Piezoresistive Sensor Applications

Thursday, November 23rd, 2017 - Piezoelectric, Pressure, Resistive Transducers

Piezoresistive Sensor Applications

Piezoresistive strain gauges are the most widely used sensors among all types of strain gauges. Capacitive and inductive strain gauges’ sensitivity to vibration, the special mounting requirements, and circuit complexity have limited their applications. Photoelectric strain gauges are costly and delicate although they can be made as small as 1/16 inch in length. Piezoresistive strain gauges are applied in measuring acceleration, force, torque, pressure, and vibration as discussed below.

Piezoresistive Accelerometers

Piezoresistive Sensor Applications

Figure 1. (a) Piezoresistive spring type accelerometer; (b) suspension arm type accelerometers.

A piezoresistive accelerometer usually contains a mass-spring system designed so that the force exerted by the spring exactly equals the force required to accelerate the mass, and the displacement of the mass (deflection) is directly proportional to the acceleration measured by the strain gauge. Other common methods for measuring the deflection of the inertial mass are capacitive (measuring the gap between a movable electrode and a fixed electrode) and piezoelectric methods (measuring the produced voltage). Figure 1 shows two piezoresistive accelerometers developed by Honeywell Sensotec, Columbus, Ohio. The first one (Figure 1.a) uses a piezoresistive spring, where the force exerted by the mass changes the resistance of the spring. The second one (Figure 1.b) measures the deflection of a seismic mass using silicon or foil strain gauges placed on the suspension arm of the mass. Both accelerometers contain a built-in Wheatstone bridge circuit to measure the resistance change and produce an electric output.

Piezoresistive accelerometers have advantages over piezoelectric accelerometers in that they can measure both dynamic and static (or zero Hz) accelerations. This is due to the fact that a resistive sensor is a nonenergy storage component (a zero-order system) and there is no energy stored within the component; therefore, its output is immediately available resulting in a near zero response time.

Piezoresistive Pressure Sensor

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Figure 2. A piezoresistive internal combustion engine sensor

Piezoresistive pressure sensors are critical devices in a variety of control and automobile applications. Figure 2 shows an internal combustion engine sensor designed by Kulite Semiconductor Products Inc., Leonia, New Jersey. It uses four piezoresistors to measure the stress in a silicon diaphragm caused by the force or pressure of the media. These four piezoresistors are connected electrically to form a Wheatstone bridge. At the corners of the diaphragm, five 0.024-mm-diameter gold bond wires (ultrasonically ball bonded to the sensor) allow electrical connections to the bridge. The sensor has a resonant frequency above 150 kHz, which also meets the stringent combustion requirements. This sensor can withstand the engine’s harsh  environment—extreme operating temperature of 500°C and high vibration.

Piezoresistive Flow Rate Sensor

A piezoresistive flow rate sensor

Figure 3. A piezoresistive flow rate sensor

Flow rate can be measured using a variety of methods. One technique is to take the differential pressure across two points in a flowing medium (e.g., one at a static point and one in the flow stream). Pitot tubes operate based on this principle and have long been used to measure flow rates. Another way is to use the Venturi effect by placing a restriction in the flow path and measuring the pressure difference. Figure 3 shows a third means to measure flow rate—a bending vane with an attached piezoresistive strain gauge, whose resistance change is proportional to the flow rate and is measured using a Wheatstone bridge circuit. The advantages of a piezoresistive flow rate sensor are:

  1. it can measure both air or water flow rate in one, two, or three dimensions and
  2. it can detect sporadic, multidimensional, or turbulent flow.

Piezoresistive Blood Pressure Sensor

A silicon piezoresistive sensor can also be used for intravascular blood pressure monitoring. When the silicon piezoresistive element (directly attached to a stainless-steel plunger) is under an applied force or pressure, the resistance of the silicon element increases, providing a low-cost, extremely compact, and fast-response blood measurement method.

Piezoresistive Force Sensor

Many force, torque, and tactile sensors are piezoresistive types. Table 1 shows the general specifications of a compact LPM 560 piezoresistive micro force sensor made by Cooper Instruments & Systems, Warrenton, Virginia, USA.

Specifications of LPM 560 Micro Force Sensor

Table 1. Specifications of LPM 560 Micro Force Sensor

Piezoresistive Imaging Sensor

Carnegie-Mellon University developed an implantable MEMS stress imager to directly measure bone strength in situ on a microlevel scale. This imaging sensor is composed of an array of piezoresistive sensors (“ohm-pixels”) with a resolution of 100 Pa and an average measurement time of 1 s. It also includes a coil antenna for RF power and remote sensing. The sensor array is embedded into a textured surface to accommodate sensor integration into the bone. The array of piezoresistive elements offers a safe and convenient way to measure bone strength in situ and provides improved and timely information for bone treatment, including prescription of drugs, fixation adjustments, rehabilitation regiments, and preemptive surgical intervention.

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