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

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to opposite ends of the cylinder, applying or venting pressure on opposite sides of

the piston. In position 1, the cylinder does not move, because the pressure is vented

to the tank. In position 2, the cylinder moves to the right, since pressure is applied to

the left side of the piston. In position 3, the cylinder moves to the left, because pres-

sure is applied to the right side of the piston.

Common types of fixed position valves are check valves, poppet valves, spool

valves, and rotary valves. Figure 10.41 illustrates check and poppet valves. The

check valve allows flow in one direction only. The poppet valve is a check valve

that can be forced open to allow reverse flow.

As illustrated in Figure 10.42 , a spool valve consists of a cylindrical spool with

multiple lobes moving within a cylindrical casing containing multiple ports. The spool

can be moved back and forth to align spaces between the spool lobes with input and

output ports in the housing to direct high-pressure flow to different conduits in the sys-

tem. The spool is balanced in each position because the static pressure is the same on

opposing internal faces of the lobes. Therefore, no force is required to hold a position.

In the left position, port A is pressurized, and port B is vented to the tank; and in the

right position, port B is pressurized and port A is vented. To move the spool between

positions, an axial force is required, from an actuator or manual control lever, to over-

come the hydrodynamic forces associated with changing the momentum of the flow.

In the design of a spool valve where large hydrodynamic forces occur, a pilot

valve is added, as shown in Figure 10.43 . The pilot valve operates at a lower pres-

sure, called pilot pressure, and at much lower flow rates and therefore requires less

force to actuate. The pilot valve directs pilot pressure to one side of the main spool,

and the force generated by the pressure acting over the main spool lobe face is large

enough to actuate the main valve. The effect of the pilot valve is to amplify force

provided by the solenoid or mechanical lever acting on the pilot spool. In the figure,

the pilot spool is in the full left position, causing pilot pressure to be applied to the

Figure 10.40 Double-acting hydraulic cylinder.

piston

4/3

valve

Figure 10.41 Check and poppet valves.

(a) check valve

(b) poppet valve

flow no flow

ball

light spring

seat

up-down plunger

no flow

unless

plunger is

down

flow

10.8 Hydraulics 471

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Figure 10.42 Spool valve.

(a) schematic

PAT PAT

left position (P-A, B-T)

right position (A-T, P-B)

force

spool lobe casing

TBTB

SEALED WET ARMATURE SOLENOIDS

Maximum protection against moisture,

corrosion and dirt.

OPERATOR PROTECTION

High temperature elements are

isolated from direct contact.

2 WIRE LEAD

50/60 Hz coils standard for

increased application flexibility.

INTERCHANGEABLE SPOOLS

Provide easy field maintenance.

No matching of parts.

LOW-PRESSURE DROPS

Reduces heat load and

increases efficiency.

SPOOL U GROOVES

Improves contamination tolerance of

spools over conventional V grooves.

Significantly reduces spool hang up

resulting in loss of production.

STANDARD

MOUNTINGS

Conforms to NFPA and

ANSI/ISO standards.

(b) actual

(Courtesy of Continental Hydraulics, Savage, MN)

Figure 10.43 Pilot-operated spool valve.

PBATT

solenoid-

operated

pilot spool

pilot

press.

pilot valve

main spool

472 CHAPTER 10 Actuators

left side of the main spool and venting fluid from the right side of the main spool to

tank, thus driving the main spool to the full right position. This applies main pressure

to port B and vents port A. Video Demo 10.24 illustrates how a pilot valve can be

used to create an amplified hydraulic force.

The discussion of spool valves so far has been limited to operation between

two positions only: on and off. Continuous operation can be achieved by using a

proportional valve, one whose spool moves a distance proportional to a mechanical

Video Demo

10.24Pilot valve

hydraulic amplifier

cut-away

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or electrical input (e.g., a lever or an adjustable-current solenoid), thus changing the

rate of flow and varying the speed and force of the actuator. When the spool position

is controlled by electrical solenoids, the proportional valve is called an electrohy-

draulic valve. These valves may be used in open-loop control situations with no

feedback, but they often include sensors to monitor spool position or actuator output.

