The surface electromyography signal is a one-dimensional voltage time-series signal obtained by the surface electrode guiding, amplifying, displaying and recording the bioelectrical changes of the neuromuscular system during random and involuntary activities, and the amplitude thereof is about 0-5000 μV. The frequency is 0-1000 Hz, and the signal shape has strong randomness and instability. Compared with the traditional needle electromyography, the spatial resolution of sEMG is relatively low, but the detection space is large and the repeatability is good. It has important academic value and application for sports science research, rehabilitation medicine clinical and basic research. significance.
Filter circuit module
The surface EMG signal is generally only a millivolt level voltage, and the signal often carries low frequency and high frequency interference signals. The truly useful myoelectric signal is roughly between 10 Hz and 500 Hz. In addition, the 50Hz power frequency signal is also an important source of interference. If it is not removed, it may cover the surface EMG signal. According to these special requirements, the dedicated filter must have DC blocking, filtering function, and requires high common mode. Suppression ratio and good anti-interference. The low pass filter uses a voltage controlled voltage source type second order low pass filter. The 50Hz power frequency signal has a great influence on the acquisition of surface EMG signals. Its frequency is just in the frequency band where the surface EMG signal energy is concentrated, and its amplitude is 1-3 orders of magnitude larger than the surface EMG signal, so it must be removed. . In this design, a dual T active filter is used to filter out the 50 Hz power frequency signal, as shown in Figure 2.
Figure 2 double T active filter circuit
The following is a way to analyze the possible introduction of power frequency interference: 1 introduced by space radiation: the electromagnetic field of the space can be induced into the corresponding frequency components by the electrode connection in the detection device, the wiring on the printed circuit board, the device pins or the device itself. The current becomes a noise mixed into the myoelectric signal. The electromagnetic field of space may come from a variety of sources, the most deadly is the power frequency interference caused by grid radiation. 2 Introduced by DC power supply: In the detection equipment, the DC power supply for the active device is usually obtained by voltage conversion, rectification and voltage regulation of the power frequency AC power supply. DC stabilized power supplies are unlikely to achieve the desired filtering effect. The power frequency (or its harmonics) current in the form of ripple is introduced into the amplifying circuit through the power supply. 3 Introduced by the subject's body: the body of the subject exposed to the space electromagnetic field will also induce an electromagnetic field to generate an induced current. The power frequency current induced by the subject's body passes through the detection electrode and is applied to the amplifier input together with the bioelectric signal. At the end, the power frequency interference is formed.
For the power frequency interference introduced by the DC power supply, the battery is used to supply the active device. The use of battery power not only avoids the problem of power frequency interference caused by the ripple of the rectified regulated power supply, but also eliminates the possibility of electric shock caused by leakage. Since the voltage of the battery is low, the multi-cell battery will appear bulky, so the DC/DC module is used to boost the power supply problem of the chip.
A/D conversion circuit module
Since the sampling frequency is not high, the 8-bit serial A/D converter ADC0832 can be used. The ADC0832 uses a sample-data-comparator structure that uses a successive approximation to convert. According to the software configuration of the multiplexer, in the single-ended input mode, the input voltage to be converted is connected to one input terminal and the ground terminal; in the differential input mode, the input voltage to be converted is connected to one input terminal and the other input terminal. The two inputs of the ADC0832 can be assigned either positive or negative and can be software configured by the multiplexer. However, it should be noted that when the input voltage connected to the distribution terminal is lower than the input voltage assigned to the negative terminal, the conversion result is all 0s. The control of the channel is selected by the serial data link connected to the control processor, and the configuration of the channel is selected by the software. The serial communication format can include more functions in the converter without increasing the package size. In addition, the converter and analog sensors can be placed together and communicated serially with the remote control processor instead of remotely transmitting analog signals at low levels. Such processing returns noiseless digital data to the processor, avoiding interference from analog signals. The hardware structure of the entire acquisition system is designed. The specific circuit is shown in Figure 3:
Figure 3 system circuit diagram
The surface EMG signal is very weak. It is necessary to amplify the weak signal to meet the requirements of the AD acquisition unit. Since the human body is an electric conductor, the power frequency interference and the external electric field and magnetic field induction will form measurement noise and interference in the human body. The detection of myoelectric information seriously affects the measurement system's work and the correct measurement of useful signals. According to the characteristics of surface EMG signal generation and the basic requirements of acquisition technology, the surface EMG signal digital sensor is designed and achieved good test results.
The POE splitter is opposite to the POE power supply. POE power supply combines data signal and power, so POE power supply is also called POE combiner. The working process of the POE splitter is just the opposite, which separates the data signal from the power. If data terminals such as network cameras (IPC), wireless APs, and IP phones support POE power supply, you do not need to use a POE splitter when wiring, and you can directly supply power via a network cable. The POE splitter is mainly used for PSE devices such as POE Switches and POE power supplies at the upper end, but the devices connected to the lower end do not support POE. The POE splitter separates the power supply into data signal and power. There are two output lines, one is the power output line, and the other is the network data signal output line, that is, the ordinary network cable. The power output has 5V/9/12V, etc., which can match various DC input non-POE power receiving terminals, and supports the IEEE 802.3af/802.3at standard. The data signal output line, that is, the ordinary network cable, can be directly connected to the network port of the non-POE power receiving terminal.
The POE splitter is used in conjunction with PSE power supply equipment to supply power to network equipment such as wireless APs, wireless routers, IP cameras, and IP phones in a network environment that lacks a power cord or is far from a power outlet. In many monitoring occasions, the best location for monitoring may not be very convenient for equipment to obtain power. The emergence of POE splitters facilitates the integration of equipment that does not support POE power supply into the POE network, and the monitoring equipment does not have to sacrifice the most to take care of the distance of the socket. Optimal installation location.
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