EMG Biofeedback Device
University of Wisconsin-Madison
College of Engineering
Biomedical Engineering 300
October 10th, 2003
Team Members
Thomas Chia
Timothy Rand
Kimberly Treml
Client
Arleigh Birchler, MDiv, BSN
Advisor
John G. Webster, Ph.D.
Biomedical Engineering
Abstract
We are designing an EMG biofeedback device that will serve as an on/off switch for a vibrating massage pad. This device will give a child with Lissencephaly some control in his life. The child will learn that he can use this device to create pleasurable sensations for himself through operant conditioning. The design utilizes two surface electrodes detecting EMG signals from the child’s quadriceps. The main design variable is the massage pad which will become a permanent fixture in the child’s chair. It must be comfortable and cohesive with the child’s ergonomics. We chose ergonomic fit as our main judgment criteria. Other judgment criteria included cost, simplicity, and durability. Our team’s conclusion was that disassembling an existing massage pad to create a customized pad is the best design option.
Problem Statement
The purpose of this project is to design a closed loop EMG biofeedback device that will serve as an on/off switch for a vibrating back massage pad. A small child with Lissencephaly will use this complete system allowing him to give himself a pleasurable tactile sensation at will. Using electric circuitry, the massage pad will turn on when a specific muscle creates a sufficiently large electric signal. Subsequent signals from the muscle will turn the system off.
Introduction
A child with a very rare condition, Lissencephaly, provides the main impetus for this project. Lissencephaly, underdevelopment of the brain due to a birth defect, affects less than 100 people worldwide (Birchler, 2003). The condition is referred to as a “smooth gray matter” condition because nerve cells migrate to abnormal positions in the brain during development and do not create normal convolutions on the surface. The severity of Lissencephaly varies but is often accompanied by respiratory complications, severe mental retardation, poor to no motor control and seizuring (Dobyns, 2001). The child our client represents retains responsiveness to tactile stimuli such as touch and vibration but retains very limited muscle control. Our client believes the child has a fully developed limbic system, which allows him a full range of emotions and the ability for stimulus-response learning.
Through a biofeedback electromyography (EMG) device our client hopes to teach the child motor control using operant conditioning. An EMG signal is a measure of electrical discharge by a muscle group (Clark, 1998). The EMG design incorporates the three important tasks that the child is capable of: creating a muscular signal, feeling and responding to tactile stimuli. It is our client’s belief that this will be a humane and charitable goal that will allow an otherwise disadvantaged child some control over his life.
Design Requirements
We have a body of information already gathered in regards to our design problem (See Appendix A). The client reported much of this information to us. He has already done a great deal of planning on the project and has passed this on directly to aid in our design process. Much of the research our group has performed deals with the feasibility and application of our client’s plan. We have delineated the design into four main components: the child, the EMG circuit, the EMG electrodes and the vibrating pad.
The client has informed the team of a few constraints on the design of our device due to the child. One major design constraint is working around devices that the child is already utilizing. This includes the chair he uses continuously, an apnea monitor, and a feeding tube directly connected to his stomach. Additionally, the client suggested the EMG electrodes be attached to the child’s thigh where there is the greatest likelihood of obtaining a strong EMG signal. The electronic circuits and controllers of the EMG biofeedback device must not interfere electronically or physically with his current monitors and assisted living devices. Comfort and unobtrusiveness are also major design concerns because of the human component of the project.
EMG Circuit
The circuit components of this project will likely be the most complicated aspect of the design. Some members of the group have experience with the desired EMG circuit design from a class taken prior to this course. Paul M. Victorey has already aided us in this circuit design by assisting in the selection of some of the amplifier components.
Paul M. Victorey is also researching and purchasing the timer, another electric circuit component. Information thus far consists of some basic timing circuits and switch components. The exact technical specifications of this component will be worked out along with the EMG circuit so a circuit board can be designed and manufactured for a prototype.
Figure 1: Block Diagram of proposed EMG circuit.
Figure 1 is a block diagram for our proposed EMG circuit. The components will be assembled with the single purpose of turning the vibrating motor on, timing its activity, and turning it off. It should also be able to turn the motor off with additional stimuli.
The electrodes collect an EMG signal from the target muscle group in units of actual voltage. The EMG signal is amplified to a higher magnitude using a differential op-amp. The amplified signal is rectified for purposes of signal averaging and passed through a filter to reduce ripple.
The comparator is the binary component which will commence the timing and activation portion of the circuit. Should the filtered EMG signal be large enough, the comparator sends an electric signal which cascades through the remaining components; the microcontroller and triac. The final portions of the circuit activate the pad, time the activity of the pad, and finally turn off the pad after a duration set in the microcontroller.
Logic
An additional component in our client’s design is a device that will be able to apply logic to the function of our device. The client’s plan involves using our device to teach the child basic action/reaction effects and muscle control. The logic component is basically a counter that would record each activity initiated by the child.
The logic component is one aspect of our design that will most likely have to be excised due to complexity. No group member has the circuit design experience to make an analog version of this logic circuit, nor do any have the programming experience necessary to make a digital logic circuit. The logic component could be constructed at a cost, by contracting the problem to another qualified person. This level of complexity would otherwise merit a whole different design project.