Proportional valves equipped with sensor and control circuitry are often called servo

valves. Electrohydraulic valves are often pilot operated where the solenoids drive the

pilot spool, which in turn controls the position of the primary spool. The pilot spool

can be driven by a single solenoid with a spring return or a set of opposing solenoids.

The solenoid currents, and therefore displacements, can be controlled by amplifiers

linked to analog or digital controllers.

10.8.2 Hydraulic Actuators

The most common hydraulic actuator is a simple cylinder with a piston driven by

the pressurized fluid. As illustrated in Figure 10.44 , a cylinder can be single acting,

where it is driven to and held in one position by pressure and returned to the other

position by a spring or by the weight of the load, or double acting, where pressure

is used to drive the piston in both directions. As illustrated in Figure 10.45 , the linear

actuator can be very versatile in achieving a variety of motions. Cylinder motion in

the hydraulic elevator drives the elevator directly. The scissor jack converts small

linear motion in the horizontal direction to larger linear motion in the vertical direc-

tion. Linear motion of the cylinder in the crane results in rotary motion of its pivoted

boom.

Rotary motion can also be achieved with hydraulic systems directly with a

rotary actuator. One type of rotary actuator, called a gear motor, is simply a gear

pump (see Figure 10.35 ) driven in reverse where pressure is applied, resulting in

rotation of a shaft.

Figure 10.44 Single-acting and double-acting cylinders.

double-actin

linder

single-acting cylinder

hydraulic elevator scissor jack

“cherr

picker” crane

Figure 10.45 Example mechanisms driven by a hydraulic cylinder.

10.8 Hydraulics 473

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474 CHAPTER 10 Actuators

Hydraulic systems have the advantage of generating extremely large forces from

very compact actuators. They also can provide precise control at low speeds and

have built-in travel limits defined by the cylinder stroke. The drawbacks of hydraulic

systems include the need for a large infrastructure (high-pressure pump, tank, and

distribution lines); potential for fluid leaks, which are undesirable in a clean environ-

ment; possible hazards associated with high pressures (e.g., a ruptured line); noisy

operation; vibration; and maintenance requirements. Because of these disadvan-

tages, electric motor drives are often the preferred choice. However, in large systems,

which require extremely large forces, hydraulics often provide the only alternative.

For more information, Internet Link 10.9 provides links to online resources and

manufacturers of hydraulic components and systems.

10.9 PNEUMATICS

Pneumatic systems are similar to hydraulic systems, but they use compressed air as

the working fluid rather than hydraulic liquid. The components in a pneumatic sys-

tem are illustrated in Figure 10.46 . A compressor is used to provide pressurized air,

usually on the order of 70 to 150 psi (482 kPA to 1.03 MPa), which is much lower

than hydraulic system pressures. As a result of the lower operating pressures, pneu-

matic actuators generate much lower forces than hydraulic actuators.

After the inlet air is compressed, excess moisture and heat are removed from

the air with an air treatment unit (see Figure 10.46 ). Unlike hydraulic pumps, which

provide positive displacement of fluid at high pressure on demand, compressors can-

not provide high volume of pressurized air responsively; therefore, a large volume of

high-pressure compressed air is stored in a reservoir or tank. The working pressure

delivered to the system can be controlled by a pressure regulator to be much lower

than the reservoir pressure. The reservoir is equipped with a pressure-sensitive switch

that activates the compressor when the pressure starts to fall below the desired level.

Control valves and actuators act in much the same way as in hydraulic systems,

but instead of returning fluid to a tank, the air is simply returned (exhausted) to the

atmosphere. Pneumatic systems are open systems, always processing new air, and

hydraulic systems are closed systems, always recirculating the same oil. This elimi-

nates the need for a network of return lines in pneumatic systems. Another advantage

of pneumatic systems is that air is “cleaner” than oil, although air does not have the

self-lubricating features of hydraulic oil.

■ CLASS DISCUSSION ITEM 10.8

Force Generated by a Double-Acting Cylinder

For a given system pressure, is the force generated by a double-acting cylinder dif-

ferent depending on the direction of actuation? How is the force determined for

each direction of motion?

