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ENTC 4350

ENTC 4350. MEDICAL INSTRUMENTATION TRANSDUCERS AND AMPLIFIERS. Although the measurement of physical parameters like force and pressure are rarely of medical interest in themselves, the determination of these parameters underlay a vast variety of medical techniques. Cardiac pulmonary function.

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ENTC 4350

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  1. ENTC 4350 MEDICAL INSTRUMENTATION TRANSDUCERS AND AMPLIFIERS

  2. Although the measurement of physical parameters like force and pressure are rarely of medical interest in themselves, the determination of these parameters underlay a vast variety of medical techniques. • Cardiac • pulmonary function

  3. To make a measurement, we must have something to measure. • Force and pressure are often difficult to measure directly and accurately. • We therefore measure these data indirectly by converting them into an electrical signal, which can be filtered, amplified, recorded, etc.

  4. SIGNAL TRANSDUCER DETECTOR AMPLIFIER RECORDER The figure shows the fundamental principles of the process of measuring physical data by means of electrical signals.

  5. SIGNAL TRANSDUCER DETECTOR AMPLIFIER RECORDER The transducer may be any device that converts physical energy into an electrical signal.

  6. SIGNAL TRANSDUCER DETECTOR AMPLIFIER RECORDER The interface is simply whatever connects or lies between the transducer and the patient.

  7. SIGNAL TRANSDUCER DETECTOR AMPLIFIER RECORDER The detector is any device used to pick out the electrical signal we want to measure. Not all transducers require a detector.

  8. SIGNAL TRANSDUCER DETECTOR AMPLIFIER RECORDER The amplifier amplifies the signal for the recorder, and the recorder records or stores the data.

  9. In most cases, the function of the transducer is to convert a physiological parameter into a voltage that is large enough to be processed accurately by the electronic equipment.

  10. Physiological parameters include • An extremely weak voltage, • A pressure, • A fluid flow rate, • A temperature, • A chemical concentration, or • An electrolyte level.

  11. To perform this task, the transducer must be properly placed on the patient, as well as strategically placed into an electronic circuit, such as a Wheatstone bridge. • Trans­

  12. CONVERSION OF PHYSIOLOGICAL PARAMETERS INTO VOLTAGES

  13. Three of the most commonly measured physiological parameters in health care are • temperature, • blood pressure, and • weight. • All of these may be measured by means of a balanced structure, such as a scale.

  14. Consider how a scale works. • Before the patient steps on it, the scale is in balance, and it reads zero. • Another way of saying this is that the scale pointer is on a null. • The patient on the scale throws it out of balance, causing a displacement of the pointer, which is calibrated in pounds.

  15. In this case, the physiological parameter of weight is transformed to a displacement of a pointer. • Here, the transducer is the platform the patient stands on, and the structure of the balance is the arrangement of levers and springs in the scale.

  16. Likewise, the physiological parameters of temperature and pressure are converted to a machine-measurable parameter—voltage—by a balanced structure. • In this case, it is a balanced circuit called a Wheatstone bridge.

  17. Wheatstone Bridge

  18. The Wheatstone bridge, which consists of four resistors arranged in a diamond shape and labeled R1, R1, R3, and Rx. • An excitation voltage, VE , is applied to two points of the diamond, and an output voltage, VOUT, is measured plus to minus from left to right across the other two points of the diamond.

  19. The two resistors on the left, Rxand R1, form a voltage divider of the VE excitation. • This produces the plus-to-minus voltage drop from node A to ground, VA.

  20. Likewise, the two resistors on the right, R2and R3, form a voltage divider that creates the voltage drop from node B to ground, VB.

  21. This circuit can be made balanced, in the simplest case, by making all four resistors the same value. • In this case, the voltage divider on the left creates the same voltage as that on the right, because they both have the same excitation voltage and the same resistor values. • Thus, VAequals VB .

  22. The voltage difference between the two nodes is defined as the output voltage, VOUT, so • In this case, VOUT is zero, and the bridge is said to be at a null point in terms of its resistance values. • That is, the bridge is balanced.

  23. This bridge can be made unbalanced by changing the value of Rx . • If Rxis caused to increase, the voltage divider on the left will cause VA to decrease in value. • Because the divider on the right is undisturbed, VBwill remain the same.

  24. Thus, VA becomes less than VBand VOUTbecomes a negative voltage.

  25. On the other hand, if Rxis caused to decrease from its null value, VOUTwill become a positive voltage drop from node A to node B. • As an exercise, prove that to yourself by studying the figure.

  26. You have learned the case where the bridge is balanced because all resistors have the same value. • In fact, the bridge can be balanced for any number of resistor value combinations given by the formula

  27. This equation is called the null condition for the bridge. • If Rxis increased above the value given by this equation, VOUTwill leave zero and be a negative voltage. • And if Rxis decreased from its null value, VOUTwill become positive.

