Introduction
- Manufacturing Challenges
- Motivation for the Study
- Project Objectives
- Scientific Contribution of the Dissertation
- Research Publications
- Outline of Dissertation
- Chapter Summary
According to the kinematic modeling method established in Section 5.5.2, the reference frames placed on the mating planes must coincide perfectly. Customization The functionality of the mechanical system can be customized by offering only the necessary modules on a platform.
Manufacturing Systems
A Review of Dedicated Manufacturing Systems
These systems are designed for the production of a single product or a limited group of products. Although DMSs are designed for high production capacity, these systems do not always operate at maximum capacity.
A Review of Flexible Manufacturing Systems
The dedicated nature of these systems means less capital investment in system initialization compared to more flexible systems. These systems use transmission line technology, dedicated machines and fixed automation for optimized, robust system performance.
The Reconfigurable Manufacturing Paradigm
Market: At the market level, lean manufacturing aims to remove product features that are unnecessary or unwanted by customers. Company: At the company level, the purpose of lean manufacturing is to reduce the amount of waste due to overproduction and holding too much inventory.
Customized Flexibility in RMSs
Scalable Production Capacity in RMSs
Reconfigurability and Expansion Flexibility in RMSs
Manufacturing System Lifecycle
Essential Characteristics of RMSs
Enabling Technologies for RMSs
Chapter Summary
Automated Production Machines
- Dedicated Machine Tools
- Computer Numerically Controlled Machines
- Reconfigurable Machine Tools: The Arch Type RMT
- Reconfigurable Machine Tools: Design Methods
- Virtual Modularity in RMT Design
- Kinematic Optimization in RMTs
- Software Aids in Virtual RMT Synthesis
- Open Architecture Control
- Chapter Summary
The software architecture of the machines is fixed, but the machine can be programmed by the end user [35]. This philosophy was applied to the creation of the world's first Reconfigurable Machine Tool (RMT), the Arch Type RMT (see Figure 3.2.a). The purpose of creating the Arch Type RMT was to illustrate the principle of constructing machine tools around part families. .
Modular Reconfigurable Machines
- Design Approach
- Design Concept
- Design Perspective
- Adopted Principles for MRM Design
- Conceptualization of MRMs: Virtual Mock-Up
- Digital Electronic Control
- Design Specifications
- Chapter Summary
MRMs are geared toward custom flexibility, as in the case of the Arch Type RMT;. As the machine architecture changes, software modules are designed to be added or removed from the host control system.
Mechanical Design and Modelling
Mechanical Modules
Mechanical Integrity and Design Objectives
Generic Architecture of MRM Modules
Mechanical Interfacing
- Bolted Interfaces
- Interface Failure
Joint separation is the separation of two bolted interface plates under the action of an external tensile force Fe. E is the modulus of elasticity of the material, At is the tensile stress range, and L is the length of either the bolt or the interface.
MRM Motion Modules (Modular Axes)
- Modular Degrees of Freedom
- Generalized Kinematic Modelling
- Axis Drive Systems
- Linear Axes
- Rotary Axes
As an alternative to the D-H method, the following sets of guidelines have been developed for attaching local reference frames to modules:. i) The placement of the local coordinate system on an interface must be such that it will coincide with the coordinate system of an adjacent interface once assembled. ii) The drive axis must be labeled according to the motion provided by the modules with respect to the global reference frame. The drive mechanism of the module consisted of the upper interface plate which is moved by a loose arm as illustrated in Figure 5.22.
MRM Process Modules (Modular Cutting Heads)
MRM Accessory Modules
Increased torques can lead to deflections in the machine structure, thereby increasing the first and second order errors in the geometric positioning of the tool. These accessories, which can be considered as commercially available modules, were used in the turning configuration for supporting long workpieces during machining. The incorporation of COTS hardware into the MRM platform highlights the potential for expanding the accessory category in the MRM library.
To facilitate the integration of COTS accessories due consideration must be given to ensure that interface elements conform to current standards in the machine building industry.
Mechanical Assembly and Reconfiguration
- Assembling a Kinematically Viable Machine Tool (An Example)
- Machine Reconfiguration
- Module Combinations
A comparison between the duty matrix (equation 5.31) and the machine transformation matrix (equation 5.32) shows a strong match in the rotational component of the HTM. An example of this is the MRM configuration illustrated in Figure 5.33, which replaces the tilt table module with the rotary cutter head module to enable drilling on the inclined surface of the sample part. In this case the MRM cutting head can be changed to enable a new process, as illustrated in the concept of Figure 4.4.
