FORCE, MECHANICAL STRESS AND TOUCH SENSORS

In the SI system main The 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. Forces are measured during mechanical research, in civil engineering, when weighing objects, in the manufacture of prostheses, etc. Determining pressure also requires measuring force. 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 force is considered when the action is applied to a specific point, and pressure is determined 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 load cells and strain gauges. Quality sensors are threshold devices whose function is not to quantify the value of a force, but to detect that a specified level of applied force has been exceeded. 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 with the force of gravity of a body of known mass

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

3. Balancing an unknown force with an electromagnetic force

4. Converting force into fluid pressure and measuring this pressure

5. Measuring the deformation of an 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 directly convert 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 reduction in length caused by the applied compressive force will be proportional to its coefficient of elasticity.

Figure 1A shows a sensor consisting of a spring and a displacement detector, implemented on the basis of a linearly controlled differential transformer (LVDT). In the linear range of change in spring length, the voltage at the LVDT output is proportional to the applied force. In Fig. Figure 1B shows another version of the force sensor, consisting of a corrugated membrane filled with liquid, directly affected by the force, and a pressure sensor. The corrugated membrane, distributing the force at the input along 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 (deformation amount). All strain gauges are based on the previously mentioned piezoresistive effect. A wire strain gauge is a resistor bonded to a flexible substrate, which in turn is attached to the object where the force or voltage is measured. In this case, a reliable mechanical connection must be ensured between the object and the strain-sensitive element, while the resistor wire must be electrically isolated from the object. The thermal expansion coefficients 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 to ensure that the sensitivity in the transverse direction does not exceed 2% of the longitudinal sensitivity. To measure voltages in different directions, the configuration of the sensors changes. It should be noted that semiconductor strain-sensitive elements have a fairly strong sensitivity to temperature changes, therefore, it is necessary to provide temperature compensation circuits in the interface circuits or in the sensors themselves.

Tactile sensors- this is a special class of force or pressure transducers, which are characterized by small thickness. These sensors are useful in applications where force or pressure is measured between two surfaces located close to each other. Such sensors are often used in robotics, for example, they are installed on the “fingers” of mechanical drives to provide feedback upon contact with an object - this is reminiscent of how tactile sensors on human skin 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 to study the pressure on a person’s legs when walking. Sometimes during prosthetic operations they are installed in artificial joints to correct the position, etc. In construction and mechanical manufacturing, tactile sensors are used to sense the forces acting on fixed devices.

Several methods are used to produce tactile sensor elements. In some of them, a special thin layer of material sensitive to mechanical stress is formed on the surface of the object. In Fig. Figure 3 shows a simple tactile sensor that provides 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 second is connected to a load resistor. If it is necessary to control several sensitive zones, a multiplexer is used. When an external force is applied to the top conductor over the hole in the pad, it bends and makes contact with the bottom conductor, thereby making electrical contact with it, grounding the load resistor. In this case, the output signal becomes zero, which indicates the applied force. The top and bottom conductors can be screen printed with conductive ink onto a substrate. The sensitive areas of such sensors are determined by rows and columns of inked conductors. Touching 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 from piezoelectric films, which are used in both passive and active modes. Many tactile sensors function as touch switches. Unlike traditional switches, the reliability of the contacts of which is greatly reduced when they are exposed to moisture and dust, piezoelectric switches, due to their monolithic design, can operate in unfavorable conditions environment.

Another type of tactile sensor 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 the elastomer is compressed between two conductive plates, or on a change in the thickness of the elastomeric layer. Depending on the magnitude of the external force acting on the sensor, the area of ​​the contact zone between the clamping device and the elastomer changes, resulting in a change in electrical resistance.

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

Piezoelectric force sensors

The considered piezoelectric tactile sensors are not intended for precise measurements strength. However, based on the same piezoelectric effect, it is possible to implement precision force sensors, both active and passive. When developing such sensors, it should always be remembered that piezoelectric devices cannot measure stationary processes. This means that piezoelectric force sensors convert changes in force into an alternating electrical signal, but they do not respond in any way to a constant value of external force. Since applied forces can change some properties of materials, the full impact of excitation signals must be taken into account when designing active sensors. In Fig. Figure 4 shows a variant of an active force sensor. When carrying out quantitative measurements using 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 when quartz crystals of certain cuts used as resonators in electronic generators are mechanically loaded, their resonant frequency shifts.