Internet Lin

10.9Hydraulic

Valves.org

(valve, pump,

motor, cylinder

manufactures and

information)

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When sources of compressed air are readily available, as they often are in

engineering-related facilities, pneumatic actuators may be a good choice. Double-

acting or single-acting pneumatic cylinders are ideal for providing low-force linear

motion between two well-defined endpoints. Since air is compressible, pneumatic

cylinders are not typically used for applications requiring accurate motion between

the endpoints, especially in the presence of a varying load. Video Demo 10.25 dem-

onstrates various types of pneumatic cylinders, and Video Demo 10.26 shows an

interesting example of an apparatus driven by a pneumatic system.

Another advantage of pneumatic systems is the possibility of replacing the

infrastructure with a high-pressure storage tank and regulator. The tank serves a

function analogous to a battery in an electrical system. This makes possible light,

mobile pneumatic systems (e.g., a pneumatically actuated walking robot). In these

applications, the capacity of the tank limits the range or operating time of the system.

QUESTIONS AND EXERCISES

Section 10.3 Solenoids and Relays

10.1. A solenoid can be modeled as an inductor in series with a resistor. Design a circuit

to use a digital output to control a 24 V solenoid.

Section 10.5 DC Motors

10.2. Why can the presence of electric motors or solenoids affect the function of nearby

electronic circuits?

10.3. If a manufacturer’s specifications for a PM DC motor are as follows, what are the

motor’s no-load speed, stall current, starting torque, and maximum power for an

applied voltage of 10 V?

■ Torque constant  0.12 Nm/A

■ Electrical constant  12 V/1000 RPM

■ Armature resistance  1.5 

filter

motor

compressor

control

valve

cylinde

infrastructure

exhaust

air

inlet

air

treatment

on/off control

reservoir

pressure

regulator

Figure 10.46 Pneumatic system components.

Video Demo

10.25Pneumatic

cylinders of various

types and sizes

10.26Pneumatic

biomechanics

exercise apparatus

overview

10.9 Pneumatics 475

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476 CHAPTER 10 Actuators

10.4. Recognizing that the H-bridge IC in Design Example 10.1 includes a current sense

output, draw the block diagram for torque control of a brushed DC motor using the

IC discussed. (Hint: The current sense output pin from the H-bridge yields 377 mA

of current for each amp that is output to the motor. Convert this output to a voltage

and use it as the input to a PWM chip like the LM3524D. Include a torque adjust

potentiometer on the LM3524D.)

Section 10.6 Stepper Motors

10.5. For the full-step drive circuit in Figure 10.29 , complete the following timing dia-

gram by adding signals B

, B

, φ

, and φ

. Using Table 10.1 , verify that the

intended motion results.

RESET

STEP

CW/CCW

10.6. Document a complete and thorough answer to Class Discussion Item 10.5.

10.7. A unipolar stepper motor full-step drive circuit is available as a single component

from some manufacturers. Explore the Internet to find a stepper motor driver, show a

labeled pin-out diagram, and indicate how to connect it to a stepper motor properly.

10.8. A designer wishes to use a stepper motor coupled to a gearbox to drive an indexed

conveyor belt to achieve a linear resolution of 1 mm and a maximum speed of 10

cm/sec. The gearbox is a speed reducer with a gear ratio of 3 to 1, and the conveyor

is driven by a 10 cm drum attached to the output shaft of the gearbox. What is the

minimum resolution required for the stepper motor? Also, what step rate would be

required to achieve the maximum speed at this resolution?

Section 10.7 Selecting a Motor

10.9. For each of the following applications, what is a good choice for the type of electric

motor used? Justify your choice.

a. robot arm joint

b. ceiling fan

c. electric trolley

d. circular saw

e. NC milling machine

f. electric crane

g. disk drive head actuator

h. disk drive motor

i. windshield wiper motor

j. industrial conveyor motor

k. washing machine

l. clothes dryer

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10.10. A permanent magnet DC motor is coupled to a load through a gearbox. If the polar

moments of inertia of the rotor and load are J

and J

, the gearbox has a N:M reduc-

tion (where N > M ) from the motor to the load, the motor has a starting torque T

and a no-load speed 

max

, and the load torque is proportional to its speed ( T

 k  ),

a. What is the maximum acceleration that the motor can produce in the load?

b. What is the steady state speed of the motor and the load?

c. How long will it take for the system to reach 95% of its steady state speed?

