proyecto de adquisicion emg

7/30/2019 PROYECTO DE ADQUISICION EMG

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Dispositivo EMGBME 200/300

Final Design Report12-10-03

Abstract

The following report outlines a design of an electromyogram (EMG) controlledmassaging pad intended for use by a child with lissensephaly. The device was designedusing criteria and constraints suggested by a client familiar with lissensephaly. The finaldesign consists of an electrode attachment device, an analog circuit to amplify andprocess the patients EMG signal and a massaging pad to provide positive tactilestimulation to the patient. A prototype circuit was built and tested and the designfunctions properly. The circuit was constructed on a printed circuit board to consolidatethe circuitry, however future work is needed to complete the final printed circuit boarddesign.

Problem Statement

The purpose of this project is to design a device that will serve as an EMG

biofeedback device to activate a vibrating pad. A child with Lissencephaly will use this

device to activate the pad by flexing a muscle in his thigh. The vibrations will provide a

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pleasurable tactile sensation at his command, providing him some control over his life

which he currently lacks. An EMG signal of a chosen magnitude will activate the pad and

the pad will deactivate after a chosen time period.

Motivation

The child with Lissencephaly currently has no control over his life. This device

will give him a way to take some control. By allowing him to activate the pad on his own

the child will be given freedom that he currently lacks. Our device will also allow us to

observe whether the child is capable of simple operant conditioning; should the child

associate flexing his muscle with activating the massage pad it will demonstrate a

capacity for cognitive activity.

We would observe this association through a deliberate increase in the flexing of

the targeted muscle. This could provide a basis for further learning. The device could be

modified to give more options to the child. For example, a component of the circuit could

be added so that the child could also turn the device off with a second flex. This process

of learning would serve to improve the quality of the patients life

Lissencephaly

Lissencephaly is a brain disorder exhibited when the brain fails to correctly

develop. Lissencephaly is also referred to as smooth grey matter due to the smooth

appearance of the brain in patients with this condition (Figure 1). During development,

nerve cells are not able to reach the outer cortex of the brain, which in turn prevents cells

from making necessary connections. The deficiency in development also accounts for the

lack of ridges normally present [1].

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Symptoms of Lissencephaly can vary from case to

case. Some of the common problems experienced are

difficulties eating, loss of bodily control, severe mental

retardation, regular seizures and reoccurring pneumonia.

The severity of these symptoms varies greatly. The

lifespan of a person with Lissencephaly is projected at

around two years, but since the disorder is quite rare and

provides few recorded cases the lifespan is difficult to

gauge [1].

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Figure 1:

(above) A normallydeveloped brain.(below) A brain from apatient with Lissencephaly.Notice the smoothappearance of thisbrain[2]


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Design Problem

The EMG biofeedback device needs to detect a proper EMG signal and then

activate the massage pad for a predetermined amount of time. There are three major

components to the EMG biofeedback device; the EMG electrodes, the EMG amplifier

circuit and the massage pad.

Either needle or surface electrodes can measure EMG signals. We will be using

the less invasive surface electrode. Surface electrodes are often composed of a Ag +/AgCl

disk and an adhesive attached to the skin [3]. However, the client has specified that the

EMG electrodes need to be reusable. Reusable EMG electrodes are already in use with

the child for his apnea monitor. The EMG pads need to be easily attached to the childs

body, reusable, and require minimal training for application. A total of three EMG

electrodes will need to be attached to the patients body.

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Figure 2: Schematic of chair Figure 3: Patients actual chair


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The EMG circuit needs to detect a proper EMG signal and respond by activating

the massage pad. A proper EMG signal is defined as a voluntary muscle contraction. This

patient will occasionally have seizures which result in involuntary muscle contractions.

Therefore, the device will need to distinguish between voluntary and involuntary muscle

contractions. Once a proper EMG signal is detected the massage pad will be activated by

a timer for a predetermined amount of time.

The final component in the EMG biofeedback device is a massaging pad. The pad

is the device that will deliver the pleasurable stimulus to the patient. There is a wealth of

small vibrating devices available through retail, ranging in function from full back

massage devices that fit inside a chair to simple massage pad motors. The massage pad

used in our biofeedback device must fit the unique shape and size of the patients chair.

