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Force, mechanical stress and touch sensors. Big encyclopedia of oil and gas

A capacitive sensor is one of the types of non-contact sensors, the principle of which is based on a change in the dielectric constant of the medium between two capacitor plates. One plate is a circuit sensor in the form of a metal plate or wire, and the second is an electrically conductive substance, for example, metal, water or the human body.

When developing a system automatic start water supply to the toilet for a bidet, it became necessary to use capacitive sensor presence and switch having high reliability, resistance to changes in external temperature, humidity, dust and supply voltage. I also wanted to eliminate the need for a person to touch the controls of the system. The requirements could only be provided by sensor circuits operating on the principle of capacitance change. Finished scheme satisfying necessary requirements I couldn't find it so I had to develop it myself.

The result was a universal capacitive touch sensor that does not require adjustment and responds to approaching electrically conductive objects, including a person, at a distance of up to 5 cm. The scope of the proposed touch sensor is not limited. It can be used, for example, to turn on lighting, systems burglar alarm, determining the water level and in many other cases.

Electrical circuit diagrams

Two capacitive touch sensors were needed to control the flow of water in the toilet bidet. One sensor had to be installed directly on the toilet, it had to give a logical zero signal in the presence of a person, and in the absence of a logical unit signal. The second capacitive sensor was supposed to serve as a water switch and be in one of two logical states.

When a hand was brought to the sensor, the sensor had to change the logical state at the output - from the initial single state to go to the state of logical zero, when the hand was touched again from the zero state to go to the state of logical one. And so on ad infinitum, until the sensor switch receives a logical zero enable signal from the presence sensor.

Capacitive touch sensor circuit

The basis of the scheme of the capacitive sensor presence sensor is the master generator of rectangular pulses, made according to classical pattern on two logic elements of the chip D1.1 and D1.2. The oscillator frequency is determined by the values of the elements R1 and C1 and is chosen at about 50 kHz. The frequency value has practically no effect on the operation of the capacitive sensor. I changed the frequency from 20 to 200 kHz and visually did not notice any effect on the operation of the device.

With 4 chip output D1.2 signal rectangular shape through the resistor R2 enters the inputs 8, 9 of the D1.3 chip and through the variable resistor R3 to the inputs 12.13 D1.4. The signal arrives at the input of the D1.3 chip with a slight change in the slope of the pulse front due to installed sensor, which is a piece of wire or a metal plate. At the input D1.4, due to the capacitor C2, the front changes for the time required to recharge it. Due to the presence of a tuning resistor R3, it is possible to set the pulse fronts at input D1.4 equal to the pulse front at input D1.3.

If you bring your hand or metal object, then the capacitance at the input of the DD1.3 microcircuit will increase and the front of the incoming pulse will be delayed in time, relative to the front of the pulse arriving at the input of DD1.4. to "catch" this delay, about inverted pulses are fed to the DD2.1 chip, which is a D flip-flop operating in the following way. On the positive edge of the pulse arriving at the input of the microcircuit C, the signal that was at the input D at that moment is transmitted to the output of the trigger. Therefore, if the signal at the input D does not change, the incoming pulses at the counting input C do not affect the level of the output signal. This property of the D trigger made it possible to make a simple capacitive touch sensor.

When the capacitance of the antenna, due to the approach of the human body to it, at the input of DD1.3 increases, the pulse is delayed and this is fixed by the D trigger, changing its output state. The HL1 LED serves to indicate the presence of the supply voltage, and HL2 to indicate the proximity to the touch sensor.

Touch switch circuit

The capacitive touch sensor circuit can also be used to operate the touch switch, but with a little refinement, since it needs not only to respond to the approach of the human body, but also to remain in a steady state after the removal of the hand. To solve this problem, it was necessary to add another D trigger, DD2.2, to the output of the touch sensor, connected according to the divider-by-two circuit.

The capacitive sensor circuit has been slightly modified. To eliminate false positives, since a person can bring and remove his hand slowly, due to the presence of interference, the sensor can output several pulses to the counting input D of the trigger, violating the necessary switch operation algorithm. Therefore, an RC chain of elements R4 and C5 was added, which for a short time blocked the possibility of switching the D trigger.

