| Air-core meters are often
favored over other movements as a result of their mechanical ruggedness
and their independence of calibration with age. A simplified diagram of
an air-core meter is shown in Figure 2. There are three basic pieces: a
magnet and pointer attached to a freely rotating axle, and two coils,
each oriented at a right angle with respect to the other. The only
moving part in this meter is the axle assembly. The magnet will tend to
align itself with the vector sum of H fields of each coil, where H is
the magnetic field strength vector. If, for instance, a current passes
through the cosine coil the magnet will align
its magnetic axis with the coil's H field. Similarly, a current in the
sine coil. If currents are applied simultaneously to both sine and cosine
coils, the magnet will turn to the direction of the vector sum of the
two H fields. H is proportional to the voltage applied to
a coil. Therefore, by varying both the polarity and magnitude of the
coil voltages the axle assembly can be made to rotate a full 360
degrees. The LM1819 provided with the kit is designed to drive the meter through a minimum of
305 degrees.
Advantages of air core meter movements include; Ruggedness - In an air-core meter the axle assembly is supported by two nylon bushings. The torque exerted on the pointer is much greater than that found in a typical d'Arsonval movement. In contrast to a d'Arsonval movement, where calibration is a function of spring and magnet characteristics, air-core meter calibration is only affected by the mechanical alignment of the drive coils. Mechanical calibration, once set at manufacture, can not change. No Case is Required - Air core meters do not nee to be mounted in a case to protect them from dust and moisture due to there ruggedness. Low power consumption - Unlike RC servos air core meters do not require large amounts of power. This is important because when many instruments are used in a simulator the power requirements can become excessive if servos or stepper motors are used. Inexpensive - RC Servos and Stepper motors cost more that the Air Core Meter Kit offered here and in addition, RC servos only have a range of 180 degrees unless gearing is used adding to the expense.
|
|
