Motion Control and Automation Systems Employed in Manufacturing

TABLE OF CONTENT

ABSTRACT

AKNOWLEDGEMENTS

LIST OF TABLES

LIST OF FIGURES

CHAPTER 1: INTRODUCTION
1.1 Motion Control Classification
1.2 Kinds of Controlled Motion

PART 1 LITERATURE REVIEW

CHAPTER 2: POSITION/PROXIMITY SENSORS FOR MOTION CONTROL
2.1 Limit Switches
2.2 Proximity Sensors
2.2.1 Inductive proximity sensors
2.2.2 Capacitive proximity Sensors
2.2.3 Ultrasonic proximity sensors
2.2.4 Photoelectric proximity sensors

CHAPTER 3: FLUID POWER (PNEUMATIC/HYDRAULIC) ACTUATORS
3.1. Valves
3.1.1 Pressure control valves
3.1.2 Reducing/regulating valves
3.1.3 Sequence valves
3.1.4 Flow control valves
3.1.5 Direction control valves
3.1.6 Check valve
3.2 Cylinders

CHAPTER 4 ELECTRICAL ACTUATORS
4.1 Mechanical switches
4.2 Solid state switches
4.3 Solenoids
4.4 Relays
4.5 Electric motors

PART 2 DEVELOPMENTS, ADVANCES AND APPLICATIONS OF MOTION CONTROL TECHNOLOGY

CHAPTER 5: ADAPTING FIELDBUS TECHNOLOGY IN MOTION CONTROL SYSTEMS
5.1 Actuator Sensor Interface (AS-i)
5.2 PROFIBUS
5.3 Industrial Ethernet

CHAPTER 6: APPLICATIONS & DEVELOPMENTS IN MOTION CONTROL & AUTOMATION TECHNOLOGY
6.1 Applications and trends of motion control in robotics
6.2 Application of fluid power in motion control technology
6.3 Application of motion control technology in plant automation

CHAPTER 6: CONCLUSIONS & RECOMMENDATIONS

REFERENCES/BIBLIOGRAPHY

ABSTRACT

Motion control has emerged as one of the most dynamic technologies in manufacturing. The current shift from mechanical control systems towards electronic servo control systems promises to increase process speeds by 50% or more, depending on application. The transfer and assembly lines have had a powerful impact in automating our factories with the primary goal of reduction of labour content while holding on to the financial justification labelled as economy of scale. Motion controllers are components that range from ON/OFF devices with simple linear controllers to complex, user programmable modules that act as controllers within complex integrated multi-axis motion systems. Applications include all types of industrial processing, packaging, and machining/forming operations. This thesis will focus on analysis of basic motion control theory, sensors and actuators used in motion control, adapting fieldbus technology in motion control systems, and developments, trends and application of motion control technology in different engineering disciplines.

ACKNOWLEDGEMENT

First, I give thanks to God for seeing me through my Masters studies.

I wish to express my sincere appreciation to my Academic Advisor Dr. Nick Karimi for his kind contributions and support. I wish also to thank Dr. Lora Rosa Hilda for her time and attention particularly during the first and second phases of my studies at the AIU. I thank all the members of the student services team of the AIU for their prompt replies to mails once clarification is sought.

I wish to thank all my friends in Automation & Control group on linked-in for their support through the online discussion forum. Same goes too to members of the Control & Automation Technical Professional Network of the Institution of Engineering & Technology (IET). I really do appreciate your efforts.

LIST OF TABLES

Table 2.1: Effect of various environmental conditions on sensor operation

Table 4.1 showing mechanical switches

Table 4.2 showing types of switches

Table 4.4 showing darlington pairs

Table 4.5 showing circuitry of DC motors

Table 4.6 showing comparisons of different types of drives

LIST OF FIGURES

Fig 1.1 This multiaxis X-Y-Z motion platform is an example of a motion control system

Fig.1.2 the right-handed coordinate system showing six degrees of freedom

Fig.1.3 Block diagram of a basic closed-loop control system.

Fig.1.4 Block diagram of an open-loop motion control system.

