Battery voltage monitoring

[5yler]

Tasks

• [x] set up voltage monitoring hardware
• [x] publish ROS message with battery voltage
• [ ] calibrate readings

[5yler]

There's a sensor_msgs/BatteryState message which is a bit more complicated than we need:

# Constants are chosen to match the enums in the linux kernel
# defined in include/linux/power_supply.h as of version 3.7
# The one difference is for style reasons the constants are
# all uppercase not mixed case.

# Power supply status constants
uint8 POWER_SUPPLY_STATUS_UNKNOWN = 0
uint8 POWER_SUPPLY_STATUS_CHARGING = 1
uint8 POWER_SUPPLY_STATUS_DISCHARGING = 2
uint8 POWER_SUPPLY_STATUS_NOT_CHARGING = 3
uint8 POWER_SUPPLY_STATUS_FULL = 4

# Power supply health constants
uint8 POWER_SUPPLY_HEALTH_UNKNOWN = 0
uint8 POWER_SUPPLY_HEALTH_GOOD = 1
uint8 POWER_SUPPLY_HEALTH_OVERHEAT = 2
uint8 POWER_SUPPLY_HEALTH_DEAD = 3
uint8 POWER_SUPPLY_HEALTH_OVERVOLTAGE = 4
uint8 POWER_SUPPLY_HEALTH_UNSPEC_FAILURE = 5
uint8 POWER_SUPPLY_HEALTH_COLD = 6
uint8 POWER_SUPPLY_HEALTH_WATCHDOG_TIMER_EXPIRE = 7
uint8 POWER_SUPPLY_HEALTH_SAFETY_TIMER_EXPIRE = 8

# Power supply technology (chemistry) constants
uint8 POWER_SUPPLY_TECHNOLOGY_UNKNOWN = 0
uint8 POWER_SUPPLY_TECHNOLOGY_NIMH = 1
uint8 POWER_SUPPLY_TECHNOLOGY_LION = 2
uint8 POWER_SUPPLY_TECHNOLOGY_LIPO = 3
uint8 POWER_SUPPLY_TECHNOLOGY_LIFE = 4
uint8 POWER_SUPPLY_TECHNOLOGY_NICD = 5
uint8 POWER_SUPPLY_TECHNOLOGY_LIMN = 6

Header  header
float32 voltage          # Voltage in Volts (Mandatory)
float32 current          # Negative when discharging (A)  (If unmeasured NaN)
float32 charge           # Current charge in Ah  (If unmeasured NaN)
float32 capacity         # Capacity in Ah (last full capacity)  (If unmeasured NaN)
float32 design_capacity  # Capacity in Ah (design capacity)  (If unmeasured NaN)
float32 percentage       # Charge percentage on 0 to 1 range  (If unmeasured NaN)
uint8   power_supply_status     # The charging status as reported. Values defined above
uint8   power_supply_health     # The battery health metric. Values defined above
uint8   power_supply_technology # The battery chemistry. Values defined above
bool    present          # True if the battery is present

float32[] cell_voltage   # An array of individual cell voltages for each cell in the pack
                         # If individual voltages unknown but number of cells known set each to NaN
string location          # The location into which the battery is inserted. (slot number or plug)
string serial_number     # The best approximation of the battery serial number

We should probably just publish a single floating point value from the Arduino code to conserve bandwidth on our Arduino serial connection, and possibly republish later.

[5yler]

This is off by some amount but otherwise the readings seem reasonable... Needs calibration.