As a critical component in many control applications, Woodward offers a variety of reliable magnetic and mechanical speed sensor options.
Magnetic pickups (MPUs) are speed sensors that detect the speed of a prime mover, typically an engine or turbine. They arethe communication link between your engine and a Woodward electronic governor control. The MPU is installed next to a drive shaft gear made of a material that reacts to a magnetic field. As each gear tooth passes the MPU, the gear interrupts the MPU's magnetic field, and an alternating voltage is developed. The frequency of this voltage is translated by the speed control into a signalthat accurately depicts the speed of the prime mover.
These speed sensors are designed for use in SIL-3, SIL-2, or SIL-1 (safety integrity level) rated turbine or engine safety systems. When applied with a SIL rated logic solver and final element, users can easily create an overspeed safety instrumented function (SIF) which meets the required safety integrity level for the specific application.
Rotating machinery requires speed sensing for rotor control, monitoring, and safety. Speed sensing is accomplished using a variety of technologies. Variable-reluctance based MPUs (magnetic pickups) are commonly selected because of their simplicity, reliability, and low cost. MPUs are passive probes in the sense that there are no active signal conditioning electronics in the probe. VR (passive) sensors also do not require an external supply, simplifying the overall system and increasing system reliability values.
To simplify application and installation concerns, these sensors have been validated to be directly compatible with Woodward’s ProTech® and MicroNet™ safety logic solvers when used in SIL-3 or SIL-2 certified safety systems, including:
ProTech-GII—Overspeed Detection System (SIL-3 certified)
ProTech-SX—Simplex Safety Logic Solver (SIL-2 certified)
ProTechTPS—Safety Logic Solver including Overspeed Detection (SIL-3certified)
MSM—MicroNet Safety Logic Solver (SIL-3 certified)
These sensors are single-channel, variable-reluctance speed sensors consisting of a single inductive coil and a permanent magnet. A ferrous gear tooth passing the sensor pole piece changes the magnetic field strength, inducing an AC voltage. The frequency of the output signal is proportional to the velocity of the gear tooth passing the sensor’s pole piece. The amplitude of the signal depends on the following:
Angular speed
Air gap between sensor face (pole piece) and target (gear tooth)
Target (gear tooth) geometry
Target (gear tooth) ferrous material properties
Load impedance of the electrical circuit used to sense output signal
Depending on the application, these sensors can be ordered with imperial or metric threads and different thread lengths (determined by part number).