Application of Mscl210 in Micro Displacement Sensor System

Micromanipulation has been applied in many application fields, especially in nano micron positioning systems, sensors are required to detect small force and displacement information. At present, there are many principles for micro displacement detection, such as optical, magnetic and inductive, capacitive and piezoelectric, but most of these are not suitable for robots to obtain micro multi-dimensional force and displacement information. In order to use the mature multi-dimensional force sensor for micro force and micro displacement information acquisition, the robot sensor laboratory of Hefei Institute of intelligence, Chinese Academy of Sciences, while transforming the structural parameters of multi-dimensional force sensor, makes full use of many new features of mscl210 single chip microcomputer of Texas Instruments (TI), such as its own 8-way 24 bit high-precision sigma delta A / D converter and programmable gain amplification (PGA) And filter, which realizes the high-precision measurement of force and displacement. This paper mainly discusses the application of microcomputer, hoping to help improve the integration degree, resolution, stability and human-computer interaction ability of sensors.

1 Characteristics of multi-dimensional force sensor and introduction to mscl210

The micro displacement sensor structure mainly draws lessons from the structural characteristics of the robot six-dimensional wrist force sensor in the laboratory, and the double E-type diaphragm structure is also used in the design. Firstly, the improved two-dimensional force sensor is used to detect the force information in the X and Y dimensions acting on the object, and then it is converted into the displacement information of the plane. At the same time, based on the six dimensional force sensor of the robot, it changes the relevant structural parameters, such as range and sensitivity, so as to obtain the small force and displacement information.

Mscl210 has some enhancement characteristics and is especially suitable for measuring weak signals output by high-precision temperature and pressure sensors. Mscl210 mainly includes enhanced 8052 microcontroller core, flash memory, high-performance simulation function and high-performance peripherals. The enhanced 8052 microcontroller core includes dual data indicators, and the execution speed of instructions is three times faster than that of the standard 8052 core. This MIPS function enables users to optimize speed, power and noise according to special needs. Fig. 1 is a block diagram of an mscl210 ADC.

The core of high precision micro displacement sensor circuit module is mscl210 single chip microcomputer. It completes the functions of weak signal multiplexing, signal buffering, PGA programming amplification, 24 bit sigma delta A / D conversion, digital filtering, data processing, signal calibration and UART communication. The following is a brief introduction to the use of micro displacement sensor.

MSC1210 input multiplexer is more flexible than general ADG, and each input pin can be configured as positive input or negative input for specific measurement. Compared with the input pair usually defined by the startup ADC component, the MSC1210 can define one pin as a negative input and the other as a positive input, making the design convenient and free. A group of bridge outputs of the micro displacement sensor are respectively connected to ain0 and ain1 of MSC1210 ADC as a differential input; The other group of bridge outputs are respectively connected to ain2 and ain3 of MSC1210 ADC as the second differential input. The initialization value of admux register is admux = ox01.

According to the data of TI's MSC1210, the input buffer can reduce the possibility of offset in ADC measurement. As long as the characteristics of the input signal allow, it should be used. The only case where the input buffer is not used is that the maximum voltage on one of the analog inputs is lower than the normal rail voltage and higher than 1.5 v. Without buffer, the input impedance of MSC1210 is 5 m Ω / PGA. When the buffer is enabled, the impedance is generally 10 g Ω. The input voltage range is reduced and the analog power supply current increases. In the case of not being the "only" mentioned above, the buffer is not used in the experiment, but the buf bit in the ADC control register (adcono. 3) is cleared, that is, the buffer is closed, but the effect is ideal, and the change of zero output deviation of the sensor is small.

The gain of the programmable gain amplifier (PGA) can be set to 1, 2, 4, 8, 16, 32, 64 or 128. The effective resolution of ADC can be greatly improved by using PGA. Of course, the gain setting should be reasonable, otherwise the noise will be amplified. In the experiment, the gain of the micro displacement sensor is set to 64 (to be exact, it should be the gain of PGA of mscl210 ADC).

MSC1210 digital filter has three types: fast establishment, sinc2 or sinc3, and there is also an automatic mode. After the input channel or PGA changes, the automatic mode can modify the sinc filter to the best available option. After switching to a new channel, it can use the fast establishment filter for the next two conversions, and the first conversion should be abandoned. Then, sinc2 and then sinc3 filters are used to improve the noise performance. This operation can combine the low noise advantage of sinc 3 filter and the fast response of fast recovery time filter at the same time. Sinc in digital filter is a kind of FIR filter in digital filter, which is commonly used in △ Σ ADC. When the input channel changes suddenly, the output takes some time to correctly represent the new input. The time required depends on the type of filter used. Sinc2 usually represents the data output time requiring 2 cycles, sinc3 represents the data output time requiring 3 cycles, and others require 1 cycle. Generally speaking, if sinc3 filter is used, when the input channel is changed, the first three data sampled and output cannot be used and should be discarded; Only the fourth output data is available. This is crucial.

