FACULTY OF ENGINEERING
ASSUIT UNIVERSITY FACULTY OF ENGINEERING MECHATRONICS & ROBOTICS ENGINEERING
on /off level control process Prepared By: Mina Medhat 26/11/2011
Submitted to: Eng. Rehab
Table of contents: Abstract ………………………………………………………………………………….. ……………...…....... 2 Introduction……………………………………………………………………… ……………………...…………3
Q1.about on/off control…………….......... …………………………………………..……….............4
Q2 Kind ofcontroller)...... …………………………………………………………………………………..… 5
Q3 one example on /off control syste...................................... ………………………….. 7
Q4 three different teqhnique of level measurement?........................................... 8
modeling and simulation of the system ..................................................................9
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refrences.......................................................................................... ....................14
Abstract:
The measurement of liquid level enables us to locate the surface of a liquid with respect to some referenced level. Level measurement is necessary in Boiler Drum Level Measurement for the safe and efficient operation of the boiler.
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Introduction: •
•
The control of liquid levels, for example in a process tank, is an important function. An example would be a hot water tank where water is removed, perhaps for washing down, and the level needs to be restored ready for the next wash cycle. Many different types of level control systems are used in industry, covering a wide range of processes. Some processes will be concerned with media other than liquids, such as dry powders and chemical feedstock. The range of media is so wide that no single instrument is suitable for all applications. Many systems are available to serve this wide range of applications
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Q1:what are you know about on/off control?
In control theory, a bang–bang controller (on–off controller), also known as a hysteresis controller, is a feedback controller that switches abruptly between two states. These controllers may be realized in terms of any element that provides hysteresis. They are often used to control a plant that accepts a binary input, for example a furnace that is either completely on or completely off. Most common residential thermostats are bang–bang controllers. The Heaviside step function in its discrete form is an example of a bang–bang control signal. Due to the discontinuous control signal, systems that include bang–bang controllers are variable structure systems, and bang–bang controllers are thus variable structure controllers. For example, a thermostat is a simple negative-feedback control: when the temperature (the "process variable" or PV) goes below a set point (SP), the heater is switched on. Another example could be a pressure switch on an air compressor: when the pressure (PV) drops below the threshold (SP), the pump is powered. Refrigerators and vacuum pumps contain similar mechanisms
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operating in reverse, but still providing negative feedback to correct errors. Simple on–off feedback control systems like these are cheap and effective. In some cases, like the simple compressor example, they may represent a good design choice. In most applications of on–off feedback control, some consideration needs to be given to other costs, such as wear and tear of control valves and maybe other start-up costs when power is reapplied each time the PV drops. Therefore, practical on–off control systems are designed to include hysteresis, usually in the form of a deadband, a region around the setpoint value in which no control action occurs. The width of deadband may be adjustable or programmable
Q2:- Mention the kind of controller that it uses as on/off control in the industry? PLC controller is a controller that is uses as on/off control in the industry.
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Microcontroller is a controller that is uses as on/off control in
the industry. A microcontroller is a computer on a single IC, designed specifically for control applications.
Q3:- Give one example on /off control system in our life in details? Solenoid Valve Basics Most solenoid valves operate on a digital principle. They therefore possess two distinct states, which are (1) - when the coil is activated ON by an electrical current, and (2) - when the valve is resting OFF (without electricity). Valve functions are defined from the resting position.
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temperature control
On/Off Control An on-off controller is the simplest form of temperature control device. The output from the device is either on or off, with no middle state. An on-off controller will switch the output only when the temperature crosses the setpoint. For heating control, the output is on when the temperature is below the setpoint, and off above setpoint. Since the temperature crosses the setpoint to change the output state, the process temperature will be cycling continually, going from below setpoint to above, and back below. In cases where this cycling occurs rapidly, and to prevent damage to contactors and valves, an on-off differential, or “hysteresis,” is added to the controller operations. This differential requires that the temperature exceed setpoint by a certain amount before the output will turn off or on again. On-off differential prevents the output from “chattering” or making fast, continual switches if the cycling above and below the setpoint occurs very rapidly. On-off control is usually used where a precise control is not necessary, in systems which cannot handle having the energy turned on and off frequently, where the mass of the system is so great that temperatures change extremely slowly, or for a temperature alarm. One special type of on-off control used for alarm is a limit controller. This controller uses a latching relay, which must be manually reset, and is used to shut down a process when a certain temperature is reached.
