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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

  • in reply to dubbie

    Hello Dubbie, the problem is that the motors spin, and do it at a considerable speed. What seems ok when you test one minute, see what happens after one day. These stuff are done to be running, not just make a short test (I don't know how long you tested, it is just a general consideration). There is also the problem that if you run the motors for a long time and the structure is weak, can suffer serious damage due to the heating of the motor body.

     

    Enrico

  • in reply to balearicdynamics

    Enrico,

     

    You are right in thinking that the motor units might be a bit on the flimsy side. This is true. I have only used a 1.5 mm thickness shell to fit around the motors to hold them in place. I did wonder whether this might be enough, but it is surprisingly strong and so far, not one has broken. I even took your advice and changed from an 80% fill to a 20% fill and it doesn't seem to have affected the strength at all. Additionally, by having four motor units the weight on each one is significantly reduced. There is a problem with the turning pivot from the direction motor which is fitted to the display where all the weight is, and the motor drive unit which causes the forward and backward motor, as it all rests on the shaft of the direction step motor just having a friction fit into the holder plastic of the driving motor. These motor shafts are only 8 mm long so even the smallest amount of tolerance in the fitting leads to the motor units playing outwards. Another contribution to this problem is that the first time I fit them together it is  tight fit, but every time I take them apart the floppier it gets. I have contemplated gluing them once I have the final assembly ready. I am reluctant to do this at present as I have to keep taking them apart to tweak them and get them to work better. I have thought about tapping the end of the motor shaft so that I can screw everything together, but it is only a 5 mm diameter shaft and that would be pretty difficult to do!

     

    I had noticed that there did seem to be some change in the acrylic just before the drill broke through, which could be heating. The display comes with the acrylic cut-outs so I didn't have the choice of using Perspex. If I was doing this again, I might be tempted to use wood (MDF) instead.

     

    Dubbie

  • in reply to three-phase

    Donald,

     

    I hadn't really thought about using smaller diameter wheels. I do need the display to be several centimetres above the ground so that the camera is able to focus on the picture being travelled over, I might have to buy a macro lese as it is. Well, I might, the camera arrived recently and I was surprised by how small the lens is and consequently how difficult it might be to replace the standard lens with a macro lens. I'll have to see what the existing camera lens can do.

     

    Dubbie

  • in reply to shabaz

    Shabaz,

     

    I haven't tested the motors to see if they get hot, as I haven't run them for long enough yet, so this I something I'll have to try. I have been thinking of running at a higher voltage to see if this help. I have some more step motors now so if I burn one out it will not be too much of a problem.

     

    Dubbie

  • 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