MSC1210 can use either internal reference voltage or external reference voltage. The power on configuration of the reference voltage is internal 2.5 v. The reference voltage can be selected through the adcon0 register. The internal reference voltage was enabled in the experiment, and 1.25 V was selected by setting adcon0.4 (vrefh). Note that enabling internal VREF does not eliminate the need for external connections. The refout pin must still be connected to VREF , and VREF - must still be connected to agnd so that the internal VREF can operate normally. Due to space limitations, other functions of MSC1210 ADC are not introduced here.

2 hardware composition of sensor

Due to the high integration of MSC1210 ADC, the hardware system composition is very simple. Fig. 2 is a partial rendering of the micro displacement sensor. The strain resistor is pasted on the surface of the E-type diaphragm through a special process and in a specific direction, and connected to form two groups of automatically decoupled Wheatstone full bridge circuits as the analog output of the original force information. The compact circuit board is placed in a circular aperture.

Fig. 3 is the circuit structure diagram of the experimental system. The experimental system is mainly composed of sensor body (output original analog signal), MSC1210 core, serial communication circuit and PC. As mentioned earlier, a group of bridge outputs of the micro displacement sensor are respectively connected to ain0 and ain1 of MSC1210 ADC as a differential input; The other group of bridge outputs are respectively connected to ain2 and ain3 of MSC1210 ADC as the second differential input. MSC1210 communicates with PC through RS 232 to display information and control MSC1210.

3 software design of MSC1210 data acquisition system

The setting of each control word of MSC1210 ADC will greatly affect the final resolution and stability of the sensor. For example, whether the input buffer is used or not, the selection of programmable amplifier gain, the selection of modulator clock speed (which determines the analog sampling rate and the selection of filter model), etc., the setting of these parameters is not completed at one time. Some of them need to be combined and tried repeatedly to finally get satisfactory results. In order to avoid that the program must be downloaded to the flash program memory again after each modification, the initialization of the ADC before the operation of MSC1210 is fully controlled by the PC. Firstly, the PC transmits various necessary control information to the MSC1210 through the serial port, and the MSC1210 initializes the ADC according to these information. Specifically, the PC sends six bytes to the MSC1210, including the filter model, the sampling values in aclk, adcon2 and adcon3 registers, the delay value after modifying the analog input channel, the number of data output cycles that must be discarded, and the gain of the programmable amplifier. Then, MSC1210 starts the initialization of ADC. After entering the normal working state, if it is found that the number of current data output cycles is greater than the preset number of data output cycles that must be discarded, MSC1210 considers that this output data and its subsequent output data are valid, but it does not immediately send the data to the PC, but immediately modify the analog input channel, Then make the output of the previous input channel invalid, and then enter the next data output cycle. The workflow of mscl210 can be simply shown in Fig. 4. In addition to the initialization control of MSC1210 ADC, PC is mainly responsible for displaying the data sent by mscl210 through RS 232 serial port at the appropriate position of the window in real time with visual graphics. MSC1210 sends 4 bytes to PC each time. These four bytes are defined as follows: the first byte represents the channel number of the sensor, and 0 represents the differential input between ain0 and ain1, that is, x-dimensional information; 1 indicates the differential input between ain2 and ain3, i.e. y-dimensional information. The last three bytes are the 24 bit binary representation of the output data and the conversion result of MSC1210 ADC. When the PC processing process detects that the input buffer is full of 4 bytes, it reads the data of the buffer and empties the input buffer at the same time. After identifying the channel according to the first byte, convert the data into voltage value and display it to the corresponding position (unit: MV), and open up two graphic display areas in the window to display the signal changes of x-axis and y-axis in real time. In order to better observe the small changes of the signal, the process provides the function of graphic amplification and display. For convenience, the magnification can be 12.5x and 125x.

4 experimental results

After repeated tests, combined with the actual requirements, the data output rate of the micro displacement sensor is finally positioned at 10 Hz, the resolution is as high as 15 bits, has high stability, and has achieved good results. The selection of relevant parameters is as follows: the filter model of MSC1210 is sinc3, aclk is 1, the sampling value in adcon2 and adcon3 registers is set to 1080, the analog input buffer is closed, three data output cycles are discarded after the analog input channel is changed, and the internal self correction of offset and gain is adopted. Fig. 5 is a graphical display of the data output of the micro displacement sensor during operation. The two curves represent the force on the x-axis and y-axis respectively. The displacement information can be easily calculated according to the force information.

Based on the multi-dimensional force sensor technology in the robot sensor laboratory of Hefei Institute of intelligence, Chinese Academy of Sciences, and using the advantages of MSC1210, a micro displacement sensor is successfully developed, which has the advantages of small volume, high resolution and good stability. MSC1210 MCU is a powerful data acquisition chip, which can be widely used in instrumentation, measurement and control, navigation and other fields.