If the kinematic structure of the machine tool does not support the change of cutting heads, the MRM structure can be decomposed into modular elements.
Cutting Conditions
- Cutting Conditions in Turning
- Cutting Conditions in Drilling
The material removal rate RMR (mm3/min) for a turning operation is calculated by equation 5.36, where V is the tangential velocity (m/s) of the surface being machined (original surface). The tangential velocity of the original surface is calculated by equation 5.37 where Do is the diameter (m) of this surface as illustrated in Figure 5.36. Based on the programmed feed rate fr (mm/min) and the thickness of the work material t (mm), the time required to machine a through hole is calculated by equation 5.38.
The approach allowance is calculated by equation 5.39, where D is the drill diameter (mm) and θ is the drill tip angle in degrees.
Forces and Torques in Machining
- Forces in Turning
- Forces in Drilling
- Forces and Torque Propagation
Mechanical Error Modelling
- First Order Errors
- Second Order Errors
The second step in modeling the accumulated assembly error is the relationship between the module-wise assembly errors and the reference frames placed at the interfaces of adjacent modules. Consideration of assembly errors is essential during machine calibration, as these errors may be compensated for by the machine controller as opposed to reassembling the machine tool. These errors contribute to the total first-order error and cause geometric inaccuracies in the machined parts.
Bends can cause or contribute to: (i) linear displacement in reference frames, or (ii) rotation of one reference frame relative to another.
Mechanical Analysis of Drilling Subassembly
By manipulating Hooke's law, we can achieve the deflection of the drill head under the action of static force. Drill head deflection is shown in Figures 5.42 and 5.43 for static vertical and horizontal forces. A simulation was performed to investigate the performance of the drill sub-assembly under a force of 50 N at 20 Hz.
This is due to the excitation frequency being close to the natural frequency of the system.
Chapter Summary
MRM Electronic System
- Electronic System: Design Considerations
- MRM Electronic Control System
- Spindle Control Modules
- The Servo Communication Module
- Servo Control Modules
- Sensors
- Process Modules
- Motion Modules
- The AC Power Box
- Power Supply System
- System Integration and Reconfiguration
- Chapter Summary
The ATmega 32L microcontroller performs the functions of receiving servo instructions, initializing the appropriate servo control routine, and finally reporting the status of the operation to the host computer (via the servo communication module). The interrupt software routine terminates the servo control routine and the operation of the motor. The ADXL 204 interfaces with the servo control module via two of the eight ADC channels present on the ATmega 32L chip (see Section 6.3).
In this case, additional servo communication modules can be added in parallel with the first one by inserting them into the USB ports of the host computer.
MRM Software System
Software Design Considerations
Machine Operating System and Development Environment
Software Reference Architecture
MRM Graphic User Interface
User Programming
Reconfiguration of the NC Command Set
Reconfiguration of Driver Module Addresses
NC Interpretation and Validation
Execution of User Programs
The progress is indicated as a percentage of the number of code blocks executed relative to the total number of blocks in the user's program. The status box is also updated with information for the machine operator, related to the current state of the machining process. When all the code blocks in the program have been executed, the software returns control to the operating system until the user activates another function on the user interface.
Key Software Subroutines Load Program Routine Function Selection Routine Status Update Routine M Word Control Routines G Word Control Routines.
M Word Software Routines
G Word Software Routines
These instructions require the execution of a suitable interpolation algorithm to divide the required travel distance into small linear segments through which a position control algorithm is applied. The smaller segmented target distances are transmitted to the servo control modules for execution after the acceleration and deceleration control is applied.
Interpolation
𝑋 = 𝑥 𝑖 + 1 − 𝑥 𝑖 (7.7) ∆𝑌 = 𝑦 𝑖 + 1 − 𝑦 𝑖 The calculation of the appropriate linear increments is dependent on the current position of a machine tool cycle after each interpolation tool cycle. This is given by equation 7.8 where R (i) is the radius of the interpolated arc from the previous interpolation cycle. In the Improved Tustin method, α is calculated by equation 7.11 and the number of required iterations by equation 7.12.
