4 Technical Development
4.1 TECHNICAL SPECIFICATION
The technical specification was based around research into available components and systems. Governing criteria based on a requirement specification were:
- To keep the total component cost as low as practicable while maintaining data reliability and consistency (<$300+GST excluding the pressure sensor);
- A system design that could be completed within a relatively short time frame;
- Maintaining a focus on potential future version including extra facilities, e.g. data logging capabilities, communication with other devices, extra sensing options;
- To use components and designs that would keep power consumption sufficiently small so that large batteries or multiple battery renewal during a season would not be required;
- A robust device taking into consideration the environment that it will be used in, and
- A simple user interface.
The requirement specification was a list of the functionality required to meet the needs of the user. This was then used to produce a design specification from which the designs of the individual sub-systems were formulated.
4.2 DESIGN OVERVIEW
Figure 1 is a block diagram of the system, which shows the micro controller and its connections to the various sub systems.
The system appearance and functionality were given a strong bias, as a practical instrument needs to be intuitive, simple to use, and featured with only what is required. A user interface comprising of a four line alphanumeric display and simple keypad were chosen. The display always shows the three main variables (current pressure, current flow rate, and accumulated volume) and any key press will bring up a simple menu (see Appendix I for display flow chart).
To reduce power consumption, the micro controller (CPU or central processing unit) and all the sub-systems, except for the real-time clock are switched off. Every minute, the real-time clock starts the CPU, which activates and reads the pressure sensor and updates the pressure. It calculates the flow by interpolating data from the table of pressure/flow characteristics that has been entered by the user and stored in secure memory. These outputs are then displayed on the LCD screen.
If a key has not been pressed, as is typically the case, the CPU immediately turns itself off. If a key has been pressed, the CPU is activated. It determines which keys are pressed and actions them accordingly (see Appendix I for display flow chart). If a key is not pressed for 30 seconds then the CPU again turns itself off.
The firmware flow chart is included as Appendix II.
Figure 1
4.4 WORK PROGRAMME
Following formulation of the research and specification stages, individual sub-systems were designed independently. They comprise:
- The external housing (machining required for display window and keypad);
- Micro controller (CPU);
- Data storage (memory) and its interface to the CPU;
- Analogue to digital signal conversion and pressure sensor interface (lightning protection) and its interface to the CPU;
- Keypad (0-9, ‘.’ And ’¿ ’) and its interface to the CPU;
- Display (with 4 by 20 characters) and interface to the CPU, and
- Power regulation (5 volts for components and 15 volts for the pressure sensor) and filtering.
The individual sub-systems were used to design a circuit board layout, which was then fabricated. The external housing was tested for water intrusion and the components mounted on the circuit board and tested. Running concurrently to the physical design, software was written and compiled. A development aid was used to debug the software.
The meter was then laboratory tested, prior to in-the-field testing. Further refinements required to obtain a satisfactory working prototype were completed, and other useful alterations to firmware or hardware noted for possible later versions.
4.5 CHANGES TO SPECIFICATIONS
The specification required that all data was to be stored securely. Non-volatile RAM was chosen, specifically EEPROM (Electrically Erasable Programmable Read Only Memory), which maintains data even when power is removed. This is a cheap, reliable way to store data. However, the number of times that the EEPROM can be written to, is limited. While this is not an issue for the look-up table values for flow and pressure and the pressure sensor type since these values will only usually be entered once, it is an issue for frequent updating of accumulated volume. For the prototype, the accumulated volume is secure except when power is lost (i.e. when the batteries are replaced).
4.6 COMPONENT AVAILABILITY/COST
The components chosen for the prototype were obtained from component suppliers based on convenience and speed of arrival, rather than lowest cost. With this in mind, it is expected that the future cost for components excluding the pressure sensor could decrease from the current $400 to approximately $200. Manufacturing costs could be expected to result in a factory cost of $400 for the meter. Marketing and distribution cost, plus a 4 to 20 mA pressure sensor (approximately $400) suggest a likely retail value of $1,000.
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