Fig 2.1 showing different operating conditions of a limit switch

Fig 2.2 showing snap-action and slow-break contact operation

Fig 2.3 showing contact arrangement for limit switches

Fig 2.4 showing the principle of operation of inductive proximity sensors

Fig 2.5 principles of operation of capacitive proximity sensors

Fig 2.6 output characteristic graph of ultrasonic proximity sensors

Fig 2.7 showing different light spectrum

Fig 2.8 scanning techniques/types of photoelectric proximity sensors

Fig 3.1 Classification diagram of valves

Fig 3.2 Pressure relief valve

Fig 3.3 Reducing valve

Fig 3.4 Sequence valve

Fig 3.5 Needle valve

Fig 3.6 Direction control valve

Fig 3.7 Check valve

Fig 3.8 directional control valve

Fig 3.9 A cross section of pneumatic/hydraulic cylinder

Fig 3.10 Schematic symbols of cylinders

Fig 3.11 showing telescopic and differential cylinders

Fig 4.1 Diode characteristics

Fig 4.2 Diode operation

Fig 4.3 Thyristor characteristics

Fig 4.4 Switching characteristics of a thyristor

Fig 4.5 Thyristor equivalent of a triac

Fig 4.6 Triac characteristics

Fig 4.7 Common-emitter circuit

Fig 4.8 Construction of a solenoid

Fig 4.9 Force stroke current

Fig 4.10 Pulse-latching solenoid

Fig 4.11 Configuration of a relay

Fig 4.12: Principle of a motor

Fig 4.13 showing the various classifications of motors

Fig 4.14 Permanent magnet D.C. motor

Fig 4.15 Analysis

Fig 4.16 Characteristic curve

Fig 4.17 Control circuit

Fig 4.18 Construction of a BLDC motor

Fig 4.19 Stepper motor

Fig 4.20 Workings of four-pole stepper motor

Fig 4.21 Single-phase motor

Fig 4.22 Characteristics of a stepper motor

Fig 4.23: Control circuit for a servo drive

Fig 4.24 Control circuit for a D.C. servomotor

Fig 4.25 Thyristor control

Fig 4.26 Triac control

Fig 5.1 showing master slave relationship in AS-I network

Fig 5.2 Technical system structure of PROFIBUS

CHAPTER 1: INTRODUCTION

A modern motion control system typically consists of a motion controller, a motor drive or amplifier, an electric motor, and feedback sensors. The system might also contain other components such as one or more belt, ball screw, or lead-screw-driven linear guides or axis stages. A motion controller today can be a standalone programmable controller, a personal computer containing a motion control card, or a programmable logic controller (PLC).

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Fig 1.1 This multiaxis X-Y-Z motion platform is an example of a motion control system

All of the components of a motion control system must work together seamlessly to perform their assigned functions. Their selection must be based on both engineering and economic considerations. Fig 1.1 illustrates a typical multi-axis X-Y-Z motion platform that includes the three linear axes required to move a load, tool, or end effector precisely through three degrees of freedom. With additional mechanical or electromechanical components on each axis, rotation about the three axes can provide up to six degrees of freedom, as shown in Fig. 1.2

Motion control systems today can be found in such diverse applications as materials handling equipment, machine tool centers, chemical and pharmaceutical process lines, inspection stations, robots, and injection moulding machines.

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Fig. 1.2 the right-handed coordinate system showing six degrees of freedom

1.1 Motion Control Classification

Motion control systems can be classified as open-loop or closed-loop. An open-loop system does not require that measurements of any output variables be made to produce error-correcting signals; by contrast, a closed-loop system requires one or more feedback sensors that measure and respond to errors in output variables.

Closed-Loop System

A closed-loop motion control system, as shown in block diagram Fig. 3, has one or more feedback loops that continuously compare the system’s response with input commands or settings to correct errors in motor and/or load speed, load position, or motor torque. Feedback sensors provide the electronic signals for correcting deviations from the desired input commands. Closed-loop systems are also called servo-systems.

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Fig. 1.3 Block diagram of a basic closed-loop control system.

Each motor in a servo-system requires its own feedback sensors, typically encoders, resolvers, or tachometers that close loops around the motor and load. Variations in velocity, position, and torque are typically caused by variations in load conditions, but changes in ambient temperature and humidity can also affect load conditions.

Open-Loop Motion Control Systems

A typical open-loop motion control system includes a stepper motor with a programmable indexer or pulse generator and motor driver, as shown in Fig. 1.4. This system does not need feedback sensors because load position and velocity are controlled by the predetermined number and direction of input digital pulses sent to the motor driver from the controller. Because load position is not continuously sampled by a feedback sensor (as in a closed-loop servosystem), load positioning accuracy is lower and position errors (commonly called step errors) accumulate over time. For these reasons open-loop systems are most often specified in applications where the load remains constant, load motion is simple, and low positioning speed is acceptable.

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Fig.1.4 Block diagram of an open-loop motion control system.

1.2 Kinds of Controlled Motion

There are five different kinds of motion control: point-to-point, sequencing, speed, torque, and incremental.

