Based in California, Merit Sensors Systems designs piezoresistive pressure senso

Question

Based in California, Merit Sensors Systems designs piezoresistive pressure sensors for medical applications. A case study of this company will provide insight into piezoresistive technology (PRT), its design limitations, calibration techniques, and medical usage. Write a 3/4 page report Based in California, Merit Sensors Systems designs piezoresistive pressure sensors for medical applications. A case study of this company will provide insight into piezoresistive technology (PRT), its design limitations, calibration techniques, and medical usage. Write a 3/4 page report Based in California, Merit Sensors Systems designs piezoresistive pressure sensors for medical applications. A case study of this company will provide insight into piezoresistive technology (PRT), its design limitations, calibration techniques, and medical usage. Write a 3/4 page report

Explanation / Answer

Piezoresistance of a semiconductor can be described as the change in resistance caused by an applied strain of the diaphragm. Thus, solid state resistors can be used as pressure and force sensors, much like wire strain gages, but with several important differences and advantages. The high sensitivity, or gage factor, is perhaps 100 times that of wire strain gages. Piezoresistors are implanted into a homogeneous single crystalline silicon medium. The implanted resistors are thus integrated into the silicon force sensing member. Typically, other types of strain gages are bonded to force sensing members of dissimilar material, resulting in thermoelastic strain and complex fabrication processes. Most strain gages are inherently unstable due to degradation of the bond, as well as temperature sensitivity and hysteresis caused by the thermoelastic strain. Silicon is an ideal material for receiving the applied force. Silicon is a perfect crystal and does not become permanently stretched. After being strained, it returns to the original shape. Silicon wafers are better than metal for pressure sensing diaphragms, as silicon has extremely good elasticity within its operating range. Silicon diaphragms normally fail only by rupturing.

The sensing element of a MICRO SWITCH solid state pressure or force sensor consists of four nearly identical piezoresistors buried in the surface of a thin circular silicon diaphragm. The thin diaphragm is formed by chemically etching a square cavity into the surface opposite the piezoresistors. The unetched portion of the silicon slice provides a rigid boundary constraint for the diaphragm and a surface mounting to some other member. A pressure or force causes the thin diaphragm to flex, inducing a stress or strain in the diaphragm and also in the buried resistors. The resistor values will change depending on the amount of strain they undergo, which depends on the amount of pressure or force applied to the diaphragm. Therefore, a change in pressure (mechanical input) is converted to a change in resistance (electrical output). The sensing element converts (transduces) energy from one form to another. The resistors can be connected in either a half-bridge or a full Wheatstone bridge arrangement. For a pressure or force applied to the diaphragm using a full bridge arrangement, the resistors can be theoretically approximated as shown in Figure 1 (non-amplified units). R ± DR and R – DR represent the actual resistor values at the applied pressure or force. R represents the resistor value for the undeflected diaphragm (P J O) where all four resistors are nearly equal in value. DR represents the change in resistance due to an applied pressure or force. All four resistors will change by approximately the same value. Note that two resistors increase and two decrease depending on their orientation with respect to the crystalline direction of the silicon material. The signal voltage generated by the full bridge arrangement is proportional to the amount of supply voltage (Vcc) and the amount of pressure or force applied which generates the resistance change DR.

MEASUREMENT TYPES

Absolute pressure is measured with respect to a vacuum reference, an example of which is the measurement of barometer pressure. In absolute devices the P2 port is sealed with a vacuum representing a fixed reference. The difference in pressure between the vacuum reference and the measurand applied at the P1 port causes the deflection of the diaphragm, producing the output voltage change. Absolute Pressure Signal conditioned sensor output is shown. One volt output represents perfect vacuum.

Differential pressure is the difference between two pressures. For instance, the measurement of pressure dropped across an orifice or venturi used to compute flow rate. In differential devices measurands are applied to both ports. Differential Pressure Signal conditioned sensor output is shown. One volt output occurs when pressures are equal on both ports

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