The Difference Between Piezoresistive and Foil-Based Pressure Sensing Technologies

Have You Ever Wondered How Pressure Sensors Work?

Pressure sensors consist of specially designed structures, that perform in a predictable and repeatable manner when force is applied. This force is translated into a signal voltage by the resistance change of the strain gages, organized in an electrical circuit and, applied to the load cell structure. A change in resistance indicates the degree of deflection and, in turn, the load on the structure.

The electrical circuit consists of strain gages or piezoresistors (silicon-based platform), typically connected in a four-arm (Wheatstone Bridge) configuration. This acts as an adding and subtracting electrical network and allows compensation for temperature effects as well as cancellation of signals caused by extraneous loading. The basic output is a low level voltage signal (i.e., mV), but through the use of signal conditioning and amplifiers, this signal can become a higher level voltage or current (i.e., 0 V to 5 V, 0 V to 10 V or 4 mA to 20 mA). These signals can be used to drive a digital/analog display, be part of a monitoring system, or form part of a closed loop feedback control system. The low level signal can also be converted into a digital output (i.e., RS-232RS-232, RS-485). However, USB is another option.

Defining Pressure

Pressure is the force per unit area exerted by a fluid or gas. The recognized International System of Units (SI) for pressure measurement is the Pascal (Pa); however, pounds per square inch (psi), inches of water (in-H2O), Newtons per millimeter squared (N/mm2) and Bar are also common. The most critical mechanical component in any pressure transducer is generally the pressure sensing structure (spring element). The pressure of the fluid or gas is a force on the pressure sensing structure. The function of the structure is to serve as the reaction for this applied force; and, in doing so, to focus the effect of the force into an isolated uniform strain field where strain gages can be placed for pressure measurement.

Figure 1: Black Diagram of Pressure Transduction

Pressure Sensing Technologies

While there are various types of pressure sensing technologies, two will be discussed in this paper: Piezoresistive-Type Pressure and Foil-Based Pressure.

Piezoresistive-Type Pressure

In piezoresistive-type pressure sensors, the transduction elements which convert the stress from the diaphragm deflection into an electrical signal are called piezoresistors. Piezoresistance equals changing electrical resistance due to mechanical stress. The pressure sensing element is a diaphragm which is made from silicon. This silicon diaphragm is attached to a glass substructure (i.e., that acts as a constraint/mounting structure for the silicon). This silicon diaphragm structure performs in a predictable and repeatable manner as the pressure is applied (i.e., a very slight deflection in the structure). This pressure is translated into a signal voltage by the resistance change of the strain gages which are doped (i.e., implanted) onto the silicon diaphragm surface, then organized in an electrical circuit.

Figure 2: Piezoresistive-Type Pressure Sensor

The silicon diaphragm, with the exposed doped Wheatstone Bridge, in test and measurement pressure sensors, is isolated from the pressure media being measured (i.e., media isolated pressure sensors). This is achieved by creating a cavity between the media being measured and the silicon diaphragm, then filling it with oil that does not attack the silicon or electrical circuit. On the opposite side of the cavity is a metal/steel diaphragm that is flexible to transmit the pressure being measured to the oil in the cavity, and the silicon diaphragm. This metal/steel diaphragm is called the isolating diaphragm. This technology is used to measure pressures from inches of water (in-H20) to 10,000 psi (6.9 kPa to 69 MPa). Note: 1kPa = 1 N/m2.

At a very top level, this technology can be described as a pressure sensor consisting of a micromachined silicon diaphragm with piezoresistive strain gages diffused into it, fused to a silicon or glass back plate. Pressure induced strain increases or decreases the value of the resistors (i.e., strain gages). This resistance change can be as high as 30 %, that typically yields one of the higher outputs from a pressure sensing technology. The resistors are connected as a Wheatstone Bridge, and the output of which is directly proportional to the pressure.

Foil-Based Pressure Sensors

Another common type of pressure sensor utilizes a bonded foil strain gage to measure an applied pressure in one of two ways. In some models, such as miniature pressure sensors, foil strain gages are bonded to the back of a steel diaphragm that is exposed to the media being measured.

Figure 3: Foil-based Pressure Sensor

The diaphragm structure performs in a predictable and repeatable manner as the pressure is applied (i.e., a very slight deflection in the structure). This pressure is translated into a signal voltage by the resistance change of the strain gages, arranged strategically around the diaphragm surface, and is organized in an electrical circuit.

