Servo - July 2008

Loki Crosses the Pond — Part 2

rangefinders, and a Devantech

SRF08 ultrasonic rangefinder in the “prototype” area of this board.

The QwikFlash controller board has an RS-232 level converter and a built-in DB- 9 connector for communications, although I currently use just the TTL lines for telemetry (via Bluetooth). With it, I run a terminal interface program called T2 on my PC to access Loki’s small monitor program. I used the monitor program to develop Loki’s gaits and postures.

There is even an 8x2 character LCD that can be added to the board, although only the board on Shelob has this part. The LCD is not currently used on Loki. The Bluetooth (Blue SMiRF) setup I’m using is from SparkFun, and is great for freeing Loki from wires (it doesn’t take much weight to off-balance a small robot such as this). Otherwise, a lightweight, three-conductor cable can be used for normal RS-232 communications.

The QwikFlash board requires 7 to 9 VDC, which I get from a 1,300 mAh 7.2V battery pack I salvaged. A 6V battery pack would be sufficient if you’d like to use a 9V battery for the controller instead. I added three 1N4001 power diodes in series to drop the voltage from the battery pack closer to 6V for the R/C servos I used (servos don’t like too much voltage). A modular RJ11 connector to match the Microchip MPLAB ICD2 in-circuit debugger cable is also on the board. I used the ICD2 to download and develop the code for Loki. MPLAB IDE v7.4 runs the ICD2 and invokes the HI-TECH Software compiler that I used.

LOKI Four-Servo Biped Robot Summary

• Cheap walking robot, low cost parts, can be made at home

• Loki is inspired by the efforts of David Buckley http://davidbuckley.net/DB/Loki.htm.

• Original construction was wood (I later found out)

• My Loki is constructed of PCB material to a similar size

• Currently using four Futaba S3004 R/C servos, need bigger knee servos

• Battery is a salvaged six-cell Ni-MH 7.2V 1,300 mAh pack

• Controller board is a $15 (bare) QwikFlash 18F4620 PIC board available from www.picbook.com

• 50 MHz, 64K Flash, 3K RAM, plus EEPROM

• Bluetooth wireless for telemetry and monitor control

• EEPROM gaits editable over wireless link

• PIC programming via ICD2

• Control program written in HI-TECH C, used to develop gaits

• Two modes, autonomous and control via a small monitor program

• Parser used to decode commands received by monitor

• EEPROM commands save, view, and execute steps in sequences

• Autonomous control currently consists of simple obstacle avoidance while doing a simple walk

• Sensors include two Sharp GP2D12 IR range finders and a SRF08 ultrasonic rangefinder

• Body and feet PCB material CAD designed and cut on my CNC’d Sherline mill

• Loki’s gait is exaggerated due to the need to clear the toes of the overlapping feet

• R/C servos are driven by two timers and interrupts in the PIC

• Servo commands set new angle for servo and time to get to new position

• Gaits are stored by strings of servo move commands, similar to those of the Lynxmotion SSC32 servo controller

• To walk, a simple array of six strings is repeated in an endless loop

• More strings allow Loki to make turns

• An FSM selects the strings to be used, and changes the string selection upon recognition of an object by a sensor

Controller Board Construction

To build the QwikFlash controller board, you can follow the instructions in the CA1.PDF document found on the previously mentioned PICbook web page. Some comments are in order. If you use a Bluetooth module,