EMG Electrodes
Either needle or surface electrodes can measure EMG signals. For this device 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 (Thompson, 1998). However, the client has expressed interest in the possibility of using a reusable carbon-silicon rubber surface electrode. The EMG pads need to be easily attached to the child’s body, are reusable, and require minimal training for application.
For proper EMG signal acquisition, two or three electrodes are needed. EMG signals generated by the muscles need to be amplified over 1000 times to generate a usable signal, rectified and filtered to eliminate ripple. Once the signal is properly conditioned, a comparator circuit can determine if the amplitude of the EMG signal is strong enough to trigger the vibrating pad.
Vibrating Pad
There is a wealth of small vibrating devices available through retail. These range in function from full back massage devices that fit inside a chair, to simple massage pad motors. Investigation into this component of the design consisted primarily of price comparisons in search of a product that would fit our size constraints. A few different feasible solutions are provided in the alternative solutions portion of this paper.
One of the primary concerns is designing the vibrating pad to properly fit between the child and the chair. His chair is designed for a 16 kg child and has guards to keep his body stable (Figure 2). The vibrating pad will need to fit the unique design of his chair and be comfortable enough for long-term use.
Figure 2: A schematic of the child’s chair.
Solutions
All of our design solutions include the same circuitry and electrodes, components that provide very little area for variation. As mentioned previously, reusable carbon rubber electrodes will be used and placed on the child’s 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.
Alternative Solution I: Homedics BK 150
Design proposal number I 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 (Figure 3) and measures 19” long x 16” wide x 3.25” thick. The BK-150 has two vibrating motors located in the center of the pad, one above the other and can be powered by an AC adapter. There is also a simple remote that adjusts the massage intensity from off, low, or high and a switch to turn the heat function on and off.
The BK-150 is a good choice as the vibrating pad for a few key reasons. Its simple design has only two vibrating motors located in the center. The motors are located so that one motor will rest near his back and the other will rest under his buttocks. There is also a lower intensity vibration setting, which may be more suitable for our patient. The remote control is simple to use and therefore hopefully easy to make controllable by an EMG amplifier circuit. Finally the cost of the pad is reasonable; multiple vendors sell the BK-150 for around $25.00.
Some downsides to this solution are that 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 child’s chair. Also, this solution leaves little opportunity to modify the padding or change the location of the vibrating
motors.
Alternative Solution II: Modified, Customized Pad
Starting with a retail massage pad, the second option requires disassembling the pad to remove the vibrating mechanism. The ConAIR HP08 Massaging Heating pad would be a simple starting material. It comes with high and low vibration settings and a heat option. This mechanism will then be built into a homemade version of a massage pad. It may be possible to use the cushioning in the original massage pad when rebuilding it. Otherwise, foam material will need to be purchased and tested for durability and comfort since the child will be constantly sitting against the pad. We will also need to purchase fabric for the covering. The fabric should be comfortable and easily washable. Suggested dimensions are 11 ½ by 8 inches with a 9 ½ by 5 inch rectangle centered on the length of the larger rectangle. A pad of these dimensions should fit very well in the chair (Figure 4).
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Figure 4. Dimensions of the customized massage pad.
The most important feature of this design option is the high degree of customizability it provides. Pad size can be made to fit exactly with the child’s chair. In addition the vibrating mechanism can be placed in whichever position evokes the greatest response from the child. The thickness of padding can also be adjusted in certain areas to provide the maximum comfort. In addition to being washable, the cover of the pad can even be created with bright and colorful fabric to make it more fun for the child. The massage pad retails for
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24.99 but additional supplies will need to be purchased. As well, this design is accompanied by an increased cost in time.
Alternative Solution III: Wraparound design
A wraparound design is similar to the custom tailored design. It is assembled from different parts including a small vibrating motor, and padding. Where this design differs from customized pad is the ergonomic design; it will be built into a pad that can be wrapped around the child’s 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, and holds the vibrating portion comfortably against the child’s back. Unwanted movement of a chair based pad is removed with this design, as the pad will stay affixed to the child. Where a normal pad could become unaligned or move out of position as the child sits on it for extended periods of time, this design will continue to operate in the intended location.
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. The client originally intended the pad be designed on the chair to avoid just this problem. Other drawbacks include others associated with a customized pad, such as time consumption, expense, and further materials testing.
Design Matrix
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). Because the child will be sitting with the pad behind him at all times, 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.
Table 1: Design Matrix
Homedics BK-150 / Modified/Customized / WraparoundCost / 3
$25.00 / 1
$25.00 plus costs to modify and cost in time / 2
similar to Modified pad
plus fastening pieces
Simplicity / 3
requires only the modification to connect to EMG circuit / 1
requires disassembly and reassembly of pad and modification to connect to EMG circuit / 1
requires disassembly and reassembly of pad and modification to connect to EMG circuit
Durability / 2
durability assured without testing / 1
would have to test for durability of pad components / 1
would have to test for durability of pad components
Ergonomics (x3) / 3
no chance for modification / 9
can be very specifically designed for child / 6
marginal opportunity for specification
Total / 11 / 12 / 10
Modification of the ConAIR HP08 to create a custom fit for the child received the largest total points making it our best design option.