Evrim Berk 030060161 4th Week

1-) Superplastic Forming

Previous Answer

Superplastic materials are thus multi-phase, which promotes pinning of
the grain boundaries during the high-temperature forming process, hence
inhibiting grain growth. Titanium and aluminium alloys have been developed
for industrial superplastic forming and it is also an accepted forming method
for producing the vanes of gas turbine engines using certain nickel-based
superalloys. With further refinement of grain size, superplasticity can be
extended to significantly higher (and hence commercially desirable) strain rates.

John Martin, Materials for engineering, P.55

New Answer

Due to the difficulties associated with forming aluminium, magnesium and other ligth-weight materials, using the standard press-based metal forming practices, the super-plastic forming technology is undergoing extensive research to explore its potential in manufacturing.

The super-plastic forming process is typically done at elevated temperatures, using low force. The main mechanism behind it is based on grain boundaru sliding, so the SPF is typically done on materials with very fine grain size.

Super plastic process offers a large straining capacity during the stretch forming mode achieved with low forming forces and low tool stress due to the low material flow stress during the forming process , typically in the order of 60 kN/m2.

(Omar M.A., The Automotive Body Manufacturing Systems and Processes, Page. 85)

2-) Multivariable Feedback Control Systems

previous one

Hardt in [560] addressed the multivariable feedback control system to control the five output variables: weld geometry variables (width, depth and height) and thermal properties (CR and HZ), for a presentation of a multivariable linear controller designed to regulate the width and throat thickness of filet welds during a GMAW process by simultaneously manipulating torch travel speed, power supply voltage, and wire-feed rate to achieve desired weld geometry. In this work the controller was designed using an empirically-derived linearized model of the welding process operating at a pre-selected operating point and using optimal control theory to ensure reference tracking, disturbance rejection, and robustness.

(Modeling, Sensing and Control of Gas Metal Arc Welding,Desineni Subbaram Naidu, S. Ozcelik, K. Moore D. S. Naidu-1st edition, (2003),P.161)

New One

In a feedback control system, when a single output is controlled by a single input then such systems are called Single-Input, Sıngle-Output (SISO) systems. Systems with more than one controlled output and one controlled input are called Multivariable Feedback Control Systems. In Multivariable Feedback Control Systems, an input that is meant to control a particular output also affects the other controlled outputs. This coupling is called loop interaction.

One of the biggest benefits of the multivariable feedback control technology is the ability to control a process in regions of operational constraints. When encountering these constraints the controller's ability to continuously drive the process toward optimal performance is the basis of this technology.

(Singh N., Process Control: Concept Dynamics and Applications, Pages: 222 - 224)

3-) Relative Gain Array

Previous One

The relative gain array (RGA) technique of process control is applied to the empirical model to design a multi-loop proportional integral controller for the process. The resulting controller pairs wire-feed speed with current and opencircuit voltage with arc voltage to regulate current and arc voltage to desired set points. Using the error between the measured values of current and arc voltage and the desired values of these variables, the controller simultaneously adjusts the wire-feed speed and the opencircuit voltage of the power supply, espectively. The basic benefit that is derived at this stage is the ability to reduce variability in the measured signals combined with the ability to force the measured outputs to their desired values. One of the distinguishing features of much of the work has been the model-based approach to the design of the controllers for the process. Also, the use of the RGA method to select controller loop pairings is unique and offers interesting insight into the best ways to control the process. A quantitative measure of interaction is needed to apply a multiloop controller and the relative gain array (RGA) is a useful technique for determining the appropriate loop pairing.It was found that the correct pairing is that wire feed speed should be used to control the current and the open-circuit voltage should be used to control the arc voltage. Based on these loop pairings, several multiloop controllers were designed.

(Modeling, Sensing and Control of Gas Metal Arc Welding,Desineni Subbaram Naidu, S. Ozcelik, K. Moore D. S. Naidu-1st edition, (2003),P.180,222

New one

One of the most important factors, common to all process control applications, is the correct (best) pairing of the manipulated and controlled variables. A number of quantitative techniques are available to assist in the selection process. One of the earliest methods proposed was the Relative Gain Array (RGA), Bristol (1966). The original technique is based upon the open loop steady state gains of the process and is relatively simple to interpret.

Consider the 2x2 system shown in Figure 2. Suppose mv2 remains constant, then a

step change in mv1 of magnitude Δmv1 will produce a change Δcv1 in output cv1.

Thus, the gain between mv1 and cv1 when mv2 is kept constant is given by:

g11|mv2 = Δcv1/mv1 | mv2

If instead of keeping mv2 constant, cv2 is now kept constant by closing the loop

between cv2 and mv2. A step change in mv1 of magnitude Δmv1 will result in another

change in cv1. The gain in this case is denoted by:

g11|y2 = Δcv1/mv1 | cv2

The gain relationships, equations and may have different values. If

interaction exists, then the change in cv1 due to a change in mv1 for the two cases

when mv2 and cv2 are kept constant, will be different.

