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ENERGY CONTROL AND SOFTWARE 177
need bother watching its receiver until the appointed time. Such a communication
protocol can save quite a bit of energy. Certainly, both ends of the link must have
accurate, free-running clocks to remain coordinated. An alternative is to use a
commonly available clock such as one transmitted by GPS satellites that is avail-
able all over the world.
SOME NOTES ABOUT SPY-HOPPING

Spy-hopping is basically a way for the robot to periodically sample the world in which
it must function. As we will see in Chapter 8, sampling can easily get the robot in trou-
ble. Two conditions make it possible to use this technique. First, conditions must be well
known for sampling to be effective. Second, the control system must be able to func-
tion properly with the limited amount of information that proper sampling techniques
afford.
We should note at this point that spy-hopping is inherently a type of polling. The
robot’s control system takes on the responsibility of watching events and catching them
as they happen. The processor software goes to each interface periodically and “polls”
it to determine if it needs attention. This control method is distinctly different from
interrupt-driven control systems where it is up to the event itself to notify the con-
trol system that action is needed. Interrupt systems also are capable of low-power
operation.
Since spy-hopping relies on sampling, an inherent response time delay is built into
the control system. If an event of interest occurs, it will be some time before the proces-
sor wakes up and polls the sensors monitoring the event. As long as the event lasts long
enough to be detected, the processor will catch it and act properly. A delay will take
place, however, which might be as long as the spy-hopping interval. As long as the con-
trol system can perform its tasks effectively in the face of the delays, no problems occur.

ADAPTIVE SPY-HOP DUTY CYCLE
The robot’s control software can adapt to a changing environment. If the control soft-
ware notes that relevant events are occurring ever more frequently, it can decrease the
spy-hopping intervals. Sampling the environment more often will help guarantee
smooth operation, but at the expense of increased energy consumption. When the
robot’s control software senses that external activity is slowing down, it can increase the
spy-hopping intervals again to save power. This technique can be used in situations
where the environment changes in a relatively predictable way. If the adaptive control
software alters the spy-hopping interval fast enough to keep track of the changing en-
vironment, then all will be fine. If the environment changes faster than the control