Figure 2 is a schematic of the chair with its dimensions and figure 3 is an actual picture of

the chair.

More detailed discussion of each component in the EMG biofeedback device is

found in the latter parts of this report. Overall, the entire device must be safe and reliable

for years of use by the child and his caretakers. The controls on the device also need to be

easy to learn and operate. Finally, throughout the design process the patients

confidentiality must be maintained. At no time can the patients name, personal

information, and/or pictures of the patient be released to the public.

Design Constraints

Designing our device specifically for this patient introduced a number of design

constraints. The first constraint was that our device must not interfere with any of the pre-

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existing equipment already in use with the patient. This includes his apnea monitor,

feeding tube, and oxygen tank.

The EMG amplifier designed for this project also needs to be designed for the

patients limited musculature. Most EMG amplifiers are designed assuming that the

subject has normal muscle tone; however for this project that assumption is not valid. To

further complicate matters, the teams access to the patient for testing and EMG

measurements is very limited. For privacy reasons, the mother of the child has not given

the team permission to work directly with him. Instead all matters involving the child

must first go through our client, Arleigh Birchler. Additional design specifications and

constraints can be found in appendix A.

Proposed Solutions

All of our proposed design solutions include the same circuitry and electrodes,

components that provide little area for variation. As mentioned previously, reusable

carbon rubber electrodes will be used and placed on the childs thigh per the request of

our client. The quadriceps is the best option for electrode placement because it is a large

muscle that the child can easily isolate and signal. Also, the thigh is not highly prone to

involuntary muscle contractions. Our design proposals vary in size, shape and application

of the vibrating pad. We will describe our best design solution in detail later in the report.

The first alternative design is to use a portable, prefabricated massage cushion for

the vibrating pad. The model of the massage cushion is the Homedics BK-150 Back

Master and measures 19 x 16 x 3.25. Even though the BK-150 is smaller then many of

the other massage pads on the market, it is still too wide to fit perfectly in the childs

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chair. Also, this solution leaves little opportunity to modify the padding or change the

location of the vibrating motors.

A second alternative design is a pad that wraps around the child. It is assembled

from different parts including a small vibrating motor, and padding. It will be built into a

pad that can be wrapped around the childs waist as opposed to attached to his chair. The

form of this design is similar to a wide belt. A Velcro piece attaches the two ends of

the pad in front of the child. A drawback to this design comes from the additional

equipment already in use. The apnea monitor and feeding tube would most likely

experience some interference from the front portion of the pad.

Table 1: Design Matrix

Homedics BK-150 Modified/Customized Wraparound

Cost 3$25.00

1$25.00 plus costs to modify

and cost in time

2similar to Modified padplus fastening pieces

Simplicity 3requires only the

modification to connect toEMG circuit

1requires disassembly and

reassembly of pad andmodification to connect to

EMG circuit

1requires disassembly and

reassembly of pad andmodification to connect

to EMG circuit

Durability 2durability assured without

testing

1would have to test for

durability of pad components

1would have to test for

durability of padcomponents

Ergonomics (x3) 3no chance for modification

9can be very specifically

designed for child

6marginal opportunity for

specification

Total 11 12 10

In order to decide which massage pad design alternative would be best we ranked

the solutions on the criteria of cost, simplicity of production, durability and

ergonomics/comfort (Table 1). The child will be sitting with the pad behind him at all

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times, so we deemed ergonomics the most important criteria weighing it three times

higher than the others. Each design option was ranked with a 1, 2, or 3; 3 being the best,

depending on how well it fit each criterion in relation to the other alternatives.

Modification of the ConAIR HP08 to create a custom fit for the child received the largest

total points making it our best design option.

EMG Circuit Design

The final circuit design contains seven main components. A functional block

diagram outlining the basic circuit design is shown below.

Figure 4: Block diagram of circuit design

Power is supplied to the integrated circuit (IC) components by two nine volt

batteries. Due to certain specifications of the IC components, two voltage regulators were

necessary to drop the power supply voltage from 9V to 7V. Attached in appendix B is the

precise circuit schematic. All components are named and all their values are shown.

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Two carbon patch input electrodes measure the potential difference created when

the patient contracts his/her muscle group. A third electrode is attached to ground to and

serves as a reference voltage. The potential difference created by the patients EMG

signal is then input into an instrumentation amplifier.

