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Value-driven design

From Wikipedia, the free encyclopedia

Value-driven design (VDD) is a systems engineering strategy based on microeconomics which enables multidisciplinary design optimization. Value-driven design is being developed by the American Institute of Aeronautics and Astronautics, through a program committee of government, industry and academic representatives.[1] In parallel, the U.S. Defense Advanced Research Projects Agency has promulgated an identical strategy, calling it value-centric design, on the F6 Program. At this point, the terms value-driven design and value-centric design are interchangeable. The essence of these strategies is that design choices are made to maximize system value rather than to meet performance requirements.

This is also similar to the value-driven approach of agile software development where a project's stakeholders prioritise their high-level needs (or system features) based on the perceived business value each would deliver.[2]

Value-driven design is controversial because performance requirements are a central element of systems engineering.[3] However, value-driven design supporters claim that it can improve the development of large aerospace systems by reducing or eliminating cost overruns[4] which are a major problem, according to independent auditors.[5]

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  • Rec 11 | MIT 6.01SC Introduction to Electrical Engineering and Computer Science I, Spring 2011
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  • ASCE Structural Engineering Institute ASCE 7-16 Presentation | March 5, 2019
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Transcription

PROFESSOR: Hi. Last time we talked about NVCC method and how to reduce the number of equations we had to deal with to solve a particular circuit. At this point, we're pretty well equipped to solve circuits in the general sense, but we really haven't talked about how to use that information or possibly use circuits in a particular way. Before we jump into both that and abstraction of circuits, we need to talk about op-amps. Op-amps is short for Operational Amplifier. And it's a tool that we can use in order to sample particular voltages from a subsection of the circuit without affecting it. Another thing we can use op-amps to do is modify our signal. Or if we're going to sample a voltage from a particular subsection of the circuit, we can then do stuff to that voltage without affecting the circuit, all within the op-amp or within the op-amp's special subset of circuitry. So first of all what is an operational amplifier? Well, an operational amplifier is a giant web of transistors. But what an operational amplifier does is act as a voltage-dependent voltage source. It can effectively sample voltages from an existing circuit and then use them to power some other object, for instance a light bulb. If you set up this kind of circuit, you will not actually be powering this light bulb with 5 Volts, because the light bulb itself acts as a resistor. And so the voltage drop across this part of the circuit is going to be different from just 5 Volts. If you want to enable a voltage drop of 5 Volts across this light bulb, then you have to stick up an op-amp. You have to use an op-amp to sample the voltage drop at this component and put it in between the light bulb and the rest of the circuit. When you see an op-amp on a schematic diagram, it'll frequently look like this. You'll have a positive input voltage, a negative input voltage, power rails, which are actually the thing that determine the range of expressivity that the op-amp has, and an output voltage. In reality, the relationship between the output voltage and then input voltages is something like this, where K is a very large number. The effect that this has is that Vout is going to be whatever Vout needs to be, such that Vplus is equal to Vminus. That's the basic rule you want to use when you're interacting with op-amps. So in this case, if we wanted to power this light bulb with 5 Volts, we would do something like this. Excuse the sloppiness of the second diagram. We still have our 10 Volt voltage source. We still have our voltage divider. This point samples 5 Volts from this sub-circuit -- and isolates this part of the circuit from the light bulb. Vout has to be whatever value is necessary such that this sample point and this sample point are equal. Since this value is 5 Volts, this value will also be driven to 5 Volts by the op-amp, which means that this value is 5 Volts. And we've successfully managed to power a light bulb with 5 Volts. The other thing you might be asked to do is to take an existing schematic, an existing circuit diagram and figure out what the operational amplifier does to a given signal or possibly what Vout is or possibly what Vout is in terms of the input signal. So let's practice using this diagram. Here's what we're after. I'm going to figure out where Vplus is going to be. This is another voltage divider. I'm now interested in Vminus, in terms of Vout, which is another voltage divider. I can set these two equations equal to one another and solve for Vout. I found a new expression for Vout in the particular case where V is 10 Volts. If my input voltage were previously unspecified, or if this voltage source were not specified or just Vin, then I would be after this expression. Some things I'd like to mention, while we're talking about op-amps, all the operational amplifiers we've been working with so far deal with Vout in terms of Vin, where Vin is driven through the positive terminal, and the negative terminal is typically connected to ground. You can do the opposite and end up with some interesting effects. But it comes at a cost. It is entirely possible that you will end up driving your op-amp to an unstable equilibrium. What you need to look at is this relationship. There may be a particular point, in which case your system is stable. But if you get any sort of minor perturbations, you'll actually end up with divergence. If this is the case, then you'll probably burn out your op-amp. You can do this by hooking it up in this way. This is expensive and could possibly burn you. The other thing to note is that the power rails on your op-amp limit its range of expressivity. And I think I've said this before, but it's worth mentioning again. If your op-amp is only powered by 10 Volts, it cannot amplify your input signal to a final value greater than 10 Volts. Likewise, if your input value is a negative voltage, and you're working with a non-inverting amplifier, if your ground is truly ground or if your ground is higher relative than your input voltage, you cannot actually express a negative voltage. The third thing I'd like to quickly mention is that there are some terms associated with op-amps that you might hear used by the staff or online, that sort of thing. A buffer and a voltage follower are the same thing. And that's explicitly when you want to sample a signal or you want to sample a particular voltage, and you don't want to multiply it or add it to something or do any kind of LTI operations that we might be able to do using op-amps in this course. You can work with amplifiers. And the thing we worked with earlier was an amplifier for a value less than 1. You can also use op-amps to some signals. And if you look for a voltage summer amplifier on the internet, you should be able to find some information. In any case, op-amps are really powerful. They allow us to both isolate a particular section of a circuit and sample a particular voltage value from that circuit without affecting that circuit, and also allow us to modify that particular voltage value before using it in another part of our overall circuit. Therefore, we're enabled to design more complicated and powerful things. Next time, I'll talk about superposition and Thevenin Norton equivalence, which will further enable modularity and abstraction in our circuit design.

