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  • Author Author: dubbie
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HoloPiBot #5 : Some Movement Has Been Achieved

dubbie

In my unofficial attempt at the Picasso Design Challenge I have made some progress and I now have two motor units fixed to the display screen. I did have some problems in drilling the acrylic sheet used to make the holder for the display as it is very brittle. I cracked both of the back sheets that make up the four layer holder, in three different places. I thought I had placed the sheets onto a stable flat surface before I started drilling, but unfortunately I forgot that I was resting the display on the heads of the four screws used to hold it all together, so of course when drilling between two of these supporting screw heads the acrylic would bend slightly. When the drill broke through the bottom of the second sheet, the bending was enough to promote crack growth so it kept snapping. It took me three attempts to work this out, but afterwards I managed to avoid any further snapping or cracks. It is not too much of a problem functionally as I have now glued everything together. It is just irritating - I should know better.

 

I plan to add four motor units to the display, one at each corner, but unfortunately I did not have enough motors so I had to order motor. While I was waiting for the extra motors to come I decided to just 3D print some supports for the two corners that did not have motor. I managed one and then my 3D printer also choose this time to run out of plastic spool so the video below just shows two motors and one support.

 

 

The system is just about working and with the two step motors powered to move forward the whole system does actually move. The step motors I used are small but they are also incredibly low power and have virtually no turning torque what so ever. They are just feeble. With only one motor if I added the battery pack to the display it just stopped moving. With two motor units it just about keeps going. I am hoping that with four units it will be acceptable. The system is moving slowly at the moment but that is the plan, so that it can display a magnified image of the Picasso painting that it is covering. Once I have more spool for my 3D printer and four motor units added I will also see about implementing the ability to turn as well as move forward.

 

 

The Raspberry Pi 3B+ starter kit and the camera V2 have been ordered so I will also soon (he said optimistically) be able to start displaying images on the display. Then it will be almost finished!

 

Dubbie

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  • Hi Dubbie,

     

    Nice updates, thanks. When I saw your robot with motors I thought, "great, I'll be able to read Dubbie's blog and see where I went wrong with my r2b4" but I'm sorry to hear you too have suffered from low torque steppers. I have little I can add to help you as I never got the expected torque from my motors either, however made some really useful points about the high frequency pulses and the coil inductance meaning full coil current (and hence torque) was never achieved....something I need to take another look at.

     

    When I've cut acrylic before I use water as a coolant/lubricant to get a better cut, which might help, and I always drill through into some scrap wood.

     

    Rod

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  • Hi Dubbie,

     

    Nice updates, thanks. When I saw your robot with motors I thought, "great, I'll be able to read Dubbie's blog and see where I went wrong with my r2b4" but I'm sorry to hear you too have suffered from low torque steppers. I have little I can add to help you as I never got the expected torque from my motors either, however made some really useful points about the high frequency pulses and the coil inductance meaning full coil current (and hence torque) was never achieved....something I need to take another look at.

     

    When I've cut acrylic before I use water as a coolant/lubricant to get a better cut, which might help, and I always drill through into some scrap wood.

     

    Rod

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  • in reply to 14rhb

    Rod,

     

    I was aware that these small step motors had a low torque output, which was one of the reasons I went for four wheel drive, it should still move forward, but I was still surprised by how feeble the torque is. I have used other step motors, 12V types, 5V types and even 3V types, which all had either a decent torque or a really high torque (the 3V step motors - which used high currents of several amps). I did know about overdriving the step motors in order to get faster rise times, which was especially important for the low voltage high current step motors but these little step motors have quite a small current so overdriving doesn't do that much you, especially at the low speeds I am using.

     

    I am going to try using a higher voltage as that should provide a higher torque, but the drawback is that it causes the motor coils to heat up and if you're not careful they can burn out. I'll also try double step which should double the torque. I'll have to see how it goes.

     

    I hadn't thought of using a coolant when drilling acrylic, it might help but obviously is more complicated to do. I did think I had been drilling onto a flat piece of work after the second breakage, only to find out that I had forgotten the screws heads holding all the layers together were protruding at the corners, so that when drilling between screws, the acrylic could still flex, which resulted in more cracks. Once I had spotted that I didn't create any more breakages. In my previous experience of drilling plastic I have never had any problems at all so it was a surprise when the breasks did occur. But on reflection I think I might have been drilling Perspex rather than acrylic and probably only drilling through a single sheet at a time, onto a flat wooden surface. There's also some new way of doing things wrong.

     

    Dubbie