Open architecture control system for a modular reconfigurable machine tool.

This research is part of a broader research initiative that addresses the development of a reconfigurable manufacturing paradigm. The processing and control efficiency of the system is evaluated on the previously developed MRMT.

Introduction

Manufacturing System Environment
Research Question
Research Objectives
MSc Research Contribution
Outline of Dissertation
Chapter Summary

Chapter One: Provides the reader with the background of the Research, the motivation for the research, and a summary of what the research entailed. Design and engineering details of the modular machine on which the OACS was tested are also presented.

Manufacturing Systems

Dedicated Manufacturing Systems
Flexible Manufacturing Systems
Intelligent Manufacturing Systems
Reconfigurable Manufacturing Systems
Manufacturing System Life Cycles
Chapter Summary

This modular concept is critical to system reconfiguration and ensures that system capacity and functionality are not fixed and predetermined. In addition, the modular nature of these systems allows the system to be flexible in both the manufacturing of parts and the structure of the system.

Open Architecture, Modular and Distributed Control Systems

Essential Characteristics of Open Architecture Controllers
Emerson DeltaV
Siemens SIMATIC
Chapter Summary

Pritschow [18] emphasizes that the performance of the operating system is affected by the degree of interoperability between the basic modules. The reconfiguration and modification of the existing system can be performed quickly and easily due to the well-defined modular nature of the system's hardware and software architectures [24].

Figure 4 [10] illustrates how these criteria affects the system both internally and externally

Mechanical Systems

Computer Numerically Controlled Machines

Live-time and online monitoring of the machining process is carried out and, where necessary, corrections are made to the CNC system to ensure accurate machining of workpieces. The CNC system interprets the part program and converts it into commands for servo motor driver circuits.

Modular Reconfigurable Manufacturing Tool

In terms of reconfigurability, the machining function of the MRMT will change by replacing process modules with others. The Z axis forms the column module of the MRMT and supports the machining module, the drilling tool for the MRMT.

Figure 10 [6] shows a conceptual library of MRMT modules. Library modules can be classified as motion, process or accessory modules

Chapter Summary

The MRMT was created and developed as part of a previous research by Padayachee et al. The development of a modular OACS for MRMT was not part of MRMT's scope, and therefore MRMT was designed and built with a fixed electronic and control software solution.

Open Architecture System for a Modular Reconfigurable Machine Tool

Research and Design Outlines
Mechatronic Design Approach
OACS High Level Design
Distributed Modules
Buffer Layers
Chapter Summary

The host computer's memory stores the control system, control algorithms, and software modules for each hardware module. An essential component of research and design is distributed modules and detailed discussion of how they add value to the system.

Figure 19: Hardware/Software Co-Design Approach

Electronic Subsystems

Overview
Host PC
Distributed Modules
Multiple Microcontroller Implementation
Communications Network
CAN bus
CAN bus Hardware
Message Packet Formatting
Spindle and Axis Speed Control
Motor Noise Cancellation
Collision Detection
Position Feedback Encoders
Vibration Sensor: Accelerometer
Power Distribution Network
Chapter Summary

The distributed modules at the different stations of the MRMT are designed around the FEZ Panda 2 board. Like Profibus, the CAN bus maintains control over the control system and links the control system to the module.

Control Algorithms for Open Architecture Control System

Kinematic Modelling

Any movement of the MRMT can be described by calculating the individual movements of each. Similarly, the HTM expressing the overall configuration of the MRMT, from position with respect to position and orientation, is represented by equation 7.3 [56]:. Since each axis of the MRMT is limited to a single DOF, equation 7.4 can be represented as equation 7.5:.

This advanced kinematic model is determined by multiplying the individual HTM of each axis by the MRMT in an ordered manner. The job transformation matrix describes the position of the MRMT tool module relative to the global reference on the work hold module at the bottom of the MRMT. Based on the derivation of the HTM of each axis, the HTM of each axis is represented in Equations 7.12 to 7.16 using the offsets due to module positioning on the MRMT:.

Figure 47: Coordinates References Between Movements

Control Theory

PID Controller

The performance of the control module therefore depends on the following: mechanical and physical modules or axes, the response times of the circuit and the control algorithms. Since the control system controls the position control loop with the slowest response time, the movement and operation of the machine is highly dependent on the performance and response characteristics of the position control loop. The main purpose of the PID controller is to minimize the error between the desired position and the actual machine position.

The output of the PID controller is used to set the duty cycle of the PWM signals required for the motor driver circuit. Therefore, with the first iteration of the PID controller, the output is set to a maximum value and thereafter the output is manipulated according to the PID calculation. From the above it can be seen that it is easy to implement a PID controller, but the performance of the PID controller is crucially dependent on the tuning and setting of the individual gains.

Figure 48: System Architecture Highlighting Control Functionality

Program Interpretation and Validation

Finally, the F-code and S-code are entered, which set the feed voltages for the axes and the spindle speed for the tool head module. The flow chart in Figure 53 illustrates the sequence of events followed for text interpretation and user program validation. Once the user has entered the program and the program is saved, the text interpretation and program validation routine is called.