The circuit proposed for repetition is an amplifier that is highly sensitive to the electromagnetic field created by external devices. When the input contact of the circuit is connected to the antenna, the LED signals the presence of electromagnetic field radiation and interference from electrical equipment. The LED will also indicate the fact of touching the contact, since the role of the antenna is in in this case performed by the human body. Hence the name - touch sensor. Another name for the circuit is active antenna.

Schematic diagram touch sensor is shown in Figure 1.

The circuit resembles a transistor oscillator n-p-n structures. One of the terminals of winding L1 is connected directly to the input pin X1. The polarity of the VD1 LED does not matter. Resistor R2 limits the current through the LED and, thereby, determines the brightness of its glow when the sensor is triggered.

The touch sensor is assembled on a breadboard measuring 40 × 40 mm. Appearance design is shown in Figure 2.

Figure 2.	Appearance of the touch sensor.

Windings L1 and L2 are located on a common frame with two winding sections and a tuning ferrite core. The outer diameter of the frame is 10 mm, the length of the core is 23 mm, the thread diameter at the base of the core is 6 mm. In the design shown in Figure 2, L1 is wound on the top section, L2 on the bottom. Each coil contains 100 turns of PEL 0.2 wire. The windings are included according to. Using a screwdriver, the core is screwed into the frame. LED VD1 - any of the AL307 series. A grounding petal is used as X1. Touching it causes the LED to light up.

VD1 can be connected in parallel measuring device, for example, a multimeter in voltage measurement mode, which will allow you to evaluate the level of field strength. In this case, the external antenna can be a piece of mounting wire several centimeters long. Setting up the circuit will come down to choosing the length of the antenna and finding the position of the core at which the voltage on the LED is maximum.

The circuit is not picky about the choice of element base. For example, in the original version of the circuit, a KT815G transistor was used, the resistance of resistor R1 was 100 kOhm. Two coils on a rod ferrite core of a long-wave magnetic antenna from a radio receiver were used as L1 and L2. The coils could be moved along the core. When moving the coils, phenomena were observed that did not contradict the law of electromagnetic induction, in contrast to the scheme proposed in. When the coils were significantly removed from each other and without a ferrite core, the circuit stopped working.

The circuit can find practical application not only in the design of field strength meters, but also in automation and signaling devices. The touch sensor can be connected to the microcontroller. To do this, you should perform an analog-to-digital voltage conversion on the VD1 LED, possibly using the resources of the microcontroller itself, if it contains a built-in ADC.

In conclusion, it should be noted that there are many touch sensor circuits based on field-effect transistors and not containing inductive elements. They may work more efficiently in many cases, but the design shown in this article is an example of the original technical solution and is aimed at beginner radio amateurs.

Literature

1. Brovin V.I. The phenomenon of transfer of energy of inductances through the magnetic moments of a substance located in the surrounding space, and its application. - M.: MetaSintez, 2003 - 20 p.
2. Krylov K. S., Lee Jaeho, Kim Young Jin, Kim Seunghwan, Lee Sang-Ha. Patent for invention No. 2395876. Active magnetic antenna with ferrite core.

Touch sensors (touch sensors) are different principles actions, such as resistive (conductive films), optical (infrared), acoustic (SAW), capacitive, etc. This project is an experiment with a capacitive touch sensor. This kind of sensor is well known as a pointing device used in tablet PCs and smartphones.

Principle of capacitive touch sensor

A capacitive touch sensor detects the change in capacitance that occurs at the electrode when covered by a conductive object, such as a finger. There are several methods for measuring capacitance. This project uses the integration method used in the capacitance meter. The change in capacitance Cx is quite small, about 1pF to 10pF, but it will be easily detected because the capacitance meter has a measurement resolution of 20pF. Also, the objects that will be detected must be grounded to create a Cx circuit in accordance with the operating principle. However, it works well even if the human body is isolated from the earth. This may be due to the following reasons.

Hardware

Software

First, calibrate each point (obtain a reference communication time with Cs) and then run a constant period scan. When the integration time has increased and exceeds the threshold, it will decide “detected”. Hysteresis requires a threshold, or the output will not be stable when half touched. The measurement time for each point is equal to the integration time, so this can be done very quickly.

