With regards to modern diesel engines revolution pick up sensor, discuss with suitable diagram the following. (16)
(a) functioning of revolution pick up sensor.
(b) adjustment of pick-up sensor
(c) adjustment of rotary encoder
(d) adjustment of pulse angle offset
Structured for DG Shipping MEO Class II examination scoring criteria.
A magnetic pick-up unit (MPU), also known as a revolution pick-up sensor, consists of a permanent magnet and an external coil winding. It's positioned a precise distance from ferrous gear teeth or flywheel teeth. As the flywheel rotates, the proximity of the teeth to the MPU creates a constantly changing magnetic field within the sensor. This fluctuating magnetic field induces an AC voltage in the coil. The frequency of this AC voltage is directly proportional to the engine's rotational speed – higher engine speed results in a higher frequency and voltage. The formula for induced frequency is:
$$F=\frac{Number\:of\:gear\:teeth\:\times Gear\:RPM}{60}$$
This frequency signal is then used by the engine's control system to determine engine speed.
The important parameter for the MPU is the air gap between the sensor and the gear teeth. This gap should be maintained within a specified range, typically 0.25 mm to 1.02 mm at the closest point. Incorrect gap adjustment significantly impacts the sensor's output. Too small a gap risks damaging the sensor, while too large a gap leads to a significant voltage drop. Adjustment involves loosening a locking nut, adjusting the sensor's position to achieve the correct gap, and then re-tightening the nut. The sensor's condition can be verified by measuring the AC voltage at a known engine speed. A new MPU should produce at least 1.5V (AC) at the ideal air gap and operational speed. A voltage below 1.5V (AC) usually indicates a faulty sensor requiring replacement.
The pulse angle offset adjustment ensures the output waveform from the pick-up sensor aligns with the engine's flywheel position.