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11.5 Case Study 2—Mechatronic Design of a Coin Counter 507
11.5 CASE STUDY 2—MECHATRONIC
DESIGN OF A COIN COUNTER
In this section, we present solutions to the following design problem: Design a coin
counter that includes an electromechanical device to accept a handful of mixed
denomination U.S. coins and that will align and present the coins in serial order to
a sensor array capable of acquiring data that can be used to determine the denomi-
nations of individual coins. The sensor output is to be interfaced to electronics that
can compute both the number of coins presented and the total value of the coins and
display those two values in some sort of multiplexed fashion on a single display vis-
ible to the user.
We presented this design challenge to a junior-level mechatronics class of
80 students broken down into groups of 4, and more than half the groups created suc-
cessful solutions given a design period of 6 weeks. All groups were able to present
the coins sequentially and display a count of the number of coins. Multiplexing the
value of the coins with the count on a single display was more challenging, requiring
creative use of digital logic, and not all groups were successful in implementing a
design for this part. Given more time to redesign, most groups would have been suc-
cessful. As you will see, there is no single correct solution to the problem, and the
designs are as varied as the people who developed them.
This problem has two significant parts: the design of an electromechanical coin
presentation system to align coins sequentially in some fashion so they may be pre-
sented to an array of sensors, and the design of an electronic calculator to use the
sensor data to display the count and value of the coins. We consider the two parts
in succession, realizing that the student design groups often divided their efforts
between the mechanical design of the coin presenter and the electronic design of
the calculator. Students with significant machine shop experience tackled the elec-
tromechanical coin presentation design while students more comfortable with the
electronic experience they gained in the course focused on the sensor and counter
design. As in all design projects, certain people have affinities for certain parts of the
overall design, and it is important to assign responsibilities, communicate progress,
document work, and assure compatibility among the various subsystems. We do not
want to digress too much on the design process itself here, other than to mention that
team coordination and communication in the design process are as critical as the
design itself.
The mechanical component of the design requires a chamber to accept a hand-
ful of mixed denomination coins and a mechanism to select coins individually and
present them to a sensor array that can read their attributes and output digital sig-
nals for computation. By observing existing mechanical coin sorters, many students
designed inclined rotating disk mechanisms that had circular holes cut in them to
accept individual coins. Examples of the mechanical systems designed to present a
handful of coins in series are shown in Figure 11.23 . Two of the three approaches
( Figures 11.23a and 11.23c) use a DC motor to rotate a perforated disk that entrained
a single coin. As the disk rotated, the coin would drop into the slot containing the sen-
sor array. Most designs included a DC motor to rotate the disk continuously. Other
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