TURCK Capacitive Sensors

TURCK 6/5/2018
Package Inspection
One of the major benefits of capacitive sensors is their ability to sense through low-dielectric materials. With the sensitivity properly adjusted, these sensors can be used to “see through” an object wall to detect its contents. From inspecting jars through a cardboard box to sensing ammonia in a vat capacitive sensors are made for these applications. In addition, capacitive sensors have the ability to sense most materials including wood, plastics, cardboard, glass, grain, all metals and most fluids. The versatility of these sensors can help you save time and run more efficiently.
The wide sensitivity band of TURCK sensors allows for detection of a variety of granular or powdered materials. Capacitive sensors are widely used to monitor the level of plastic pellets in the hoppers of injection molding machines. TURCK’s new BCC and BCF line of sensors are ideal for this application. TURCK Intrinsically Safe NAMUR sensors are also used in grain elevators to monitor the levels of materials ranging from rice and barley malt to corn and soybeans.
Small Parts Detection
Another great use for capacitive sensors is to detect small items as they come down the assembly line. They can be used to count product or sense proper operation of the line. Choose from many styles with short-circuit and overload protection in AC, DC and Intrinsically Safe NAMUR.
Capacitive Sensors Work Where Others Don’t!
You may know that TURCK has the broadest product offering in inductive sensors, but did you also know that we have the most extensive line of capacitive sensors? TURCK’s Q-Pak capacitive sensors are available in packages up to 10 times narrower than conventional barrel-style sensors. Also our PVDF sensors offer incredible resistance to harsh chemical environments found in the semiconductor and chemical industries.
Liquid Level Detection
Capacitive sensors have the ability to “see through” lower dielectric materials, suchas 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. With TURCK’s Intrinsically Safe NAMUR sensors, PVDF models and PTFE tank wells, even explosive or corrosive materials can be safely sensed.
Wire Break Detection
TURCK capacitive sensors are ideal for sensing wire breakage. Our sensors will detect even the smallest wires of any metal. The long sensing ranges allow the wire to bounce during the process without causing false outputs.
Sensitivity Adjustments
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. 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.
Noise Immunity
Capacitive sensors were originally designed for use in level detection applications in areas that were generally far away from other electrical equipment. As factory automation has become more prevalent throughout industrial markets, these capacitive sensors have gravitated into new environments where electrical noise levels are greatly increased. Electrical noise can be produced by various sources including variable frequency drives, electromechanical motors and standard walkie-talkie devices. These “noisy” environments can have adverse effects on sensing devices causing them to operate improperly and unreliably. TURCK recognizes this and has developed a new circuit for its capacitive sensors. These new “BCF” sensors incorporate a unique filter principle, making them immune to most industrial noise. This principle involves a fixed oscillator frequency combined with a rectifier filter providing superior noise immunity over the competition.
TURCK’s fixed oscillator allows the sensor to maintain a constant frequency regardless of sensitivity adjustment. This fixed frequency is high enough to ignore most of the “standard” noise levels seen on plant floors. Electrical noise is mostly symmetrical which makes it easier to identify and separate from the sensor’s input signal. The TURCK rectifier filter is able to block this noise allowing only the “useful” input signal, which is in phase with the oscillator frequency, to pass.
These two innovative electrical techniques give TURCK the best defense against 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 new “BCF” sensors from TURCK to ensure your process operates smoothly.
Test Type CE "Product" Standard CE "Generic" Standard TURCK "BCF" Noise Immune Capacitive Sensors
Immunity to Electrostatic Discharge (ESD) IEC 1000-4-2, EN 61000-4-2 4 kV Direct Contact, 8 kV Airborne 4 kV Direct Contact, 8 kV Airborne 8 kV Direct Contact, 30 kV Airborne
Immunity to Radiated Electromagnetic Fields. Radio Frequency Interference (RFI) IEC 1000-4-3, EN 61000-4-3 3 V/M, 80-1000 MHz 10V/M, 80-1000 MHz 15 V/M, 80-1000 MHz
Immunity to Electrical Fast Transients (Burst-High Voltage) IEC 1000-4-4, EN 61000-4-4 2000 V 2000 V 3000 V
Immunity to Conducted R.F. Voltage (Line coupled Noise) IEC 1000-4-6, EN 61000-4-6 Undefined 10 V, 150 kHZ-80 MHz 10+ V, 150 kHZ-230 MHz
Immunity to Surges (lightning strike) IEC 255-5 1kV, 500Ω DC Undefined 1kV, 500Ω DC, 5kV, 500Ω AC
• Liquid Level Control for both explosive and non-explosive materials.
• 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; greater sensing range than comparable inductive sensors.
Operating Principle
The active element is formed by two metallic electrodes positioned much like an “opened” capacitor (Figure 1). 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 2).
Capacitive Sensors
Capacitance is a function of the surface area of either electrodes (A or B), the distance between the electrodes (d), and the dielectric constant of the material (ε) between the electrodes (Figure 1).
C= (ε x A)/d
C = capacitance of sensor
A = surface area of either electrode
d = distance between two electrodes
ε = dielectric constant of material between the electrodes
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 3). 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.