The trigger DD2.2 works in the same way as DD2.1, but the signal to the input D is not supplied from other elements, but from the inverse output of DD2.2. As a result, on the positive edge of the pulse arriving at input C, the signal at input D changes to the opposite. For example, if in the initial state there was a logical zero at pin 13, then by bringing your hand to the sensor once, the trigger will switch and a logical unit will be set at pin 13. The next time the sensor is acted upon, a logical zero will again be set at pin 13.

To block the switch in the absence of a person on the toilet, a logical unit is supplied from the sensor to the input R (setting zero at the trigger output, regardless of the signals at all its other inputs) of the DD2.2 microcircuit. At the output of the capacitive switch, a logical zero is set, which is fed through the harness to the base of the switching switching transistor solenoid valve in the power supply and switching unit.

Resistor R6, in the absence of a blocking signal from the capacitive sensor in the event of its failure or a break in the control wire, blocks the trigger at input R, thereby eliminating the possibility of spontaneous water supply to the bidet. Capacitor C6 protects input R from interference. The HL3 LED is used to indicate the water supply in the bidet.

Structure and details of capacitive touch sensors

When I started designing a bidet sensor system, the most difficult task for me seemed to be the development of a capacitive presence sensor. This was due to a number of restrictions on installation and operation. I did not want the sensor to be mechanically connected to the toilet lid, since it must be removed periodically for washing, and did not interfere with sanitization the toilet itself. Therefore, I chose the capacitance as the reacting element.

Presence sensor

According to the above published scheme, I made a prototype. The details of the capacitive sensor are assembled on a printed circuit board, the board is placed in a plastic box and is closed with a lid. To connect the antenna, a single-pin connector is installed in the housing, and a four-pin RSh2N connector is installed to supply the supply voltage and signal. PCB connected with solder connectors copper conductors in PTFE insulation.

The touch capacitive sensor is assembled on two microcircuits of the KR561 series, LE5 and TM2. Instead of the KR561LE5 chip, you can use the KR561LA7. Chips of the 176 series, imported analogues, are also suitable. Resistors, capacitors and LEDs will fit any type. Capacitor C2, for stable operation of the capacitive sensor when operating in conditions of large temperature fluctuations environment you need to take with a small TKE.

The sensor is installed under the platform of the toilet, on which it is installed drain tank in a place where, in the event of a leak from the tank, water cannot enter. The sensor body is glued to the toilet with double-sided adhesive tape.

The antenna sensor of the capacitive sensor is a piece of copper stranded wire 35 cm long in PTFE insulation, glued with transparent adhesive tape to the outer wall of the toilet bowl a centimeter below the plane of the glasses. The sensor is clearly visible in the photo.

To adjust the sensitivity of the touch sensor, it is necessary, after installing it on the toilet, by changing the resistance of the tuning resistor R3 to make the HL2 LED go out. Next, put your hand on the toilet lid above the location of the sensor, the HL2 LED should light up, if you remove your hand, go out. Since the human thigh by mass more hands, then during operation the touch sensor, after such a setting, will work guaranteed.

The design and details of the capacitive touch switch

The capacitive touch switch circuit has more details and a housing was needed to accommodate them bigger size and for aesthetic reasons, appearance the housing in which the presence sensor was placed was not very suitable for installation in a conspicuous place. The rj-11 wall socket for connecting the phone drew attention to itself. It fit true to size and looks good. Having removed everything superfluous from the outlet, I placed the printed circuit board of the capacitive touch switch in it.

For fixing printed circuit board a short rack was installed at the base of the case and a printed circuit board with touch switch parts was screwed to it with a screw.

The capacitive sensor sensor was made by gluing a sheet of brass to the bottom of the socket cover with Moment glue, after cutting out a window for the LEDs in them. When the lid is closed, the spring (taken from a flint lighter) contacts the brass sheet and thus provides electrical contact between the circuit and the sensor.

The capacitive touch switch is attached to the wall using one self-tapping screw. For this, a hole is provided in the body. Next, the board, connector is installed and the cover is fixed with latches.