1. In point-to-point motion control the load is moved between a sequence of numerically defined positions where it is stopped before it is moved to the next position. This is done at a constant speed, with both velocity and distance monitored by the motion controller. Point-to-point positioning can be performed in single-axis or multi-axis systems with servomotors in closed loops or stepping motors in open loops. X-Y tables and milling machines position their loads by multi-axis point-to-point control.
2. Sequencing control is the control of such functions as opening and closing valves in a preset sequence or starting and stopping a conveyor belt at specified stations in a specific order.
3. Speed control is the control of the velocity of the motor or actuator in a system.
4. Torque control is the control of motor or actuator current so that torque remains constant despite load changes.
5. Incremental motion control is the simultaneous control of two or more variables such as load location, motor speed, or torque.

This thesis has been divided into two parts; the first treats the necessary literature on position sensors, fluid power actuators and electrical actuators used in motion control applications. The second part treats the developments trends and applications of motion control technology in various engineering disciplines.

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PART I. LITERATURE REVIEW

CHAPTER 2: POSITION SENSORS FOR MOTION CONTROL

Sensor technology is very vital in the fast growing field of industrial automation. There is an increasing need for detection of the state and position of a process or object. This could be level sensing, metal detection, presence or absence of an object, among others.

In the world of factory automation, discrete, position, or proximity sensors —whatever they are called – have come to stay. They serve in all areas of manufacturing including continuous processing, batch processing, utilities, and discrete products. Technology has changed the mechanical limit switch into an intelligent, rugged, accurate device with the ability to sense a wide variety of objects that can be wired into a sensor network if needed. In fact, flexibility has become a very important factor to proximity sensors’ adaptability in a control system.

Sensors form a critical part of active field devices, which communicates with master-drives such as Programmable Logic Controllers (PLC), and other field devices. Sensors are used to obtain a constant feedback, which is frequently needed by industrial control systems to obtain the position of one or more components of the operation being controlled. This thesis will treat discrete proximity sensors used in factory automation systems.

2.1 LIMIT SWITCHES

Limit switches requires the aid of mechanical actuator input which changes output when an object is physically touching the switch.

A limit switch is made up of two components; a switch body and an operating head. The switch body is made up of electrical contacts to energize and de-energize a circuit. The operating head makes use of some type of lever arm or plunger, known as an actuator.

The standard limit switch is a mechanical device that uses physical contact to detect the presence of an object or target. When the target comes in contact with the actuator, the actuator is rotated from its normal position to the operating position. This mechanical operation activates contacts within the switch body.

Principle of operation

The following terms are associated with the mechanical operation of a limit switch

- The free position is the position of the actuator when no external force is applied.
- Pre-travel is the distance or angle travelled in moving the actuator from the free position to the operating position.
- The operating position is where contacts in the limit switch change from their normal state (NO or NC) to their operated state.
- Over-travel is the distance the actuator can travel safely beyond the operating point.
- Differential travel is the distance travelled between the operating position and the release position.
- The release position is where the contacts change from their operated state to their normal state.
- Release travel is the distance travelled from the release position to the free position.

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Fig 2.1 showing different operating conditions of a limit switch

When the target comes in contact with the actuator, it rotates the actuator from the free position, through the pre-travel area, to the operating position. At this point, the electrical contacts in the switch body changes state. A spring returns the actuator lever and electrical contacts to their free position when the actuator is no longer in contact with the target.

It is desirable, in many applications, to have the actuator lever and electrical contacts remain in their operated state after the actuator is no longer in contact with the target. This is referred to as maintained operation. With this type of operation, the actuator lever and contacts return to their free position when a force is applied to the actuator in the opposite direction.

Types of contacts for limit switches

There are two types of contacts used in limit switch operation. They are;

- Snap-action contact
- Slow-break contact

Regardless of actuator speed, Snap-action contacts open or close by a snap action. When force is applied to the actuator in the direction of travel, pressure builds up in the snap spring. When the actuator reaches the operating position of travel, a set of moveable contacts accelerates from its normal position towards a set of fixed contacts. As force is removed from the actuator, it returns to its free position. When the actuator reaches the release position, the spring mechanism accelerates the moveable contact back to its original state.

Snap-action contacts are particularly suited for low actuator speed applications since the opening or closing of the contacts is not dependent on the speed of the actuator. Snap-action contacts are the most commonly used type of contact.

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Fig 2.2 showing snap-action and slow-break contact operation

Switches with slow-break contacts have moveable contacts that are located in a slide and move directly with the actuator. This ensures the moveable contacts are forced directly by the actuator. Slow-break contacts can be either break-before-make or make-before-break.

In break-before-make contacts type of slow-break switches, the normally closed contact opens before the normally open contact closes. This allows the interruption of one function before continuation of another function in a control sequence.

