AFRON Design Challenge 2012 (Traditional Category)

Aulë – One kit, two robots; Many kits, infinite possibilities

1. High level design description

Most robot kits allow a user to build one robot of fixed design, which usually turns out to be a differential drive robot with some sensors on board. At the other end of the spectrum, there are more general purpose kits that enable the construction of a wide variety of robotic contraptions, like Lego’s Mindstorms. The concept of Aulë the robot kit is motivated by the desire to address the inflexibility associated with many robot kits, while avoiding the challenges (and perhaps even the constraints) involved in a very versatile kit like Mindstorms.

Aulë is not so much a robot platform as it is a kit that allows the user to construct one of two very different robots, depending on the manner in which the parts are assembled. The first is the ‘standard’ differential drive robot, and the second is a 2-DOF robot ‘arm’ which has no end-effector. Together, these two robots expose the student to two major aspects of current robotics technology and research: mobile robotics and robotic manipulation.

As a mobile robot.

As a 2-DOF ‘arm’.

Above and beyond the ability to build two robots, the parts of multiple kits can be combined to build more complicated robots with greater degrees of freedom. This is the key feature of Aulë, and that makes it possible to keep the cost of individual kits low, without constraining the user to building simple robots with highly limited functionality. That will greatly extend its useful lifespan in supporting the education of its user.

As a 2-DOF module of a larger, modular robot.

All this is made possible by the modular design of Aulë, which is inspired by the emerging field of modular self-reconfigurable robotics. Each kit consists of two modules, each of which serves as the main structural component of the robot. Each module allows for the mounting of a single motor in two possible ways, and behind the motor there is space for either the control board or the battery pack. Thus, between the two modules, a single robot with two degrees of freedom is made, and the configuration of these DOFs is entirely due to the manner in which the modules are connected and the manner in which the motors are mounted.

Radial and axial motor mounts (to the left and right of image respectively) enable motors to be mounted in two different orientations.

The mechanical design of Aulë is relatively simple and can certainly be mass-manufactured, but it may also be built from scratch using mostly wood and foam for the mechanical parts (though possession of the right tools will greatly influence the quality of the final product and the difficulty of fabrication).

Contents of the kit at a glance.

In addition, the robots are supported by a sensor (i.e. the ‘eye patch’), which has sockets for inserting 4 components. The circuit is designed to allow either an LDR or an LED to be inserted into a socket, and the corresponding microcontroller I/O pin set either as an ADC input to read the LDR, or as a digital output to control the LED.This allows for a few possibilities. A) 4 sensors, when arranged in a cross, allows for tracking of a light source in two DOF. B) a sensor may be substituted with an LED to provide illumination. C) in a modular robot, with one LED and one LDR on each eye patch, communication is achieved by bringing the eye patches of two modules to face each other.

The eye patch board. Either LEDs or LDRs can be plugged into the sockets, with the corresponding I/O pins on the micrcontroller configured accordingly.

Eye patch with example configuration – two LDRs and two LEDs.

IMPORTANT NOTE: Honestly, the practical realisation of this design is not a complete work, and indeed some design details have not even been addressed yet! The most glaring omissions may well be: “how to mount the circuit board and the batteries and the eye patch?!” and “the mobile robot will fall over without a castor wheel!”. When in doubt, imagine the use of hot glue gun and magic. For the glaring absence of a motor encoder, I can but offer these words for now: homebrew potentiometer with from ESD dissipating plastic bag-lets (use the black ones with the matte rubbery feel!).

2. Educational applications

Build the kit from scratch to learn practical skills

Many parts of the kit can be built from scratch (save for the motors and electronic components and screws and shaft couplers!). The mechanical components are made from wood and foam, and fabricating these parts is a great opportunity/excuse for students to pick up skills for working with these materials. This is especially so for woodworking, which dominates the effort involved in building the parts from scratch. Depending on pedagogical objective, this can be done in different ways than simply providing students with the drawings and instructions to fabricate the robot.

