Page 59 - Mechatronics with Experiments

CHAPTER 2

CLOSED LOOP CONTROL

THIS CHAPTER contains the fundamental material on closed loop control systems.
Before one uses feedback, that is closed loop control, one should explore the option of open
loop control. We will address the following questions:

What are the advantages and disadvantages of closed loop control versus open loop

control?
Why should we use feedback control instead of open loop control?

In what cases may open loop control be better than closed loop control?

A control system is designed to make a system do what we want it to do. Therefore,
a control system designer needs to know the desired behavior or performance expected
from the system. The performance specifications of a control system must cover certain
fundamental characteristics, such as stability, quality of response, and robustness. Despite
the great variety and richness of control theory, more than 90% of the feedback controllers
in practice are of the proportional-integral-derivative (PID) type. Due to its wide usage in
practice, PID control is considered a fundamental controller type. PID control is discussed
in the last section of this chapter.
The control decisions can be made either by an analog control circuit, in which case the
controller is calledan analog controller, or by a digital computer, in which case the controller
is called a digital controller. In analog control, the control decision rules are designed
into the analog circuit hardware. In digital control, the control decision rules are coded
in software. This software code implementing the control decisions is called the digital
control algorithm.
The main advantages of digital control over analog control are as follows:

1. Increased flexibility: changing the control algorithm is a matter of changing the
software. Making software changes in digital control is much easier than changing
analog circuit design in analog control.
2. Increased level of decision making capability: implementing nonlinear control func-
tions, logical decision functions, conditional actions to be taken, and learning from
experience can all be programmed in software. Building analog controllers with these
capabilities would be a prohibitive task, if not impossible.
It is important to identify the place of the control of dynamic systems in the big picture
of control systems. Real-world control systems involve many discrete event controls using
sequencing and logic decisions. Discrete event control refers to the control logic based
on sensors and uses actuators which have only two level states, ON/OFF, (i.e., pneumatic
cylinders controlled by an ON/OFF solenoid, relays). The sequence controllers use sensors
and actuators which have only an ON/OFF state, and the control algorithm is a logic

Mechatronics with Experiments, Second Edition. Sabri Cetinkunt.
© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.
Companion Website: www.wiley.com/go/cetinkunt/mechatronics

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