Build a Mini Walking Bot!

Don Wilcher
Copyright © 2006

Building robots doesn't require an elaborate machine shop or sophisticated electronics tools and accessories. What is really required is imagination, some basic electronic components, a solderless breadboarding station and a good motorized construction kit. In building bots, I've found using a motorized construction kit helps speed up build time [1]. To illustrate this technique I'll describe how to rapidly build a table-top walking robot using a readily available motorized construction kit, a solderless breadboarding station, a small robotics controller, and several electronic components. The concept for the Mini Walking Bot is to create a distributed controller using two individual computing platforms and motorized walker for mechanical propulsion. Figure 1 shows a Concept Block Diagram for the Mini Walking Bot.

Figure 1. The Mini Walking Bot Concept Block Diagram.

Collecting the Parts

After determining the type of robot to build, I put together a parts list or what we in the engineering community call a Bill Of Materials (BOM).I have several motorized construction kits laying around my lab, but for this project I wanted to build a bot in less than 3 hours. For this robot project, I decided to use the ROBOTIX motorized construction kit as the core mechanical component for my Mini Walking Bot because there is an available 2 Legged Walker. With this pre-built walking mechanism I would be able to meet my build time goal of 3 hours. Here is the BOM for the Mini Walking Bot.

Bill Of Materials (BOM)

1 – 2 Legged Walker (ROBOTIX kit)

2 – Elbow connectors (ROBOTIX kit)

2 – Wheel Foots (ROBOTIX kit)

2 – 75mm Arms (ROBOTIX kit)

1 – Drive Axle (ROBOTIX kit)

1 – Low Speed Motor (ROBOTIX kit)

Figure 2 shows the ROBOTIX pieces used in building the Mini Walking Bot. The assembly process will take 3 steps: Mechanical Build, Electrical-Electronics Build, and Software Build.

Figure 2. ROBOTIX parts for Mini Walking Robot (L) Elbow connectors, Wheel Foot(s), Arms (75mm), 2 Legged Walker, Drive Axle, and Low Speed Motor.

Mechanical Build

When the main mechanical parts are available as shown in Fig. 2, the Mini Walking Bot can be built quite easily. The photo-assembly instructions will show how the bot is built. This motorized construction kit was chosen because of the ease in which table top robots can be built as well as the additional parts that can be purchased [2]. Also, LEGO parts can be integrated with the ROBOTIX motorized construction set, thereby allowing the creation of hybrid machines.

Assembly Instructions

Here are the following steps for building the Mini Walking Bot using the ROBOTIX parts shown in Fig. 2.

Step1. Attach the Drive Axle to the Low Speed Motor shaft (Fig. 3).

Step 2.Place the Low Speed Motor/Drive Axle on top of the 2 Legged Walker (Fig. 4).

Step 3. Attach Elbow Connectors and Wheel Foot(s) to 2 Arms (Fig. 5).

Figure 5. Building the Mini Walking Bot's Back Legs.

Step 4. Attach the left leg part to the 2 Legged Walker (Fig. 6).

Figure 6. Attaching the left leg part to the 2 Legged Walker.

Step 5. Attach the right leg part to the 2 Legged Walker (Fig. 7).

Figure 7.Attaching the right leg part of the 2 Legged Walker.

With all the parts attached, the Mini-Walking Bot is completed as shown in Fig. 8. That completes the mechanical build for the Mini-Walking Bot. Next, the electrical-electronics & software builds and how the construction of a distributed controller is created using 2 different computing platforms will be explained.

Figure 8. The completed Mini Walking Bot!

Electrical-Electronics Bill Of Materials

1 – Handy Cricket Embedded Controller

1- Handy Cricket IR (Infrared) Transceiver

1 – PICAXE microcontroller

1 – 22 K, 1/4W resistor

1 -10K, 1/4W resistor

1- 9 Pin “D” Shell connector

Solderless breadboard

Pre-cut solid wires (22 AWG)

The electrical-electronics build for the Mini-Walking Bot consist of using a distributed technique of control, wherein two embedded controllers used for processing an input signal and driving the robot's low speed motor will communicate to each other using hardwired connections. In prototyping this embedded controller, I used the Radio Shack Electronics Learning Lab because of the pre-mounted parts and the solderless breadboard for circuit building. The solderless breadboard on the electronics learning lab was partitioned to accommodate the PICAXE microcontroller circuit and the Handy Cricket embedded controller. By using the electronics learning lab breadboard, both the PICAXE Microcontroller circuit and the Handy Cricket embedded controller can easily be wired together. Figure 9 shows the circuit schematic diagram of the Distributed Controller.