The setting of the capacitive switch is practically the same as the setting of the presence sensor described above. For tuning, you need to apply the supply voltage and adjust the resistor so that the HL2 LED lights up when a hand is brought to the sensor, and goes out when it is removed. Next, you need to activate the touch sensor and bring and remove your hand to the switch sensor. The HL2 LED should blink and the red HL3 LED should light up. When the hand is removed, the red LED should remain lit. When the hand is brought up again or the body is removed from the sensor, the HL3 LED should go out, that is, turn off the water supply in the bidet.

Universal PCB

The capacitive sensors presented above are assembled on printed circuit boards, which are slightly different from the printed circuit board shown in the photo below. This is due to the combination of both printed circuit boards into one universal one. If you assemble the touch switch, then you only need to cut track number 2. If you assemble the presence sensor, then track number 1 is removed and not all elements are installed.

The elements necessary for the operation of the touch switch, but interfering with the operation of the presence sensor, R4, C5, R6, C6, HL2 and R4, are not installed. Instead of R4 and C6, wire jumpers are soldered. The chain R4, C5 can be left. It will not affect work.

Below is a drawing of a printed circuit board for knurling using the thermal method of applying tracks to the foil.

It is enough to print the drawing on glossy paper or tracing paper and the template is ready for the manufacture of a printed circuit board.

Trouble-free operation of capacitive sensors for the touch control system of water supply in the bidet has been confirmed in practice for three years of continuous operation. No failures were recorded.

However, I want to note that the circuit is sensitive to powerful impulse noise. I received an email asking for help with setup. It turned out that during the debugging of the circuit there was a soldering iron with a thyristor temperature controller nearby. After turning off the soldering iron, the circuit worked.

There was another case. The capacitive sensor was installed in the lamp, which was connected to the same outlet as the refrigerator. When you turn it on, the light turns on and when you turn it off again. The issue was resolved by connecting the lamp to another outlet.

A letter came about the successful application of the described capacitive sensor circuit for adjusting the water level in storage tank from plastic. In the lower and upper parts, it was glued with silicone along the sensor, which controlled the on and off of the electric pump.

Touch sensor for Arduino

The module is a touch button; a digital signal is generated at its output, the voltage of which corresponds to the levels of logical ones and zero. Refers to capacitive touch sensors. We encounter such input devices when working with the display of a tablet, iPhone or a touchscreen monitor. If on the monitor we press the icon with a stylus or a finger, then for this we use an area of the board surface the size of a Windows icon, which is touched only by a finger, the stylus is excluded. The basis of the module is the TTP223-BA6 chip. There is a power indicator.

Melody playback rhythm control

When installed in the device, the sensor area of the module board surface is covered with a thin layer of fiberglass, plastic, glass or wood. The advantages of a capacitive touch button include a long service life and the possibility of sealing the front panel of the device, anti-vandal properties. This allows the touch sensor to be used in outdoor appliances in direct contact with water droplets. For example, a doorbell button or Appliances. Interesting application in equipment smart House- replacement of light switches.

Characteristics

Supply voltage 2.5 - 5.5 V
Touch response time in various current consumption modes
low 220 ms
typical 60 ms
Output signal
Voltage
high log. level 0.8 X supply voltage
low log. level 0.3 X supply voltage
Current at 3 V supply and logic levels, mA
low 8
high -4
Board dimensions 28 x 24 x 8 mm

Contacts and signal

No touch - the output signal has a low logic level, touch - a logical unit at the sensor output.

Why it works or some theory

The human body, like everything that surrounds us, has electrical characteristics. When the touch sensor is triggered, our capacitance, resistance, inductance appear. On the bottom side of the module board there is a section of foil connected to the input of the microcircuit. Between the operator's finger and the foil on the bottom side there is a dielectric layer - the material of the carrier base of the printed circuit board of the module. At the moment of contact, the human body is charged with a microscopic current flowing through a capacitor formed by a piece of foil and a person's finger. In a simplified view, the current flows through two capacitors connected in series: a foil, a finger located on opposite surfaces of the board, and the human body. Therefore, if the surface of the board is covered with a thin layer of insulator, this will lead to an increase in the thickness of the dielectric layer of the foil-finger capacitor and will not disrupt the operation of the module.
The TTP223-BA6 microcircuit captures a tiny microcurrent pulse and registers a touch. Due to the properties of the microcircuit, this technology does not cause any harm to work with such currents. When we touch the case of a working TV or monitor, microcurrents of greater magnitude pass through us.

