Appliance Design - March 2009 - (Page 32)

SWITCHES & RELAYS lar optical or reed-sensor devices. The recent inclusion of push-pull outputs of opposite polarity in Hall-effect switches allows for use of these sensors with a single external bypass capacitor. The elimination of output pull-up resistors reduces bill-ofmaterial costs and saves PC board area in end-user applications. Finally, Hall sensors are sold in a wide variety of very small and lightweight packages, including leaded packages, surfacemount packages, and wafer-level chip-scale packages (WLCSP). The size and overall weight of these packages generally meet the needs of even the most discerning appliance design engineer. Table 1 shows a comparison of commonly available proximity sensor and switch technologies as a function of common demands in the appliance market. and release points of the sensor (BOP and BRP respectively). However, in this case BOP occurs at a positive field level while the BRP transition occurs at a negative magnetic field level. This is what is commonly referred to as Hall-effect latching behavior. Magnetic design Some system designers are unnecessarily intimidated by the need to design a magnetic circuit for use with an appropriate Hall-effect sensor. In most cases, very simple magnetic circuits can be developed for use with a Hall-effect sensor. For example, the “head-on” mode of operation refers to a system where a single-pole magnet moves perpendicular to the active face of the Hall device, as shown in Fig 3. Here we see a change in the incident field through the Hall sensor (y-axis) as a function of the distance between the magnet and the sensor (x-axis). In systems requiring high accuracy, this non-linear transfer function can be compensated for through the use of a lookup table (or other translational mathematic formulae) in the system microcontroller. Alternatively, the “slide-by” mode of operation refers to movement of a magnet in a plane that is parallel to the active face of the Hall device, as seen in Fig. 4. The slideby method typically results in better sensing precision than the head on method as a result of the enhanced linearity in the central portion of the field versus distance transfer function. The large magnetic slope between the poles makes it possible to obtain very precise switch-point locations when Halleffect switches or latches are used. Another method for actuating a Hall device is known as vane-interrupt switching. A vane is a ferromagnetic object that has a unique configuration of notches cut into it. With vane-interrupt switching, a magnet and Hall device are mounted in a stationary position such that the Hall device is forced into the “on” state by the activating magnet. When the ferromagnetic material of the vane passes between the Hall sensor and activating magnet, the vane forms a magnetic shunt to divert the field away from the Hall device. (See Fig. 5). Therefore, as one or more vanes pass by the sensor, the digital sensor-output signal can be used to determine the speed of rotation, or the crude position of a rotating shaft or knob. www.applianceDESIGN.com Fig. 2. Transfer function for typical Hall-effect sensor switches and latches. Sensor design At a fundamental level, Hall-effect sensors can be classified based on the transfer function between an input magnetic field and the output signal of the sensor. Table 2 segregates various types of Hall-effect sensors by output type, while also discussing typical applications for these types of sensors. The output transfer function of a linear Hall-effect proximity sensor is extremely simple to understand. In short, the output voltage (or duty cycle in the case of a PWM output) of a linear Hall sensor is directly proportional to an applied magnetic field. In order to understand the output transfer function for Hall-effect switches and latches, consider a simple description of unipolar Hall-effect sensor switches. (See the red transfer function in Fig. 2.) Here we see that the digital output of the sensor is in the high state until a pre-determined level of magnetic field (BOP) is exceeded. When the applied magnetic field exceeds the BOP level, the sensor output transitions to the low state and will remain in this state until the field level falls below the BRP level. The fact that BOP and BRP are both greater than or equal to zero Gauss (where Gauss is a unit of magnetic field) denotes one of the defining characteristics of a unipolar Hall sensor switch. The defining characteristics of a Halleffect latch can also be seen in Fig. 2, which shows the same references to the operate Fig. 3. Head-on Hall sensor actuation. TEAG is the total effective air gap, or the distance between the sensor and the magnet. Fig. 4. Slide-by Hall sensor actuation. 32 applianceDESIGN March 2009 http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - March 2009

Appliance Design - March 2009 Contents Editorial Shipments/Forecasts News Watch Transmitting Power Wirelessly Promises to Reduce the Ever-Growing Tangle of Cords and Cables Found in the Modern Home. There Are Different Technological Approaches for Achieving this Goal, and Some Are Already on the Market. Electrochemical Capacitors Can Provide an Extra Peak-Power Boost in Battery-Operated Appliances, Allowing Product Designs to Have Smaller Battery Packs. Metallic Foams Put the Properties of Metals into Lightweight Packages. Applications for this Emerging Technology Include Gas Burners, Heat Exchangers, and Electronics Housings. Hall-Effect Switches Are Compact and Provide a High Degree of Reliability and Durability as They Virtually Eliminate Mechanical Wear, Shock, and Contact Oxidation. Blowing Agents for Polyurethane Foam Insulation Face Increased Scrutiny Due to Global Warming Concerns, but Alternatives with Lower GWP Values May Provide Solutions to Meet These Challenges. Design Marts Association Report: AHAM Advertisers' Index

Appliance Design - March 2009

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