The instrumentation amplifier has high input impedance so it does not draw any

current from the patient. A differential amplifier then amplifies the potential difference

across the two electrodes and has a gain of 13. This value is determined by one external

resistor, R1 on the circuit schematic.

The signal is then passed through a passive high pass filter with a low corner

frequency of 1.6 Hz. This filter blocks any DC offset present in the signal. The final stage

of the instrumentation amplifier is an active low pass filter that has a gain of 158 and a

2027 Hz corner frequency. Overall, the gain of the entire circuit is about 2050.

The next stage of the circuit is a precision full wave rectifier [4]. The rectifier

takes the absolute value of the amplified signal. After the rectifier is a passive low pass

filter. The low pass filter serves to effectively average the signal.

At this point in the circuit the EMG input signal has been processed from a signal

similar to high frequency noise, to essentially a DC signal with amplitude proportional to

the strength of the patients muscle contraction. This signal now can be easily compared

to a set threshold voltage.

The next stage in the circuit, a comparator, performs the threshold operation. The

threshold voltage, or reference voltage, can be manually controlled via a potentiometer

that feeds the positive input of the comparator. The reference voltage set on the

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comparator stage of the circuit determines the strength of muscle contraction needed to

trigger the timer device.

If the signal is strong enough, the comparator outputs positive saturation (+7V) to

the timer stage of the circuit. However, if the amplitude of the input signal is not greater

than the set reference voltage then the output of the comparator will stay at negative

saturation (-7V).

A 555 timer IC component was used to establish a specific time to trigger the

massage pad. The input to the timer determines whether the output is either high or low.

A high output passes the signal to the next stage of the circuit and triggers a timed

countdown, while a low output gives no input to the next circuit stage. If the timer

receives a positive signal from the comparator the output will be high for about two and

half minutes. The time constant of the timer stage is determined by the product of the

values of R9 and C10 on the circuit schematic.

After the duration of one time constant, the output of the timer will return to low.

The high output of the timer is about 7 volts and is sent to the next stage of the circuit, a

solid state relay device.

The relay serves as a switch that safely activates the massage pad unit upon

receiving a high input from the timer. The relay is optically isolated so that there is no

opportunity for the high voltage and current supplied by a wall outlet to reach the patient.

The final stage is the massage pad unit. The pad contains a transformer that steps

down the 120 volt wall voltage to 12 volts for pad operation. One of the massage pad

cord wires was severed and the two output wires of the relay device were attached across

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the severed lead to complete the design. See appendix B for an exact schematic of the

circuit design.

Circuit Board

A circuit board was drawn to match specifications laid out in the circuit

schematic. Guidelines for drawing a circuit board were used [5]; thicker traces for power

supply and ground, no closed-loop ground trace, and sufficient spacing for traces and

pads.PCB Express was the program used to draw and order the circuit boards used in the

final design.

Most of the components for the schematic were ordered as SMT (surface mount)

components through Digi-Key. Custom footprints were drawn for components that

were unavailable in SMT, requiring common push-through parts such as diodes, wire

terminals, potentiometers and capacitors used with the voltage regulators. Components

were attached to the circuit board in the final design using a simple soldering procedure.

The design of the circuit board condensed the circuitry in to as little space as

possible. The remainder of the board was used as space for the battery casing. These

batteries were used to power the circuitry components independent of the wall source,

which powered the vibrating pad. See appendix C for printed circuit board design.

Consolidated Circuitry

A black plastic box was selected to house the various electrical components

separate from the pad. This protects the components from damage, and protects a user

from accidental electrical exposure. The physical dimensions of the box were

approximately 3 3 6. The limited volume was used to contain the circuitry,

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batteries, and SSR in a protected compartment. The three components needed to be as

compact as possible, and this was mostly accomplished through condensing the circuit-

board design.

The circuit board was mounted inside the black box using four bolts attached to

the corners. The SSR was attached alongside the circuit board. Openings in the black box

would have been drilled for the electrode leads, and the massage pad wire fed in series

through the SSR. These holes were not completed in the prototype due to the un-

functional operation of the circuitry upon assembly.

Figure 5: Printed circuit board mounted in black box.

