Abstract: This paper designs a wireless multi-channel surface electromyography (SEMG) acquisition system, which includes multi-channel wireless sensor and signal receiving part. The sensor can be independently worn on the surface of the human body, and the surface electromyography signal is obtained by the linear differential electrode, amplified, filtered, A/D converted, and transmitted to the receiving portion in a wireless manner according to the communication protocol designed herein. The receiving part integrates the data of each sensor and transmits it to the computer for storage, display and processing through the USB interface. Each sensor has a volume of 35mm×20mm×11mm, weighs only 13g (including battery), can work for 9 hours on a single charge, wireless communication distance is 7.5m, and the collected signal noise is lower than -70dB (1mV of myoelectric signal represents 0dB) ). The design greatly improves the convenience of electrode placement, the portability of the acquisition device and the safety of the human body, and avoids power frequency interference, and can meet the requirements of research on gesture or gesture recognition based on surface myoelectric signals. 1 Introduction Surface electromyography (SEMG) signals are a combination of temporal and spatial effects of muscle electrical activity on the surface of the skin [1]. The surface EMG signals generated by the contraction of muscle groups in different movements will show different characteristics, so that people's postures and movements can be recognized, thereby realizing the interaction between humans and computers. In recent years, SEMG has not only been widely used in disease diagnosis, rehabilitation medicine, sports and sports, but also as a novel human-computer interaction input method [2]. Literature [3] realized the control of the manipulator with the EMG signal on the arm surface. Literature [4] realized the use of surface EMG signals from three parts of the human body to drive electric vehicles. Most of the existing surface electromyography acquisition systems use a wired method. The electrodes are connected to control devices such as acquisition devices and computers, and are powered by an AC power source. For example, the literature [5-6] uses a wired SEMG electrode. This kind of scheme is easy to introduce power frequency interference of power lines [7], and the wired electrodes are bound to the human body and are not convenient for use in sports. In order to make the SEMG acquisition system more portable, easy to use, and good in electrode wearability, it can be placed in the whole body to meet the needs of human body posture recognition and other applications. In this paper, the sensor is designed as a separate module, the volume is as small as possible, and the weight is as light as possible. Battery powered, wirelessly transmitting data. The wireless solution avoids power frequency interference and isolates people from AC mains and has good electrical safety. At the same time, this paper designs a wireless receiving circuit to customize a set of wireless communication protocols for the needs of multi-channel SEMG signal acquisition. In addition, a related program on the PC has been developed to realize multi-channel signal storage, waveform display and processing. The experiment proves that the acquisition system has the ability of real-time acquisition, wireless transmission and storage of multi-channel SEMG, featuring micro, portable security and high signal-to-noise ratio. 2 Implementation method 2.1 system hardware design The acquisition system includes a plurality of independent wireless sensors and receiving portions that can be worn on the surface of the human body. Multi-channel wireless sensors can work together as described in Figure 4 of Section 2.2. In order to demonstrate the complete process of EMG signal acquisition, Figure 1 shows a block diagram of a channel's wireless sensor and a receiving part that contains only one receiver. The wireless sensor uses the differential electrode to collect the surface EMG signal, and then performs amplification, filtering and A/D conversion, and the signal is transmitted by the RF chip under the control of the single chip microcomputer, and the power source is provided by the battery. The receiving end uses the same RF chip to receive the signal, and uses the single-chip microcomputer to control the entire communication process in an orderly manner according to a certain communication protocol. At the same time, the data is transmitted to the PC by the USB controller provided by the single-chip microcomputer, and the signal storage is completed by the PC. , waveform display and algorithm processing. Figure 1 hardware block diagram Fig.1 Block diagram of hardware The wireless sensor module uses a three-layer structure on the hardware, as shown in Figure 2. The lower layer is a signal acquisition and pre-processing board, which includes an electrode in contact with the surface of the human body, a signal pre-processing circuit, and a single-chip microcomputer for A/D conversion and wireless transceiving control; a rechargeable lithium battery is integrated in the middle; It is a wireless transceiver circuit that integrates a radio frequency chip, an antenna matching network, and a PCB antenna. Figure 2 sensor three-layer structure Fig.2 Three-layer structure of sensor In many application fields of SEMG, the electrodes directly in contact with the human body mostly adopt a differential structure, and materials such as Ag/AgCl, Ag and Au are used, and the spacing of the electrodes is mostly 10 mm to 30 mm [8]. Department The electrode part of the wireless sensor module consists of two long strips of gold-plated pads. The two pads are 10mm long, 2.5mm wide and 10mm apart. They are located on the bottom surface of the sensor. Next to the differential electrode is a large pad. The circuit ground leads to the skin as a reference electrode, so that the acquired EMG signal voltage range is within the input range allowed by the op amp. In signal preprocessing, considering the characteristics of surface EMG signals: the amplitude is very small (tens of microvolts to thousands of microvolts), the frequency range is 20~1000Hz, and the main energy is concentrated in the range of 50~100Hz; It is easy to generate motion artifacts, and its energy is mainly concentrated in 0~20Hz [7]. The filter amplification circuit shown in Figure 3 is designed. The signal is input in differential mode, and two high-input-impedance, high-CMRR, low-current noise amplifier chips are selected. The two operational amplifiers and the peripheral components complete the two-stage total 1000-fold amplification and the 20-1000 Hz band-pass filter. . The front stage uses the instrumentation op amp AD8220, its current noise is only Figure 3 schematic diagram of the amplification filter circuit Fig.3 Principle of amplification and filter circuit
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It occupies a small area of ​​the board, and the amplifier is set to 34 times by a 1.5k resistor and a 10μF capacitor. The rear stage adopts the op amp OPA364, which has SOT23-5 package and only occupies 3mm×3mm on the PCB board, which is suitable for the miniaturization design of the sensor. The OPA364 and two capacitors and two resistors form a Chebyshev I-type 1st-order bandpass filter with a passband of 20~1000Hz, and the magnification is about 30 times. 
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