Servo - July 2008

• POnn Servo offset

• Cnn Canned sequence

• Knn Continuous canned sequence

• R Reset (stop) sequence

• U Range request (sonar)

• Z.. Pot update of a servo

• I Range request (IR)

• X Execute step of stored EEPROM sequence

• Y Execute (back) step of EEPROM sequence

• V View stored EEPROM steps

• E Save current position into EEPROM step

• A Set address of execution step

Loki can execute either one of the canned sequences, or the EEPROM sequence that’s been saved. I developed the simple six-step gait and turn sequences used by the autonomous mode (and the canned sequences) with the EEPROM commands.

Either the ‘Z’ or ‘+/-’ offset commands can be used to position a servo, and then store all servos with the ‘E’ save command.

Sequences and Autonomous Mode

The SW3 pushbutton is pressed to start up the autonomous mode of Loki. Loki starts by walking forward, and will take random turns at random intervals. Loki turns away if the IR sensors encounter an obstacle. The forward walk is accomplished by continuously looping through canned sequence #1.

A full left turn is really three #3 sequences, and a full right turn is three #5 sequences. Although not very efficient on space, the sequences are “man readable,” and easily cut and pasted into a program or manually sent via a terminal.

Wondering what sequences #2, #4, and #6 are? They are backwards gaits of #1, #3, and #5, respectively. Sequence #7 reads the pot to determine position (more on that later), and #8 is a waving posture.

To control the transitions between the sequences, a FSM (finite state machine) is used. This FSM is scheduled to run periodically by the main ISR. The FSM can be called the first “behavior” for Loki. I intend to add additional behaviors such as following a light source. These too are implemented as FSMs. All FSMs run in parallel and all are scheduled from the main ISR via the system ticks. This parallel FSM operation allows a new behavior to be easily programmed (that’s the theory). The multiple FSMs will be controlled by subsumption. Loki doesn’t have all that implemented yet, however.

Sensors

Loki has three sensors currently. The two IR distance sensors (also called proximity sensors or rangefinders) have analog outputs. The processor reads the IR sensors with the A/D and is controlled by interrupts. The IR data is conditioned and results in left and right range values in inches. The ultrasonic sensor is interfaced by I²C and

Loki Crosses the Pond — Part 2

controlled by another FSM. The ultrasonic sensor data read is in inch range readings. The behavior FSM checks these ranges for use in deciding when to make turns and when to stop. If a terminal is connected (either by a direct RS-232 connection or by Bluetooth), the telemetry (actually, just a lot of printf statements) will be available, and the ranges seen by the sensors will be displayed, along with some sequence information.

LEDs

The controller board has five LEDs. D1 is the power on LED. D2 is the “heart beat” LED which flashes at a one second rate to indicate that the program is still running. D4 is the message LED that indicates when a command is being received. D5 is the ERROR LED, which gets set when a received command is in error. D6 is the move LED that indicates when a servo move is in progress.

Switches

There are three switches on the controller board. SW1 is a miniature toggle switch for +5V. SW2 (small pushbutton) is the reset switch and resets the processor. SW3 (small pushbutton) is the data switch used to initiate autonomous activity. There is also the miniature servo power switch on the body.

Servo Operation

The main timer already generates interrupts for the main ISR at the frame rate of the servos (2,500 μs). This rate is also divided down to generate system ticks at a 1/10 second rate for other background tasks. A second timer times the pulse width for each of the four (or eight) servos sequentially. We don’t just suddenly “jump” from one servo position to the next. What we do is “sweep” the servo(s) to