The following factors should be considered when selecting a displacement transducer:

1. Measurement range

2. Armature type

3. AC-AC versus DC-DC

4. Environment

Measurement range: Displacement transducers are available with ranges from 0,25 mm to 470 mm [±.01 in to ±18.5 in]. A displacement transducer with a ±470 mm [±18.5 in] range can be used in one direction to measure up to 939,8 mm [37 in]. If accuracy is important, the range selected should not be any larger than necessary.

Armature type: The three available armature types are free unguided armatures, captive guided spring return armatures and captive guided armatures.

1. Free unguided armatures are recommended for applications in which the target being measured moves parallel to the transducer body, as well as, those that require frequent or continuous measurements. This armature type is often well suited for dynamic applications. When using a free unguided armature, the armature and the displacement transducer body must be mounted so that their correct relative positions are maintained. This type of displacement transducer features an armature/threaded push rod assembly that is completely separable from the displacement transducer body. Since the free unguided armature involves no mechanical coupling between the armature and the displacement transducer body, there are no springs or bearings to fatigue. This unit often has a virtually unlimited fatigue life.

2. Captive guided, spring return armatures are often well suited for those applications requiring the measurement of multiple targets or applications in which the target moves transverse to the armature and changes in a structure’s surface are being measured. In this type of displacement transducer, the armature moves over bearings in the displacement transducer body. The armature is biased by an internal spring so that the ball-ended probe bears against the surface of the target whose displacement is being measured. The displacement transducer is held in position by clamping the body alone. The armature is not attached to the target being measured.

3. Captive guided armatures are designed for applications requiring a longer working range. The armature moves freely over machined bearings but cannot be removed from the body. The displacement transducer body has a threaded mounting hole and the armature is attached to the structure being measured. The armature end is threaded so that special adapters such as spherical bearings or rollers can be attached.

AC-AC versus DC-DC: The major advantages of DC-DC displacement transducers are the ease of installation, the ability to operate from dry cell batteries in remote locations and lower system cost. AC-AC displacement transducer advantages include enhanced accuracy and a smaller body size. An AC-AC displacement transducer can be equipped with more sophisticated electronics, such as Honeywell’s SC instrumentation. The SC instrument provides an AC power supply, a phase sensitive demodulator, a scaling amplifier and DC output. The AC-AC displacement transducer system has less residual noise at minimum readings than DC-DC units that use internal electronics.

Environment: For applications involving very high humidity or requiring submersion of the displacement transducer, a submersible displacement transducer is required. Submersible units are available for either AC-AC or DC-DC operation and with free unguided or captive spring, return armatures. The unit selected should also operate and survive at the temperatures dictated by the application. Note that AC-AC units will operate at higher temperatures (up to 125 °C [257 °F]) than the DC-DC units (up to 70 °C [158 °F]) that have internal electronics.

Side loads: Side loads must be kept to a minimum since they will cause rubbing between the armature and the displacement transducer body. This friction will cause excessive wear of bearings and parts and in extreme cases, the armature may bend. At a minimum, side loads will reduce the unit’s life and accuracy.

Mounting blocks: Mounting blocks come in two sizes. The large size (order code AA937) accommodates displacement transducers with an outside diameter of 20,6 mm [0.80 in]. The small size (order code AA945) is for units with outside diameters of 9,5 mm [0.37 in] or 8 mm [0.32 in]. The mounting blocks are designed to be bolted to a flat surface. The sensor is clamped with a captive cap head screw. Two mounting cap screws are furnished. Both units are made from glass-filled nylon and have an operating temperature of -29 °C to 110 °C [-20 °F to 230°F]. Strokes up to 10,16 mm [0.4 in] require one mounting block, strokes over 12,7 mm [0.5 in] require two mounting blocks.