  28. Thermistor • A thermistor is a transducer that makes it possible to convert the physiological parameter of temperature into a voltage. • A thermistor may be constructed of a cube of material, about 0.1 inch on a side, embedded in glass whose electrical resistance varies with its temperature. • Almost all electrical conductors exhibit this property to some degree.

  29. For example, if copper is heated, the atoms will vibrate harder, making it more difficult for free electrons to get past without a collision. • This increases its resistance. • Thus, copper has a positive temperature coefficient, because an increase in temperature causes an increase in resis­tance.

  30. Some metals act similarly, but in the opposite direction. • For example, an increase in temperature in a semiconducting metal like silicon will break more electrons free from their crystal bonds and increase the number of free electrons, so that an increase in temperature will decrease the resistance. • Because of this, silicon is said to have a negative temperature coefficient.

  31. Commonly used thermistor elements are made from oxides of nickel, copper, or aluminum. • This gives the thermistor elements a relatively high temperature coefficient.

  32. Temperature Transducer • A thermistor mounted in a Wheatstone bridge can function as the transducer that converts body temperature to a voltage. • This may be used as the transducer for an electronic thermometer. • Its advantage over the traditional mercury thermometer is its fast response time and ease of reading, not to mention the fact that mercury from a broken thermometer is a hazardous material.

  33. In a blood donor screening, for example, reducing the three minutes it takes to do a temperature with a mercury thermometer becomes important. • On the other hand, the electronic thermometer is more complicated, bulkier, and may not last as long as the mercury thermometer.

  34. Pressure Transducer • Blood pressure is most commonly measured with an air cuff and stethoscope using a device called a sphygmomanometer. • This is the noninvasive test given in a blood donor screening.

  35. For intensive care situations, however, it may be necessary to use an invasive procedure. • Here, the focus is on how the physiological parameter of pressure is transformed into a voltage.

  36. Diaphragm A B Armature C D Strain-gage wires • A commonly used pressure transducer is shown. • The dome on the top may be filled with a saline solution that articulates to a catheter, as in the heart to measure the blood pressure in a ventri­cle. • The other fluid coupling connection is blocked off.

  37. Changes in blood pressure propagate through the catheter and cause small displacements in the diaphragm. • These displacements move a plunger to which are connected four wires, called strain gauges.

  38. With each displacement, two of these wires lengthen and the other two get shorter. • Lengthening the wire increases its resistance, while shortening the wire decreases its resistance by the same amount.

  39. Lengthening a wire causes it to increase in resistance both because it gets longer and because its cross-sectional area reduces. • These high resistance wires are arranged in the form of a Wheatstone bridge.

  40. In the figure, each of the strain gauge wires is represented by a resistor, R, plus a change in resistance, DR, imposed by changes in pressure on the diaphragm. • Notice on the left branch of the bridge that a positive DR increases the upper resistance and decreases the lower re­sistance.

  41. Thus, VAwould decrease. • Because of the change in sign of the DRs on the right branch, VB would go in the opposite direction and increase. • The net result is that VOUT, defined as plus to minus from node A to node B, would be a negative voltage.

  42. If the pressure on the diaphragm changes to the opposite direction, VOUTwould become a positive voltage. • Thus, you have a mechanism that converts the pressure changes into voltage changes. • This voltage could be used to drive electrical meters and monitoring equipment.

  43. Pressure Transducer Sensitivity • In general, the sensitivity of a pressure transducer, SV, is defined as the change in output voltage per volt of excitation per millimeter of mercury of applied pressure (V/V/mmHg). • A typical commercially available pressure transducer has a sensitivity ranging from 5 mV/V/mmHg to 40 mV/V/mmHg, depending upon the manufacturer and model.

  44. Some disposable pressure transducers work on the same electrical principle just described. • The manufacturing process for these transducers is inexpensive enough that the unit can be disposed of rather than put through an expensive sterilization process. • In fact, in some cases, trying to sterilize a disposable unit can damage it and make it inaccurate.

  45. VOLTAGE AMPLIFIERS • Amplifiers are as old as history. • A lever with a fulcrum for prying up stone is a force amplifier.

  46. A force down on one side of the lever will cause a larger force going in the opposite direction to be exerted on the other side of the lever. • The closer the fulcrum is to what is being pried up, the larger that force will be.

  47. Notice that the output force is in the opposite direction from the input force. • This is an example of an inverting amplifier.

  48. A pressure amplifier is illustrated. • It consists of two disks attached to either end of a rod.

  49. If a pressure is exerted on the larger disk in the direction shown in the figure, the smaller disk will exert a larger pressure in the same direction. • For example, if PINon the disk on the left is 1 pound per square foot on a 1-square-foot area, the rod will transmit that 1 pound to the smaller disk at a pressure of 1 pound per square inch. • This converts to a pressure of 144 pounds per square foot.

  50. This, therefore, is an example of a pressure amplifier with a gain of 144. • In this case, the output pressure, POUTis in the same direction as PIN. • This is an example of a noninverting amplifier.

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