Servo - January 2007

FIGURE 2. An LCD screen and multi-state LEDs provide internal state information — in this case, low motor voltage (red LED, left, and LCD), but adequate main voltage (blue LED, right). Control buttons are not shown.

components and microcontrollers. One of my favorite processor boards — the ATMega128-based Mavric-IIB — features a watchdog timer. This independent timer can be used to reset the microcontroller if the program executing on the controller hangs. Using the watchdog timer, you can assure that the microcontroller resets in a controlled manner following a power glitch, robot impact with something in the environment, or a variety of other causes.

The less powerful but easier to use Parallax BS2 and related chips don’t provide this independent watchdog timer function. However, safety routines needn’t reside exclusively in the microcontroller. Consider the Parallax HB- 25 motor controller, which has a communications timeout function that stops motor activity if it doesn’t receive a refresh pulse every four seconds or less. With the timeout function enabled, the HB- 25 will render the robot immobile following an accident or component failure that results in the loss of refresh pulses.

Although several motor controllers support a timeout feature, most components require custom safety subroutines to automatically disable autonomous functions. The basis of operation can be a specified time of inactivity or sensor data, such as a tilt reading from an onboard accelerometer.

Physical Sensors

Software subroutines assume functional sensor-microcontroller communications, but this assumption frequently doesn’t hold following a mishap. For this reason, it’s a good idea to supplement soft safety subroutines with hard wired physical sensors that are independent of

microprocessor operation.

I like to install a hard tilt switch across the control input of solid-state relays that control motor current. The physical tilt switch cluster shown in Figure 1 is composed of four mercury switches, each accompanied by a 1K series resistor. When the tilt of the robot platform is greater than the angle defined by the position of the mercury switch on a given axis, the microcontroller control signal is shunted to ground, thereby opening the relay output circuit.