Low Power Mode

After power is applied, the touch sensor is in a low power mode. After triggering for 12 seconds, the module goes into normal mode. If further contact does not occur, the module will return to the reduced current consumption mode. The response speed of the module to touch in various modes is given in the specifications above.

Work with Arduino UNO

Download the following program to the Arduino UNO.

#define ctsPin 2 // Contact for connecting the touch sensor signal line
int ledPin = 13; // Pin for LED

Void setup() (
Serial.begin(9600);
pinMode(ledPin, OUTPUT);
pinMode(ctsPin, INPUT);
}

void loop() (
int ctsValue = digitalRead(ctsPin);
if (ctsValue == HIGH)(
digitalWrite(ledPin, HIGH);
Serial.println("TOUCHED");
}
else(
digitalWrite(ledPin,LOW);
Serial.println("not touched");
}
delay(500);
}

Connect the touch sensor and Arduino UNO as shown. The circuit can be supplemented with an LED that turns on when the sensor is touched, connected through a 430 Ohm resistor to pin 13. Touch buttons are often equipped with a touch indicator. This makes it easier for the operator to work. When a mechanical button is pressed, we feel a click regardless of the reaction of the system. Here, the novelty of the technology is a little surprising due to our motor skills that have developed over the years. The pressure indicator saves us from an excessive feeling of novelty.

Elector 2008 №7-8

The operation of capacitive touch sensors is based on the electrical capacitance of the human body. For example, when a finger is brought close to the sensor, this creates a capacitance between the sensor and the ground, which lies in the range of 30 ... 100 pF. This effect can be used in proximity sensors and touch switches.

Touch capacitive sensors have obvious advantages over other sensors (for example, triggered by 50/60 Hz interference or measuring resistance), but they are more laborious to implement. Chip manufacturers such as Microchip have created custom ICs for this purpose in the past. However, even now it is possible to create a reliable capacitive detector and/or switch using only a small number of standard components.

In this circuit, we detect changes in signal pulse width that occur when a contact is touched. In Figure 1, the following nodes can be considered (from left to right):

Rice. 1. IC1 - 561TL1

Rectangular pulse generator, made on the Schmitt trigger (IC CD4093);
RC circuit with quenching diode followed by Schmitt trigger/terminal plate with 470 pF isolation capacitor;
- Integrating RC circuit that converts changes in pulse width into voltage. This voltage lies in the region of 2.9 ... 3.2 volts when the plate is touched, and 2.6 volts otherwise.
- The LM 339 comparator is used to compare the voltage at point C with the reference voltage at point D. The latter is about 2.8 V and is set by a voltage divider.

As soon as the sensor plate is touched, the output of the circuit will become active. To explain the operation of the circuit, Figure 2 shows waveforms of signals in different points. The dotted line shows the state when the sensor plate is touched, the solid line shows when there is no touch.

Rice. 2. Oscillograms of signals at different points.

The reference voltage at point D is adjusted once using the divider R4/R5 (by changing the value of R4). The magnitude of this voltage is highly dependent on the surface area of the sensor plate (usually a few square centimeters). Big square The surface of the plate increases capacitance and the voltage at point C will still be greater than when the plates were not touching. The reference voltage at point D should be set closer to 3.4 V. The touch sensor can also work with plates large area(for example, you can use the entire body as a sensor).

The output signal can be used to turn on various loads. In many cases it is recommended to add one Schmitt trigger to the output, especially if the output is connected to a digital input.

Wim Abuys

Rice. 4. The location of the components on the printed circuit board.

Rice. 5. PCB.

Rice. 6. Printed circuit board (mirror view).

When developing a system for automatically turning on the water supply to the toilet bowl for a bidet, it became necessary to use a capacitive presence sensor and a switch with high reliability, resistance to changes in external temperature, humidity, dust and supply voltage. I also wanted to eliminate the need for a person to touch the controls of the system. The requirements could only be provided by sensor circuits operating on the principle of capacitance change. I did not find a ready-made scheme that met the necessary requirements, I had to develop it myself.

