T_Y's Build Log

[EddieJ]

Quick update, HYTTIOAOA is now complete, aside from the aluminium wedge. For that, I'm waiting for the aluminium servo horn to arrive (should be soon), as the plastic one currently attached is slightly shorter.

In the meantime, I've been procrastinating about the mechanum wheel bot. While browsing the Sketchup model library, I found a mechanum wheel model, so I'm going to have a go at modifying that and see if I can print it off.

@EddieJ I might not need you to send me a set of wheels in the end, depending on how this goes. Sorry about any inconvenience.

No probs at all! Let us know how the print and modify job works out!

[T_Y]

So I found an omni wheel on the Sketchup model library (search "Omni Wheel 2.0", model is by "machinebot...") and had a go at modifying it for 3D printing.





Original is on the left, modified is on the right.

I've bulked out the roller supports, hollowed out the inside of the wheel and added the 3mm D-shaft hole. I'm going to thread everything with 1mm brass rod, so the holes in the rollers have been made 1.4mm to give plenty of freedom to roll. I've also carved out part of the internal struts on one side to recess the motor into, to reduce the width of the motor + wheel. The wheel will be cut down the middle for printing.

Next is to print it off and see if it goes together, I just need to wait for the brass rod to arrive.

[MarkR]

Or use paperclips. They're probably as robust as 1mm brass rod.

It looks amazing, I'd print it as a single piece with some support material.

[T_Y]

So, CAD work has progressed, I now have the design of the whole bot and have a chassis printed off ready for assembly.





It prints off in a single piece with no supports, taking nearly 8 hours!

It took me a while to settle on the design, as I had to cram in all the components (2x Arduino Pro Mini, 2x Adafruit DRV8833, battery and receiver). After AWS60 I'll look into finding a better microcontroller solution. The plan is to have a hinged 0.5mm titanium plough on the front, hopefully there is enough weight.

I've also printed off the parts for the wheels and started assembling them.





I will probably have to redo the wheels, as I've noticed that as the wheel rotates, the side parts holding the rollers in place will contact the ground. I may also have to add grip to the rollers. I might do this by using O rings like I normally do, or find some grippy tape. I'll also have to remove the back shafts on these motors.

Lastly, I have finally decided on a name for the bot. Because I've spent a long time trying to think of a name, it will be called Error 404 - Robot Name Not Found.

[MichaelG92]

This looks great, I love your designs! I'll be very interested to see how the mechanum wheels work. Is the front plough stationary or will it rotate like Shockwave's?

[T_Y]

The plough will be hinged, so it scrapes the ground, but alas, I won't be able to power the plough - there's just not enough room or weight to do so!

[retropunk]

This is looking totally sick, although i’m slightly terrified of a mechanum (spelling?) wheel ant! good luck

[MarkR]

I don't think it's necessary to use two arduino pro minis.

You will need 2 GPIOs for each H-Bridge, i.e. 8

Then just need one pin to connect a S-BUS / i-bus receiver (use the Arduino's UART RX pin), which is 9 pins in total, which is less than the number of gpios on a single Arduino pro mini.

I haven't used those specific components, but it should work.

But even if you wanted 1 PWM pin for each channel (4 channels) then that's still only 12 pins.

The /SLEEP line can be tied high so it doesn't use a gpio.

Mark

[T_Y]

I thought I needed 2 PWM pins per H-bridge :S *Checks Adafruit DRV8833 documentation*

Huh. That makes things a bit simpler! So it looks like I only need 1 PWM pin per H-bridge - the Arduino Pro Mini has 6 so I'm good then, thanks for pointing that out!

Each DRV8833 board also has a sleep pin, which I'll connect to the same pin on the Arduino for failsafe control. Plus two more pins for receiver input.

I've printed off some larger rollers which should reduce the amount of contact the roller supports make with the ground... I'll have to assemble the bot and program the Arduino before I can assess how badly that will affect drive.

[MarkR]

I'm not an expert on the Arduino's capabilities - there is probably a limit to how many / which pins can do *hardware* PWM, but software PWM (bit-twiddling) can be done on as many pins as you have cpu time.

For those H-bridges, and most of the ones I've seen, there are two signals - essentially, forward and reverse - you need to PWM them individually to run forward and backwards, but not both at the same time.

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I think it should be possible to have the cpu spin in a loop waiting for the time to turn off the pin(s) to generate the correct duty cycle, and have interrupts (or some other code which runs at part of the loop) update the data from the receiver.

I also don't think it's necessary to connect /SLEEP to a gpio, you can turn off all the motors by setting all pins off, the /SLEEP is probably intended for low power applications to save battery (e.g. long-lived primary cells, think of Banksy's girl with balloon thing)