~: Liquid level Controller :~

Abstract: - Keeping the liquid up to certain level in tank is one of the widely used applications in petroleum and consumable goods producing industries. Certain amount of liquid should be always there in the tank to keep the liquid pressure as it is. To maintain this level there is a need of liquid level controller.

Its a close loop control system. It may be ON-OFF type of or continuous type system. There are two valves in tank. One is inlet and other is outlet. If one wants constant flow of liquid from tank then outlet valve must be fully open and liquid level in the tank should be above minimum specified limit. And this limit is maintained by controlling flow from inlet valve.

ON-OFF type: - in this system there are two limits. 1) max limit 2) min limit. If liquid level go beyond max limit then inlet valve is closed. Now as outlet valve is fully open the liquid level will start decreasing. As it reaches min limit and go below this inlet valve is again open. So liquid level will remain within these two limits.

Continuous type: - in this system there is only one limit. That is called set value. The liquid level in the tank is continuously compared with this set value. The opening of inlet valve is varied according to the change in the level in tank. Opening becomes wider if level falls and becomes shorter if level go beyond set value. So continuous correction is made to maintain set value of level.

Here I am giving you an example of very simple controller in which first the tank is filled up to required level using pump and then after some time it is flushed completely by reversing pump. Again when tank is empty the pump automatically starts filling tank. And this cycle continuous. One can set the required level every time or he can change it in every cycle. This is demo application in which I am using Simulated water tank from multisim software to illustrate this application.

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So first let us understand the operation of this water tank. Here the figure given below shows the arrangement of water tank with pump and other assembly.

As shown in above figure the full capacity of tank is 50 later. The set point (required level) is fixed at 35 letter. The rate at which pump fills the tank that depends upon valve setting. Flow control (which is not the part of this system) can be achieved by changing voltage applied at this terminal. There are three inputs to this system that controls pump operation

1. flow in - when it is given a positive pulse, water will start flowing inside tank
2. flow out - when it is given a positive pulse, water will start flowing outside tank
3. stop flow - when given positive pulse flow will be stopped

Three outputs form the system are

1. set level - this o/p will become high when tank is filled upto set level. It will remain high till level is maintained. As the level decreases o/p becomes low
2. tank empty - as tank is flushed completely this o/p becomes high. It immediately becomes low as tank starts filling.
3. analog o/p - it gives DC o/p 0 - 5 V corresponding to the liquid level in tank. That means 0V when tank is empty and 5V when tank is full

So actually the system is very simple. Only we have to send pulses to input signals and use the outputs for indications and control actions. So lets first see the block diagram of the system.

Block diagram of system

Let us see all the major building blocks one by one

89C51 - this controller chip is the heart of entire system. It receives the inputs from system, it generates suitable outputs to control the system, it displays the data on LCD and indicates various on going processes on LEDs

ADC 0801 - it converts analog input 0-5V from system in to digital equivalent

LCD panel - it displays the liquid level in tank as well as the set point value

buffer and latch - they are provided for protection of port pins

LED indicators - they indicate various processes going on like water flow inside-outside tank, water flow stop etc.

So all four ports 89C51 are engaged for various peripheral interfacing.

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