Make a simple start in mechatronics with this moving model of an endangered Aussie animal.

BUILD TIME: 3 Hours
DIFFICULTY RATING: BEGINNER**

**Gearbox assembly and cutting of cardboard parts may need adult help. Glue Gun use WILL require adult supervision.

When many people think of mechatronics, they think of advanced robots which can move independently like humans, or perform sophisticated tasks automatically, like robotic vacuum cleaners which map the floor and make sure all of it is cleaned. However, at its heart, mechatronics is just a combination of mechanical and electronic technologies to perform a task. While the task of advanced versions can be performing surgery or remote rescue, the task of simple versions can be purely ornamental.

Mechatronic devices can be designed to do a job instead of a human, in which case the design is dictated by the task to be performed. At other times, mechatronic devices are designed to mimic something or someone, as happens in the movie industry. Ours is a model designed to mimic something, just for fun. It’s a simple toy but the process behind it is important. In Australian NSW schools, the design process is taught in Science from Kindergarten, although it gets more complex and more obvious as you approach year 6.

While our moving marsupial model will not be featured in a David Attenborough documentary mounting a hidden camera and moving with a group of real wild animals, it will still guide you through some of the factors involved in designing a mechatronic device. Additionally, the ideas we used to get there will be a starting point for you to think about bigger projects.

The circuit in this month’s project is really just batteries, a motor, and a switch. Instead of describing how it works (which would be a very short paragraph), we are going to build the model first, then describe the process we went through to arrive at this design. The processes used and decisions made will be very similar to those you will need to approach your own mechatronics projects, or indeed any other design project.

The Build:

Tools Required:
Small Pliers
Small Side Cutters
Hot Melt Glue Gun and Glue
Glue Stick
Scissors
Colouring Materials of Choice
Pencil
Phillips Head Screw Driver
Bead at least 15mm diameter and 10mm long

Parts Required:	Jaycar	Altronics	Core Electronics
1 x Tamiya Single-Motor Gearbox	YG2740*	-	POLOLU-118
1 x Battery Holder, 2xAA	PH9202	S5057	POLOLU-1151
1 x Toggle Switch, Minature	ST0335	S1315	POLOLU-1407
100mm Light Duty Hook-Up Wire	WH3010	W2251	PRT-08867
2 x AA Batteries	SB2424	S4955B	CE04629

* The Tamiya Branded Gearbox is important. It is available from many hobby shops.

Step 1:

Use a ruler to mark a line on the hexagonal shaft that is 39mm from one end. Slide the hexagonal brass nut up to the line so that there is 39mm between the edge of it and the end of the shaft. Use the supplied hex key to tighten the grub screw, which is supplied loose. Apply a smear of the supplied grease to both sides of the shaft.

Step 2:

Install the final gear, which is the yellow one with a notched hexagon on one side. Follow it with a blue two-step gear, and the yellow crown gear. A crown gear has teeth that face to one side, rather than outwards like the other gears.

Step 3:

Take the small round shaft, and mark a line at 4.5mm from one end. Install the longer of the two yellow plastic spacers so that there is 4.5mm between the spacer edge and the end of the shaft. Apply a smear of grease to the shaft.

Step 4:

Install two blue two-step gears so that the smaller side faces away from the yellow spacer.

Step 5:

Sit the two shafts together so that the three blue gears end up side by side, and the two yellow gears are on the outsides, and slide the brass bushes onto the ends. The bushes have the flange (the raised bit) closest to the gears.

Step 6:

Slide the deeper half of the housing onto the shafts, taking care that the gears stay aligned.

Step 7:

Slide the other half of the grey gearbox cover onto the other sides of the shafts, and align it with the locating lugs.

Step 8:

Screw the gearbox together with the three supplied 12mm self-tapping screws. There are also two 10mm self-tapping screws and three 3mm bolts supplied, but these are not used here. The 12mm screws are the longer of the ones with more widely spaced threads.

Step 9:

Slide the purple pinion gear onto the motor shaft, then press down on a table or work surface until the gear seats home. It should not slide all the way to the motor, but needs to stay flush with the end of the shaft.

