Technical notes of interest to Marine Engineers
Magnetic pickup sensors
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Selection and installation guidelines for magnetic pickup sensors
Magnetic pickup (MPU) sensors are devices which can be used in conjunction with electronic control modules for monitoring of parameters such as speed, direction of rotation and a variety of alarm conditions. MPU sensors are available in various sizes and types. Following are some guidelines for their selection and installation on industrial engines.
MPUs are located near a flywheel, camshaft or other type of gear with rotation relative to engine speed. The passing of the gear teeth through the MPU tip (pole) generates a voltage and frequency. This frequency is converted to a speed reference by a control module. It should be noted here that most MPUs will develop similar signals. However, the magnitude and strength of the signal is dependent on the size of the sensor in relation to the gear teeth, the clearance between the pole piece and the gear teeth, and various other factors. As a general rule of thumb, the following standards should be met:
- Dimension of tooth top surface should be equal to or more than the pole piece diameter.
- Tooth height should be equal to, or more than the space between teeth.
- The space between teeth should be approximately three times the pole piece diameter.
- Air gap (clearance) between tooth and pole piece should be between .015" to .030".
MPUs normally have 3/8", 5/8" or ¾" threads and 2", 3" and 4" lengths. Determining which size is appropriate depends on the space available for installation, the availability of pre-drilled holes and consideration to those rules mentioned above. In most cases, these will be the only factors affecting the selection of a MPU. In some instances, the MPU will be "shared" between an isochronous governor and another engine control module, such as a speed switch. In such cases, the voltage output from the MPU to the controller may be too low (a minimum 3VAC is often required by controllers during engine cranking). It is always a good idea to contact the MPU manufacturer to discuss applications as these.
MPU installation is normally performed in the engine flywheel housing. Mounting should accommodate several threads and be heavy enough to avoid excessive vibration. The mounting material should be non-magnetic to avoid signal interference. To install, drill a hole slightly smaller than the MPU diameter in the flywheel cover, perpendicular to the gear teeth. Hole location should be towards the top of the cover and away from the starter motor. This is recommended to avoid metal chips from accumulating between the MPU pole and the teeth. Once the hole is drilled, it should be tapped to the MPU thread size with a bottoming tap. Note that a tapered tap will only allow the MPU to travel so far, which may be confused with the MPU actually making contact with the tooth. A tooth should be aligned to the hole, and the MPU threaded carefully to reach the tooth. Once contact is made, the MPU is backed up to allow for the proper clearance (typically ¼ to ½ turn). After securing the MPU in its position, turn the flywheel a full turn to insure no contact with the remaining teeth. Now in position, the MPU is locked in place by the lock nut. Take care not to overtighten the lock nut which could result in stretching and damage of the MPU. Once installed, the MPU is wired to the control module according to the manufacturers instructions.
A magnetic pickup is the most accurate means of detecting engine speed as it provides a clean high frequency signal (many pulses per revolution) with higher resolution than that seen on alternator tap or tachometer output signals. MPU uses range from industrial and marine to automotive applications.