SENSING TECHNOLOGIES
SENSING
TECHNOLOGIES SENSORS
are used heavily in fluid power applications, to measure critical functions such as position, flow, pressure, temperature, and more. The variety of devices that exist to gauge these functions are many, but here, we take an in-depth look at some of the more commonly used devices — position and pressure sensors and transducers.
PRESSURE TRANSDUCERS
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Pressure is defined as the force per given area required to stop a fluid from expanding. Pressure transducers, which are a subset of pressure sensors, can be any number of devices that sample and record the pressure in a system. A pressure transducer converts a pressure measurement into an analog electrical output signal, which can be used by sensing instrumentation such as microprocessors and computers. Most often, this is accomplished simply through physical deformation or mechanical deflection. Important criteria to consider when selecting a pressure transducer are the general mechanism type, input and output, and performance specifications. The most common types of pressure transducers are strain gauge, and thick/thin film. Strain gauge transducers use the mechanical deformation under pressure of strain-sensitive variable resistors, which may be integrated into measurement circuits such as a wheatstone bridge. In a thick/thin film transducer, a titanium nitride or polysilicon film may be applied to sensing equipment to impart the circuit with piezoelectric sensitivity to pressure. Almost all pressure transducers require a source of electrical input. The transducer input voltage can vary but typically falls under 10 V, while the output is typically in the hundreds of thousandths of volts. A change in the system’s pressure would cause a change in the transducer’s
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resistance on the electrical circuit and would result in a change to the output voltage. With the aid of an analog to digital converter (ADC), the transducer’s output signal can be used in systems that require digital signals. For example, a programmable logic controller (PLC) or a programmable automation controller (PAC) can use the digital signal to monitor the pressure and take action if needed. Some pressure transducers output current rather than voltage, and are then often referred to as transmitters. These values typically fall within tens of thousandths of amps. When choosing the output of a pressure transducer, it is important to keep in mind the input requirements of the device that will be accepting the signal, the distance the signal must travel and possible interference that can be found in the environment around the system. Important performance criteria to consider are the pressure transducer’s operating pressure range, maximum rated pressure, accuracy and operating temperature range. The operating pressure range demarcates the intended pressure bounds at which the transducer has been designed to perform optimally. The maximum rated pressure is the highest allowable pressure that the pressure transducer is rated to withstand. The accuracy of the transducer is usually represented by suppliers in terms of ASME B40.1 grades: 4A (0.1%), 3A (0.25%), 2A (0.5%), A (1%), B (2%), C (3%) and D (4%) deviance from the true pressure value. A good pressure transducer is designed to operate independently of temperature; however, the operating temperature specifies a “safe” range; 7 • 2020
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