90 ton 1/14 metal excavator scratch build w/embedded PC

[SteinHDan]

Yes, true, I just thought the 3kg would be less violent when it engages. It should probably be a pulling spring instead of a pushing spring so that the backlash is only seen when pushing hard with the stick and not when pulling hard. I'll see what I can do about it.

I also found that the set screw was loose, so the backlash is only about half of what you see in the video when the set screw is tight.

I have done a few things that I haven't documented here yet. The gas spring on the stick, a gas spring on the bucket, a mount for the CPU (the Raspberry Pi) and angle sensors for the bucket and stick. I'll get those updates up here shortly. :-)


Stein :-)

[Lil Giants]

Originally Posted by SteinHDan

I also found that the set screw was loose, so the backlash is only about half of what you see in the video when the set screw is tight.

Do you have flat spots on the shaft for the set screws to anchor to? Blue locktite is adequate to keep set screws tight! But the locktite will also seep around shaft & mounted part too making it a real bytch! to get apart again. Maybe you don't want to add that just yet during testing phases, eh?

[SteinHDan]

Added a simple mount for the brain (CPU) of the excavator. At the same time I replaced the Raspberry Pi 2 with the version 3. It's mostly the same, but a little bit quicker and has WiFi and BlueTooth built in.

- Raspberry Pi 3


The mount is very simple. Four self-tapping screws to hold the PCB and then two nylon M4 bolts to hold the mount on the base plate.








The audio (engine sound) has its output through that 90-degree 3.5mm jack on the top.

[SteinHDan]

Bolting together the arm

The arm sandwich of 3mm aluminum sheets with 3D-printed ABS in the middle is held together by 34 M4 countersunk stainless steel bolts and sleeve nuts.

There are different length bolts used throughout the arm:
- 23 x 50mm (for the main part)
- 5 x 75mm (80mm cut to length) for the stick hub
- 4 x 80mm for the stick mount
- 2 x 55mm for the stick angle sensor mount

Ebay links:
- Bolts: http://www.ebay.com/itm/231787670429
- Sleeve nuts: http://www.ebay.com/itm/191771467782

[SteinHDan]

I added a gas spring from the boom to the stick. I only found the correct length, stroke and force on Aliexpress:

- 6kg (60N) gas spring: http://www.aliexpress.com/snapshot/7601366858.html

The sleeves and spacers are cut from 8mm and 12mm stainless steel tubes. The pin is also gotten from Ebay:

- 8mm (ID 4mm) tube cut to 13mm length: http://www.ebay.com/itm/121600172085
- 12mm (ID 8mm) tube cut to 4mm legnth: http://www.ebay.com/itm/121762512929
- 8x30mm pin: http://www.ebay.co.uk/itm/121851516316

[SteinHDan]

I also added a gas spring to the bucket, mostly for the visual appearance as the bucket is already way more powerful than it needs to be.

The spacers are made the same way as previously, cut tube to length.

Ebay links:
- 8mm (ID 6mm) tube cut to 4mm length: http://www.ebay.com/itm/121762445745
- 12mm (ID 8mm) tube cut to 4mm legnth: http://www.ebay.com/itm/121762512929
- 6x30mm pin: http://www.ebay.co.uk/itm/121868709865
- 8x80mm pin: http://www.ebay.com/itm/291511678072
- Eyelets: http://www.ebay.com/itm/151838223022

The lower eyelet is drilled out to 8mm to fit the 8mm pin in the bucket linkage.

[SteinHDan]

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:
- 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