miniature capacitive sensor Archives

Liquid Handling Solutions in Action

On Sensortech, we have posted several entries about the trend toward miniature sensors including, Let’s Get Small: The Drive Toward Miniaturization and Trending Now: Miniature Sensors. At the end of January Balluff attended SLAS in San Diego, CA and saw this trend firsthand. Automation in the clinical lab is growing by leaps and bounds. Bioscience engineers are facing pressure to reduce cost, increase the number of samples run, and improve the speed at which lab tests are performed.

As an exhibitor at the event, we were able to showcase our solutions with a great functional demo. Below is a brief video of the demo with our Life Science Industry Manager, Blake DeFrance explaining the technology.

For more information on solutions for the Life Science Industry visit www.balluff.us.

Multiple Sensing Modes for Miniature Capacitive Sensors

In a previous blog post we discussed miniature capacitive sensors and their use for precision and small-part sensing. Here we will discuss the different sensing modes available with separately amplified miniature capacitive sensors.

Standard Switching Mode

Std_Switch_Mode

This is the most commonly used teach method for most sensing applications. As an object is placed statically in front of the sensor at its desired detection point, the amplifier is triggered to teach-in this value as its switch point (SP1). Once the value is taught, the output will then switch when the switch point is reached.

Two-Point Switching Mode

TwoPoint_Switch_Mode

As the name sug
gests this teach method has two separate teach-in points, a switch-on point (SP1) and a switch-off point (SP2). These points can be taught wide apart or close together, depending on the application need. One application example is for fill-level control by teaching in min. and max. fill-level points.

Window Function Mode

Window_Mode

This teach method creates a window between two separate switch points (SP1 and SP2). If the sensor value falls inside this window, the output will switch on. If the sensor value is outside of this window, the output remains off. An application example is material thickness (or multiple layer) detection. If the material is too thin or too thick (i.e., sensor value is outside the window) the output remains off; however, if the material is at the correct thickness (i.e., sensor value falls inside the window) the output switches on.

Dynamic Operation Mode

This mode only responds to moving objects and ignores static conditions. This mode is commonly used to ignore a close background, and only detect objects moving in front of the sensor.

Analog Output Mode
Analog_Mode

Additionally, an analog output (either voltage or current) is available. To utilize the whole analog range, two separate teach points are needed. SAHi, analog signal high, and SALo, analog signal low, are taught accordingly to obtain the full range. An application example would be continuous fill-level detection across the sensing area.

For more information on capacitive sensors and their remote amplifiers, click h
ere.

Miniature Capacitive Sensors for Small Part Detection

As discussed in a previous blog post, miniature sensors are an ongoing trend in the market as manufacturing and equipment requirements continue to demand smaller sensor size due to either space limitations and/or weight considerations. However, size and weight aren’t the only factors. The need for more precise sensing — higher accuracy, repeatability, and smaller part detection — is another demanding requirement and, often times, the actual main focus point.

This post will look specifically at capacitive sensors and how smaller capacitive sensors can lead to better detection of smaller parts.

cap4 — Parallel-plate capacitor equation

Capacitive sensors provide non-contact detection of all types of objects, ranging from insulators to conductors and even liquids. A capacitive sensor uses the principle of capacitance to detect objects. The equation for capacitance takes into account the surface area (A) of either electrode, the distance (d) between the electrodes, and the dielectric constant (ε_r) of the material between the electrodes. In simple terms: a capacitive sensor detects the change in capacitance when an object enters its electrical field. Internal circuitry determines if the gain in capacitance is above the set threshold. Once the threshold is met the sensor’s output is switched.

When looking at small part detection, the size of the capacitive sensor’s active sensing surface plays a significant part. Now there isn’t a defined formula for calculating smallest detectable object for a capacitive sensor because of the numerous variables that need to be considered (as seen in the equation above). However, the general rule for optimal sensing is that the target size should be at least equal to the size of the sensor’s active surface. The reason behind this is if the target size is smaller than the sensor’s active surface, the electric field would travel around the target and cause unreliable readings.

Taking the general rule into consideration and comparing a miniature 4mm diameter capacitive sensor to a standard 18mm diameter capacitive sensor, it’s simple to determine that the 4mm diameter capacitive sensor can reliably detect a much smaller target (4mm) than the 18mm diameter capacitive sensor (18mm).

So when looking at small part detection, the smaller the sensor’s active sensing surface is, the better its ability for small part detection. Therefore, if an application requires detection of a small part, it’s best to start with miniature capacitive sensor.

For more information on miniature capacitive sensors click here.