How It Works  [Back ]

A magnetic pickup is essentially a coil wound around a permanently magnetized probe.  When discrete ferromagnetic objects—such as gear teeth, turbine rotor blades, slotted discs, or shafts with keyways—are passed through the probe's magnetic field, the flux density is modulated.  This induces AC voltages in the coil.  One complete cycle of voltage is generated for each object passed.

If the objects are evenly spaced on a rotating shaft, the total number of cycles will be a measure of the total rotation, and the frequency of the AC voltage will be directly proportional to the rotational speed of the shaft.

(Output waveform is a function not only of rotational speed, but also of gear-tooth dimensions and spacing, pole-piece diameter, and the air gap between the pickup and the gear-tooth surface.  The pole-piece diameter should be less than or equal to both the gear width and the dimension of the tooth's top (flat) surface; the space between adjacent teeth should be approximately three times this diameter.  Ideally, the air gap should be as small as possible—typically 0.005 inch.  A number of steel or cast-iron gears, precisely manufactured to AGMA standards, are available for use with the Model MP1A.  The standard solid gear comes with various dimensions and with 48, 60, 72, 96, or 120 teeth.  For assistance in selecting proper gear type and size, contact the factory.)

The above diagram shows a magnetic pickup used in conjunction with a 60-tooth gear to measure the rpm of a rotating shaft.  Such a gear is often selected because the output frequency (in Hz) is numerically equal to rpm—a situation that allows frequency meters to be employed without calibration.  For very high rotational speeds, a smaller number of teeth may be called for.

The following diagram shows how a turbine flowmeter can measure the volumetric flow of a fluid.  The fluid flow exerts a force on the turbine blades, causing the meter to rotate.  In properly designed flowmeters, the output frequency produced by the magnetic pickup is a linear function of the volumetric flow rate.  Each output cycle therefore represents the passage of a known volume of fluid, and the flowmeter can be accordingly calibrated in cycles per gallon or similar units.  This rating is known as the "K factor" of the flowmeter.  It will vary somewhat with viscosity and flow rate, but is usually quite predictable, with repeatability to within 0.1% in many units.

A magnetic pickup may also be used as a timing or synchronization device—as, for example, in ignition timing of gasoline engines, angular positioning of rotating parts, or stroboscopic triggering of mechanical motion.

The Model MP1A  [Back ]

The Model MP1A Magnetic Pickup is a fast, general-purpose sensor, providing an effective, accurate means of measuring the speed and frequency of mechanical rotary motion without the necessity of mechanical linkage—and the contact, wear, cabling, and alignment problems such linkage entails.  It is recommended for maximum-sensitivity applications with low speed and/or large air gaps.

The MP1A is a "passive" or "self-generating" device, requiring no external excitation.  When mounted in proximity to the teeth (or blades) of a conventional rotating gear (or turbine), it produces an approximately sinusoidal AC voltage-signal output with a frequency directly proportional to RPM.  The amplitude of the voltage is also generally proportional to the speed of rotation, as shown in the following graph:

Housed in a stainless-steel shell, the MP1A is reliable over a wide temperature range, at repetition rates exceeding one megahertz, and under severe environmental conditions of mechanical shock, vibration, humidity, immersion in water or oil, salt spray, sand and dust, radiation, and pressure.  It has a threaded mounting shank and locking nut.

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