For example, the process of cutting a complicated shape (such as the foam shell, which is a key structural component of a robot module) from a block of foam involves making the appropriate sequence of cuts with a hot wire cutter, using a set of jigs which must be designed and used in a particular order. Thus, students may be presented with the foam shell and be asked to figure out the process by which they can make a copy of it themselves. In this case they will be designing their own jigs instead of replicating them from the fabrication instructions. This is a useful exercise for mentally visualising shapes in 3D.

Learn about two major areas of robotics

The parts of the kit can be used to assemble a mobile robot with differential drive, or a robot ‘arm’ with no end effector. The ‘eye patch’ that is included with the kit allows the same application for both mobile robot and robot arm: tracking a light source. The manner in which the actuators must be driven so that the robot ‘gets to the light’ is very different, however.

In particular, the arm will teach about degrees of freedom, and how their configuration will affect the way the robot moves. E.g. if we swapped the order of the DOFs, so that the one at the base allows for tilting instead of panning, then the workspace of the arm is drastically reduced.

Work together to build a more awesome robot

Robot modules from multiple kits can be used to make more complicated robots. Each module contains a single motor, which may be mounted in one of two orthogonal directions. By mounting the motors and connecting the modules appropriately, various mechanical configurations can be achieved.

Since this is possible only with multiple kits, it provides an incentive for students to work together, share parts from their kits and build something more interesting. With hope, this may gather its own momentum when students see what their peers are capable of and attempt to do likewise.

Students, working in teams, can explore the space of possible robots that can be built in this manner. They will expand upon their understanding of the concept of degrees of freedom, and relate between the mechanical configuration of the motors and the modules, and the movements that their robots can make. The students will also appreciate firsthand the difficulties of some configurations, caused by the limitations of structural strength and actuator torque.

An introduction to microprocessors

The control board for the robot is essentially a general-purpose microcontroller board in its own right, and can be used as such in the same manner as other similar boards like the Arduino. Therefore the control board may be used as part of an introduction to programming/microcontrollers/electronics, either as a set of lessons in its own right, or as a prelude to its use in the context of controlling a robot.

For the microcontroller I have chosen the 28-pin PIC32MX150F128B from Microchip. Admittedly, having a 32-bit microcontroller on board appears to be overkill for the types of robots that this kit aims to support. But as a general-purpose microcontroller board, its greater processing power may come in handy.

Moreover, the ample processing power of a 32-bit microcontroller opens up the possibility of executing an interpreter for a higher-level language with built-in libraries, analogous to Parallax’s Basic Stamps. This would serve as a layer of abstraction over the hardware of the microcontroller, and make the microcontroller board/robot kit more accessible to beginning programmers.

The key feature of the particular microcontroller chosen that makes this all feasible, is that it comes in a 28-pin DIP package, so soldering is not a problem.

Get a taste of contemporary research

As more complicated robots must be built by putting together modules from several robot kits, this necessarily means that one such robot will have multiple control boards. Thus, the nature of the kit forces students to approach the control of their robot in a modular and distributed manner (i.e. as a set of interacting parts/tasks), and to coordinate their development efforts to ensure that the individual modules now work together as part of a single robot.

I believe that thinking about things in a distributed manner is a very important challenge for engineering, and even as we grapple with ‘distributed-ness’ in many fields of research, Aulë can help the next generation make earlier acquaintance with what presently seems to be a novel way of solving engineering problems. They will also appreciate the fact that the modular robots they are building are also the subject of contemporary robotics research.

Furthermore, as students learn about conceiving a robot’s behaviour as the result of the interaction of many smaller software tasks/agents/behaviours, the groundwork is laid in their minds for extending those ideas to other domains, such as in the design of embedded systems, where functionality is organised around tasks executed by some real-time operating system.

