03-10-2012, 12:31 PM
LECTURES ON PID CONTROLLERS
[attachment=35119]
CONVENTIONAL CONTROLLERS
Today, a number of different controllers are used in industry1 and in many other
fields. In quite general way those controllers can be divided into two main groups:
· conventional controllers
· unconventional controllers
As conventional controllers we can count a controllers known for years now, such
as P, PI, PD, PID, Otto-Smith, all their different types and realizations, and other
controller types2. It is a characteristic of all conventional controllers that one has to know
a mathematical model of the process in order to design a controller. Unconventional
controllers utilize a new approaches to the controller design in which knowledge of a
mathematical model of a process generally is not required. Examples of unconventional
controller are a fuzzy controller and neuro or neuro-fuzzy controllers.
Manny industrial processes are nonlinear and thus complicate to describe
mathematically. However, it is known that a good many nonlinear processes can
satisfactory controlled using PID controllers providing that controller parameters are
tuned well. Practical experience shows that this type of control has a lot of sense since it
is simple and based on 3 basic behavior types: proportional (P), integrative (I) and
derivative (D). Instead of using a small number of complex controllers, a larger number
of simple PID controllers is used to control simpler processes in an industrial assembly in
order to automates the certain more complex process. PID controller and its different
types such as P, PI and PD controllers are today a basic building blocks in control of
various processes. In spite their simplicity, they can be used to solve even a very complex
control problems, especially when combined with different functional blocks, filters
(compensators or correction blocks), selectors etc. A continuous development of new
control algorithms insure that the time of PID controller has not past and that this basic
algorithm will have its part to play in process control in foreseeable future. It can be
expected that it will be a backbone of many complex control systems.
Basic controller types
PID controllers use a 3 basic behavior types or modes: P - proportional, I -
integrative and D - derivative. While proportional and integrative modes are also used as
single control modes, a derivative mode is rarely used on it’s own in control systems.
Combinations such as PI and PD control are very often in practical systems. It can be also
shown that PID controller is a natural generalization of a simplest possible controller -
On-off controller.
Choice of the controller type
Insofar were described proportional, integrative and derivative modes of the
controllers and a rational behind their use was explained. However, excerpt for a few tips,
an attention was not given to a question when to use different types of controllers. The
rest of this section will give some answers on that particular topic.
On-off controller
On-off controller is the simplest controller and it has some important advontages.
It is economical, simple to design and it does not require any parameter tuning. If
oscillations will hamper the operation of the system and if controller parameter tuning is
to be avoided, on-off controller is a good solution. In addition, if actuators work in only
two modes (on and off), then it is almost always only controller that can be used with
such actuators. That is a reason why on-off controllers are often used in home appliances
(refrigerators, washers etc.) and in process industry when control quality requirements are
not high (temperature control in buildings etc.). Additional advantage of on-off
controllers is that they in general do not require any maintenance.
P controller
When P controller is used, large gain is needed to improve steady state error.
Stable system do not have a problems when large gain is used. Such systems are systems
with one energy storage (1st order capacitive systems). If constant steady state error can
be accepted with such processes, than P controller can be used. Small steady state errors
can be accepted if sensor will give measured value with error or if importance of
measured value is not too great anyway. Example of such system is liquid level control in
tanks when exact approximate level of liquid suffice for the proper plant operation. Also,
in cascade control sometime it is not important if there is an error inside inner loop, so P
controller can a good solution in such cases.
Derivative mode is not required if the process itself is fast or if the control system
as whole does not have to be fast in response. Processes of 1st order react immediately on
the reference signal change, so it is not necessary to predict error (introduce D mode) or
compensate for the steady state error (introduce I mode) if it is possible to achieve
satisfactory steady state error using only P controller.
[attachment=35119]
CONVENTIONAL CONTROLLERS
Today, a number of different controllers are used in industry1 and in many other
fields. In quite general way those controllers can be divided into two main groups:
· conventional controllers
· unconventional controllers
As conventional controllers we can count a controllers known for years now, such
as P, PI, PD, PID, Otto-Smith, all their different types and realizations, and other
controller types2. It is a characteristic of all conventional controllers that one has to know
a mathematical model of the process in order to design a controller. Unconventional
controllers utilize a new approaches to the controller design in which knowledge of a
mathematical model of a process generally is not required. Examples of unconventional
controller are a fuzzy controller and neuro or neuro-fuzzy controllers.
Manny industrial processes are nonlinear and thus complicate to describe
mathematically. However, it is known that a good many nonlinear processes can
satisfactory controlled using PID controllers providing that controller parameters are
tuned well. Practical experience shows that this type of control has a lot of sense since it
is simple and based on 3 basic behavior types: proportional (P), integrative (I) and
derivative (D). Instead of using a small number of complex controllers, a larger number
of simple PID controllers is used to control simpler processes in an industrial assembly in
order to automates the certain more complex process. PID controller and its different
types such as P, PI and PD controllers are today a basic building blocks in control of
various processes. In spite their simplicity, they can be used to solve even a very complex
control problems, especially when combined with different functional blocks, filters
(compensators or correction blocks), selectors etc. A continuous development of new
control algorithms insure that the time of PID controller has not past and that this basic
algorithm will have its part to play in process control in foreseeable future. It can be
expected that it will be a backbone of many complex control systems.
Basic controller types
PID controllers use a 3 basic behavior types or modes: P - proportional, I -
integrative and D - derivative. While proportional and integrative modes are also used as
single control modes, a derivative mode is rarely used on it’s own in control systems.
Combinations such as PI and PD control are very often in practical systems. It can be also
shown that PID controller is a natural generalization of a simplest possible controller -
On-off controller.
Choice of the controller type
Insofar were described proportional, integrative and derivative modes of the
controllers and a rational behind their use was explained. However, excerpt for a few tips,
an attention was not given to a question when to use different types of controllers. The
rest of this section will give some answers on that particular topic.
On-off controller
On-off controller is the simplest controller and it has some important advontages.
It is economical, simple to design and it does not require any parameter tuning. If
oscillations will hamper the operation of the system and if controller parameter tuning is
to be avoided, on-off controller is a good solution. In addition, if actuators work in only
two modes (on and off), then it is almost always only controller that can be used with
such actuators. That is a reason why on-off controllers are often used in home appliances
(refrigerators, washers etc.) and in process industry when control quality requirements are
not high (temperature control in buildings etc.). Additional advantage of on-off
controllers is that they in general do not require any maintenance.
P controller
When P controller is used, large gain is needed to improve steady state error.
Stable system do not have a problems when large gain is used. Such systems are systems
with one energy storage (1st order capacitive systems). If constant steady state error can
be accepted with such processes, than P controller can be used. Small steady state errors
can be accepted if sensor will give measured value with error or if importance of
measured value is not too great anyway. Example of such system is liquid level control in
tanks when exact approximate level of liquid suffice for the proper plant operation. Also,
in cascade control sometime it is not important if there is an error inside inner loop, so P
controller can a good solution in such cases.
Derivative mode is not required if the process itself is fast or if the control system
as whole does not have to be fast in response. Processes of 1st order react immediately on
the reference signal change, so it is not necessary to predict error (introduce D mode) or
compensate for the steady state error (introduce I mode) if it is possible to achieve
satisfactory steady state error using only P controller.