1. introduction
EMG (electromyography, surface sEMG) signal is a neuromuscular system for arbitrary and non-arbitrary activities of bioelectrical changes via surface electrode boot, zoom, display and record the time series of one-dimensional voltage signal, the amplitude of approximately 0-5000 μ V, frequency 0-1000Hz, signal shape with strong randomness and instability.
Unlike traditional Needle EMG, sEMG compared to the spatial resolution is relatively low, but detection space larger, better repeatability, for Sport Science research, rehabilitation medicine, clinical and basic research with important academic value and application [1].The human body is a conductor, the frequency interference and in vitro induction of electric and magnetic fields are formed in the human body measurement noise, interference, sEMG detection signal filtering and circuit of shielding a surface EMG digital sensor design focus.
Divided into several sections: electrodes, amplifier, filter, A/D conversion.2. electrode design
This article of the electrode plate substrate surface of copper, silver, in the form of a common bipolar, and in the middle of two electrodes inserted a reference electrodes, also known as unrelated to the electrodes in order to facilitate the reduction of noise, and improve the common mode signal suppression capabilities.
In order to eliminate noise from power supply line, using the differential amplifier.EMG consists of two electrodes to detect, two input signal "Subtract", get rid of the same element of "common-mode", only zoom in different "differential mode".
Any noise if far from detection points, in detection point will behave as a "common-mode signal detection", and near the surface of the performance of the signal into different, is magnified. Therefore, the relatively distant power line noise is eliminated, while relatively near the EMG is magnified. Its accuracy by common mode rejection ratio (CMRR) to measure [2].EMG signal information in human tissues (volume conductor) is passed in, as the distance increases the decaying soon.
So stick electrode should be placed in the electrical distribution the strongest muscles in the abdomen in order to reduce the adjacent muscles of electrical interference (crosstalk). A smaller electrode can improve selectivity, but increases the electrodes with skin contact impedance between.3 amplifying circuit design
Human muscle tissue is skin table of signal source, electromyogram its issuance of electromyography after subcutaneous tissue of the body resistance to the skin surface, some hundreds of resistance, Ohm, surface electrodes with skin contact impedance is high, some thousands of ohm.
Contact resistance is also affected by contact with tension, skin cleaning, humidity, changes in seasonal seasons and many other factors, changed much [3]. Thus, for the amplifier, EMG signal source is a high internal resistance of the signal source.In the design of the EMG amplifier, focused on the following issues: 1. high gain: surface EMG amplitude about distribution of μ V ~ mV magnitude, is a very weak signal, to enlarge it to a volt about ease of use, it will gain settings in the amplifier 80dB.
2. high common-mode rejection ratio: surface EMG collection vulnerable 50Hz frequency power supply and other high-frequency electrical noise interference. But the interference signal to the input of the amplifier performance for the same amplitude-phase signal — the common mode signal, therefore choosing high common-mode rejection ratio of amplifier circuit on interference signal suppression. 3. high input impedance: muscle and electrode contact impedance may be considerable scope changes, dry areas, contact resistance even up to tens of ohm, in those circumstances, even when the amplifier is extremely high common mode, if the input impedance is not high enough, common mode interference signals can also generate output error. Therefore you must raise the input impedance of the amplifier.According to the above, the design of EMG collection circuit requires a high gain, high input impedance, high total touch rejection ratio (CMRR), low drift, low offset, low power consumption, especially low voltage 1/f noise.
This selection of TI (Texas Instruments) company Burr-Brown series phase three parallel differential amplifier amp INA128PA as core components to build a preamplifier, get a good circuit. The internal principle circuit diagram shown in Figure 1.Figure 1 schematic INA128 internal
Surface Electromyography is very weak, boot from the electrode signals with very strong interference signal, in order to avoid interference with strong nonlinear area when signal into cause serious distortion, should adopt a two-level zoom.
