**4.1. Filtering**

As discussed earlier, there are many concerns regarding the proper detection of the EMG signal. Once the electrode is properly placed and the signal is extracted, noise plays a major role in hampering the recording of the EMG signal. For this purpose, the signal has to be properly filtered, even after differential amplification [18, 19].

The noise frequencies contaminating the raw EMG signal can be high as well as low. Low frequency noise can be caused from amplifier DC offsets, sensor drift on skin and temperature fluctuations and can be removed using a high pass filter. High frequency noise can be caused from nerve conduction and high frequency interference from radio broadcasts, computers, cellular phones etc. and can be deleted using a low pass filter.

In order to remove these high and low frequencies, high pass and low pass bio-filters will be discussed in adequate detail in this section.

#### *4.1.1. High pass filter*

Computational Intelligence in Electromyography Analysis – 438 A Perspective on Current Applications and Future Challenges

bipolar EMG electrode configuration is shown is Figure 13.

Bipolar configuration is used to acquire EMG signal using two EMG detecting surfaces with the help of a reference electrode. The signals from the two EMG surfaces are connected to a differential amplifier. The two detecting surfaces are placed only 1-2 cm from each other. The differential amplifier suppresses the common noise signals to both inputs and then amplifies the difference [5, 14]. The limitations of the monopolar configuration are catered for by this configuration. This is the most commonly used electrode configuration. The

This configuration uses more than two detecting surfaces to acquire the EMG signal with the help of a reference electrode. This configuration further reduces crosstalk and noise concerns [14]. A much more enhanced EMG signal is acquired from this configuration. The signals from three or more EMG detecting surfaces, placed 1-2 cm from each other, are passed through more than two stages of differential amplification. For example if three

This configuration is used in comprehensive researches carried out to study EMG muscle

This section will discuss the electrical design considerations in order to synthesize the best possible EMG signal from the muscles of the human body in thorough detail. The basic circuitry for signal acquisition or preamplification circuitry is explained in due detail in the previous section. In this section we will discuss the circuitry implemented after the

As discussed earlier, there are many concerns regarding the proper detection of the EMG signal. Once the electrode is properly placed and the signal is extracted, noise plays a major

detecting surfaces are used then double differential technique is employed.

fiber orientation, conduction velocity and motor point localization.

*3.6.2. Bipolar configuration* 

**Figure 13.** Bipolar Configuration

*3.6.3. Multipolar configurations* 

**4. Electrical design considerations** 

preamplification stage.

**4.1. Filtering** 

A high pass filter is used to remove low frequency component from a particular electrical signal. A term 'cut-off frequency', denoted by '*fc*', is the frequency below which all frequencies are eliminated. All frequencies above *fc* are carried forward. The frequency range where the filter response is '1' and the signals are transmitted is known as 'passband' region. On the contrary, the frequency range where the filter response is '0' and the signals are attenuated is known as 'stop band' region [18]. A high pass filter response is shown in Figure 14.

**Figure 14.** A high pass filter response

A high pass filter can be developed by using a resistor and a capacitor. This circuit will then be known as a CR circuit [20]. This circuit is a first order high pass filter. It is the simplest high pass filter possible. The high pass filtered signal is gathered across the resistor. The filter is shown in Figure 15.

The cut-off frequency of the high pass filter is given in Eq. 3.

$$f\mathbf{c} = \frac{1}{2\pi\text{RC}}\tag{3}$$

A second order high pass filter can also be designed. An effective design can employ an active electronic component [20]. The design uses two first order filters in series and is facilitated by an operational amplifier. The circuit is given Figure 16.

For this circuit, if R1 = R2; C1 = C2 then *fc* is given as:-

$$f\mathbf{c} = \frac{1}{2\pi RC} \tag{4}$$

Signal Acquisition Using Surface EMG and Circuit Design Considerations for Robotic Prosthesis 441

**Figure 17.** Low Pass Filter Response

**Figure 18.** 1st Order Low Pass Filter

**Figure 19.** 2nd Order Low Pass Filter

given in Figure 19.

The cut-off frequency equation for the circuit in Figure 18 is the same as that of Eq. 3.

A 2nd order low pass filter can be more effective as compared to a 1st order one. It can be designed by cascading two 1st order filters attached to an operational amplifier. The circuit is

For R1 = R2 and C1 = C2, the cut-off frequency of the circuit in Figure 19 is the same as that of Eq. 4. R3 and R4 are optional as they are required for separate gain settings as given in Eq. 5.