Figure 9. The Mini-Walking Bot Distributed Controller circuit schematic diagram.


The PICAXE microcontroller [3] is an 8-bit chip capable of performing I/O (Input/Output) monitoring and control suitable for small table top robots. Computer programs are written in a version of PBasic [4] and are downloaded to the 8 bit microcontroller using serial communications (RS23). A comprehensive circuit schematic of the PICAXE microcontroller is shown in Fig.10.

Figure 10. The complete PICAXE Microcontroller Circuit Schematic.

The prototype device assembled on the breadboard allows for ease in testing, troubleshooting, and quick changes as well. Figure 11 is the experimental controller built using the 2 computing platforms.

Figure 11. The assembled Mini Walking Bot Distributed Controller using a PICAXE microcontroller, a Handy Cricket, and a Radio Shack Electronics Learning Lab.

As seen in the picture a yellow power cable is attached to the Mini Walking Bot's low speed motor which completes the final assembly of the sci-tech project. Figure 12 shows the yellow power cable attached to the Mini Walking Bot's low speed motor. The final step of the robo project is to provide intelligence with embedded control software.

Figure 12. The yellow power cable from the Distributed Controller attached the Mini Walking Bot's Low Speed Motor.

The Software

Software is the “glue” that brings the electrical-electronics and mechanical structure layers together for the final intelligent machine product. Several embedded controller platforms on the market useng various computer programming languages like Visual Basic, Java, C, C++, and Visual C. Each computer language has its own unique set of instructions and programming primitives allowing the software developer to create interesting embedded controller devices. I decided to use the Handy Cricket Logo & Logo Blocks for software development of this embedded controller because of its easy programming style and function capability. Likewise, the PICAXE microcontroller's Basic language is easy to use and it has a Flowchart Modeling feature allowing control logic to be simulated before downloading the software code to the chip. The software formats of the 2 computer languages are shown in Figures 13 and 14.

Figure 13. The Logo Blocks and Cricket Logo Control Software for the Mini Walking Bot.

Figure 14. PICAXE Control Software for Mini Walking Bot.

Embedding software codes into their respective computing platforms requires unique communication download methods. The Handy Cricket [4] uses an IR (Infrared Transceiver) to send binary data from a computer serial port (RS232) to the small embedded controller over a beam of invisible light. Figure 15 shows the Handy Cricket's IR transceiver.

Figure 15. The Handy Cricket IR Transceiver.

The communication download media is wireless. The PICAXE microcontroller, on the other hand, uses the serial port exclusively for loading the Basic programming code onto the 8-bit chip. The software structure of the code is such that the electrical switch attached to pin 4 of the PICAXE microcontroller will turn on the output at pin 6 creating a +5V signal. The +5V signal is received by the Handy Cricket at Sensor port A, allowing the embedded controller to turn on the DC motor at Port A. This output will stay on for 40 seconds and allow the Mini Walking bot with mobility until the time has expired. This distributed method of control will repeat each time the electrical switch is triggered.

Further Thoughts

In this article I explained the three phases of building a mini walking robot: mechanical, electrical-electronics, and software. Although the ROBOTIX motorized construction kit was used in creating the walking robot, with a little ingenuity, any electrical building set can be substituted for the mechanical structure. I like the ROBOTIX sets because of the many parts and electrical accessories that can be purchased and modified for electrical-electronics interfacing control projects. The PICAXE microcontroller and the Handy Cricket embedded controller are easy to use computing platforms for building small table top robots and intelligent machines. The computer programming languages that each computing platform uses are easy to build and download into the target microcontroller device. In future articles, I'll dive deeper into the inner workings of these two computing devices with additional robotics projects. Enjoy!!!

Resources

1. “A Quick Guide to Building Bots: Rapid Prototyping Techniques for the Novice Robot Hobbyis,” D. Wilcher, The Citizen Scientist, http://www.sas.org/tcs/weeklyIssues_2005/2005-12-09/feature1/index.html .

2. The ROBOTIX motorized construction kit can be purchased at http://www.roboticsandthings.com

3. The 8-bit PICAXE microcontroller can be purchased at http://www.phanderson.com/picaxe/index.html

4. “The BASIC Stamps are programmed in a simple version of the BASIC language, called PBASIC.” BASIC Stamp Programming Manual 1.7 , Parallax, Inc http://www.parallax.com

5. The Handy Cricket starter kit and accessories can be purchased at http://www.family-science.net/store.htm