Sensors and Detectors Tutorial


To design a circuit for measurement and control of a physical quantity, we must first know the sensors are available for this purpose. Typically, each physical size there is more than one sensor. It should thus be an appropriate choice of the sensor, depending on the requirements of the application, and then to proceed to the design of the circuit.

In this tutorial we will look at some key applications, such as bending and pressure measurement, control of liquid level and control level. We will first look at the sensors can be used in any application, and then we will proceed to design a minimum, measurement and control circuit of the corresponding quantity. Then we describe the main causes of electrical noise in circuit automation and will look at ways of reducing it. Finally we examine the importance of grounding and shielding the circuits of automation.





THE construction of automation we have to face a number of problems common to many applications. The first problem is how to connect the sensor to the rest of the circuit. Then the input signal must be enhanced and eventually usually compared with another mark which is the reference signal (set point). In this section we will see some simple solutions to these problems. You also get some items needed to manufacture a automation.


Circuit adjustment sensor


The first problem to be addressed is the connection of the transducer measuring circuit. When the sensor is passive (i.e. it needs external power source), a common way is to bridge connection.



Figure 3.2.1 Connection of sensors in bridge



A connection bridge is that of Figure 3.2.1. The sensor may be one or more of the resistors of Fig. Thus, we have:

{C}ü  Connection Bridge with a sensor. In this case, the sensor is placed in the position R4. Assuming that the (variable) resistance of the sensor is of the form R (1 + d) and set R1 = R2 = R3 = R, then VAB = VCC d / 4, that is independent of R. Therefore, change in temperature does not affect the measurement.

{C}ü  Connection Bridge with two sensors. The sensors are placed in positions R2 and R4 the form R (1 + d). So, if we set R1 = R3 = R, show that d VAB = VCC / 2, ie twice the previous case. This connection is used if it is to monitor the change in one size (e.g. force measurement to a bar, two type sensors strain gauge, placed opposite the bar).

{C}ü  Connection Bridge with four sensors. In this connection, the roles of all sensors are resistors. In positions of R2 and R4 type R (1 + d), while the other two are of the form R (1-d). It turns out that VAB = VCC d


Amplifier circuit of the input signal


The signal generated by the previous circuits should be strengthened. To drive the signal to the amplifier, usually use an operational amplifier in differential amplifier wiring. Figure 3.2.2 shows one such amplifier




Figure 3.2.2 Differential Amplifier


It turns out that Vout = {C}{C}

This circuit gives rise to several drawbacks (problems with compensation, linearization) and so it is usually used more complex circuits.


comparison circuit


Almost all automation systems compare the value of a physical quantity, which we can control, a set point of this magnitude. This comparison can be made by analog either digital manner.

The analog comparator is an analog circuit with two inputs and one output. In an input have the desired value as a preset value (preset value), usually via a potentiometer. The second input is driven by the output of the sensor, usually through an amplifying circuit and adaptation. Depending on the design of a comparator, its output is minimum (min) or maximum (max), when the value of the sensor signal is greater than the desired value. That is:


{C}{C}{C}{C} {C}{C}


The desired value can be either the w, either x. A theoretical circuit that implements this function is that of Figure 3




Figure 3.2.3. Schematic diagram analog comparator



The digital or logical comparator also has two inputs and one output. And here we have the signal from the sensor and a preset value.


Figure 3.2.4. Block diagram of digital comparator



However, the signals are led to the comparator circuit, once converted to digital (with converters Analog to Digital A / D), and the circuit of the comparator is digital and this *.

Lately we have released digital comparator integrated circuits. Thus the integrated 74LS85 compare two words of 4bits and activates one of three outputs (for <,>, =). Figure 3.2.4 we can see the block diagram of a digital comparator.



Materials automation


Each circuit consists of automation various electrical, electronic, mechanical and hydraulic components, properly connected. The main elements of the auxiliary circuit are:


{C}Ø  Relay or relays (H / N). This is a mechanism which, when stimulated, restore or close a connection. Figure 3.2.5 see schematically the construction of the relay.




Figure 3.2.5. Schematic diagram relay


At rest the contact is closed a relay, so there is contact between points 3 and 5. When the input relay (points 1 and 2) applied voltage, the inductor current flows through and pulls the blade 6 (arming relay). Then the arm 7 is rotated, as shown by arrow, with the result that a contact to open and close the contact b. Now there is contact between points 3 and 4 of the output relay. That is, depending on the value of the input current, the arm 7 is switched from one contact to another, providing the ability to put on or to break a circuit, causing a change in the state of the power system. Note that a relay usually has more than one changeover contacts.


{C}Ø  Comms. Are components, which are given by the circuit manually some commands (such as start or stop). These are divided into buttons and switches. The button has a contact, which at rest is either open (normal open-NO) and pressure closes instantaneously, or closed (normal close-NC) and pressure opens instantaneously. Figure 3.2.6 shows the function of the buttons.



NC <--------> NO


Figure 3.2.6. Schematic diagrams buttons


The switches have one or more contacts. The difference is that the buttons change their situation is permanent and needs a new push to return to their original state (no return spring).


{C}Ø {C}Displays measurements: These help to know at all times the prices we control sizes. Such bodies are the voltmeter, the ammeter, the frequency meter, the meter, pressure gauge (manometer) etc.




The pressure is an important parameter in many automation circuits, especially in industrial applications, and the measurement value is used in the measurement of fluid flow velocity and calculating the level. A major problem is that it cannot be measured directly. Generally, a mechanical circuit can provide information only on difference pressure between two points. So, to have absolute measurement, one must assume reference point (viewpoint), In which the measurement is made.

The pressure sensors are usually available in three formats, allowing for the measurement absolute (absolute), differential (differential) or relative (gauge) pressure.

Sensing differential pressure measure the pressure difference between two points (source pressure), the pressure of a point is considered as reference. If the reference pressure is that of the environment (atmospheric pressure), then the measurement is in relative pressure. If the reference pressure is the pressure of vacuum is (by definition) equal to zero, measuring the absolute pressure.



Pressure Sensors

In all the transducers (transducers) pressure, energy must be converted by the system to be measured in another form.

There are two categories of pressure transducers. In the first category the pressure measured directly converted into mechanical motion and thereto belonging manometers (manometers) and equalizers pressure (pressure-balances) whose operation is based on the equation

Pressure = force / area unit

In the second category, the pressure is measured indirectly by converting it into an electrical signal. The instruments of this group is based on the conversion of the pressure into an electrical signal suitable for processing, as the fluid to be studied pushing a resilient surface (e.g. septum) that causes the change of resistance or capacity of an element which in turn causes a change in voltage or current of an element. Figure 3.3.1 shows the principle of operation of such a converter

Figure 3.3.1 Principle of pressure transducer



The inverters commonly used are those that give electric signal. The main types used are based on the change of resistance, inductance or capacity. It is more accurate than those of the first category and give directly an electrical signal (e.g. electricity intensive 4-20mA). Table 3.3.1 shows a comparison of these three types.

Except converters mentioned, there are several others, leading to the piezoelectric. These are based on the phenomenon that some materials exhibit, show potential difference across, when exerted on the pressure.



TABLE 3.3.1