How Magnetic Sensors Are Helping to Drive Changes in Appliance Designs

By: Gwenn Gmeinder

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From measuring electricity usage to turning off lights via remote control, smart technology is part of many of today’s consumer electronic devices.

Within the home appliance industry, sensors are playing a significant role in how consumers interact with smart appliances like washing machines, dishwashers and refrigerators. Temperature sensing and control are among the most important and well-established sensor functions in modern home appliances.

Among a variety of factors, the introduction of microprocessor control systems in appliances has helped drive the application of sensing techgraphnologies within these smart devices. Large mechanical switches, contactors and relays are no longer required for switching applications that historically operated on 120 VAC. Now microprocessor controls allow for the use of much lower power, noncontact sensing technologies like reed switches and Hall effect sensors.

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After a brief introduction to household appliance sensors, this article will highlight the top four factors driving the application of sensors in this growing market. Then, it will explain the impact of noncontact sensing technologies on various home appliance designs, including the resulting energy savings and performance efficiencies.

Household Appliance Sensors

The projected volume for the global household appliance industry for 2014 was 760 million small and large appliances. As more microcontrollers are used in a wide range of household appliances, the annual number of appliances produced is expected to increase to 990 million units in 2017, based on current market data.

Home appliances that use sensor technologies include both small and large household appliances as well as those used for heating, ventilation and air-conditioning (HVAC). As shown in the table below, sensor technologies for position, proximity, level and speed controls are used across a broad array of appliances. Multiple sensors are required in each appliance.

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Four Factors Driving Appliance Sensor Applications

1. The use of microprocessor-based sensing and control systems has diminished the need for large mechanical contactors/relays and switching technologies that traditionally switched several amps and incandescent light bulbs. Based on the current technology demand, smart sensing requires low-voltage and low-current switching/sensing devices. Noncontact (touchless) sensing devices are ideal for replacing the mechanical units. Low-cost technology options that can give both digital and ratio-metric outputs are also desirable.

2. Rising energy prices and government regulations demand energy-efficient appliances. The use of sensors in multiple applications, including smart power grids, smart buildings and smart industrial process control, helps enable a more efficient use of resources and a reduction in greenhouse gas emissions. Modern appliances with built-in efficiencies from advanced sensors dramatically reduce the required levels of energy and water, while minimizing the amount of waste fluids for disposal.

3. Worldwide demand continues to grow in Asia, South America and other continents for new appliances with improved efficiencies and energy savings—at low prices.

4. Home automation and consumer convenience are driving the need for household appliances with smart sensing technology. As modern homes become more dependent on smart technology, smart appliances with noncontact sensing will be in the best position to support future smart home infrastructures, allowing synchronized communication and control.

Noncontact Sensing Technologies

Reed switches and Hall effect sensors are noncontact sensing technologies that are suitable for millions of operation cycles. They are considered noncontact because the switching device no longer has to make physical contact with the appliance to change the switching state such as the traditional microswitches used in a refrigerator door. The following section discusses key details about reed switches and Hall effect sensors and how they impact today’s home appliance designs.

Reed Switches

The reed switch has remained a popular choice for low-power applications because of its simplicity and reliable performance. In the home appliance sector, reed sensors are used in a refrigerator or oven door to detect when the door is open or closed. The sensor sends a signal to the control unit to promptly activate or deactivate the LED in the appliance.

A reed switch is an electrical switch operated by an applied magnetic field. It is a passive component that does not require any power to operate. The switch consists of two or three thin metal pieces called reeds, with plated contacts at their tips that are spaced a small distance apart. The reeds are typically enclosed in a sealed glass tube filled with inert gas.

The magnetic sensitivity switchpoints of reed switches are more precise than solid-state digital switches. High accuracy is especially important when designing the system to function under all operating conditions. Since the reed switch contacts are hermetically sealed, they are protected from the extreme temperatures, humidity and moisture found in many appliances without impacting their contact stability or operating life.

Reed switch customizations, such as mechanical packaging and termination options, are available to easily fit these sensors into a wide variety of applications, including appliances. Reed sensor packages are very adaptable to unique or challenging mounting positions in an appliance. For example, flat flange mount reed sensors can be attached to the appliance with screws or mounting posts. Magnetic sensors can be packaged in unique, custom-designed capsules that fit an application’s specific mounting location.