3. Parts, availability and prices

In the following, the parts of the robot kit as they should be found ‘out-of-the-box’ are listed and categorised as mechanical or electrical/electronic. Where it is possible to fabricate a part from scratch, a list of materials/parts needed to make it is appended.

Mechanical components

  • Foam shells (2x)
    • Extruded polystyrene foam (so-called blue foam, widely used as insulation and padding in the building and construction industry). Obtain a cube with sides that are 10cm long, failing which it is necessary to make such a cube by sticking together thinner pieces of foam.
    • 5mm thick cork sheet (approx. 2 A4 sized sheets required)
    • 7mm dia. X 30mm wood dowels
    • Epoxy (either in liquid or putty form)
    • UHU Por (Contact adhesive for sticking foam together, if the foam cube must be built from scratch. Epoxy can do this too, but much slower)
  • Base plates for foam shell (2x)
    • 6mm thick wood sheet, with dimensions of at least 100x100mm to make a single base plate
      • 2.4x13mm self-tapping wood screws
  • Axial and Radial motor mounting brackets (2x radial, 2x axial)
    • Metal rulers, preferably 30cm long, with a width of at least 2.5cm, and made of luminium. (Stainless steel rulers are possible, though they will be much harder to work with) ($1)
    • 6mm thick wood sheet, with dimensions of at least 50x60mm to make a single mounting bracket
    • M3 screws
    • 2.4x13mm self-tapping wood screws
    • Epoxy (in liquid form)
  • Axial and Radial motor shaft adaptors (for attachment to connecting rods) (2x radial, 2x axial)
    • Shaft collar suitable for approx. 3mm dia. motor shaft
    • 15mm X 30mm X 60mm rectangular wood blocks
  • Connecting rods (2x long – 180mm, 2x short – 60mm)
    • 9mm dia. wood rods
  • Wheels
    • Shaft collar suitable for approx. 3mm dia. motor shaft
    • 9mm thick wood sheet, with dimensions of at least 100x100mm to make a single wheel
    • Epoxy (either in liquid or putty form)

Materials for building the hot wire cutter tool

A hot wire foam cutter is required to make the foam shell from a block of foam, but this tool should be built from scratch rather than purchased. A suitable power source can be salvaged to power the foam cutter (an old wireless router’s mains adapter, in my case). Other materials required to build this tool are:

  • 0.28mm dia. steel wire
  • 5mm thick wood strip with dimensions of at least 400mm x 20mm
  • Small block of wood, at least 10mm on each side
  • M3x20mm screw and M3 nut
  • 2.4x13mm self-tapping wood screws
  • Contact cement (recommended) / epoxy (stronger but long cure time)
  • Aluminum salvaged from an old drink can
  • Power resistor or other circuit to control heating of the steel wire (according to time and taste)

Electrical and electronic components

  • Control board
    • PIC32MX150F128B 28-pin microcontroller ($5)
    • L293D H-bridge motor driver ($3)
    • L7805 (or equivalent) 5-Volt regulator ($1)
    • LM3940 3.3-Volt regulator ($2)
    • LM358 op amp (80 cents)
    • 10k resistor (1x)
    • 100uF, 22uF, 10uF (tantalum), 1uF capacitors (1x each value)
    • Header pins
    • Wires and prototyping board
  • Eye patch board
    • 10k Ohm, 330 Ohm resistors, 4x each value
    • sockets for plugging in components, 4 pairs of holes
    • 6-pin row of header pins
    • Wires and prototyping board
    • Handful of LEDs and LDRs
  • DC motors (2x)
    • I have used 12V 315rpm geared motors with stall torque of 3.8kg-cm. They cost SGD19 each! That’s about USD16.
  • AA Battery holder for 6 batteries
  • 6 AA batteries (Rechargeable ones more environmentally friendly)

Further notes overall

  • A total of 6 shaft collars are required, and each costs about $1.60. Epoxy is about $5.
  • All wood materials were obtained from shop called Daiso, where everything costs $2.
  • Costs are dominated by the motors, shaft collars and epoxy adhesive. It may be possible to reduce motor cost if the robot is scaled down in size so smaller motors are required, or alternatively if someone comes up with a clever way to hack cheap RC servos to make them capable of both continuous rotation as well as position control at the flick of a switch/lever.

