Conceptual Design of Subsystems

The drill encompasses many areas of engineering design, summarised by the following chart:

The following are the conceptual design area that have been considered in this chapter.

X-Y Drive System Concepts

An important consideration to take into account in the early design stage is what exactly will be moving, i.e. the drill or the PCB. In order to keep the stepper motor sizes down to a minimum, the payload should be kept to a minimum, for example keeping the drill stationary and using a flexible drive shaft, etc.

A Cartesian XY system because of accumulative positioning errors using cylindrical or polar co-ordinates. For example, if the polar positional accuracy is 0.5° this results in a linear positioning resolution of 0.785mm (Tan0.5*100) at a distance 100mm from the fulcrum. See figure 4.2 below.

Figure 4.2 Linear distance moved by 0.5° step

This does not take into account backlash and slop, so it is not really a feasible option, even though it would get rid of any linear bearing, as all joints could be rotary.

Using Cartesian co-ordinate methods, the layout of the X-Y drive system can be one of 3 options, as shown below.

    Figure 4.3 Twin axis arrangement, drill moves

    Figure 4.4 Separate split axis arrangement

    Figure 4.5 Twin axis arrangement, PCB moves

There are several drive possibilities that can be used to gain the required accuracy, namely

The best in terms of accuracy is probably the leadscrew, but its is by far the most expensive of all the options. Although expensive, an anti-backlash nut could be used to greatly enhance repeatability. A possible arrangement for single axis motion is shown below.

Figure 4.6 - Leadscrew and nut coupled direct to motor

Timing belt and gears

This arrangement gives good positional accuracy and repeatability together with low cost and 'off the shelf' components. The belt used would be 'non-stretch' polyurethane, with aramid tensioning fibres. The pulleys teeth pitch match the belt, resulting in minimal backlash.

Figure 4.7 - Timing belt and gears

As it is the belt that moves, it needs to be attached to the mounting plate, or what ever needs positioning. This is achieved with a stock belt clip as shown below.

Figure 4.8 Belt clip

Wire cable and pulleys

This option is the cheapest of the three discussed, but is also the least rugged, and does not give very good accuracy or repeatability without additional components. This type of arrangement is seen in many plotters, printers and scanners where the mass to be moved is low.

Figure 4.9 Wire rope and pulleys

Figure 4.10 Wire rope and pulleys, top down view

Bearing Possibilities

The carriage needs to run smoothly with minimal play to meet the performance specification. Circular shaft with self lubricating bearings e.g. oilite or plastic. This type of bearing on a circular shaft would give good performance but bearing life is a consideration, and needs to be addressed in testing. Plastic bearings would be cheaper, but have less life expectancy.

Figure 4.11 Bearing possibilities

Dovetail/Vee with linear ball bearing

An excellent and accurate method, but expensive. This method is standard in many lathes and mills.

Precision Linear Slide

This is a very good solution in terms of accuracy, but it is expensive. An initial enquiry found a vendor that sells it for £30 per 200mm, excluding the sliding carriage.

Figure 4.12 Precision linear slide
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PCB Clamp Mechanism

In order for the user to place and remove a PCB quickly, a quick release method is desirable. The PCB fixing mechanism needs to hold the PCB rigid and give support to the force of the drill. Depending on the depression of the PCB under the force of the drill, it may be necessary to employ some form of disposable backing, which would also improve hole quality. This will be addressed in the technical feasibility and testing.

Sliding V-edge vice

The diagram below shows a concept using a sliding quick release lever mechanism.

Figure 4.13 - Sliding V-edge vice

Spring loaded clips

This method could be used as a quick and cheap method for retaining the PCB. It can be duplicated for each corner of the PCB.

Figure 4.14 Spring loaded clips

Drill Up/Down Mechanism

If the max thickness to be drilled is 3mm, and allowing 1mm tip clearance and 1mm thru overlap, then the total distance to be moved by the up/down mechanism is at least 5mm. See dia.

Figure 4.15 Drilling clearance

Mechanism Concepts

Figure 4.16 Solenoid with hinged or linear bearing

                  Figure 4.17 - Geared DC Motor driven, with circular to linear converter mechanism with feedback

Figure 4.18 - Lead screw, with reversible DC motor

Concept Choice and Justification

The final concept that chosen was a split axis timing belt driven system. The reason was the balance between accuracy and cost. The wire and pulley option could be feasible but it would take testing to confirm that opinion. Timing belts are used frequently for medium loads, and where good accuracy is required.