Massage Pad

The massage pad is the second major component of the biofeedback device,

which provides a stimulating and pleasurable feedback. We chose the ConAIR HP08, an

off-the-shelf massage pad to modify for the project. This particular pad was chosen

because its dimensions (11 x 24) match the childs unique chair. In addition, the pad

is less than one inch thick so it should not disturb the comfortable fit of the chair that the

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child is accustomed to. The ConAIR HP08 offers two massage intensities and a heat

option that can be adjusted to create the most pleasurable sensation for the child.

To improve the quality and functionality of the pad we created a fabric cover

similar to a pillow case to put over the pad (Figure 6). The cover serves many functions.

Made with a fabric with a child friendly pattern, it appears less like a medical device than

the plain massage pad would and new covers can be made as the childs interests change.

More importantly, the cover is removable and washable to increase sanitation since the

child will be sitting on it twenty-four hours a day, seven days a week. Finally, the cover

will reduce the amount of wear on the pad increasing its useful life.

Figure 6: Modified massage pad with protective cover.

Electrodes

Electrodes and means of attaching them are the final component of the full design.

We chose to use carbon-rubber electrodes to fulfill the request of our client that they be

reusable and inexpensive. The child is already using carbon-rubber electrodes on other

devices so he and his caretakers are already familiar with them. Moreover, the carbon-

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rubber electrodes are an excellent choice because they are non-invasive. They simply rest

on the skin rather than penetrate it or adhere to it. With the addition of a thin layer of

conductive gel they work nearly as well as an adhesive electrode.

The electrodes will be attached to the child with a soft, felt strap that will

Velcro around the childs thigh and can be adjusted for a perfect fit as the child grows

(Figure 7). The strap is washable for the same reason as the massage pad cover. The

electrodes can also be removed from the strap for washing. The electrodes attach to the

strap with Velcro to assure simple replacement in the correct position after they have

been removed. This is important because the electrodes must be in approximately the

same place each time in order to obtain consistent results.

Figure 7: Electrode attachment device.

Ethics

Our project has presented us with some important ethical considerations. These

considerations stem from the fact that our device is designed for a specific patient rather

than the general public. Of utmost importance was honoring the clients request that all

sensitive patient information be kept confidential. All operating components of our

design must be safe for use within specific design parameters for this patient. Additional

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technical support could be necessary, but is possible with such exclusive patient use. All

possible emphasis was placed on meeting these ethical requirements.

Conclusion

The EMG biofeedback project has been very successful in proving that an EMG

signal can be obtained, amplified, and used to control the activation of a massage pad.

Our large degree of accomplishment shows that a project of this sort can be completed to

the given specifications. There are, however, a number of steps that need to be taken

before this prototype can be implemented with the child.

The first step towards completion is an accurate measurement of the childs EMG.

This measurement needs to include measurements in multiple locations to find an ideal

position on the body. From these measurements the EMG amplifier can be calibrated to

properly amplify his biological signal to proper amplitude to activate the massage pad.

Voluntary and involuntary EMG signals will also need to be measured. These

measurements could allow the design of an IC to distinguish between a conscious versus

involuntary muscle contraction, such as during a seizure.

The patient will finally need to be monitored to see if he is adapting to the device.

This will be the ultimate test of this projects success. If clients predictions are correct,

the patient will eventually condition himself to understand that a particular muscle

contraction will elicit a pleasurable stimulus.

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References

[1] Dobyns, William.Lissencephaly: Information for Parents. 1991. Online:http://www.Lissencephaly.org/about/lissen.htm

[2] Lissencephaly Newsgroup. Acessed: Oct 2003.http://groups.yahoo.com/group/Lissencephaly/

[3] Webster, John.Bioinstumemtation.New York: John Wiley and Sons, 2004. Chapter 2

[4] Medical Instrumentation Application and Design. 3rd Ed. John G. Webster Editor.John Wiley and Sons Inc. NY 1998.

[5] Victorey, Paul. Personal consultation. Oct-Dec 2003.

Sources Consulted

Webster, John. Personal consultation. Oct-Dec 2003

Birchler, Arleigh. Personal consultaion. 12 Sept. 2003

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Appendix A

Product Design Specification:

EMG Biofeedback Device

Team Members:

Brent Geiger LeaderJason Ethington CommunicationsTom Chia BSACKim Treml BSACTim Rand BWIG

Advisor: John G. WebsterRevision date December 7, 2003

Function: An EMG biofeedback device will allow a child with Lissencephaly to exhibitsome control over his life.