 4. Tools and equipment

The prototype for this kit was built at home with whatever tools that I already had and could find in a hardware/DIY store.

Tools for fabricating mechanical parts:

  • Electric power drill and a complete set of HSS drill bits (say, 1.5mm to 10mm)
  • Electric jig saw (OPTIONAL, if lazy to use hand saw)
  • Hack saw
  • Suitable table equipped with a vice
  • Rulers and set squares
  • C and F-clamps
  • Screwdrivers (Phillips and flat head ones, various sizes)
  • Penknife, scissors, and a special penknife for cutting circles would be quite necessary
  • Printer (for printing out the templates used to fabricate jigs and mechanical parts)
  • File
  • Sandpaper

Tools for fabricating control board electronics:

  • Soldering iron
  • Multimeter
  • Cutters and pliers
  • Microchip In-Circuit Debugger 3 (ICD3)

5. Drawings and dimensions

(*Not being a mechanical engineer nor trained in technical drawing, I regret that the drawings presented below do not meet the proper conventions that I am vaguely aware of. Hopefully, they’re still good enough!)

[CLICK IMAGE FOR HIGHER RESOLUTION]

Foam Shell Drawings

1. The foam shell

2. Jigs for making foam shell using a hot wire cutter

Base Plate Drawings

Motor Mount Drawings

Motor Shaft Adapter Drawings

Connecting Rods

The connecting rods are simply cylindrical wooden rods of diameter 9mm, cut to two lengths: 60mm and 180mm.

Wheels

Control Board

1. Circuit schematic

2. Control board suggested layout

Eye patch

6. Instructions for making the robot

Assuming that the robot will be built from scratch, there are two major parts to the effort. The first is fabricating all the parts that make up the kit, which is the more daunting task, and the second is putting the parts together to create the desired robot, i.e. either the mobile robot or the robot arm.

Part 1 – Building the parts

In my own effort at building the robot parts, I have been hampered most by the absence of a drill press. This is reflected a little in the instructions, which document these difficulties. A second difficulty is the need to make straight, perpendicular cuts. This requires some skill with a saw, but that may be made easier with some jigs, in the absence of jig saws and/or circular saws. Overall, resolution of these issues are left to the wisdom of someone versed in carpentry, which is beyond the powers of this electrical engineer.

The mechanical parts of the robot, (also found in the list of parts in section 3), are as follows:

  1. Foam shells (2x)
  2. Base plates for foam shell (2x)
  3. Axial and Radial motor mounting brackets (2x radial, 2x axial)
  4. Axial and Radial motor shaft adaptors (2x radial, 2x axial)
  5. Connecting rods (2x long, 2x short)
  6. Wheels (2x)

The electrical/electronic parts of the robot that have to be fabricated are as follows:

  1. Control board
  2. Eye patch

Instructions for fabricating each of these mechanical and electrical parts will now be described in turn, and we begin with instructions for building the hot wire foam cutting tool, which is indispensible for making the foam shells.

Making the hot wire cutter

Foam can be cut cleanly using a hot wire cutter, which can be purchased from art and craft shops. However, this tool can and should be built from scratch, in order to save costs and ensure that the tool is capable of cutting something of the foam shell’s dimensions.

This is a picture of the hot wire cutter that I made:

The frame is made with 3 strips of wood stuck together with contact cement and fastened with screws, a crude and quick arrangement that can certainly be improved. More importantly, it must be large enough so that the wire has the length to cut the foam shell from a 10cm cube. The handle must be set deep enough so that it is not too difficult to cut the slit in the foam shell.