Capacitive Sensors
When a Non-Conductive Target enters the sensor’s field, it acts as an electrical insulator between electrodes A and B (Figure 4). The dielectric constant of the material (ε) 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.
Capacitive Sensors
Sensitivity Adjustment
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 5). It should be adjusted in increments of no greater than a quarter-turn (Figure 6). Increasing the sensitivity results in a greater operating distance to both conductive and non-conductive targets.
Capacitive Sensors
When sensing non-conductive targets, the larger the dielectric constant of a material, the greater the achievable operating distance (Figure 7). 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, the greater the achievable operating distance. 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 Sensors
Example Application 1 - Adjustment
Problem: Can a BC20-K40SR-FZ3X2 be used to sense the presence of ammonia from behind a .125" glass panel?
Solution: The dielectric constants for these materials can be found on pages 15 and 16.
Dielectric (εr) of ammonia: 20
Dielectric (εr) of glass: 10
From Figure 7, εr = 20 corresponds to 80% Sn; εr = 10 corresponds to 60% Sn.
Since Sn = 20 mm for a BC20:
S for ammonia = 16 mm
S for glass = 12 mm
The difference is 4 mm. The glass thickness = .125", or 3.1 mm. This application will work with a 0.9 mm margin. This means that by adjusting the potentiometer there should be a reasonable distinction between the glass and the ammonia as seen by the sensor. To set up the sensor for this application, the sensing face of the sensor should be flush against the sight glass.
1. With no ammonia present (if possible) turn the potentiometer clockwise until the sensor turns on. If the sensor is already on, skip step one.
2. Next, turn the potentiometer counter clockwise until the sensor turns off.
3. Now add the ammonia so that it covers the glass panel.
4. Once again, turn the potentiometer counter clockwise, counting the number of turns until the sensor turns off.*
5. Divide the number of turns by two and turn the potentiometer back clockwise that amount. Using this process will allow for a margin of error in either direction. If this application had called for something other than ammonia, like molasses, that tends to leave buildup behind, step 1 above should be performed with the buildup present (if possible).
* If sensor does not turn off after 10 full turns, turn back the potentiometer clockwise between 3 to 5 turns. Minor adjustments may need to be made to achieve desired setting.
Example Application 2 - Mounting
Problem: A metal tank containing a water-based solution has a 1" outside diameter sight glass. What sensor and bracket could be used for monitoring the liquid level?
Solution: The QF 5.5 flat style can used on non-conductive tubing up to 1.0 inch in diameter with the standard mounting straps provided with the sensor (Figure 8). Other mounting straps for larger diameters are available upon request (consult factory).
Capacitive Sensors
Temperature and Environmental Conditions
Compensation Electrode
In practice, sensors can be affected by water droplets, humidity, dust, etc., causing false outputs. To combat this effect each TURCK sensor incorporates a compensating electrode (C) which forms part of a negative feedback circuit (Figure 9). When contaminants are on the sensor face, they affect the sensor’s main field, as well as its compensation field. The negative feedback circuit detects the increase in both fields, and can filter out the effects of the contaminants. When a large target comes into the sensor’s main field, the compensation field is not affected, thus the negative feedback circuit can distinguish a difference between the two fields, and the sensor generates an output.
Capacitive Sensors
Most capacitive sensors manufactured by TURCK are embeddable, which ensures that the electric field is only effective in front of the active face. They are suitable for flush mounting at the factory setting in any material (conductive & nonconductive). When sensors are flush mounted, the effect on the operating distance is minimal and can be overcome by adjustment of the potentiometer. Minimum separation distances must be observed to avoid the possibility of interference between the two sensors’ fields (Figure 10).
Capacitive Sensors
Operating Distance (Sensing Range) Considerations
The operating distance (S) of the different models is basically a function of the diameter of the sensing coil. Maximum operating distance is achieved with the use of a standard or larger target. Rated operating distance (Sn) for each model is given in the manual.
Standard Target
An earth-grounded square piece of carbon steel having a thickness of 1 mm (0.04 in) is used as a standard target to determine the following operating tolerances. The length and width of the square is equal to three times the rated operating distance.
Operating Distance = S
The operating distance is the distance at which the target approaching the sensing face along the reference axis causes the output signal to change.
Rated Operating Distance = Sn
The rated operating distance is a conventional quantity used to designate the operating distance. It does not take into account either manufacturing tolerances or variations due to external conditions such as voltage and temperature. (Figure 10)
Effective Operating Distance = Sr 0.9 Sn ≤ Sr ≤ 1.1Sn
The effective operating distance is the operating distance of an individual proximity sensor at a constant rated voltage and 23°C (73°F). It allows for manufacturing tolerances.
Usable Operating Distance = Su 0.72 Sn ≤ Su ≤ 1.32Sn
The usable operating distance is the operating distance of an individual proximity sensor measured over the operating temperature range at 85% to 110% of its rated voltage. It allows for external conditions and for manufacturing tolerances.
Assured Operating Range = Sa 0 ≤ Sa ≤ 0.72Sn
The assured actuating range is between 0 and 72% of the rated operating distance. It is the range within which the correct operation of the proximity sensor under specified voltage and temperature ranges is assured. (Figure 11)
Capacitive Sensors