I've had one issue with the operation, and that is that I need some way to stop the motors at the end of travel. I could have done this with limit switches like I did for my MAN truck build:
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http://www.scale4x4rc.org/forums/showthread.php?t=80393
An alternative is to use angle sensors in each joint, and I've selected this method for this build because it also allows for a host of other cool applications and features:
- End stops
- Stall detection
- Recording of movement
- Replay of movement
- Full servo capability at the joint
- Alternative control modes (e.g. for beginners), like absolute bucket movement forward/backward and up/down, not angle control
- Automatic bucket leveling
- Automatic grading
- GPS assisted digging
- GPS controlled digging
- .. and probably more.
So I've started to add some angle sensors to the arm. First the stick and the bucket. (This approach could also very well be used on hydraulic arms, btw.)
I'm reading the sensor on the corresponding motor controller. That way, the control of the motor based on the angle sensor can be completely controlled on the controller without having to bother the main CPU.
As I was already modifying the motor controller, I added some heat sinks for good measure, but those are most likely not required at all.
Ebay link:
- 18x18x13.5mm heat sink:
http://www.ebay.com/itm/361352931009
To read the sensor, I'm using the unused analog inputs 4 and 5 on the Arduino. I've added longer pins to these positions so that I can attach wires on the front side. I did the same for GND and 5V.
The angle sensor itself is just a 10k Ohm linear potetiometer. I'm using a thumb wheel version to get it as flat as possible. I bought these ones at SparkFun:
- Thumbwheel potentiometer, 10k linear:
https://www.sparkfun.com/products/retired/11173
The potentiometer attaches to the hub adapter using one very small self-tapping screw.
The hub adapter has a D-shape that fits exactly onto the D-shape of the gear motor axle:
I've printed a part that mocks up a stick mount that has two purposes:
- Cover and hold the angle sensor
- Be a base when running the control an power lines to the stick.
The sensor connector has three pins, that connect to the Arduino cable from the first step:
- Pin1: GND
- Pin2: Sensor output (0-5V)
- Pin3: 5V
