Conditional operation is the cornerstone of device functionality. It’s something most people don’t consider. Industrial devices, familiar appliances and cutting-edge mobile technologies must be able to detect specific external stimuli or user input, and then respond appropriately.
The form factors are endless. Common conditional operations include:
- Smartphone touchscreens responding to mechanical stress
- Water valves closing and opening on washing machines
- Small-scale devices detecting ultrasonic vibrations in continuously operating machinery
In each case, successfully capturing conditional output depends on device sensors that offer the best-fit combination of size, structure and sensitivity. Three of the most common types found in modern devices include electromagnetic, piezoelectric and electrostatic.
Here’s everything you need to know about these conditional capture categories.
Stick the Landing: Electromagnetic Sensors
Durability is the defining characteristic of electromagnetic sensors. Here’s why: These devices leverage the reliable nature of electromagnetism to detect state changes and deliver specific outcomes.
In its most basic form, this means wrapping a length of copper wire around a piece of ferrous metal. When current is run through the wire, the metal produces a magnetic field. It’s a straightforward operation that’s often found in home appliances such as washing machines and dishwashers to control the flow of water by opening or sealing valves.
Electromagnetic Benefits
- High output currents
- Long life span and minimal maintenance required
- Rugged and durable, thanks to the reliability of electromagnetic effects
Sticky Situations
- Low output voltages
- Materials are often costly
- Inefficient current production and capture at small sizes and in low-frequency applications
Stressed to Impress: Piezoelectric Solutions
Piezoelectric sensors make use of the piezoelectric effect — the ability of crystalline structures under mechanical stress to generate a charge differential that can then be harvested and used.
Here’s how it works: At rest, crystal structures are stable and charge is evenly distributed. When mechanical stress is applied and the structure begins to deform, positive and negative charges migrate to opposite faces, creating an imbalance and empowering high-output voltages.
The same is true in reverse — electrical charge applied to one side of the crystal will cause a resulting deformation. Piezoelectric sensors are ideal for crystal-based applications such as smartphone touchscreens and vibration sensors.
Piezoelectric Possibilities
- High output voltages
- High capacitances
- No need to control physical gaps for current capture
- Easy to produce at small sizes required for mobile technologies
Structural Shortfalls
- Materials needed to produce crystal-based sensors are expensive
- Coupling coefficient of piezoelectric devices is directly linked to material properties
Push-Pull Paradigms: Electrostatic Options
If you’re unfamiliar with electrostatic sensor options, you’re not alone. This sensor type often takes a backseat to electromagnetic and piezoelectric solutions at scale. This is in large part because the technology required to create viable, reliable electrostatic sensors has only become widely available in the past few years.
The principle is simple: Electrostatic devices capture and convert potential energy into useable voltages and/or currents. The application is slightly more complex. Oppositely charged metal plates are placed in proximity and then pulled apart — in turn, releasing stored potential energy that held the plates in place.
The result is high-current or high-voltage output (depending on device design) that can be captured and used to power small-scale sensors. Electrostatic devices are often used in laser printers, air cleaners and as ultrasonic transducers.
Electrostatic Advantages
- High-output voltages or currents
- Devices can be built at low cost
- Coupling coefficients are easily adjustable
- High coupling coefficients can be achieved with the right design
- As sensor size decreases, capacitance increases
Static Struggles
- Lower overall capacitances compared to electromagnetic and piezoelectric sensors
- Parasitic capacitances can lower performance
- No ability to deliver direct mechanical-to-electrical conversion
- Must control dimensions at micrometer scale to help ensure effective operation
Choosing Wisely
Sensors are everywhere — but not every sensor type works for every application. For example, while it’s possible to use piezoelectric solutions for ultrasonic detection across industrial applications at scale, the associated costs can quickly become prohibitive. Electrostatic options are often the better choice. While these potential energy producers excel in high-voltage applications, they’re not ideal for repetitive tasks in high-stress environments. In this case, electromagnets are most effective.
Bottom line, if you need something rugged and reliable, go with the electromagnetic effect. If you prefer small-scale precision, choose piezoelectric products. If you’re looking for high outputs or voltages at lower cost, select electrostatic solutions. Be sure to view the accompanying resource for more information on pros, cons and applications of each device sensor.
Author bio: Margaret Bezerko is President of SensComp, a world leader in ultrasonic sensors. She has 18 years of experience in the industry and currently focuses on leading the company in second stage growth.