A Brush-Creeper

Contents

• BODY OF ARTICLE
• How Does It Work?
• Modified Designs
• Epilogue
• REFERENCES
• About the Author
• FIGURES

In this note, we introduce a simple homemade toy called the brush-creeper, which can glide forward with no propellers, limbs, wheels, and seemingly no movement of any kind that can push forward against the ground. The toy arouses pupils' interest and their incentive to ask "Why?" in lessons related to friction.

The brush-creeper is converted from a clothing/shoe brush with slightly tilted bristles and a vibrator mounted on the top (Fig. 1). Bristles were pressed to tilt permanently by inserting the brush into a gap between two hard surfaces for one week or so (Fig. 2). The vibration is generated by a 0.2-W, 3.0–6.0 V toy motor with an eccentric mass about 3 g on the shaft and powered by a battery of matched emf.

Figure 1.

Figure 2.

The vibrator causes the brush to move up and down with an amplitude of the order of 0.1 mm on a flat surface at a frequency of about 300 Hz. Microscopically, even a mirror surface has many rough spots invisible to the eye. Each bristle changes its shape periodically by bending more as the brush presses down and restores its shape when the brush bumps up (Fig. 3). The bristle advances a few rough spots each time as it falls back and pushes the brush along. A brush-creeper can move with a maximum speed up to 0.5 m · s⁻¹.

Figure 3.

The force experienced by the tip from the "rough spots" depends on the position of landing. If it lands on a surface that slants forward [Fig. 4(a)], it will experience a normal force with a forward component. Only the normal force needs to be considered, because the frictional force has been taken into account by the rough spots. On the other hand, if the tip lands on a surface that slants backward, it will experience force acting with a backward component [Fig. 4(b)]. The average force experienced by each tip over a large number of up-down strokes would be zero. Hence, once the brush starts creeping, it will move with a uniform speed in the forward direction.

Figure 4.

Very often, due to uneven compression in the shaping process, a single brush-creeper does not run straight. However, two nearly identical brush-creepers coupled together by a pair of hinged light rods can be made to run straight or turn in any direction by alternating the speed of one of the motors (Fig. 5).

Figure 5.

Using a robot command system (RCX) and a light sensor such as those provided by the LEGO Pico-blocks,¹ each motor can be powered and switched on or off directly by the RCX according to the intensity of light reflected from the ground. In this way, the coupled brush-creepers can be programmed to move along a curved black track in exactly the same way as a wheeled robot.²

This work was actually initiated by a project called LEAD (Leading through Engineering, Art, and Design), a joint venture of the Hong Kong Federation of Youth Groups and The Chinese University of Hong Kong. We were among the investigators studying the perception of students in pilot schools who were invited to design robots using LEGO Picoblocks. As we had no previous experience in the RCX and the programming language used, in order to get a feel of how things work and go through the learning process that kids might experience, we decided to build a toy ourselves and set our target on a similar robot we learned about from the facilitators of the MIT Media Laboratory. What we had in mind was an animal robot without limbs, like a snail or an earthworm, which can be programmed to move along a black track. Besides physics teaching, the toy sheds new light on the learning of robotics by establishing an example of delivering commands from an RCX to control the movement of a motorized robot made from toys and home appliances.

Reference

1. E.g. LEGO Mindstorms, Robotics Invention System 1.5. first citation in article
   
2. LEGO Mindstorm for Schools 9793 and 9794, accompanying user's guide, pp. 14 –15. first citation in article
   

Se-yuen Mak got his B.Sc. CUHK (Physics), Hong Kong, M.Sc. and Ph.D. (Theoretical Physics) from Brown University. He currently is on the teaching staff, Faculty of Education, CUHK. He earned a position in Marquis Who's Who, Science and Engineering this year. Mak's research interests include designing teaching aids and experiments for science and physics teaching. Mak publishes his papers mainly in TPT, AJP, and Physics Education (UK).Faculty of Education, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR; symak@cuhk.edu.hk

Siu-ling Wong was formerly a physics research fellow in the University of Oxford. She joined The University of Hong Kong in 2002 and is currently an assistant professor in the Faculty of Education, HKU.

Full figure

Fig. 1. A single brush-creeper. First citation in article

Full figure

Fig. 2. Reshaping the bristles of the brush. First citation in article

Full figure

Fig. 3. Changes in the shape and the position of a bristle tip in one cycle as the brush presses down and bumps up on rough spots (enlarged) of an apparently smooth surface. First citation in article

Full figure

Fig. 4. Force acting on the bristle tip depends on the position of landing. First citation in article

Full figure

Fig. 5. A pair of coupled brush-creepers. First citation in article

Up: Issue Table of Contents
Go to: Previous Article | Next Article
Other formats: HTML (smaller files) | PDF (208 kB)