INTRODUCTION
In the closed-loop control system, sometimes the shape of the output signal energy plant not the same as the shape of the input signal so that energy can not be compared, therefore, needed to change sensors or transducers form output signal energy into a form of energy similar to the reference input signal. This section will explain about sensors and transducers including the functions in the transducer arrangement and classification systems and sensors based on signal energy forms measurable.
Sensor And transducer
Transducer is a physical device that is used to transform a signal from one form of energy into another form of energy or of the magnitude a physical one physical quantity to another. In general, the output transducer is an electrical signal that can be current, voltage, resistance, capacitance or frequency. Basically the sensor is also a transducer. Distinguishing between sensor with trnduser is application and use.
Transducer is one component in the system settings in particular, the closed-loop arrangement. Location of transducer or sensor in the system settings can be seen in the block diagram of a closed loop system the following settings:
In the closed-loop control system, sensor or transducer transform energy output signal from the plant into a form of energy similar to the reference input signal (setpoin).
Transducer or sensor response to the input signal can be either a first-order system or second-order. In general, transducer or sensor is a first-order system, the relationship between input and output transducers in the domain of s can be expressed in the transfer function as follows:
Where C * (s) and C (s) respectively are the input and output transducers, K is the gain and time constant
t is the transducer or sensor. If the transducer time constant faster than the time constant of the plant, in this case 
the transducer time constant can be ignored so that the transfer function of the transducer is the proportional gain 
Example : tachogenerator ( velocity transducer )
In general, the transducer or temperature sensor has a time constant very slow . Performance of a transducer or sensor that is expressed in 2 specifications technical specifications and dynamic specifications . Technical specifications stating how well the correlation between input and output transducer or sensor . Technical specifications consist of :
1 . accuracy
Accuracy is used to determine the maximum error expected from a transducer or sensor ( in % error )
2 . sensitivity
Sensitivity indicates the ability of the transducer or sensor can provide output to small input changes .
3 . resolution
Resolution stating that menyebabkab perubahab smallest input changes output transducer or sensor
4 . hysteresis
Hysteresis shows different output values of the variable value measurement input from low to high and from high to low .
5 . repeatability
Repeatability expressed how well transducers or sensors provide output the same for a given input repeatedly . Dynamic specification states how fast the output changes occur to changes in input . Dynamic specification consists of :
1.Rise time
2.Time constant
3.Dead time
4.Respon frequency
5.Parameter such second-order damping ratio , natural frequency , settling time , and maximum overshoot . Based on the form of energy or the magnitude of the input signal , the transducer or sensor in the system settings can generally be classified into :
1 . Mechanical transducer or sensor . Including displacement sensors , location or position , level sensors , and motion sensors ( speed ) .. Mechanical transducers are widely used in the system servomekanik
2 . Transducer or temperature sensor Transducer or temperature sensor is widely used in the process control system .
Examples of displacement sensors , location or position :
1 . linear potentiometer
Linear potentiometer convert linear motion into a variable resistance can be converted directly into a voltage signal and / or currents .
The magnitude of the linear potentiometer output is :
2. rotary potentiometer
Rotary potentiometer change the angular motion into a variable resistance can be converted directly into a voltage signal and / or currents.
The magnitude of the linear potentiometer output is:
The use of rotary potentiometer on the motor position control system of AC 2 phase can you look like the following block diagram:
3. capacitive sensors
Capacitive sensors are used to change the capacitance changes into current signal or voltage.
The basic operation of a capacitive sensor can be seen from the equation for a capacitor parallel plates as follows:
dielekrik where K is a constant, 0 is the permittivity e0 = 8.85 pF / m, A is the area cross plate, and d is the inter-plate distance.
4. inductive sensor
If a nucleus (core) permeable put in an inductor as shown on the image, then the associated inductance will rise. Each new position of the nuclei produced different inductance. In this condition, a combination of the inductor and movable The core can be used as a displacement sensor.
Examples of level sensor:
1. float
One of the most common techniques for measuring level, especially for liquids, is a buoy that can go up and down according to changes in liquid level. This buoy is connected by linkage (liaison) to the measurement system secondary displacement as a potentiometer
2. ultrasonic
The use of reflection (reflection) to measure the level of ultrasonic preferred because is a technique that "noninvasive" (no damage), ie with no put anything in the material.
There are 2 techniques to measure levels using ultrasonic reflection, namely:
1. External technique: for solid / liquid material
2. Internal technique: for liquid material
Speed sensor example: 1. tachometer
Tachometer is a transducer that is used to change the velocity signal into a voltage signal. The use of a tachometer on a DC motor speed regulation system can be seen such as the following block diagram:
Temperature sensor example:
1. bimetallic
Bimetal sensor that converts a temperature value changes to energy mechanics. Bimetal sensor consists of two different metals with a coefficient of thermal expansion (expansion temperature) that is attached to a different one (bonded together). coefficient of expansion thermal unequal on the two pieces of metal that will cause the bimetal curved.
If the temperature is decreased below the temperature setpoin t0, bimetal pieces will curve to the metal which has a lower coefficient of thermal expansion (bottom). Conversely, if the temperature rises above the temperature of the bimetallic pieces setpoin t0 will curve to the metal that has a higher coefficient of thermal expansion (above).
2. Resistance Temperature Detector (RTD) Resistance temperature detector (RTD) is a transducer that converts the change temperature value to the value of resistance in a metal.
The relationship between temperature and resistance is expressed by the following equation:
If the temperature change is small, can be approximated by the following linear equation:
It appears that the greater the RTD temperature the greater its resistance so called as Positive Temperature Coefficient (PTC)
3. thermistor
Thermistor is a transducer that converts a temperature value changes resistance value in a semiconductor.
The relationship between temperature and resistance is expressed by the following equation:
It appears that the thermistor temperature the greater the smaller the value resistance so called Negative Temperature Coefficient (NTC)
4. thermocouple
Thermocouple is a transducer that converts a temperature value changes emf (electromotive force).
Here's a thermocouple types and range of materials along with the area of work:
SUMMARY
1.Pada regulatory system, sensor or transducer signal energy remodel output of the plant to be similar to that of the reference input signal energy. Widely used in mechanical.
2.Sensor servomekanik system and temperature sensor widely used in the process control system.
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