Concept

Value-driven design creates an environment that enables and encourages design optimization by providing designers with an objective function and eliminating those constraints which have been expressed as performance requirements. The objective function inputs all the important attributes of the system being designed, and outputs a score. The higher the score, the better the design.[6] Describing an early version of what is now called value-driven design, George Hazelrigg said, "The purpose of this framework is to enable the assessment of a value for every design option so that options can be rationally compared and a choice taken."[7] At the whole system level, the objective function which performs this assessment of value is called a "value model."[8] The value model distinguishes value-driven design from Multi-Attribute Utility Theory applied to design.[9] Whereas in Multi-Attribute Utility Theory, an objective function is constructed from stakeholder assessments,[10] value-driven design employs economic analysis to build a value model.[11] The basis for the value model is often an expression of profit for a business, but economic value models have also been developed for other organizations, such as government.[8]

To design a system, engineers first take system attributes that would traditionally be assigned performance requirements, like the range and fuel consumption of an aircraft, and build a system value model that uses all these attributes as inputs. Next, the conceptual design is optimized to maximize the output of the value model. Then, when the system is decomposed into components, an objective function for each component is derived from the system value model through a sensitivity analysis.[6]

A workshop exercise implementing value-driven design for a GPS satellite was conducted in 2006, and may serve as an example of the process.[12]

History

The dichotomy between designing to performance requirements versus objective functions was raised by Herbert Simon in an essay called "The Science of Design" in 1969.[13] Simon played both sides, saying that, ideally, engineered systems should be optimized according to an objective function, but realistically this is often too hard, so that attributes would need to be satisficed, which amounted to setting performance requirements. But he included optimization techniques in his recommended curriculum for engineers, and endorsed "utility theory and statistical decision theory as a logical framework for rational choice among given alternatives".

Utility theory was given most of its current mathematical formulation by von Neumann and Morgenstern,[14] but it was the economist Kenneth Arrow who proved the Expected Utility Theorem most broadly, which says in essence that, given a choice among a set of alternatives, one should choose the alternative that provides the greatest probabilistic expectation of utility, where utility is value adjusted for risk aversion.[15]

Ralph Keeney and Howard Raiffa extended utility theory in support of decision making,[10] and Keeney developed the idea of a value model to encapsulate the calculation of utility.[16] Keeney and Raiffa also used "attributes" to describe the inputs to an evaluation process or value model.

George Hazelrigg put engineering design, business plan analysis, and decision theory together for the first time in a framework in a paper written in 1995, which was published in 1998.[7] Meanwhile, Paul Collopy independently developed a similar framework in 1997, and Harry Cook developed the S-Model for incorporating product price and demand into a profit-based objective function for design decisions.[17]

The MIT Engineering Systems Division produced a series of papers from 2000 on, many co-authored by Daniel Hastings, in which many utility formulations were used to address various forms of uncertainty in making engineering design decisions. Saleh et al.[18] is a good example of this work.

The term value-driven design was coined by James Sturges at Lockheed Martin while he was organizing a workshop that would become the Value-Driven Design Program Committee at the American Institute of Aeronautics and Astronautics (AIAA) in 2006.[19] Meanwhile, value centric design was coined independently by Owen Brown and Paul Eremenko of DARPA in the Phase 1 Broad Agency Announcement for the DARPA F6 satellite design program in 2007.[20] Castagne et al.[21] provides an example where value-driven design was used to design fuselage panels for a regional jet.

Value-based acquisition

Implementation of value-driven design on large government systems, such as NASA or European Space Agency spacecraft or weapon systems, will require a government acquisition system that directs or incentivizes the contractor to employ a value model.[22] Such a system is proposed in some detail in an essay by Michael Lippitz, Sean O'Keefe, and John White.[23] They suggest that "A program office can offer a contract in which price is a function of value", where the function is derived from a value model. The price function is structured so that, in optimizing the product design in accordance with the value model, the contractor will maximize its own profit. They call this system Value Based Acquisition.