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Q4:- Describe in details three different teqhnique of level measurement?
1)Photoelectric Sensor
The sensor uses invisible infrared light beam to determine the distance to the top level. The sensor can
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be very reliably used to maintain sufficient material in the hopper by controlling the inlet valve. This sensor is available in M36 brass chrome plated housing and works on 12VDC or 24VDC supply. Sensing range is between 100mm and 600mm. The switch is designed very user-friendly by providing a ‘TEACH-IN’ button and red LED for selection of Lower level and Upper level. The levels are stored in non-volatile memory, so whenever power fails or switched off, the level settings remain intact. A green LED is provided for output ON/OFF indication. NPN or PNP transistor output is available to drive inlet valve and is protected for short circuit, overload and Inductive load. The specialty of this switch is, it’s functioning is independent of object colour. If you set levels on white object and use for red, black, blue or any colour object you get the same level control. Except for water-clear material it works fine for all other colour material. This switch is very useful for Pharmaceutical, Foods, Packaging and Plastic industries.
2)Conductivity probes Consider an open tank with some water in it. A probe (metal rod) is suspended in the tank (see Figure 3.16.3). If an electrical voltage is applied and the circuit includes an ammeter, the latter will show that: •
•
With the probe immersed in the water, current will flow through the circuit. If the probe is lifted out of the water, current will not flow through the circuit
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3)Electro Mechanical Float Switch
Basic Principle: The horizontal float level switch consists of stainless steel Float hinged in such a way that it can vertically
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have angular movement. As the liquid level varies in the storage tank, the float moves angularly, up or down, transferring the mechanical movement to the outside mechanism thereby completing the external electrical circuit. A float switch is an electro-mechanical switch which allows for an electrical switch to be opened or closed depending on the fluid level in a container. The float switch allows for automatic operation of devices depending on the level of fluid, such as the operation of pumps, or the opening or closing of valves. Float switches range from small to large and may be as simple as a mercury switch inside a hinged float or as complex as a series of optical or conductance sensors producing discrete outputs as the liquid reaches many different levels within the tank. Perhaps the most common type of float switch is simply a float raising a rod that actuates a micro switch.Float switches of numerous configurations have been used for various marine and industrial applications. Most float switches contain an electrical switch imbedded within the body of the float switch device. The electrical switch is actuated upon physical movement of the portion of the float switch device containing the electrical switch or upon physical movement of another portion of the float switch device. Such switches typically include a base member having mounted thereon a buoyant arm or float member.
modeling and simulation of the system (tank model)
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qo
=
h
R
( t )
q q
( t )
( t )
q q
q
( t )
d
=
dt
( t )
h
−
−
( t )
h
ss
=
R
q
=
ss
q
=
=
1
( t )
d h( t ) dt
(h( t ) − h ss ) + A
R
1
H
( A h( t ) )
0
R
ss
(h( t ) − h ss ) + A
+
R
( t )
A
=
R
−
( t )
Q
qo
−
−
ss
( t )
A
d (h( t ) − h ss ) dt
d ( h( t ) − h ss ) dt
d H ( t ) dt
where :
Q
= ( t )
H
( t )
=
H Q
∴
q
( s )
( s )
− ( t )
h
( t )
=
q
ss
−
h
ss
R s +1 τ
where : τ = A * R
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valve model
X I
m ( s )
=
( s )
K
m
τ m s
+
1
−τ om s
X = K e V τ s + 1 Q = K * X m ( s )
m
i ( s )
m
m ( s )
v
( s )
Level Sensor model:
V
s ( s )
K * H
=
s
(s)
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The system transfer function
REFRENCE http://www.spiraxsarco.com/resources/steamengineering-tutorials/control-applications/leveland-flow-control-applications.asp
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http://en.wikipedia.org/wiki/Control_system
http://www.google.com.eg/search? q=Solenoid+valve+operational+principle&btnG= %D8%A8%D8%AD %D8%AB&hl=ar&gbv=2&gs_sm=c&gs_upl=1185l585 2l0l7366l12l12l0l5l5l0l407l2145l0.1.2.3.1l7l0&spell=1 &sa=X&oq=Solenoid+valve+operational+principle&a q=f&aqi=&aql=
http://parts.digikey.com/1/parts-datasheet/sensorcoupling-probe-2-npt-3b-gems-sensors-datasheet
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