SEMG signal is a one-dimensional voltage time series signal obtained by the bioelectrical changes of neuromuscular system during random and non random activities guided, amplified, displayed and recorded by surface electrode, and its amplitude is about 0-5000 μ 5. The frequency is 0-1000hz, and the signal shape has strong randomness and instability. Compared with traditional needle electromyography, sEMG has relatively low spatial resolution, but large detection space and good repeatability. It has important academic value and application significance for sports scientific research, rehabilitation medicine clinical and basic research.
Filter circuit module
Surface EMG signal is generally only millivolt voltage, and the signal is often mixed with low-frequency and high-frequency interference signals. The really useful EMG signal is about 10hz-500hz. In addition, 50Hz power frequency signal is also an important interference source. If it is not removed, it may mask the surface EMG signal. According to these special requirements, the special filter must have straight separation and filtering functions, and high common mode rejection ratio and good anti-interference. The low-pass filter adopts voltage controlled voltage source type second-order low-pass filter. 50Hz power frequency signal has a great impact on the acquisition of sEMG signal. Its frequency is just in the frequency band where the energy of sEMG signal is concentrated, and its amplitude is 1-3 orders of magnitude larger than that of sEMG signal, so it must be removed. In this design, dual t active filter is used to filter 50Hz power frequency signal, as shown in Figure 2.
Figure 2 Dual t active filter circuit
The following is to analyze the possible ways to introduce power frequency interference: 1. Introduced by space radiation: the space electromagnetic field can become a noise mixed EMG signal through the electrode wiring in the detection equipment, the wiring on the printed circuit board, the device pin or the device itself to induce the current of the corresponding frequency component. The electromagnetic field in space may come from many sources, and the most fatal is the power frequency interference caused by power grid radiation. 2. Introduced by DC power supply: in the detection equipment, the DC power supply for active devices is usually obtained by transforming, rectifying and stabilizing the power frequency AC power supply. DC regulated power supply can not achieve ideal filtering effect. The power frequency (or its harmonic) current in the form of ripple will be introduced into the amplification circuit through the power supply. 3 introduced by the subject’s body: the subject’s body exposed to the space electromagnetic field will also induce the electromagnetic field and generate induced current. The power frequency current induced by the subject’s body is added to the input end of the amplifier together with the bioelectric signal through the detection electrode to form power frequency interference.
Aiming at the power frequency interference introduced by DC power supply, battery is used to supply power to active devices. The use of battery power supply not only avoids the power frequency interference caused by the ripple of rectifier regulated power supply, but also eliminates the possibility of electric injury to subjects due to leakage. Because the voltage of the battery is low and the volume will be huge with multiple batteries, the DC / DC module is used to boost the voltage to solve the power supply problem of the chip.
A / D conversion module
Since the sampling frequency is not high, the 8-bit serial A / D converter ADC0832 can be selected. ADC0832 uses the structure of sampling data comparator and adopts successive approximation for conversion. According to the software configuration of the multiplexer, in the single terminal input mode, the input voltage to be converted is connected to one input terminal and the ground terminal; In differential input mode, the input voltage to be converted is connected to one input and another input. The two inputs of ADC0832 can be assigned as positive or negative, and can be configured by multiplexer software. However, it should be noted that when the input voltage connected to the assigned terminal is lower than the input voltage assigned to the negative terminal, the conversion result is all 0. The control command is transmitted through the serial data link connected to the control processor, and the channel selection and input configuration are configured by software. The serial communication format can include more functions in the converter without increasing the package size. In addition, the converter and analog sensor can be put together to communicate with the remote control processor in serial, rather than the remote transmission of low-level analog signals. Such processing makes the digital data without noise returned to the processor, and avoids the interference of analog signal remote transmission. The hardware structure of the whole acquisition system is designed, and the specific circuit is shown in Figure 3:
Fig. 3 system circuit diagram
The surface EMG signal is very weak. The weak signal must be amplified first to meet the requirements of AD acquisition unit. Moreover, because the human body is a conductor, power frequency interference and external electric and magnetic field induction will form measurement noise in the human body, interfere with the detection of EMG information, and seriously affect the work of the measurement system and the correct measurement of useful signals. According to the characteristics of surface EMG signal generation and the basic requirements of acquisition technology, a digital sensor of surface EMG signal is designed, and good experimental results are obtained.