4 𝑅 (7.12) The required feed rates for individual axes to achieve synchronized circular motion are given by equation 7.13:.
After Interpolation Acc/Dec Control
The sum of the final j increments is used to calculate a declaration constant D that provides the linear decrement in the reference position. Note that deceleration control is only applied from interpolation cycle time numbers (kfinal – j+1) to (kfinal+n), where kfinal+n is the new number of interpolation cycles required to complete the axes movement. After the Acc/Dec check has been performed, control passes to the serial communication software routine.
This software routine then transmits the axial increments stored in FIFO.dat as part of the instruction set to the servo control modules.
Control Protocols and Data Communication
Servo Communication Module: Software Routines
After all instructions have been transferred, the servo communication module issues a general call on the I2C bus. After the general call has been issued, the servo communication module goes into the feedback receiving mode. In feedback receiving mode, the servo communication module receives feedback from the servo control modules.
The individual servo control modules transmit their feedback data packets to the communication module via the I2C bus.
Servo Control Modules: Software Routines
The transfer function of the PID controller is defined by equation 7.20, where Kp, Ki and Kd are the gains of the controller (coarse-tuned by the Ziegler-Nichols method and fine-tuned by trial). The Input Capture Register (ICR) was used to set the TOP value, which defines the frequency of the PWM signal. The frequency of the PWM is calculated by equation 7.39, where N is a pre-scalar and fclk_I/O is the operating frequency of the chip [67].
The Output Compare Register (OCR1A) was used to set the duty cycle of the PWM according to equation 7.40.
Spindle Control Modules: Software Routines
Chapter Summary
System Assembly and Performance
- System Assembly and Reconfiguration – An Overview
- Pairing of Mechanical Modules and Control Modules
- Machine Calibration
- System performance Criteria and Errors
- Control Performance: Interpolated Motion
- Control Performance: Rapid Point to Point Motion Control
- Control Performance: Synchronized Motion
- Accuracy and Repeatability of Axes
- Chapter Summary
Both the rotary and tilting table modules of the cutting head showed a "slip-stick" effect in the frictional response of the axis. These errors are relatively small compared to the errors that occur during axis movement. The maximum resulting drill spindle position error was 0.429 mm, and the trajectory was completed with an average position error of 0.069 mm.
The maximum position error Ep is the final axis position error as measured by the control system.
Discussion
- Performance Summary: Mechanical Systems
- Performance Summary: Positioning Systems
- Comparative Analysis of the Properties of MRMs
- MRMs in Reconfigurable Manufacturing Systems
- Reconfigurable Functionality in MRMs
- Initial Capital Investment in Hardware
- Scalable System Capacity
- High Product Variety and Product Customization
- Expansion Flexibility and System Life Span
- MRMs and the Five Essential Characteristics of RMSs
- Problems Associated with MRMs
- Geometric Proportions of Machine Slides
- Mass Distribution in MRM Structures
- Module Weights, Stiffness and Actuation Power
- Power Supply Systems
- Problems Associated with Reconfigurations
- Chapter Summary
A test was performed for the synchronized movement of the base module and the sliding worktable module in a 3 DOF drilling configuration. System performance can therefore be improved by adding a fine moving average interpolator to the software system. Reconfigurability of this nature has three important consequences:. i) The flexibility of individual machines can be extended to enable RMSs to produce different families of parts with minimal investment in additional hardware. ii) MRMs need only have the exact level of functionality required to complete the operation, any redundant modules can be removed and distributed to other machines in the system. iii).
As the system's product portfolio evolves, the machines in the system can be expanded with additional modules.
Conclusion
For preemptive collision detections on the host PC side, a more extensive reconfiguration of the software system is required. This is a fundamental problem and modules may need to be designed with generally high motors and high stiffness; which will ultimately reduce the financial viability of the technology. The final issue identified is the complexity of the reconfiguration process for MRMs.
MRMs must be taken offline from a system that needs to be reconfigured and the profit losses associated with machine downtime further reduce the economic viability and utility of the technology.
Module Interfaces
Library of Modules
Mechanical Calculations
MRM Assemblies
Electronic Control Circuits and Protocols
Test Results
Mechanical Drawings (Sample Only)
Final Specifications for MRM Modules and Assemblies
C++ Code for Host PC (Sample Only)
