Power Source

I have estimated that each motor will draw an average of about 500mA. There will be 5 motors so this give 5 * 0.5 = 2.5A. Lawna used NiMH cells but for this larger current drain, I believe a lead-acid battery would be better. A smallish 14AH battery, should provide the weight over the wheels and even with a 50% charge margin (7AH), would give me over two hours of mowing time. In practice, anything over hour would be good but being able to do the whole lawn on one charge would be fantastic.

Lead acid batteries are also very easy to charge and lend themselves easily to solar charging. With a 20 Watt panel in bright sunlight, it should be possible to recharge in an afternoon, and even in overcast conditions, it should only take two or three days. However, it might be more challenging to make a docking station and I have experimented with homing beacons with Lawna. However, we need to focus on keeping Mo v1 simple fo now.

Measuring Battery Charge

To determine if the robot needs to recharge, it is necessary to monitor its battery voltage. The arduino can do this easily with the minimum of external components.

A 12v lead-acid battery can have a charge in excess of 14v when charging. When its terminal voltage drops to below 12 volts, it is totally discharged and this is not a good place to be. We need to monitor the battery voltage with the arduino which has a 5v rail. Therefore, we can use a potential divider circuit to bring the battery voltage down within the range of the arduino. Two series resistors of 68k and 33k achieve this effectively. Given this as a starting point, we can calculate the equivalent digital thresholds.

/* Calculating Battery Thresholds.
 ** We use a 33k + 68k potential divider. This gives 33/(33 +68) = 0.327 reduction.
 ** So maximum voltage measurable = 5volts/0.327 = 15.3volts. Maximum is usually 14.4 for pb 
 ** A fully charged battery has a terminal voltage of 12.7v
 ** For leadAcid, 12.4v for 75% charge, 12.2v for 50% charge. I will use 12.3v
 ** So we need to go to the FEED state when we drop to 12.3v and start work again at 12.7v.
 ** Going through our potential divider, this gives:
 ** 12.7 * 0.327 = 4.15 volts = Max Charge.
 ** 12.3 * 0.327 = 4.02 volts = Min Charge
 ** 5v  is represented by 1023 in 16 digital bits.
 ** So representing charge levels in 8 bit thresholds:
 ** (4.15/5) * 1023 = 849 for max threshold.
 ** (4.02/5) * 1023 = 822 for minimum threshold.
 ** When the battery is being charged, the voltage will tend to be higher if charged from a stiff
 ** source. This is why many chargers charge to 13.8v. Given this information, I will initially try
 ** charging to 13.2v for the max threshold. This gives:
 ** (13.2 * 0.327)/5  * 1023 = 883. This goes some way to ensuring full charge. I may need to tweak.
 ** So the values for min and max of 822 and 883 are defined in mo.ino 
 */

Measuring Daylight

A simple Light Dependant Resistor (LDR) can be used to detect daylight hours. There are a few approaches that could be taken here:

• Analogue Comparator to digital input.
• LDR triggered transistor to digital input.
• Simple potential divider analogue input.

The first two of these require some simple external electronics to convert the analogue output from the LDR into a digital on/off signal. There a several spare analogue input pins on the arduino so it is simpler to use a potential divider and switch on a level in software so this is the approach taken.

The LDR used has the following characteristics:

As dawn (or dusk) approaches, the output from the LDR will start to approach the threshold value and will fluctuate around that value for a period. This could cause the mower to switch between sleep and mow states (especially with passing clouds), so some form of hysteresis needs to be added so it transitions cleanly.

A potential divider of the LDR and a 27k resistor across the 5V supply with a threshold level of 800 works well.