Influencing factors and measures of capacitance
Affect factors
Because the dielectric constant changes with temperature, capacitive sensors are very sensitive to environmental changes such as temperature and humidity, temperature fluctuations during sensing operation have the most significant impact on performance.
In addition, many external factors can affect capacitive level sensor elements due to edge effects.
The human body itself introduces a lot of parasitic capacitance, which can interfere with the measurement. Capacitive level sensor exhibit non-liner behaviors due to edge effects.
KSLV605 how to improve (solution)
To avoid those influence factors, we designed KSLV605 capacitive level sensor.
How to avoid temperature affects
In liquid-level sensing applications, temperature compensation is more difficult because we cannot assume that the liquid sensor is not covered with liquid; it may be covered to any level for any length of time.
We must, therefore, compensate for temperature variation through the use of algorithms and optimized sensor designs.
This approach minimizes the impact due to temperature drift, which affects a number of parameters in a liquid-level sensing system. First is the capacitance to be measured.
Drift impacts other system parameters, such as the integrating capacitor and the current used by the capacitive sensing engine.
As a result of these variations, the raw count also increases or decreases because of temperature (Fig. 2.1 ). Capacitive sensing circuitry converts the measured capacitance to a digital count, which is known as raw count.
X-axis is temperature, Y-axis is raw count
There are two possible ways to overcome the issue:
1: Fixed temperature compensation capacitor
A temperature compensation capacitor is a sensor that has similar characteristics as the other sensors used for detecting the liquid level.
However, it is not placed in direct contact with the liquid. In other words, this sensor must remain unaffected by the liquid level. The raw count of this capacitor is used as a reference for the actual sensors.
Since the temperature compensation sensor and the actual sensor have the same characteristics, the effect of temperature on both of the sensors will be the same.
In this way, the impact of temperature on the liquid-level detecting sensors can be nullified by using the temperature compensation sensor as a reference.
It’s worth noting that temperature compensation on the liquid-level detecting sensor using the temperature compensating sensor has to be performed both when the tank is empty and when there is liquid in the tank.
In case permittivity and temperature are unknown, a reference sensor can be used to replace the reference capacitance.
Figure2.2 shows an example of a container with two sensor plates and a reference sensor at the bottom. This is to make sure, the reference is measuring the liquid even with a low fill level.This configuration produces ratio metric measurements. This means the ratio between level sensor and reference sensor will be always the same independent of material properties or temperature.
In this configuration, both parameters will affect both sensors and therefor balances itself amongst them.
2: Software algorithm on baseline
Software algorithms to detect a finger touch with capacitive sensing comprise a reference that is a filtered version of the raw count. This reference keeps track of slow environmental changes in raw count, and it’s typically called a baseline.
The baseline is used to detect the presence of a finger in case of touch detection. With liquid-level sensing, the baseline can be used to track changes in raw count. Temperature drift is often slow; hence, its effect on raw count is also slow.
In such cases, with appropriate values of baseline update parameters, the baseline can be used to compensate for temperature variations in raw count.
If the effect of temperature on raw count is very fast, then a temperature compensation capacitor should be used instead.
A secondary effect of temperature is condensation. A liquid that is significantly colder than the ambient air temperature may cause condensation to form on the sensor surface. Condensation may cause a higher capacitance, which, in turn, causes an increased error.
Condensation during low-temperature testing can be reduced by insulating the surface of the sensor.
Another approach is to provide a small insulating air gap between the liquid container and the sensor substrate. The air gap should not be larger than 3 mm for the best performance.
How to avoid edge effects
The human body itself can introduce a lot of parasitic capacitances and therefore disturb the measurements.
The high content of water in the human body causes a potential difference between the sensor plates and therefor influences the measurement.
This additional parasitic capacitance will be measured and would bring a distortion to the fill level relation, as can be seen in the simulation in Figure 2.4.
One way to reduce external parasitic influences is to build up a shielding around the level sensor, as shown in Figure 13.
An additional conductive container, that is not connected to the sensor ground acts as a shield against parasitic capacitances coming from outside.
It is important to keep the sensor ground and the container isolated from each other to have the measurement system isolated from the environmental ground.
The part 1 focuses on the principle, application scenarios, and advantages of the sensor;
