Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems

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11.5 Case Study 2—Mechatronic Design of a Coin Counter 511

When this project was assigned, we had not yet started teaching microcontroller

programming and interfacing in our course. In lieu of this, we had students develop

solutions using basic TTL ICs. Combinational and sequential logic is required to

determine the denomination of the coin and increment the displayed output by a

value corresponding to the denomination. There were as many solutions for this

problem as there were design groups. Figures 11.26 and 11.27 display two student

group solutions for the coin counter processing and display circuit. The outputs were

transmitted to a digital display driver to multiplex the current number of coins and

the accumulated value.

■ CLASS DISCUSSION ITEM 11.2

Coin Counter Circuits

Figure 11.26 shows the coin counter electrical schematic from a student group

report. The design uses one-shots to latch the value corresponding to the coin and

adds that value to the previous sum. There are mistakes in the input portion of the

first half of the circuit used to determine whether the coin is a penny, nickel, or

quarter. The error is in the combinational logic portion of the circuit. Based on the

timing diagrams shown in Figure 11.25 , convince yourself that the logic circuit will

not identify the coin correctly.

Even if the logic were correct, the design could still have problems due to tim-

ing because the one-shot pulse lengths are fixed but the sensor pulse widths depend

on how fast the coin rolls past. A sequential logic circuit that would unequivocally

identify the coin as a penny, nickel, or quarter follows. Draw a timing diagram that

supports the argument that this sequential logic circuit is a robust design. Include

signals S

low

(low sensor), S

middle

(middle sensor), S

high

(high sensor), B, C, L, R, X, Y,

and Z in your diagram. Refer to Figure 11.25 for the phototransistor sensor signal

timing for each type of coin.

Y (nickel)

Z (penny)

X (quarter)

high

middle

low

CLR

one-shot one-shot

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Figure 11.26 Counter design 1.

(a) first half of circuit

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(b) second half of circuit

Figure 11.26 (continued)

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Figure 11.27 Counter design 2.

(a) first half of circuit

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(b) second half of circuit

Figure 11.27 (continued )

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516 CHAPTER 11 Mechatronic Systems—Control Architectures and Case Studies

11.6 CASE STUDY 3—MECHATRONIC

DESIGN OF A ROBOTIC

WALKING MACHINE

In this section, we present a case study of the mechatronic design of an articulated

walking machine. It was executed by undergraduate engineering students in 1994,

following the completion of our introductory mechatronics course for which this

book is used. In 1987, the Society of Automotive Engineers (SAE) began sponsor-

ing an annual Robotic Walking Machine Decathlon, pitting teams from different

universities in a challenge to design articulated walking machines that could execute

10 different performance events including a dash, a slalom, obstacle avoidance, and

crossing a crevasse. Half of the events included walking motion and obstacle avoid-

ance under tether control, that is, control from a human-operated switch box with

an electrical umbilical to the machine. The other half of the events required auton-

omous control via onboard, preprogrammed systems with no human intervention.

Over the years we have seen scores of different walking machine designs, some very

simple and capable of completing just a few events, and others of great sophistica-

tion and creativity capable of completing all events. The excitement and fun of such

competitions is in seeing the fruit of design concepts actually functioning to speci-

fications. Our intent here is not to examine the varieties of walking machines but to

focus on a specific design example to illustrate the mechatronic aspects. We begin

by displaying three different walking machines, all of which won the national SAE

contest (see Figure 11.28 ).

We now present as a case study the design of the 1994 Colorado State walk-

ing machine that the students, applying their ever-present wit, named Airratic. This

design was a refinement of the first pneumatic design from 1992, which was a dra-

matic break from the evolving electromechanical designs of the previous seven years.

With the air-powered designs, the students had to contend with a whole new set of

design constraints: providing an onboard source of stored, pressurized air; control-

ling the mechanical motion of the articulated legs with pneumatic cylinders; distrib-

uting and controlling the air; controlling the pressure; reducing coordinated walking

motion to sets of computer commands; interfacing a computer to the pneumatic con-

trol system; minimizing the weight and size of the machine; ensuring the safety of

the system, including sensors on the machine for obstacle avoidance; and making the

design changeable and adaptable during test trials on the competition floor.