The wire is 0.28mm diameter steel, and must be kept taut to keep the cutting process straightforward and ensures that the piece produced is accurate. Cut a length of steel wire in excess of 10cm (say, 15cm), and first tie one end to the head of the M3 screw. This screw passes through a small piece of wood, to which it is secured at the other side of the hole by a nut. This arrangement is used to perform final tightening of the steel wire.

Using a pair of pliers, now pull the other end and twine it around the two screws that act as posts in a figure-of-eight fashion several times, taking care to keep the wire fairly taut.

Finally, tighten the M3 screw to make the wire really taut. This is evident when the wire can be strummed like a guitar string. Note that the string expands and sags a little when it heats up, so you may find it necessary to make a final adjustment after powering up the hot wire cutter.

If there is any concern that the hot wire may contact the wood (and hence cause smouldering and panic or even ignition in a dry climate), it helps to place some metal in between the two. Aluminum salvaged from a drink can is particularly suitable. Take care when cutting the metal to avoid metal splinters piercing the skin.

A voltage is applied across the steel wire from a power source that can source suitable amounts of current. In my haste, I have used a 12V DC, 1A mains adapter that used to power an old router. You may find, as I did, that this heats the wire too much (turns red hot!). A quick hack was to limit the current with the heftiest power resistor I had in my stash, a 5 Ohm 50W resistor. This is of course not an elegant hack. Some commercial foam cutters for hobby use, require two D-sized alkaline batteries, so that alternative may be considered to power the hot wire cutter.

Making the foam shell

Two foam shells must be made.

The foam shell is cut to shape from a cube of blue foam with dimensions of 10x10x10cm. Finding a foam cube of such dimensions appears to be a daunting task, whereas the white expanded polystyrene ones found in art and craft supply shops are too flimsy to make a robot with. Therefore, it may be necessary to make up a cube by stacking up 10x10cm foam squares cut from a thinner sheet of blue foam, often 2.5cm thick.

Cutting the foam squares can be tricky. A suggestion is to build a really long foam cutter, which is really just a much-lengthened version of the design described above, and maybe add a nice drillpress-like lever. I have not tried this myself, but I expect it to be a much faster, neater and safer way than what I did, which was to cut the squares with a jig saw. (If you try this yourself, note that from my past experience, the sag of the hot wire upon power-on is so incredibly significant that kids will finally believe when you tell them that metals expand when heated.)

With the foam squares ready, they can be stacked together to make up a cube.  It is important to note that the thickness tolerance of these foam sheets is atrocious, so blindly stacking them up will lead of a cube with gaps between the layers, and the cube will be rather misshapen. Thus, some effort at matchmaking the squares’ mating surfaces is required.

Then adhesive may be applied to stick them together. Use a suitable adhesive that does not destroy foam, like UHU Por (a contact adhesive), otherwise the foam cube will be full of holes and structural integrity of the foam shell will be compromised. Contact cement may also work. When applying the contact adhesive, remember that the right surfaces should be mated with each other to obtain a nice cube.

Despite all that effort, the cube will not look perfect, but that is probably quite okay since precision is not a critical factor in this design. Now the hot wire foam cutter can be used to trim the cube to get the shape of the foam shell, and this is done with the guidance of two jigs that will be used in sequence.

First, fasten jig #1 to any two opposing surfaces of the foam cube, using masking tape. Take care to align the right-angled corners of the cube with those of the jig, so that ultimately, the shapes of the two jigs on the opposite sides project onto each other without skew. Note that the tape should be pasted so that it does not obstruct the foam cutter.

With everything in position, the foam cutter can now be used. Ensure that the wire of the foam cutter is taut. Even so, the wire will sag during the cutting when the wire is dragged across the foam, so take care when transiting from one edge to another; wait for the middle portion of the wire to ‘catch up’ before pulling it along an edge at a different angle.

