What is technical budgeting?
This page gives an introduction to technical budgeting. We describe what technical budgeting is, why you should apply it in your projects and how it is done.
Technical budgeting is an engineering method on translating critical performance requirements to (sub) element performance requirements explicitly. Technical budgeting is about how to subdivide performance over sub-elements and where to put the safety margins into the design. Iterative top-down and bottom-up approaches are used to converge to a balanced design.
Technical budgets are made for, amongst others, the mass of a device, positioning accuracy, cost-of-goods, mean-time-between-failure, noise.
Why should you use technical budgets?
Actively managing technical budgets will maximize the chance that a device or system will have the expected performance. Technical budgets provide a good decomposition of requirements and insight why a device or system performs. This insight will shorten the development time and reduce the overall design costs.
Benefits of technical budgets are:
- Discover early if requirements will be met (or not)
- Make explicit priority setting (look for the big fishes first…)
- Make assumptions explicit
- Providing robust design by having attention to spread and variation early in the design phase
- Having early on a quantitative underpinning of performance at milestones
- Having an understanding of the critical parameters and sensitivity for small deviations from the specifications
How is technical budgeting done?
Technical budgeting is an iterative process in which refinements to the budgets are made based on progressing insights when the design is getting more detailed (flow down from system to parts) and more evidence is gathered (detailed estimations of and measurements of contributions).
Key area of expertise
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- Customer Objectives, Application, Functional, Conceptual, Realization (CAFCR)
- Requirements engineering & management
- Tech assessments
- Technical budgeting
- Modular design
- Interface definition & management
Typical steps that are taken during technical budgeting are shown below:
Technical budgeting step 1
Determine Critical to Quality parameters (CtQs)
Define a list of CtQ (Critical to Quality) features that will make this device or system stand out in the market. Important features that are (or have become) obvious are not differentiating and thus not a CtQ. Possibly they belong to a design guide.
In order to be useful for Technical Budgeting, the CtQs must be formulated as (or translated into) a numerical requirement.
Examples of such specifications could be:
- Total machine mass
- Machine accuracy
- Mean time between failures (MTBF)
- Cost of goods
- Maximum noise level
- Failure rate during assembly (e.g. tolerance stack)
Technical budgeting step 2
Create (initial) model of the system
A system model has:
- An overall function
- A list of external entities that the system interacts with (the environment). Think of air cooling, power from the wall socket, support from the floor, etc.
- A list of internal (typically 5 – 10) internal subsystems with a functionality description for each subsystem
- For each subsystem a list of interactions with the environment or with other subsystems
Once the top level description is done (typically by the system architect), the internal architecture of each of the sub-elements is done by subsystem architects. This continues recursively down to the part level.
Technical budgeting step 3
A design guide describes the most important design knowledge for a certain aspect. It is typically inspired by common failures of the past. The design guide dictates which external standards are applicable (CE, FDA, MDD) and it can extend these for aspects that routinely come up in the current business.
If a design guide is missing, the design team should make the first draft of this.
Examples of how to calculate a tolerance chain, what vendors to use, what materials are strongly discouraged, etc. can all be part of the design guide. Other elements of the design guide can be design rules such as which small accuracy level can be safely ignored.
The main advantage of a growing design guide is that it concentrates the collected knowledge over the years. It brings the design team at a higher maturity level.
Technical budgeting step 4
Create initial technical budget (top-down)
The budgeting is done by the system architect by looking at each CtQ item and by choosing how each of the sub-elements should contribute to the total budget. The top-level CtQs are the input specifications for the system architect. The distribution of these specifications over the sub-elements set the sub-element requirements.
Each of the sub-element architects does this too (recursively).
The details of the distribution of the specifications depend a lot on the nature of the CtQ item and on how the specification is formulated. However, it should be clear to the architect that the proposed subdivision will result in meeting the CtQ.
Technical budgeting step 5
Estimate/measure contributions (bottom-up)
Once the budget has been allocated, the architects can ask in down-flow direction what the estimated performance will be for each of the budget items. If the budget is tight, some subsystems will meet their budget and others will fail.
The feedback can be based on a back of the envelope estimate, based on a detailed FEM calculation or based on a measurement. This means that the feedback number has an attribute that expresses a level of numeric maturity.
With this information, the architect has feedback on the challenges in the design. The architect can decide to re-adjust the current budget such that the challenges are more fairly distributed. Alternatively, other (less critical) budget items could be relaxed such that more room is created for finding a working solution.
Technical budgeting step 6
Decide if refinement is needed (do the math)
With the collected information and the known way of adding the numbers, the architect has feedback on all the top-level CtQ items in the design.
The architect can report back to the stakeholders what the expected performance level will be and the architect can explain the confidence of the numbers on which the estimate is based.
Also the architect can decide to change the sublevel specs such that a better balance is made between the design efforts in the team and meeting the CtQ specifications; The CtQ items might be competing for the same resources. This informed balancing of the specifications is the core of technical budgeting.
Technical budgeting step 7
Refine/optimize the balance
Care must be taken that architects remain transparent. An architect might be tempted to reserve margins in the distribution of the budgets so that if one of the subsystems fails to meet the specification, that the architect is can use the margin to help out. Of cause, this makes sense from a design stability point of view, but if every architect does this the whole system becomes over specified. So if margins used, it should be done openly and preferably only at one place (e.g. at the top level).
Usually, the CtQ items are in competition with each other. If one is relaxed, it will also be easier to meet the other CtQ items. Around the optimum, for small perturbations, this sensitivity could be modeled as linear. The coefficients of these linear relationships are the sensitivities between the CtQ items. The full list of all sensitivities will help the architects to propose improvements for a next design towards the stakeholders or it can be used to see what the most effective or cheapest way is to bring all the CtQ items in spec.
Examples of technical budgets
Technical budget is used in many sub-domains. The Center of Expertise has examples from actual projects in the following sub-domains:
- Tolerance stack-ups
- Timing budget of moving process/parts in a device
- Positioning accuracy and sensitivity analysis
- (Electrical) power budgets
- Mass budget
- Cost of goods
- Thermal budgets
- Mean time between failures (MTBF) and availability
- Water flow/pressure drop
- Acoustic budget
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