Abstract
The use of EMG as a means for control is a topic that has been studied by many different groups. The most common forms of myographic control allow the user to control prosthetics using a number of muscles. Controls are designed in a way such that the movement of the prosthetic is determined, and preprogrammed to follow certain routines when the state of each of the control muscles fit predetermined patterns. This method of control is safe and user friendly, however; limits the user feedback and the versatility of the prosthetic. The goal of this project is to create an EMG amplifier and servo motor controller to be used with a two degree of freedom mechanical gripper. The control of the gripper will be accomplished by directly relating the magnitude of the EMG signal obtained from a forearm muscle to the servo motors torque and the grippers position. The objective set out to achieve is to provide a user with the ability to manipulate the gripper with both fine and coarse adjustments and also have the ability to increase the gripping power. In addition, feedback via a light vibrating motor will allow the user to get a feel for the grip strength being applied.
The use of EMG as a means for control is a topic that has been studied by many different groups. The most common forms of myographic control allow the user to control prosthetics using a number of muscles. Controls are designed in a way such that the movement of the prosthetic is determined, and preprogrammed to follow certain routines when the state of each of the control muscles fit predetermined patterns. This method of control is safe and user friendly, however; limits the user feedback and the versatility of the prosthetic. The goal of this project is to create an EMG amplifier and servo motor controller to be used with a two degree of freedom mechanical gripper. The control of the gripper will be accomplished by directly relating the magnitude of the EMG signal obtained from a forearm muscle to the servo motors torque and the grippers position. The objective set out to achieve is to provide a user with the ability to manipulate the gripper with both fine and coarse adjustments and also have the ability to increase the gripping power. In addition, feedback via a light vibrating motor will allow the user to get a feel for the grip strength being applied.
Electromyographic (EMG) signals provide a very useful means of control of robotic prosthetics. In recent years research in biomedical engineering and other similar areas, along with advancements in electro mechanics, have led to the creation of advanced and accurate robotics prosthetics. The foremost problem encountered when dealing with physiological signals is isolating the desired signal from that of its like counterparts. When measuring motor unit action potentials, it can be hard to assure that the acquired signal is that of the muscle under examination.
Once obtained the EMG signal can be used for several applications. In applications such as determining muscle fatigue, the frequency spectrum of the signal possesses the desired information, whereas, for control applications the overall energy of the signal provides the preferred control parameter. The goal for this project is to use EMG signals obtained from the forearm to control a mechanical gripper. Not only will the focus be on obtaining an accurate signal from the muscle, but also creating an easily controllable servo mechanism that will provide user feedback. For gripping tasks, a drawback for the user can be their inability to identify grip strength. The goal of this project is to provide the user with a vibration sensation to indicate gripper strength, while allowing the user to vary this strength accordingly.
The process of muscle contraction begins in the brain and is transmitted through the spinal cord. From the spinal cord, motor neurons or axons branch several times before coming in contact with muscle fibers. The combination of these motor neurons and the muscle fibers which they connect to, make up what is called a motor unit. The motor neurons, however; do not necessarily connect exclusively to separate muscle fibers, any given segment of muscle may have fibers belonging to 20 – 50 motor units . The number of fibers in a motor unit dictates the muscles precision, generally muscles that make more robust movements contain more fibers per motor unit. An action potential propagates through the axons via an electrochemical gradient created within the cell body. This action potential causes a contraction, and depolarization, of the muscle fiber and thus the creation of an electrical potential within the muscle fiber. It is this electrical field that is the source of the signal that will be detected from the surface of a muscle using an electrode. When all of the fibers in a motor unit fire a motor unit action potential (MUAP) is created. This process is repeated many times by multiple motor units, giving the myoelectric signal its characteristic of a dense frequency spectrum up to around 500 hertz.
Almost every signal amplification, and even more commonly with physiological signals, noise removal plays a major role in signal acquisition. Generally an amplifiers quality is its ability to remove such noise and more accurately represent the desired signal. There are commonly 3 main types of noise to consider when obtaining EMG signals through surface electrodes. Electromagnetic radiation, inherent noise in electronics, and motion artifact will adequately cover the types of noise that will be encountered in obtaining the EMG signal. Maximizing the signal-to-noise ratio will come as a direct result of minimizing these factors when obtaining the desired signal.