Step 10:

Insert the motor into the gearbox housing until the retainer clicks into place behind it. You may need to take the motor out and turn the purple gear slightly to make it go in, as the teeth may not line up with the crown gear inside.

Step 11:

Print out and colour in the design from our website. You can use ours as a colour guide, or find images of real Bilbies. Glue the design to a sheet of cardboard. The only exception is the tail, one side of which you should leave until later. The cardboard should be thick enough to hold its own weight, so choose something thicker than a cereal box. Most shoe boxes are good enough, and we used a post parcel box.

Step 12:

Cut out the design carefully using scissors. It is better to cut away from the lines first, then trim closer little bit by little bit until you are happy.

Step 13:

Use a pencil to make a hole in each of the body pieces at the mark, big enough to slide the shaft of the motor through.

Step 14:

Cut a piece of cardboard measuring 10cm x 15cm. Mark a line down each long side that is 1.5cm from the edge. On these lines, measure at 2cm and 3cm from the left end and the same on the outside. Join the marks to gain the shape shown.

Step 15:

Cut out the shaded area between the 2cm and 3cm lines, then fold along the long lines. A ruler pressed into the cardboard should help you get a straight fold.

Step 16:

Sit the motor on the cardboard with the shafts in the gap and the motor facing forward. Pencil draw around the motor frame, then turn it upside down and glue around the edges. Be careful, as the frame has places where glue should not go, like close to the gears.

Step 17:

Load two AA batteries into your battery pack, being careful the wires do not touch together. Briefly touch the wires to the motor terminals, and note which way the shafts turn. They need to rotate so that the top of the shaft rolls forwards. Mark the positive + terminal with a texta and remove the batteries.

Step 18:

Slide one side of the motor shaft through the hole on one body piece, and glue the cardboard on with hot melt glue. The tray with the motor on it needs to be aligned so that the end is close to the bottom edge of the Bilby’s body.

Step 19:

Cut another piece of cardboard 10cm x 6cm. Draw lines 1.5cm from the edge along the 6cm length, then lines 2cm apart along the 10cm length. Fold the 6cm lines with a ruler, then flatten and cut along the 2cm lines.

Step 20:

Glue these to the body pieces above the motor. These will space the body pieces and give structure to the body.

Step 21:

Twist the black wire from the battery pack to the negative terminal of the motor as you marked earlier. Twist the positive wire to the middle terminal of the switch. Twist a piece of insulated hook-up wire between one of the outer terminals of the switch, and the positive terminal of the motor. Wrap each connection with insulation tape. If you have an adult who can solder, this would be a better option instead of twisting wires. Not everyone has access to such an adult, so twist if you need to.

Step 22:

Add glue to the other sides of all the spacers and the motor tray, then attach the other body piece before the glue sets. Use the table surface to keep the sides straight and aligned.

Step 23:

Cut two 12cm lengths of 1.25mm or similar strong wire thick enough to have structure but thin enough to shape and work with. It comes on a roll, so you’ll need to straighten it with pliers. Mark 2.5cm from each end, and bend one end of each at a right (square) angle.

Step 24:

On one wire, slide on a bead. You can get these from craft stores and dollar shops and most are big enough. It needs to be at least 1cm diameter. Bend the other end of this wire, and glue it with hot melt glue so that the wire is just at the lower edge of the Bilby’s paws. A couple of drops of hot melt glue about 3cm apart either side of the bead will keep it centred.

Step 25:

Cut out the tail pieces and glue one side onto cardboard. Cut it out, then glue the other piece onto the same cardboard. Cut a piece of cardboard 2cm x 3cm, and cut a slit in each end, 1cm long.

Step 26:

Take the other piece of wire, and wrap the 2cm x 3cm piece of cardboard around it to fold in half. Slide the end of the tail into the notch and glue in place.

Step 27:

Take the leg pieces and push a pin or pencil tip through them so the centre mark from the paper transfers. Glue the grey arms from the motor kit onto the cardboard so that they face up and down. Allow the glue to set fully.

