A magnetic displacement transducer operates upon the principle of null-balance between a magnetic flux induced by the displacement of a permanent magnet and the other magnetic flux produced by the output current. A transducer of this type is very much suited for use on industrial process measurement and control because of its stable output obtained from the use of a permanent magnet, lack of contact failure, rugged construction, and wide range of operative ambient temperature.
The magnetic displacement transducer as described here employs a saturable magnetic core (μ-metal) for detection of magnetic flux and a new self-oscillating magnetic modulator. This new modulator functions both as an excitation oscillator and as a signal amplifier, eliminating the need of an external AC power source. The transducer is therefore simpler in structure and operates on less power consumption than usual models. Thus a two wire system transducer becomes feasible, which contributes to reduction of wiring cost of process instrumentation. In addition, the transducer's magnetic circuitry is so devised as to provide a linear output over a wide angular displacement range.
The characteristics of standard displacement transducer are as follows.
Output: DC 4-20mA for 40° angular displacement
Power supply: DC 24V
Load resistance: 400Ω maximum
Nonlinearity: ±0.1%
Hysteresis: less than 0.05%
Ambient temperature: -40°C- +80°C
This transducer is being incorporated in such instruments as a 2-wire system pressure indicating transmitter arid a servo recorder. The type has been in operation on process control and measurement for more than five years and has been proven in its high stability and reliability. It can also be applied as a process controller. A modification in shape of the permanent magnet will produce a linear displacement transducer.
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