POTENTIOMETER & LINEAR VARIABLE DIFFERENTIAL TRANSFORMER Engr. Raymond J. Pidor, PECE TRANSDUCER a device that converts energy from one form to another common examples include microphones, loudspeakers, position, light intensity, speed, acceleration, temperature and pressure sensor INPUT TRANSDUCERS All industrial control depends on the ability to accurately and swiftly measure the value of controlled variable. It has been found that the best way to measure the value of a controlled variable is to convert it into an electrical signal of some sort and to detect it with the electrical measuring device. This approach is superior to converting the value of the controlled variable into a mechanical signal because electrical signals have certain advantages over mechanical signals: electrical signals can be transmitted from place to place much more easily than mechanical signals electrical signals are easier to amplify and filter than the mechanical signals electrical signals are easy to manipulate to find out such things as the rate of change of variable. POTENTIOMETER A potentiometer (also known as a 'pot') is a modified resistor. It can be used to allow a change in the resistance in a circuit or as a variable voltage divider (in the case of a volume control). It is the most common electrical transducer where it can be used alone or attached to a mechanical sensor to convert a mechanical motion into an electrical variation. A potentiometer generally has 3 terminals. 2 of the terminals are connected to the opposite ends of a resistive element. The 3rd terminal (usually, is physically in-between the other 2 terminals) is called the wiper. The wiper is a contact (actually, generally many very small contacts) that slides along the resistive element. SCHEMATIC SYMBOL POTENTIOMETER LINEARITY The term linear means that a given mechanical movement of the wiper produces a given change in resistance no matter where the wiper happens to be on the element. It also means that the resistance is evenly distributed along the length of the element. POTENTIOMETER RESOLUTION Resolution in a pot means it is the smallest possible change is resistance as the wiper moves along the element. A POTENTIOMETER AS A VARIABLE VOLTAGE DIVIDER: A pot connected as it is in the following diagram will act to divide the voltage like the 2 individual resistors. In the middle of travel, the resistance from the sliding terminal to either of the other terminals is half of the total resistance. The output is simply the voltage at the point where the wiper contacts the resistive element. RESISTANCE TAPER The potentiometer had a 'linear' taper. These are general-purpose potentiometers and may be used for controlling DC voltage or to control the levels of the individual bands on an equalizer. For volume controls you need to use a potentiometer with a 'log' (short for logarithmic) taper. This is because the human auditory system works logarithmically. If we'd use a linear taper pot for a volume control, the low end of the volume control's range would be 'touchy'. ROTARY POTENTIOMETERS: A single turn pot will only travel through about 270º of a complete revolution. There are other potentiometers that have some sort of mechanism to increase the number of times the shaft must be turned to travel through its entire range of motion. This is generally done with gears or bearings. The multi-turn potentiometers are used to make it easier to precisely set the output level. WHEATSTONE BRIDGE CIRCUIT LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT) A common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. A passive inductive transducer that require an external source of power. It consists basically of a primary winding and two secondly windings, wound over a hollow tube and positioned so that the primary is between two of its secondary windings. LVDT An iron core slides within the tube and therefore affects the magnetic coupling between the primary and two secondary windings. When the core is in the centre , the voltage induced in the two secondary windings is equal. When the core is moved in one direction of centre, the voltage induced in one winding is increased and that in the other is decreased. Movement in the opposite direction reverse this effects. the winding is connected ‘series opposing’ -that is the polarities of V1 and V2 oppose each other as we trace through the circuit from terminal A to B. Consequently, when the core is in the center so that V1=V2, there is no voltage output, Vo = 0V. ASSIGNMENT In a one whole sheet of newsprint: What types of industries and applications use LVDTs?