The Role of Controls in Mechatronics“Mechatronic design deals with the integrated and optimal design of a mechanical system and its
embedded control system.” This definition implies that the mechanical system is enhanced with electronic
components in order to achieve a better performance, a more flexible system, or just reduce the cost of
the system. In many cases the electronics are present in the form of a computer-based embedded (control)
system. This does not imply that every controlled mechanical system is a mechatronic system because
in many cases the control is just an add-on to the mechanical system in a sequential design procedure.
A real mechatronics approach requires that an optimal choice be made with respect to the realization of
the design specifications in the different domains. In control engineering the design of an optimal control
system is well understood and for linear systems standard methods exist. The optimization problem is
formulated as: given a process to be controlled, and given a performance index (cost function), find
optimal controller parameters such that the cost function is minimized. With a state feedback controller
and a quadratic cost function, solutions for the optimal controller gains can be found with standard
controller design software, such as Matlab
1
(Figure 12.1).
Mechatronic design on the contrary requires that not only the controller be optimized. It requires
optimization of the system as a whole. In the ideal case all the components in the system: the process itself,
the controller, as well as the sensors and actuators, should be optimized simultaneously (Figure 12.2).
In general this is not feasible. The problem is ill defined and has to be split into smaller problems that
can be optimized separately. Later on the partial solutions have to be combined and the performance of
the complete system has to be evaluated. After eventually readjusting some parts of the system this leads
to a sub-optimal solution.
In the initial conceptual design phase it has to be decided which problems should be solved mechanically
and which problems electronically. In this stage decisions about the dominant mechanical properties
have to be made, yielding a simple model that can be used for controller design. Also a rough idea about
the necessary sensors, actuators, and interfaces has to be available in this stage. When the different partial
designs are worked out in some detail, information about these designs can be used for evaluation of
the complete system and be exchanged for a more realistic and detailed design of the different parts.
Although the word mechatronics is new, mechatronic products have been available for some time. In
fact, all electronically controlled mechanical systems are based on the idea of improving the product by
adding features realized in another domain. Good mechatronic designs are based on a
real systems
approach
. But mostly, control engineers are confronted with a design in which major parameters are
already fixed, often based on static or economic considerations. This prohibits optimization of the system
as a whole, even when optimal control is applied.
In the last days of gramophones, the more sophisticated designs used tacho feedback in combination
with a light turntable to achieve a constant number of revolutions. But a really new design was the
compact disc player. Instead of keeping the number of revolutions of the disc constant, it aims for a
constant speed of the head along the tracks of the disc. This means that the disc rotates slower when
tracks with a greater diameter are read. The bits read from the CD are buffered electronically in a buffer
that sends its information to the DA converter, controlled by a quartz crystal. This enables the realization
of a very constant bit rate and eliminates all audible speed fluctuations. Such a performance could never
be obtained from a pure mechanical device only, even if it were equipped with a good speed control
system. In fact, the control loop for the disc speed does not need to have very strict specifications. It
should only prevent overflow or underflow of the buffer. The high accuracy is obtained in an open loop
mode, steered by a quartz crystal (Figure 12.3).
The flexibility introduced by the combination of precision mechanics and electronic control has
allowed the development of CD-ROM players, running at speeds more than 50 times faster than the
original audio CDs. A new way of thinking was necessary to come to such a new solution. On the other
hand, the CD player is still a sophisticated piece of precision mechanics. No solid-state electronic memory device can compete yet economically with the opto-mechanical storage capabilities of the CD and its
successor the DVD. But this may change rapidly.
12.2 Key Elements of Controlled Mechatronic Systems
A mechatronic system consists by definition of a mechanical part that has to perform certain motions
and an electronic part (in many cases an embedded computer system) that adds intelligence to the system.
In the mechanical part of the system power plays a major role. This in contrast to the electronic part of
the system where information processing is the main issue. Sensors convert the mechanical motions into
electrical signals where only the information content is important or even into pure information in the
form of numbers (if necessary, through an AD converter). Power amplifiers convert signals into modulated
power. In most cases the power supply is electrical, but other sources such as hydraulic and pneumatic
power supplies are possible as well. A controlled mechanical motion system thus typically consists of a
mechanical construction, one or more actuators to generate the desired motions, and a controller that
steers the actuators based on feed-forward and sensor-based feedback control (Figure 12.4).
12.3 Integrated Modeling, Design, and Control
Implementation
Modeling
During the design of mechatronic systems it is important that changes in the construction and the
controller be evaluated simultaneously. Although a proper controller enables building a cheaper construction,
a badly designed mechanical system will never be able to give a good performance by adding
a sophisticated controller. Therefore, it is important that during an early stage of the design a proper
choice can be made with respect to the mechanical properties needed to achieve a good performance of
the controlled system. On the other hand, knowledge about the abilities of the controller to compensate
for mechanic imperfections may enable that a cheaper mechanical construction be built. This requires
that in an early stage of the design a simple model is available that reveals the performance limiting
factors of the system. Still there is a gap between modeling and simulation software used for evaluation
of mechanical constructions and software used for controller design. Mechanical engineers are used to
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