Describing the Desired Outcome of a Project

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Identifying and Prioritzing Project Parameters

Prioritiztion

In order to achieve a workable system, some aspects of a design will usually be modified in order that more important aspects can be implemented. Trade-offs are based on identifying and prioritizing as many useful parameters as possible. Whether a design variable is a dependent variable or an independent variable depends on its priority and context. Of course it is possible to describe combinations of parameters and priorities that cannot exist!

 Two classes of parameters are at work. Material parameters are physical descriptions of the raw materials at ones disposal. The physical parameters of system components define what will work and what will not work in a limited application context. Project parameters are a list of desired outcomes.

 Solar work starts with three sets of material parameters. They belong to light sources, loads, and battery systems. I have evaluated several lighting conditions which are enumerated in a separate document. The load characterizations and the battery system evaluations are in progress. Any new design that comes out of this work will result from combining elements from the material parameter evaluations.

 Project Parameters (P) are prioritized from 0 to 5. In many cases, the operational value of a parameter will change as a result of modifying other elements of the system. Here is the scale.

Identifying and Prioritzing Project Parameters

Prioritiztion

In order to achieve a workable system, some aspects of a design will usually be modified in order that more important aspects can be implemented. Trade-offs are based on identifying and prioritizing as many useful parameters as possible. Whether a design variable is a dependent variable or an independent variable depends on its priority and context. Of course it is possible to describe combinations of parameters and priorities that cannot exist!

 Two classes of parameters are at work. Material parameters are physical descriptions of the raw materials at ones disposal. The physical parameters of system components define what will work and what will not work in a limited application context. Project parameters are a list of desired outcomes.

 Solar work starts with three sets of material parameters. They belong to light sources, loads, and battery systems. I have evaluated several lighting conditions which are enumerated in a separate document. The load characterizations and the battery system evaluations are in progress. Any new design that comes out of this work will result from combining elements from the material parameter evaluations.

 Project Parameters (P) are prioritized from 0 to 5. In many cases, the operational value of a parameter will change as a result of modifying other elements of the system. Here is the scale.

Priority rank (low to high)

Description

0

Don't Care, Not Applicable, or is fully controlled by another parameter

1

P is potentially affected by many factors, but itself does not have a bearing on overall quality. Max and min limits can be exceeded within reason.

2

P has significant influence on the system but is potentially affected by many factors. Max and min values span a wide range of acceptable values, and the limits are negotiable.

3

P is has a working range of values with hard limits.

4

P is critical, but can be slightly adjusted to avoid conflict with P. 5

5

P is critical and should not be adjusted


An example test-case project suggested by Shawn Decker:

Consider an Arduino microcontroller programmed to produce a set of autonomous behaviors in a sample group of simple electrical devices. For this example, a load consisting of low power devices capable of producing effects of movement, light, and sound will be used with a minimum of external components [3 LED's (3 x 10mA) + 1 pager motor (50mA) + 1 MCU (25mA)]. The power source for the system will be a solar cell and storage battery combination and may make use of ancillary circuits to help integrate the solar source and the battery. Other ancillary circuits may be used to help connect the storage battery to the load.

 As the MCU is controlling the behavior of its connected loads, it is also monitoring its own power supply in order to discern the state of charge and amount of available power at its disposal. The monitoring function will be aided by external circuitry. When the power available to the system drops below a set threshold, the microcontroller suspends its outputs and enters a charge mode. At this point, other actions may be taken for whatever functional or aesthetic purpose might connect with an immanent loss of power. While the storage battery is charging, the microcontroller switches to a low-power mode where it continues to communicate periodically with its battery monitor circuit. When the battery monitor function detects sufficient charge accumulation in the battery, a signal causes the microcontroller to wake up and return to its normal behavior.

 Below are two groups of project parameters which should, once they are certified, serve as a set of experimental goals. One key component of the system is missing in the parameter list. The battery “gas gauge” circuit (priority level 5) is a big unknown. There are many ways to get this functionality, but the conditions in this solar circuit are unusual in some ways. There exist a number of battery monitor IC's (they really are called “gas gauge” chips), and a number of charge control and battery monitoring circuits, both integrated and discrete. One task in getting this system up and running will be to find or invent, and then implement, a good gas gauge.

 Here, then is a description of the test device which is currently in design. I pulled several of the numbers out of the air. If anyone wants to change them, let me know soon.




Power supply characterization:







Parameter

Value (if numeric)

max-min

Priority

 

Description

 

Available light intensity

10-80 ft-candles

5

Uses unmodified classroom lighting

Duration of time when light is available for charging

24 hr

4

Allow long charging time

(1) run on mix of battery and solar, (2) solar charge then run on battery, (3) solar only

1 or 2

0

 

Maximum time from discharged to rated charge

5hr (aesthetic choice)

THIS IS THE HARD ONE, #1 of 2

2

Discharged means it's time to charge. A dead battery is a different matter.

Minimum available discharge time at average load

5-10 min (aesthetic choice)

THIS IS THE HARD ONE, #2 of 2

 

3

 

Maximum dimensions - length and width of the solar panel(s)

4” x 8” (aesthetic choice)

1

 

Voltage at the load (a max-min range if not regulated)

3.3V or 5V

3

 

Minimum milliamp-hours available to the final load from one charge cycle

8.3

0

 

Minimum milliamp-hours available at the battery from one charge cycle – includes losses from step-up conversion

?

 

 

Type of solar cell to storage battery conversion: (1) None. Cell voltage and battery voltage are matched; (2) boost regulator

2

0

Try garden light boost regulators in series


 




Load Characterization, Electrical and Mechanical, and other:





Parameter

Value (if numeric)

max-min

Priority

 

Description

 

Voltage at the load (a max-min range if not regulated)

3.3V or 5V

3

 

average load consumption (mA) at rated load voltage

50mA

4

3 LED's (3 x 10mA) +
1 pager motor (50mA)+
1 MCU (25mA)

(Aesthetic judgement)

peak load consumption (mA) at rated load voltage

105mA

5

 

volumetric size of circuitry (by LxWxH dimensions)

0

 

 

cost

0

 

 

availability of materials

0

 

 

fabrication skill required for assembly

0

 

 

build-or-buy?

0