The result was a universal capacitive touch sensor that does not require adjustment and responds to approaching electrically conductive objects, including a person, at a distance of up to 5 cm. The scope of the proposed touch sensor is not limited. It can be used, for example, to turn on lighting, alarm systems, detect water levels and in many other cases.

Electrical circuit diagrams

Capacitive touch sensor circuit

The basis of the circuit of the capacitive sensor presence sensor is the master rectangular pulse generator, made according to the classical scheme on two logic elements of the microcircuit D1.1 and D1.2. The oscillator frequency is determined by the values of the elements R1 and C1 and is chosen at about 50 kHz. The frequency value has practically no effect on the operation of the capacitive sensor. I changed the frequency from 20 to 200 kHz and visually did not notice any effect on the operation of the device.

From the 4 outputs of the D1.2 chip, a rectangular signal is fed through the resistor R2 to the inputs 8, 9 of the D1.3 chip and through the variable resistor R3 to the inputs 12.13 D1.4. The signal arrives at the input of the D1.3 chip with a slight change in the slope of the pulse front due to the installed sensor, which is a piece of wire or a metal plate. At the input D1.4, due to the capacitor C2, the front changes for the time required to recharge it. Due to the presence of a tuning resistor R3, it is possible to set the pulse fronts at input D1.4 equal to the pulse front at input D1.3.

If you bring a hand or a metal object closer to the antenna (touch sensor), then the capacitance at the input of the DD1.3 microcircuit will increase and the front of the incoming pulse will be delayed in time relative to the front of the pulse coming to the input of DD1.4. to "catch" this delay, about inverted pulses are fed to the DD2.1 chip, which is a D flip-flop that works as follows. On the positive edge of the pulse arriving at the input of the microcircuit C, the signal that was at the input D at that moment is transmitted to the output of the trigger. Therefore, if the signal at the input D does not change, the incoming pulses at the counting input C do not affect the level of the output signal. This property of the D trigger made it possible to make a simple capacitive touch sensor.

Touch switch circuit

To block the switch in the absence of a person on the toilet, a logical unit is supplied from the sensor to the input R (setting zero at the trigger output, regardless of the signals at all its other inputs) of the DD2.2 microcircuit. A logical zero is set at the output of the capacitive switch, which is fed through the harness to the base of the key transistor for turning on the solenoid valve in the Power and Switching Unit.

Structure and details of capacitive touch sensors

When I started designing a bidet sensor system, the most difficult task seemed to me to be the development of a capacitive presence sensor. This was due to a number of restrictions on installation and operation. I did not want the sensor to be mechanically connected to the toilet lid, since it must be periodically removed for washing, and did not interfere with the sanitization of the toilet itself. Therefore, I chose the capacitance as the reacting element.

Presence sensor

According to the above published scheme, I made a prototype. The details of the capacitive sensor are assembled on a printed circuit board, the board is placed in a plastic box and is closed with a lid. To connect the antenna, a single-pin connector is installed in the housing, and a four-pin RSh2N connector is installed to supply the supply voltage and signal. The printed circuit board is connected to the connectors by soldering with copper conductors in fluoroplastic insulation.

The touch capacitive sensor is assembled on two microcircuits of the KR561 series, LE5 and TM2. Instead of the KR561LE5 chip, you can use the KR561LA7. Chips of the 176 series, imported analogues, are also suitable. Resistors, capacitors and LEDs will fit any type. Capacitor C2, for stable operation of the capacitive sensor when operating under conditions of large fluctuations in ambient temperature, must be taken with a small TKE.

A sensor is installed under the toilet platform, on which a drain tank is installed in a place where water cannot enter in the event of a leak from the tank. The sensor body is glued to the toilet with double-sided adhesive tape.

To adjust the sensitivity of the touch sensor, it is necessary, after installing it on the toilet, by changing the resistance of the tuning resistor R3 to make the HL2 LED go out. Next, put your hand on the toilet lid above the location of the sensor, the HL2 LED should light up, if you remove your hand, go out. Since the human thigh is larger than the arm in mass, then during operation the touch sensor, after such a setting, will be guaranteed to work.

