Electrical and Control System Details

I've written the Wi-Fi Bot Driver software application which handles the comms over the wireless network. This allows the joystick control signals to be communicated from the fixed PC to the mobile robot and also allows a live read-back of the condition of the motor speed controllers on the bot. This is a key part of the control system. It is made up of two programs - Sender and Receiver, which run on the fixed and mobile PC's respectively. The Sender program reads the state of the joystick on the fixed PC and transmits this using UDP protocols to the receiving PC. If required the Sending program can instead output the control signals directly to a serial port on the fixed PC. This is useful for wired debugging of the bot or for onward transmission by wireless telemetry links.

The Receiver program running on the board notebook/laptop converts commands received over the network into a local serial output which is read by a PICAXE micro controller based processor unit (SPU) to provide the local control over the machine. The Receiver program exports the state of three axes and 8 button buttons on the sending joystick and could be used to provide a serial feed to any microcontroller based applications that can read serial data.
 

The bot driver software uses standard network IP addresses and port numbers to identify the participating PC's. The data flow can be interrupted at either PC to disable the robot.

 

 

The bot Signal Processor Unit reads the serial data from the notebook PC (or direct from the fixed PC) and mixes the joystick forward/reverse and left/right axis data to determine speed and direction required of each drive motor, it then sends the speed and direction instructions to the motor speed controllers on the robot. It also instructs RC style servos which provide pan and tilt for the onboard webcam (so you can see where you are going if the robot is in a different room or off exploring in the back garden).

 

The overall system wiring for the robot is shown above. The SPU is built on the picaxe experimenter board and uses two 28X1 picaxe chips. The master chip handles the serial comms with the laptop and drives two Devantech MD03 motor controllers on an I2C bus. It also passes the state of the designated pan and tilt joystick buttons on the the 2nd or slave 28X1 chip which issues servo pulse commands to drive two camera position servos.
 

Monitoring the Robot's Condition

The communications provided by the robot driver software is two-way. The on-board SPU reports the motor speed controller speed settings, current flow and temperature back to either the Notebook PC carried on the bot (for onward transmission to the master PC) or directly to the master PC. This feedback data is displayed live on the sender program window. This gives the driver live feedback of the current draw and temperatures of the speed controllers - it makes for interesting watching.

 

 

The Bot Driver Software is also able to dump this feedback data to a text file where the data can be inspected more closely. For example the trace below shows the current flow in the MD03 speed controllers during simple straight line travel and then during full on-the-spot skid-steer rotations. It is generally known that skid steering consumes much more power than more conventional steering approaches but it's interesting to see the effect directly.

 

The trace is a bit noisy but small up/down steps can be seen over the initial part of the trace matching the initial simple forward and reverse straight line motion of the bot. The current draw in each controller looks like it is approximately 0.3/0.4 Amps - ie it requires little effort to travel on the flat in a straight line. However the jump in the current draw during the skid-steer turns can be seen clearly, The current draw rises to roughly 5 to 6 Amps with some peaks higher as the speed of the turns gets faster. I haven't included a temperature trace but it shows the controller temperatures climbing during the period of higher current draw. The MD03's are rated for 20 Amps and the 250W motors for 14 Amps so the currents are still well within capacity but it is still interesting to see the scale of the higher power demand as the wheels are dragged and spun during the turns.
 

 

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