Servo - July 2008

Loki Crosses the Pond — Part 2

the new position over the time period allocated for the move. An increment in position for each servo frame is calculated for each servo to be moved. Each frame we increment each servo position as required. The result is a smooth transition of position for the servos. And it all runs in the background! The new positions, of course, come from the servo commands parsed. The code also reads and parses the canned sequences. The action of turning on a servo output line for the time determined by the second timer results in a PWM (pulse width modulated) signal suitable for controlling R/C servos.

Checkout

Before powering up Loki, make sure his legs are positioned left foot first and he is standing up. This is the starting position, and all of the canned move sequences both start and stop here.

Initially, both the servo and the controller board power switches should be off and the battery disconnected. Remove the PIC chip U3 if installed. It is recommended that the servos and sensors be unplugged at this time.

You may now connect the battery and turn on the controller power switch, and observe the LED power indicator come on. Measure +5V on the output terminal of the 5V regulator REG1.

Turn the power off and insert the PIC (the end with pin 1 and the little notch faces the three LEDs). When you turn the power back on, you should again observe the LED power indicator come on and still read +5V on the output of the regulator. The “heart beat” LED D2 should be flashing once a second, which indicates that the program is running.

Terminal Tests

Power down again and connect either a checked-out Bluetooth module (TTL) with a viable wireless link to the PC or install a MAX232 level converter chip into U1 (any

Loki in a posture.

56 SERVO 07.2008

Bluetooth module should be disconnected) and a 1:1 three-wire DB9 cable to the PC. Run a suitable terminal emulator program such as T2 or Docklight (even Hyper Terminal). Loki expects a 115K baud rate, no parity, and no handshake. Set your terminal software (and Bluetooth if used) accordingly. (Because of the variety of Bluetooth modules available, no detailed description of using the Bluetooth or SmiRF will be given here).

Assuming the terminal program is configured properly and the Bluetooth (if used) is operational, then you should observe the sign-on message “Loki 1.0 here!” when the board power is turned on. Insure that you are familiar with Bluetooth before attempting to use it! I suggest you try RS-232 first (I did).

Loopback Tests

In case no sign-on message is seen, and assuming you have the proper voltages and the chip(s) in right, you should first check out communications. It is normally easier to start off with a simple RS-232 serial cable known to work. If no message is seen upon power-up of the board, disconnect the RS-232 cable and plug in a loopback plug into the end of the cable instead. You should see your keystrokes echoed through the loopback plug. In the case of Bluetooth, the Tx and Rx jumper leads (blue) can be unplugged from the controller board and connected together with a small piece of .025” header pin. This “loops” the Rx out of the Bluetooth module and right back in. This is a typical way of checking out a terminal program and cabling, and also the Bluetooth. After verifying the communications, standard troubleshooting techniques apply. Check for shorts, grounds, damaged traces, and the like. Pins are easy to bend over on the IC chips. Mind the polarity of the electrolytic caps, LEDs, and diodes.

Loki Here!

On your terminal, send the string REV<CR>. <CR> is a RETURN keystroke. It is often represented as ‘\r,’ as well. Loki should reply with “Loki 1.0 here!” This is the message also seen on power-up, but it is a good fast test of a valid two-way connection to the terminal. And with Bluetooth, it’s nice to have a quick way to verify that you’ve still got a connection.

You are now ready to send some initial servo commands to Loki to test him out. Insure Loki’s legs are positioned with the left foot first. (Loki always puts his left foot forward!)

WARNING! Servos can rapidly jump when first turned on. Positioning the feet as mentioned will minimize any undesired jerking motion of the feet upon application of servo power.