Section 10.8 Hydraulics

10.11. Document a complete and thorough answer to Class Discussion Item 10.8.

10.12. What is the maximum force a 1 inch inner diameter single acting hydraulic cylinder

can generate with a fluid pressure of 1000 psi?

10.13. If a machine requires that a 1 cm inner diameter single-acting hydraulic cylinder

produce 2000 N of force, what is the minimum required system pressure in MPa?

Section 10.9 Pneumatics

10.14. You are designing a pneumatic system that requires use of a pneumatic valve.

On the Internet, find the specifications for a pneumatic valve capable of handling

100 psi, and draw a schematic showing how you would interface it to a digital

system.

10.15. You are designing a pneumatic actuator using a dual-acting cylinder to produce

100 lbf using a 2000-psi scuba tank as a pressure source. Specify the elements

required in the system in block diagram form. Be as specific as possible about

component specifications.

BIBLIOGRAPHY

Kenjo , T. , Electric Motors and Their Controls, Oxford Science Publications, Oxford,

England, 1994 .

Khol , R. , editor, “Electrical & Electronics Reference Issue,” Machine Design, v. 57, n. 12,

May 30, 1985 .

McPherson , G. , An Introduction to Electrical Machines and Transformers, John Wiley, New

York, 1981 .

National Semiconductor Corporation, “National Power ICs Databook,” 2900 Semiconductor

Drive , P.O. Box 58090, Santa Clara , CA 95052.

Norton , R. L. , Design of Machinery, 3rd ed., McGraw-Hill, New York, 2003 .

Shultz , G. P. , Transformers and Motors, Macmillan , Carmel, IN, 1989 .

Westbrook , M. H. and Turner , J. D. , Automotive Sensors, Institute of Physics Publishing ,

Philadelphia, 1994 .

Williamson , L. , “What You Always Wanted to Know About Solenoids,” Hydraulics and

Pneumatics, September, 1980 .

Bibliography 477

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478

Mechatronic Systems—

Control Architectures

and Case Studies

his chapter provides a summary of mechatronic system digital control archi-

tectures, introduces the concepts of PID control, and presents three case

studies of mechatronic system design. ■

INPUT SIGNAL

CONDITIONING

AND INTERFACING

- discrete circuits

- amplifiers

- filters

- A/D, D/D

OUTPUT SIGNAL

CONDITIONING

AND INTERFACING

- D/A, D/D

- PWM

- power transistors

- power op amps

GRAPHICAL

DISPLAYS

- LEDs

- digital displays

- LCD

- CRT

SENSORS

- switches

- potentiometer

- photoelectrics

- digital encoder

- strain gage

- thermocouple

- accelerometer

- MEMs

ACTUATORS

- solenoids, voice coils

- DC motors

- stepper motors

- servo motors

- hydraulics, pneumatics

MECHANICAL SYSTEM

- system model - dynamic response

DIGITAL CONTROL

ARCHITECTURES

logic circuits

microcontroller

SBC

PLC

sequencing and timing

logic and arithmetic

control algorithms

communication

- amplifiers

CHAPTER OBJECTIVES

After you read, discuss, study, and apply ideas in this chapter, you will:

1. Know why many engineering designs today can be classified as mechatronic

systems

2. Be aware of the various types of control architectures possible in mechatronic

system design

CHAPTER

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11.2 Control Architectures 479

3. Understand the basic principles of feedback control systems

4. Appreciate the variety of designs that can result from limited specifications of

a mechatronic system

5. Understand the basics of programmable logic controller (PLC) programming

using ladder logic

6. Recognize that many aerospace, automotive, and consumer products are

mechatronic systems

11.1 INTRODUCTION

In the previous chapters of this book, we developed the foundations for the integra-

tion of mechanical devices, sensors, and signal and power electronics into mecha-

tronic systems. As we progressed through the chapters, we included increasingly

challenging mechatronic design examples to help you connect the theory and

analysis with actual applications to expand your knowledge and design skills. To

obtain completeness in the integration of mechanical devices, sensors, and sig-

nal and power electronics into the most advanced mechatronic systems, we must

include microprocessor-based control systems. Chapter 7 presented the basics of

microcontroller programming and interfacing. Here, we briefly introduce other con-

trol architectures also useful in mechatronic system design. Then we present three

case studies of complete mechatronic systems: a myoelectrically controlled robotic

arm, an articulated walking machine for movement in unusual environments, and a

coin counter. These projects have been used as mechatronic projects in coursework

at Colorado State University. To provide a final perspective, we list examples of

mechatronic systems in many industries.

