Sensors Products Page 14 Sensors Products

2017 Littelfuse, Inc. Specifications are subject to change without notice 14 Reed switch-based sensors are used as highly effective speed sensors in low-cycle- rate applications with rates up to a few thousand revolutions per minute (RPM). For example, in treadmills, a reed sensor is used to detect the speed of the moving belt. Two permanent magnets are placed on diametrically opposite ends of the belt pulley, and a reed sensor is mounted to the treadmill frame perpendicular to one end of the pulley. As the treadmill pulley rotates, the reed sensor detects when each of the magnets is close to the sensor. The time interval between the detection of each magnet is used to compute the speed at which the pulley is rotating. Reed sensors are highly accurate in speed-sensing applications where there is a rotating part. They are commonly used in fitness equipment where there is a rotating part with a variable speed. SPEED SENSING USING REED SENSORS- Treadmill Pulley Display Board Power Supply Motor Drive Main Motor Board User Interface Reed Sensor Figure 3-Treadmill pulley Effective, noncontact solution used to switch the sensor Digital output signal is used to compute speed with high accuracy Long lifetime and reliable sensing for millions of operations Low-cost alternative to infrared/ optical sensors Overmolded sensors provide additional protection against mechanical stresses and vibration 59025 (Firecracker) 59065 (Threaded barrel) 59140 (Miniature flange mount) 59150 (Flange mount) Similar Applications: Fitness equipment, including treadmill, stationary bicycle and elliptical cross-trainer Hall effect sensors make very effective rotary speed sensors. The strengths of these sensors are their reliability, repeatability, accuracy and high speed. For example, in a clothes washing machine, a digital or latching Hall effect sensor can be used to detect a rotating ring magnet to determine the rotating speed of the drum. Multipole magnets (e.g., 8-40 poles) are typically used as the ring magnets in these designs. The frequency and resolution of speed are affected by the number of magnet poles. Direction, as well as speed, can be measured. The direction signal can be provided by either quadrature signaling or with the direction encoded in the pulse width. Quadrature signaling involves two square wave speed signals where the direction of rotation is determined by the lead vs. lag phase relationship between the two signals. Another popular method of detecting speed is to use Hall effect speed sensors to detect rotating ferrous gear teeth such as in an automotive transmission speed sensor application. The Hall effect sensor has a back-bias magnet and detects the variation in magnetic flux density between the gear tooth and the Hall effect sensor. SPEED SENSING USING HALL EFFECT SENSORS- Washing Machine Drum Speed User Interface Display Board Main Motor Board Power Supply Main Motor Hall Effect Sensor Figure 4-Washing machine drum speed sensor with ring magnet Digital output signal is used to compute speed with high accuracy Long lifetime and reliable sensing for millions of operations Effective for high-speed applications 55140 (Flange mount) 55310 (Flat pack digital) 55505 (Geartooth sensor) 55075 (Geartooth sensor) Custom-designed sensor Similar Applications: DC motors, vacuum cleaners and wheel speed S N Flux Concentration-Tooth Position Single Low S N Flux Concentration-Valley Position Single High Figure 5-Geartooth sensing Features and Benefits: Suggested Products: Features and Benefits: Suggested Products: APPLICATIONS

Previous Page
Next Page