After cutting the foam with jig #1, remove it and apply jig #2, which should wrap around the entire curved surface of the nascent foam shell. This jig is used to cut the slit in the foam shell. Make sure that the slit extends all the way to the back, so that after the cut it is possible to see right through the rear of the foam shell.

Finally, before removing jig #2, use a pen knife to cut a shallow well through the front of the slit, as specified by the notch of jig#2. This well is required to accommodate mounting the motor in the ‘axial’ orientation. Exploit the length and flexibility of the pen knife blade, which will be of use in this effort.

Mark and drill the 8 holes at the rear of surfaces of the foam shell, according to the drawing. These holes are meant to accommodate the 7mm dia. by 30mm long wood dowels, and can be made with a 6.5mm drill bit (the resulting hole usually is larger than that because the foam gives way too easily). Correct depth is ensured by marking the drill bit with masking tape. Note that if a handheld electric drill is used, it is easy to mess up the drilling and remove too much foam, so care is required to guide the drill bit.

With the holes done, the dowels should now be inserted. Epoxy putty is the recommended adhesive, to fill up the excess space that will undoubtedly be made in the foam by the drilling process. A good way to do this is to first deposit a lump of epoxy putty into the hole, and coax it to the bottom with a bamboo skewer. Then wrap some putty around the dowel, before forcing it into the hole with the help of something hard (say, by pressing it in with a block of wood). This should fill up the surrounding spaces with epoxy. The remaining spaces closer to the top of the hole can then be filled in with more putty, to be forced into the crevices with fingers. Excess putty can be scraped away with a blunt knife, or sanded down later.

Tip when using epoxy putty: the working life of the putty is fairly short, so mix enough for only about 2 holes at a time to avoid wastage of putty that has cured before it can be used. My experience is that sticking 3 dowels at a time demands a rather frantic pace of work.

Making the base plates for the foam shell

Two base plates must be made.

The base plate is made of two parts, a 10x10cm square of 6mm thick wood sheet, and a receptacle for the fastening of a connecting rod to the module.

The wood sheet may be cut with a hand saw, jig saw or something more awesome. Use a suitable saw blade to keep the cut edges smooth. Note that if the edges are too frayed and filing them down becomes necessary, the dimensions of the square will be less than 10x10cm and that will make drilling and hence alignment of base plate holes with the dowels on the foam shell difficult.

Mark and drill the holes in the base plate according to the drawing, using a 2mm drill bit.

The receptacle can be made from a ready-cut block of wood measuring 15x30x30mm that can be purchased off-the-shelf in my local context (see parts list section 3 for more information). The block of wood I have used measures 15x30x60mm (the reason is lack of foresight at the shop). It first needs to be trimmed from a length of 60mm to 30mm.

Then a 9mm hole of depth 25mm needs to be bored into the wood in the manner as shown in the drawing, in order to fit the connecting rod, which is 9mm in diameter. To prevent excessive chipping of the wood, which is rather soft, it was necessary to drill the hole using a sequence of drill bits in order of increasing diameter, e.g. 3mm, 4.8mm, 8mm and 10mm. The final drill bit is 10mm because I don’t have a 9mm drill bit! Nevertheless, the set screw will ensure it fits snugly, though possibly not perpendicularly to the base plate. Also, without a drill press, it is likely that the 10mm bit will ruin the hole somewhat, because the bit needs to be in contact with the wood when it accelerates from rest, and its bite is quite ‘chunky’.

Finally, stick the receptacle to the centre of the base plate, using epoxy. Epoxy in either liquid or putty form may be used. In putty form, it must be applied around the edges, whereas in liquid form it should be applied to both mating surfaces. If more peace of mind is desired, drill two holes and fasten with screws as extra measure, but only after the epoxy has fully cured.

Making the motor mounts

There are two types of motor mounts: radial and axial. Two of each type should be made (i.e. a total of 4 motor mounts).