Step 28:

Glue the ears on using hot melt glue. These can go at any angle you like, because, like a dog, the Bilby’s ears change position depending on its behaviour and its surroundings.

Step 29:

Install the batteries, being careful that the switch is off. If the toggle switch is off, the toggle arm will be above the two wires, and the unused terminal will be all on its lonesome.

Step 30:

Push the leg armatures onto the shafts. The angle doesn’t matter, as long as long as both legs are lined up. Be careful to press only over the centre of the shaft. You can also glue on the tail now, the same way the bead holder went on at the front.

Now all that remains is to turn on your creation. If it hops forward, you’ve done everything correctly. If nothing happens, check your wiring and connections. If it hops backwards, swap the wiring on the motor connections.

USING THE DESIGN PROCESS

There are many versions of the design process, each specific to a different industry or field. The one taught in New South Wales primary schools is not directly defined by any syllabus document, but most are very similar. Some details will change, but most primary school students, regardless of which state you live in, will find this familiar. Of course, you may not have got to the point in your school career where you are told that this is called the design process, but you will have started learning the skills and thinking processes since your first year.

When designing anything, a series of steps should be followed.

First, define the task or problem. Do this in clear, simple terms. If there is more to say, do so in a list of criteria rather than in the task or problem.

Secondly, collect additional information, such as other things that may be not stated in the original problem. For example, the task may be to make a device to cross a river. Additional information may be that the river is too shallow to use a boat.

Thirdly, research. Look at ways people have solved similar problems, what materials are available to use, what skills you can learn or skilled people you can borrow.

The fourth step is to brainstorm initial ideas. Anything goes here, no matter how crazy.

Next, you’ll need to choose some ideas that seem to be the best, and then compare them to the task and criteria, including what materials and skills you have access to.

Now, you can refine one or more chosen ideas, always going back to the task and the criteria.

If you had more than one idea, now is the time to choose one based on which best fits the task and criteria, rather than which is the coolest idea.

At this point, it is a good idea to produce a model or prototype of your idea. You can then assess whether or not your idea has answered the task or not. You may need to go back a step or two, or even to the beginning. It isn’t unusual for students in a year 5 or 6 class to go back to look at their research or additional information, then work forward again, three or four times. This ensures a sound, quality product at the end. Of course, the complexity of the task has a strong influence on this, and even the process presented here is a simplified version.

OUR PROCESS

Defining the Task

We wanted to build a basic moving model of an animal, as an introduction to mechatronics.

That’s it, it’s that simple. As we said above, however, there are some criteria. In Kids’ Basics, we cannot rely on soldering or specialised tools. Materials need to be readily available and parts available from our major suppliers at a retail, not a trade, level. The task needs to be able to be completed by a primary-school aged student.

There wasn’t really any additional information regarding the task, until we chose a subject. We chose a Bilby, because we originally had this idea for an Easter craft project. We saw a picture of a chocolate Easter Bilby. We decided to go for a Bilby because we like local. Rabbits are an environmental problem here in Australia, and the Bilby is one of the species affected by rabbits. They’re a captivating creature and are the subject of several research and conservation programs. We’re not going to rabbit on about the Bilby itself (yes, that was a terrible pun), but instead provide some links to external articles. The lack of additional information means we’re moving straight on to research.

Research

When the task is to mimic a living thing, the next part of the design process will be to study that thing to find out how it behaves. Because the task was to make a moving model of a Bilby, we started to look at how a real Bilby moves. Watching several videos showing the Bilby move revealed that they have multiple movement patterns. When moving slowly across short distances, they almost walk, although both hind legs often move together. However, when at speed or moving across any significant distance, they have a very pronounced hop. The back legs do most of the work, with the front legs moving together to provide a pivot and a small amount of motive force. The movement really does look like a rabbit, with hind legs moving high and feet in a somewhat circular motion.

