The Scooter has three wheels, which gives it stability on slightly uneven surfaces. It is interesting to contrast the wheel system of the Scooter with the very different system of the Quester. The Scooter’s two front wheels are on the same shaft so it needs only one drive motor and only one motor control circuit. This makes the robot easier and cheaper to build.
The third wheel is a castor. How this is constructed depends on what type of small wheel is available. We used a pair of Lego® wheels, with tyres, 25 mm diameter. The wheels clip on to two extensions on a 16 mm square brick. A second brick of the same type is clipped on above this so that the assembly can be bolted to a strip of expanded PVC board (or a strip of brass). The other end of this strip pivots on a bolt that projects from the lid.
plastic that is easy to drill and to cut with a craft knife. This type is suitable for the project. Sometimes they soften with the heat of drilling, which leaves a rather ragged edge to the hole, but on the whole it is usually no problem to get a tidy finish.
The robot is steered by an extremely simple mechanism. When moving forward, the castor lever turns to the position shown in the photo above. The Scooter always travels straight ahead. To turn, it must reverse a short distance, which makes the castor lever turn to an angled position, as in the photo opposite and in the drawing on p. 179. As a result, the robot backs and turns at the same time.
If it continues to back several times, this action takes it round in a small circle about 400 mm in diameter. Normally it turns through only a small angle before the robot moves forward again, but in a different direction.
This technique is not true steering ability but it is simple to build, works well and is easy to program. You may decide to adopt two-motor steering instead. If so, refer to the description of the Quester. Its wheel and motor system can readily be fitted into a plastic box.
The Scooter’s wheel assembly is on the lid, seen here from below (the outer side). It runs on three wheels, in effect, but the rear castor wheel actually is a pair of wheels.
The castor is pivoted at a point to one side of the fore-aft centre line. In this photo the castor is in the position it takes up when the robot is running straight ahead.
We chose a typical food storage box, with a snap-on lid. The corners are rounded (see photo). The best way to use this type of box is upside down. The lid (now underneath) has the motor, the drive wheels and the castor mounted on it. The circuit boards and parts of the robot are housed in the box.
An electric motor spins too fast to drive the wheels directly. Use a motor with a built-in gearbox. We chose a 6 V (nominal) DC motor which included a kit of plastic gear wheels.
These are assembled to give two different reduction ratios, 1:60 and 1:288. We made up the 1:60 gearing as the Scooter is intended to be a fast mover. The axis of the motor is at right-angles to the drive shaft. The drive shaft projects from both sides of the gearbox and each end is coupled to a wheel.
A top (inside) view of the wheel assembly on the lid. The castor wheels are mounted inside the lid but project through a fan-shaped aperture.
The lever on which the wheels are mounted turns on a pivot; a long bolt fixed to the inside of the lid. In this photo the lever is in the position
it takes when the robot is reversing. The stop peg limits the lever from turning further when in the ‘ahead’ position (photo opposite). There is no need for a stop peg for the ‘reverse’ position because the casing of
the motor prevents the lever from turning further,
A recurring problem for robot builders is that the drive shaft of the chosen motor and and the hubs of the chosen wheels are not the same diameter. Yet it is essential for the wheels to fit firmly on to the shaft, without slipping. Now may be the time to improvise.
We chose a pair of wheels with a sporty look. They are sold by Tamiya as a 56 mm diameter Sports Tire set. The set includes hubs for attaching the wheels to the shaft, but does not include the shaft. To give a stable base, the drive wheels need to be about 170 mm apart, but the output shaft of the gearbox is not long enough. Aluminium tubing, 4 mm diameter was used to extend the shaft at both ends. This is a push-in fit into the hubs and also a push-fit on to the output shaft. The tubing must be long enough to hold the wheel well away from the side of the box.
The 4 mm tubing fits firmly to the output shaft and does not need any support. The tubing did not slip as the drive shaft rotated, but friction grip has a habit of gradually working loose. To prevent this, push the tubing on to the shaft and drill a 1 mm (or smaller) hole through the tubing and shaft. Push a short length of connecting wire through the hole and bend its ends to prevent it from dropping out.
Coupling one of the drive wheels to the output drive shaft of the gearbox, The wire, threaded through a hole bored in
the drive shaft and tubing, eliminates slipping. (Not to scale)
Consider this alternative. As described above, the robot can go only straight ahead or back and turn right. The main pair of wheels could be mounted at an angle so that the robot continuously veers to the left as it goes forward. To go ahead it continually corrects this bias by backing a short way and then continuing forward. To go right it backs a greater distance before continuing. In effect this gives it left-right steering, essential when trying to follow a wall.
There are several ways of building the castor. The essential feature is that the wheel(s) is mounted on a lever that is pivoted to the left or right of the fore-aft centre line of the robot. The axle of the wheel(s) is parallel to the lever.
The principle of the eccentric castor.
The view is from above.
The wheel unit is made from two identical wheel-mounting bricks. The wheels rotate on two projections from the sides of the lower brick. The upper brick also has two projections, visible in the photo. These are aligned at right-angles to the projections on the lower brick. A long M3 bolt runs through the central holes in each brick and is used to
attach the assembly to the lever.
The wheel assembly is bolted to the lever, which is cut from metal strip or sheet plastic.
The wheels are at right-angles to the length of the lever. At the other end the lever pivots on a bolt (see photo overleaf). The length of the bolt and the position of the lever on the bolt are adjusted so that the lid is level when resting on the drive and castor wheels.
The lever may sag under the weight of the robot for two reasons. One reason is that the plastic of the lid is too flexible. In this case, reinforce the lid by bolting to the lid a small square of stiffer plastic at the point where he pivot is mounted.
The prototype Scooter did not need this reinforcement. Because the lever has to turn freely on the pivot it can not be bolted firmly to it, so allowing it to sag. It needs to be thicker.
The photo shows the solution.
Before drilling the hole for the pivot, two small squares of plastic board are bolted to the lever, using two shorter bolts. Then the pivot hole is drilled
through all three layers. The forward stop peg is seen in this photo.
An alternative design for a castor has a single wheel. The mounting could be made from strip aluminium or brass about 10 mm wide.
This completes the mechanical side of the robot apart from mounting the circuit boards when they are ready, and a few other items such as LEDs. Boards are bolted to the lid and the bottom of the box, using 15 mm M3 bolts and nuts.