However, in many other models, the foil strain gages are bonded to an element that is mechanically connected to a diaphragm, then exposed to the media being measured. The strain gaged element is measuring the force transmitted from the diaphragm by the mechanical linkage. This element acts as a load cell (i.e., designed to measure force that is directly proportional to the load applied to the diaphragm). This technology is typically used to measure pressures from 10 psi 174,000 psi (69 kPa to 1,206 MPa). Note: 1kPa = 1 N/m2.

Types of Pressure Sensors

In addition to the various sensor categories and designs available, there are a multitude of specific types within each sensor category. Pressure measurement applications can be divided as follows:

• Absolute: This is the pressure measured as referenced to an absolute vacuum. An example would be a barometer that measures the absolute pressure of the atmosphere.

• Gage: This is the pressure measured as referenced from an atmospheric pressure to a pressure being measured. An example would be a pressure gauge on a process pipeline.

• Sealed Reference: Very similar to gage pressure, except the back side of the sensing element is sealed during the fabrication of the sensor. The gage pressure is now a measure from this reference atmospheric pressure to the pressure being measured. This type of sensor will be susceptible to changes in atmospheric pressure and will give significant error measurements at lower pressures. More suited for higher pressure applications where the sensor has to be sealed from the environment (i.e., eliminating fluid or gas ingress).

• Vacuum: Similar to gage pressure, except the pressure in the system is below atmospheric pressure.

• Differential: This is the pressure difference measured between two different pressures. Typically wet/dry differential pressure is a measure of a liquid on the sealed side of the sensing element, and a dry gas on the back exposed side of the sensing element. Wet/wet differential pressure is typically a measure of two liquids, and both sides of the sensing element is isolated from the media.

Figure 4: Types of Pressure Sensors

Wheatstone Bridge

Pressure sensors become a transducer with the utilization of the Wheatstone Bridge. When a force is applied (i.e., in this case a tensile force) to a structure with a single strain gage bonded to it (could also be a piezoresistive gage doped to a silicon substrate), the induced strain field will cause a resistance change (ΔR) in the gage (i.e., in this case, an increase in resistance).

So we can see that strain ε is equal to the change in length of the structure (ΔL) divided by the original length (L) of the structure, which in turn is proportional to the change in resistance (ΔR) divided by the original resistance (R) when no force is applied.

A measure of this conversion from strain to resistance is called the Gage Factor (F), and it can be defined as the ratio of change in resistance, divided by the ratio of change in length of the structure, which is the strain induced in the element.

In a load cell (also applies to pressure and torque sensors), these strain gages are strategically placed on the force sensing element, and wired in such a fashion to make up the electrical circuit called a Wheatstone Bridge. The gages are placed to ensure that some of the resistance changes are increasing (+ resistance), and some are decreasing (- resistance). The end result is to unbalance the output of the bridge which will be proportional to the force applied to the load cell structure.

The Wheatstone Bridge is powered by a fixed input voltage (V in), which is typically, but not limited to, 0 Vdc to 10 Vdc applied across points A and C, and the output from the bridge is measured as a voltage between points B and D. In the unloaded condition, the output voltage (V out) measured between points B and D will be approximately 0, and when the full scale load is applied, the output voltage (V out) is measured in millivolts (mV). Typically the sensitivity of a load cell is 2mV/V (i.e., if the input voltage was 10 V, then the output would be 20 mV in the fully loaded condition).

Figure 5: Wheatstone Bridge

Working with Instrumentation

Typically, the output from Wheatstone Bridge-based pressure sensors is used to monitor and/or control an application or process. Since the full-scale output from a bridge-based sensor is typically low level voltage (i.e. 20 mV to 300 mV), additional signal conditioning (SC), and conversion to a digital signal (A/D) is typically required.

Two options are shown below, with both being very similar in that the mV signal from a Honeywell sensor is converted into the digital signal. The difference between the two revolve around combining the SC and A/D functions for Option 1, with separate instrumentation, and separating them in Option 2 (i.e., keeping the SC on the sensor and providing the A/D with separate instrumentation). The digital signal can then be supplied to a PC, where a variety of software programs can process the input, store the data, make decisions, etc. Honeywell provides pressure, load and torque sensors that have an amplified output (i.e., signal conditioned signal), and this is shown in Option 2, where the signal can be converted into a digital signal.

Figure 6: Instrumental Options for Pressure Sensors

Examples of Pressure Sensors