08/08/2015 at 18:31 •
"Angstwiderstand" (literally "fear resistor") is a German expression for resistors you put in your design that do nothing in normal operation. Here's why you should put them in there anyways:
Today I had a few problems with the h-bridge controlling the height adjustment. Later I foolishly left the robot unsupervised in the garage with the high power circuitry on to let the usb power banks charge. After some time the l298 h-bridge went nuts:
Now that's toast. On the high power side the l298 is connected to the 14.8V power rail (+BATT), the input pins are connected to the microcontroller:
It seems like while bursting in smoke the l298 shorted +BATT to the micro, which destroyed it. Have a look at that bulge:
As you can see in the schematic the micro also controlls 4 FETs, two of which power the mowing motors.
As a result I had a smoking robot running it's mowing blades at full speed despite all controllers powered down
When I discovered that scene gaining control again luckily was as simple as flipping the switch for the high power circuit. Things learned from this:
- Put current limiting resistors in datalines (Angstwiderstand) to protect components from each other in case one blows up. In this case 1k resistors in all connections between l298 and microcontroller could have prevented this scenario
- Don't leave weird contraptions unattended with only a mosfet preventing them from becoming hazardous
12/30/2014 at 22:43 •
Well, it has been some time since the last update.. - but with vacation now there's stuff happening:
As you can see the casing on the back made some progress and there's a snow shield now!
I assure you it does work reasonably well, but I missed to shot a video and took it apart shortly after. The reason is that now the drive system has to do some heavy work and it revealed some issues:
- The mounting of the wheels to the motors is not sufficient for snow plowing
- The drive system supports two modes: a) regulate torque: measure a torque-ish value from the front wheels, do some calculus and apply it to the back whells b) regulate speed: the speed for all wheels is regulated individually, this isn't ideal for flat surfaces (more motors than there are degrees of freedom -> slightly tearing tires due to inaccuracies) but very nice for snow. Problem: b) doesn't seem to work as nice as it once did, not sure why though. I suspect the BackEMF-speed-measurement to have some trouble
this is the new mounting system for the wheels. Yes, I drilled wrong :-D No problem though.. Thanks to the screws there is no slipping no more and the axis is quite straight as well.
Another problem with the motors was that the connections between gearbox and motor itself were loose in all cases and therefore the whole assembly was rather noisy. To solve that I opened the boxes. Sadly they're not meant to be opened so you have to drill-open them (6-7mm drill, go down ~2mm in each corner). This is what you're greeted with:
Cleaned of the nasty grease (<- honestly the most work) and disassembled:
The motors are mounted with (slightly oversized) retainer rings, originally only loctite was used.
And here's the clue regarding the speed measurement:
BackEMF is going to be supported by photoelectric sensors, hence the two holes in the gear. Why not replace BackEMF completely? Well, these are the numbers of teeth of all the gears:
- Motor: 9
- Small gear: 9, 41
- Middle gear: 13, 58
- Big gear: 67
Therefore the number degrees per either rising or falling edge from the photoelectric sensor are:
0.5 * 9/58 * 13/67 * 360° = 5.4°
With the 14cm wheels that's a distance of :
0.5 * 9/58 * 13/67 * 2 * Pi * 70mm = 6.6mm
And I'm pretty certain that that's not enough for proper regulation on very low velocity. Now I could drill more holes into the gear, but those are so damn rock solid I ruined all my 3mm HSS-G drill bits with the 4*2 holes already - so no, won't do that..
For the sensors itself I ordered 3mm (like LEDs) phototransistors which get inserted in a 3mm hole on one side of the gear and a 3mm LED on the other.
In case you're interested the the motors: CHM-2435-1 is their model number and they're quite cheap. Got mine for 9€ each on http://www.pollin.de/shop/dt/Nzc1OTg2OTk-/Motoren/DC_Getriebemotoren/Gleichstrom_Getriebemotor_CHM_2435_1.html
And here's another minor thing I added to height adjustment mechanism:
Before in case the connector of the height measurement would have become loose in theory the motor could have teared the assembly apart or made the circuit burst in smoke. Now it gets shut off by a limit switch and a diode allows it to drive down only
10/10/2014 at 21:09 •
The last few days I was busy with rebuilding the mowing system. Now there's a lot less noise and vibration. It also got the laser cut hood mounted with springs to make it flexible.
The gain in power is just astonishing: