09
02.2020

Capacitive Sensors - Concept of capacitive measurement technology

親子育兒   數碼科技(周邊)  
標籤: frequency  Sensing 

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Principle of operation

A capacitive sensor operates with a high frequency oscillation circuit which generates an electric field at the active sensor surface by means of a capacitor. As a solid or liquid substance approaches this field, a change in capacity occurs and thus a change in the gain in this oscillation circuit. When this gain exceeds a threshold, a signal is generated.

Sensing

The sensing distance is the distance between an object and the active sensor surface when a signal is generated. The sensing distance is in relation to the diameter of the sensor surface, therefore larger sensors are required for greater sensing distance. The sensing distance can be set for many EGE sensors.

Capacitive sensors react to conductive and nonconductive substances with a dielectric constant ε> 1. The sensing distance is in relation to the material of the object. At constant dimensions, the sensing distance to grounded steel St37 changes.

The table below gives approximate values ​​for material-related reduction factors. For practical use, for example, the mounting conditions can lead to deviations.

 

The water content of an object or liquid has a decisive influence on the sensing distance. A greater moisture content significantly increases the sensing distance.

When the sensor is moistened with conductive substances, its function can be affected when a conductive film is formed which creates an electrically conductive connection between the sensor electrode and a metallic wall.

Capacitive sensors can sense goods through non-conductive container walls. Safe detection of the goods occurs when the dielectric constant of the goods is at least as large as that of the container wall.

Nominal sensing distance sn

The nominal sensing distance is a nominal value in which variations in production and external factors such as temperature and supply voltage are neglected.

Actual sensing distance sr

The actual sensing distance is the effective sensing distance at rated voltage and a rated temperature of 23 ° C. It is between 90% and 110% of the nominal sensing distance.

Effective sensing distance su

The effective sensing distance for the entire permissible temperature and voltage range is between 80% and 120% of the actual sensing distance.

Working distance said

The guaranteed sensing distance includes all external factors and variations in production. It is between 0% and 72% of the nominal sensing distance. Safe sensing is guaranteed in this area.

Operation with temperature-related sensing point

The sensing distances are indicated for an ambient temperature of 23 ° C. Within the permissible temperature range, the sensing distance varies by less than 15% compared to the value at 23 ° C. The sensing point is not affected by the temperature of the measuring object.

Hysteres H

The hysteresis is the distance difference between the point of impact when an object approaches and the point of departure when the object moves away from the sensor. The hysteresis is defined with a maximum of 20% of the actual sensing distance according to EN 60947-5-2, for EGE sensors it is normally 10 to 15% of the actual sensing distance sr.

Repeat accuracy R

The repetition accuracy describes how closely the sensing point is followed when an object repeatedly approaches under defined conditions. EGE sensors typically have tolerances of less than 3% of the actual sensing distance.

Switching frequency

The maximum switching frequency of the sensor is determined in accordance with EN 60947-5-2 with standardized measuring plates ST37 at a half-nominal sensing distance sn.

operating voltage

The operating voltage is the voltage range within which EGE sensors operate safely. When supplying direct voltage, the limit values ​​must be followed, including ripples.

Switching current

By this term, the maximum permissible continuous current for the sensor coupling output is denoted at an ambient temperature of 25 ° C and resistive load. At elevated ambient temperature, the permissible continuous current decreases.

For analog outputs, the limit values ​​and especially the permissible values ​​for the load resistors specified in the respective technical data must be followed.

Short circuit protection

The short-circuit protection protects the sensor from being damaged by short-circuit output. When the fault is fixed, the output is reactivated. When a maximum shock current is specified, this must not be exceeded

Overcurrent trip

This value indicates the mean value of the current when the short-circuit protection is activated with a tolerance of ± 20%.

Protection against reverse polarity

The protection prevents the sensor from being destroyed during voltage supply with reverse polarity.

Voltage drop Ud

The voltage drop arises on the internal resistance of semiconductor elements located in the current direction of the active switching output. It depends on the load current and is stated in accordance with EN 60947-5-2 for an average current of 50 mA.

Residual current Ir

The residual current flows in the load circuit when the output is blocked. When connecting sensors in parallel, the residual current must be taken into account.

Minimum load current Im

A minimum load current is required for faultless operation with two-conductor sensors.

Power consumption

Power consumption specifies the maximum value of the idle current Io consumed by the sensor without load.

ambient temperature

The ambient temperature indicates the maximum permissible temperature range for the sensor.

Electromagnetic compatibility EMC

The EMC class is a measure of the sensor's resistance to external electrical and magnetic interference. The information refers to the standard EN 61000-6-2.

Output delay

EGE sensors are equipped with an output delay that blocks the output when the operating voltage is applied.

Protection

The enclosure class indicates the sensors' protection against intrusion of solids and water, according to EN 60529.

LED display

EGE sensors with yellow LED display the detection status optically.

The casing material

The casing material determines the chemical resistance of the sensor to external influences. For special applications, sheaths in other materials can be supplied.

Connection

 

The sensors are connected by plug or cable. On request, other cable types and lengths can also be supplied.

發表於2020.2.09
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