Magnetic Speed Pickup (MPU)
A magnetic pickup is a passive sensor that senses the flywheel teeth passing in front of it and produces one voltage pulse for each tooth. It requires no supply; the signal amplitude increases with speed, so the reading gets weak at very low speed. It is the standard method of speed measurement in generator and engine applications.
The sensor consists of a permanent magnet and a coil. As a tooth approaches, the magnetic flux changes and an alternating voltage is induced in the coil. The frequency is directly the rate at which the teeth pass, and therefore proportional to speed.
- The gap setting is critical: the common practice is to screw the sensor in until it touches the tip of a tooth and then back it off about one turn. If the gap is too large the signal weakens, if too small there is a risk of hitting the tooth.
- The signal cable must be run screened; the screen is connected at one end only, to the reference on the control side.
- A weak or intermittent signal leads to the speed being read wrongly and to oscillation in the governor.
A note regarding AXI products: the standard GS740 kit does not measure engine speed. Speed reading only comes into play with the optional active-feedback speed stabilisation module. The GS740-3P's feedback, on the other hand, is not speed but the actuator position measured with the AS5600.
Context in which this term is used: Electronic speed and throttle control on a diesel engine