Electromagnetic Radiation
Figure - Differential Amplification of EMG Signal
The human body acts much like an antenna for many different types of signals, the most important one of these signals to consider is that of the standard North American electrical power line, located at a frequency of 60 Hz. The magnitude of this noise at the skin surface can be in the range of 3 times that of the EMG signal making its removal inparative. Also as discussed in the previous section the majority of the EMG signal is located from 0 – 500 Hz, thus this 60Hz ambient noise must be removed to insure that the EMG signal is accurately represented. A high Q value notch filter would suffice for removing this interference, however; because the desired signal has energy located at this frequency a differential amplification process is a superior solution to this problem. The design that was implemented for this project uses just such a method. Figure below shows how a differential amplifier can be used to remove noise. Due to the fact that the electromagnetic interference is common to both electrodes subtracting the signals can remove the commonalities in the signal.
All electronics have inherent noise which can range from 0 to many thousand of hertz. As this noise cannot be removed, its existence must be considered and the choice of high quality components can be the best solution to creating a better amplifier.
Another common source of noise is motion artifact. The source of motion artifact is both by electrodes moving on skin surface and due to electrode wire movement. This motion usually causes lower frequency noise in the range of 0 – 20 Hz. The removal of this motion artifact is accomplished via a simple high pass filter with a cutoff frequency somewhere between 15-40 hertz. Due to the fact that this frequency band is in the range of the EMG signal eliminating noise of this type will ultimately cause EMG signal energy loss as well. A tradeoff must be made between signal energy loss and the elimination of motion artifact.
There are two types of electrodes for obtaining EMG signals, inserted (invasive) electrodes and surface (evasive) electrodes. The ease of use and lack of pain associated with surface electrodes makes their implementation for this project preferable. Surface electrodes come in many varieties, with most characterized by the number of contacts. Some different types of surface electrodes are monopolar, bipolar, tripolar and multipolar, all of whose geometry is described by there name. For the purpose of this project a bipolar electrode will be used along with a reference electrode in order to implement the differential amplifier.
Electrode placement is important when using any of the electrode types described above. With the bipolar electrode the optimal position of the electrodes is parallel to the muscle fibers in order to maximize the probability of reading the same signal. electrode placements and the resulting potentials. the desired position for electrodes is on the belly of the muscle and not on the outer edge of the muscle where other muscles could interfere with the muscle under examinationThere are two types of electrodes for obtaining EMG signals, inserted (invasive) electrodes and surface (evasive) electrodes. The ease of use and lack of pain associated with surface electrodes makes their implementation for this project preferable. Surface electrodes come in many varieties, with most characterized by the number of contacts. Some different types of surface electrodes are monopolar, bipolar, tripolar and multipolar, all of whose geometry is described by there name. For the purpose of this project a bipolar electrode will be used along with a reference electrode in order to implement the differential amplifier.
Electrode placement is important when using any of the electrode types described above. With the bipolar electrode the optimal position of the electrodes is parallel to the muscle fibers in order to maximize the probability of reading the same signal. electrode placements and the resulting potentials. the desired position for electrodes is on the belly of the muscle and not on the outer edge of the muscle where other muscles could interfere with the muscle under examination.
Electrode placement is important when using any of the electrode types described above. With the bipolar electrode the optimal position of the electrodes is parallel to the muscle fibers in order to maximize the probability of reading the same signal. electrode placements and the resulting potentials. the desired position for electrodes is on the belly of the muscle and not on the outer edge of the muscle where other muscles could interfere with the muscle under examinationThere are two types of electrodes for obtaining EMG signals, inserted (invasive) electrodes and surface (evasive) electrodes. The ease of use and lack of pain associated with surface electrodes makes their implementation for this project preferable. Surface electrodes come in many varieties, with most characterized by the number of contacts. Some different types of surface electrodes are monopolar, bipolar, tripolar and multipolar, all of whose geometry is described by there name. For the purpose of this project a bipolar electrode will be used along with a reference electrode in order to implement the differential amplifier.
Electrode placement is important when using any of the electrode types described above. With the bipolar electrode the optimal position of the electrodes is parallel to the muscle fibers in order to maximize the probability of reading the same signal. electrode placements and the resulting potentials. the desired position for electrodes is on the belly of the muscle and not on the outer edge of the muscle where other muscles could interfere with the muscle under examination.