The second phase of researching is to look at how other people have approached a similar situation. Now that we know our Bilby moves like a rabbit (to a point), we can look at how moving models of rabbits, and any other similar animal, have been made. We found two distinct trends. One type of model uses a motor, straight lengths of rigid materials, and some elastics, to create a leg that can spring up and down. The other uses a shaped leg that rotates on a motor shaft, like a wheel with bits missing in a way. This one is far simpler, and is the most suitable for Kids’ Basics.

Other things to research were the availability of suitable materials. There weren’t many motors around from retail suppliers that fit the set Kids’ Basics criteria of running on AA batteries or a 9V battery, so the choice was made for us there. The same goes for construction materials: Cardboard was the first thing that came to mind, and it fit the criteria of available, cheap, and workable.

All of this means that the design process steps of brainstorming, choosing, and refining ideas were already done for us. They still happened, but they were shaped and led by the research results. This was because of the simplicity of the task and criteria, and wouldn’t normally happen. It did mean that we could jump straight to prototyping. The research had shown that the leg should be drawn inside a circle, so that the correct rotation is achieved. After we had done this, we drew an appropriately-sized body, and cut out the pieces.

Our prototype was going to run on a 9V battery, but this proved too fast. When we tried to fit 4xAA batteries in, we found we needed the side-by-side, end-to-end arrangement. This highlights the value of prototyping. We also discovered that we needed the cardboard spacers you see in the assembly steps, but our original model was much thinner and had a different motor glued straight to the body. Prototyping also showed us that we needed tie wire or similar to mount the tail and front wheel. This is a hardware store item and should be accessible to everyone, but we still analysed the choice against our criteria before we glued anything into the model.

Having revisited earlier steps of the design process multiple times during our prototyping stage, we were pretty happy with the results. That is, until we applied power. The Bilby promptly fell over, not something a real Bilby does very often. When we took a closer look, other things weren’t quite right either. We found that the tail, which had the wire pass straight through it, flopped sideways. The whole model was too tall and top heavy.

We revisited the criteria. We chose the original motor and gearbox because it is pre-assembled, and runs on 5 -10V. While simplicity is a criteria, the options just weren’t going to work. We needed something wider so the model wasn’t so unstable. This led us to the Tamiya gearbox in the final model. It requires assembly, runs on 3V, and is more expensive, but otherwise, the criteria are all answered. It altered the design to be wider, which is what physics demanded.

The same goes for the tail. We went back to the criteria, which said we were building a model of a real animal, and that had to behave similarly as much as possible. The real tail does not turn side to side, and we couldn’t just delete the feature. A re-brainstorming session came up with the current design. We also simplified the ears. We had attached them with flat elastic so they flopped around, but they had too much movement and usually fell backwards too far. The criteria said ‘as realistic as practical’, so we accepted defeat and glued them on rigidly.

Hopefully, as you read through this description, you can see our thinking and decision making, and use it to develop your own projects. This doesn’t apply just to basic mechatronics projects. We’ve used a slightly more detailed version, with support material like flow charts, with a year 5 class to develop new playground equipment, and a year 6 class to design an accessible nature walk across varied terrain including a swamp. So get to brainstorming and share your ideas!

WHERE TO FROM HERE?

In terms of the design process, you are free to let your imagination run wild. The process we described should help lead you toward achievable results.

In terms of the Bilby, you could try to detail it more. Maybe a felt cover, a different body structure that is more rounded, like a real Bilby. You could also add a rope/fabric tail and whiskers. Double layers of cardboard for the legs may make it stronger, too.

Warning:

We are going to use hot melt glue for this project. Please get an adult to help you with this project, as hot melt glue can give serious burns if it is misused. If it gets on your skin, it is hard to remove quickly, and keeps heating the contact area. Some tips to use hot melt glue safely are:

• Keep fingers clear of the hot end of the glue gun
• Make sure no skin is ever under the glue gun, where glue could drip. This includes while moving the glue gun around the project.
• Turn the glue gun off if it gets too hot. Glue only needs to be melted enough to flow and stick. If left heating for too long, the glue melts too much, becoming very hot and very runny. It is more dangerous this way.
• Be careful of glue squeezing out the sides when joining two things. Keep your fingers away from where the glue can ooze from.