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Grundprinzip

Der Roboter findet seine Ladestation, wo er wieder aufgeladen wird mit Hilfe der Perimeter-Schleife. Er fährt also solange in Uhrzeiger-Richtung an der Schleife entlang bis er eine Ladespannung an den Ladeanschlüssen feststellt. Hier stoppt der Roboter und lädt seinen Akku wieder auf.

Charger

For the charger, we are using a Lithium Ion e-bike charger (29.4V, 1.5A current limitting) that can be purchased via the shop Shopping.png. The charger is placed in a protected area (house etc.) and connected to the robot charging station. The charger should accomplish the following things (in this case Lithium-Ion, lead battery is similar, but less critical):

  • Charge battery pack via the charging pins of the robot
  • Maximum cutoff voltage / charge voltage compliance (charging voltage limit)
  • Maximum charging current compliance (charging current limit)

If you are using an existing charger, these things are implemented with high probability already in it.

Total power balance

  • 2 x Gear motors each 1A (under heavy load): 2A, 27 volt (measured at max. 80% accuracy by motor drivers)
  • 1 x Mower motor 1A (under normal load): 1A, 27 volt (measured at max. 80% accuracy by motor drivers)
  • PCB: 1A, 5 volt (not measured by PCB)


Total: 3A * 27v + 1A * 5v = 81W + 5W = 86W

Battery

For the battery, we are using a 'Sony Konion 7S2P' Lithium Ion battery pack (Sony Konion US18650V3 2250 mAh cells, Li-Mn), 29.4V x 4500 mAh = 132 Wh, 500 recharge cycles, 126 x 36 x 65 mm (LBH).

Using this battery with the Ardumower motors, the mowing time is about 1.5 hours (132 Wh / 86W).

battery charge/discharge conditions for optimal battery life time:

  • charging per cell: max 4.15v (0.3A - 0.1A charging cutt-off)
  • discharging per cell: min 3.1v

Standby-off / Under-voltage protection

There are two reasons for a battery switch (standby-off mechanism):

Reason1: If the robot was not started within 5 minutes, it should turn battery off to save energy.

Reason2: Modern batteries should not be completely discharged. When the robot isn't able to charge for some reason and when the battery is below a certain threshold, it should be able to switch off the battery itself (undervoltage protection)

Idea:

  1. User presses existing START/POWER button, that switches on the MOSFET for the time of pressing
  2. Arduino starts and immediately also switches on MOSFET (via Arduino pinBatterySwitch)
  3. If undervoltage detected, Arduino switches off MOSFET (via Arduino pinBatterySwitch)
  4. Emergy button is still used for completely switching ON/OFF power
  5. POWER/START button: switches-on operation voltage (1st one second press), starts robot in automatic mode (2nd one second press)
BOM:
1x IRF9540N  P-MOSFET   T1   (or IRF5210)
1x BC337 NPN   T2
1x Zener Diode 15V ZD15  Z1
2x Diode 1N4148 D2, D3
2x Diode 1N5819 50V D1, D4
1x 22 uF C1
1x 1k R4
2x 10k (R3, R8)
1x 30k R6
2x 100k  (R2, R7)

Robot charging

In the robot itself, the battery voltage is monitored (see diagram below). So it can be determined when the robot must go to the charging station. The robot and the voltage and the current during charging is controlled. So it can be determined whether the robot has reached the charger and when the battery is fully charged again.

robot charging circuit:

 Robot charging (+)---+------+-- relais ---- current sensor ----- battery (+)
                             |
                             ---- voltage sensing
 Robot charging (-)-- +------------------------------------------ battery (-)

Charging station

Ideally, the charging station also powers the perimeter loop sender.

charging station circuit:

 AC power supply => Charger 24V (+)----- Charging station charger pin(+)
                    GND         (-)----- Charging station charger pin(-)
                    Charger 24V (+)----- DC-DC converter 12V => Perimeter sender MC motor driver
                                                             => Perimeter sender Arduino Nano Vin

Charge monitoring/Battery settings

Via pfodApp (Android) you can monitor the charging process.

Charging station ideas

Videos

  1. Driving into charging station
  2. Drive in and out
  3. Tracking and docking


Further links