The design of control systems is a specific example of engineering design.Again, the
goal of control engineering design is to obtain the configuration, specifications, and
identification of the key parameters of a proposed system to meet an actual need.
The first step in the design process consists of establishing the system goals. For
example, we may state that our goal is to control the velocity of a motor accurately.
The second step is to identify the variables that we desire to control (for example,
the velocity of the motor).The third step is to write the specifications in terms of the
accuracy we must attain.This required accuracy of control will then lead to the identification of a sensor to measure the controlled variable.
As designers, we proceed to the first attempt to configure a system that will result in the desired control performance. This system configuration will normally
consist of a sensor, the process under control, an actuator, and a controller, as shown
in Figure 1.9. The next step consists of identifying a candidate for the actuator. This
will, of course, depend on the process, but the actuation chosen must be capable of
effectively adjusting the performance of the process. For example, if we wish to control the speed of a rotating flywheel, we will select a motor as the actuator. The sensor, in this case, will need to be capable of accurately measuring the speed. We then
obtain a model for each of these elements.
The next step is the selection of a controller, which often consists of a summing
amplifier that will compare the desired response and the actual response and then
forward this error-measurement signal to an amplifier.
The final step in the design process is the adjustment of the parameters of the
system in order to achieve the desired performance. If we can achieve the desired
performance by adjusting the parameters, we will finalize the design and proceed to
document the results. If not, we will need to establish an improved system configuration and perhaps select an enhanced actuator and sensor. Then we will repeat the
design steps until we are able to meet the specifications, or until we decide the specifications are too demanding and should be relaxed. The control system design
process is summarized in Figure 1.22.
The performance specifications will describe how the closed-loop system
should perform and will include (1) good regulation against disturbances, (2) desirable responses to commands, (3) realistic actuator signals, (4) low sensitivities, and
(5) robustness.
The design process has been dramatically affected by the advent of powerful
and inexpensive computers and effective control design and analysis software. For
example, the Boeing 777, which incorporates the most advanced flight avionics of
any U.S. commercial aircraft, was almost entirely computer-designed [62, 63]. Verification of final designs in high-fidelity computer simulations is essential. In many applications, the certification of the control system in realistic simulations represents a
significant cost in terms of money and time. The Boeing 777 test pilots flew about
2400 flights in high-fidelity simulations before the first aircraft was even built.
Another notable example of computer-aided design and analysis is the McDonnell Douglas Delta Clipper experimental vehicle DC-X, which was designed, built,
and flown in 24 months. Computer-aided design tools and automated code-generation
contributed to an estimated 80 percent cost savings and 30 percent time savings [64].
24 Chapter 1 Introduction to Control Systems
If the performance does not meet the specifications,
then iterate the configuration and the actuator.
If the performance meets the
specifications, then finalize the design.
1. Establish control goals
7. Optimize the parameters and
analyze the performance
5. Obtain a model of the process, the
actuator, and the sensor
4. Establish the system configuration
and identify the actuator
3. Write the specifications
for the variables
2. Identify the variables to control
6. Describe a controller and select
key parameters to be adjusted
FIGURE 1.22
The control system
design process.
In summary, the controller design problem is as follows: Given a model of the
system to be controlled (including its sensors and actuators) and a set of design
goals, find a suitable controller, or determine that none exists. As with most of engineering design, the design of a feedback control system is an iterative and nonlinear
process. A successful designer must consider the underlying physics of the plant
under control, the control design strategy, the controller design architecture (that is,
what type of controller will be employed), and effective controller tuning strategies.
In addition, once the design is completed, the controller is often implemented in
hardware, hence issues of interfacing with hardware can surface. When taken together, these different phases of control system design make the task of designing
and implementing a control system quite challenging