Electronic Design
Transforming Circuit Board Into Robot

Transforming Circuit Board Into Robot

Texas Instruments (TI) has delivered a lot of great dev kits. One of its latest is the Stellaris Evalbot kit that is based on a 32-bit Stellaris microcontroller. The Stellaris line is built around Arm's Cortex-M3 platform. The kit is designed to highlight a range of features from motor control to Micrium's latest RTOS, uC/OS III.

The Evalbot (Fig. 1) starts out unassembled as a circuit board (Fig. 2). The board holds the Cortex-M3 microcontroller and all the interfaces it provides including Ethernet. There is no electrical construction but there is mechanical work for the new roboticist. This starts by removing the components from the circuit board that has been designed for just this purpose.

The most complex piece are the Evalbot wheels (Fig. 3). This is done using tiny screws. The final Evalbot wheel/motor assembly (Fig. 4) is then bolted the bottom of the Evalbot. The holes in the wheels are not just for looks but by design. They are combined with LED and sensors on the main circuit board to implement the Evalbot's wheel encoders (Fig. 5). This design is actually very impressive in its simplcity and elegance.

The Evalbot also has a set of bump sensors on its front end. Again, the design is simple and elegant. There are a set of pushbutton switches that push the bumper arms out. Unfortunately there are no long range sensors. A pity since incorporating some IR LEDs and sensors would have been a simple chore.

One other issue I ran into was the tight tolerances on the drive system. The Evalbot uses two pegs on the front and rear along with the wheels to balance the robot. It these are off by a fraction of an inch then the Evalbot will spin its wheels. Adding a washer helped in my case but don't run it on anything but a perfectly flat surface.

Wireless Robot Control

The Evalbot includes a pair of headers that are designed to work with TI's wireless modules . An Evalbot equipped with one of these modules (Fig. 6) can interact with similarly equipped robots or with a control station. I had a number of units to experiment with including the CC2500EM, CC2520EM and CC2530EM. The CC2500 is a very low power, 2.5 GHz transceiver that has a data bandwidth up to 250 kbuad. It can be used with TI's SimpliciTI protocol stack. The CC2520EM and CC2530 are 802.15.4 modules. These differ primarily in their on-board microcontroller although the radio side differs some as well. They support 802.15.4 as well as ZigBee.

Interfacing to the modules is done using an SPI interface. The host handles the protocol stack with the CC2500. The others move most of the protocol stack to the on-chip microcontrollers. The CC2500 is also found on the eZ420-RF2500 kit. This USB-based development module is handy as a host control unit. Of course, at the Embedded Systems Conference TI was showing off the Evalbot using the EZ430 Chronos (see Low-Cost Kits Make Evaluation Faster).

Micrium uC/OS III

Micrium's uC/OS has been around for a long time handling RTOS chores on 8-bit to 32-bit platforms. The latest incarnation, uC/OS III (Fig. 7), includes a number of books by Jean Labrosse tailored for specific platforms like TI's Evalbot. Another platform is Renesas' new 32-bit RX line. About 90% of the books are common and cover the RTOS. The differences are in chapters that address the specific platforms with examples.

The Evalbot can be purchased without the book and it can be programmed without using the uC/OS III operating system. On the other hand, there are significant advantages to using a solid RTOS like this on a robot. This is especially true since the framework for handling the various peripherals is already in place. It made the job of enhancing the basic applications much easier for me.

Overall, the Evalbot/Micrium combintation is impressive for the cost. Many will be interested in the plaform from a robotics standpoint but it is a useful motor control experimentation platform as well. I had only a couple quibbles with the package. On the software side, you will need to turn to online resources to get deep into the robotics and communication aspect of the system. The Micrium book is a good introduction but its primary purpose is to explain the RTOS. It does a great job there but leaves the roboticist wanting more guidance.

The hardware issue is more of a challenge. There is a small number of I/O pins available via a 0.1-in center header but it is located on the side, not the front of the robot. It is also a challenge to solder the header pins in after the robot is assembled. It is not impossible but patience is needed along with an expert soldering hand.

Finally, the robot has a lot of neat design aspects that novices are not likely to pick up immediately such as the design of the wheel encoders and the bump sensors. Having worked with robots in the past, I can appreciate the elegant solutions TI designers came up with.

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