3.4. Quadrature encoders
Each drive motor on the Zumo 32U4 has a
corresponding quadrature encoder system consisting
of a magnetic disc attached to the extended motor
shaft and a pair of Hall effect sensors mounted to the
underside of the main board. Other than the sensor
orientation, these encoders work similarly to our
magnetic
encoder
kits
for
micro
metal
gearmotors
[https://www.pololu.com/product/2598]
.
They can be used to track the rotational speed and
direction of the robot’s drive sprockets.
The encoders provide a resolution of 12 counts per
revolution of the motor shaft when counting both
edges of both channels. To compute the counts per
revolution of the drive sprockets, multiply the
gearboxes’ gear ratio by 12. For example, if
75:1
motors
[https://www.pololu.com/product/2215]
(which
have gear ratios more accurately specified as
75.81:1) are used, the encoders provide 75.81 × 12 ≈
909.7 CPR
.
Quadrature encoder transitions are often detected by monitoring both encoder channels directly. However, since
transitions on the Zumo’s encoders can occur at high frequencies (several thousand per second) when its motors
are running, it is necessary to use the AVR’s pin change interrupts or external interrupts to read the encoders. To
reduce the required number of interrupt pins, the Zumo 32U4 main board XORs together both channels of each
encoder and connects the resulting signal to an interrupt pin, while channel B of each encoder is connected to a
non-interrupt pin:
•
Digital pin 7
, or PE6, reads the
right encoder XORed signal
using external interrupt INT6.
•
Digital pin 8
, or PB4, reads the
left encoder XORed signal
using pin change interrupt PCINT4.
•
Digital pin 23
(analog pin 5), or PF0, reads the
right encoder channel B
.
• Pin
PE2
reads the
left encoder channel B
.
Pololu Zumo 32U4 Robot User’s Guide
© 2001–2015 Pololu Corporation
3. The Zumo 32U4 in detail
Page 15 of 76