The design and details of the capacitive touch switch

The capacitive touch switch circuit has more details and a larger case was needed to accommodate them, and for aesthetic reasons, the appearance of the case in which the presence sensor was placed was not very suitable for installation in a conspicuous place. The rj-11 wall socket for connecting the phone drew attention to itself. It fit true to size and looks good. Having removed everything superfluous from the outlet, I placed the printed circuit board of the capacitive touch switch in it.

To fix the printed circuit board, a short stand was installed at the base of the case, and a printed circuit board with touch switch parts was screwed to it with a screw.

Universal PCB

Below is a drawing of a printed circuit board for knurling using the thermal method of applying tracks to the foil.

It is enough to print the drawing on glossy paper or tracing paper and the template is ready for the manufacture of a printed circuit board.

A letter came about the successful application of the described capacitive sensor circuit for adjusting the water level in a plastic storage tank. In the lower and upper parts, it was glued with silicone along the sensor, which controlled the on and off of the electric pump.

FORCE, MECHANICAL STRESS AND TOUCH SENSORS

In the SI system main units are mass, length and time, while force and acceleration are derivatives units. In the British and American systems of units, the basic units are force, length and time. The unit of force is one of the fundamental physical quantities. The measurement of forces is also carried out during mechanical research, and in civil engineering, and when weighing objects, and in the manufacture of prostheses, etc. When determining pressure, a force measurement is also required. It is believed that when working with solid objects, force is measured, and when working with liquids and gases, pressure is determined. This means that the force is considered when the action is applied to a specific point, and the pressure is defined when the force is distributed over a relatively large area.

Force sensors can be divided into two classes: quantitative and qualitative. Quantitative sensors measure force and represent its value in electrical units. Examples of such sensors are torque cells and strain gauges. Quality sensors are threshold devices whose function is not to quantification force values, but in detecting the excess of a given level of applied force. An example of such devices is a computer keyboard, each key of which closes the corresponding contact only when pressed with a certain force. High-quality sensors are often used to detect the movement and position of objects. A door mat that responds to pressure applied to it and a piezoelectric cable are also examples of quality pressure sensors.

Methods for measuring force can be divided into the following groups:

1. Balancing an unknown force by the gravity of a body of known mass

2. Measurement of the acceleration of a body of known mass, to which an unknown force is applied

3. Balancing an unknown force by an electromagnetic force

4. Converting force into fluid pressure and measuring this pressure

5. Measurement of deformation of the elastic element of the system caused by an unknown force

In modern sensors, method 5 is most often used, and methods 3 and 4 are used relatively rarely.

Most sensors do not direct conversion force into an electrical signal. This usually requires several intermediate steps. Therefore, as a rule, force sensors are composite devices. For example, a force sensor is often a combination of a force-displacement transducer and a position (displacement) detector. This may be a simple coil spring, the decrease in length of which, caused by the applied compression force, will be proportional to its coefficient of elasticity.

Figure 1A shows a sensor consisting of a spring and a displacement detector based on a linearly adjustable differential transformer (LRDT). In the linear range of change in the length of the spring, the voltage at the output of the LVDT is proportional to the applied force. On fig. 1B shows another version of the force sensor, consisting of a corrugated membrane filled with liquid, which is directly affected by the force, and a pressure sensor. The corrugated membrane, distributing the force at the input over the surface of the sensitive element of the pressure sensor, plays the role of a force-pressure converter.

load cell is a flexible resistive sensing element, the resistance of which is proportional to the applied mechanical stress (strain value). All strain gauges are based on the previously mentioned piezoresistive effect. A wire load cell is a resistor glued to a flexible substrate, which in turn is attached to an object where force or voltage is measured. In this case, a reliable mechanical connection between the object and the strain-sensing element must be ensured, while the resistor wire must be electrically isolated from the object. The coefficients of thermal expansion of the substrate and wire must be matched. To obtain good sensitivity, the sensor must have long longitudinal sections and short transverse ones (Fig. 2). This is done so that the sensitivity in the transverse direction does not exceed 2% of the longitudinal sensitivity. For measuring voltages in different directions changing the configuration of the sensors. It should be noted that semiconductor strain-sensing elements are quite sensitive to temperature changes, therefore, in the interface circuits or in the sensors themselves, it is necessary to provide for temperature compensation circuits.

