第二组：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 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: ①Dimension of tooth top surface should be equal to or more than the pole piece diameter. ②Tooth height should be equal to, or more than the space between teeth. ③The space between teeth should be approximately three times the pole piece diameter. ④Air gap (clearance) between tooth and pole piece should be between 0.015" to 0.030". MPUs normally have 3/8", 5/8" 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 selcetion of a MPU. MPUs can be used to ____.①sense speed ②supply alarm ③sense direction of rotation.