When it comes to measuring fluid levels in appliances, it pays to think outside the box—or in this case, the tank.
Traditionally, fluid measurement in applications such as icemaker water tanks and beverage dispenser tanks has been accomplished via systems employing mechanical sensors, floats and switches. These systems are immersed in the fluid inside the tank, where they can be subject to corrosion and possible cross-contamination. Also, due to their mechanical design, system accuracy is limited and they require standard tank shapes, such as cylinders, in order to provide consistent measurements.
Next-generation sensors are beginning to change that paradigm. For example, capacitive fluid level sensors can be mounted outside plastic and glass containers with pressure-sensitive adhesives, eliminating contact with the fluid being measured. Using capacitive technology, the sensor can distinguish between air and water or other liquids and, combined with a microprocessor, transmit signal data to a control unit.
Because they are solid-state and have no moving parts, the sensors are more accurate and reliable than mechanical devices.
Capacitive fluid level sensors also give appliance designers more freedom, enabling the use of unconventionally shaped tanks that would have been impossible with traditional mechanical sensors. Sensor circuitry substrate options include PCB, polyester flexible circuit and polyimide flexible circuit. A design can use a traditional PCB for a flat surface container or a thin, flexible circuit to accommodate curved surfaces or space-constrained applications. This can reduce the overall footprint of appliances and enable more efficient product designs.
Capacitive single-point sensor used to detect water in a reservoir of a stand-alone residential ice maker. The assembly incorporates wiring to the pump motor for reduced system cost.
INSIDE THE SENSOR
Capacitive fluid level sensors measure the levels of fluid or granular material in non-conductive (non-metallic) containers. Combined with configurable software, the sensors provide higher accuracy and easier installation than traditional mechanical sensors. They provide the ability to measure caustic liquids that may be difficult or impossible to measure with traditional mechanisms.
Capacitive fluid level sensors have multiple output interface options, including USB, I2C, discrete signals or even wireless protocols, and provide a customized interface. The embedded software can be configured for maintenance-free auto-calibration or manual calibration to maximize sensor accuracy.
SENSATIONAL SENSORS
Capacitive sensors are not new. They are what make your smartphone respond to your touch and let you punch in cook times on a high-end microwave oven. What is new is using the sensors in appliances to detect and measure fluids, and there appears to be a high level of interest in the technology from designers of next-gen appliances.
For appliance designers interested in using capacitive fluid level sensors, it’s important to work with their sensor supplier early in the design process. It saves time and effort to integrate the sensor with the initial concept rather than modifying the system design later. Also, if capacitive sensors are considered in the initial design, they can be easily customized for specific applications, and several different sensor options can be considered.
One application on the market is a refrigerator icemaker where the capacitive fluid sensor measures the water reservoir used in the ice-making process. There are many other potential applications. In a washing machine, a capacitive sensor could be used to measure the level of the water in the basin or the level of liquid detergent. A dishwasher could be equipped with a sensor regulating water and detergent levels during washing and rinsing cycles.
Another application of a capacitive fluid sensor is as a point sensor. Rather than measuring the fluid level, it simply reports whether or not fluid is present at the point where it is mounted to the tank. Two such sensors can be used to regulate the fluid in a container, one sensor that indicates when a tank is full and turns off flow into the tank, and one that indicates when the tank is empty and triggers a refill.
NEW DESIGN OPTIONS
Capacitive fluid sensors enable appliance designers to create new designs that are not constrained by traditional tank geometries. For example, unconventionally shaped tanks can be employed to fit in tight spaces or in uniquely shaped appliances. A capacitive sensor can convert the tank level to tank volume, which is not a simple equation when using unconventionally shaped tanks. If, for example, there is more volume at one level of the tank than another, a capacitive fluid level sensor with customized software can automatically compensate for that. Also, if designers want to use a tank in a remote part of the product, they can use a wireless protocol to communicate between the sensor and the control unit. Most importantly, this technology can help OEMs break down design barriers and allow them to create enhanced features in completely new products.
ENABLING THE IOT
The era of smart, connected appliances is on the horizon as the Internet of Things begins to take shape. The IoT will include sensors that can perform functions such as alerting maintenance when an appliance needs liquids added and monitoring fluid levels so they can be purchased and added before they run out. Capacitive fluid level sensors will be a key part of the IoT by making appliances smarter and enabling more efficient decision making, delivery scheduling, maintenance and purchasing. Capacitive fluid level sensors deliver higher accuracy, reliability, and flexibility than their mechanical counterparts, and they will be a key part of the future of appliance design.
