Variable reluctance pressure (VRP) transducers (see Photo 1) are not a new entry in the pressure transducer field. The demand for these devices, however, is increasing in low pressure range applications such as pulmonary fuction testing and HVAC.

Photo 1. Variable reluctance (VRP) transducers cover a wide pressure range, but because they use integral motion as the sensing mechanism, they are particularly suited to very low pressure applications.

     This new interest in VRP transducer technology derives in part from its design; the devices use integral motion as the sensing mechanism rather than strain measurement requiring some form of amplifying mechanism. Full-scale diaphragm deflection is usually limited to about 0.001 in. Regardless of design pressure range, 0.01 psi or 5000 psi, only diaphragm thickness and diameter change.

     Futhermore, excessive deflection of the diaphragm is mechanically prevented, making bursting nearly impossible and the transducer inherently safe to use. The rugged devices withstand environmental qualification requirements such as vibration, shock, and acceleration testing. Operational cycles exceeding 10⁶ are routinely experienced wheras lead wire bending causes some other types of pressure transducers to fail after far fewer cycles.

     Because VRP transducers have no internal wiring and expose no mechanisms to the pressure media, both sides of the diaphragm can handle most liquids and gases without resorting to diaphragm isolators, which are impractical in low-pressure applications. Where required by a special pressure medium, nickel/gold plating can be applied to the diaphragm for added corrosion resistance.

Figure 1. Installing a new diaphragm to change pressure range in a variable reluctance pressure transducer is a simple matter of removing the bolts that hold the case halves together. If a particular installation requires multiple VRP transducers, a supply of spare diaphragms should be kept on hand.

     As a related bonus, VRP transducers are by design less susceptible to radiation damage than are most other transducer technologies. Users can easily disassemble the transducers and change their pressure ranges in the field (see Figure 1). For in stallations using multiple VRP transducers, a small supply of spare diaphragms can be maintained.

     The VRP transducer's force-summing component is a magetic stainless steel diaphragm clamped or welded between two case halves that form two symmetrical pressure cavities. Keeping the volumetric displacement of these cavities very small provides the highest possible frequency response. Close matching and small volumetric displacement also reduce line-pressure coefficients.

     VRP transducers can be used to measure differential, gauge, or absolute pressure, depending on the pressure maintained in the reference volume side. The reference volume side of an absolute pressure transducer is evacuated and sealed; atmospheric pressure is admitted to the reference side so that gauge pressure can be measured.



VRP TRANSDUCER OPERATING PRINCIPLES

Figure 2. This section of a VRP shows the E-cores and the small pressure media volumes separated by the stiff stainless steel diaphragm.

     An E-core/coil assemply is imbedded in each of a VRP transducer's case halves and is electrically connected as a variable transformer (usually one-half of a variable reluctance bridge) that forms the transduction device (see Figure 2). The coils are encapsulated in a hard compound to maintain maximum stability under very high pressure. The exposed E-core assembly is protected by a thin Inconel shield that is welded in place to provide compatibility with most media. When excited by an AC carrier, a magnetic flux is produced in each core and the air gaps formed by the diaphragm. The transducer can therefore be thought of as a half of a variable-reluctance bridge.

Figure 3. Reluctance is the flux resistance in the electromagnetic circuit components L1 and L2 and is the basis for the operation of a VRP transducer.

     The operating principle of the VRP transducer is based on the reluctance of the L₁ and L₂ coils (see Figure 3). This reluctance is directly proportional to the length of the flux path and inversely proportional to its permeability. Each electromagnetic circuit associated with coil L₁ and coil L₂ contains two reluctance elements, iron and air-gap paths (see Figure 4). The permeability of the E-core magnetic materials is ~100 x the air gap permeability. Since the length and permeability do not change appreciably, the reluctance of the magnetic material path can be considered constant. As a differential pressure is applied, the diaphragm deflects, one side decreasing and the other increasing, and the air gap reluctances in the electromagnetic circuits change proportionally to the differential pressure applied. A F.S. pressure on the diaphragm, ~0.001 in. deflection, will produce a large output signal that is easily differentiated from spurious noise signals.

     VRP sensor output is proportional to the reluctance in each of the arms of a half-active Wheatstone inductive bridge that uses the equivalent inductive reactances (X_L) for L₁ and L₂ as the active elements. The inductance of any coil is determined by the number of turns and the geometry of the coil itself. When a permeable material such as iron is introduced into the flux field of the coil, the lines of magnetic flux are redirected and concentrated in the permeable material.

     This alters the apparent inductance, or self-inductance, of the coil. If the major portion of the reluctance of a magnetic circuit is the result of an air gap, the reluctance is proportional to that air gap. Every effort is made in VRP technology to approach this ideal condition. The inductance of a circuit is inversely proportional to its magnetic reluctance, and the inductive reactance (X_L) is proportional to inductance. It is apparent, then, that X_L = k/g where k is a numerical constant and g is the air gap.

Figure 4. The circuit diagram indicates the reluctance bridge of a VRP transducer in a conventional manner using inductances as the bridge components.

     When the bridge is excited by an AC carrier, the output (E_O) is obtained from the junction between the two inductance arms and the center tap of the carrier oscillator's output transformer, or completion network. As pressure is applied to the transducer, the charge in inductance of the two active arms modulates the amplitude of the carrier directly proportional to the pressure.

     The bridge output signal is the amplitude modulation of the carrier excitition, and the amplitude is proportional to the bridge imbalance. The phase of the carrier changes with the direction of the inductance arm imbalance as the applied transducer differential pressure changes from plus to minus.

     To put this signal into a usable form, it must be demodulated to produce a direction-sensitive DC output directly proportional to the pressure applied to the transducer. For DC operation, these VRP transducers can be physically combined with a miniture IC carrier/demodulator in a single unit.

     The diversity of recent variable reluctance pressure transducer applications indicates their broad range of potential uses in high-technology fields, especially in low-pressure applications.

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