Instrument amplifier INA128 as level zoom ratio, designed as a secondary operation amplifier.4 filter design
Surface Electromyography generally only Millivolt signal voltage, often with low frequency entrainment (near DC) and high-frequency interference signal, really useful EMG roughly between the 10Hz-500Hz.
In addition, 50 Hz frequency signal is also an important source of interference, if not removal may conceal surface EMG, according to these special requirements, special filter must have every other straight, filtering, and high common-mode rejection ratio and good immunity. Low-pass filter using voltage-controlled voltage source second-order low-pass filter.50Hz frequency signal on surface EMG collection have a great influence, its frequency just surface EMG energy set by the band, and its amplitude ratio surface EMG big 1-3 level, must therefore be removed.
This design uses double T filter to filter out 50 Hz mains frequency signal, as shown in Figure 2.Figure 2 double T Activo
The following analysis may introduce ways of line frequency interference: 1 Introduction by space radiation: space
The electromagnetic field can detect device electrode wires, printed circuit board wiring, device PIN or device themselves induction into the corresponding frequency components of the current, as noise mixed with EMG. Space of electromagnetic fields may come from multiple sources, the most fatal is the power line frequency interference caused by radiation. 2 introduction by DC power: testing equipment for active devices powered DC power supplies are often low frequency AC power transformer, Rectifier and voltage. DC power supply is not possible to achieve the desired effect of the filter. A ripple in the form of power frequency (or its harmonic) current to pass power into the amplifier circuit. 3 introduced by volunteers: the body exposed to the electromagnetic field will also be rewarded with the body to produce induction electromagnetic coils, subjects the body induction by detecting electrode frequency currents, together with the biological signals to the amplifier input, voltage-frequency interference.For DC power line frequency interference, introduced with the adoption of the battery to supply power for active devices.
Operates on battery not only avoids the rectifier voltage ripple by frequency interference problems, but also eliminates the subjects due to leakage caused by electrical injury. Because the battery voltage is low, with multiple batteries will look bulky, so use DC/DC module power supply for boosting solution chip.5 A/D conversion
Since the sampling rate is not high, the selection of the 8-bit serial A/D converter ADC0832.
ADC0832 use sampling — data comparer structure, the use of successive approximation method for conversion. According to the multiplier of software configuration, single-ended input mode you want to convert the input voltage is attached to an input terminal and ground terminal; differential input mode you want to convert the input voltage is attached to an input terminal and another input. The two input ADC0832 can be assigned as the cathode or anode by multiplier for software configuration. However, it should be noted that, when attached to the allocation for the end of the input voltage is lower than the assigned negative side of input voltage, the conversion result is 0. Adoption and control processor connected serial data link control command, use the software on the channel selection and configuration of the input. Serial communication format without increasing the size of the package, you can converter contains more features. In addition, you can put the converter and analog sensors, and remote control processor serial communication, rather low level of remote transmission of analog signals. This treatment allows to return to the processor is no noise from digital data, avoiding the analog signal long-distance transmission interference. The entire collection system hardware structure design is complete, the specific circuit diagram 3 shows:Figure 3 system circuit diagram
6 conclusions
Surface EMG signal very weak, to zoom in on the weak signal, in order to meet the requirements of AD acquisition unit, and because the human body is a conductor, the frequency interference and in vitro induction of electric and magnetic fields are formed in the human body measurement noise, interference electromyography information detection, seriously affecting the measuring system of work and the correct measurement of the useful signal.
This article in accordance with the characteristics of surface EMG signal generation and acquisition of the essential requirements, the design of the surface semg digital sensor and achieved good test results.Reference documents
[1] Division-Suk, Li Jie, Wang Xu. EMG signal acquisition system and the computer's serial port [J]. Micro-computer information, 2006, 1: 226-227.
[2] Surface Electromyography: Detection and Recording，Delsys Incorporated, 2002
[3] money xiaojin, Zheng as ice, Andy Lam. High performance electrical testing preamplifier design. Modern scientific instruments, 2003, 19 (3): 50-52
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