Client Requirements: Client would like the device to serve as a learning tool for thechild. The device will consist of four components: electrodes, amplifier, informationprocessor, and massage pad.

Electrodes

Two/three carbon patch electrode placed on skin on childs thigh muscle. The

electrodes must not be intrusive or irritating to the child. They must require littleskill to attach and remove.Amplifier

A basic EMG amplification circuit will be used.Information Processor

This stage must distinguish between voluntary and involuntary musclecontractions by establishing a baseline. The amplified EMG signal will trigger themassage pad. This stage will control the duration of vibrations, and record howmany voluntary contractions occur in a certain time period. The processor willterminate the massage after a specified time period. It should regulate massagepad to eliminate over stimulation by child.

Massage PadThe massage pad must fit in the childs chair. The pad should be comfortable forthe child and not change the environment to which he has became accustomed to.The massage must not be too strong and must not cause the child any discomfort.

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Design Requirements:

Physical and Operational Characteristics

a. Performance requirements

Must be able to perform at any given time 24/7. Massage mechanism will beactivated 5-6 times a day. Must be easy to operate and have minimal learningcurve for operator.

b. SafetyMust not harm patient in any way. Device will not have any sharp areas, exposedwires, or abrasive material. EMG pads will not draw current from the patientsbody. Massage pad vibration will be noticeable, but not cause pain. Entire devicemust not interfere and be compatible with current devices he is hooked up to.

c. Accuracy and ReliabilityMassage feedback must trigger every time an intentional muscle contraction isperformed. Device must accurately distinguish between involuntary (C-posturing)

and voluntary muscle contractions. Vibrational feedback must only be turned onfor 5 minutes each time a voluntary muscle contraction is detected.

d. Life in ServiceMust be in service for as long as patient is able to utilize the device. Properoperation should be checked for each week. Major calibration checked for every 6months.

e. Shelf LifeThe shelf life of our product will be five years stored at room temperature in a drylocation.

f. Operating EnvironmentThis device will be used in a home environment. It will be subjected to constant

pressure caused by the patient sitting on top of the vibrating pad. The device maybe subjected to fluids and dust caused by everyday use. This device will behandled by a nurse or caretaker for the patient.

g. ErgonomicsThe patient will be sitting on this device for most of the day so it is important thatthe device does not change the current environment as to annoy or agitate thepatient. The device must as comfortable as the chair that the patient currentlyuses. A washable, protective pad will cover the massage pad to minimize wearand increase sanitation.

h. SizeThe device must fit in the current chair of the patient. The bottom of the chair is

11.5x11.5. The device must fit between two side guards that are 9.5 apart and8 above the bottom on the back of the chair. The back of the chair is 15 tall.

i. WeightThe information processor should be no more than 5 pounds so that it is easy totransport with the child. The electrodes and device used to hold them in placeshould be just a few ounces so that they do not inhibit or encumber the child.There are no weight specifications for the massage pad because we will assumewhichever model we use is already at a suitable weight.

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j. MaterialsThere are no restrictions on materials.

k. Aesthetics,Appearance, and FinishThe massage pad must be soft and comfortable. The apparatus for applying theelectrodes must also be comfortable so that it does not bother the child in any

way. The massage pad cover has been designed with a colorful pattern that ischild orientated.

Production Characteristics

a. Quantity

One. Childs guardian gets to keep prototype if she wishes.b. Target Production Cost

$100-$200. More if digital microcomputer processors are incorporated intodesign.

Miscellaneous

a. Standards and SpecificationsHuman subjects testing approval may be required. Approval from childs doctorsand mother also required.

b. CustomerConfidentiality is of utmost importance for both the child and the childs mother.No information whatsoever regarding their whereabouts can be disclosed.

c. Patient-related concernsChilds EMG signal may be somewhat irregular, amplifier must be able toproperly amplify signal. Device must fit into childs chair. Device must be able tobe cleaned periodically due to constant contact with child. Child could possiblyfind device uncomfortable and irritable. Again, all patient information must be

held in confidence.d. Competition

Lissencephaly is an extremely rare. There are no devices designed for thispurpose.

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