Tactile sensors- this is a special class of force or pressure transducers, which are characterized by a small thickness. These sensors are useful when force or pressure is being measured between two surfaces that are close together. Such sensors are often used in robotics, for example, they are installed on the "fingers" of mechanical actuators to ensure feedback upon contact with an object - this is reminiscent of how human skin tactile sensors work. Touch sensors are used in touch displays, keyboards, and other devices that need to respond to physical touch. Tactile sensors are widely used in biomedicine, to determine the bite of teeth and the correct installation of crowns in dental practice, as well as in the study of pressure on a person's legs when walking. Sometimes, during prosthetics operations, they are installed in artificial joints to adjust the position, etc. In construction and mechanical industries, tactile sensors are used to determine the forces acting on fixed devices.

Several methods are used to manufacture tactile sensing elements. In some of them, a special thin layer material sensitive to mechanical stress. On fig. 3 shows a simple tactile sensor providing on-off functions, consisting of two sheets of foil and a spacer. Round (or any other necessary shape) holes are made inside the gasket. One of the foil sheets is grounded and the other is connected to a load resistor. If multiple sensitive areas need to be monitored, a multiplexer is used. When an external force is applied to the top conductor over the hole in the gasket, it flexes and contacts the bottom conductor, thereby making electrical contact with it, grounding the load resistor. In this case, the output signal becomes zero, indicating the applied force. The top and bottom conductors can be screen printed with conductive ink on a substrate. The sensitive zones of such sensors are determined by rows and columns of ink-marked conductors. Touching in a certain area of the sensitive surface leads to the closure of the corresponding row and column, which shows the localization of the applied force. Good tactile sensors are obtained on the basis of piezoelectric films, which are used in both passive and active modes. Many tactile sensors act as touch switches. Unlike traditional switches, the reliability of contacts of which is greatly reduced when moisture and dust get on them, piezoelectric switches, due to their monolithic design, can operate in adverse environmental conditions.

Another type of tactile sensors is piezoresistive sensitive element. It is made of materials whose electrical resistance depends on the applied mechanical stress or pressure. Such materials include conductive elastomers or pressure sensitive pastes. Conductive elastomers are made from silicone rubber, polyurethane, and other materials that contain conductive particles or fibers. For example, conductive rubber is obtained by introducing carbon powder into ordinary rubber. The principle of operation of elastomeric sensors is based either on a change in the contact area when an elastomer is squeezed between two conductive plates, or on a change in the thickness of the elastomeric layer. Depending on the size external force acting on the sensor, the area of the contact zone between the clamping device and the elastomer changes, as a result of which the electrical resistance changes.

Thinner piezoresistive tactile sensors are made from semiconductor polymers, the resistance of which also depends on pressure. The design of such sensors resembles a membrane switch. Compared to strain gauges, piezoresistive sensors have a wider dynamic range.

Piezoelectric Force Sensors

The considered piezoelectric tactile sensors are not intended for accurate force measurements. However, based on the same piezoelectric effect, it is also possible to implement precision force sensors, both active and passive. When designing such sensors, always keep in mind that piezoelectric devices cannot measure stationary processes. This means that piezoelectric force sensors convert force changes into an alternating electrical signal, but they do not respond in any way to a constant external force. Because applied forces can change some properties of materials, the design of active sensors must take into account the comprehensive influence of excitation signals. On fig. 4 shows a variant of the active force sensor. When carrying out quantitative measurements with such sensors, it should be remembered that its measurement range depends on the mechanical resonance frequency of the piezoelectric crystal used. The principle of operation of such sensors is based on the fact that under the mechanical load of quartz crystals of certain sections used as resonators in electronic generators, their resonant frequency shifts.

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Force, mechanical stress and touch sensors. Big encyclopedia of oil and gas

Electrical circuit diagrams

Capacitive touch sensor circuit

Touch switch circuit

Structure and details of capacitive touch sensors

Presence sensor

The design and details of the capacitive touch switch

Universal PCB

Electrical circuit diagrams

Capacitive touch sensor circuit

Touch switch circuit

Structure and details of capacitive touch sensors

Presence sensor

The design and details of the capacitive touch switch

Universal PCB

FORCE, MECHANICAL STRESS AND TOUCH SENSORS

What else to read

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