Designing a Control for a Material Fatigue Testing Machine

Escuela Superior de Ingenieros de Sevilla, Spain

DESIGNING A CONTROL FOR A
MATERIAL FATIGUE TESTING MACHINE

by

Joerg Henseler

A thesis submitted in fulfilment of the
requirements for an intermediate
student research project

Studienarbeit

2001

TABLE OF CONTENTS

Chapter 1: Project description ... 1

Chapter 2: Main concept ... 4

Chapter 3: Stepper Motors ... 6
3.1 Types ... 7
3.1.1 Variable Reluctance Motors ... 7
3.1.2 Unipolar Motors ... 8
3.1.3 Bipolar Motors ... 9
3.1.4 Multiphase Motors ... 11
3.2 Stepper Motor Parameters ... 11
3.3 Characteristics of the particular motors chosen ... 12

Chapter 4: Low-level Control ... 14
4.1 H-Bridge ... 14
4.1.1 Switching elements: IGBT vs. MOSFET ... 15
4.1.2 Low-side Switch ... 16
4.1.3 High-side Switch ... 17
4.1.4 Circuit design of the H-Bridge ... 19
4.2 Short Circuit Avoidance ... 20
4.3 Current Control ... 23
4.3.1 Pulse Width Modulation ... 24
4.3.2 The 555 timer circuit ... 26
4.3.3 Reference Voltage ... 28

Chapter 5: Mid-level Control ... 30
5.1 Moore Machine ... 31
5.2 Step Synthesis ... 32
5.3 Sequential Logic ... 33
5.4 Reset and Enable ... 38
5.5 Interface ... 38
Intersection: The Low- and Mid-level-PCB ... 40

Chapter 6: High-level Control ... 46
6.1 Timing Devices ... 46
6.1.1 Interrupts ... 47
6.1.2 Timing Hardware ... 50
6.1.3 Timing Routines ... 52
6.1.3.1 The Procedure SetTimerSpeed ... 52
6.1.3.2 The Function GrabTimer ... 53
6.1.3.3 The Procedure SetTimerHandler ... 54
6.1.3.4 The Procedure InternalHandler ... 54
6.1.3.5 The Procedure DisableHandler ... 56
6.1.3.6 The Procedure NormTimerSpeed ... 56
6.1.3.7 The Procedure ResetTimer ... 57
6.2 I/O-Management ... 57
6.2.1 Developing an ISA I/O-Card ... 58
6.2.2 The Parallel Port ... 63
6.2.3 I/O-Routines ... 64
6.2.3.1 The Procedure MotorWrite ... 64
6.2.3.2 The Procedures Motor1Move and Motor2Move ... 65
6.3 Control Routines ... 66

Chapter 7: Human Machine Interface: ... 72
7.1 Turbo Vision ... 74
7.2 The TFatigueControl Object ... 75
7.2.1 Desktop and Menu Design ... 78
7.2.2 Options-Window ... 79

Chapter 8: Summary and outlook ... 83

Bibliography ... 85

LIST OF FIGURES
1.1. Biaxial Material Fatigue Testing Machine
2.1. Main Concept
3.1. Schematic of a variable reluctance stepper motor
3.2. Schematic of a unipolar stepper motor
3.3. Schematic of a bipolar stepper motor
3.4. The two control sequences of bipolar motors
3.5. Half step modus
3.6. Schematic of a multiphase stepper motor
4.1. H-Bridge
4.2. Symbol of an n-channel MOSFET
4.3. Symbol of an n-channel IGBT
4.4 H-Bridge circuit diagram
4.5. Forward mode
4.6. Fast decay mode
4.7. Dynamic braking mode
4.8. Short circuit avoidance
4.9. Pulse Width Modulation
4.10. Current mode control
4.11. Pin specification of a 555
4.12. 555 circuit
4.13. Reference voltage circuit
5.1. Moore machine state diagram
5.2. KV-Diagram for X1*
5.3. KV-Diagram for Y1*
5.4. KV-Diagram for X2*
5.5. KV-Diagram for Y2*
5.6. Sequential logic
5.7. KV-Diagram for the decrease signal
5.8. Mid-level control circuits
X.1. Assembly Top Layer
X.2. Top Layer Routes
X.3. Assembly Bottom Layer
X.4. Bottom Layer Routes
X.5. Simulation Result Example
6.1. Interrupt driven program control transfer
6.2. Interrupt Vector Table
6.3. 8253/8254 Programmable Interval Timer
6.4 Circuit diagram for an ISA I/O-card
6.5 Assembly Top View of the ISA I/O-card
6.6 Routes on the top layer of the ISA I/O-card
6.7 Routes on the bottom layer of the ISA I/O-card
6.8 Status window
7.1 Desktop
7.2 Options window

C h a p t e r 1

PROJECT DESCRIPTION

One main field of activity of the Department of Mechanical Engineering and Material Science at the Escuela Superior de Ingenieros de Sevilla is material fatigue testing. In order to evaluate material characteristics under specified conditions, material probes are outset to biaxial movements, i.e.

• traction or pressure, and
• torsion.

The machines for this particular application typically use hydraulics for the power generation; the exact movement is achieved by servo-ventils. Industrial manufacturers demand at least 300.000 € for building a machine like that. This appears very expensive, keeping in mind the very basic task of the machine. Whereas a university of western industrialized countries may bear inversions like that, for universities of third world it is hardly possible to procure such a machine.

This was the motivation for the Department of Mechanical Engineering and Material Science at the Escuela Superior de Ingenieros de Sevilla to start a project in order to find a cheap as well as reliable solution for biaxial material fatigue testing.

The main change in respect to the up to now common machine design (an example is shown in figure 1.1) is that the biaxial movement of the machine to be build shall be realized by two stepper motors. This evokes two main advantages:

• From the mechanical point of view, the power transmission becomes easier: The deviation via the pressure generation is no longer needed, instead the power of the stepper motors can be transformed by screws into the desired movement.
• Regarding the control, stepper motors give the opportunity of a precise movement without the necessity of a control loop.* Most of all the costs of sensors (most suitable would be LVDT’s) can be avoided in this way.

The project was divided into two main parts, each of them treated by a student with a related field of studies:

• The mechanical part: A student of Mechanical Engineering was to design the mechanical structure of the machine, including the selection of suitable stepper motors.
• The control part: As a student of Electrical Engineering it became my part to design the control for the machine.

The components of the control were not specified. However, certain parts were strongly recommended:

• For high-level control old computers (80286-80486) should be used; these are easily to get for free, as they are no longer useful for modern office applications.
• The control must be simple as well as robust.
• Regarding the low target costs the control should consist of as few components as possible.

C h a p t e r 2

MAIN CONCEPT

In order to reduce the complexity of the given task the control is divided into three categories:

• Low-level control:
  It is responsible for the current supply of the motors. Additionally, it includes sequential logic for short circuit avoidance.
• Mid-level control:
  It has two main tasks: Keeping track of the motor state and providing four input lines: Clock, Direction, Enable and Reset.
• High-level control:
  It gives the reset impulse within an initialisation process and continuously takes three main decisions:

[...]

* Chapter 3 will explain the principle of stepper motors and their advantages.