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EMBC.2017.8036842

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Design of sEMG-Detecting Circuit for EMG-Bridge
Xiao-Bin Chen, Yu-Xuan Zhou, Hai-Peng Wang, Xiao-Ying Lü, Zhi-Gong Wang, Senior Member, IEEE *

Abstract— A surface electromyography (sEMG) signal typically
results from the electrical activities of many muscle fibers, and
can be utilized as a signal source in prostheses due to its
abundance of movement information. This paper proposes an
sEMG--detection circuit for the acquisition of the controlling
signal in EMG-Bridge (EMGB) systems. The detection circuit
mainly comprises a preamplifier, a driven right leg (DRL)
circuit, a high-pass filter (HPF), a low-pass filter (LPF), and a
gain adjustable amplifying circuit. The common-mode rejection
ratio (CMRR) of the circuit is higher than 120 dB, the input
impedance is greater than 100 MΩ, the passband range is
20~450 Hz, and the frequency attenuation in stopband is not less
than 120dB/dec.
I. INTRODUCTION
Limb paralysis is a severe loss of muscle function often
caused by stroke and spinal cord injury (SCI). For example,
nearly 80% of stroke survivors suffer from limb paralysis due
to the brain damages [1], an increasing problem with aging
(b) High-Pass
(a) Preamplifier
population. The recovery of motor function after stroke and
SCI has been a difficult problem in the field of rehabilitation
[2].The electromyography bridge (EMGB) proposed by our
researching team [3] is an advanced technique to help patients
rebuild their limb function. The principle is using the real-time
sEMG signals of a healthy limb to control functional electrical
stimulation (FES) on the paralytic limb, allowing voluntary
and flexible rehabilitation. Because sEMG signals are
normally the sum of the electrical contributions of the active
motor units (MUs) as detected by electrodes placed on the
skin superficial to the muscle, they have the following two
main characteristics:
1. Small amplitudes: Most sEMG signals are relatively weak:
their amplitude range is 0.01~5 mV, with peak to peak and
root mean square values of less than 6 mV and, 1.5 mV,
respectively.
2. Low frequency: The frequency range is 0~500 Hz, with
most of the energy concentrated between 50~150 Hz [4].
Therefore, a key component of the EMGB system is the
sEMG
Filter (HPF)
(c) Low-Pass
Filter (LPF)
OPA132
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(e) Gain
(d) Driven Right Leg (DRL) Circuit
OPA132
Adjustable Amplifier
R20
TLC2252
(f) Level
C10
A
D
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Lift Circuit
Fig.1 The schematic of the sEMG-signal detecting circuit.
*
This work was supported by National Natural Science Foundation of
China (No. 61534003) and the Jiangsu Province Science and Technology
Project of China (No. BE2016738)
Xiao-Bin Chen, is with State Key Laboratory of Bioelectronics, Southeast
University, Nanjing, P. R. China. ([email protected])
Yu-Xuan Zhou, is with State Key Laboratory of Bioelectronics, Southeast
University, Nanjing, P. R. China.
Hai-Peng Wang, is with the Institute of RF- & OE-ICs, Southeast
University, Nanjing, P. R. China.
Xiao-Ying Lü, corresponding author, is the vice director of State Key
Laboratory of Bioelectronics, Southeast University, Nanjing, 210096, P.R.
China
(phone:
025-83793430;
fax:
025-83793430;e-mail:
[email protected]).
Zhi-Gong Wang, corresponding author, is the director of the Institute of
RF- & OE-ICs, Southeast University, Nanjing, 210096, P. R. China (phone:
025-83792882; fax: 025-83792882; e-mail: [email protected]).
Xiao-Ying Lü and Zhi-Gong Wang are also with the Co-innovation
Center of Neuroregeneration, Nantong University, 226001 Nantong, China.
978-1-5090-2809-2/17/$31.00 ©2017 IEEE
detection circuit. In this work, we present a novel design with
high performance in terms of the correct decoupling of
the sEMG signals and implementation of the channel
mapping.
II. SEMG DETECTING CIRCUIT
Fig. 2 shows the block diagram of the sEMG detection
circuit, and the detailed schematic we designed is presented in
Fig. 1. The circuit includes five parts: a preamplifier, a DRL
circuit, a high-pass filter (HPF), a gain adjustable amplifier,
and a low-pass filter (LPF).
382
Fig.2 The block diagram of the sEMG-detecting system.
A. Preamplifier
In order to improve the anti- interference performance of
the circuit, the preamplifier and the sEMG detecting
electrodes should be located as close as possible. Thus, we
combined the sEMG electrodes and preamplifier as a sensor
by packaging them in one rectangular plastic box with a size
of 20×35×7 mm3. There are two kinds of sEMG electrodes:
dry metal electrodes [5] and wet gel electrodes [6]. The wet
gel electrodes are mostly disposable electrodes, whereas the
dry metal electrode can be repeatedly used. Therefore, we
used two filamentary silver electrodes with cylindrical shapes
of s of 10-mm length, 1-mm diameter and 10-mm separation
interval(Fig. 3)
Fig.3 Illustration of the sEMG sensor with placement on the forearm (not to
scale).
common mode interferences on the skin and match the body's
reference potential to the circuit's reference voltage. The input
of the DRL can be obtained from the node of In-Amp’s gain
resistors R1 and R2 as shown in Fig.2(a).