See also

References

  1. ^ "AIAA Program Committees". Retrieved 2009-05-24.
  2. ^ Sliger, Michele; Broderick, Stacia (2008). The Software Project Manager's Bridge to Agility. Addison-Wesley. p. 46. ISBN 978-0-321-50275-9.
  3. ^ Kapurch, Stephen J.; et al. (2007). NASA Systems Engineering Handbook (PDF) (Rev 1 ed.). National Aeronautics and Space Administration. p. 43. Archived from the original (PDF) on 2012-07-09. Retrieved 2009-05-24.
  4. ^ "Value-Driven Design Aerospace America". American Institute of Aeronautics and Astronautics, Reston, VA. December 2008. p. 109. Retrieved 2009-05-25.
  5. ^ Mullins, Brian (March 31, 2008). Defense Acquisitions: Assessments of Selected Weapon Programs (Report). US Government Accountability Office. Retrieved 2009-05-24.
  6. ^ a b Collopy, Paul (2001). "Economic-Based Distributed Optimal Design" (PDF). American Institute of Aeronautics and Astronautics, Reston, VA. Retrieved 2009-05-24.
  7. ^ a b Hazelrigg, G. A. (1998). "A Framework for Decision-Based Engineering Design". Journal of Mechanical Design. 120 (4): 653–656. doi:10.1115/1.2829328.
  8. ^ a b Collopy, Paul; Horton, Randy (2002). "Value Modeling for Technology Evaluation" (PDF). American Institute of Aeronautics and Astronautics. Retrieved 2009-05-25.
  9. ^ Thurston, D. L. (1990). "Multiattribute utility analysis in design management". IEEE Transactions on Engineering Management. 37 (4): 296–301. doi:10.1109/17.62329.
  10. ^ a b Keeney, Ralph L.; Raiffa, Howard (1976). Decisions with Multiple Objectives: Preferences and Value Tradeoffs. John Wiley & Sons, New York. p. 96. ISBN 978-0-521-43883-4. Retrieved 2009-05-25.
  11. ^ Collopy, Paul (1997). Surplus Value in Propulsion System Design Optimization (PDF). American Institute of Aeronautics and Astronautics, Reston VA. Retrieved 2009-05-25.
  12. ^ Collopy, Paul (2006). "Value-Driven Design and the Global Positioning System" (PDF). American Institute of Aeronautics and Astronautics, Reston, VA. Retrieved 2009-05-24.
  13. ^ Simon, Herbert A. (1969). "3". The Sciences of the Artificial: The Science of Design. The MIT Press, Cambridge MA. ASIN B000UDMTJM.
  14. ^ von Neumann, John; Morganstern, Oskar (1947). Theory of Games and Economic Behavior. Princeton University Press, Princeton NJ. pp. 17–31. ISBN 0-691-00362-9. Retrieved 2009-05-25.
  15. ^ Arrow, Kenneth J. (1971). "2". Essays in the Theory of Risk Bearing, Exposition of the Theory of Choice under Uncertainty. Markham Publishing, Chicago. ISBN 978-0-444-10693-3. Retrieved 2009-05-25.
  16. ^ Keeney, Ralph L. (1992). "5". Value-Focused Thinking: A Path to Creative Decisionmaking, Quantifying Objectives with a Value Model. Harvard University Press, Cambridge MA. ISBN 978-0-674-93198-5. Retrieved 2009-05-25.
  17. ^ Cook, Harry E. (1997). Product Management: Value, Quality, Cost, Price, Profit and Organization. Chapman & Hall, London. ISBN 0-412-79940-5.
  18. ^ Saleh, Joseph H. (March 2003). "Flexibility and the Value of On-Orbit Servicing: New Customer-Centric Perspective". Journal of Spacecraft and Rockets. 40 (2): 279–291. Bibcode:2003JSpRo..40..279S. doi:10.2514/2.3944. Archived from the original on 2009-06-06. Retrieved 2009-05-25.
  19. ^ "Value-Driven Design VDD". Archived from the original on 2011-07-28. Retrieved 2009-05-26.
  20. ^ http://webext2.darpa.mil/tto/solicit/BAA07-31/F6_BAA_Final_07-16-07.doc[dead link]
  21. ^ Castagne, S.; Curran, R.; Collopy, P. (2009). "Implementation of value-driven optimisation for the design of aircraft fuselage panels". International Journal of Production Economics. 117 (2): 381–388. doi:10.1016/j.ijpe.2008.12.005.
  22. ^ Brown, Owen; Eremenko, Paul (2008). "Application of Value-Centric Design to Space Architectures: The Case of Fractionated Spacecraft" (PDF). American Institute of Aeronautics and Astronautics. pp. 29–31. Retrieved 2009-05-24.
  23. ^ Carter, Ashton B.; White, John P. (2000). "7". Keeping the Edge: Managing Defense for the Future. The MIT Press, Cambridge, Massachusetts. pp. 194–202. ISBN 0-262-03290-2. Retrieved 2009-05-24.
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