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Pressure is a key factor in almost every fluid-power circuit. Pressure transducers convert fluid pressure to an electrical signal for monitoring and control of pressure in electronic control systems.
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Pressure transducers, when connected to an appropriate electrical source and exposed to a pressure source, will produce an electrical output signal (voltage, current, or frequency) proportional to the pressure. Most transducers are designed to produce output that is linear with the applied pressure and independent of other system variables — the most important of these being temperature. Most outputs are mV, V, mA, and, sometimes, as a frequency.
Pressure transducers have a sensing element of constant area and respond to force applied to this area by the fluid pressure. This force deflects a diaphragm, bellows, or Bourdon tube. In turn, these deflections, strains, or tensions are converted to electrical outputs through any of a variety of different transduction methods. Figure 1 illustrates three of these.
To operate, most pressure transducers require an electrical input (usually called excitation). Many operate from a 5- to 10-Vdc input and produce full-scale outputs from, say, 0 to 20 mV and 0 to 100 mV. Transducers that produce high level voltage outputs operate from voltage sources. Typical outputs are 0 to 5, 1 to 5, 1 to 6, and 1 to 11 Vdc. Digital control circuits can be interfaced by routing transducer output through an analog-to-digital (A/D) converter or by using a transducer with a frequency output. This allows pressure to be monitored by microprocessors, programmable controllers, computers, and similar electronic instruments.
Pressure transducers that generate a current output usually are called transmitters. By definition, they are variable-current devices and produce 4- to 20-mA outputs with supplies of widely varying voltage.
Outputs are chosen with the following factors in mind:
- the type of device that will receive the transducer's output signal (programmable controller, panel meter, signal conditioner, etc.)
- the distance between the transducer and its receiving device
- presence of electromagnetic interference (EMI) in the environment, which can come from sources such as power lines, welding equipment, solenoid valves, motors, 2-way radios, etc., and
- cost as it relates to the entire installed system (not just the transducer).
Pressure transducers are mechanical structures made from more than one material. Because of this, they respond not only to changes in pressure, but to changes in temperatures as well. These changes can affect both the zero and full scale output (FSO) of the transducer, regardless of its type. The term temperature effect upon zero refers to the change in output at constant pressure as temperature is varied over a stated range. Extreme temperature fluctuations may change a transducer's output signal even though pressure remains constant.
Many other characteristics — such as linearity, hysteresis, repeatability, etc. — help to determine the measurement accuracy of a pressure transducer, Figure 2. Additional factors, equally important, are more elusive; these include packaging, configuration, construction materials, and internal design. Each of these can be appraised on the basis of field testing and/or experience.
Overall, the best pressure transducer for one application is not necessarily the best for another. In fact, a transducer with the second best performance may be the best choice for an application if its price is significantly lower.
Pressure transducer terminology
The following definitions are used to put a quantitative value on transducer performance.
Range refers to minimum through maximum pressures that can be accurately measured by a transducer. Usually transducers are selected so that system operating pressure is 50 to 60% of the transducer's maximum rated pressure. For example, a hydraulic system, normally operating in the 2500- to 3000-psi region, would usually use a 5000 psi transducer. In addition to providing a safety margin, this practice also makes a good compromise among performance characteristics.
Over-range capability is the maximum magnitude or pressure that can be applied to a transducer without causing a change in performance beyond specified tolerance.
Burst pressure refers to fluid pressure at which mechanical failure and/or fluid leakage from the transducer is expected. Do not confuse burst pressure with over-range capability. Exceeding over-range capability can affect a transducer's ability to function; exceeding burst pressure can destroy it.
FSO (full-scale output) refers to the variation in the output signal as the transducer performs over its calibrated range from minimum to maximum pressure at a specified temperature. A tolerance and a temperature are generally given. Output, with maximum pressure applied and rated excitation, is FSO. Example: 5 Vdc ± 0.05 Vdc at 77° F
Zero unbalance is the residual output of an excited transducer that has no pressure applied. For a sealed gage transducer, tolerance must account for temperature and atmospheric pressure. Examples are 0.0 V ± 5mV at 77° F and ± 0.4 mA at 77° F.
If zero unbalance is large in respect to FSO, the user may have to zero a transducer through an external zero-adjust circuit. This ensures that the transducer will produce no output signal when no pressure is present. Depending on accuracy requirements, the user also may have to calibrate the transducer's output circuit to correct for deviations from nominal value at FS.
Most manufacturers specify accuracy as plus or minus a percentage of FSO, including the mathematically combined effects of linearity, hysteresis, and repeatability errors. Note that this accuracy does not include the effects of environment - especially temperature - and system dynamics. Operation at a constant temperature is implicit in this concept of accuracy.
Resolution refers to the smallest change in pressure that can be detected in the transducer's output. It is usually expressed as a percentage of FSO. For example, if two transducers each have a resolution of 0.1% of FSO, a 100 psi (6.8 bar) transducer could detect a pressure increase or decrease of 0.1 psi (0.007 bar). A 5000 psi (340 bar) transducer could detect a pressure change of 5 psi (0.34 bar).
Resolution generally is not constant throughout a transducer's range. Manufacturers may publish values for maximum resolution or average resolution. Users should be aware of the difference between them when comparing one transducer's performance with another.
Maximum resolution describes the best resolution that can be expected. Average resolution represents a value between the best and worst values throughout a transducer's range. Even though a pressure transducer may have infinite resolution, electrical noise — contained in power supplies and introduced by other sources — can limit resolution. Also, instrumentation, such as analog-to-digital (A/D) converters, can limit resolution.