Custom Magnetic Sensor Selection Guide for White Goods Applications
Find digital and analog sensors that drive efficient, reliable appliance designs.
The demand for energy efficiency and convenience is driving the application of sensor technology in the home appliance/white goods market. Within this industry, sensors are used for detecting and controlling temperature, position, proximity, level and speed. As more sensors are integrated into appliance designs, engineers must identify advanced sensing solutions that deliver the improved accuracy, reliability and efficiency demanded by consumers.
When an off-the-shelf sensing solution will not meet the unique requirements of a white goods application, the design engineer must specify a custom sensor. To find the best custom sensor for an appliance design, the engineer should consider the full magnetic circuit as well as the environmental, mechanical, electrical and magnetic parameters of the application. When all key factors are known, the engineer can recommend a custom magnetic circuit design with a robust sensing technology.
This article will discuss how to select digital and analog sensors for appliance designs that utilize magnetic sensing technology. In addition to defining and describing the benefits of Hall effect sensors and reed switches, it will provide examples of appliance designs that incorporate these sensors.
Analog Sensors for Superior Accuracy and Stability
Analog sensing applications allow the end user to collect instantaneous feedback on the magnet’s position. Analog Hall effect sensors deliver a high-accuracy, high-resolution ratiometric output signal in large and small appliances. Unlike mechanical switches, these sensors can be hidden behind the appliance walls, increasing the appliance’s aesthetic appeal. Since analog Hall effect devices utilize noncontact magnetic sensing, they do not exhibit the mechanical wear associated with lever arm or push-button technologies.
Recent design advancements in analog Hall effect technology have enabled the Hall effect chip to measure the angle of the flux field instead of the traditional amplitude. This improvement makes it much less susceptible to temperature changes, allowing the sensor to deliver a more stable analog output signal across a large temperature range.
The following section provides details on two types of Hall effect sensors that can be selected for analog appliance applications:
Rotary Hall Effect Sensor
Combining a Hall effect sensing element with electrical circuitry, this semiconductor-based sensor provides an analog output signal that corresponds to the change in a rotating magnetic field.
Rotary Hall effect sensors offer two output options: analog or pulse-width modulation (PWM). The engineer can program the sensor to associate a specific output voltage or PWM output value with a precise degree of rotation. Multiple programming points are available up to 360 degrees of rotation. Each programming point represents a voltage or PWM output value that corresponds to a given angle of the magnetic field. This results in a ratiometric output signal relative to the degree of rotation.
Benefits: Unlike a mechanical rotary or resistive film rotary device, the rotary Hall effect sensor does not experience mechanical wear or changing resistance values. In addition, it is very stable over normal operating temperatures up to +105°C. Units are accurate from 0 to 360 degrees rotation with 0.5 V–4.5 VDC output or 10–90% duty cycle for PWM.
Appliance Applications: Rotary Hall effect sensors can be used to detect the dial position in washing machines, clothes dryers, stoves/ranges and blenders. For consumers who prefer the detent/clicking sound and feel of a mechanical rotary dial, the design engineer can add these mechanical features to the external packaging that encapsulates the noncontact rotary Hall effect sensor. Thus, the end user gets the mechanical experience while still enjoying the high reliability of a Hall effect sensor.
Linear Hall Effect Sensor
Instead of measuring rotation like the rotary Hall effect sensor, the linear Hall effect sensor measures the linear movement of a magnetic field. The Hall effect sensor is programmable for a set output voltage that is ratiometric for a given travel distance. Its output options are the same as the rotary Hall effect sensor. The sensor measures the linear movement and relative flux angle of a magnetic actuator up to 30 mm of travel distance with a single Hall effect chip. This results in a ratiometric output signal relative to the precise movement of the sensor.
Benefits: Before programming the output voltage (or PWM value) to correspond to the relative magnetic field value of the actuation magnet, the sensor and actuator can be placed in their final mounting area within the application, considering all magnetic influences from the nearby surroundings. This allows the engineer to optimize the application’s output since any shunting, mechanical tolerances or stack-up tolerances of the magnetic field will be included within the programming environment.
Appliance Applications: Linear Hall effect sensors are often used to monitor fluid levels in refrigerator ice makers, dishwashers, washing machines and coffee makers.
Digital Sensors for Precise, High-Reliability Applications
Most white goods applications demand digital output to verify that an object is in its proper position. The use of microprocessor-based sensing and control systems in appliances require low-voltage, low-current switching devices such as reed switches and Hall effect sensors. The following noncontact sensors provide outstanding reliability and durability in digital appliance applications:
The reed switch is an electrical switch that does not require power to operate like an integrated circuit. Its electrical contacts are hermetically sealed within a glass tube with precious metal contact material.
Benefits: This sealed switch is highly reliable because it is unaffected by moisture and other environmental elements. Because its electrical contacts will not oxidize, the reed switch can deliver reliable operation for millions of cycles with logic-level loads.
Appliance Applications: Since reed sensors are very reliable with logic-level loads, they are useful for digital on/off operations. For example, they can be used to detect door/lid closure in refrigerators, dishwashers, washing machines, clothes dryers, stoves/ranges, coffee makers and food processors.
In a refrigerator, the magnet is mounted to the door while the reed sensor is attached to the fixed frame hidden behind the external wall of the refrigerator. When the door is open, the magnetic field cannot be sensed by the reed sensor, causing the LED bulb to turn on. When the door is closed, the sensor detects the proper magnetic field and the LED turns off. In this application, a microcontroller within the appliance receives the signal from the reed sensor. Then, the control unit activates or deactivates the LED.
Digital Hall Effect Sensor
Combining a Hall effect sensing element with electrical circuitry, this semiconductor-based device varies the output voltage relative to a change in the magnetic field. The digital Hall effect sensor provides a digital on/off output signal that corresponds to a change in the magnetic field. Since the Hall effect sensor is a contactless component with no moving parts, its operational lifetime is significantly longer than a mechanical switch. The active circuitry within the Hall effect device draws a small amount of current at all times. This sensor can only be used in applications characterized by low DC voltage and current.
Benefits: Digital Hall effect sensors offer high reliability and can be programmed to activate at a given magnetic field tolerance for precise sensing requirements.
Appliance Applications: Digital Hall effect sensors are used in speed-sensing applications such as washing machines, clothes dryers and blenders.
In a washing machine, a rotating 16-pole ring magnet activates the Hall effect chip with each passing red (north pole) segment. It deactivates the chip with each passing white (south pole) segment. This process produces a very accurate high-speed signal.
The need for energy-efficient, conveniently designed appliances has triggered a demand for advanced sensors. When design engineers must address custom sensing requirements, reed switches and Hall effect sensors can be used in complex appliance designs. Analog applications benefit from the stability and accuracy of rotary and linear Hall effect sensors. On the other hand, reed switches and digital Hall effect sensors provide exceptional precision and reliability in digitally-driven white goods applications. Building a magnetic circuit that incorporates all known parameters and a robust custom sensor will result in a highly reliable, efficient appliance design.