The ratio: λ11 = g11|mv2 / g11|cv2 is a dimensionless value and it defines the relative gain between the output cv1 and the input mv1.

Interpretation of the relative gain

1. λij = 1. There is no interaction with other control loops.

2. λij = 0. Manipulated input, i, has no affect on output, j.

3. λij = 0.5. There is a high degree of interaction. The other control loops have the

same effect on the output, j, as the manipulated input, i.

4. 0.5 < λij < 1. There is interaction between the control loops. However, this would

be the preferable pairing as it would minimise interactions.

5. λij > 1. The interaction reduces the effect gain of the control loop. Higher

controller gains are required.

6. λij > 10. The pairing of variables with large RGA elements is undesirable. It can

indicate a system sensitive to small variations in gain and possible problems

applying model based control techniques.

7. λij < 0. Care must be taken with negative RGA elements. Negative off-diagonal

elements indicate that closing the loop will change the sign of the effective gain.

More importantly, negative diagonal elements can indicate ‘integral instability’ i.e.

the control loop is unstable for any feedback controller.

(Willis M.J., Multivariable Control: An Introduction, Pages: 9 - 11)

4-) Weld Pool Oscillation

Previous One

Weld pool oscillations are caused by high frequency external forces on the weld pool. It was first suggested that the ripple formation in solidified welds is explained by the oscillatory behavior of the weld pool. It is worth noting that the weld pool oscillation frequency will be influenced by the droplet frequency. Weld pool oscillations can also be induced by current pulsing and monitored using optical sensing. This approach is applied for the GTAW process in. In particular, the oscillations are induced by a phase-locked loop (PLL) which consists of a phase detector, low-pass filter, and oscillator

(Modeling, Sensing and Control of Gas Metal Arc Welding,Desineni Subbaram Naidu, S. Ozcelik, K. Moore D. S. Naidu-1st edition, (2003),P.110)

New One

Weld Pool Oscillation could be triggered in a number of ways. For instance: by mechanical vibrations, by the impact of droplets entering the weld pool, by plasma arc force, by gas bubbling, and by sudden changes in arc current.

The concept of using weld pool motion as a pool geometry sensing method was proposed by Hardt and later demonstrated by Zacksenhouse, Richardson, Renwick and Sorensen. The concept is based on the fluid dynamics of a pool constrained by a solid container and by significant surface tension forces. Such a pool will exhibit a surface motion that is function of external forces, the properties of the fluid, the surface tension, and the shape of the container. Thus, if this motion can be excited, measured and related to the pool geometry, a meaning of sensing pool shape will exist.

(Chen X., Devanathan R., Fong M.A., Advanced Automation Techniques in Adaptive Material Processing, Page: 169)

5-) Preventive Maintenance Costs

Previous One

1- Computers & Instrumentation

2- Software

3- Staff

Justifying Predictive Maintenance, Nichol R., p.4

New One

Preventive maintenance can be defined as follows: Actions performed on a time- or machine-run-based schedule that detect, preclude, or mitigate degradation of a component or system with the aim of sustaining or extending its useful life through controlling degradation to an acceptable level.

The U.S. Navy pioneered preventive maintenance as a means to increase the reliability of their vessels. By simply

expending the necessary resources to conduct maintenance activities intended by the equipment designer, equipment life is extended and its reliability is increased. In addition to an increase in reliability, dollars are saved over that of a program just using reactive maintenance. Studies indicate that this savings can amount to as much as 12% to 18% on the average. Depending on the facilities current maintenance practices, present equipment reliability, and facility downtime, there is little doubt that many facilities purely reliant on reactive maintenance could save much more than 18% by instituting a proper preventive maintenance program.

While preventive maintenance is not the optimum maintenance program, it does have several advantages over that of a purely reactive program. By performing the preventive maintenance as the equipment designer envisioned, we will extend the life of the equipment closer to design. This translates into dollar savings. Preventive maintenance (lubrication, filter change, etc.) will generally run the equipment more efficiently resulting in dollar savings. While we will not prevent equipment catastrophic failures, we will decrease the number of failures. Minimizing failures translate into maintenance and capital cost savings.

Advantages

• Cost effective in many capital-intensive processes.

• Flexibility allows for the adjustment of maintenance periodicity.

• Increased component life cycle.

• Energy savings.

• Reduced equipment or process failure.

• Estimated 12% to 18% cost savings over reactive maintenance program.

Disadvantages

• Catastrophic failures still likely to occur.

• Labor intensive.

• Includes performance of unneeded maintenance.

• Potential for incidental damage to components in conducting unneeded maintenance.

( U.S. Department of Energy, Operations & Maintenance Best Practices: A Guide to Achieving Operational Efficiency, Release 3.0, 5.3)