EMG Control dexterous Hand (iii)

Properties and configuration of EMG electrodes

EMG electrode refers to the EMG control interface, which is designed for the acquisition of EMG signals, which has included the necessary amplification, filtering, rectification and other signal processing processes, typical of which is the independent dry electrode module, which integrates electrodes, power supply, signal amplification, modulation and noise shielding processes in a certain size volume.

The amplitude of EMG signal is random and obeys Gaussian distribution basically. The noise of the surface skin signal mainly originates from internal and external, the internal mainly is the conduction influence of skin tissue, the potential issue crosstalk, etc., the external is mainly contact surface state and power frequency, electromagnetic interference and other effects.

The EMG electrodes are widely used in bipolar differential amplification. The amplification of the potential difference between the two poles can effectively reduce common mode noise and improve the common mode rejection ratio.

The concept of differential mode signal, common mode signal and common mode rejection ratio

Differential mode, also known as serial mode, refers to the signal difference between the two lines;

Common mode noise, also known as ground noise, refers to the noise of two wires to ground respectively.

For a pair of signal lines A, B, differential mode interference is equivalent to A and B with a disturbance voltage, common-mode interference is equivalent to A and ground, B and the ground between a disturbance voltage.

As usual to see the use of twisted-pair transmission differential signal is to eliminate common mode noise, the principle is very simple, two wires screwed together, by the common mode interference voltage is very close, Ua-ub

There is still no change, of course this is the ideal situation.

In practice, the influence of various environmental noises can be regarded as common mode noise signal, but if the noise attenuation of the two lines is not the same as that in the transmission process, there is an additional voltage difference between the two lines, which is the common mode noise is transformed into the differential mode noise.

Differential amplifier, differential mode input, differential mode is relative to common mode. Differencing is a way.

Differential mode, common mode signal, difference amplifier circuit

For example, if an ADC has two analog inputs and the ad conversion results depend on the difference between these two input voltages, then we say that the ADC is a differential input and that the two analog inputs are combined together called differential inputs. However, the voltages added to the differential inputs are not always equal in size and in opposite directions, even in many cases the same symbol. (Note: That is not necessarily a positive and negative) we call their difference in differential mode input, and the amount they share (that is, the average) is called the common-mode input.

Differential is a form of circuit called ....
Differential mode is the definition of a signal

Differential mode signal: equal size, opposite AC signal, common mode signal: equal in size. The same direction.

In the differential amplifier circuit, there are two inputs, when the two terminals on the same size, phase opposite signal, (this is a useful signal) amplifier can produce a large magnification, we call this signal differential mode signal, then the magnification is called differential mode magnification. If a signal of equal size and phase is entered on the two inputs, (this is actually a signal from the last level due to temperature change, it is a harmful thing), we call this signal a common mode signal, at which point the magnification is called the common mode amplification factor. Due to the characteristics of the differential amplification circuit, the circuit has a strong negative feedback on the common mode signal, so the common mode magnification is very small.

Common mode signal: Two signals are the same when double-ended input.
Differential mode signal: Two-terminal input, two signal phase difference of 180 degrees.

Any two signals can be decomposed into common-mode signals and differential-mode signals. The input signals of the two channel are: A, B.
The m,n is the common-mode signal component and the differential-mode signal component of the input signal, a, B. Input signal A, B can be expressed as: A=m+n;b=m-n
The input signal, a, B, can be considered as a synthesis of the common-mode signal m and the differential-mode signal n. where m= (a+b)/2;n= (A-B)/2.
The difference amplifier makes two signals as the output signal. The output signal is a-B, compared with the common-mode signal and the differential mode signal in the original two signals, it can be found that:
The common-mode signal m= (A+B)/2 is missing, while the differential mode signal n= (A-B)/2 gets twice times the magnification. This is how the differential mode amplifier works.

Common-mode rejection ratio

In order to illustrate the ability of the differential amplifier to suppress common-mode signals, common-mode rejection ratios are measured as a technical indicator, which is defined as the ratio of the amplifier's voltage magnification to the differential-mode signal to the voltage magnification of the common-mode signal, AUC.

When expressed in decibels, the

Factors affecting common-mode rejection ratio of differential amplifiers

◇ Circuit symmetry-the symmetry of the circuit determines the amplitude of the residual common mode interference of the amplified signal, the lower the symmetry of the circuit, the smaller the common-mode rejection ratio, the less the ability to suppress the common-mode signal (interference).
◇ the linear operating range of the circuit itself-the actual circuit its linear range is not infinite, when the common-mode signal outside the circuit linear range, even if the normal signal can not be normal amplification, not to mention the common-mode rejection capability. The actual circuit of the linear operating range is less than its operating voltage, which is why the common-mode rejection of high-demand device front-end circuit also uses higher operating voltage reasons

After the correct selection of the applicable electrodes, is it necessary to select the exact electrode position according to the function of the EMG control to collect the EMG signal of the specific muscle? Does pattern recognition use complex data to classify? Can we use the combination of PCA, SVM, filter and so on to realize the pattern recognition of electrode-free position selection?

Reference Baidu Library article the concept of differential mode signal, common mode signal, common mode rejection ratio

Yang Dapeng's doctoral dissertation "Research on EMG control of multi-exercise mode of humanoid prosthetic prosthesis"

EMG Control dexterous Hand (iii)