Application Note VRS Industrial Magnetic Speed Sensors 1.0 General Information Some of the unique features that make the use of VRS Completely self-powered, VRS (Variable Reluctance Speed) sensors valuable in these potential applications include: sensors are simple, rugged devices that do not require an external voltage source for operation. (See Figure 1.) Self-powered operation Error-free conversion of actuator speed to output frequency Figure 1: VRS Sensor with Cutaway View Simple installation No moving parts Useable over a wide range of speeds Adaptable to a wide variety of configurations These properties have led to wide-spread use in many industries. As a result, VRS sensors are known by many use- related names such as: Magnetic-pickups Transducers Speed sensors Magnetic probes Motion sensors, Timing probes Pulse generators Monopoles Frequency generators Pick-offs Variable reluctance sensors 3.0 Principles of Operation The output signal of a VRS sensor is an ac voltage that varies in amplitude and wave shape as the speed of the monitored device changes, and is usually expressed in peak-to-peak voltage (Vp-p). One complete waveform (cycle) occurs as each VRS sensors are generally used to provide speed, timing or actuator passes the sensing area (pole piece) of the sensor. synchronization data to a display (or control circuitry) in the form of a pulse train. The most commonly used actuator is a metal gear however, bolt heads (cap screws are not recommended), keys, keyways, 2.0 Potential Applications magnets, holes in a metal disc and turbine blades are all Engine RPM (Revolutions Per Minute) measurement on appropriate. In all cases, the target material must be a ferrous aircraft, automobiles, boats, buses, trucks and rail vehicles metal, preferably unhardened. Motor RPM measurement on drills, grinders, lathes and automatic screw machines A permanent magnet is the heart of a VRS sensor and Process speed measurement on food, textile, woodworking, establishes a fixed magnetic field. An output signal is paper, printing, tobacco and pharmaceutical industry generated by changing the strength of this field. This is caused machinery by the approach and passing of a ferrous metal target near the Motor speed measurement of electrical generating pole piece. The alternating presence and absence of ferrous equipment metal (gear tooth) varies the reluctance, or resistance of flow Speed measurement of pumps, blowers, mixers, exhaust of the magnetic field, which dynamically changes the magnetic and ventilating fans field strength. This change in magnetic field strength induces a Flow measurement on turbine meters current into a coil winding which is attached to the output Motor RPM measurement on precision camera, tape terminals. (See Figures 2 and 3.) recording and motion picture equipment Wheel-slip measurement on autos and locomotives Flow measurement on turbine meters MPH measurement on agricultural equipment Sensing and Control VRS Industrial Magnetic Speed Sensors Figure 2: High Reluctance Position of Magnetic Circuit Figure 5: Steel Coarse Tooth Gear Figure 3: Low Reluctance Position of Magnetic Circuit Figure 6: Steel Sprocket 4.0 Potential Application Considerations VRS sensors are not designed for sensing extremely low If a standard gear is used as an actuator, this output signal speeds. The target passing the pole piece of the sensor must resembles a sine wave if viewed on an oscilloscope. (See be traveling at a minimum velocity, or surface speed, to Figures 4, 5, 6.) provide an adequate output voltage. Typical minimum surface speeds for each of Honeywells VRS sensor product offerings Figure 4: Steel Fine Tooth Gear can be found in the VRS sensor product documentation. Proper VRS sensor selection requires that the sensor: Provide the required Vp-p at the minimum speed of interest. Will still function properly at the maximum operating frequency of the application. 2 Honeywell Sensing and Control