Microsoft LPF-00004 Manuel de l'opérateur télécharger pdf (Page 67)

TRANSDUCERS Page 49

YBER

380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Pressure

High quality pressure transducers have historically been very expensive and suffered from a

significant temperature sensitivity. A variety of semiconductor pressure transducers having improved

temperature sensitivity are now available at low cost.

When measuring very low pressures the height of the pressure transducer should be the same as the

sense location, avoiding hydrostatic errors introduced by fluid filled catheters. Pressure transducers

should also be kept at a constant temperature by isolating them from heat sources such as heat lamps.

Force

Force measurements are so diverse that many laboratories make their own force transducers, although

some are commercially available. User-designed force transducers are usually made with resistive or

semiconductor strain gauges. Resistive gauges are cheaper and generally more rugged while

semiconductor strain gauges are smaller and about 10 times more sensitive.

Acceleration

Three types of accelerometers are manufactured. One uses a piezoelectric pressure sensor attached to

a test mass, another uses a strain gauge attached to a mass, and a third type uses feedback to prevent

displacement of the test mass with the amount of the applied feedback force generating the output

signal.

Length

Implantable length gauges

Implantable length gauges are made from silicone tubing filled with mercury or saline (Lemon

and Prochazka, 1984). As the tubing is stretched the impedance of the transducer increases. To

avoid the electrolytic generation of bubbles in the tubing, a high frequency AC signal is used with

an AC bridge circuit. In practice these gauges have many difficulties and are not superior to

cinematography techniques.

Linear potentiometers

Linear potentiometers are available in short and long lengths (at least 4 inches) with linearities of

0.1%. They are inexpensive and simple to use but must be closely aligned to avoid internal

damage.

Linear Variable Differential Transformers (LVDT)

Linear variable differential transformers (LVDTs) are used for accurate and stable measurement

of linear position and displacement. Rotary variable differential transformers (RVDTs) are

similarly used to measure rotation. These devices are very rugged and because there is no contact

between the core and the body of the LVDT (or RVDT) there is no friction or wear. A linearity

of 0.25% over the full range is typical (e.g., Transicoil). Full scale LVDT ranges are typically

limited to ±1 inch or less.

1 2 ... 62 63 64 65 66 67 68 69 70 71 72 ... 156 157

Pas de commentaire

Microsoft LPF-00004 Manuel de l'opérateur Page 67