In make-before-break contacts type of slow-break switches, the normally open contact closes before the normally closed contact opens. This allows the initiation of one function before the interruption of another function.

The two most common type of contact arrangements associated with limit switch operations and their configurations are;

- Single Pole Double Throw (SPDT)
- Double Pole Double Throw (DPDT)

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Fig 2.3 showing contact arrangement for limit switches

Advantages of limit switches

1. High current capability
2. Low cost
3. Familiar ”low tech” sensing

Disadvantages of limit switches

1. Requires physical contact with target
2. Very slow response
3. Contact bounce

Applications of limit switches

1. Interlocking
2. Basic end-of-travel sensing

2.2 PROXIMITY SENSORS

Proximity sensors detect the presence of nearby objects without any physical contact. Proximity sensors often emits an electromagnetic or electrostatic field, or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. The object sensed is referred to as the proximity sensor target. Different proximity sensor targets demand different sensors. For example, a capacitive or photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor requires a metal target. Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between sensor and the sensed object.

The following sections describe in details the various types of proximity sensors commonly in use, their principle of operation and applications.

2.2.1 Inductive proximity sensors

Inductive proximity sensors only detect the presence of metallic objects with the use of its electromagnetic field. They operate under the electrical principle of inductance. Inductance is the phenomenon where a fluctuating current, which by definition has a magnetic component, induces an electromotive force (emf) in a target object.

An inductive proximity sensor has four components; a coil, an oscillator, a trigger (detection) circuit and an output

The oscillator is an inductive-capacitive tuned circuit that creates a radio frequency. The electromagnetic field produced by the oscillator is emitted from the coil away from the face of the sensor. The circuit has just enough feedback from the field to keep the oscillator going.

Eddy current circulates within the target once a metal target enters the field. This causes a load on the sensor, decreasing the amplitude of the electromagnetic field. As the target approaches the sensor the eddy currents increases thus, increasing the load on the oscillator and further decreasing the amplitude of the field. The trigger circuit monitors the oscillator’s amplitude and at a predetermined level switches the output state of the sensor from its normal condition (on or off). As the target moves away from the sensor, the oscillator’s amplitude increases. At a predetermined level the trigger switches the output state of the sensor back to its normal condition (on or off).

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Fig 2.4 showing the principle of operation of inductive proximity sensors

The standard target for inductive proximity sensors is mild steel which must have a flat smooth surface and is 1mm thick. The standard target for shielded sensors has sides equal to the diameter of the sensing surface while that used with unshielded sensors has sides equal to the diameter of the sensing surface or three-times the rated operating range, whichever is greater. If the target is larger than the standard target, the sensing range does not change. However, if the target is smaller or irregular shaped the sensing distance decreases. The smaller the area of the target the closer it must be to the sensing surface to be detected.

Three major factors that affect sensing distance are; target size, target thickness and target material

Advantages of inductive proximity sensors

1. Resistant to harsh environments
2. Very predictable
3. Long life
4. Easy to install

Disadvantage of inductive proximity sensors

1. Distance limitation
Applications of inductive proximity sensors
1. Industrial and machines applications
2. Machine tools
3. Senses metal-only targets

2.2.2 Capacitive proximity Sensors

Capacitive proximity sensors work using the principle of electrostatics. They work by measuring changes in electrical property called capacitance. Capacitance describes how two conductive objects with a space between them respond to a voltage difference applied to them. When voltage is applied to the conductors, an electric field is created between them causing positive and negative charges to collect on each object. If the polarity of the voltage is reversed, the charges will also reverse.

Capacitive sensors use an alternating voltage, which causes the charges to continually reverse their positions. The moving of the charges creates an alternating electric current, which is detected by the sensor. The amount of current flow is determined by the capacitance, and the capacitance is determined by the area and proximity of the conductive objects.

Capacitive proximity sensors sense metallic and non-metallic objects such as paper, glass, liquids, and cloth. When an object nears the sensing surface it enters the electrostatic field of the electrodes and changes the capacitance in an oscillator circuit. As a result, the oscillator begins oscillating. The trigger circuit reads the oscillator’s amplitude and when it reaches a specific level the output state of the sensor changes. As the target moves away from the sensor the oscillator’s amplitude decreases, switching the sensor output back to its original state.

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Fig 2.5 principles of operation of capacitive proximity sensors

Capacitive proximity sensors are mostly used for level detection through barrier. For example, water has a much higher dielectric than plastic. This gives the sensor the ability to “see through” the plastic and detect the water.

Advantages of capacitive sensors

1 Detects through some containers
2 Can detect non-metallic targets

Disadvantage of capacitive sensors

1 Very sensitive to extreme environmental changes

Application of capacitive sensors

1 Level sensing

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