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PROCESS CONTROLLER

for temperature, flow, pressure etc

How do Temperature Controllers work?

• A temperature controller gets input from temperature sensor such as a thermocouple or RTD.

• It compares the actual temperature to the desired control temperature, or set point,

• and provides an output in the form of transistor output or relay

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What Are the Different Types of Controllers

There are three basic types of controllers:

• on-off, • proportional • PID.

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on-off controller is the simplest form of temperature control device.

• The output from it is either ON or OFF, with no middle state. • It will switch the output only when the temperature crosses the

set point. • For heating control, the output is ON when the temperature is

below the set point, and OFF above set point. • Since the temperature crosses the set point to change the

output state, the process temperature will be cycling continually, going from below set point to above, and back below.

• On-off control is usually used where a precise control is not necessary, where the mass of the system is so great that temperatures change extremely slowly, or for a temperature alarm.

Proportional Control• Proportional controls are designed to eliminate the cycling associated with on-

off control. • A proportional controller decreases the average power supplied to the heater as

the temperature approaches set point. • This has the effect of slowing down the heater so that it will not overshoot the set

point, but will approach the set point slowly and maintain a stable temperature. • This proportioning action can be accomplished by turning the output ON and

OFF for short time intervals. • This “time proportioning” varies the ratio of “on” time to “off” time to control the

temperature. • The proportioning action occurs within a “proportional band” around the set

point temperature. Outside this band, the controller functions as an on-off unit, with the output either fully on (below the band) or fully off (above the band). However, within the band, the output is turned on and off in the ratio of the measurement difference from the set point.

• At the set point (the midpoint of the proportional band), the output on:-off ratio is 1:1; that is, the on-time and off-time are equal. if the temperature is further from the set point, the on- and off-times vary in proportion to the temperature difference.

• If the temperature is below set point, the output will be ON longer; if the temperature is too high, the output will be OFF longer.

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PID Control• PID controller provides proportional action with l with two

additional adjustments, integral and derivative, which helps the unit automatically compensate for changes in the system.

• Integral and derivative, are expressed in time-based units; they are also referred as RESET and RATE.

• The proportional, integral and derivative terms must be individually adjusted or “tuned” to a particular system using trial and error.

• It provides the most accurate and stable control of the three controller types, and is best used in systems which have a relatively small mass and those which react quickly to changes in the energy added to the process.

• It is recommended in systems where the load changes often and the controller is expected to compensate automatically the amount of energy available, or the mass to be controlled. PROLIFIC

PID CONTROLLER PID controllers are process controllers with the

following characteristics:

• Continuous process control Analog input (also known as "measurement" or "Process

Variable" or "PV") • Analog output• (referred to simply as "output") • Set point (SP) • Proportional (P), Integral (I), and / or Derivative

(D) constants

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PID CONTROLLER• Once the PID controller has the

process variable equal to the set point, a good PID controller will not vary the output.

• It is desired to maintain the output very steady (not changing). 

• If the valve (motor, or other control element) are constantly changing, instead of maintaining a constant value, this could case more wear on the control element. 

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PID CONTROLLER

• Examples of "continuous process control" are temperature, pressure, flow, and level control.

• PID controller functionality is a common feature of programmable logic controllers (PLC). Software PID loops are the most stable, because they do not wear out as compared to mechanical control systems.

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• Proportional Band is referred to as Gain

• Integral Band is referred to as Reset

• Derivative Band is referred to as Rate

PID CONTROLLER

• The analog output is often simply referred to as "output“ and is given as 0 to 100%.

(In this heating example, it would mean is the valve totally closed,(0 %) or totally open (100 %). 

• The set point (SP) is simply -- what process value do you want. 

• Set point - the desired value of the controlled variable X

• Error - control error. This is the difference between the set point and the measured real controlled value.

• Y - the controller output

• V - delayed controller output ( the Delay and Process blocks form the model of the controlled process)

• X - controlled value. This is the output of the controlled process, for example temperature, pressure, motor velocity, flow etc.

• Xm - the measurement result. The measurement instrument may have the gain different from zero and a first order inertia, given by its time constant.  

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In the diagram the valve could be

• controlling the gas going to a heater,• the chilling of a cooler, • the pressure in a pipe, • the flow through a pipe, • the level in a tank, • or any other process control system.   

 

PID CONTROLLER

• So there are these two contradictory goals. 

• Fast response (fast change in output) when there is a "process upset",

• but slow response (steady output) when the PV is close to the set point

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PID CONTROLLER• When there is a "process upset",

meaning, when the process variable OR the set point quickly changes  -- the PID controller has to quickly change the output to get the process variable back equal to the set point.  For example in a walk-in cooler with a PID controller someone opens the door and walks in, the temperature (process variable) could rise very quickly.  Therefore the PID controller has to increase the cooling (output) to compensate for this rise in temperature. 

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PID CONTROLLER

1.What the PID controller is looking at is the difference (or "error") between the PV and the SP. 

2. It looks at the absolute error and the rate of change of error. 

• Absolute error means -- is there a big difference in the PV and SP or a little difference? 

• Rate of change of error means -- is the difference between the PV or SP getting smaller or larger as time goes on. 

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PID CONTROLLER- Explanation

• Note that the output often goes past (over shoots) the steady-state output to get the process back to the set point. 

• For example, a cooler may normally have it's cooling valve open 34% to maintain zero degrees (after the cooler has been closed up and the temperature settled down). 

• If someone opens the cooler, walks in, walks around to find something, then walks back out, and then closes the cooler door -- the PID controller is freaking out because the temperature may have raised 20 degrees! 

• So it may crank the cooling valve open to 50, 75, or even 100 percent -- to hurry up and cool the cooler back down -- before slowly closing the cooling valve back down to 34 percent. 

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PID CONTROLLER GAS HEATER EXAMPLE

• The PID controller would receive as input the actual temperature and control a valve that regulates the flow of gas to the heater. 

• The PID controller automatically finds the correct (constant) flow of gas to the heater that keeps the temperature steady at the set point.  Instead of the temperature bouncing back and forth between two points, the temperature is held steady. 

• If the set point is lowered, then the PID controller automatically reduces the amount of gas flowing to the heater. 

• If the set point is raised, then the PID controller automatically increases the amount of gas flowing to the heater. 

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PID CONTROLLER Accuracy Explanation

• The analog input (measurement) is called the "process variable" or "PV".  You want the PV to be a highly accurate indication of the process parameter you are trying to control. 

For example, if you want to maintain a temperature of + or - one degree then we typically strive for at least ten times that or one-tenth of a degree.  If the analog input is a 12 bit analog input and the temperature range for the sensor is 0 to 400 degrees then our "theoretical" accuracy is calculated to be 400 degrees divided by 4,096 (12 bits) = 0.09765625 degrees.  We say "theoretical" because it would assume there was no noise and error in our temperature sensor, wiring, and analog converter -- even with the usual amount of noise and other problems -- one degree of accuracy should easily be attainable. 

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