Pololu 3pi Robot - Manual (Page 12)

• Linear regulators use a simple feedback circuit to vary how much energy is passed through and how much is
discarded. The regulator produces a lower output voltage by dumping unneeded energy. This wasteful, inefficient
approach makes linear regulators poor choices for applications that have a large difference between the input and
output voltages, or for applications that require a lot of current. For example, 15 V batteries regulated down to
5 V with a linear regulator will lose two-thirds of their energy in the linear regulator. This energy becomes heat,
so linear regulators often need large heat sinks, and they generally don’t work well with high-power applications.
• Switching regulators turn power on and off at a high frequency, filtering the output to produce a stable
supply at the desired voltage. By carefully redirecting the flow of electricity, switching regulators can be much
more efficient than linear regulators, especially for high-current applications and large changes in voltage. Also,
switching regulators can convert low voltages into higher voltages! A key component of a switching regulator is
the inductor, which stores energy and smooths out current; on the 3pi, the inductor is the gray block near the ball
caster labeled “100”. A desktop computer power supply also uses switching regulators: peek through the vent in
the back of your computer and look for a donut-shaped piece with a coil of thick copper wire wrapped around it
– that’s the inductor.
The power management subsystem built into the 3pi is shown in this block diagram:
The voltage of 4 x AAA cells can vary between 3.5 – 5.5 V (and even to 6 V if alkalines are used). This means it’s
not possible simply to regulate the voltage up or down to get 5 V. Instead, in the 3pi, a switching regulator first boosts
the battery voltage up to 9.25 V (Vboost), and a linear regulator regulates Vboost back down to 5 V (VCC). Vboost
powers the motors and the IR LEDs in the line sensors, while VCC is used for the microcontroller and all digital
signals.
Using Vboost for the motors and sensors gives the 3pi three unique performance advantages over typical robots,
which use battery power directly:
• First, a higher voltage means more power for the motors, without requiring more current and a larger motor
driver.
• Second, since the voltage is regulated, the motors will run the same speed as the batteries drop from 5.5 down
to 3.5 V. You can take advantage of this when programming your 3pi, for example by calibrating a 90° turn based
on the amount of time that it takes.
• Third, at 9.25 V, all five of the IR LEDs can be powered in series so that they consume the lowest possible
amount of power. (Note that you can switch the LEDs on and off to save even more power.)
Pololu 3pi Robot User's Guide © 2001–2014 Pololu Corporation
5. How Your 3pi Works Page 12 of 63