Applications of Resistive Transducers in Biomedical Instrumentation

Applications of Resistive Potentiometer

Caliper Myograph

It measures change in displacement at various applications. One such application is to detect the changes in the chest (Thoracic) circumference. It operates by connecting a rotary potentiometer to a chest band on a person. In this setup, the transducer acts as a pivot to the caliper arms. They attach to the muscles as shown in the figure below. Therefore, now when there is a contraction of muscles, it makes the arm driven apart. Finally, the potentiometer measures the motions of the arm.
caliper myograph

Spirometer

Spirometer is a device used for studying respiratory functions. It has a water tank, bell jar immersed upside down inside water and an external tube connecting the mouthpiece. One end of the string connects the bell jar and another to weight via a pulley. The subject breathes air to the tube-using mouthpiece. For every cycle of inspiration and expiration, bell jar moves up and down. The jar movement depends on the volume of air exhaled or inhaled. Accordingly, the weight attached on another side of the string also moves up and down. Pen recorder draws the output continuously resulting in a graph called Kymograph. In this device, a potentiometer mounted on the axis of the pulley measures the movement of the bell-jar.
kymograph

Tremor Detection of Parkinson’s Detection

A miniature low-torque potentiometer detects the tremor in patients’ diagnosed with Parkinson’s disease. Two rods get attached to the hand and lower arm, to detect the flexion and extension of the patient’s wrist. One rod connects the rotor and another to potentiometer body. The setup is supplied with 1.4V using mercury battery. This detects the 0 – 10 degree tremor at 3 – 6 Hz and recorded. The recorded values used to evaluate drug therapy.

Applications of Strain Gauge

  • The strain gauge is mounted on the cardiac muscle wall. It continuously measures the force exerted on the cardiac muscle due to contraction of the heart chambers.
  • Strain gauge measures the blood pressure inside the heart or blood vessels to diagnose cardiovascular abnormalities. Attach strain gauge to the tip of the catheter. Inject the catheter into the heart through veins. The catheter is a tube used for administering medicines. On the front side of the strain gauge diaphragm is mounted. The diaphragm is an elastic material, which undergoes deflection due to the applied force. Therefore, when blood pressure inside the heart varies, it deflects the diaphragm that in turn changes the strain gauge resistance.
  • While using artificial limbs, pressure (load) acts on the surface. The applied load is measured with a strain gauge.
  • Strain gauges measure the change in blood flow through arm and legs. Place the strain gauges around the arm or leg. When blood flows through arm or leg the diameter changes. This, in turn, changes the strain gauge resistance. A plethysmograph is a device used to measure blood volume. Strain gauge measures pressure or stress in terms of strain. It makes use the principle of Piezo – Resistive Effect, which states that due to mechanical stress resistivity changes.

Applications of Photoelectric Type Resistive Transducers

Photoresistor or LDR uses transmittance method to measure the pulsatile blood volume changes. A small lamp and photoresistor are placed inside a box, which can be worn on the fingertip. The entire setup acts a bridge circuit. Along one of the bridge arm, the LDR is fastened. The resistance of the photoresistor is proportional to the light intensity that falls on it. The photoresistor is made of semiconductor material. So, when light falls on the LDR, resistance decreases. The bridge output varies based on the intensity of transmitted light. In our body, when heart muscle contracts, blood flow in finger increases. This results in a change of optical density in the finger. As optical density varies, the amount of light being transmitted through finger decreases. The resistance of photoresistor gets increased.
resistive transducers

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