A motor mount has two parts: the wooden mounting plate and the metal mounting bracket, which are stuck to each other with epoxy.

The metal mounting bracket can be made starting from a strip of metal. This strip should measure 85mm long for the radial bracket, and 135mm long for the axial bracket. The strip should be approximately 25mm wide. Aluminium rulers bought from a stationery shop may be a good choice of material that is easy to work with for beginners (i.e. easy to cut and bend). These rulers are approximately 25mm wide, but some tolerance is permissible. To cut the strips from a ruler, mark the required length and score both sides deeply with a pen knife across the width, then bend the ruler along the resulting line of weakness to break it. A neat cut results from a neat score with the knife on opposite sides of the ruler.

Print the drilling and bending templates for the motor mount brackets and stick them onto the aluminium strips.

Assuming that a handheld drill is used, first make right-angle bends on the aluminium strips along the lines marked on the template. Note the direction of these bends in the case of the axial motor mounts.

After the bending, it is now easier to clamp the bracket and drill the three holes in it. Do the drilling first and the bending later if a drill press is employed instead. The large hole in the centre is 10mm diameter, while the small holes at the side are for M3 screws and should be 3mm in diameter. The large hole can be drilled with a progression of drill bits with increasing diameter. Once again, drilling is very tricky without a drill press.

The motor mounting plate is made of 6mm thick wood sheet, and the plate measures 50x60mm. Like with the base plate, accurate sawing of the mounting plate facilitates drilling of the 2mm diameter holes at the four corners, and hence the fastening of the motor mount onto the foam shell dowels.

Finally, stick the motor mount bracket onto the motor mounting plate with epoxy. Liquid epoxy is the recommended adhesive.

Making the motor shaft adapter

There are two types of motor shaft adapters: radial and axial. Two of each type should be made (i.e. a total of 4 motor shaft adapters).

The motor shaft adapter is a block of wood that has holes for inserting a connecting rod and the motor shaft collar. The latter is purchased off-the-shelf.

The shaft adapters can be made from a ready-cut block of wood measuring 15x30x60mm that can be purchased off-the-shelf in my local context (see parts list section 3 for more information). For the radial shaft adapter, the dimensions of the block are just right.

For the axial shaft adapter, the length of the block should be shortened down to 35mm with say, a hack saw. A little filing/sanding can help ensure that the cut is perpendicular, which is an important factor in ensuring that the axis of the connecting rod is in line with the axis of the motor shaft collar, to avoid wobble when the motor rotates the adapter and whatever else is connected to it.

With wood blocks of the correct dimensions ready, the holes can be drilled.

First, the 9mm hole of 25mm depth should be bored at one end of the wood block for both radial and axial motor shaft adapters. This is similar to the task already encountered when producing the receptacle of the base plate for the foam shell. As usual, a progression of drill bits should be used (e.g. 3mm, 4.8mm, 8mm, 9mm).

Second, the recess in which the motor shaft collar should sit within the block of wood that is the motor shaft adapter should be drilled.

For the radial shaft adapter, the entire shaft collar should fit into the recess. This involves first boring two concentric holes of different width to fit the profile of the shaft collar: a 5mm diameter hole of depth 11mm, followed by a shallower, 11mm diameter hole of depth 6mm. The latter may be drilled with a sequence of ascending drill bit diameters as usual.

In practice, the bottom of the hole formed by regular drill bits is not flat. My attempt at the stunt of drilling the recess resulted overall in an 11mm diameter hole that went all the way down, with a cone-shaped bottom. It may save time to simply drill an 11mm hole instead.

Second, a 3mm diameter hole has to be drilled right through the wood into the recess, to provide access to the set screw of the shaft collar which will be buried within the wood block.

For the axial shaft adapter, only the narrower end should go in, so a 5mm diameter hole of depth 5mm is sufficient.

