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Chapter 7: Ser v o Motors and Extending the Ser v o Control System 167

Feature Analog Digital
Adjust pulse parameters Unable to adjust; fixed by Able to dynamically adjust for
circuit component values optimal performance
Update frequency or rate Fixed at the incoming Receives at 50 Hz but
frequency, normally 50 Hz updates to motor at 300 Hz
Deadband Fixed by component value Adjustable to suit dynamic
operating conditions
Torque Low to moderate; slow to build Moderate to high; very rapid
to peak value build-up to peak
Power consumption Low to moderate Moderate to high
Cost Low to moderate Moderate to high

Table 7.1 Feature Comparisons between Analog and Digital Servos

can clearly be seen on the right side of the figure. I have already discussed the ATmega8L
chip in Chapter 5 because it is the common controller used in many ESCs. In this application,
the chip does ADC as well as real-time numeric calculations to generate the appropriate
power-control pulse in much the same fashion that the ESC did in response to its PWM
signals.
The digital servo has several significant advantages and one big disadvantage as
compared to the analog servo, all of which are listed in Table 7.1.
The digital servo outperforms the analog servo in all areas except power consumption,
which can be a factor if your aircraft uses many servos. This is actually not that great a
concern given the current availability of high-energy and high-capacity LiPo batteries.
I have also included Figure 7.9, which is an excerpt from the Fubata datasheet that shows
quite clearly the relationship of the R/C control pulses as they apply to both analog and
digital servos.

Figure 7.9 R/C command pulses applied to both analog and digital servos.