Just one more thing....November 2013

OK, I can't help myself..

I had finished Lawna and been busying myself with beacon experiments. The summer was over so Lawna was due to go into hibernation until next year. However, its been mild, the grass keeps growing, the lawn is covered in leaves so there is work to do. So when it finally stopped raining, I set Lawna out to do her stuff and found a problem. When the grass is wet, the wheels skid, the robot is not heavy enough to gain purchase. Ok, the grass is pretty wet, but this is not good.

I have considered adding spikes to the wheels so they gain more purchase, but as the battery life is so good now, I'm considering beefing up the drive motors to get extra weight, extra torque and perhaps even extra speed to help ot plough through piles of wet leaves. I know the mower was originally designed just for a maintenance cut but having to decide what is 'good grass to cut' and 'bad grass' is not really labour saving. Bad grass means I have to mow the lawn myself and that is not the plan.

Locomotion Revisited

The original locomotion motors are MFA RE 280/1 with a 250:1 gearbox. These give a maximum working torque of 6250g/cm with a stall torque of 25000 g/cm. They are essentially 1 Watt motors that have a current drain between 100mA to 300mA. With the wheels I am using, I get a speed of about 20cm/sec which seems about right to me.

I've been looking at the next range up for MFA motors, the 6 Watt devices, one of which I used to replace the original cutting motor. I'm considering the RE 385 with a 148:1 gearbox. These give a maximum working torque of 9664 g/cm with a stall torque of 61805 g/cm. They have a current drain between 150mA to 800mA. With the same wheels, they will give a speed of 53 cm/sec. This is well over twice the speed and may be excessive although it would be possible to reduce this with software. The extra speed has safety implications and the mower may well damage itself hitting things faster than it currently does, and the eye-stalks may need redesign to prevent false triggering. It is also questionable as to how efficient the mower blade will be at this speed. The extra current consumption may mean all the gains I have made in battery life will be lost on the locomotion motors. However, increased speed means it will get the job done more quickly and will be able to plough through wet autumn leaves as needed.

The new motors are also physically larger and there is limited space to play with on the mower so things will need to move around a bit. There are other motors I've looked at but either their gearing was too high or they were physically much larger so the RE385 looks like my best option. All, in all, the argument against replacing the motors seems to outweigh and benefits gained. However, I'm still going to try, especially if I can adjust the speed in software without sacrificing too much torque.

New Locomotion Motors

The new motors were put in place and wired up to replace the existing motors. A few measurements showed Lawna now moves at 49cm/sec, pretty close to the estimated speed. I will need to replace the front bumper as the wheels now protrude further from the sides than they did previously so the bumper needs to be lengthened.

Here is a picture comparing the old motors with new ones. They are considerably larger.

I let her loose on the wet leaf-covered grass to see how she behaved.

Firstly, she moves at quite a pace, the torque in fantastic, pushing through huge mounds of leaves and clumps of long grass effortlessly. However, there were lots of undesirable side effects, many of which I had predicted.

• Erratic Behaviour. This was due to the eye-stalks vibrating due to the higher speed. I disabled them for a while and the mower behaved as expected.
• When reversing/turning, the mower backed up quite a way and span more than previously, making its mow-pattern rather odd. This was due to the higher speed.
• The grass cutting was degraded. I suspect this is due to the speed-over-ground. There must be a sweet-spot where the blade rotation speed and speed-over-ground give the best cut in a particular time. The mower moves fast now but tends to not cut as well as before.

I left the mower running to get an idea of how well the battery lasted and it looks like just over an hour which is good news indeed, total current drain is about 1 Amp with both motors turning and cutting grass simultaneously.

Fortunately, all of these shortcoming could be addressed easily by just adjusting the mower speed in software. I reduced it to just under half. The mower still moves faster than it used to but the cut improved dramatically. The eye stalks still give false triggering occasionally but I think I can overcome this by shortening them slightly. Reversing and turning was still more pronounced than with the original motors but is perfectly acceptable. I can always adjust that in software if needs be.

Running at < 50% PWM, has reduced the motor torque somewhat, but it is still very acceptable and Lawna happily ploughs through wet leaves and long grass, but perhaps not so eagerly as at full speed. I am hoping that after a recharge, I will get extra life from the batteries as well, since the motors are being throttled now. This would be an unexpected bonus as the battery life is very good as it stands.

November 2013