The text interpretation and validation routine is then terminated and the .dat file is deleted. Depending on the MRMT requirements, a more complex text interpretation may be implemented on the OACS. If necessary, OACS can be modified by simplifying the editing of the text interpretation routine, ensuring that the pointers to input data and output data in the routines follow the conventions of the original text interpretation routine.

Table 12: Reduced NC command Instruction Set

Interpolation

Linear Interpolation
Circular Interpolation

The interpolation routine calculates the required feed rate for each axis by the following equation 7.26, where is the lowest feed rate of all the axes. Central to the interpolation routine calculation is division of the axis displacement by the interpolation time. The interpolation routine runs and calculates the motion requirements for each axis based on these calculations and the results are stored in a First In First Out (FIFO) buffer.

The reference word interpolator for circular interpolation is implemented, and the speed of the MRMT during circular interpolation can be calculated according to the following derivation. The shaft speed is calculated by the interpolation routine and used as a reference input for the position controllers. The following equations can be obtained from Figure 57, using the Pythagorean theorem for right triangles.

Figure 54: (a) 2 Axis Input Parameters (b) Interpolated results

Acceleration and Deceleration Control

The S-shape method was chosen and implemented for the acceleration and deceleration control routine, as the S-shape translates into smoother acceleration and deceleration control of the axis. Furthermore, as shown in an example, the S-Shape method can optionally result in a linear-type response through manipulation of the routine constants. The S-shaped method is best described with reference to Figure 61, which shows a hardware implementation for the algorithm, but can easily be converted to a software calculation.

From Figure 61, the output ∆ is calculated as Equation 7.53, where the multiplication values and the input data shifter are at the iteration.

Figure 60: Input and Output Pulse Train Profiles

Encoder Position Algorithms

Chapter Summary

Open Architecture Control System Software

Overview
Development Environment
User Interface
Set-Up and Configuration
Data Downloading and Module Information
Contoller Selection
User Programming
Motor Control, Debugging and Performance Evaluation
Algorithm Editing and Tuning
Distributed Module Software
Chapter Summary

This is because the host PC does not yet know the physical configuration of the MRMT. Using this list, the host PC then prompts the user to enter the details of the physical configuration of the hardware modules. To evaluate the performance, the user can track the response of the control algorithms by plotting the feedback response.

The programming environments for each of the distributed modules differ depending on the type of microcontroller. We discussed the various OACS features that the user can access and the added value of each feature was highlighted. In addition, a flowchart of the sequence of events that the user is expected to follow is presented, which is closely related to the OACS function.

System Testing and Performance

MRMT Setup
Reconfiguration Times
PC Load
Acceleration and Deceleration Control Example
Accuracy and Repeatability
Vibrations
Power Distribution Network Loading
Response Times
Chapter Summary

OACS reconfiguration and configuration is faster when performed by a user familiar with the system;. A Windows performance monitor was used to test the load of OACS on the PC. To evaluate the performance of the OACS in terms of accuracy and repeatability, a set of test results was required.

The Z-axis accuracy is within acceptable 1 mm for the three of the four worst possible measurements; To evaluate the load on the power distribution network, the current usage and energy efficiency of the OACS was analyzed. Label C shows the responses to the data download messages from each of the distributed modules.

Figure 78: Distributed Module - Arduino UNO (Linear Axis Z)

Discussion

Performance of Electronic Subsystems

Position Control: Accuracy and Repeatability
Axes Speeds and Time to Execute Commands
Power Usage
Vibrations
Distributed Module Response Times

Performance of OACS on PC

Reconfiguration Times
CPU Load

OACS Integration with Mechanical System
OACS and OA standards
OACS on MRM for RMS
Challenges and Limitations for OA Systems
Chapter Summary

This can be achieved with minimal interference to the existing system design which is part of the scalable features of the OACS. This implies that despite the clock frequency, the performance of the distributed module microcontroller is also dependent on the bootloader. The performance results of the multiple microcontroller implementations of the distributed modules on the OACS showed one of the areas of concern for OA systems.

The operating frequencies of distributed modules and microcontroller loaders present an additional challenge for: operation, performance and system integration. Therefore, the performance of the OACS also depends on the mechanical integrity of the mechanical systems. The number of active modules in OACS and MRMT has an impact on the load of the power supply system.

Figure 100 shows the MRMT configured with 2 modules, the Tool module and Z axis, whereas Figure 101 shows a 4 module configuration

Conclusion

The operation of the OACS is also independent of the types of modules on each layer or what is on each layer, provided that the data exchange between the layers is constant. Finally, the number of active modules on OACS and the MRMT affects the load on the power system. Additionally, as the life cycle of the MRMT increases, users can upgrade and optimize modules on the MRMT by replacing that component.

Research has shown that the flexible, reconfigurable, and modular nature of the system in its electronic, control, and software architecture helps the system meet OACS goals. The following sample code shows the discrete-time implementation of the PID control routine called during the axis movement, as well as the function called to update the three gains. 96: // Similarly, the rest of the program can be read and interpreted from the text file.

Figure A102: X Axis with i and i+1 Reference Points Table A23: X Axis Design Data

Sample Code for Generic Servo Module Class