turck blog

Capacitive Level Sensors

TURCK 6/5/2018
 
Capacitive sensors have the ability to “see through” lower dielectric materials, such as plastic or glass, to detect higher dielectric ones. This allows capacitive sensors to detect levels of many types of materials either directly through the wall for plastic tanks, or by utilizing a sight glass or tank well for metal tanks.
 
WHY CHOOSE TURCK’S CAPACITIVE SENSORS?
 
 • Q-Pak capacitive sensors are available in packages up to 10 times narrower than conventional barrel-style sensors.
 • Extensive NAMUR offering for intrinsically safe applications; from flatpack housings to 12-30 mm barrels.
 • Wide sensitivity band allows for detection of a variety of granular or powdered materials.
 • NAMUR, PVDF and PTFE tank wells can be used in explosive or corrosive materials.
 
WHERE CAN I USE TURCK’S CAPACITIVE SENSORS?
 
 • Liquid level control for explosive and non-explosive environments.
 • Package inspection for product content and/or fill level.
 • Wire-break detection for wire sizes down to .003”.
 • Plastic pellet detection in a hopper for injection molding processes.
 • Grain or food products level detection; intrinsically safe models available.
 • Small metal parts detection for greater sensing range than comparable inductive sensors.
 • Semiconductor and chemical industries.
 
HOW DO I ADJUST THE SENSITIVITY OF A CAPACITIVE SENSOR?
 
Many applications require adjusting the sensitivity of the capacitive sensor in order to reliably detect the target material. Although the potentiometer is factory set for an operating distance of 0.7 to 0.8 times the rated operating distance, it can be easily changed. The rated operating distance is set using the standard target which is a grounded piece of mild steel.
 
Most TURCK capacitive sensors are listed as embeddable. By increasing the sensitivity, the embeddable sensor can be changed into a non-embeddable version with enhanced sensing capabilities. Capacitive sensors can be adjusted two ways in order to sense a target consistently.
 
1.  Physical adjustment – moving the sensor towards or away from the target is the preferred method of adjusting sensitivity when the sensor is not in direct contact with the target. This allows materials to be moved into or out of range while leaving the sensor at the factory setting or after re-calibration to the nominal operating distance Sn.

2.  Adjustment of the potentiometer – turning the potentiometer in a clockwise direction increases the sensitivity of the sensor. The potentiometer is factory-set for an operating distance of 0.7 to 0.8 Sn to a grounded standard target (Figure 1). It should be adjusted in increments of no greater than a quarter-turn (Figure 2). Increasing the sensitivity results in a greater operating distance to both conductive and non-conductive targets.
 
When sensing non-conductive targets, the larger the dielectric constant of a material, the greater the achievable operating distance (Figure 3). Adjusting the potentiometer affects the total curve; for example, if the potentiometer is adjusted for less sensitivity, it will have less operating distance to all materials.
 
In general terms, the larger the dielectric constant of a material (ε), the greater the achievable operating distance. The dielectric constant of the material  is a measure of its insulation properties. All liquids and solids have a greater dielectric constant than air (εair = 1). Therefore, the capacitance with a non-metallic target present is always greater than the capacitance of the circuit in the absence of the target. When detecting organic materials the sensing distance will depend largely on the water content (εwater =  88). It should be noted that a large increase in sensitivity will cause the sensor to become nonembeddable, and may result in an unstable switching point that can be influenced by environmental changes such as temperature, humidity, dust, etc.  At  adjustments of S > Sn, the differential travel (hysteresis) can also increase.
 
Capacitive Level Sensors
Capacitive Level Sensors
Capacitive Level Sensors
 
Operating Principle
 
The active element is formed by two metallic electrodes positioned much like an “opened” capacitor (Figure 5). Electrodes A and B are placed in a feedback loop of a high frequency oscillator. When no target is present, the sensor’s capacitance is low, therefore the oscillation amplitude is small. When a target approaches the face of the sensor, it increases the capacitance. This increase in capacitance results in an increased amplitude of oscillation. The amplitude of oscillation is measured by an evaluating circuit that generates a signal to turn on or off the output (Figure 4).
 
Capacitance is a function of the surfacerea of either electrodes (A or B), theistance between the electrodes (d), and the dielectric constant of thematerial (ε) between the electrodes (Figure 5).
 
When a Conductive Target enters the sensor’s field, it forms a counter electrode to the active face of the sensor, thus decreasing the distance between the electrodes (d) and increasing their average surface area (Figure 6). The capacitance with a metal target present is always greater than the capacitance of the circuit in the absence of the target. Reduction factors for different metals are not a consideration when using capacitive sensors. When a non-Conductive Target enters the sensor’s field, it acts as an electrical insulator between electrodes A and B (Figure 7).
 
Capacitive Level Sensors
Capacitive Level Sensors
 
CAN I USE CAPACITIVE SENSORS IN AREAS WITH HIGH ELECTROMAGNETIC NOISE?
 
Capacitive sensors were originally designed for use in areas that were generally far away from other electrical equipment. As factory automation has become more prevalent throughout industrial market, these capacitive sensors have gravitated into new environments where electrical noise levels are present. Electrical noise can be produced by various sources, including variable frequency drives, electro-mechanical motors and standard walkie-talkie devices. These “noisy” environments can have adverse effects on sensing devices causing them to operate improperly.
 
TURCK developed a new circuit for its BCF capacitive sensors that incorporates a fixed oscillator frequency combined with a rectifier filter that makes the sensors immune to most industrial noise. The list of specifications and test results below demonstrates how TURCK meets or exceeds all of the rigid standards established by CE. In fact, the criteria set forth by CE is so stringent that most capacitive sensors offered on the market today cannot pass any or all of these testing requirements. If you have a capacitive sensor application located in a “noisy” environment, choose the BCF sensors from TURCK to ensure your process operates smoothly.
 
Test Type BCC Capacitive Sensors CE "Product" Standard CE "Generic" Standard
Immunity to Electrostatic Discharge (ESD) IEC 1000-4-2, EN 61000-4-2 8 kV Direct Contact, 30 kV Airborne 4 kV Direct Contact, 8 kV Airborne 4 kV Direct Contact, 8 kV Airborne
Immunity to Radiated Electromagnetic Fields. Radio Frequency Interference (RFI) 3 V/M, 80-1000 MHz 15 V/M, 80-1000 MHz 3 V/M, 80-1000 MHz 3 V/M, 80-1000 MHz
Immunity to Electrical Fast Transients (Burst-High Voltage) IEC 1000-4-4, EN 61000-4-4 3000 V 2000 V 2000 V
Immunity to Conducted R.F. Voltage (Line coupled Noise) IEC 1000-4-6, EN 61000-4-6 10+ V, 150 kHZ-230 MHz Undefined 10 V, 150 kHZ-80 MHz
Immunity to Surges (lightning strike) IEC 255-5 1 kV, 500 Ω DC, 5 kV, 500 Ω AC 1 kV, 500 Ω DC Undefined