C. Filtering and Gain Circuits
The task of the sEMG detecting circuit is to extract clean
EMG signals from a noisy background. In this work, we used
an In-Amp as the first-stage amplifier (preamplifier) to
achieve significant rejection of the common mode noises and
interferences. Because the frequency band of sEMG signals
ranges from 20 Hz to 500 Hz and most of the energy is
concentrated between 50~150 Hz, a high-pass filter and a
low-pass filter (Fig. 1 (b) and Fig. 1 (c)) were designed to
suppress the spectral components outside of that range. Two
Sallen-Key filters are used as the basic unit for the
fourth-order high pass and low pass filter circuits due to their
high input impedance and low output impedance
characteristics and lower requirements for operational
amplifier and precision resistors and capacitors. The high pass
and low pass filter circuits compose a pass band ranging from
20 Hz to 450 Hz, which can help to remove baseline wander
and motion artifacts. The Sallen-Key filter circuits are shown
in Fig. 4.
The signal amplitude detected by the filamentary silver
electrodes is very low, typically in the mV-range. In this work,
a preamplifier (Fig.2 a) has been designed. It consists of an
instrument amplifier and a voltage follower circuit. The gain
was fixed using an INA128 instrumentation amplifier
(In-Amp) and is calculated as:
with RG = R1 + R2 ( R1 = R2) . The fixed gain in this stage
provides the lowest value within our desired range, and higher
gains are provided by the gain adjustable amplifying circuit.
B. Driven Right Leg (DRL) Circuit
The sEMG-detecting circuit can detect small EMG
signals. Unfortunately, the patient's body also receives electric
interferences from the environment, especially at the
50/60-Hz frequency of electrical power lines. The use of DRL
has been suggested to eliminate the interferences by active
cancellation [7].
In this work, a DRL circuit is implemented to reduce the
Fig. 4 Sallen-Key low-pass filter (a) and high-pass filter (b).
In this work, the gain circuit is difficult to realize in the
first stage due to large DC offset without AC coupling. Our
design used a low gain amplifier in the first stage and then a
high gain in the final stage. Therefore, we have chosen a
two-stage gain adjustable amplifier (Fig.2 (e)) where the gain
can be increased beyond 100.
III. RESULTS AND DISCUSSION
A. sEMG Detection Circuit
In the fabrication of the sEMG detecting circuit, the
electrodes and the preamplifier were packaged into one sensor,
whereas the other circuits were made into an sEMG-signal
processing unit. The components of the sEMG detection
circuit are shown in Fig. 5.
383
Fig. 5 The components of sEMG detecting circuit
B. Experiments and Results
In order to verify the frequency selectivity of the filters,
its amplitude-vs.-frequency characteristic was measured. The
input signal is a sinusoidal signal with amplitude of 1 V. The
signal frequency ranged from 0 Hz to 40 Hz and from 300 Hz
to 4 kHz for the high-pass and low-pass filters, respectively.
The transfer functions of the filters are shown in Fig. 6. For the
high-pass filter, the transfer function tapers off at 17 Hz. For
the low-pass filter, the transfer function tapers off at 450 Hz .
(b)
Fig.6. Amplitude-vs.-frequency characteristic of the
highpass filter (a) and low pass filter (b)
(a)
The experimental procedure of detecting sEMG signals
of the extensor carpi ulnaris (ECU) using the proposed circuit
are described by following protocol:
1. Use ethanol to clean the right lower arm of the subject.
2. Attach the sensor to the skin superficial to the extensor
carpi ulnaris (ECU).
3. The subject performs repeated free wrist extensions.
4. A 0.5kg load is lifted, then the subject performed wrist
extensions
The output of the circuit during the wrist extension is
shown in Fig. 7 Analysis revealed a high signal noise ratio
(SNR) of amplitude. Accordingly, our circuit demonstrated
reliable sEMG signal detection, and the signals meet the
requirements as input for the EMGB system.
384
[7]
(a)
(b)
Fig.7 sEMG signal obtained during wrist extension exercise.
IV. CONCLUSION
We described the complete sEMG detection system and
presented a general circuit topology of a sEMG-signal
detecting circuit with detailed description of each component.
The experiment demonstrated that our circuit has a signal
detection capability with a high signal-to-noise ratio can be
reliably used in an EMGB system.
In our future work, the sensor can be further improved
with emphasis on further miniaturization of the circuit and the
investigation of using other electrode surface materials (e.g.,
gold and copper). A wireless interface between the PCB and
the computer may also be included in future designs..
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