The preferred method of amplification for this application is differential amplification using a bipolar electrode and instrumentation amplifier. It is this methods ability to remove electromagnetic noise that the body has picked up that make it the most attractive for this application. The main specification that must be considered when selecting an instrumentation amplifier for this task is its CMRR or common mode rejection ratio. When the input signals are subtracted and the difference amplified, there is a residual signal amplified that is common to both inputs. The CMRR, represented in dB, is the measure of the ratio of the amplified signal to the amount of amplification of the signal common to the inputs. In the case of EMG amplification the common signal is most commonly noise so this specification plays a critical role in acquiring an accurate signal. A reference electrode is used like that seen in figure this electrode is used to create a common body reference signal which will increase the amplifiers ability to remove the unwanted commonalities of the inputs.
The power for the device will come via two standard nine volt batteries supplying the positive and negative rail voltages. The logical voltage and motor voltage will be supplied using a LM7805 +5 voltage regulator.
The burr-brown INA 2128 instrumentation amplifier was selected for this design due to its high CMRR at high gain. The CMRR of the INA2128 at a gain of 1000 is about 130 dB, which is more than sufficient for this design. The first stage of the amplifier is shown in the figure 4, to calculate the resistance for a certain gain, equation 5.1 can be used
Ga =1+50k ohm /2R where,R =R1+R2 ....................[5.1]
for the first amplification stage a gain of around 1000 is desired
R = 50 K ohm/999*2 = 25 ohm ...............................[5.2]
figure 4. preamplifier stage
The initial amplified signal is high passed filtered; this filtering is used to remove the motion artifact of the signal. For this a cutoff frequency of 30 hertz was chosen. This value was chosen to make certain that all motion artifacts would be removed. It was decided that the frequency content that will be removed from the signal was negligible. For the reason that an active electrode will be used in this design, it is preferred to have a minimal amount of components in the first stage that will be located on the electrode. For this reason an active filter was not used for this high pass filter and a simple RC type filter was implemented.
After the initial amplification and filtering stage the signal is amplified further. Further amplification is needed due to the small magnitude of the signal being acquired. As stated earlier the signal can be in the microvolt range so an amplification of 20k or more is in the desired range. Using equation 5.3, the gain of this amplification stage was set to be variable between 0 and 200, giving a final maximum gain of 197k
Gpre = 1+50 k ohm/54 Gvar = 1+ r7/r5 ............[5.3]
Gpre = 926. Gmax var = 1+ 100 k ohm / 470 ohm
Gmax var = 213
Gtotal = Gvar*(means , multiplication) Gpre
= 197 k
figure . variable gain amplifier.........
Rectifier and Low Pass Filter
The control system that will be used for the servo gripper will read an analogue voltage.
figure : half wave rectifier
Since the nature of the EMG signal is alternating with many different frequency components the signal must be processed such that we obtain a smooth analog output. The first step to accomplishing this task is to rectify the signal and obtain the positive portion of the voltage waveform. Following this the signal must be low passed filtered to smooth the ripples in the signal. There is a tradeoff in selecting the low pass filter cutoff frequency however; that is a lower frequency will take longer to respond to changes in the users actual muscle force. A higher frequency cutoff on the other hand, will cause a higher ripple voltage which will give skewed data when an analog to digital conversion is made. A cutoff of 0.3 hertz was chosen experimentally as an optimal value for this application. Figure 6 and 7 show the implementation of this stage.
figure : low pass filter.
Bias Adjustment
The final stage of the EMG amplifier is a bias adjustment. The signal may have some DC offset at this point which is undesirable for the next stage of the servo control system. This stage, shown in figure 8 will be used to eliminate this DC offset using a summing amplifier with a variable, negative supply input. Equation 5.4 shows how this amplifier is used to adjust the dc offset of the signal.
figure :- bias adjustment.
V emg_ out = V in* Av + V bias.
where , Av = 1 + R20 / R19 + R 22 and ,
0<= V bias <= -9
Therefore it can be seen that this stage adds a negative voltage to an amplified version of the input signal, where the gain can vary from unity to 1.319.
control system part in second post. ( contd.)
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