Overview of Sensor Signal Paths
Abstract: This tutorial explains the sensor signal chain for the most popular sensor transducer types for pressure, temperature, current, light, and proximity sensing. The article introduces the intricacies of selecting a signal path. Example circuits and block diagrams help the reader select an optimal set of parts to meet their design needs.
Pressure Sensors and Weigh Scales (Force Sensing)
The need to detect and measure pressure and weight is a very common requirement for modern industrial control and system monitoring. Pressure measurement is especially important, as it is also used indirectly to measure flow, altitude, and other properties. Pressure- and weight-measurement devices can be regarded as “force sensors,” since force is the property that affects the transducers’ outputs. The applications for force sensors are vast and range from vacuum gauges, to heavy machinery weigh scales, industrial hydraulic equipment, and manifold absolute pressure (MAP) sensors for internal-combustion engines. Each application has its own diverse needs for precision, accuracy, and cost.
Although there are several methods and technologies for measuring pressure and weight (force sensing), the most commonly used measurement element is the strain gauge.
The two most common types of strain gauges are the metal-foil type used in a variety of weight/pressure sensors, and the semiconductor-based peizoresistive transducers, widely used to measure pressure. Compared to metal-foil-type transducers, piezoresistive transducers are more sensitive with better linearity, but have large temperature dependence and large initial offsets.
In principle, all strain gauges react to an applied force by varying a resistance value. Therefore, in the presence of electrical excitation they effectively convert a pressure or weight to an electrical signal. Usually, one, two, or four of these active resistive elements (strain gauges) are arranged in a Wheatstone bridge configuration (sometimes called a load cell) to produce a differential output voltage in response to pressure or weight.
Engineers can design a sensor module that meets the unique requirements of diverse force-sensing applications. A successful design will include the suitable sensing element for the physical property and an appropriately designed signal chain.
Complete Signal-Chain Solutions
The sensor signal chain must handle extremely small signals in the presence of noise. Accurately measuring changes in the output voltage from a resistive transducer requires circuitry that provides the following electrical functions with precision: excitation, amplification, filtering, and acquisition. Some solutions may also require the use of digital-signal processing (DSP) techniques for signal manipulation, error compensation, digital gain, and user programmability.
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