Servo - July 2008

Loki Crosses the Pond — Part 2

Top view of Loki (new legs). Sonar and IR sensors.

connector. The red +V wire goes to the center connector; the black -V wire goes to the shell. This connector allows easy disconnect from the controller board. The battery connector can be disconnected to allow connection of the battery charger.

Software

Now down to the software issues. I’m an old hand at the C language, and I naturally grabbed my trusty HI-TECH compiler for the job. (Note that any one of several different languages would work.) Although I’m not using one, a boot loader would be a good choice for a ‘bot like this. As mentioned, I use an ICD2 for programming, and have to plug in a modular cable for this. Obviously with a boot loader and wireless connection, no cables would be needed! But don’t dismiss this simple umbilical cord; it is an easy way to get started. In fact, I initially ran my Loki with a battery pack lying on my desk. Having the weight of the battery pack off-loaded allowed me to use some old Futaba S3004 R/C servos I had laying around until I could determine what size servos I needed. The S3004 servos at the knees are too weak to lift up the body of Loki. You could also use the Hitec RCD servos listed here:

Front view of Loki (new legs).

Sonar and IR sensors.

(interrupt service routine).

The ISR is called at a regular interval by a timer interrupt.

The ISR generates “system ticks” that initiate various background tasks. Also, several peripherals are supported by an ISR in the background. The USART and A/D converter are examples of this, as are the updates for the R/C servo positions. The I²C FSM for the ultrasonic rangefinder relies on multiple calls from the scheduler to avoid

“blocking” while waiting for I²C events. This RTOS is not pre-emptive and only supports a fixed list of tasks; hence, the baby moniker.

The current controller program (RTOS) is a bit eclectic in that it has a monitor that accepts commands from a console and also supervises an autonomous mode for when the robot is on its own. I’ve mentioned the monitor command to move the servos; there are also commands to save/view/execute the servo positions stored in EEPROM, and to read the sensors. The current autonomous mode is in its infancy; it’s merely a simple obstacle avoidance behavior for now. More is planned for it in the future.

Parser

Hitec HS-645MG 107 oz-in 4.8V, 133 oz-in 6.0V Hitec HS-475HB 61 oz-in 4.8V, 76 oz-in 6.0V

Note: Futaba 3004 servos rotate the opposite direction to Hitec HS-475HB servos! Be sure you account for this if you change between the servo brands. I’m currently making modifications to the code to accommodate both rotations.

The heart of the monitor is a simple hacked-together parser to decode commands for Loki. Perhaps the most important command is the R/C servo command which takes position parameters for up to four servos (although I’ve planned to allow up to eight servos) at a time. A servo command is a simple string of ASCII characters (like “#0P1200 #1P1500”) ending in a CR (carriage return) in a format similar to that used by SSC (Serial Servo Controllers) such as the Lynxmotion SSC32 servo controller. This is convenient for me, as Shelob — my main hexapod — uses it. Add to the command a parameter for the required time of the move (“T1000”), and we have a means of commanding 1-4 servos to move in what’s called in CNC parlance a “coordinated motion” (all axis motion starts and ends at the same time). This is very convenient for moving the legs of Loki, or any robot, for that matter.

Loki currently recognizes the following commands or parameters:

RTOS

Loki’s controller program is basically a “baby RTOS” (real time operating system), which means that there are several tasks that run at the same time, managed by an ISR