Current to Pressure Transducer
Current to Pressure Transducer Working:- A transducer is a device which is used to convert energy in one form to some other form. When it is used in control systems, it is used to convert signal in one form to signal in some other form. Current to pressure (I/P) transducers convert analogue type signal to a pneumatic type signal. Its function is to translate analogue type output from the controller to proportional pneumatic values in order to control an actuator in the system. The actuator is the final control element.
Usually the current input signal to the current to pressure transducer is in the range of 4-20 mA and the proportional pneumatic output values are in the range of 3-15 psi.
Current to pressure transducers works on the principle of electromagnetic force balance. There is a coil in the system. When the current signal flows from the controller to the coil, a force is produced between the coil and the flapper.
The main elements of the current to pressure transducers are coil, flapper and nozzle. The flapper is pivoted in one end and free at other end so that it has some degree of flexibility to move up and down, also, a magnetic material is attached to the free end of the flapper. The coil is present in the system to act as an electromagnet. When current is input in the system from the controller then when it passes through the coil, a magnetic field is produced. The electromagnet tends to attract the magnetic material attached to the free end of the flapper. It causes the flapper to move downwards towards the coil. Stronger the current flowing through the coil, stronger will be attraction between the coil and the magnetic material.
A nozzle is present near the flapper. There is some gap between the nozzle and the flapper. Air flows through the nozzle. A standard supply of about 20 psi is maintained as supply pressure. If the nozzle is covered and no air is allowed to pass through it then all the 20 psi of pressure will output from the opposite end of the nozzle. If the nozzle is not completely covered then the pressure output from the opposite end of the nozzle will be less than 20 psi.
The size of the gap between the nozzle and the flapper depends on how much current is passing through the coil. If strong current is passing through the coil then there will be strong attraction between the coil and the magnetic material thus flapper will be pulled downwards and when it moves downwards the gap between the nozzle and the flapper gets reduced. Less gap means less air will pass through the nozzle end and thus most of the air of the supply pressure will move out from the end opposite to that of nozzle. If weak current passes through the coil, the gap between the nozzle and the flapper increases and relatively more air can pass through the nozzle end.
The system is setup in such a way that if 4 mA current passes through the coil then the pressure at the end opposite to that of nozzle is 3 psi. If 20 mA current passes through the coil then the pressure at the end opposite to that of nozzle is 15 psi. They follow a proportional linear relationship thus all the intermediate values of pressure is equal to 0.75 times the value of current. Thus if current value is 6 mA then the pressure value will be 0.75*6=4.5 psi.
Why 4-20 mA is dominant industry standard?
The 4-20 mA current loops are very good for transmitting signal at large distances. They use less wiring than similar systems and are quite simple to connect, configure and troubleshoot for problems. Another important reason is that low current means safety in operation. It is especially useful near areas which have vapours or dust in the surroundings because low power consumption implies that a dangerous situation will not be reached where the vapours or dust particles may combust.