Electricity + Control July 2018

FLOW MEASUREMENT + INSTRUMENTATION

Simply put, a transducer converts one type of physical property into an electrical output signal. An example of this is the conversion of hydraulic pressure into a vari- able voltage or amperage output. Demystifying transducers in fluid power Paul J Heney, Design World/ WTWH Media

T he manner in which a transducer works will depend primarily on the type of measure- ment being observed. Some transducers measure pressure, temperature or flow, while oth- ers determine fluid level, position and even water or particle contamination. Pressure transducers Pressure transducers are the most common type of transducer used in hydraulic systems. Oth- er properties are easily monitored via analogue gauges or indicators. Because pressure is dynam- ic in range and rate of change, the accuracy and responsive measurement – as is provided by an electronic device – is required. A pressure transducer is often a strain gauge, which measures changes in resistance, induct- ance or capacitance. Here, strain refers to the deformation of the metallic membrane, as its shape changes while exposed to the force of pres- sure. Electrical resistance is the expression of how much the conducting material resists the flow of electrons. Inductance describes the inertia of electrical current. A big coil of wire, for example, will resist changes in current flow, either increasing or de- creasing. Inductors are less practical in modern electronics, mostly because of their size. Capacitors are much more common than coils, although they work differently. A capacitor stores excess incoming electrical energy, ready to re- lease it when downstream energy drops again. If this description sounds familiar, you’re thinking about hydraulic accumulators. They absorb incom- ing hydraulic energy, ready to blast it out when

downstream pressure drops below accumulator pressure. Capacitors, just like accumulators, are capable of extremely high bursts of energy, and should be carefully applied. Regardless of the electrical change caused by strain force, an active pressure transducer will use its strain gauge to modify incoming electrical volt- age into a usable variable analogue output. Unlike some other electrical sensing devices, like a ther- mocouple or microphone, an electronic pressure transducer uses various electrical components, such as a wheatstone bridge, to create an ac- curate variable output. The electronics on board these transducers require a power source, which is typically 12, 24 or 120 V, although some sensors will take a range of input, such as 8-30 V. The wheatstone bridge common to most mod- ern transducers uses either bonded strain gauges, or piezoelectric types, which are less common in hydraulic applications. In its signature diamond shape, the wheatstone bridge is a combination of resistors and strain gauges that accurately meas- ures strain force, and therefore pressure. Howev- er, because of the harsh environmental conditions of a hydraulic system, pressure transducers re- quire other electronics to improve their accuracy, repeatability and reliability. In the electrical circuit, Figure 1 , it is easy to spot the wheatstone bridge. However, all the oth- er componentry is there for other purposes. Part of it is to ensure changes in temperature (which changes the electrical resistance) and does not af- fect accuracy. If the resistance of the wheatstone bridge changes, so does the pressure signal. Other parts of the circuit improve hysteresis. Hysteresis

Take Note!

A temperature transducer is capable of: Reading the changes in resistance as caused by a change in temperature. It uses on-board elec- tronics to improve ac- curacy, resolution and response time. 1

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40 Electricity + Control

JULY 2018