where
7、the inductance value point. This is also the largest frequency curve frequency impedance value of the corresponding point. Under normal circumstances, for example, in static conditions, the sensor L, R and C has a unique value, in the resonant frequency impedance to Department with the greatest valu
8、e. When a moving object near the sensor, then sensor L and C values will be changed and a new resonant frequency. By monitoring the changes in resonant frequency (and thus lead to changes in impedance), it is possible to speculate that the relative movement of objects moving away from the sensor. Ca
9、lculated resonant frequency: calculation circuit measuring the resonant frequency of the need for the relationship between frequency and impedance, in particular, need a certain frequency range with the ability to scan the waveform generator. A simple, low-cost method is based on the AD5930 waveform
10、 generator. AD5930 with a group of pre-set in the frequency range of the ability to provide a linear scan. Once the conditions for setting, on the need for further control, in addition to a frequency scan for the start of the trigger. AD5930 has many advantages: the output frequency resolution of 28
11、 bit, so you can be less than the control accuracy of 0.1 Hz output frequency. The output frequency range of 0 10 MHz, thus the selection of sensors with a lot of flexibility. For example, some sensors a very narrow frequency range, but the requirements in this frequency range with high resolution.
12、Some sensors may require a wide frequency range, but lower resolution requirements. This approach is easy to calculate the resonant frequency of the sensor. System block diagram: typical block diagram of such a system as shown in Figure 3. Through the BF-535 DSP processor AD5930 digital waveform gen
13、erator set. AD5930 needs arising from the sinusoidal output voltage waveform for low-pass filtering and amplification in order to eliminate the master clock (MCLK), mirroring the frequency and high frequency noise generated by feed through. After filtering the sensor signal can be used as a source o
14、f excitation frequency. According to the impedance of the sensor response signal amplification may be needed in order to enter the ADC (ADC) dynamic range. Sensor output and the frequency of the source of both incentives into the AD7266 12 bit, 2 MSPS dual simultaneous sampling ADC. ADC output data
15、will be stored in memory in order to do further analysis to calculate the phase and amplitude of the sensor offset. Complete integrated sensor solutions: separation described above is a common solution for impedance measurement of the sensor solution. The program may require many discrete components
16、, so the sensor is a cost-analysis solution. These separate components will increase their own sources of error. The design of active components will increase the number of phase error, which is the need for correction. In addition, the DSP also need to deal with some complex mathematical calculatio
17、ns, this may require external memory to store the original data of the ADC, which would further increase costs. Address the above-mentioned analysis of the issue of low-frequency sensor solutions AD5933 / 4 device, it will by the main processing module are integrated into one chip. The core of the c
18、hip, including three main modules: the frequency of scanning for the direct digital synthesizer (DDS) waveform generator； used to measure the sensors response to the 12 bit, 1 MSPS ADC； and, finally, to the ADC data for 1024 point Discrete Fourier Transform (DFT) calculations of the DSP engine. The
19、results of DFT calculations to provide a real part (R) and an imaginary part (I) data, which can easily calculate the impedance. Using the following formula is easy to calculate the impedance amplitude and phase: In order to determine the actual value of the real impedance Z (), is typically require
20、d to perform frequency scanning. Can calculate the impedance of each frequency point, which can draw a relationship between frequency and amplitude curves. So it is easy to measure 100 20 M resistance within the scope. The system allows users to set up a 2 V peak-to-peak (PK-PK) of the sinusoidal si
21、gnal as an external frequency source excitation load. Output range can be set to 1V, 500 mV and 200 mV. Frequency resolution can be 27 bit (0.1 Hz). The realization of the frequency of scanning: In order to achieve the frequency of scanning, the user must first set up the required frequency of scann
22、ing conditions: the need for a start frequency, frequency interval and sweep points. Then the need for a start command to start scanning. Frequency points in each scan, ADC completed the first 1024 samples, and then calculating the DFT in order to provide the waveform of the real and imaginary parts
23、 of the data. The real and imaginary parts of the data through the I2C interface in the form of two 16 bit words available to the user. DSP-chip processing unit user does not have the advantage of complex mathematical calculations, and need not store ADC raw data, only two 16 bit data. Therefore, it
24、 allows the DSP to choose cheaper solutions, as greatly reduce the processing power of the final requirements. Without calibration of the system as a result can only use the typical value of sensitivity and offset the output voltage is converted to pressure, the pressure measured will have a margin
25、of error as shown in Figure . This initial error without calibration by the following components: 1.offset error: As the pressure in the entire range of vertical shift to maintain a constant, so the proliferation and laser conditioning converter changes the amendment would have offset error. 2.The s
26、ensitivity of error, resulting in errors in direct proportion to the size and pressure: If the device is higher than the typical value of the sensitivity, the sensitivity of the error will be incremental pressure function (see Figure 1). If the sensitivity is lower than the typical value, then the s
27、ensitivity of the error will be decreasing function of pressure. The cause of the error diffusion process is to change. 3.Linearity Error: This is an initial error factor less affected, the error is the cause of the physical non-linear silicon, but with the sensor amplifier, should also include non-
28、linear amplifier. Linear error curve can be concave curve, it could be a convex curve. 4.Lag Error: In most cases, the lag error can be ignored completely, because silicon has a higher degree of mechanical stiffness. Changes in general just a lot of pressure to consider the case of hysteresis error.
29、 Calibration can eliminate or greatly reduce these errors, and compensation technique is usually required to identify the parameters of the actual transfer function, rather than simply the use of typical values. Potentiometer, adjustable resistance, and other hardware can be used in the compensation
30、 process, while the software is able to achieve more flexibility in the work of this error compensation. Calibration method that can eliminate the transfer function against the Agency to compensate the offset drift error, such as the auto-zero calibration method. Offset zero calibration is usually c
31、arried out under pressure, especially in the differential sensor, because under the conditions of the nominal differential pressure is usually 0. For pure sensor offset calibration will be difficult, because it either needs to read a pressure system to measure the atmospheric pressure in the environ
32、ment under the conditions of the calibration of pressure or need to obtain the pressure of expectations of the pressure controller. Zero differential pressure sensor is very accurate calibration, because the pressure of strict demarcation is 0. On the other hand, the pressure of 0:00 is not the accuracy of the calibration depends on the pressure controller or the performance measurement system.
