INVESTIGATION OF CAPACITIVE SENSORS FOR DETECTING OBJECTS IN WALLS

INVESTIGATION OF CAPACITIVE SENSORS FOR DETECTING OBJECTS IN WALLS

Vladimir Abramyan

Student, National Polytechnic University of Armenia,

Armenia, Yerevan

ABSTRACT

In this article were investigated capacitive sensors which are largely used in the devices called Stud Finders for finding and identifying the objects under the drywall. The analysis was performed and the relatively optimal dimensions for the sensors were selected, and was simulated the operation of sensors. The sensor matrix is simulated in the COMSOL environment. By means of simulation, the change in the capacitance during the operation of the sensor is determined for different thicknesses of base material. Also were simulated the capacitance change of sensors related to objects that were occurring in sensing region of sensors. With help of simulation in COMSOL were Identified the optimal thick ness of dielectric material between capacitive sensor plates. As a result of the investigation was the dependencies of capacitance change from dielectric material thickness, which is clearly showing the thickness that will allow getting the highest change in capacity when the object will occur in the sensing area of sensors.

Keywords: capacitive sensor, COMSOL simulation environment, capacitance change, capacitive sensor array.

Introduction: The operation of the predominant part of the equipment that detects an object in the wall is based on capacitive sensors. There is also similar equipment on the market, the operation of which is based on a magnet and inductance. Recently, attempts have been made to integrate the radar in this equipment. In this work, we have studied and developed a model of capacitive sensors that can successfully work in equipment for detection and identification of objects under drywall.

In the late 1970s, Robert Franklin designed a capacitor plate to determine the density of a wall. [1] In the 90s, when the patent expired, other companies such as DeWalt, Bosch, and Franklin Sensors began offering similar and sometimes better devices for finding supports.

Devices for detecting objects in walls work by using an internal capacitor sensors. The detection of objects is based on the capacitive sensors own capacitance change when an object is occurring in their working region. These devices are designed to detect poles, pipes, power lines, and other objects under drywall. This type of devices are able to detect objects at a depth of about 4 .

After studying the available equipment and the principle of their operation, a model of a square sensor was selected. For getting correct results during the simulation in the model were integrated the environment in which is working these devices.

The sensor matrix was designed to detect an object under the drywall with thickness maximum 50 mm's.. In many European countries and in the United States of America the entire structure of the house is assembled from wood or in some cases metal poles coated with big lists of drywall,. Depending on the manufacturer, the thickness of the drywall can vary, in most cases they simply fix several layers for strength.. During the investigation, the average wall thickness was chosen to be 30 mm. Since the permittivity of drywall Is varying in big range from 2.5 - 6.0 [2]. During the simulation, the dielectric permittivity were taken 4 and all simulation were done using this value.

During the simulation, a wooden support was taken as an object in the wall, which has a width of 20 mm and a depth of 40 mm. The dielectric permittivity of wooden stud were chosen 6 and kept the same value during all simulation.

According to the simulation results, for the sensors to work with this method, it is necessary that the dielectric material of the capacitive sensors has 5.5 or more dielectric permittivity. In addition, it is also important for this equipment that they were light and not too thick, so that their use was comfortable in the hand because of it there is a necessity to identify the optimal thickness of dielectric material.. High dielectric permittivity and big loss tangent factor is allowing to get expected results In order to detect any object behind the drywall by sensors, the required capacity(approximately 3 to 4 pF) is obtained with an insulator thickness of 1.5 mm and a dielectric permittivity of 5.5. As higher will be the dielectric permittivity of capacitive sensor dielectric material as thinner can be made sensors and make the device compact.

EXPERIMENT

Figure 1. View of the capacitive sensor from the top (a) and from the side (b)

The sensor itself is obtained on a printed circuit board. The sensor has the structure of a capacitor, the essence of which is to fix the change in charge between its two plates. The sensor is built from two lower platforms-external and internal (Fig. 1). The external platform serves as a signal generator, and the internal platform serves as a receiver [3]. Thus, when working on such a sensor, when there is an object in the wall, this object causes a parasitic capacitance, which changes the main capacitance of the sensor. By recording the change can be identified whether there is an object in the wall or not.

Figure 2. Capacitive sensor array

In the figure 2 is shown the 3D look of capacitive sensor array. It consists from sensors(1), dielectric material(2), and metal plate in the opposite side of sensors array(3) which is a uniform layer of metal and serves as a screen to reduce the environmental impact on the sensor.

During the work also were investigated the contribution of the matrix of capacitive sensors. Prior to this well-known equipment operating on the basis of capacitive sensors, it is very difficult to determine the shape of an object. The use of a matrix of capacitive sensors will allow to build on it more affordable equipment, which can be used to detect objects under the drywall, identify their position and shape with higher accuracy.

RESULTS AND DISCUSSION

During the simulation, three models were studied in which the thickness of the dielectric material of the capacitive sensor board changed. Were done the following simulations for different thicknesses of dielectric material.

1. Capacitive sensor array in the air
2. Capacitive sensor on drywall
3. Capacitive sensor array on the drywall and under it is a wooden support

During the simulation, the results of three sensors were taken for comparison. The thickness of the drywall sheet did not changed. The wooden support was installed under the 4th sensor and its position was not changed during the simulation. For comparison, the capacitances of sensors 3, 4, and 5 were taken (counting from the left). During the simulation, the thickness of the insulator changed from 1 to 3 mm in 0.5 mm step[4]

In figure 3 is shown the view of the electromagnetic field that is generated by sensors. Studs that are appearing in the electromagnetic field are changing the capacitance of sensors. This change occurs as a result of appearing of the other material, in the electrical field of sensors which have different dielectric permittivity or conductance. Both dielectric permittivity and conductance of stud are effecting in change of sensors capacitance.

Figure 3. Electromagnetic field of the capacitive sensor

Table 1 shows the capacitances of the sensors depending on the changes in the thickness of the insulator when the sensors are in the air.

Table 1.

Sensor capacities in the air

During the simulation, the electric field generated by the capacitive sensor was also obtained (Fig. 4) and is shown in figure 4.

Figure 4. Electric field generated by capacitive sensors

Graph 1 shows 3 graphs showing the dependence of the sensor capacity on the insulator thickness. The graph clearly shows the difference in capacity when there is something under the sensor.

Graph 1. Dependence of the sensor capacity on the insulator thickness

After the work done and the results obtained, the following conclusions can be made.

1. It is desirable that the DP of the PCB insulator for capacitive sensors is 5.5 and more so that it is possible to make a printed circuit board with a thickness of 1.5 mm or thinner
2. With the help of capacitive sensors, you can accurately detect objects in the wall, and their position and shape.
3. Graph 1 shows that the sensor with the best characteristics is obtained with a PCB thickness of 1.5 mm.
4. Using a matrix of capacitive sensors, it is possible to develop an inexpensive support pole finder.

References:

1. How do stud finders work? https://www.tooltally.com/how-do-stud-finders-work/
2. https://www.clippercontrols.com/pages/Dielectric-Constant-Values.html
3. Frequency Domain Modeling of a Capacitor. https://www.comsol.ru/model/frequency-domain-modeling-of-a-capacitor-12693
4. Modeling a Capacitive Position Sensor Using FEM.