While tuning the PID loop and debugging the ultrasonic range finding code (more on that to come) in oroboto I’ve been writing log files to its flash for subsequent analysis using dotVision.

A log line contains the following data:

timestamp x y logtype waypoint

timestamp

x

y

logtype

waypoint

There were two reasons for recording this data:

1. Keep track of where the robot thought it was (x,y co-ordinates) at a given time (to allow for comparison to where it really was)
2. For ultrasonic range finding, keep track of where the robot detected obstacles (to allow for comparison to where they actually are)

While the timestamps are useful, because they allow dotVision to play the log back “in realtime” I still had to try and remember where the robot was at that time in order to make any meaningful comparisons for #1. This turned out to be a fruitless effort.

Better solution: pipe the data back to dotVision in realtime over a wireless connection.

dotVision (updated on git) has now been updated with a UDP server (uses Grand Central Dispatch to put a dispatch source around the socket and then schedules “dot” additions into the dotLog on the main UI thread for drawing) and the DotLog class on the BBB has a mini UDP client in it.

Which brings up the next question: what’s the ideal physical layer to send this over? The two obvious choices are 802.11(whatever) or BlueTooth. I had a spare D-Link DWA-131 revB USB wifi adapter lying around so I gave that a shot…

DWA-131 revB Power Requirements

First things first, you’re gonna need more power. I had limited success getting the DWA-131 revB running when tethered via USB and no success at all when running off the measly 5x AA cell power supply I had jury rigged. Scratch that, time for a LiPo.

Lithium-based battery chemistry has come a long way and as any RC fan can attest, LiPo (lithium polymer) batteries can deliver a significant amount of current in a small form factor (just be careful charging them, they also make great fireworks).

I picked up a few Zippy 2200mAh 2S batteries (2S meaning 2 cells in series, so 7.4V at full charge) and rejigged the robot chassis a bit to house one of them (and we now have an on/off switch, what progress!). This keeps the BBB, wifi adapter and motors happy for at least an hour in the testing I’ve been doing.

“Look ma, no hands!”

Now for the meaty bit. Once this is done the robot can boot the default BBB Angstrom Linux distro and be immediately accessible directly via WiFi. This assumes you’re connecting to an infrastructure mode network using WPA2.

• Download the firmware for the WiFi chipset used by the DWA-131 (an RTL8192CU) from here and copy it into /lib/firmware/rtlwifi
• Load the kernel module at boot:

# echo rtl8192cu > /etc/modules-load.d/rtl8192cu.conf
# update-modules

• Disable connman, it’s a con, man:

# systemctl disable connman.service

• Define the properties for your wireless network and the NIC you’ll be using (wlan0). Note that unlike as mentioned in the references, there is no pre-up command here. This happens in /etc/network/interfaces:

auto wlan0
iface wlan0 inet static
   address X.X.X.X
   netmask mask.mask.mask.mask
   gateway your.gateway.ip
   wireless_mode managed 
   wireless_essid "YOUR_ESSID"
   wpa-driver nl80211
   wpa-conf /etc/wpa_supplicant.conf
   post-down killall -q wpa_supplicant

• Generate the hash for your WPA pre-shared key using wpa_passphrase, follow the prompts and then look for the psk line, you’ll be wanting to put that in your wpa_supplicant configuration file, /etc/wpa_supplicant.conf:

fast_reauth=0
ap_scan=1
network={
 scan_ssid=1
 proto=WPA2
 key_mgmt=WPA-PSK
 pairwise=TKIP
 group=TKIP
 ssid="YOUR_ESSID"
 psk=long_hex_string_from_wpa_password

}

• We now need to ensure the DWA-131 firmware is loaded at boot time by configuring the kernel module with the USB device ID of the adapter. Angstrom is using systemd for boot time initialisation scripts so I just wrote a little service that runs when the networking stack comes up. The key here is to ensure it runs before we try to bring the interface up (see wifi.service below), note the Before condition.  The contents below goes into /lib/systemd/system/wifidriver.service:

[Unit]
Description=wifidriver
Wants=network.target
Before=wifi.service

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/home/root/wifidriver.sh

[Install]
WantedBy=multi-user.target

• The /home/root/wifidriver.sh it references is simple:

#!/bin/sh
echo "2001 330D" | tee /sys/bus/usb/drivers/rtl8192cu/new_id

• Enable the service so it runs at boot:

# systemctl enable wifidriver.service

• The last step is to get the interface itself to come up (as we disabled connman which technically was meant to do it on demand, but seems to be rather average at that job). This is again done with a simple systemd service. The contents below goes into /lib/systemd/system/wifi.service:

[Unit]
Description=wifi
Wants=network.target
Before=network.target
BindsTo=sys-subsystem-net-devices-wlan0.device
After=sys-subsystem-net-devices-wlan0.device

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/home/root/wifi.sh
ExecStop=/bin/sh -c "ifdown wlan0"

[Install]
WantedBy=multi-user.target

• The /home/root/wifi.sh it references (the sleep is just to give the driver time to load the firmware and boot the device):

#!/bin/sh
sleep 10
ifup wlan0

• Finally, enable the service so it runs at boot:

# systemctl enable wifi.service

The act of bringing up the interface should cause wpa_supplicant to run and authenticate against your network (it will remain running as a daemon to handle rekeying etc).

Reboot and hopefully you’ll be able to SSH in directly without the USB tether.

References:

1. Ubuntu 12.04 – Install D-Link DWA-131 rev. B1 wifi adapter
2. Auto connect to a wireless network (Ångström Linux)

Next research project: low power BlueTooth instead of WiFi. But before then, some notes on ultrasonic range finding.