Sensors For Mobile Robots -— Part 2
FIGURE 11. MinIMU
mounted on a breadboard.
FIGURE 12. Parallax
reading all of the applicable literature available on the
Pololu website, as well as all the other company websites.
There are hundreds of pages, and some of that material
may be skimmed over. The interfacing, mounting,
specifications, register description, and a raft of very
descriptive tables are available for each chip.
I had a bit of trouble with testing the L2G4200D
device; not because it was defective in any way, but
because one of the jumper wires on my solderless
breadboard was making intermittent contact as I moved
the board in different orientations with my hands.
Unfortunately, that happened several times with different
modules before I discovered the problem causing the error
messages. It was the shaking of the board to test the
accelerometers that loosened the wire intermittently. Use
your breadboard for initial testing, but securely plug in or
solder the board for your finished robot. Figure 11 shows a
good way to secure a module on a breadboard for testing
using a straight six-pin header. Note the X-Y-Z orientation
markings on the back of the IMU board.
If you’re looking for an incredibly small dual-axis
accelerometer, the Parallax 0.42” square Memsic 2125 is a
great choice. The low cost, dual-axis thermal accelerometer
78 SERVO 06.2012
shown in Figure 12 is capable of measuring tilt,
acceleration, rotation, and vibration with a range of ± 3 g,
and is electrically compatible with other popular
accelerometers. Its six pins spaced at 0.1” work great for
solderless breadboards such as the Boe-Bot or the many
experimental platforms such as the Propeller-based PropBOE
and Arduino boards. Operating from 3. 3 or 5 VDC 4.0
mA, it outputs a TTL compatible 100 Hz PWM signal
proportional to acceleration, and is a great choice for
autonomous robotics applications.
Parallax also has a three-axis accelerometer module (the
MMA7455) shown in Figure 13. Utilizing the Freescale
Semiconductor MMA7455L MEMS chip, it operates from 2.5V
to 5.5V with a very low 26 µA in standby, and less than 3 mils
when operating. It features an ADC, digital low pass filter,
and selectable sensitivity ranges of ± 2 g, ± 4 g, or ± 8 g. This
device can easily be configured to detect quick motion pulses
as single taps, double taps, and 0 g free-fall conditions.
Communicating via an eight-bit Serial Peripheral Interface (SPI)
or Inter-Integrated Circuit (I2C) bus at a max of 8 MHz, it
worked well with my Propeller setup and should interface with
BASIC Stamps or any of the Arduino series. Quad rotor or
any mobile robots should find this inexpensive module useful
for three-axis motion sensing, as well as vibration analysis.
Magnetic compasses have been and still are an
excellent way for people, robots, planes, and many other
moving vehicles to determine their direction in relation to
the earth’s magnetic field. However, there are a few
hiccups in using a magnetic compass and knowing which
direction you are facing. Sailors have long known that the
needle or “N” on a compass rarely points north as the
geographic North Pole is nowhere near the magnetic north
pole that is in northern Canada. For example, you could be
in northern Alaska facing true north and your compass
needle could be pointing 90º to your right. There is a wavy