R3 and R4 are optional and are required for separate gain settings as:-

$$\mathbf{A}\rho = \mathbf{1} + \mathbf{R}\rho \mathbf{R}\rho \tag{5}$$

Using a 2nd order filter is recommended as they provide a roll-off of 40 dB/dec as compared to 20 dB/dec provided by 1st order filters [18]. The use of active components can isolate the filter from the rest of the circuitry.

**Figure 15.** First order high pass filter

**Figure 16.** A 2nd Order High Pass Filter

#### *4.1.2. Low pass filters*

The concept of low pass filters is entirely opposite to that of high pass filters. In these filters, the frequencies less than the cut-off frequency are transmitted and above that are removed [18]. A low pass filter response is shown in Figure 17.

The simplest low pass filter can be designed with the help of a resistor and a capacitor called as a 1st order RC circuit [20]. The low pass filtered signal is detected across the capacitor. The 1st order low pass filter circuit is shown in Figure 18.

**Figure 17.** Low Pass Filter Response

Computational Intelligence in Electromyography Analysis – 440 A Perspective on Current Applications and Future Challenges

For this circuit, if R1 = R2; C1 = C2 then *fc* is given as:-

filter from the rest of the circuitry.

**Figure 15.** First order high pass filter

**Figure 16.** A 2nd Order High Pass Filter

[18]. A low pass filter response is shown in Figure 17.

1st order low pass filter circuit is shown in Figure 18.

*4.1.2. Low pass filters* 

facilitated by an operational amplifier. The circuit is given Figure 16.

R3 and R4 are optional and are required for separate gain settings as:-

A second order high pass filter can also be designed. An effective design can employ an active electronic component [20]. The design uses two first order filters in series and is

�� � �

A0 = 1 + R4/R3 (5)

Using a 2nd order filter is recommended as they provide a roll-off of 40 dB/dec as compared to 20 dB/dec provided by 1st order filters [18]. The use of active components can isolate the

The concept of low pass filters is entirely opposite to that of high pass filters. In these filters, the frequencies less than the cut-off frequency are transmitted and above that are removed

The simplest low pass filter can be designed with the help of a resistor and a capacitor called as a 1st order RC circuit [20]. The low pass filtered signal is detected across the capacitor. The

���� (4)

**Figure 18.** 1st Order Low Pass Filter

The cut-off frequency equation for the circuit in Figure 18 is the same as that of Eq. 3.

A 2nd order low pass filter can be more effective as compared to a 1st order one. It can be designed by cascading two 1st order filters attached to an operational amplifier. The circuit is given in Figure 19.

**Figure 19.** 2nd Order Low Pass Filter

For R1 = R2 and C1 = C2, the cut-off frequency of the circuit in Figure 19 is the same as that of Eq. 4. R3 and R4 are optional as they are required for separate gain settings as given in Eq. 5. A 2nd order low pass filter is again recommended as compared to a 1st order one for the same reasons mentioned for a 2nd order high pass filter.

Signal Acquisition Using Surface EMG and Circuit Design Considerations for Robotic Prosthesis 443

The EMG signal, as mentioned before, is very weak i.e. only 1-10 mV. For certain muscles, for which the signal response is very strong e.g. Biceps Brachii, a gain of 500-1000 can be enough. But for muscles, whose EMG response is weak e.g. Flexor Palmaris Longus (ring

The proper gain setting solely depends upon the signal response observed from the subject's target muscle. It is to be noted that every subject gives a separate signal response. Some subjects will give weak responses as compared to others. So, in that case, appropriate gain

In order to successfully achieve robotic prosthesis, an effective control technique is very important in order to drive the electric motors in the mechanism. With the advent of modern microcontroller technology, the control options available today have never been so effective. For implementing the desired control to the motors, the amplified EMG signal in analog form has to be converted into digital format. After this, the motors are driven with the help of a microcontroller through the thresholding technique. These techniques will be discussed

The digitization process of the analog signal is carried out with an Analog to Digital Converter (ADC). Nowadays, the ADC has become a common component of modern electronic devices. Their use has become highly varied and widespread. Before using the ADC, its specifications, advantages and limitations have to be analyzed in order to select the most appropriate one for the application. In the same way, important considerations have to

Control of the motor will be developed after the EMG signal is converted into digital format. A particular ADC has a specific range of conversion i.e. there are maximum and minimum levels defined for an ADC over which it can operate. An ADC can convert the analog signal over a certain number of bits. The number of bits which an ADC can convert is known as its

be taken into account while converting EMG signals into digital format.

value should be set once the subject's EMG signal response is properly observed.

finger muscle), the gain settings should be very high i.e. 10000.

**Figure 21.** A Non-Inverting Amplifier

**5. Control technique** 

in detail in this section.

**5.1. Analog to digital conversion** 
