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UW-Madison BME 301 - Research Paper - Device

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Problem StatementTo develop a light weight and compact device that will give the orientation andposition of a testing mouse's head relative to a sound speaker.Client ResearchOur client, Dr. Matt Banks of the Department of Anesthesiology, is currentlystudying how drugs affect cognition and perception of certain auditory stimuli. Usingmice as research subjects, the lab has connected electrodes to specific parts of eachmouse’s brain and measures stimulation due to an audio signal. The mice in the study areallowed to move freely in an 8in. x 4in. x 4in. cage for an extended period of time whilebefore and after drug administration data is collected. Since the position and orientationof the mouse’s ears effect how they interpret sound frequencies, it is crucial to know theposition and orientation of the mouse’s head, when the stimulus is emitted. Background InformationPeople perceive sound by processing information collected by the ear. Sound waves create vibrations in the inner ear, specifically at the cochlea, which are transduced into electrical signals that are sent along different neural pathways in the brain. These action potentials will fire to different parts of the brain depending on how the inner ear vibrates.The cochlea is filled with fluid that is divided into two parts by the basilar membrane. Emitted sound flexes the basilar membrane and produces traveling waves along its length (Elsea, 1996). The shape of the basilar membrane allows the sound waveto increase to a certain amplitude along its length and then quickly die out. The location and amplitude of the sound wave depends upon its frequency.Along the basilar membrane are tiny hairs which are connected to bundles of nerves. Motion in the basilar membrane bends the hairs which stimulate the associated nerve fibers which carry sound information to the brain. The location of the hair cells which are stimulating the nerve bundles along the basilar membrane depends upon the frequency of the sound.The mechanics of the ear are well know, however the mechanics of interpretation of sounds are currently being studied. We know that a sound of a particular waveform Figure 1. A sliced diagram of the ear. (Elesea, 1996)Figure 2. Diagram shows the middle and inner ear. The oval window is connected thecochlea which stimulates the basilar membrane. Certain hairs on the membrane then stimulate nerve fibers which fire information to different parts of the brain. (Elesea, 1996)and frequency gives a characteristic pattern of active locations on the basilar membrane (Elsea, 1996). It is speculated that if the pattern repeats enough we learn to recognize that pattern and associate it to a certain sound. We are also able to perceive the direction of the sound source. Directional perception is done by the brain processing the arrival time of the sound to each ear or perceiving the difference in phase of sounds at each ear. Hence where one’s head is positioned relative to a sound will stimulate different parts of the brain.Design SpecificationsThe design must be lightweight and small since the mice themselves approximately weigh 20 grams. The transmitter or receiver which is located on the mouse should be less than 1 gram in weight and no bigger than 1cm x 1cm x 1cm. It should be time efficient (easy and quick to calibrate) and must not require constant maintenance. The device must be able to operate in an enclosed sound proof chamber with limited dimensions. The orientation and position sensor must also not interfere withthe current testing equipment and have an accuracy of at least one quadrant ( 90° accuracy). The final product should be safe to use and cause no harm to the testing mice. In addition, the cost of the device cannot exceed $5,000. Design OptionsUltrasoundUltrasonic or acoustic signals are used in many applications for determining location of an object. This option is low cost and is functional in the testing conditions. For this design there would be a transmitter located on the mouse’s head. This will mostlikely be mounted on the head cap of the mouse where the electrodes are placed (approx. 1cm x 1cm). The transmitter will produce a series of ultrasonic pulses. These pulses willbe picked up by transducers which will be placed around the cage which the mouse is located. By measuring the voltages produced by the pulsing from the various transducersthe position of the head can be determined. To determine the orientation two transmittersmust be used where the angle in the x-y plane can be determined by the voltage change between the two emitters given at one receiver.The transmitters are of lightweight and are relatively cheap, costing around $10 each. The transducers are also small and cost effective (approx. $10). Each is on a scale of about 0.5in x 0.5in base. The lightest transmitters we have found weigh 6 grams which is much too heavy for the mouse. It is possible to decrease the weight by striping the casing of the emitter and replacing it with a lighter material.A driving circuit will have to be assembled to produce a pulsing signal from the transmitter. This circuit will consist of at least a pulsing generator to produce the pulsing waves and an amplifier to increase the gain of the circuit. Other components will be added to increase the efficiency of the circuit.Figure 3. An example of a transmitter/transducer. (Massa, 2003)An amplifying circuit must be also be made to amplify the signal received by the transducer to a readable quantity. This circuit will consist of at least one amplifier to increase the magnitude of the signal. It will also contain a potentiometer to obtain the best common mode ratio and a high pass filter to reduce noise. An example of a driving circuit is shown in figure 5. The benefits of using this design are that it is low cost, lightweight, feasible, and small. Some problems that may arise for this design are interference from signals Figure 4. A sample driving circuit. (Massa, 2003)Figure 5. A sample amplifing circuit.bouncing back to the receiver, interference from cage material, and weight constrictions. Also the system to interpret the signal is yet to be determined. Magnetic SensorThe basic concept of the electromagnetic sensor is that a current will be produced in wires by a magnetic field. These sensors consist of magnetic wire wrapped around a core which produces a capacitance. Depending upon the size, order, and


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