The capacitance meter measures integration time with a resolution of one clock cycle (100 ns) with an analog comparator and input clamp function. However, this feature is not available on all I/O ports. To implement a touch sensor on any I/O port, the integration time is measured by polling software, and the resolution becomes 3 clock cycles (375ns). In normal condition, the time reporting number is about 80, which is enough for touch buttons.

Conclusion

As a result, I can confirm that a capacitive sensor can be easily implemented on a regular microcontroller. The plastic cover can be up to 1mm thick (depending on the dielectric constant) to work well. When ATtiny2313 is used for touch sensor module, it can have 15 touch points. The control program used in this project is experimental and has not been tested in dirty environments such as noise and interference, so any anti-noise algorithm may be required for actual use.

List of radioelements

Designation	Type	Denomination	Quantity	Note	Shop	My notepad
U?	MK AVR 8-bit	ATtiny2313-20PU	1			To notepad
R1-R8	Resistor	1 MOhm	8			To notepad
R9-R16	Resistor	R9-R16	8			To notepad
C1	Electrolytic capacitor	100 µF	1			To notepad
C2	Capacitor	100 nF	1			To notepad
D1-D8	Light-emitting diode		8

Elector 2008 No. 7-8

Capacitive touch sensors work 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 ground, lying in the range of 30...100 pF. This effect can be used in proximity sensors and touch-controlled switches.

Sensory capacitive sensors have obvious advantages compared to other sensors (for example, those triggered by interference with a frequency of 50/60 Hz or those measuring resistance), but they are more labor-intensive to implement. Chip manufacturers such as Microchip have created custom ICs for this purpose in the past. However, it is now 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 you can consider the following nodes (from left to right):

Rice. 1. IC1 - 561TL1

Rectangular pulse generator based on a Schmitt trigger (IC CD4093);
RC circuit with a suppression diode followed by a Schmitt trigger/contact plate with a 470 pF isolating capacitor;
- An 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.
- Comparator LM 339 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 the voltage divider.

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

Rice. 2. Oscillograms of signals at different points.

The reference voltage at point D is adjusted once using the R4/R5 divider (changing the value of R4). The magnitude of this voltage strongly depends on the surface area of ​​the sensor plate (usually several square centimeters). The larger surface area of ​​the plate increases the capacitance and the voltage at point C will nevertheless be greater compared to the voltage 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 large area plates (for example, the entire body can be used as a sensor).

The output signal can be used to switch 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 Abuis

Rice. 4. Arrangement of components on the printed circuit board.

Rice. 5. Printed circuit board.

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

As is known, any metal surface, for example, a metal object, plate or door knob. The sensors have no mechanical elements, which in turn gives them significant reliability.

The scope of use of such devices is quite wide, including turning on a bell, light switch, control electronic devices, a group of alarm sensors, etc. When necessary, the use of a touch sensor allows for hidden placement of the switch.

Description of the touch sensor operation

The operation of the sensor circuit below is based on the use of the electromagnetic field present in houses, which is created by electrical wiring located in the walls.

Touching the sensor sensor with your hand is equivalent to connecting an antenna to the sensitive input of the amplifier. As a result, the induced network electricity enters the gate of the field-effect transistor, which plays the role of an electronic switch.

The touch touch sensor quite simple due to the use of field-effect transistor KP501A (B, C). This transistor provides current transmission up to 180 mA at a maximum source-drain voltage of up to 240V for letter A and 200V for letters B and C. To protect against static electricity there is a diode at its input.

The field-effect transistor has a high input resistance, and in order to control it, a static voltage that is greater than the threshold value is enough. For this type of field-effect transistor, the nominal threshold voltage is 1...3 V, and the maximum permissible is 20 V.

When you touch sensor E1 with your hand, the degree of induced potential on the gate is sufficient to open the transistor. In this case, at the drain VT1 there will be electrical pulses lasting 35 ms, and having a frequency electrical network 50 Hz. Most electromagnetic relays require only 3…25 ms to switch. To prevent the relay contacts from bouncing at the moment of contact, capacitor C2 is included in the circuit. Due to the accumulated charge on the capacitor, the relay will be turned on even during that half-cycle of the mains voltage when VT1 is closed. As long as there is a touch to the sensor sensor, the relay will be on.

Capacitor C1 increases the sensor's immunity to high-frequency radio interference. You can change the sensitivity of touching the sensor by changing the capacitance C1 and resistance R1. Contact group K1.1 controls external electronic devices.

By adding a trigger and a network load switching node to this circuit, you can get.