Rather than doing matrix analysis to decide a concept for each mechanical software, and electronic design area, I have primarily used the performance criteria as an ongoing 'sanity check' for each design option, and have not spent much time on outsider concepts. This is because of the nature of the project, with its many design areas, more time could be spent formalising a common sense decision process. In any case I am confident that my concept choices will perform to specification.

Appearance and System Integration

Considerations:

    · Painted sheet metal chassis/frame, low cost

    · Vacuum formed plastics for covers where necessary.

    · Bought in plastic components, e.g. gears

    · Wiring out of sight where possible, and harnessed.

    · Logo on sticker or painted

    A rendering of the concept development to date is shown overleaf.

System Integration

Considerations:

    · PCB to be housed in drill chassis

    · Watch out for electromagnetic emissions, provide shielding

    · Foot print less than 300mm x 300mm

Software User Interface and Human Factors

There are many programming language possibilities including:

    · Quickbasic, Qbasic etc..

    · Turbo C, ANSI C etc..

    · C++

    · Delphi

    · Visual Basic

There are many considerations to take into account when choosing a language including:

    · Existing programmer knowledge-base

    · Difficulty in programming and debugging

    · Is there a requirement for language specific libraries, e.g. vbrun400.dll etc.?

    · Does the language support I/O to the parallel port?

    · What is the portability of the code to other platforms, e.g. UNIX

    · What is the speed of the compiled executable?

    · What operating system is required to run the program?

Prototyping has been undertaken in Qbasic and final application development will be in C. The reason for this is because of Basic's easy to use code, and ability to get results quickly, although maybe not elegantly. Basic allows for rapid development modifications and changes while not having to worry about code syntax as much as C. My existing knowledge of Quickbasic will allow rapid development to get the hardware up and running, while not being frustrated by lack of programming knowledge. In other words, I want to get a program fully functional and running in Basic before I convert it to C. Perhaps an unconventional practice, but an effective one, given the situation.

Windows uses interrupts, and allocates time slices to each program that is running, therefore I've initially decided against using it as a final platform because of the slower operation than in DOS.

Interface Style

Required: Pull down menu, text/graphic, hybrid mouse & keyboard driven

Considerations:

    · Intuitive menu structure, copy mainstream applications in format

    · On line help

    · Full diagnostics and testing

    · Hotkeys

    · Preferences file for portability

Initial development in Quickbasic:

Figure 4.19 Menu driven interface

Figure 4.20 - Initial Menu Command Structure

Electronics Interface and Driver

As the project involves controlling high voltages and currents, using low TTL voltages, there is the danger of damage to the PC I/O board should an error occur. Therefore it is good practice to isolate the power electronics from the low power side. There are three feasible possibilities, namely:-

Optoisolated I/O interface to PC

This method provides a very high degree of over voltage protection. The cost of an optoisolator chip is relatively expensive when compared to a 7407 or a 7404.

Buffer/driver chip, e.g. 7407

This method employs a buffer to handle the difference in voltages between both sides of the electronics (5V TTL, and 12V for the motor). The 7407 is a cheap chip, providing 6 buffers and current drain of up to 30mA

Figure 4.21 7407 Buffer driver

In order to provide the high curren ts required for the coils of the motors, a driver is needed, and a cheap and reliable method is to use a power transistor such as the TIP122. It can sink up to 5Amps, and together with heat sinks will be a reliable and cheap choice.

Figure 4.22 TIP122 Driver transistor

Driver Module

For information on the driver circuit, see the technical specification.

Hardware User Interface and Ergonomics

Hardware controls required

    · Power on/off switch for CNC drill

    · On/off line switch

    · Everything else controlled by software and PC

Ergonomic Considerations

    · Ease of changing drill bits

    · Ease of changing PCB material

    · Ease of removal of swarf tray

    · Not catch fingers on moving parts or any holes, slots etc.

    · Safety guard / Hardware interlock switch

    · No sharp edges on sheet metal

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