11.2 CONTROL ARCHITECTURES

Many mechatronic systems have multiple inputs and outputs related by determinis-

tic relationships that result in some form of control of the outputs. A designer can

choose from a wide spectrum of control architectures, ranging from simple open-

loop control to complex feedback control. Implementation of the control can be as

simple as using a single op amp or as complicated as programming massively paral-

lel microprocessors. Here, we describe a hierarchy of basic control approaches you

may consider in the design of a mechatronic system.

11.2.1 Analog Circuits

Many simple mechatronic designs require a specific actuator output based on an

analog input signal. In some cases, analog signal processing circuits consisting of op

amps and transistors can be employed to effect the desired control. Op amps can be

used to perform comparisons and mathematical operations such as analog addition,

subtraction, integration, and differentiation. They can also be used in amplifiers for

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480 CHAPTER 11 Mechatronic Systems—Control Architectures and Case Studies

linear control of actuators. Analog controllers are often simple to design and easy to

implement and can be less expensive than microprocessor-based systems.

11.2.2 Digital Circuits

If the input signals are digital or can be converted to a finite set of states, then com-

binational or sequential logic controllers may be easy to implement in mechatronic

design. The simplest designs use a few digital chips to create a digital controller. To

generate complex Boolean functions on a single IC, specialized digital devices such

as programmable array logic (PAL) controllers and programmable logic arrays

(PLAs) can be used to reduce design complexity. PALs and PLAs contain many

gates and a gridwork of conductors that can be custom connected using a program-

ming device. Once programmed, the ICs implement the designed Boolean function

between the inputs and outputs. PALs and PLAs may offer an alternative to complex

sequential and combinational logic circuits that require many ICs.

Another type of programmable logic-gate-based device is the field-

programmable gate array (FPGA). Like PALs and PLAs, an FPGA contains a

large number of reconfigurable gates that can be programmed to create a wide range

of logic functions. FPGAs are different from PALs and PLAs because they also can

include memory, I/O ports, arithmetic functions, and other functionality found in

microcontrollers. Furthermore, FPGAs are usually programmed with a high-level

software language (e.g., VHDL) that allows for fairly sophisticated functionality.

Video Demo 11.1 shows an example of an FPGA development system used to con-

trol a simple device.

Sometimes, it may be economically feasible to design an application-specific

integrated circuit (ASIC) that provides unique functionality on a single IC. Logic

functions, memory, computation, signal processing, and other digital and analog fea-

tures can be custom built onto a single ASIC. Design and setup for manufacturing

can be expensive, but in high-volume manufacturing applications, an ASIC solution

can be cheaper in the long run. ASICs are also attractive because the integrated solu-

tion will usually be smaller in size and consume less power.

11.2.3 Programmable Logic Controller

Programmable logic controllers (PLCs) are specialized industrial devices for

interfacing to and controlling analog and digital devices. They are designed with

a small instruction set suitable for industrial control applications. They are usu-

ally programmed with ladder logic, which is a graphical method of laying out the

connectivity and logic between system inputs and outputs. PLCs are designed with

industrial control and industrial environments specifically in mind. Therefore, in

addition to being flexible and easy to program, they are robust and relatively immune

to external interference.

The symbols, notation, and basic constructs used to define and create ladder

logic diagrams are shown in Figure 11.1 . Programming a PLC is a simple matter of

constructing a diagram like this by interactively dragging and dropping components

in a graphical user interface provided by the manufacturers. Referring to the figure,

Video Demo

11.1Table

tennis assistant

controlled by an

FPGA

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