Because the designers elected to power the walking machine pneumatically, an

onboard fiber-wound pressure tank was selected as the energy source for the stra-

tegically placed pneumatic pistons that controlled the position and movements of

the legs. As shown in Figure 11.29 , the skeletal structure of the walking machine

consisted of welded aluminum members with 16 double-acting pneumatic cylinders.

The four corner legs have 3 degrees of freedom, and the front and rear-center legs

have 2 degrees of freedom. The mechanical design of the six legs was such that

control algorithms could easily produce forward, reverse, sidewise, and diagonal

motion, as well as full-up and full-down motion of the main frame, by simple coor-

dinated control of the 16 cylinders. Each leg included an axial cylinder that could be

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(b) “Airachnid”—First pneumatic design (1992)

(a) “Lurch”—Scotch yoke mechanism leg design (1989)

Figure 11.28 Student-designed walking machines from Colorado State University.

11.6 Case Study 3—Mechatronic Design of a Robotic Walking Machine 517

extended or retracted. Ten other cylinders were arranged to position the legs in dif-

ferent patterns in the horizontal plane.

For static stability the machine must be supported by a minimum of three legs at

all times, requiring a total of six legs for locomotion. Most walking motions could be

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Figure 11.29 Aluminum frame and telescoping pneumatic legs.

(a) side view

(b) front view

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divided into the coordinated movement of two sets of three legs each, called Group

1 Legs and Group 2 Legs. Figure 11.30 displays the flowchart used to develop the

control code for one class of coordinated motion: forward motion of the machine.

The essence of the control system involves the coordination of 16 pneumatic

cylinders using computer-controlled solenoid valves. Each solenoid valve switches

air from the main cylinder to one side of the pneumatic cylinder and exhausts pres-

surized air from the other side, as shown in Figure 11.31 . Needle valves were manu-

ally adjusted on each solenoid valve to govern cylinder actuating speed.

The complexity of the flowchart in Figure 11.30 warrants a microprocessor-

based solution. This and similar flowcharts for each motion command were very

helpful in designing the computer code necessary to control and coordinate the

motion. The onboard computer controls the solenoid valves via 74373 tristate invert-

ing buffer interface chips receiving signals from an 8-bit port on the computer. As

shown in Figure 11.32 , the 8-bit port is multiplexed to provide 18 bits of information

for coordinated control of the cylinders.

The controller selected was a Motorola 68HC811E2 EVM 8-bit microprocessor-

based single-board computer. It has 128 K of RAM, and 64 K acts as pseudo-ROM

for downloading control programs from an external PC or laptop computer. Thirty

I/O bits are provided and expanded via multiplexing to the 48 bits required for the

control of all of the components of the machine. Computer code was created on a

laptop computer in the high-level language C, then complied and downloaded to the

Motorola EVM for testing. This provided a convenient development environment for

testing and modifying the control strategies.

In addition to various walking motions, the machine included sensor feedback

using two optical retroreflective sensors. The sensors were mounted on a rotating

platform to allow scanning a portion of the region in front of the machine. A digital

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Figure 11.30 Flowchart for forward motion routine.

11.6 Case Study 3—Mechatronic Design of a Robotic Walking Machine 519

encoder provided position information corresponding to the sense state of the two

sensors. The sensor data was read from the I/O bus of the microprocessor, and soft-

ware control routines used the data to avoid obstacles.

In summary, we examined an example of a student-designed mechatronic sys-

tem that received first place in the 1994 SAE Walking Machine Decathlon. It illus-

trated the use of pneumatic actuators for motion, optical sensors for feedback, and

an onboard microcomputer for controlling the machine in both operator-controlled

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Figure 11.31 Pneumatic system.

Figure 11.32 Computer ports and I/O board.

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