With the holes prepared, the motor shaft collars can now be stuck into the wood blocks. For the radial shaft adapter, due to the inability to bore a recess that fits the shaft collar snugly, epoxy putty is necessary to fill up the unwanted gaps and crevices. As in the task of inserting the dowel into the foam shell, drop a lump of epoxy to pad the bottom, wrap some around the shaft collar, and press it in. ENSURE that the shaft collar is rotated so that its set screw can be accessed via the 3mm diameter hole.

For the axial shaft adapter, liquid epoxy is most suitable for the job.

Making the connecting rods

There are two lengths of connecting rods: 6mm and 18mm. Two of each type should be made (i.e. a total of 4 connecting rods).

The connecting rods are made from 9mm diameter wood rods that can be purchased off-the-shelf in my local context (see parts list section 3 for more information). They can be cut to length using a hack saw. The cut should preferably be perpendicular, and this may be facilitated with the help of some filing/sanding.

Making the wheels

Two wheels must be made.

First, cut a 45mm diameter circle out of 6mm thick wood sheet. This may be done using a handheld electric drill with a hole-saw or a circle cutter, and a nice side-effect is the drilling of a hole in the centre of a circle, which is necessary to stick it to a motor shaft coupler using epoxy adhesive.

I have used the hole-saw to cut this circle, and unfortunately, the 8mm diameter hole it created in the centre of the circle is too large for a snug fit of the narrower end of the motor shaft coupler. In this case, epoxy putty should be used, and with care to ensure that it does not clog the hole meant for the motor shaft.

Next, cut a100mm diameter circle out from foam sheet using a hot wire cutter.

Finally, stick the wood circle to the foam circle, so that their centres are aligned, which ensures that the wheel does not wobble when it spins.

Making the control board

Refer to schematic and suggested circuit board layout outline in section 5, ‘Drawings and Dimensions’.

Making the eye patch

Refer to schematic and suggested circuit board layout outline in section 5, ‘Drawings and Dimensions’.

Part 2a – Assembling a mobile robot

  • Attach both motors to the axial motor mounts.
  • Fasten the motors in their motor mounts, to the foam shells, using the self tapping wood screws.
  • Fasten the base plate to the foam shell using the wood screws. The battery pack should be fastened to one of the base plates, and the control board to the other.
  • Connect the two modules back-to-back by Inserting each end of a short (6cm) connecting rod into the receptacle on each module’s base plate, and fasten it with a wood screw, which functions as a set screw.
  • Fix the wheels onto the motor shafts
  • Fix the eye patch onto the robot in an appropriate place, with say masking tape

Part 2b – Assembling a robot arm

  • The module with the axially mounted motor will actuate the azimuth DOF (i.e. panning), while the other lighter module will actuate the elevation DOF (i.e. tilt).
  • Attach one motor to an axial motor mount, and the other to a radial motor mount.
  • To the shaft of the motor on the axial mount, attach the axial motor shaft adapter.
  • To the shaft of the motor on the radial mount, attach the radial motor shaft adapter.
  • Fasten the motors in their motor mounts, to the foam shells, using the self tapping wood screws.
  • Fasten the base plate to the foam shell using the wood screws. The battery pack should be fastened to one of the base plates, and the control board to the other. The base plate with the battery goes to the foam shell with the motor mounted axially while that with the control board, being lighter in weight, goes to the foam shell with the motor mounted radially.
  • Fasten the module with axially mounted motor to the ground, and fasten a long connecting rod to the axial motor shaft adapter.
  • Fasten the other end of that connecting rod to the base plate of the other module (i.e. with radially mounted motor).
  • Fasten a second long connecting rod to the radial motor shaft adapter of that module.

Acknowledgements

A great amount of gratitude goes to my parents for their support and encouragement, and to my mum for doing most of the gluing of the foam squares to make the foam cubes from which the foam shells are cut.

Many thanks also to Rush and Michelle of AISCube for their support, and particularly for lending me the PIC32 microcontroller chip and their Microchip ICD3!

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