Hall Effect Sensors

Hall effect sensors are semiconductor-based transducers that vary the output voltage in response to changes in the magnetic field. These sensors combine a Hall effect sensing element with circuitry to provide a digital on/off or analog output signal that corresponds to the change of a magnetic field without involving any moving parts. Unlike a reed switch, a Hall effect device contains active circuitry, so it draws a small amount of current at all times.

Hall effect sensors are quite popular in washing machine speed sensor applications. The rotational speed of the machine tub is monitored by a multipole magnet (16 or 32 poles) that is attached to the motor shaft. The magnet rotates over a Hall effect sensor with excellent speed measurement capabilities. In turn, this digital speed signal is sent to the control unit that internally controls the motor speed for the various speed cycles.

Hall effect devices are programmable and can be used in a variety of ways to obtain multiple outputs, including digital, analog and speed sensing. The output voltage or pulse width modulation (PWM) signal can be programmed between 0 and 360. Fifteen degrees rotation could be 0.5 V; 160 degrees rotation could be 4.5 V. Options for multiple programming points or segments up to four points per 360-degree rotation are also available.

The following list outlines three primary functions of Hall effect technology:

1. Speed sensing: Hall effect technology is commonly used in measuring the rotational speed of a passing gear tooth or magnet. Hall effect sensors are very efficient for pulse speeds up to 20 KHz and are widely used in drum rotation applications such as clothes washing machines.

2. Rotary selector position: An analog rotary Hall effect sensor provides a contactless solution for control knob position sensing. A programmable Hall effect sensor can be used to control washing machines, dryers and oven rotary controls with high resolution and accuracy.

3. Linear sensing: Analog Hall effect sensors can measure linear length up to 30 mm with one Hall chip. This lends itself to some level-sensing applications and linear movement actuation measurement. The programmable design of the sensors makes it easy to adjust resolution and precision.

Impact of Noncontact Sensing Technologies

Flexibility and reliability: With noncontact sensing technologies, appliance design options are much more flexible for applications requiring tight tolerances—especially in confined areas where the space available for mounting sensors is very limited. These technologies deliver improved reliability and durability over the life of the appliance because they are unaffected by mechanical wear or oxidization buildup on the mechanical contacts. In addition, noncontact sensors are protected from changes in temperature or humidity that are common in many household appliances.

Enhanced accuracy: Output signal options for noncontact sensors include digital signals for position sensing and ratio-metric output values for linear or rotational sensing. These signals can deliver instantaneous values, which enable tighter controls and better resolution for enhanced sensing accuracy.

Aesthetic appeal: Unlike mechanical devices that have visible levers or push-buttons, the sensor and magnet actuators of the noncontact technologies are hidden behind the appliance panels—improving the aesthetic appeal of the appliance.

Energy efficiency: Noncontact sensing solutions help deliver the energy efficiencies needed for household appliances to earn the Energy Star rating. The increased use of Energy Star appliances has had a positive effect on the environment. The EPA estimates that Energy Star products prevent over 150 million metric tons of greenhouse gas emissions each year. These products also save more than 200 billion kilowatt-hours of electricity per year — which is 15 percent of U.S. residential use. The Energy Star program has resulted in billions of dollars in savings on energy usage since its inception in 1992.

Conclusion

As household appliances become smarter, so do the sensor technologies used in their designs. The demand for energy-efficient technologies and the use of microcontrollers have helped position noncontact sensors in the home appliance market. Instead of large mechanical switches and relays, low-power, noncontact sensors are being used to improve the efficiency and performance of modern appliances. Reed switches and Hall effect sensors are contactless technologies that deliver flexible sensing solutions with tighter tolerances. These advanced sensors are enabling more reliable, durable and attractive appliance designs that have a lower impact on the environment.

About the Author

Gwenn Gmeinder is the North American Business Development Manager for sensor products within the Electronic Business Unit at Littelfuse. For over 30 years, he has worked with sensor technologies at several companies. Gmeinder joined Hamlin in 1978 as a product design engineer and began managing its global business development in 2007. He has held his current position within the company (now part of Littelfuse) since June 2013. Gmeinder earned his BS in industrial technology from the University of Wisconsin-Stout.

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